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SWITCHING REGULATORS
Introduction
The switching regulator is increasing in popularity because it offers the advantages
of higher power conversion efficiency and increased design flexibility (multiple output
voltages of different polarities can be generated from a single input voltage).
This paper will detail the operating principles of the four most commonly used
switching converter types:
Buck: used the reduce a DC voltage to a lower DC voltage.
Boost: provides an output voltage that is higher than the input.
Buck-Boost (invert): an output voltage is generated opposite in polarity to the input.
Flyback: an output voltage that is less than or greater than the input can be
generated, as well as multiple outputs.
Also, some multiple-transistor converter topologies will be presented:
Push-Pull: A two-transistor converter that is especially efficient at low input voltages.
Half-Bridge: A two-transistor converter used in many off-line applications.
Full-Bridge: A four-transistor converter (usually used in off-line designs) that can
generate the highest output power of all the types listed.
Application information will be provided along with circuit examples that illustrate
some applications of Buck, Boost, and Flyback regulators.
Switching Fundamentals
Before beginning explanations of converter theory, some basic elements of power
conversion will be presented:
THE LAW OF INDUCTANCE
If a voltage is forced across an inductor, a current will flow through that inductor (and
this current will vary with time). Note that the current flowing in an inductor will
be time-varying even if the forcing voltage is constant.
It is equally correct to say that if a time-varying current is forced to flow in an
inductor, a voltage across the inductor will result.
The fundamental law that defines the relationship between the voltage and current in
an inductor is given by the equation:
v = L (di/dt)
31
Two important characteristics of an inductor that follow directly from the law of
inductance are:
1) A voltage across an inductor results only from a current that changes with
time. A steady (DC) current flowing in an inductor causes no voltage across it
(except for the tiny voltage drop across the copper used in the windings).
2) A current flowing in an inductor can not change value instantly (in zero
time), as this would require infinite voltage to force it to happen. However, the
faster the current is changed in an inductor, the larger the resulting voltage
will be.
Note: Unlike the current flowing in the inductor, the voltage across it can change
instantly (in zero time).
The principles of inductance are illustrated by the information contained in Figure 25.
The important parameter is the di/dt term, which is simply a measure of how the
current changes with time. When the current is plotted versus time, the value of
di/dt is defined as the slope of the current plot at any given point.
The graph on the left shows that current which is constant with time has a di/dt value
of zero, and results in no voltage across the inductor.
The center graph shows that a current which is increasing with time has a positive
di/dt value, resulting in a positive inductor voltage.
Current that decreases with time (shown in the right-hand graph) gives a negative
value for di/dt and inductor voltage.
It is important to note that a linear current ramp in an inductor (either up or down)
occurs only when it has a constant voltage across it.
i(t)
+ v -
i(t)
di/dt = 0
v = 0
I
T
i(t)
di/dt > 0
v > 0
I
T
i(t)
di/dt < 0
v < 0
I
T
FIGURE 25. INDUCTOR VOLTAGE/CURRENT RELATIONSHIP
32
TRANSFORMER OPERATION
A transformer is a device that has two or more magnetically-coupled windings. The
basic operation is shown in Figure 26.
The action of a transformer is such that a time-varying (AC) voltage or current is
transformed to a higher or lower value, as set by the transformer turns ratio. The
transformer does not add power, so it follows that the power (V X I) on either side
must be constant. That is the reason that the winding with more turns has higher
voltage but lower current, while the winding with less turns has lower voltage
but higher current.
The dot on a transformer winding identifies its polarity with respect to another
winding, and reversing the dot results in inverting the polarity.
Example of Transformer Operation:
An excellent example of how a transformer works can be found under the hood of
your car, where a transformer is used to generate the 40 kV that fires your cars
spark plugs (see Figure 27).
VA V B
IA IB
N1 N2
V B VA N1
N2
= I B IA
N2
= N1
VA V B
IA IB
N1 N2
V B -VA N1
N2
= IB -IA
N2
= N1
+
-
+
-
+
-
+
-
FIGURE 26. TRANSFORMER THEORY
N1 N2
POINTS CLOSED
12V
(N2 >>> N1)
SPARK
GAP
STORING ENERGY
POINTS OPEN
12V
SPARK FIRES
VP VS
C C
CURRENT
LIMITING
RESISTOR
COIL
FIGURE 27. SPARK FIRING CIRCUIT
33
The "coil" used to generate the spark voltage is actually a transformer, with a very
high secondary-to-primary turns ratio.
When the points first close, current starts to flow in the primary winding and
eventually reaches the final value set by the 12V battery and the current limiting
resistor. At this time, the current flow is a fixed DC value, which means no voltage is
generated across either winding of the transformer.
When the points open, the current in the primary winding collapses very quickly,
causing a large voltage to appear across this winding. This voltage on the primary is
magnetically coupled to (and stepped up by) the secondary winding, generating a
voltage of 30 kV - 40 kV on the secondary side.
As explained previously, the law of inductance says that it is not possible to instantly
break the current flowing in an inductor (because an infinite voltage would be
required to make it happen).
This principle is what causes the arcing across the contacts used in switches that
are in circuits with highly inductive loads. When the switch just begins to open, the
high voltage generated allows electrons to jump the air gap so that the current flow
does not actually stop instantly. Placing a capacitor across the contacts helps to
reduce this arcing effect.
In the automobile ignition, a capacitor is placed across the points to minimize
damage due to arcing when the points "break" the current flowing in the low-voltage
coil winding (in car manuals, this capacitor is referred to as a "condenser").
PULSE WIDTH MODULATION (PWM)
All of the switching converters that will be covered in this paper use a form of output
voltage regulation known as Pulse Width Modulation (PWM). Put simply, the
feedback loop adjusts (corrects) the output voltage by changing the ON time of the
switching element in the converter.
As an example of how PWM works, we will examine the result of applying a series of
square wave pulses to an L-C filter (see Figure 28).
VPULSE
L
C
VPULSE
T P
VPK T V ON OUT = VPK
T P
X
TON
FIGURE 28. BASIC PRINCIPLES OF PWM
34
The series of square wave pulses is filtered and provides a DC output voltage that
is equal to the peak pulse amplitude multiplied times the duty cycle (duty cycle
is defined as the switch ON time divided by the total period).
This relationship explains how the output voltage can be directly controlled by
changing the ON time of the switch.
Switching Converter Topologies
The most commonly used DC-DC converter circuits will now be presented along with
the basic principles of operation.
BUCK REGULATOR
The most commonly used switching converter is the Buck, which is used to
down-convert a DC voltage to a lower DC voltage of the same polarity. This is
essential in systems that use distributed power rails (like 24V to 48V), which must be
locally converted to 15V, 12V or 5V with very little power loss.
The Buck converter uses a transistor as a switch that alternately connects and
disconnects the input voltage to an inductor (see Figure 29).
The lower diagrams show the current flow paths (shown as the heavy lines) when
the switch is on and off.
When the switch turns on, the input voltage is connected to the inductor. The
difference between the input and output voltages is then forced across the inductor,
causing current through the inductor to increase.
During the on time, the inductor current flows into both the load and the output
capacitor (the capacitor charges during this time).
VIN
VOUT
PWM
CONTROL
SWITCH
LOAD
L
D C
+
+
SWITCH
VIN ON
SWITCH
VIN OFF
FIGURE 29. BUCK REGULATOR
35
When the switch is turned off, the input voltage applied to the inductor is removed.
However, since the current in an inductor can not change instantly, the voltage
across the inductor will adjust to hold the current constant.
The input end of the inductor is forced negative in voltage by the decreasing current,
eventually reaching the point where the diode is turned on. The inductor current
then flows through the load and back through the diode.
The capacitor discharges into the load during the off time, contributing to the total
current being supplied to the load (the total load current during the switch off time is
the sum of the inductor and capacitor current).
The shape of the current flowing in the inductor is similar to Figure 30.
As explained, the current through the inductor ramps up when the switch is on, and
ramps down when the switch is off. The DC load current from the regulated output
is the average value of the inductor current.
The peak-to-peak difference in the inductor current waveform is referred to as the
inductor ripple current, and the inductor is typically selected large enough to keep
this ripple current less than 20% to 30% of the rated DC current.
CONTINUOUS vs. DISCONTINUOUS OPERATION
In most Buck regulator applications, the inductor current never drops to zero during
full-load operation (this is defined as continuous mode operation). Overall
performance is usually better using continuous mode, and it allows maximum output
power to be obtained from a given input voltage and switch current rating.
In applications where the maximum load current is fairly low, it can be advantageous
to design for discontinuous mode operation. In these cases, operating in
discontinuous mode can result in a smaller overall converter size (because a smaller
inductor can be used).
Discontinuous mode operation at lower load current values is generally harmless,
and even converters designed for continuous mode operation at full load will
become discontinuous as the load current is decreased (usually causing no
problems).
TON TOFF TON TOFF
INDUCTOR
CURRENT
EQUIVALENT DC
LOAD CURRENT
FIGURE 30. BUCK REGULATOR INDUCTOR CURRENT
36
BOOST REGULATOR
The Boost regulator takes a DC input voltage and produces a DC output voltage that
is higher in value than the input (but of the same polarity). The Boost regulator is
shown in Figure 31, along with details showing the path of current flow during the
switch on and off time.
Whenever the switch is on, the input voltage is forced across the inductor which
causes the current through it to increase (ramp up).
When the switch is off, the decreasing inductor current forces the "switch" end of the
inductor to swing positive. This forward biases the diode, allowing the capacitor to
charge up to a voltage that is higher than the input voltage.
During steady-state operation, the inductor current flows into both the output
capacitor and the load during the switch off time. When the switch is on, the load
current is supplied only by the capacitor.
OUTPUT CURRENT AND LOAD POWER
An important design consideration in the Boost regulator is that the output load
current and the switch current are not equal, and the maximum available load
current is always less than the current rating of the switch transistor.
It should be noted that the maximum total power available for conversion in any
regulator is equal to the input voltage multiplied times the maximum average
input current (which is less than the current rating of the switch transistor).
Since the output voltage of the Boost is higher than the input voltage, it follows
that the output current must be lower than the input current.
VOUT
SWITCH LOAD
L
D
C
PWM
CONTROL
VIN
+
+
SWITCH
ON
VIN
SWITCH
OFF
VIN
FIGURE 31. BOOST REGULATOR
37
BUCK-BOOST (INVERTING) REGULATOR
The Buck-Boost or Inverting regulator takes a DC input voltage and produces a DC
output voltage that is opposite in polarity to the input. The negative output voltage
can be either larger or smaller in magnitude than the input voltage.
The Inverting regulator is shown in Figure 32.
When the switch is on, the input voltage is forced across the inductor, causing an
increasing current flow through it. During the on time, the discharge of the output
capacitor is the only source of load current.
This requires that the charge lost from the output capacitor during the on time be
replenished during the off time.
When the switch turns off, the decreasing current flow in the inductor causes the
voltage at the diode end to swing negative. This action turns on the diode, allowing
the current in the inductor to supply both the output capacitor and the load.
As shown, the load current is supplied by inductor when the switch is off, and
by the output capacitor when the switch is on.
SWITCH
L LOAD
D
C
-VOUT
+
VIN - PWM
CONTROL
+
-
+
+ -
-
SWITCH
VIN ON +
-
VIN
+
-
SWITCH
OFF
FIGURE 32. BUCK-BOOST (INVERTING) REGULATOR
38
FLYBACK REGULATOR
The Flyback is the most versatile of all the topologies, allowing the designer to
create one or more output voltages, some of which may be opposite in polarity.
Flyback converters have gained popularity in battery-powered systems, where a
single voltage must be converted into the required system voltages (for example,
+5V, +12V and -12V) with very high power conversion efficiency.
The basic single-output flyback converter is shown in Figure 33.
The most important feature of the Flyback regulator is the transformer phasing, as
shown by the dots on the primary and secondary windings.
When the switch is on, the input voltage is forced across the transformer primary
which causes an increasing flow of current through it.
Note that the polarity of the voltage on the primary is dot-negative (more
negative at the dotted end), causing a voltage with the same polarity to appear at the
transformer secondary (the magnitude of the secondary voltage is set by the
transformer seconday-to-primary turns ratio).
VOUT
PWM
CONTROL
SWITCH
LOAD
D
C
+
+ VIN
+
+
SWITCH
ON
VIN
+
+
SWITCH
OFF
VIN
FIGURE 33. SINGLE-OUTPUT FLYBACK REGULATOR
39
The dot-negative voltage appearing across the secondary winding turns off the
diode, preventing current flow in the secondary winding during the switch on time.
During this time, the load current must be supplied by the output capacitor alone.
When the switch turns off, the decreasing current flow in the primary causes the
voltage at the dot end to swing positive. At the same time, the primary voltage is
reflected to the secondary with the same polarity. The dot-positive voltage occurring
across the secondary winding turns on the diode, allowing current to flow into both
the load and the output capacitor. The output capacitor charge lost to the load
during the switch on time is replenished during the switch off time.
Flyback converters operate in either continuous mode (where the secondary
current is always >0) or discontinuous mode (where the secondary current falls to
zero on each cycle).
GENERATING MULTIPLE OUTPUTS
Another big advantage of a Flyback is the capability of providing multiple outputs
(see Figure 34). In such applications, one of the outputs (usually the highest
current) is selected to provide PWM feedback to the control loop, which means this
output is directly regulated.
The other secondary winding(s) are indirectly regulated, as their pulse widths will
follow the regulated winding. The load regulation on the unregulated secondaries is
not great (typically 5 - 10%), but is adequate for many applications.
If tighter regulation is needed on the lower current secondaries, an LDO
post-regulator is an excellent solution. The secondary voltage is set about 1V above
the desired output voltage, and the LDO provides excellent output regulation with
very little loss of efficiency.
FIGURE 34. TYPICAL MULTIPLE-OUTPUT FLYBACK
PWM
CONTROL
+
VIN
+
+
+
FEEDBACK
-12V LDO
REGULATOR
12V LDO
REGULATOR
-12V
+12V
+5V
GND
40
PUSH-PULL CONVERTER
The Push-Pull converter uses two to transistors perform DC-DC conversion (see
Figure 35).
The converter operates by turning on each transistor on alternate cycles (the two
transistors are never on at the same time). Transformer secondary current flows
at the same time as primary current (when either of the switches is on).
For example, when transistor "A" is turned on, the input voltage is forced across the
upper primary winding with dot-negative polarity. On the secondary side, a
dot-negative voltage will appear across the winding which turns on the bottom diode.
This allows current to flow into the inductor to supply both the output capacitor and
the load.
When transistor "B" is on, the input voltage is forced across the lower primary
winding with dot-positive polarity. The same voltage polarity on the secondary turns
on the top diode, and current flows into the output capacitor and the load.
An important characteristic of a Push-Pull converter is that the switch transistors
have to be able the stand off more than twice the input voltage: when one transistor
is on (and the input voltage is forced across one primary winding) the same
magnitude voltage is induced across the other primary winding, but it is "floating" on
top of the input voltage. This puts the collector of the turned-off transistor at twice
the input voltage with respect to ground.
The "double input voltage" rating requirement of the switch transistors means the
Push-Pull converter is best suited for lower input voltage applications. It has been
widely used in converters operating in 12V and 24V battery-powered systems.
VOUT
+ +
VIN
GND
A B
PWM CONTROL
Np C
Np Ns
Ns
FIGURE 35. PUSH-PULL CONVERTER
41
Figure 36 shows a timing diagram which details the relationship of the input and
output pulses.
It is important to note that frequency of the secondary side voltage pulses is twice
the frequency of operation of the PWM controller driving the two transistors. For
example, if the PWM control chip was set up to operate at 50 kHz on the primary
side, the frequency of the secondary pulses would be 100 kHz.
The DC output voltage is given by the equation:
VOUT = VPK X (TON / TPER)
The peak amplitude of the secondary pulses (VPK) is given by:
VPK = (VIN - VSWITCH) X (NS / NP) - VRECT
This highlights why the Push-Pull converter is well-suited for low voltage converters.
The voltage forced across each primary winding (which provides the power for
conversion) is the full input voltage minus only the saturation voltage of the switch.
If MOS-FET power switches are used, the voltage drop across the switches can be
made extremely small, resulting in very high utilization of the available input voltage.
Another advantage of the Push-Pull converter is that it can also generate multiple
output voltages (by adding more secondary windings), some of which may be
negative in polarity. This allows a power supply operated from a single battery to
provide all of the voltages necessary for system operation.
A disadvantage of Push-Pull converters is that they require very good matching of
the switch transistors to prevent unequal on times, since this will result in saturation
of the transformer core (and failure of the converter).
A
B
C
TON TPER
VPK
FIGURE 36. TIMING DIAGRAM FOR PUSH-PULL CONVERTER
42
HALF-BRIDGE CONVERTER
The Half-Bridge is a two-transistor converter frequently used in high-power designs.
It is well-suited for applications requiring load power in the range of 500W to 1500W,
and is almost always operated directly from the AC line.
Off-line operation means that no large 60 Hz power transformer is used, eliminating
the heaviest and costliest component of a typical transformer-powered supply. All of
the transformers in the Half-Bridge used for power conversion operate at the
switching frequency (typically 50 kHz or higher) which means they can be very small
and efficient.
A very important advantage of the Half-Bridge is input-to-output isolation (the
regulated DC output is electrically isolated from the AC line). But, this means that all
of the PWM control circuitry must be referenced to the DC output ground.
The voltage to run the control circuits is usually generated from a DC rail that is
powered by a small 60 Hz transformer feeding a three-terminal regulator. In some
designs requiring extremely high efficiency, the switcher output takes over and
provides internal power after the start-up period.
The switch transistor drive circuitry must be isolated from the transistors, requiring
the use of base drive transformers. The added complexity of the base drive circuitry
is a disadvantage of using the Half-Bridge design.
If a 230 VAC line voltage is rectified by a full-wave bridge and filtered by a capacitor,
an unregulated DC voltage of about 300V will be available for DC-DC conversion. If
115 VAC is used, a voltage doubler circuit is typically used to generate the 300V rail.
The basic Half-bridge converter is shown in Figure 37. A capacitive divider is tied
directly across the unregulated DC input voltage, providing a reference voltage of
1/2VIN for one end of the transformer primary winding. The other end of the primary
is actively driven up and down as the transistors alternately turn on and off.
V OUT
GND
AC
AC
+
-
+
+
FWB
A
B
PWM CONTROL
C
NP
NS
NS
VIN
FIGURE 37. HALF-BRIDGE CONVERTER
43
The switch transistors force one-half of the input voltage across the primary winding
during the switch on time, reversing polarity as the transistors alternate. The
switching transistors are never on at the same time, or they would be destroyed
(since they are tied directly across VIN). The timing diagram for the Half-Bridge
converter is shown in Figure 38 (it is the same as the Push-Pull).
When the "A" transistor is on, a dot-positive voltage is forced across the primary
winding and reflected on the secondary side (with the magnitude being set by the
transformer turns ratio). The dot-positive secondary voltage turns on the upper
rectifier diode, supplying current to both the output capacitor and the load.
When the "A" transistor turns off and the "B" transistor turns on, the polarity of the
primary voltage is reversed. The secondary voltage polarity is also reversed, turning
on the lower diode (which supplies current to the output capacitor and the load).
In a Half-Bridge converter, primary and secondary current flow in the
transformer at the same time (when either transistor is on), supplying the load
current and charging the output capacitor. The output capacitor discharges into the
load only during the time when both transistors are off.
It can be seen that the voltage pulses on the transformer secondary side (applied to
the L-C filter) are occurring at twice the frequency of the PWM converter which
supplies the drive pulses for the switching transistors.
The output voltage is again given by:
VOUT = VPK X (TON / TPER)
The peak amplitude of the secondary pulses (VPK) is given by:
VPK = (1/2 VIN - VSWITCH) X (NS / NP) - VRECT
A
B
C
TON TPER
VPK
FIGURE 38. TIMING DIAGRAM FOR HALF-BRIDGE CONVERTER
44
FULL-BRIDGE CONVERTER
The Full-Bridge converter requires a total of four switching transistors to perform
DC-DC conversion. The full bridge is most often seen in applications that are
powered directly from the AC line, providing load power of 1 kW to 3 kW.
Operating off-line, the Full Bridge converter typically uses about 300V of unregulated
DC voltage for power conversion (the voltage that is obtained when a standard 230
VAC line is rectified and filtered).
An important feature of this design is the isolation from the AC line provided by the
switching transformer. The PWM control circuitry is referenced to the the output
ground, requiring a dedicated voltage rail (usually powered from a small 60 Hz
transformer) to run the control circuits.
The base drive voltages for the switch transistors (which are provided by the PWM
chip) have to be transformer-coupled because of the required isolation.
Figure 39 shows a simplified schematic diagram of a Full-Bridge converter.
The transformer primary is driven by the full voltage VIN when either of the
transistor sets ("A" set or "B" set) turns on. The full input voltage utilization means
the Full-Bridge can produce the most load power of all the converter types.
The timing diagram is identical to the Half-Bridge, as shown in Figure 38.
Primary and secondary current flows in the transformer during the switch on times,
while the output capacitor discharges into the load when both transistors are off.
The equation for the output voltage is (see Figure 38):
VOUT = VPK X (TON / TPER)
The peak voltage of the transformer secondary pulses (VPK) is given by:
VPK = (VIN - 2VSWITCH) X (NS / NP) - VRECT
VOUT
GND
AC
AC
+
-
+
+
FWB
A
B
PWM CONTROL
C
NP
NS
NS
VIN
A
B
FIGURE 39. FULL BRIDGE CONVERTER
45
Application Hints For Switching Regulators
Application information will be provided on topics which will enhance the designers
ability to maximize switching regulator performance.
Capacitor Parasitics Affecting Switching Regulator Performance
All capacitors contain parasitic elements which make their performance less than
ideal (see Figure 40).
INPUT CAPACITORS
All of the switching converters in this paper (and the vast majority in use) operate as
DC-DC converters that "chop" a DC input voltage at a very high frequency. As the
converter switches, it has to draw current pulses from the input source. The source
impedance is extremely important, as even a small amount of inductance can
cause significant ringing and spiking on the voltage at the input of the converter.
The best practice is to always provide adequate capacitive bypass as near as
possible to the switching converter input. For best results, an electrolytic is used
with a film capacitor (and possibly a ceramic capacitor) in parallel for optimum high
frequency bypassing.
ESR
ESL
R C
Summary of Effects of Parasitics:
ESR: The ESR (Equivalent Series Resistance) causes
internal heating due to power dissipation as the ripple
current flows into and out of the capacitor. The capacitor
can fail if ripple current exceeds maximum ratings.
Excessive output voltage ripple will result from high ESR,
and regulator loop instability is also possible. ESR is
highly dependent on temperature, increasing very quickly at
temperatures below about 10 °C.
ESL: The ESL (Effective Series Inductance) limits the
high frequency effectiveness of the capacitor. High ESL is
the reason electrolytic capacitors need to be bypassed by
film or ceramic capacitors to provide good high-frequency
performance.
The ESR, ESL and C within the capacitor form a resonant
circuit, whose frequency of resonance should be as high as
possible. Switching regulators generate ripple voltages on
their outputs with very high frequency (>10 MHz)
components, which can cause ringing on the output voltage
if the capacitor resonant frequency is low enough to be near
these frequencies.
FIGURE 40.
CAPACITOR PARASITICS
46
OUTPUT CAPACITOR ESR EFFECTS
The primary function of the output capacitor in a switching regulator is filtering. As
the converter operates, current must flow into and out of the output filter capacitor.
The ESR of the output capacitor directly affects the performance of the switching
regulator. ESR is specified by the manufacturer on good quality capacitors, but be
certain that it is specified at the frequency of intended operation.
General-purpose electrolytics usually only specify ESR at 120 Hz, but capacitors
intended for high-frequency switching applications will have the ESR guaranteed at
high frequency (like 20 kHz to 100 kHz).
Some ESR dependent parameters are:
Ripple Voltage: In most cases, the majority of the output ripple voltage results
from the ESR of the output capacitor. If the ESR increases (as it will at low
operating temperatures) the output ripple voltage will increase accordingly.
Efficiency: As the switching current flows into and out of the capacitor (through the
ESR), power is dissipated internally. This "wasted" power reduces overall regulator
efficiency, and can also cause the capacitor to fail if the ripple current exceeds
the maximum allowable specification for the capacitor.
Loop Stability: The ESR of the output capacitor can affect regulator loop stability.
Products such as the LM2575 and LM2577 are compensated for stability assuming
the ESR of the output capacitor will stay within a specified range.
Keeping the ESR within the "stable" range is not always simple in designs that must
operate over a wide temperature range. The ESR of a typical aluminum
electrolytic may increase by 40X as the temperature drops from 25°C to -40°C.
In these cases, an aluminum electrolytic must be paralleled by another type of
capacitor with a flatter ESR curve (like Tantalum or Film) so that the effective ESR
(which is the parallel value of the two ESR's) stays within the allowable range.
Note: if operation below -40°C is necessary, aluminum electrolytics are probably
not feasible for use.
BYPASS CAPACITORS
High-frequency bypass capacitors are always recommended on the supply pins of
IC devices, but if the devices are used in assemblies near switching converters
bypass capacitors are absolutely required.
The components which perform the high-speed switching (transistors and rectifiers)
generate significant EMI that easily radiates into PC board traces and wire leads.
To assure proper circuit operation, all IC supply pins must be bypassed to a clean,
low-inductance ground (for details on grounding, see next section).
47
Proper Grounding
The "ground" in a circuit is supposed to be at one potential, but in real life it is not.
When ground currents flow through traces which have non-zero resistance, voltage
differences will result at different points along the ground path.
In DC or low-frequency circuits, "ground management" is comparatively simple: the
only parameter of critical importance is the DC resistance of a conductor, since
that defines the voltage drop across it for a given current. In high-frequency circuits,
it is the inductance of a trace or conductor that is much more important.
In switching converters, peak currents flow in high-frequency (> 50 kHz) pulses,
which can cause severe problems if trace inductance is high. Much of the "ringing"
and "spiking" seen on voltage waveforms in switching converters is the result of high
current being switched through parasitic trace (or wire) inductance.
Current switching at high frequencies tends to flow near the surface of a conductor
(this is called "skin effect"), which means that ground traces must be very wide on a
PC board to avoid problems. It is usually best (when possible) to use one side of the
PC board as a ground plane.
Figure 41 illustrates an example of a terrible layout:
The layout shown has the high-power switch return current passing through a trace
that also provides the return for the PWM chip and the logic circuits. The switching
current pulses flowing through the trace will cause a voltage spike (positive and
negative) to occur as a result of the rising and falling edge of the switch current. This
voltage spike follows directly from the v = L (di/dt) law of inductance.
It is important to note that the magnitude of the spike will be different at all
points along the trace, being largest near the power switch. Taking the ground
symbol as a point of reference, this shows how all three circuits would be bouncing
up and down with respect to ground. More important, they would also be moving
with respect to each other.
VIN
LOGIC
CIRCUITS
POWER
SWITCH
PWM
CONTROL
CHIP
LTRACE LTRACE LTRACE
ISW
FIGURE 41. EXAMPLE OF POOR GROUNDING
48
Mis-operation often occurs when sensitive parts of the circuit "rattle" up and down
due to ground switching currents. This can induce noise into the reference used to
set the output voltage, resulting in excessive output ripple.
Very often, regulators that suffer from ground noise problems appear to be unstable,
and break into oscillations as the load current is increased (which increases ground
currents).
A much better layout is shown in Figure 42.
A big improvement is made by using single-point grounding. A good
high-frequency electrolytic capacitor (like solid Tantalum) is used near the input
voltage source to provide a good ground point.
All of the individual circuit elements are returned to this point using separate
ground traces. This prevents high current ground pulses from bouncing the logic
circuits up and down.
Another important improvement is that the power switch (which has the highest
ground pin current) is located as close as possible to the input capacitor. This
minimizes the trace inductance along its ground path.
It should also be pointed out that all of the individual circuit blocks have "local"
bypass capacitors tied directly across them. The purpose of this capacitor is RF
bypass, so it must be a ceramic or film capacitor (or both).
A good value for bypassing logic devices would be 0.01 mF ceramic capacitor(s),
distributed as required.
If the circuit to be bypassed generates large current pulses (like the power switch),
more capacitance is required. A good choice would be an aluminum electrolytic
bypassed with a film and ceramic capacitor. Exact size depends on peak current,
but the more capacitance used, the better the result.
VIN + POWER
SWITCH
PWM
CONTROL
CHIP
LOGIC
CIRCUITS
FIGURE 42. EXAMPLE OF GOOD GROUNDING
49
Transformer/Inductor Cores and Radiated Noise
The type of core used in an inductor or transformer directly affects its cost, size, and
radiated noise characteristics. Electrical noise radiated by a transformer is
extremely important, as it may require shielding to prevent erratic operation of
sensitive circuits located near the switching regulator.
The most commonly used core types will be presented, listing the advantages and
disadvantages of each.
The important consideration in evaluating the electrical noise that an inductor or
transformer is likely to generate is the magnetic flux path. In Figure 43, the slug
core and toroidal core types are compared.
The flux in the slug core leaves one end, travels through the air, and returns to the
other end. The slug core is the highest (worst) for radiated flux noise. In most
cases, the slug core device will give the smallest, cheapest component for a given
inductor size (it is very cheap to manufacture).
The magnetic flux path in the toroid is completely contained within the core. For
this reason it has the lowest (best) radiated flux noise. Toroid core components
are typically larger and more expensive compared to other core types. Winding a
toroid is fairly difficult (and requires special equipment), driving up the finished cost
of the manufactured transformer.
SLUG CORE
TOROID CORE
FIGURE 43. FLUX PATHS IN SLUG AND TOROID CORES
50
There is another class of cores commonly used in magnetic design which have
radiated flux properties that are much better than the slug core, but not as good as
the toroid. These cores are two-piece assemblies, and are assembled by gluing the
core pieces together around the bobbin that holds the winding(s).
The cores shown are frequently "gapped" to prevent saturation of the Ferrite core
material. The air gap is installed by grinding away a small amount of the core (the
gap may be only a few thousandths of an inch).
Figure 44 shows the popular E-I, E-E and Pot cores often used in switching regulator
transformers. The cores show the locations where an air gap is placed (if required),
but the bobbins/windings are omitted for clarity.
The air gap can emit flux noise because there is a high flux density in the vicinity of
the gap, as the flux passing through the core has to jump the air gap to reach the
other core piece.
The E-E and E-I cores are fairly cheap and easy to manufacture, and are very
common in switching applications up to about 1 kW. There is a wide variety of sizes
and shapes available, made from different Ferrite "blends" optimized for excellent
switching performance. The radiated flux from this type of core is still reasonably
low, and can usually be managed by good board layout.
The Pot core (which is difficult to accurately show in a single view drawing), benefits
from the shielding effect of the core sides (which are not gapped). This tends to
keep the radiated flux contained better than an E-E or E-I core, making the Pot core
second best only to the toroid core in minimizing flux noise.
Pot cores are typically more expensive than E-E or E-I cores of comparable power
rating, but they have the advantage of being less noisy. Pot core transformers are
much easier to manufacture than toroid transformers because the windings are
placed on a standard bobbin and then the core is assembled around it.
E-I CORE
GAP
GAP
E-E CORE
GAP
POT CORE
FIGURE 44. FLUX PATHS IN E-I, E-E AND POT CORES
51
Measuring Output Ripple Voltage
The ripple appearing on the output of the switching regulator can be important to the
circuits under power. Getting an accurate measurement of the output ripple voltage
is not always simple.
If the output voltage waveform is measured using an oscilloscope, an accurate result
can only be obtained using a differential measurement method (see Figure 45).
The differential measurement shown uses the second channel of the oscilloscope to
"cancel out" the signal that is common to both channels (by inverting the B channel
signal and adding it to the A channel).
The reason this method must be used is because the fast-switching components in a
switching regulator generate voltage spikes that have significant energy at very high
frequencies. These signals can be picked up very easily by "antennas" as small as
the 3" ground lead on the scope probe.
Assuming the probes are reasonably well matched, the B channel probe will pick up
the same radiated signal as the A channel probe, which allows this "common-mode"
signal to be eliminated by adding the inverted channel B signal to channel A.
It is often necessary to measure the RMS output ripple voltage, and this is usually
done with some type of digital voltmeter. If the reading obtained is to be meaningful,
the following must be considered:
1) The meter must be true-RMS reading, since the waveforms to be measured
are very non-sinusoidal.
2) The 3dB bandwidth of the meter should be at least 3X the bandwidth of the
measured signal (the output voltage ripple frequency will typically be > 100 kHz).
3) Subtract the "noise floor" from the measurement. Connect both meter leads
to the negative regulator output and record this value. Move the positive meter lead
to positive regulator output and record this value. The actual RMS ripple voltage
is the difference between these two readings.
O-SCOPE
CHAN A
CHAN B
+
-
SWITCHING
REGULATOR
POS OUT
NEG OUT
NOTE: INVERT CHANNEL B AND ADD TO CHANNEL A TO REMOVE COMMON-MODE SIGNAL
MATCHED PROBE SET
FIGURE 45. DIFFERENTIAL OUTPUT RIPPLE MEASUREMENT
52
Measuring Regulator Efficiency of DC-DC Converters
The efficiency (h) of a switching regulator is defined as:
h = PLOAD / PTOTAL
In determining converter efficiency, the first thing that must be measured is the total
consumed power (PTOTAL). Assuming a DC input voltage, PTOTAL is defined as
the total power drawn from the source, which is equal to:
PTOTAL = VIN X IIN (AVE)
It must be noted that the input current value used in the calculation must be the
average value of the waveform (the input current will not be DC or sinusoidal).
Because the total power dissipated must be constant from input to output, PTOTAL
is also equal to the load power plus the internal regulator power losses:
PTOTAL = PLOAD + PLOSSES
Measuring (or calculating) the power to the load is very simple, since the output
voltage and current are both DC. The load power is found by:
PLOAD = VOUT X ILOAD
Measuring the input power drawn from the source is not simple. Although the input
voltage to the regulator is DC, the current drawn at the input of a switching
regulator is not. If a typical "clip-on" current meter is used to measure the input
current, the taken data will be essentially meaningless.
The average input current to the regulator can be measured with reasonable
accuracy by using a wide-bandwidth current probe connected to an oscilloscope.
The average value of input current can be closely estimated by drawing a horizontal
line that divides the waveform in such a way that the area of the figure above the
line will equal the "missing" area below the line (see Figure 46). In this way, the
"average" current shown is equivalent to the value of DC current that would produce
the same input power.
IAVE
I PK
I IN
A
B
AREA "A" EQUALS AREA "B"
FIGURE 46. AVERAGE VALUE OF TYPICAL INPUT CURRENT WAVEFORM
53
If more exact measurements are needed, it is possible to force the current in the line
going to the input of the DC-DC converter to be DC by using an L-C filter between
the power source and the input of the converter (see Figure 47).
If the L-C filter components are adequate, the current coming from the output of the
DC power supply will be DC current (with no high-frequency switching
component) which means it can be accurately measured with a cheap clip-on
ammeter and digital volt meter.
It is essential that a large, low-ESR capacitor be placed at CIN to support the input
of the switching converter. The L-C filter that the converter sees looking back into
the source presents a high impedance for switching current, which means CIN is
necessary to provide the switching current required at the input of the converter.
Measuring Regulator Efficiency of Off-Line Converters
Off-Line converters are powered directly from the AC line, by using a bridge rectifier
and capacitive filter to generate an unregulated DC voltage for conversion (see
Figures 37 and 38).
Measuring the total power drawn from the AC source is fairly difficult because of the
power factor. If both the voltage and current are sinusoidal, power factor is
defined as the cosine of the phase angle between the voltage and current
waveforms.
The capacitive-input filter in an off-line converter causes the input current to be very
non-sinusoidal. The current flows in narrow, high-amplitude pulses (called
Haversine pulses) which requires that the power factor be re-defined in such cases.
For capacitive-input filter converters, power factor is defined as:
P.F. = PREAL / PAPPARENT
DC
SUPPLY
+
-
+
CIN
L L
C C
+ IN
- IN
SWITCHING
REGULATOR
CIN MUST BE LARGE VALUE
CURRENT HERE
IS NOT DC
CURRENT HERE IS DC
(MEASURE HERE)
VIN
IIN
P TOTAL = VIN X IIN
FIGURE 47. L-C FILTER USED IN DC INPUT CURRENT MEASUREMENT
54
The real power drawn from the source (PREAL) is the power (in Watts) which
equals the sum of the load power and regulator internal losses.
The apparent power (PAPPARENT) is equal to the RMS input current times the
RMS input voltage. Re-written, the importance of power factor is shown
IIN (RMS) = PREAL / ( VIN (RMS) X PF )
The RMS input current that the AC line must supply (for a given real power in
Watts) increases directly as the power factor reduces from unity. Power factor for
single-phase AC-powered converters is typically about 0.6. If three-phase power is
used, the power factor is about 0.9.
If the efficiency of an off-line converter is to be measured, power analyzers are
available which will measure and display input voltage, input power, and power
factor. These are fairly expensive, so they may not be available to the designer.
Another method which will give good results is to measure the power after the
rectifier bridge and input capacitor (where the voltage and current are DC). This
method is shown in Figure 48.
The current flowing from CIN to the converter should be very nearly DC, and the
average value can be readily measured or approximated (see previous section).
The total power drawn from the AC source is the sum of the power supplied by CIN
(which is VIN X IDC) and the power dissipated in the input bridge rectifier. The
power in the bridge rectifier is easily estimated, and is actually negligible in most
off-line designs.
AC
AC
+
-
+
FWB
V IN
TRANSFORMER
AND
RECTIFIERS
MEASURE CURRENT HERE
I DC
P TOTAL = V X + PFWB IN IDC
CIN
FIGURE 48. MEASURING INPUT POWER IN OFF-LINE CONVERTER
55
Application Circuits
Introduction
Application circuits will be detailed which will demonstrate some examples of
switching regulator designs.
LM2577: A Complete Flyback/Boost Regulator IC
The LM2577 is an IC developed as part of the SIMPLE SWITCHERÔ product
family. It is a current-mode control switching regulator, with a built-in NPN switch
rated for 3A switch current and 65V breakdown voltage.
The most commonly used applications are for Flyback or Boost regulators (see
Figure 49). In the Boost regulator, the output is always greater than the input. In the
Flyback, the output may be greater than, less than, or equal to the input voltage.
The theory of operation of the Flyback and Boost Converters has been previously
covered, and will not be repeated here.
The LM2577 is targeted for applications with load power requirements up to a
maximum of about 25 Watts, and can be used to implement Boost or Flyback
regulators (with multiple output voltages available if Flyback is selected).
The SIMPLE SWITCHER product family is supported by design-aid software titled
"Switchers Made Simple", which allows finished designs to be generated directly
from the computer.
The next sections will show the LM2577 being used in circuits which are more
advanced than the typical applications (these circuits were generated as solutions
for specific customer requirements).
C + + IN
COUT
VIN VOUT
GND GND
COMP
VIN SW
GND
FB
VOUT > VIN
LM2577
+ +
C IN
COUT
VIN VOUT
GND GND
COMP
VIN SW
GND
FB
LM2577
BOOST REGULATOR FLYBACK REGULATOR
FIGURE 49. BASIC APPLICATION CIRCUITS FOR THE LM2577
56
Increasing Available Load Power in an LM2577 Boost Regulator
One of the most frequently requested circuits is a method to squeeze more power
out of a boost converter. The maximum load power available at the output is directly
related to the input power available to the DC-DC converter.
When the input voltage is a low value (like 5V), this greatly reduces the amount of
power that can be drawn from the source (because the maximum input current is
limited by what the switch can handle). In the case of the LM2577, the maximum
switch current is 3A (peak).
Increased load power can be obtained with the LM2577 by paralleling two devices
(see Figure 50). Because current-mode control is used in the LM2577, the two
converters will automatically share the load current demand.
The right-hand regulator (which is a fixed 12V version) is the master that sets the
duty cycle of both regulators (tying the Compensation pins together forces the duty
cycles to track).
The master regulator has direct feedback from the output, while the other regulator
has its Feedback pin grounded. Grounding the Feedback pin makes the regulator
attempt to run "wide open" (at maximum duty cycle), but the master regulator
controls the voltage at both Compensation pins, which adjusts the pulse widths as
required to hold the output voltage at 12V.
+
VOUT
GND
COMP
VIN SW
GND
FB
LM2577
ADJ
VIN
COMP
VIN SW
GND
FB LM2577
12
+
+12V
@ 1.5A
1 mF 1 mF
470 mF
47 mH 47 mH
1000 mF
5V
0.22
mF
2.2K
SCHOTTKY (2 PL.)
FIGURE 50. DUAL LM2577 BOOST CIRCUIT
57
LM2577 Negative Buck Regulator
The LM2577 can be used in a Buck regulator configuration that takes a negative
input voltage and produces a regulated negative output voltage (see Figure 51).
The LM2577 is referenced to the negative input, which means the feedback signal
coming from the regulated output must be DC level shifted. R1, D1, and Q1 form a
current source that sets a current through R2 that is directly proportional to the
output voltage (D1 is included to cancel out the VBE of Q1).
Neglecting the base current error of Q1, the current through R2 is equal to:
IR2 = VOUT / R1 (which is 300 mA for this example.)
The voltage across R2 provides the 1.23V feedback signal which the LM2577
requires for its feedback loop.
The operation of the power converter is similar to what was previously described for
the Buck regulator:
When the switch is ON, current flows from ground through the load, into the
regulator output, through the inductor, and down through the switch to return to the
negative input. The output capacitor also charges during the switch ON time.
When the switch turns OFF, the voltage at the diode end of the inductor flies positive
until the Schottky diode turns on (this allows the inductor current to continue to flow
through the load during the OFF time). The output capacitor also discharges
through the load during the OFF time, providing part of the load current.
VOUT
GND
VIN SW
VIN
COMP
FB GND
LM2577
ADJ
+
+
-15V @ 3A
-20 TO -40V
150 mH
470 mF R1
49.9K
100K
R2
4.12K
2N3906
100 mF
3.3K
0.22 mF
3A/50V
SCHOTTKY
1N4148 Q1
R3
1.23V
D1
FIGURE 51. NEGATIVE BUCK REGULATOR
58
LM2577 Three-Output, Isolated Flyback Regulator
Many applications require electrical isolation between the input and output terminals
of the power supply (for example, medical monitoring instruments require
isolation to assure patient safety).
Figure 52 shows an example of a three-output Flyback regulator2 built using the
LM2577 that has electrical isolation between the input and output voltages.
Three output voltages are obtained from three separate transformer secondary
windings, with voltage feedback being taken from the 5V output.
To maintain electrical isolation, the feedback path uses a 4N27 opto-coupler to
transfer the feedback signal across the isolation barrier.
2K
INPUT
GND
VIN SW
VIN
FB COMP GND
LM2577
ADJ
+
+
+
+
51V
LM385
680
mF
0.1
mF
16 - 36V 18T
16T
24T
24T
100mF
220mF
470mF
0.1
mF
R1
15K
R2
45.3K
220
+
-
FB
OUTPUT
GND
4N27
47K
0.47mF
100k
4.7V
10K
470
5V@
150mA
7.5V@
100mA
-7.5V@
70mA
ISOLATION
FIGURE 52. THREE-OUTPUT ISOLATED FLYBACK REGULATOR
59
The 5V output is regulated using an LM385 adjustable reference, whose voltage is
set by R1 and R2. The LM385 operates by forcing a 1.24V reference voltage
between the positive terminal and the feedback pin, so the set voltage across the
LM385 is given by:
VREF = 1.24 X (R2/R1 + 1)
For the values shown in this example, the voltage will be 5V.
The function of the LM385 in the circuit can be described as an "ideal" Zener diode,
because the current flowing through the LM385 is very small until the voltage at its
positive terminal reaches 5V with respect to ground. At that point, it tries to regulate
its positive terminal to 5V by conducting current (which flows out of the negative
terminal of the LM385 and through the 470W resistor into the diode side of the
opto-coupler).
When the LM385 starts conducting current through the opto-coupler diode, the
collector of the transistor in the opto-coupler pulls down on the compensation pin of
the LM2577, which reduces the duty cycle (pulse widths) of the switching converter.
In this way, a negative feedback loop is established which holds the output at 5V.
The feedback signal from the collector of the opto-coupler is fed into the
compensation pin (not the feedback pin) of the LM2577 in order to bypass the
internal error amplifier of the LM2577. The gain of the LM385 is so high that using
the error amplifier inside the loop would make it difficult to stabilize (and is not
necessary for good performance).
Test data taken with the input voltage set to 26V and all outputs fully loaded showed
the frequency response of the control loop had a 0dB crossover point of 1kHz with a
phase margin of 90°.
The 7.5V and -7.5V outputs are not directly regulated, which means their voltages
are set by the pulse width of the regulated (5V) winding. As a result, the load
regulation of these two outputs is not quite as good as the 5V output.
Summary of test performance data:
Output Voltages
5V
7.5V
-7.5V
Line Regulation
(@ Full Load)
0.2%
0.3%
0.3%
Load Regulation
(VIN=26V)
0.04%
(30mA - 150mA)
3%
(20mA - 100mA)
2%
(12mA - 70mA)
Output Ripple
Voltage (25°C)
50mV
50mV
50mV
60
LM2575 and LM2576 Buck Regulators
The LM2575 and LM2576 products are Buck regulators developed as part of the
SIMPLE SWITCHERÔ product family.
The LM2575 is rated for 1A of continuous load current, while the LM2576 can supply
3A. The maximum input voltage for the parts is 40V (60V for the "HV" versions),
with both adjustable and fixed output voltages available. Both parts are included in
the "Switchers Made Simple" design software.
The basic Buck regulator application circuit is shown in Figure 53.
The LM2575 and LM2576 can also be used in an inverting (Buck-Boost)
configuration which allows a positive input voltage to be converted to a negative
regulated output voltage (see Figure 54).
+
+
LM2575-ADJ
V IN LM2576-ADJ
GND FB
OUT
ON/OFF
C IN
C OUT
L
D
R1
R2
V IN
V OUT = 1.23 X (1 + R2/R1)
GND GND
V OUT
FIGURE 53. LM2575 AND LM2576 BUCK REGULATOR APPLICATION
+
+
LM2575-ADJ
V IN LM2576-ADJ
GND FB
OUT
ON/OFF
C IN
C OUT
L
D
V IN
GND
- V OUT
FIGURE 54. LM2575 AND LM2576 INVERTING APPLICATION
61
Low Dropout, High Efficiency 5V/3A Buck Regulator
A circuit was developed which provides a 5V/3A regulated output voltage with very
high efficiency and very low dropout voltage (see Figure 55). The customer required
that the circuit be able to operate with an input voltage range of 6V to 12V, allowing
only 1V of dropout at the lowest input voltage.
An unusual feature of this circuit is that it can stay in regulation with only 300 mV
across the regulator. Also, the efficiency is highest (89%) at the lowest input
voltage (buck converters are typically more efficient at higher input voltages).
The low (<300mV) dropout voltage is achieved by using an external PNP power
transistor (Q2) as the main switching transistor (the other transistors in the circuit are
drivers for Q2). With components values shown, Q2 has a saturation voltage of
200mV @ 3A, which allows the 300 mV input-output differential requirement for the
regulator to be met.
The switch inside the LM2575 drives the base of Q1 through R2. Note that the
maximum collector current of Q1 (and the maximum base drive available for Q2) is
limited by Z1 and R4. When Z1 clamps at 5V, the maximum current through Q1 is:
I Q1 (MAX) = (5 - VBE) / R4 = 215mA
+
+
VIN LM2575-5
ON/ GND
OFF
GND
100
mF
5.3V TO
12V
VOUT
FB
330
200
1N5818
39
2N2222
1N4734
5V
20 1N5821 1000mF
68mH
D45H1
5V/3A
GND
Q1
Q2
R1 Z1
R2
R3
R4
D1
D2
FIGURE 55. LOW-DROPOUT 5V/3A REGULATOR
62
The maximum Q1 current (215mA) limits the amount of base drive available to Q2,
forcing the collector current of Q2 to "beta limit" as the output is overloaded (this
means the maximum collector current of Q2 will be limited by the gain of the
transistor and the base drive provided). Although this is not a precise current limiter,
it is adequate to protect Q2 from damage during an overload placed on the output.
If the regulator output is shorted to ground, the output short-circuit current flows
from the output of the LM2575 (through D1 and the inductor), which means the
regulator short-circuit current is limited to the value set internally to the
LM2575 (which is about 2A).
Note also that when the regulator output is shorted to ground, the cathode of D1 will
also be near ground. This allows D1 to clamp off the base drive to Q1 off,
preventing current flow in the switch transistor Q2.
If the input voltage does not exceed 8V, R2 and Z1 are not required in the circuit.
This circuit was tested with 6V input and was able to deliver more than 4A of load
current with 5V out. Other test data taken are:
Measured Performance Data:
Line Regulation:
5.3V to 12V @ 1A 32mV
5.3V to 12V @ 3A 45mV
Load Regulation:
0.3A to 3A @ 5.3V Input 10mV
0.3A to 3A @ 12V Input 17mV
Efficiency @ 3A Load:
VIN = 5.3V 89%
VIN = 12V 80%
Output Ripple Voltage:
VIN = 7.2V, IL = 3A 35 mV(p-p)
SWITCHING REGULATORS
Introduction
The switching regulator is increasing in popularity because it offers the advantages
of higher power conversion efficiency and increased design flexibility (multiple output
voltages of different polarities can be generated from a single input voltage).
This paper will detail the operating principles of the four most commonly used
switching converter types:
Buck: used the reduce a DC voltage to a lower DC voltage.
Boost: provides an output voltage that is higher than the input.
Buck-Boost (invert): an output voltage is generated opposite in polarity to the input.
Flyback: an output voltage that is less than or greater than the input can be
generated, as well as multiple outputs.
Also, some multiple-transistor converter topologies will be presented:
Push-Pull: A two-transistor converter that is especially efficient at low input voltages.
Half-Bridge: A two-transistor converter used in many off-line applications.
Full-Bridge: A four-transistor converter (usually used in off-line designs) that can
generate the highest output power of all the types listed.
Application information will be provided along with circuit examples that illustrate
some applications of Buck, Boost, and Flyback regulators.
Switching Fundamentals
Before beginning explanations of converter theory, some basic elements of power
conversion will be presented:
THE LAW OF INDUCTANCE
If a voltage is forced across an inductor, a current will flow through that inductor (and
this current will vary with time). Note that the current flowing in an inductor will
be time-varying even if the forcing voltage is constant.
It is equally correct to say that if a time-varying current is forced to flow in an
inductor, a voltage across the inductor will result.
The fundamental law that defines the relationship between the voltage and current in
an inductor is given by the equation:
v = L (di/dt)
31
Two important characteristics of an inductor that follow directly from the law of
inductance are:
1) A voltage across an inductor results only from a current that changes with
time. A steady (DC) current flowing in an inductor causes no voltage across it
(except for the tiny voltage drop across the copper used in the windings).
2) A current flowing in an inductor can not change value instantly (in zero
time), as this would require infinite voltage to force it to happen. However, the
faster the current is changed in an inductor, the larger the resulting voltage
will be.
Note: Unlike the current flowing in the inductor, the voltage across it can change
instantly (in zero time).
The principles of inductance are illustrated by the information contained in Figure 25.
The important parameter is the di/dt term, which is simply a measure of how the
current changes with time. When the current is plotted versus time, the value of
di/dt is defined as the slope of the current plot at any given point.
The graph on the left shows that current which is constant with time has a di/dt value
of zero, and results in no voltage across the inductor.
The center graph shows that a current which is increasing with time has a positive
di/dt value, resulting in a positive inductor voltage.
Current that decreases with time (shown in the right-hand graph) gives a negative
value for di/dt and inductor voltage.
It is important to note that a linear current ramp in an inductor (either up or down)
occurs only when it has a constant voltage across it.
i(t)
+ v -
i(t)
di/dt = 0
v = 0
I
T
i(t)
di/dt > 0
v > 0
I
T
i(t)
di/dt < 0
v < 0
I
T
FIGURE 25. INDUCTOR VOLTAGE/CURRENT RELATIONSHIP
32
TRANSFORMER OPERATION
A transformer is a device that has two or more magnetically-coupled windings. The
basic operation is shown in Figure 26.
The action of a transformer is such that a time-varying (AC) voltage or current is
transformed to a higher or lower value, as set by the transformer turns ratio. The
transformer does not add power, so it follows that the power (V X I) on either side
must be constant. That is the reason that the winding with more turns has higher
voltage but lower current, while the winding with less turns has lower voltage
but higher current.
The dot on a transformer winding identifies its polarity with respect to another
winding, and reversing the dot results in inverting the polarity.
Example of Transformer Operation:
An excellent example of how a transformer works can be found under the hood of
your car, where a transformer is used to generate the 40 kV that fires your cars
spark plugs (see Figure 27).
VA V B
IA IB
N1 N2
V B VA N1
N2
= I B IA
N2
= N1
VA V B
IA IB
N1 N2
V B -VA N1
N2
= IB -IA
N2
= N1
+
-
+
-
+
-
+
-
FIGURE 26. TRANSFORMER THEORY
N1 N2
POINTS CLOSED
12V
(N2 >>> N1)
SPARK
GAP
STORING ENERGY
POINTS OPEN
12V
SPARK FIRES
VP VS
C C
CURRENT
LIMITING
RESISTOR
COIL
FIGURE 27. SPARK FIRING CIRCUIT
33
The "coil" used to generate the spark voltage is actually a transformer, with a very
high secondary-to-primary turns ratio.
When the points first close, current starts to flow in the primary winding and
eventually reaches the final value set by the 12V battery and the current limiting
resistor. At this time, the current flow is a fixed DC value, which means no voltage is
generated across either winding of the transformer.
When the points open, the current in the primary winding collapses very quickly,
causing a large voltage to appear across this winding. This voltage on the primary is
magnetically coupled to (and stepped up by) the secondary winding, generating a
voltage of 30 kV - 40 kV on the secondary side.
As explained previously, the law of inductance says that it is not possible to instantly
break the current flowing in an inductor (because an infinite voltage would be
required to make it happen).
This principle is what causes the arcing across the contacts used in switches that
are in circuits with highly inductive loads. When the switch just begins to open, the
high voltage generated allows electrons to jump the air gap so that the current flow
does not actually stop instantly. Placing a capacitor across the contacts helps to
reduce this arcing effect.
In the automobile ignition, a capacitor is placed across the points to minimize
damage due to arcing when the points "break" the current flowing in the low-voltage
coil winding (in car manuals, this capacitor is referred to as a "condenser").
PULSE WIDTH MODULATION (PWM)
All of the switching converters that will be covered in this paper use a form of output
voltage regulation known as Pulse Width Modulation (PWM). Put simply, the
feedback loop adjusts (corrects) the output voltage by changing the ON time of the
switching element in the converter.
As an example of how PWM works, we will examine the result of applying a series of
square wave pulses to an L-C filter (see Figure 28).
VPULSE
L
C
VPULSE
T P
VPK T V ON OUT = VPK
T P
X
TON
FIGURE 28. BASIC PRINCIPLES OF PWM
34
The series of square wave pulses is filtered and provides a DC output voltage that
is equal to the peak pulse amplitude multiplied times the duty cycle (duty cycle
is defined as the switch ON time divided by the total period).
This relationship explains how the output voltage can be directly controlled by
changing the ON time of the switch.
Switching Converter Topologies
The most commonly used DC-DC converter circuits will now be presented along with
the basic principles of operation.
BUCK REGULATOR
The most commonly used switching converter is the Buck, which is used to
down-convert a DC voltage to a lower DC voltage of the same polarity. This is
essential in systems that use distributed power rails (like 24V to 48V), which must be
locally converted to 15V, 12V or 5V with very little power loss.
The Buck converter uses a transistor as a switch that alternately connects and
disconnects the input voltage to an inductor (see Figure 29).
The lower diagrams show the current flow paths (shown as the heavy lines) when
the switch is on and off.
When the switch turns on, the input voltage is connected to the inductor. The
difference between the input and output voltages is then forced across the inductor,
causing current through the inductor to increase.
During the on time, the inductor current flows into both the load and the output
capacitor (the capacitor charges during this time).
VIN
VOUT
PWM
CONTROL
SWITCH
LOAD
L
D C
+
+
SWITCH
VIN ON
SWITCH
VIN OFF
FIGURE 29. BUCK REGULATOR
35
When the switch is turned off, the input voltage applied to the inductor is removed.
However, since the current in an inductor can not change instantly, the voltage
across the inductor will adjust to hold the current constant.
The input end of the inductor is forced negative in voltage by the decreasing current,
eventually reaching the point where the diode is turned on. The inductor current
then flows through the load and back through the diode.
The capacitor discharges into the load during the off time, contributing to the total
current being supplied to the load (the total load current during the switch off time is
the sum of the inductor and capacitor current).
The shape of the current flowing in the inductor is similar to Figure 30.
As explained, the current through the inductor ramps up when the switch is on, and
ramps down when the switch is off. The DC load current from the regulated output
is the average value of the inductor current.
The peak-to-peak difference in the inductor current waveform is referred to as the
inductor ripple current, and the inductor is typically selected large enough to keep
this ripple current less than 20% to 30% of the rated DC current.
CONTINUOUS vs. DISCONTINUOUS OPERATION
In most Buck regulator applications, the inductor current never drops to zero during
full-load operation (this is defined as continuous mode operation). Overall
performance is usually better using continuous mode, and it allows maximum output
power to be obtained from a given input voltage and switch current rating.
In applications where the maximum load current is fairly low, it can be advantageous
to design for discontinuous mode operation. In these cases, operating in
discontinuous mode can result in a smaller overall converter size (because a smaller
inductor can be used).
Discontinuous mode operation at lower load current values is generally harmless,
and even converters designed for continuous mode operation at full load will
become discontinuous as the load current is decreased (usually causing no
problems).
TON TOFF TON TOFF
INDUCTOR
CURRENT
EQUIVALENT DC
LOAD CURRENT
FIGURE 30. BUCK REGULATOR INDUCTOR CURRENT
36
BOOST REGULATOR
The Boost regulator takes a DC input voltage and produces a DC output voltage that
is higher in value than the input (but of the same polarity). The Boost regulator is
shown in Figure 31, along with details showing the path of current flow during the
switch on and off time.
Whenever the switch is on, the input voltage is forced across the inductor which
causes the current through it to increase (ramp up).
When the switch is off, the decreasing inductor current forces the "switch" end of the
inductor to swing positive. This forward biases the diode, allowing the capacitor to
charge up to a voltage that is higher than the input voltage.
During steady-state operation, the inductor current flows into both the output
capacitor and the load during the switch off time. When the switch is on, the load
current is supplied only by the capacitor.
OUTPUT CURRENT AND LOAD POWER
An important design consideration in the Boost regulator is that the output load
current and the switch current are not equal, and the maximum available load
current is always less than the current rating of the switch transistor.
It should be noted that the maximum total power available for conversion in any
regulator is equal to the input voltage multiplied times the maximum average
input current (which is less than the current rating of the switch transistor).
Since the output voltage of the Boost is higher than the input voltage, it follows
that the output current must be lower than the input current.
VOUT
SWITCH LOAD
L
D
C
PWM
CONTROL
VIN
+
+
SWITCH
ON
VIN
SWITCH
OFF
VIN
FIGURE 31. BOOST REGULATOR
37
BUCK-BOOST (INVERTING) REGULATOR
The Buck-Boost or Inverting regulator takes a DC input voltage and produces a DC
output voltage that is opposite in polarity to the input. The negative output voltage
can be either larger or smaller in magnitude than the input voltage.
The Inverting regulator is shown in Figure 32.
When the switch is on, the input voltage is forced across the inductor, causing an
increasing current flow through it. During the on time, the discharge of the output
capacitor is the only source of load current.
This requires that the charge lost from the output capacitor during the on time be
replenished during the off time.
When the switch turns off, the decreasing current flow in the inductor causes the
voltage at the diode end to swing negative. This action turns on the diode, allowing
the current in the inductor to supply both the output capacitor and the load.
As shown, the load current is supplied by inductor when the switch is off, and
by the output capacitor when the switch is on.
SWITCH
L LOAD
D
C
-VOUT
+
VIN - PWM
CONTROL
+
-
+
+ -
-
SWITCH
VIN ON +
-
VIN
+
-
SWITCH
OFF
FIGURE 32. BUCK-BOOST (INVERTING) REGULATOR
38
FLYBACK REGULATOR
The Flyback is the most versatile of all the topologies, allowing the designer to
create one or more output voltages, some of which may be opposite in polarity.
Flyback converters have gained popularity in battery-powered systems, where a
single voltage must be converted into the required system voltages (for example,
+5V, +12V and -12V) with very high power conversion efficiency.
The basic single-output flyback converter is shown in Figure 33.
The most important feature of the Flyback regulator is the transformer phasing, as
shown by the dots on the primary and secondary windings.
When the switch is on, the input voltage is forced across the transformer primary
which causes an increasing flow of current through it.
Note that the polarity of the voltage on the primary is dot-negative (more
negative at the dotted end), causing a voltage with the same polarity to appear at the
transformer secondary (the magnitude of the secondary voltage is set by the
transformer seconday-to-primary turns ratio).
VOUT
PWM
CONTROL
SWITCH
LOAD
D
C
+
+ VIN
+
+
SWITCH
ON
VIN
+
+
SWITCH
OFF
VIN
FIGURE 33. SINGLE-OUTPUT FLYBACK REGULATOR
39
The dot-negative voltage appearing across the secondary winding turns off the
diode, preventing current flow in the secondary winding during the switch on time.
During this time, the load current must be supplied by the output capacitor alone.
When the switch turns off, the decreasing current flow in the primary causes the
voltage at the dot end to swing positive. At the same time, the primary voltage is
reflected to the secondary with the same polarity. The dot-positive voltage occurring
across the secondary winding turns on the diode, allowing current to flow into both
the load and the output capacitor. The output capacitor charge lost to the load
during the switch on time is replenished during the switch off time.
Flyback converters operate in either continuous mode (where the secondary
current is always >0) or discontinuous mode (where the secondary current falls to
zero on each cycle).
GENERATING MULTIPLE OUTPUTS
Another big advantage of a Flyback is the capability of providing multiple outputs
(see Figure 34). In such applications, one of the outputs (usually the highest
current) is selected to provide PWM feedback to the control loop, which means this
output is directly regulated.
The other secondary winding(s) are indirectly regulated, as their pulse widths will
follow the regulated winding. The load regulation on the unregulated secondaries is
not great (typically 5 - 10%), but is adequate for many applications.
If tighter regulation is needed on the lower current secondaries, an LDO
post-regulator is an excellent solution. The secondary voltage is set about 1V above
the desired output voltage, and the LDO provides excellent output regulation with
very little loss of efficiency.
FIGURE 34. TYPICAL MULTIPLE-OUTPUT FLYBACK
PWM
CONTROL
+
VIN
+
+
+
FEEDBACK
-12V LDO
REGULATOR
12V LDO
REGULATOR
-12V
+12V
+5V
GND
40
PUSH-PULL CONVERTER
The Push-Pull converter uses two to transistors perform DC-DC conversion (see
Figure 35).
The converter operates by turning on each transistor on alternate cycles (the two
transistors are never on at the same time). Transformer secondary current flows
at the same time as primary current (when either of the switches is on).
For example, when transistor "A" is turned on, the input voltage is forced across the
upper primary winding with dot-negative polarity. On the secondary side, a
dot-negative voltage will appear across the winding which turns on the bottom diode.
This allows current to flow into the inductor to supply both the output capacitor and
the load.
When transistor "B" is on, the input voltage is forced across the lower primary
winding with dot-positive polarity. The same voltage polarity on the secondary turns
on the top diode, and current flows into the output capacitor and the load.
An important characteristic of a Push-Pull converter is that the switch transistors
have to be able the stand off more than twice the input voltage: when one transistor
is on (and the input voltage is forced across one primary winding) the same
magnitude voltage is induced across the other primary winding, but it is "floating" on
top of the input voltage. This puts the collector of the turned-off transistor at twice
the input voltage with respect to ground.
The "double input voltage" rating requirement of the switch transistors means the
Push-Pull converter is best suited for lower input voltage applications. It has been
widely used in converters operating in 12V and 24V battery-powered systems.
VOUT
+ +
VIN
GND
A B
PWM CONTROL
Np C
Np Ns
Ns
FIGURE 35. PUSH-PULL CONVERTER
41
Figure 36 shows a timing diagram which details the relationship of the input and
output pulses.
It is important to note that frequency of the secondary side voltage pulses is twice
the frequency of operation of the PWM controller driving the two transistors. For
example, if the PWM control chip was set up to operate at 50 kHz on the primary
side, the frequency of the secondary pulses would be 100 kHz.
The DC output voltage is given by the equation:
VOUT = VPK X (TON / TPER)
The peak amplitude of the secondary pulses (VPK) is given by:
VPK = (VIN - VSWITCH) X (NS / NP) - VRECT
This highlights why the Push-Pull converter is well-suited for low voltage converters.
The voltage forced across each primary winding (which provides the power for
conversion) is the full input voltage minus only the saturation voltage of the switch.
If MOS-FET power switches are used, the voltage drop across the switches can be
made extremely small, resulting in very high utilization of the available input voltage.
Another advantage of the Push-Pull converter is that it can also generate multiple
output voltages (by adding more secondary windings), some of which may be
negative in polarity. This allows a power supply operated from a single battery to
provide all of the voltages necessary for system operation.
A disadvantage of Push-Pull converters is that they require very good matching of
the switch transistors to prevent unequal on times, since this will result in saturation
of the transformer core (and failure of the converter).
A
B
C
TON TPER
VPK
FIGURE 36. TIMING DIAGRAM FOR PUSH-PULL CONVERTER
42
HALF-BRIDGE CONVERTER
The Half-Bridge is a two-transistor converter frequently used in high-power designs.
It is well-suited for applications requiring load power in the range of 500W to 1500W,
and is almost always operated directly from the AC line.
Off-line operation means that no large 60 Hz power transformer is used, eliminating
the heaviest and costliest component of a typical transformer-powered supply. All of
the transformers in the Half-Bridge used for power conversion operate at the
switching frequency (typically 50 kHz or higher) which means they can be very small
and efficient.
A very important advantage of the Half-Bridge is input-to-output isolation (the
regulated DC output is electrically isolated from the AC line). But, this means that all
of the PWM control circuitry must be referenced to the DC output ground.
The voltage to run the control circuits is usually generated from a DC rail that is
powered by a small 60 Hz transformer feeding a three-terminal regulator. In some
designs requiring extremely high efficiency, the switcher output takes over and
provides internal power after the start-up period.
The switch transistor drive circuitry must be isolated from the transistors, requiring
the use of base drive transformers. The added complexity of the base drive circuitry
is a disadvantage of using the Half-Bridge design.
If a 230 VAC line voltage is rectified by a full-wave bridge and filtered by a capacitor,
an unregulated DC voltage of about 300V will be available for DC-DC conversion. If
115 VAC is used, a voltage doubler circuit is typically used to generate the 300V rail.
The basic Half-bridge converter is shown in Figure 37. A capacitive divider is tied
directly across the unregulated DC input voltage, providing a reference voltage of
1/2VIN for one end of the transformer primary winding. The other end of the primary
is actively driven up and down as the transistors alternately turn on and off.
V OUT
GND
AC
AC
+
-
+
+
FWB
A
B
PWM CONTROL
C
NP
NS
NS
VIN
FIGURE 37. HALF-BRIDGE CONVERTER
43
The switch transistors force one-half of the input voltage across the primary winding
during the switch on time, reversing polarity as the transistors alternate. The
switching transistors are never on at the same time, or they would be destroyed
(since they are tied directly across VIN). The timing diagram for the Half-Bridge
converter is shown in Figure 38 (it is the same as the Push-Pull).
When the "A" transistor is on, a dot-positive voltage is forced across the primary
winding and reflected on the secondary side (with the magnitude being set by the
transformer turns ratio). The dot-positive secondary voltage turns on the upper
rectifier diode, supplying current to both the output capacitor and the load.
When the "A" transistor turns off and the "B" transistor turns on, the polarity of the
primary voltage is reversed. The secondary voltage polarity is also reversed, turning
on the lower diode (which supplies current to the output capacitor and the load).
In a Half-Bridge converter, primary and secondary current flow in the
transformer at the same time (when either transistor is on), supplying the load
current and charging the output capacitor. The output capacitor discharges into the
load only during the time when both transistors are off.
It can be seen that the voltage pulses on the transformer secondary side (applied to
the L-C filter) are occurring at twice the frequency of the PWM converter which
supplies the drive pulses for the switching transistors.
The output voltage is again given by:
VOUT = VPK X (TON / TPER)
The peak amplitude of the secondary pulses (VPK) is given by:
VPK = (1/2 VIN - VSWITCH) X (NS / NP) - VRECT
A
B
C
TON TPER
VPK
FIGURE 38. TIMING DIAGRAM FOR HALF-BRIDGE CONVERTER
44
FULL-BRIDGE CONVERTER
The Full-Bridge converter requires a total of four switching transistors to perform
DC-DC conversion. The full bridge is most often seen in applications that are
powered directly from the AC line, providing load power of 1 kW to 3 kW.
Operating off-line, the Full Bridge converter typically uses about 300V of unregulated
DC voltage for power conversion (the voltage that is obtained when a standard 230
VAC line is rectified and filtered).
An important feature of this design is the isolation from the AC line provided by the
switching transformer. The PWM control circuitry is referenced to the the output
ground, requiring a dedicated voltage rail (usually powered from a small 60 Hz
transformer) to run the control circuits.
The base drive voltages for the switch transistors (which are provided by the PWM
chip) have to be transformer-coupled because of the required isolation.
Figure 39 shows a simplified schematic diagram of a Full-Bridge converter.
The transformer primary is driven by the full voltage VIN when either of the
transistor sets ("A" set or "B" set) turns on. The full input voltage utilization means
the Full-Bridge can produce the most load power of all the converter types.
The timing diagram is identical to the Half-Bridge, as shown in Figure 38.
Primary and secondary current flows in the transformer during the switch on times,
while the output capacitor discharges into the load when both transistors are off.
The equation for the output voltage is (see Figure 38):
VOUT = VPK X (TON / TPER)
The peak voltage of the transformer secondary pulses (VPK) is given by:
VPK = (VIN - 2VSWITCH) X (NS / NP) - VRECT
VOUT
GND
AC
AC
+
-
+
+
FWB
A
B
PWM CONTROL
C
NP
NS
NS
VIN
A
B
FIGURE 39. FULL BRIDGE CONVERTER
45
Application Hints For Switching Regulators
Application information will be provided on topics which will enhance the designers
ability to maximize switching regulator performance.
Capacitor Parasitics Affecting Switching Regulator Performance
All capacitors contain parasitic elements which make their performance less than
ideal (see Figure 40).
INPUT CAPACITORS
All of the switching converters in this paper (and the vast majority in use) operate as
DC-DC converters that "chop" a DC input voltage at a very high frequency. As the
converter switches, it has to draw current pulses from the input source. The source
impedance is extremely important, as even a small amount of inductance can
cause significant ringing and spiking on the voltage at the input of the converter.
The best practice is to always provide adequate capacitive bypass as near as
possible to the switching converter input. For best results, an electrolytic is used
with a film capacitor (and possibly a ceramic capacitor) in parallel for optimum high
frequency bypassing.
ESR
ESL
R C
Summary of Effects of Parasitics:
ESR: The ESR (Equivalent Series Resistance) causes
internal heating due to power dissipation as the ripple
current flows into and out of the capacitor. The capacitor
can fail if ripple current exceeds maximum ratings.
Excessive output voltage ripple will result from high ESR,
and regulator loop instability is also possible. ESR is
highly dependent on temperature, increasing very quickly at
temperatures below about 10 °C.
ESL: The ESL (Effective Series Inductance) limits the
high frequency effectiveness of the capacitor. High ESL is
the reason electrolytic capacitors need to be bypassed by
film or ceramic capacitors to provide good high-frequency
performance.
The ESR, ESL and C within the capacitor form a resonant
circuit, whose frequency of resonance should be as high as
possible. Switching regulators generate ripple voltages on
their outputs with very high frequency (>10 MHz)
components, which can cause ringing on the output voltage
if the capacitor resonant frequency is low enough to be near
these frequencies.
FIGURE 40.
CAPACITOR PARASITICS
46
OUTPUT CAPACITOR ESR EFFECTS
The primary function of the output capacitor in a switching regulator is filtering. As
the converter operates, current must flow into and out of the output filter capacitor.
The ESR of the output capacitor directly affects the performance of the switching
regulator. ESR is specified by the manufacturer on good quality capacitors, but be
certain that it is specified at the frequency of intended operation.
General-purpose electrolytics usually only specify ESR at 120 Hz, but capacitors
intended for high-frequency switching applications will have the ESR guaranteed at
high frequency (like 20 kHz to 100 kHz).
Some ESR dependent parameters are:
Ripple Voltage: In most cases, the majority of the output ripple voltage results
from the ESR of the output capacitor. If the ESR increases (as it will at low
operating temperatures) the output ripple voltage will increase accordingly.
Efficiency: As the switching current flows into and out of the capacitor (through the
ESR), power is dissipated internally. This "wasted" power reduces overall regulator
efficiency, and can also cause the capacitor to fail if the ripple current exceeds
the maximum allowable specification for the capacitor.
Loop Stability: The ESR of the output capacitor can affect regulator loop stability.
Products such as the LM2575 and LM2577 are compensated for stability assuming
the ESR of the output capacitor will stay within a specified range.
Keeping the ESR within the "stable" range is not always simple in designs that must
operate over a wide temperature range. The ESR of a typical aluminum
electrolytic may increase by 40X as the temperature drops from 25°C to -40°C.
In these cases, an aluminum electrolytic must be paralleled by another type of
capacitor with a flatter ESR curve (like Tantalum or Film) so that the effective ESR
(which is the parallel value of the two ESR's) stays within the allowable range.
Note: if operation below -40°C is necessary, aluminum electrolytics are probably
not feasible for use.
BYPASS CAPACITORS
High-frequency bypass capacitors are always recommended on the supply pins of
IC devices, but if the devices are used in assemblies near switching converters
bypass capacitors are absolutely required.
The components which perform the high-speed switching (transistors and rectifiers)
generate significant EMI that easily radiates into PC board traces and wire leads.
To assure proper circuit operation, all IC supply pins must be bypassed to a clean,
low-inductance ground (for details on grounding, see next section).
47
Proper Grounding
The "ground" in a circuit is supposed to be at one potential, but in real life it is not.
When ground currents flow through traces which have non-zero resistance, voltage
differences will result at different points along the ground path.
In DC or low-frequency circuits, "ground management" is comparatively simple: the
only parameter of critical importance is the DC resistance of a conductor, since
that defines the voltage drop across it for a given current. In high-frequency circuits,
it is the inductance of a trace or conductor that is much more important.
In switching converters, peak currents flow in high-frequency (> 50 kHz) pulses,
which can cause severe problems if trace inductance is high. Much of the "ringing"
and "spiking" seen on voltage waveforms in switching converters is the result of high
current being switched through parasitic trace (or wire) inductance.
Current switching at high frequencies tends to flow near the surface of a conductor
(this is called "skin effect"), which means that ground traces must be very wide on a
PC board to avoid problems. It is usually best (when possible) to use one side of the
PC board as a ground plane.
Figure 41 illustrates an example of a terrible layout:
The layout shown has the high-power switch return current passing through a trace
that also provides the return for the PWM chip and the logic circuits. The switching
current pulses flowing through the trace will cause a voltage spike (positive and
negative) to occur as a result of the rising and falling edge of the switch current. This
voltage spike follows directly from the v = L (di/dt) law of inductance.
It is important to note that the magnitude of the spike will be different at all
points along the trace, being largest near the power switch. Taking the ground
symbol as a point of reference, this shows how all three circuits would be bouncing
up and down with respect to ground. More important, they would also be moving
with respect to each other.
VIN
LOGIC
CIRCUITS
POWER
SWITCH
PWM
CONTROL
CHIP
LTRACE LTRACE LTRACE
ISW
FIGURE 41. EXAMPLE OF POOR GROUNDING
48
Mis-operation often occurs when sensitive parts of the circuit "rattle" up and down
due to ground switching currents. This can induce noise into the reference used to
set the output voltage, resulting in excessive output ripple.
Very often, regulators that suffer from ground noise problems appear to be unstable,
and break into oscillations as the load current is increased (which increases ground
currents).
A much better layout is shown in Figure 42.
A big improvement is made by using single-point grounding. A good
high-frequency electrolytic capacitor (like solid Tantalum) is used near the input
voltage source to provide a good ground point.
All of the individual circuit elements are returned to this point using separate
ground traces. This prevents high current ground pulses from bouncing the logic
circuits up and down.
Another important improvement is that the power switch (which has the highest
ground pin current) is located as close as possible to the input capacitor. This
minimizes the trace inductance along its ground path.
It should also be pointed out that all of the individual circuit blocks have "local"
bypass capacitors tied directly across them. The purpose of this capacitor is RF
bypass, so it must be a ceramic or film capacitor (or both).
A good value for bypassing logic devices would be 0.01 mF ceramic capacitor(s),
distributed as required.
If the circuit to be bypassed generates large current pulses (like the power switch),
more capacitance is required. A good choice would be an aluminum electrolytic
bypassed with a film and ceramic capacitor. Exact size depends on peak current,
but the more capacitance used, the better the result.
VIN + POWER
SWITCH
PWM
CONTROL
CHIP
LOGIC
CIRCUITS
FIGURE 42. EXAMPLE OF GOOD GROUNDING
49
Transformer/Inductor Cores and Radiated Noise
The type of core used in an inductor or transformer directly affects its cost, size, and
radiated noise characteristics. Electrical noise radiated by a transformer is
extremely important, as it may require shielding to prevent erratic operation of
sensitive circuits located near the switching regulator.
The most commonly used core types will be presented, listing the advantages and
disadvantages of each.
The important consideration in evaluating the electrical noise that an inductor or
transformer is likely to generate is the magnetic flux path. In Figure 43, the slug
core and toroidal core types are compared.
The flux in the slug core leaves one end, travels through the air, and returns to the
other end. The slug core is the highest (worst) for radiated flux noise. In most
cases, the slug core device will give the smallest, cheapest component for a given
inductor size (it is very cheap to manufacture).
The magnetic flux path in the toroid is completely contained within the core. For
this reason it has the lowest (best) radiated flux noise. Toroid core components
are typically larger and more expensive compared to other core types. Winding a
toroid is fairly difficult (and requires special equipment), driving up the finished cost
of the manufactured transformer.
SLUG CORE
TOROID CORE
FIGURE 43. FLUX PATHS IN SLUG AND TOROID CORES
50
There is another class of cores commonly used in magnetic design which have
radiated flux properties that are much better than the slug core, but not as good as
the toroid. These cores are two-piece assemblies, and are assembled by gluing the
core pieces together around the bobbin that holds the winding(s).
The cores shown are frequently "gapped" to prevent saturation of the Ferrite core
material. The air gap is installed by grinding away a small amount of the core (the
gap may be only a few thousandths of an inch).
Figure 44 shows the popular E-I, E-E and Pot cores often used in switching regulator
transformers. The cores show the locations where an air gap is placed (if required),
but the bobbins/windings are omitted for clarity.
The air gap can emit flux noise because there is a high flux density in the vicinity of
the gap, as the flux passing through the core has to jump the air gap to reach the
other core piece.
The E-E and E-I cores are fairly cheap and easy to manufacture, and are very
common in switching applications up to about 1 kW. There is a wide variety of sizes
and shapes available, made from different Ferrite "blends" optimized for excellent
switching performance. The radiated flux from this type of core is still reasonably
low, and can usually be managed by good board layout.
The Pot core (which is difficult to accurately show in a single view drawing), benefits
from the shielding effect of the core sides (which are not gapped). This tends to
keep the radiated flux contained better than an E-E or E-I core, making the Pot core
second best only to the toroid core in minimizing flux noise.
Pot cores are typically more expensive than E-E or E-I cores of comparable power
rating, but they have the advantage of being less noisy. Pot core transformers are
much easier to manufacture than toroid transformers because the windings are
placed on a standard bobbin and then the core is assembled around it.
E-I CORE
GAP
GAP
E-E CORE
GAP
POT CORE
FIGURE 44. FLUX PATHS IN E-I, E-E AND POT CORES
51
Measuring Output Ripple Voltage
The ripple appearing on the output of the switching regulator can be important to the
circuits under power. Getting an accurate measurement of the output ripple voltage
is not always simple.
If the output voltage waveform is measured using an oscilloscope, an accurate result
can only be obtained using a differential measurement method (see Figure 45).
The differential measurement shown uses the second channel of the oscilloscope to
"cancel out" the signal that is common to both channels (by inverting the B channel
signal and adding it to the A channel).
The reason this method must be used is because the fast-switching components in a
switching regulator generate voltage spikes that have significant energy at very high
frequencies. These signals can be picked up very easily by "antennas" as small as
the 3" ground lead on the scope probe.
Assuming the probes are reasonably well matched, the B channel probe will pick up
the same radiated signal as the A channel probe, which allows this "common-mode"
signal to be eliminated by adding the inverted channel B signal to channel A.
It is often necessary to measure the RMS output ripple voltage, and this is usually
done with some type of digital voltmeter. If the reading obtained is to be meaningful,
the following must be considered:
1) The meter must be true-RMS reading, since the waveforms to be measured
are very non-sinusoidal.
2) The 3dB bandwidth of the meter should be at least 3X the bandwidth of the
measured signal (the output voltage ripple frequency will typically be > 100 kHz).
3) Subtract the "noise floor" from the measurement. Connect both meter leads
to the negative regulator output and record this value. Move the positive meter lead
to positive regulator output and record this value. The actual RMS ripple voltage
is the difference between these two readings.
O-SCOPE
CHAN A
CHAN B
+
-
SWITCHING
REGULATOR
POS OUT
NEG OUT
NOTE: INVERT CHANNEL B AND ADD TO CHANNEL A TO REMOVE COMMON-MODE SIGNAL
MATCHED PROBE SET
FIGURE 45. DIFFERENTIAL OUTPUT RIPPLE MEASUREMENT
52
Measuring Regulator Efficiency of DC-DC Converters
The efficiency (h) of a switching regulator is defined as:
h = PLOAD / PTOTAL
In determining converter efficiency, the first thing that must be measured is the total
consumed power (PTOTAL). Assuming a DC input voltage, PTOTAL is defined as
the total power drawn from the source, which is equal to:
PTOTAL = VIN X IIN (AVE)
It must be noted that the input current value used in the calculation must be the
average value of the waveform (the input current will not be DC or sinusoidal).
Because the total power dissipated must be constant from input to output, PTOTAL
is also equal to the load power plus the internal regulator power losses:
PTOTAL = PLOAD + PLOSSES
Measuring (or calculating) the power to the load is very simple, since the output
voltage and current are both DC. The load power is found by:
PLOAD = VOUT X ILOAD
Measuring the input power drawn from the source is not simple. Although the input
voltage to the regulator is DC, the current drawn at the input of a switching
regulator is not. If a typical "clip-on" current meter is used to measure the input
current, the taken data will be essentially meaningless.
The average input current to the regulator can be measured with reasonable
accuracy by using a wide-bandwidth current probe connected to an oscilloscope.
The average value of input current can be closely estimated by drawing a horizontal
line that divides the waveform in such a way that the area of the figure above the
line will equal the "missing" area below the line (see Figure 46). In this way, the
"average" current shown is equivalent to the value of DC current that would produce
the same input power.
IAVE
I PK
I IN
A
B
AREA "A" EQUALS AREA "B"
FIGURE 46. AVERAGE VALUE OF TYPICAL INPUT CURRENT WAVEFORM
53
If more exact measurements are needed, it is possible to force the current in the line
going to the input of the DC-DC converter to be DC by using an L-C filter between
the power source and the input of the converter (see Figure 47).
If the L-C filter components are adequate, the current coming from the output of the
DC power supply will be DC current (with no high-frequency switching
component) which means it can be accurately measured with a cheap clip-on
ammeter and digital volt meter.
It is essential that a large, low-ESR capacitor be placed at CIN to support the input
of the switching converter. The L-C filter that the converter sees looking back into
the source presents a high impedance for switching current, which means CIN is
necessary to provide the switching current required at the input of the converter.
Measuring Regulator Efficiency of Off-Line Converters
Off-Line converters are powered directly from the AC line, by using a bridge rectifier
and capacitive filter to generate an unregulated DC voltage for conversion (see
Figures 37 and 38).
Measuring the total power drawn from the AC source is fairly difficult because of the
power factor. If both the voltage and current are sinusoidal, power factor is
defined as the cosine of the phase angle between the voltage and current
waveforms.
The capacitive-input filter in an off-line converter causes the input current to be very
non-sinusoidal. The current flows in narrow, high-amplitude pulses (called
Haversine pulses) which requires that the power factor be re-defined in such cases.
For capacitive-input filter converters, power factor is defined as:
P.F. = PREAL / PAPPARENT
DC
SUPPLY
+
-
+
CIN
L L
C C
+ IN
- IN
SWITCHING
REGULATOR
CIN MUST BE LARGE VALUE
CURRENT HERE
IS NOT DC
CURRENT HERE IS DC
(MEASURE HERE)
VIN
IIN
P TOTAL = VIN X IIN
FIGURE 47. L-C FILTER USED IN DC INPUT CURRENT MEASUREMENT
54
The real power drawn from the source (PREAL) is the power (in Watts) which
equals the sum of the load power and regulator internal losses.
The apparent power (PAPPARENT) is equal to the RMS input current times the
RMS input voltage. Re-written, the importance of power factor is shown
IIN (RMS) = PREAL / ( VIN (RMS) X PF )
The RMS input current that the AC line must supply (for a given real power in
Watts) increases directly as the power factor reduces from unity. Power factor for
single-phase AC-powered converters is typically about 0.6. If three-phase power is
used, the power factor is about 0.9.
If the efficiency of an off-line converter is to be measured, power analyzers are
available which will measure and display input voltage, input power, and power
factor. These are fairly expensive, so they may not be available to the designer.
Another method which will give good results is to measure the power after the
rectifier bridge and input capacitor (where the voltage and current are DC). This
method is shown in Figure 48.
The current flowing from CIN to the converter should be very nearly DC, and the
average value can be readily measured or approximated (see previous section).
The total power drawn from the AC source is the sum of the power supplied by CIN
(which is VIN X IDC) and the power dissipated in the input bridge rectifier. The
power in the bridge rectifier is easily estimated, and is actually negligible in most
off-line designs.
AC
AC
+
-
+
FWB
V IN
TRANSFORMER
AND
RECTIFIERS
MEASURE CURRENT HERE
I DC
P TOTAL = V X + PFWB IN IDC
CIN
FIGURE 48. MEASURING INPUT POWER IN OFF-LINE CONVERTER
55
Application Circuits
Introduction
Application circuits will be detailed which will demonstrate some examples of
switching regulator designs.
LM2577: A Complete Flyback/Boost Regulator IC
The LM2577 is an IC developed as part of the SIMPLE SWITCHERÔ product
family. It is a current-mode control switching regulator, with a built-in NPN switch
rated for 3A switch current and 65V breakdown voltage.
The most commonly used applications are for Flyback or Boost regulators (see
Figure 49). In the Boost regulator, the output is always greater than the input. In the
Flyback, the output may be greater than, less than, or equal to the input voltage.
The theory of operation of the Flyback and Boost Converters has been previously
covered, and will not be repeated here.
The LM2577 is targeted for applications with load power requirements up to a
maximum of about 25 Watts, and can be used to implement Boost or Flyback
regulators (with multiple output voltages available if Flyback is selected).
The SIMPLE SWITCHER product family is supported by design-aid software titled
"Switchers Made Simple", which allows finished designs to be generated directly
from the computer.
The next sections will show the LM2577 being used in circuits which are more
advanced than the typical applications (these circuits were generated as solutions
for specific customer requirements).
C + + IN
COUT
VIN VOUT
GND GND
COMP
VIN SW
GND
FB
VOUT > VIN
LM2577
+ +
C IN
COUT
VIN VOUT
GND GND
COMP
VIN SW
GND
FB
LM2577
BOOST REGULATOR FLYBACK REGULATOR
FIGURE 49. BASIC APPLICATION CIRCUITS FOR THE LM2577
56
Increasing Available Load Power in an LM2577 Boost Regulator
One of the most frequently requested circuits is a method to squeeze more power
out of a boost converter. The maximum load power available at the output is directly
related to the input power available to the DC-DC converter.
When the input voltage is a low value (like 5V), this greatly reduces the amount of
power that can be drawn from the source (because the maximum input current is
limited by what the switch can handle). In the case of the LM2577, the maximum
switch current is 3A (peak).
Increased load power can be obtained with the LM2577 by paralleling two devices
(see Figure 50). Because current-mode control is used in the LM2577, the two
converters will automatically share the load current demand.
The right-hand regulator (which is a fixed 12V version) is the master that sets the
duty cycle of both regulators (tying the Compensation pins together forces the duty
cycles to track).
The master regulator has direct feedback from the output, while the other regulator
has its Feedback pin grounded. Grounding the Feedback pin makes the regulator
attempt to run "wide open" (at maximum duty cycle), but the master regulator
controls the voltage at both Compensation pins, which adjusts the pulse widths as
required to hold the output voltage at 12V.
+
VOUT
GND
COMP
VIN SW
GND
FB
LM2577
ADJ
VIN
COMP
VIN SW
GND
FB LM2577
12
+
+12V
@ 1.5A
1 mF 1 mF
470 mF
47 mH 47 mH
1000 mF
5V
0.22
mF
2.2K
SCHOTTKY (2 PL.)
FIGURE 50. DUAL LM2577 BOOST CIRCUIT
57
LM2577 Negative Buck Regulator
The LM2577 can be used in a Buck regulator configuration that takes a negative
input voltage and produces a regulated negative output voltage (see Figure 51).
The LM2577 is referenced to the negative input, which means the feedback signal
coming from the regulated output must be DC level shifted. R1, D1, and Q1 form a
current source that sets a current through R2 that is directly proportional to the
output voltage (D1 is included to cancel out the VBE of Q1).
Neglecting the base current error of Q1, the current through R2 is equal to:
IR2 = VOUT / R1 (which is 300 mA for this example.)
The voltage across R2 provides the 1.23V feedback signal which the LM2577
requires for its feedback loop.
The operation of the power converter is similar to what was previously described for
the Buck regulator:
When the switch is ON, current flows from ground through the load, into the
regulator output, through the inductor, and down through the switch to return to the
negative input. The output capacitor also charges during the switch ON time.
When the switch turns OFF, the voltage at the diode end of the inductor flies positive
until the Schottky diode turns on (this allows the inductor current to continue to flow
through the load during the OFF time). The output capacitor also discharges
through the load during the OFF time, providing part of the load current.
VOUT
GND
VIN SW
VIN
COMP
FB GND
LM2577
ADJ
+
+
-15V @ 3A
-20 TO -40V
150 mH
470 mF R1
49.9K
100K
R2
4.12K
2N3906
100 mF
3.3K
0.22 mF
3A/50V
SCHOTTKY
1N4148 Q1
R3
1.23V
D1
FIGURE 51. NEGATIVE BUCK REGULATOR
58
LM2577 Three-Output, Isolated Flyback Regulator
Many applications require electrical isolation between the input and output terminals
of the power supply (for example, medical monitoring instruments require
isolation to assure patient safety).
Figure 52 shows an example of a three-output Flyback regulator2 built using the
LM2577 that has electrical isolation between the input and output voltages.
Three output voltages are obtained from three separate transformer secondary
windings, with voltage feedback being taken from the 5V output.
To maintain electrical isolation, the feedback path uses a 4N27 opto-coupler to
transfer the feedback signal across the isolation barrier.
2K
INPUT
GND
VIN SW
VIN
FB COMP GND
LM2577
ADJ
+
+
+
+
51V
LM385
680
mF
0.1
mF
16 - 36V 18T
16T
24T
24T
100mF
220mF
470mF
0.1
mF
R1
15K
R2
45.3K
220
+
-
FB
OUTPUT
GND
4N27
47K
0.47mF
100k
4.7V
10K
470
5V@
150mA
7.5V@
100mA
-7.5V@
70mA
ISOLATION
FIGURE 52. THREE-OUTPUT ISOLATED FLYBACK REGULATOR
59
The 5V output is regulated using an LM385 adjustable reference, whose voltage is
set by R1 and R2. The LM385 operates by forcing a 1.24V reference voltage
between the positive terminal and the feedback pin, so the set voltage across the
LM385 is given by:
VREF = 1.24 X (R2/R1 + 1)
For the values shown in this example, the voltage will be 5V.
The function of the LM385 in the circuit can be described as an "ideal" Zener diode,
because the current flowing through the LM385 is very small until the voltage at its
positive terminal reaches 5V with respect to ground. At that point, it tries to regulate
its positive terminal to 5V by conducting current (which flows out of the negative
terminal of the LM385 and through the 470W resistor into the diode side of the
opto-coupler).
When the LM385 starts conducting current through the opto-coupler diode, the
collector of the transistor in the opto-coupler pulls down on the compensation pin of
the LM2577, which reduces the duty cycle (pulse widths) of the switching converter.
In this way, a negative feedback loop is established which holds the output at 5V.
The feedback signal from the collector of the opto-coupler is fed into the
compensation pin (not the feedback pin) of the LM2577 in order to bypass the
internal error amplifier of the LM2577. The gain of the LM385 is so high that using
the error amplifier inside the loop would make it difficult to stabilize (and is not
necessary for good performance).
Test data taken with the input voltage set to 26V and all outputs fully loaded showed
the frequency response of the control loop had a 0dB crossover point of 1kHz with a
phase margin of 90°.
The 7.5V and -7.5V outputs are not directly regulated, which means their voltages
are set by the pulse width of the regulated (5V) winding. As a result, the load
regulation of these two outputs is not quite as good as the 5V output.
Summary of test performance data:
Output Voltages
5V
7.5V
-7.5V
Line Regulation
(@ Full Load)
0.2%
0.3%
0.3%
Load Regulation
(VIN=26V)
0.04%
(30mA - 150mA)
3%
(20mA - 100mA)
2%
(12mA - 70mA)
Output Ripple
Voltage (25°C)
50mV
50mV
50mV
60
LM2575 and LM2576 Buck Regulators
The LM2575 and LM2576 products are Buck regulators developed as part of the
SIMPLE SWITCHERÔ product family.
The LM2575 is rated for 1A of continuous load current, while the LM2576 can supply
3A. The maximum input voltage for the parts is 40V (60V for the "HV" versions),
with both adjustable and fixed output voltages available. Both parts are included in
the "Switchers Made Simple" design software.
The basic Buck regulator application circuit is shown in Figure 53.
The LM2575 and LM2576 can also be used in an inverting (Buck-Boost)
configuration which allows a positive input voltage to be converted to a negative
regulated output voltage (see Figure 54).
+
+
LM2575-ADJ
V IN LM2576-ADJ
GND FB
OUT
ON/OFF
C IN
C OUT
L
D
R1
R2
V IN
V OUT = 1.23 X (1 + R2/R1)
GND GND
V OUT
FIGURE 53. LM2575 AND LM2576 BUCK REGULATOR APPLICATION
+
+
LM2575-ADJ
V IN LM2576-ADJ
GND FB
OUT
ON/OFF
C IN
C OUT
L
D
V IN
GND
- V OUT
FIGURE 54. LM2575 AND LM2576 INVERTING APPLICATION
61
Low Dropout, High Efficiency 5V/3A Buck Regulator
A circuit was developed which provides a 5V/3A regulated output voltage with very
high efficiency and very low dropout voltage (see Figure 55). The customer required
that the circuit be able to operate with an input voltage range of 6V to 12V, allowing
only 1V of dropout at the lowest input voltage.
An unusual feature of this circuit is that it can stay in regulation with only 300 mV
across the regulator. Also, the efficiency is highest (89%) at the lowest input
voltage (buck converters are typically more efficient at higher input voltages).
The low (<300mV) dropout voltage is achieved by using an external PNP power
transistor (Q2) as the main switching transistor (the other transistors in the circuit are
drivers for Q2). With components values shown, Q2 has a saturation voltage of
200mV @ 3A, which allows the 300 mV input-output differential requirement for the
regulator to be met.
The switch inside the LM2575 drives the base of Q1 through R2. Note that the
maximum collector current of Q1 (and the maximum base drive available for Q2) is
limited by Z1 and R4. When Z1 clamps at 5V, the maximum current through Q1 is:
I Q1 (MAX) = (5 - VBE) / R4 = 215mA
+
+
VIN LM2575-5
ON/ GND
OFF
GND
100
mF
5.3V TO
12V
VOUT
FB
330
200
1N5818
39
2N2222
1N4734
5V
20 1N5821 1000mF
68mH
D45H1
5V/3A
GND
Q1
Q2
R1 Z1
R2
R3
R4
D1
D2
FIGURE 55. LOW-DROPOUT 5V/3A REGULATOR
62
The maximum Q1 current (215mA) limits the amount of base drive available to Q2,
forcing the collector current of Q2 to "beta limit" as the output is overloaded (this
means the maximum collector current of Q2 will be limited by the gain of the
transistor and the base drive provided). Although this is not a precise current limiter,
it is adequate to protect Q2 from damage during an overload placed on the output.
If the regulator output is shorted to ground, the output short-circuit current flows
from the output of the LM2575 (through D1 and the inductor), which means the
regulator short-circuit current is limited to the value set internally to the
LM2575 (which is about 2A).
Note also that when the regulator output is shorted to ground, the cathode of D1 will
also be near ground. This allows D1 to clamp off the base drive to Q1 off,
preventing current flow in the switch transistor Q2.
If the input voltage does not exceed 8V, R2 and Z1 are not required in the circuit.
This circuit was tested with 6V input and was able to deliver more than 4A of load
current with 5V out. Other test data taken are:
Measured Performance Data:
Line Regulation:
5.3V to 12V @ 1A 32mV
5.3V to 12V @ 3A 45mV
Load Regulation:
0.3A to 3A @ 5.3V Input 10mV
0.3A to 3A @ 12V Input 17mV
Efficiency @ 3A Load:
VIN = 5.3V 89%
VIN = 12V 80%
Output Ripple Voltage:
VIN = 7.2V, IL = 3A 35 mV(p-p)
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