Introduction to Power
Supplies
Introduction
Virtually every piece of electronic equipment, e.g., comput-
ers and their peripherals, calculators, TV and hi-fi equip-
ment, and instruments, is powered from a DC power source,
be it a battery or a DC power supply. Most of this equipment
requires not only DC voltage but voltage that is also well
filtered and regulated. Since power supplies are so widely
used in electronic equipment, these devices now comprise a
worldwide segment of the electronics market in excess of $5
billion annually.
There are three types of electronic power conversion de-
vices in use today which are classified as follows according
to their input and output voltages: 1) DC/DC converter; 2) the
AC/DC power supply; 3) the DC/AC inverter. Each has its
own area of use but this paper will only deal with the first two,
which are the most commonly used.
A power supply converting AC line voltage to DC power must
perform the following functions at high efficiency and at low
cost:
1. Rectification: Convert the incoming AC line voltage to
DC voltage.
2. Voltage transformation: Supply the correct DC voltage
level(s).
3. Filtering: Smooth the ripple of the rectified voltage.
4. Regulation: Control the output voltage level to a con-
stant value irrespective of line, load and temperature
changes.
5. Isolation: Separate electrically the output from the input
voltage source.
6. Protection: Prevent damaging voltage surges from
reaching the output; provide back-up power or shut
down during a brown-out.
An ideal power supply would be characterized by supplying
a smooth and constant output voltage regardless of varia-
tions in the voltage, load current or ambient temperature at
100% conversion efficiency. Figure 1 compares a real power
supply to this ideal one and further illustrates some power
supply terms.
Linear Power Supplies
Figure 2 illustrates two common linear power supply circuits
in current use. Both circuits employ full-wave rectification to
reduce ripple voltage to capacitor C1. The bridge rectifier
circuit has a simple transformer but current must flow
through two diodes. The center-tapped configuration is pre-
ferred for low output voltages since there is just one diode
voltage drop. For 5V and 12V outputs, Schottky barrier
diodes are commonly used since they have lower voltage
drops than equivalently rated ultra-fast types, which further
increases power conversion efficiency. However, each diode
must withstand twice the reverse voltage that a diode sees in
a full-wave bridge for the same input voltage.
The linear voltage regulator behaves as a variable resis-
tance between the input and the output as it provides the
precise output voltage. One of the limitations to the efficiency
of this circuit is due to the fact that the linear device must
drop the difference in voltage between the input and output.
Consequently the power dissipated by the linear device is
(Vi–Vo) x Io. While these supplies have many desirable
characteristics, such as simplicity, low output ripple, excel-
lent line and load regulation, fast response time to load or
line changes and low EMI, they suffer from low efficiency and
occupy large volumes. Switching power supplies are becom-
ing popular because they offer better solutions to these
problems.
01006101
FIGURE 1. Real Power Supply has error compared to Ideal Power Supply
National Semiconductor
Application Note 556
September 2002
Introduction
to
Pow
erSupplies
AN-556
© 2002 National Semiconductor Corporation AN010061 www.national.com
Linear Power Supplies (Continued)
Switching vs Linear Power
Supplies
Switching power supplies are becoming popular due to high
efficiency and high power density. Table 1 compares some of
the salient features of both linear and switching power sup-
plies. Line and load regulation are usually better with linear
supplies, sometimes by as much as an order of magnitude,
but switching power supplies frequently use linear post-
regulators to improve output regulation.
DC-DC Converters
DC-DC converters are widely used to transform and distrib-
ute DC power in systems and instruments. DC power is
usually available to a system in the form of a system power
supply or battery. This power may be in the form of 5V, 28V,
48V or other DC voltages. All of the following circuits are
applicable to this type of duty. Since voltages are low, isola-
tion is not usually required.
TABLE 1. Linear vs Switching Power Supplies (typical)
Specification Linear Switcher
Line Regulation 0.02%–0.05% 0.05%–0.1%
Load Regulation 0.02%–0.1% 0.1%–1.0%
Output Ripple 0.5 mV–2 mV RMS 10 mV–100 mVP-P
Input Voltage Range ±10% ±20%
Efficiency 40%–55% 60%–95%
Power Density 0.5 W/cu. in. 2W–10W/cu. in.
Transient Recovery 50 µs 300 µs
Hold-Up Time 2 ms 34 ms
Switching Power Supplies
PULSE WIDTH MODULATION
In the early 60’s, switching regulators started to be designed
for the military, who would pay a premium for light weight and
efficiency. One way to control average power to a load is to
control average voltage applied to it. This can be done by
opening and closing a switch in rapid fashion as being done
in Figure 3.
The average voltage seen by the load resistor R is equal to:
Vo(avg) = (t(on)/T) x Vi (1)
Reducing t(on) reduces Vo(avg). This method of control is
referred to as pulse width modulation (PWM).
BUCK REGULATOR
As we shall see, there are many different switching voltage
regulator designs. The first one to be considered because of
its simplicity is the buck regulator (Figure 4), also known as
a step-down regulator since the output voltage as given by
Equation (1) is less than the input voltage. A typical applica-
tion is to reduce the standard military bus voltage of 28V to
5V to power TTL logic.
At time t(o) in Figure 4, the controller, having sensed that the
output voltage VO is too low, turns on the pass transistor to
build up current in L, which also starts to recharge capacitor
C. At a predetermined level of Vo, the controller switches off
the pass transistor Q, which forces the current to free wheel
around the path consisting of L, C, and the ultra-fast rectifier
D. This effectively transfers the energy stored in the inductor
L to the capacitor. Inductor and capacitor and capacitor sizes
are inversely proportional to switching frequency, which ac-
counts for the increasing power density of switching power
supplies. Power MOSFETs are rapidly replacing bi-polar
transistors as the pass transistor because of their high fre-
quency capability. Since the pass transistor must not only
carry load current but reverse recovery current of diode D,
an ultra-fast recovery diode or Schottky diode is mandatory.
01006102
a. Center Tap Transformer Input
01006105
b. Full-Wave Bridge Input
FIGURE 2. Linear Voltage Regulator
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Switching Power Supplies (Continued)
BOOST REGULATOR
A second type of regulator shown in Figure 5 is capable of
boosting the input voltage. Applications for this circuit would
be to increase 5V battery sources to 12V for interface circuits
or even to 150V for electro-luminescent displays.
The concept of this circuit is still the same as the previous,
namely to transfer the energy stored in the inductor into the
capacitor. The inductor current can ramp up quickly when
the transistor switch is closed at time t(0) since the full input
voltage is applied to it. The transistor is turned off at time (1)
which forces the inductor current to charge up the capacitor
through the ultra-fast diode D. Since the energy stored in the
inductor is equal to L x I x 1⁄2, the PWM IC can increase Vo by
increasing its own on-time to increase the peak inductor
current before switching. The transfer function is:
Vo = VIN (T/(T – t(on))) (2)
01006103
FIGURE 3. Example of Pulse Width Modulation
01006104
FIGURE 4. Buck Regulator Circuit with Voltage and Current Waveforms
01006106
FIGURE 5. Boost Regulator and Associated I/V Waveforms
AN-556
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Switching Power Supplies (Continued)
INVERTING REGULATOR
Figure 6 shows a switching circuit which produces an output
voltage with the opposite polarity of the input voltage. This
circuit works in the same fashion as the boost converter but
has achieved the voltage inversion by exchanging positions
of the transistor and inductor. The circuit is also known as a
buck-boost regulator since the absolute magnitude of the
output voltage can be higher or lower than the input voltage,
depending upon the ratio of on-time to off-time of the pass
transistor.
FLYBACK CONVERTER
The three previous regulators are suitable for low voltage
control when no electrical isolation is required. However in
off-line switchers operating from 110V/220V mains, electrical
isolation is an absolute must. This is achieved by using a
transformer in place of the inductor. The flyback converter
shown in Figure 7 is commonly used in power supplies up
through 150W, which is sufficient for most personal comput-
ers, many test instruments, video terminals and the like.
Since the transformer operates at high frequency, its size is
much smaller than a 50 Hz/60 Hz transformer shown in
Figure 2. Within certain frequency limits, transformer size is
inversely proportional to frequency.
Inspection of the switching waveforms in Figure 7 shows that
the circuit behaves very similarly to the boost regulator. The
transformer should be regarded as an inductor with two
windings, one for storing energy in the transformer core and
the other for dumping the core energy into the output capaci-
tor. Current increases in the primary of the transformer dur-
ing the on-time of the transistor (t(0) – t(1)) but note that no
secondary current flows because the secondary voltage re-
verse biases diode D. When the transistor turns off, the
transformer voltage polarities reverse because its magnetic
field wants to maintain current flow. Secondary current can
now flow through the diode to charge up the output capaci-
tor. The output voltage is given by the basic PWM equation
times the transformer turns ration (N2/N1):
Vo = VIN x (t(on))/(T – t(on)) x (N2/N1) (3)
Voltage control is achieved by controlling the transistor on-
time to control the peak primary current.
The flyback converter is well suited for multiple output and
high voltage power supplies since the transformer induc-
tance replaces the filter inductor(s). The major disadvan-
tages which limit its use to lower wattage supplies are:
1. The output ripple voltage is high because of half-wave
charging of the output capacitor.
2. The transistor must block 2 x VIN during turn-off.
3. The transformer is driven in only one direction, which
necessitates a larger core, i.e., more expensive, in a
flyback design than for an equivalent using a forward or
push-pull design.
OFF-LINE SWITCHING SUPPLY
Based on the flyback regulator circuit, a complete off-line
switching supply is shown in Figure 8. The supply is called
“off-line” because the DC voltage to the switch is developed
right from the AC line.
The circuit also shows the feedback loop completed from the
output back to the switching transistor. This feedback loop
must have isolation in order for the DC output to be isolated
from the AC line. This is normally accomplished by a small
transformer or opto-coupler.
01006107
FIGURE 6. Inverting Regulator
01006108
FIGURE 7. Flyback Converter
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Switching Power Supplies (Continued)
Switching power supplies designed for international usage
must have selectable AC input voltage ranges of 115V and
230V. Figure 9 shows how this is accomplished for many
switching power supplies.
FORWARD CONVERTER
Although the forward converter is not as well-known as the
flyback converter, it is becoming increasingly popular for
power supplies in the 100W–500W range. Figure 10 shows
the basic circuit of the forward converter. When the transistor
is switched on, current rises linearly in the primary and
secondary current also flows through diode D1 into the
inductor and capacitor. When the transistor switch is
opened, inductor current continues to free-wheel through the
capacitor and diode D2. This converter will have less ripple
since the capacitor is being continuously charged, an advan-
tage of particular interest in high current supplies.
The relationship between input and output for this circuit
configuration is:
Vo = VIN x (N2/N1) x (t(on)/T) (4)
Note that the transformer shown in the above figure has
been wound with a third winding and series diode D3. The
purpose of this winding is to transfer the magnetizing energy
in the core back to the DC supply so it does not have to be
dissipated in the transistor switch or some other voltage
suppressor. The turns ratio N3/N1 limits the peak voltage
seen by the transistor and is normally chosen equal to 1 so
that the forward converter can run at 50% duty cycle. Under
this condition, the transistor must block 2 x VIN during
turn-off.
01006109
FIGURE 8. Complete Isolated Flyback Switching Supply
01006110
FIGURE 9. Selector Switch for 115V/230V Inputs
AN-556
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Switching Power Supplies (Continued)
Symmetrical Converters
PUSH-PULL CONVERTER
The circuit for this widely used converter is shown in Figure
11.
Transistors Q1 and Q2 are alternately switched on for time
period (t(on). This subjects the transformer core to an alter-
nating voltage polarity to maximize its usefulness. The trans-
fer function still follows the basic PWM formula but there is
the added factor 2 because both transistors alternately con-
duct for a portion of the switching cycle.
Vo = 2 x VIN x (N2/N1) x (t(on)/T) (5)
The presence of a dead time period t(d) is required to avoid
having both transistors conduct at the same time, which
would be the same as turning the transistors on into a short
circuit. The output ripple frequency is twice the operating
frequency which reduces the size of the LC filter compo-
nents. Note the anti-parallel diodes connected across each
transistor switch. They perform the same function as diode
D3 in the forward converter, namely to return the magneti-
zation energy to the input voltage whenever a transistor
turns off.
Compared to the following symmetrical converters, this cir-
cuit has the advantage that the transistor switches share a
common signal return line. Its chief disadvantages are that
the transformer center-tap connection complicates the trans-
former design and the primary windings must be tightly
coupled in order to avoid voltage spikes when each transis-
tor is turning off.
HALF-BRIDGE CONVERTER
This converter (Figure 12) operates in much the same fash-
ion as the previous push-pull circuit.
01006111
FIGURE 10. Forward Converter
01006112
FIGURE 11. Push-Pull Converter
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Symmetrical Converters (Continued)
The input capacitors C1 and C2 split the input voltage
equally so that when either transistor turns on, the trans-
former primary sees Vin/2. Consequently note no factor of “2”
in the following transfer equation:
Vo = VIN x (N2/N1) x (t(on)/T) (6)
Since the two transistors are connected in series, they never
see more than the input voltage VIN plus the inevitable
switching transient voltages. The necessity of a dead time is
even more obvious here since the simultaneous conduction
of both transistors results in a dead short across the input
supply. Anti-parallel ultra-fast diodes return the magnetiza-
tion energy as in the push-pull circuit but alternately to
capacitors C1 and C2. This circuit has the slight inconve-
nience of requiring an isolated base drive to Q1, but since
most practical base drive circuits use a transformer for iso-
lation, this shortcoming is hardly worth noting.
FULL-BRIDGE CONVERTER
Because of its complexity and expense, the full-bridge con-
verter circuit of Figure 13 is reserved for high power convert-
ers. Ideally, all voltages are shared equally between two
transistors so that the maximum voltage rating of the device
can approach VIN.
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into the body, or (b) support or sustain life, and
whose failure to perform when properly used in
accordance with instructions for use provided in the
labeling, can be reasonably expected to result in a
significant injury to the user.
2. A critical component is any component of a life
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can be reasonably expected to cause the failure of
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01006113
FIGURE 12. Half-Bridge Converter Circuit
01006114
FIGURE 13. Full-Bridge Converter Circuit
Introduction
to
Pow
erSupplies
AN-556
National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.
Introduction
FIGURE 1. Real Power Supply has error compared to Ideal Power Supply
Linear Power Supplies
FIGURE 2. Linear Voltage Regulator
Switching vs Linear Power Supplies
DC-DC Converters
TABLE 1. Linear vs Switching Power Supplies (typical)
Switching Power Supplies
PULSE WIDTH MODULATION
BUCK REGULATOR
FIGURE 3. Example of Pulse Width Modulation
FIGURE 4. Buck Regulator Circuit with Voltage and Current Waveforms
BOOST REGULATOR
FIGURE 5. Boost Regulator and Associated I/V Waveforms
INVERTING REGULATOR
FIGURE 6. Inverting Regulator
FLYBACK CONVERTER
FIGURE 7. Flyback Converter
OFF-LINE SWITCHING SUPPLY
FIGURE 8. Complete Isolated Flyback Switching Supply
FIGURE 9. Selector Switch for 115V/230V Inputs
FORWARD CONVERTER
FIGURE 10. Forward Converter
Symmetrical Converters
PUSH-PULL CONVERTER
FIGURE 11. Push-Pull Converter
HALF-BRIDGE CONVERTER
FULL-BRIDGE CONVERTER
FIGURE 12. Half-Bridge Converter Circuit
FIGURE 13. Full-Bridge Converter Circuit
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