02-11-2012, 01:25 PM
Introduction to Power Supplies
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Introduction
Virtually every piece of electronic equipment, e.g., computers
and their peripherals, calculators, TV and hi-fi equipment,
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 devices
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.
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 preferred
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.
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 supplies.
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 postregulators
to improve output regulation.
DC-DC Converters
DC-DC converters are widely used to transform and distribute
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, isolation
is not usually required.
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.