Introduction to Power Supplies

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.
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 constant
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 variations
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.