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.