Page 297 - Op Amps Design, Applications, and Troubleshooting

278 POWER SUPPLY CIRCUITS

tion is computed in the same basic way, the results are quite different because the
transistor is operated in either saturation or cutoff at all times. Thus, although the
power at any given time is expressed as

either Z c is very low (at cutoff I c « 0) or V CE is very low (during saturation V CE is
ideally 0). Therefore, the only time the switching transistor dissipates significant
power is during the actual switching time (a few microseconds).
The reduced power dissipation results in other advantages. Since the cooling
requirement is less for a given output power, both size and cost of the associated
circuitry and support components are less. It is reasonable to expect size reduc-
tions on the order of five or more.
Another advantage of switching regulators is that the output voltage can be
stepped up, stepped down, and/or changed in polarity in the process of being
regulated. This can simplify some designs.
Switching regulators are not without their disadvantages, however. First,
they require more complex circuitry for control, although this is becoming less dis-
advantageous as more specialized regulator ICs are being provided to the power
supply designer.
Another major disadvantage of switching regulators is electrical noise gen-
eration. Anytime a circuit changes states quickly, high-frequency signals are gen-
erated. You may recall from basic electronics theory that a square wave is made up
of an infinite number of odd harmonics. So, if we have a 100-kilohertz square
wave, we will be generating harmonic frequencies of 300 kilohertz, 500 kilohertz,
700 kilohertz, and so on. The Federal Communications Commission (FCC) in the
United States and similar agencies in other countries restrict the amount of
electromagnetic emissions that may leave an electronic device. For example, sup-
pose you have designed a new computer that fits in the palm of your hand. The
FCC will prevent you from marketing your new computer unless it can pass the
FCC-defined emissions tests. Many new computer designs fail to pass these tests
because of the electrical noise generated by switching power supplies. Now, this
doesn't mean you can never use a switching regulator in a computer. Quite the
contrary, most computers do use switching regulators. But additional components
will have to be included to filter the high-frequency noise that is generated. This
noise can easily extend into the 450-kilohertz to 150-megahertz band.
Finally, although switching regulators are good, they cannot respond as
quickly to sudden changes in line voltage or load current. That is, they do not reg-
ulate as well as their linear counterparts if the line and load changes are rapid.

6.4.3 Classes of Switching Regulators
We can categorize switching regulators into four general groups based on the
method used to control the switching transistor:

1. Fixed off time, variable on time
2. Fixed on time, variable off time