Page 298 - Op Amps Design, Applications, and Troubleshooting

Over-Current Protection 279

3. Fixed frequency, variable duty cycle
4. Burst regulators

First, it should be noted that all of the types listed work by switching the regulator
transistor from full off to full on. Additionally, they all regulate by altering the ratio
of on time to off time of the transistor. The various methods refer to the actual cir-
cuitry and waveform driving the switching transistor.
The first two regulator types listed are similar in that one alternation of the
transistor drive signal is fixed. Regulation is achieved by adjusting the time for the
remaining alternation. Because one alternation is fixed and one is variable, the fre-
quency of operation inherently varies. These types are sometimes called variable
frequency regulators.
The third class of switching regulators uses a constant frequency, but alters
the duty cycle of the signal applied to the switching transistor. That is, if the on
time is increased, the off time is decreased proportionately, so the output voltage
can be controlled without altering the basic frequency of operation. This is one of
the most common classes of switching regulators.
Finally, the burst regulator operates by gating a fixed-frequency, fixed-pulse-
width oscillator on and off. The duty cycle of the switching waveform is such that
the output voltage would be too high if the switching were continuous. The circuit
senses this excessive output voltage and interrupts or stops the switching com-
pletely. With the switching transistor turned off continuously, the output voltage
will quickly decay. As soon as it decays to the correct voltage, the switching is
resumed. Thus, the regulation is actually achieved by periodically interrupting
the switching waveform going to the switching transistor.

6.5 OVER-CURRENT PROTECTION

Regulated power supplies are often designed to be short-circuit protected. That is,
if the output of the supply is accidentally shorted to ground or tries to draw exces-
sive current, the supply will not be damaged. There are several classes of over-
current protection:

1, Load interruption
2, Constant current limiting
3, Foldback current limiting

6.5.1 Load Interruption
The simplest form of over-current protection is shown in Figure 6.18. The protec-
tive device is generally a fuse (as shown in the figure), a fusible resistor, or a circuit
breaker. In any case, once a certain value of current has been reached the protec-
tive device opens and completely isolates the load from the output of the supply.
As long as the protective device is designed to operate at a lower current value
than the absolute maximum safe current from the supply, the power supply will