Page 294 - Op Amps Design, Applications, and Troubleshooting

276 POWER SUPPLY CIRCUITS

The basic behavior of capacitors is to oppose changes in voltage. That is, we
cannot change the voltage across a capacitor instantaneously; the voltage can only
change as fast as the capacitor can charge or discharge. The capacitor in Figure
6.16 is a filter and smooths the otherwise pulsating load voltage. So, even though
the switch is interrupting the DC supply at regular intervals, the voltage is steady
across the load because of the combined effects of the coil and capacitor. By vary-
ing the ratio of on time to off time (i.e., the duty cycle), we can vary the DC voltage
across the load. If we sample the actual load voltage and use its value to control
the duty cycle of the switch (a transistor in practice), then we will have con-
structed a switching regulator circuit.
Figure 6.17 shows a more accurate representation of a switching voltage reg-
ulator. Here the DC input voltage is provided by a standard transformer-coupled,
bridge-rectifier circuit followed by a brute filter (Q). The interrupting device is an
n-channel, power MOSFET (Q^. Gate drive for the MOSFET comes from a pulse
width modulator circuit. This can be built around an op amp comparator/oscillator
circuit, but is generally an integral part of an integrated circuit designed specifi-
cally for use in switching regulators.
The pulse width modulator has two inputs. One is the sample output volt-
age derived from a voltage divider (Ri and R 2); the other is provided by a stable
voltage reference. The pulse width modulator compares the reference voltage
with the sampled output voltage (just as the series and shunt-linear regulators dis-
cussed in previous sections do) and alters the pulse width (effective duty cycle) of
the signal going to the MOSFET. As the duty cycle of the MOSFET is altered, the
average (i.e., DC) output voltage is adjusted and maintained at a constant value. If
the load voltage tried to change, perhaps in response to a changing current
demand, then this change would be fed back through the voltage divider to the
pulse width modulator circuit. The pulse width going to the MOSFET would
quickly be adjusted to bring the load voltage back to the correct value.

6.4.2 Switching versus Linear Voltage Regulators
Why go to all the trouble of interrupting the DC voltage and then turning right
around and smoothing it back into DC again? Well, switching regulators do have
some outstanding advantages over linear regulators. One of their primary advan-
tages as compared to their linear equivalents is lower power dissipation.
In a linear regulator, the series pass transistor or the shunt regulator transis-
tor dissipates a significant amount of power. Typical efficiencies for linear regula-
tors are 30 to 40 percent, which means, for example, that a linear supply designed
to deliver 12 volts at 3 amps DC actually draws at least 90 watts from the power
line. The internal power loss results in heat, which in turn leads to cooling require-
ments like fans and heat sinks. Most of the power loss in a linear regulator is in the
regulator transistors. Recall that their power dissipation is computed as

PD ~ JC^CE
The switching regulator, on the other hand, typically achieves efficiencies on the
order of 75 percent. This improvement is caused primarily by a dramatic reduc-
tion in power dissipation in the regulator transistor. Although the power dissipa-