682 Chapter 19
use of the nomograph in Tables 14-2 and 14-3. Since
regulated power supplies are designed to control the
output at the power supply output terminals, the
conductors used for the supply line must be considered
as a part of the power supply load.
19.4.3 Switching Regulators
In a switching regulator, the pass transistor operates in
an on-off mode, increasing efficiency and reducing heat.
The simple switching regulator shown in Fig. 19-13
incorporates a pulse generator circuit that pulses on the
pass transistor as the output voltage decreases. As the
output voltage increases, the comparator circuit reduces
the pulse generator, reducing the on time of the pass
transistor and, therefore, reducing the average output
voltage. Since the output voltage is a series of pulses, a
filter is required to smooth the dc output. An induc-
tance-capacitance filter is commonly used. Switching
regulators normally operate at 20 kHz or higher and
have the following advantages:
- Switching regulators are on-off devices, so they
avoid the higher power dissipation associated with
the rheostat like action of a series regulator. Transis-
tors dissipate very little power when either saturated
(on) or nonconducting (off); most of the power losses
occur elsewhere in the supply. Efficiency to 85% is
typical for switching supplies, as compared to
30–45% for linear supplies. Less wasted power
means switching supplies run cooler, cost less to
operate, and have smaller regulator heat sinks. - Size and weight reductions for switching supplies are
achieved because of their high switching rate. Typi-
cally, a switching supply is less than one-third the size
and weight of a comparable series-regulated supply. - Switching supplies can operate under low ac input
voltage (brownout) conditions and sustain a rela-
tively long carryover (or holdup) of its output if input
power is lost momentarily because more energy is
stored in its input filter capacitors. In a switching
supply, the input ac voltage is rectified directly, and
the filter capacitor charges to the ac voltage peaks.
The ac voltage input of the standard linear supply is
stepped down through a power transformer and then
rectified, resulting in a lower voltage across its filter
capacitor. Since the energy stored in a capacitor is
proportional to CV^2 and V is higher in switching
supplies, their storage capability (and thus their
holdup time) is better.
Switching disadvantages include the following:
- A switching supply transient recovery time (dynamic
load regulation) is slower than that of a series-regu-
lated supply. In a linear supply, recovery time is
limited only by the speeds of the semiconductors
used in the series regulator and control circuitry. In a
switching supply, recovery is limited mainly by the
inductance in the output filter. - Electromagnetic interference (emi) is a natural by-
product of the on-off switching. This interference can
be conducted to the load (resulting in higher output
ripple and noise), it can be conducted back into the ac
line, and it can be radiated into the surrounding atmo-
sphere.
High-Power Regulated Supply. Regulation of a high-
power switching regulator is accomplished by push-pull
switching transistors operating under control of a feed-
back network consisting of a pulse-width modulator and
a voltage comparison amplifier, Fig. 19-14. The feed-
back elements control the onperiods of the switching
transistors to adjust the duty cycle of the bipolar wave-
form (E) delivered to the output rectifier filter. Here the
waveform is rectified and averaged to provide a dc out-
put level that is proportional to the duty cycle of the
waveform, varying the ontimes of the switches.
The waveforms of Fig. 19-14 provide a more
detailed picture of circuit operation. The voltage
comparison amplifier continuously compares a fraction
of the output voltage with a stable reference voltage,
Vref,, to produce the Vcontrol level for the turn-on compar-
ator. This device compares the Vcontrol input with a trian-
gular ramp waveform A, occurring at a fixed 40 kHz
rate. When the ramp voltage is more positive than the
control level, a turn-on signal (B) is generated. Notice
that an increase or decrease in the Vcontrol voltage varies
the width of the output pulses at B and thus the on time
of the switches.
Steering logic within the modulator chip causes
switching transistors Q 1 and Q 2 to turn on alternately soFigure 19-13. Basic switching regulator.Vin Pulse Vout
generator Comparator