Handbook for Sound Engineers

724 Chapter 20

The operation of the negative half mirrors that of the
positive half for negative swings of the output signal.
One is not restricted to the employment of just four
voltage supply values consisting of two positive and
two negative. One might in fact employ three or more
on each side by increasing the number of fixed voltage
taps on the power supply, adding more comparator-
operated switches, and reverse-biased diodes as appro-
priate for each switch. In this fashion, the voltage
supply of the moment more accurately tracks the instan-
taneous requirements of the signal and the dissipation in
the active output devices is kept near a minimum.

20.3.4 Switch Mode Power Supplies

Switching power supplies are now employed in practi-
cally all high-power audio amplifiers be they of the
analog type of classes AB + B, G, and H or of the
various manifestations of switching amplifier stem-
ming from class D. A variety of considerations have
compelled this change from former practice. When the
power requirements are large, a switching power supply
offers significant size and weight advantage over a
conventional supply employing an ac-main-operated
power transformer, full-wave bridge rectifier, and
capacitor filter bank. More importantly, a switching
power supply offers the advantage of active power
factor correction. This latter feature is crucial for
obtaining the maximum power from a single 120 Vac
outlet having limited current capability. Additionally,
most switching supplies feature a convenient selection
feature that allows operation from either 120 V or

240 V mains. In fact, the more advanced designs of

switching supplies can operate over an ac voltage range

of 90 V to 270 V rms without any internal circuit

changes.

These desirable features of switching power supplies

come with a price in that the design of such units is

highly specialized and much more engineering effort

must be expended in yielding a viable unit. High-power

units require auxiliary supplies for the control circuitry

that must be fully active during main supply startup as

well as normal operation. The fundamental switching

frequencies fall in the range of 30 kHz to 100 kHz with

the switching pulses being pulse width modulated. As a

result, harmonics are generated in the radio-frequency

range. This property necessitates the incorporation of a

sophisticated electromagnetic interference (EMI) filter

to prevent the appearance of the switching frequency

and its harmonics as common-mode signals on the ac

supply lines. Fig. 20-31 is a block diagram of a typical

switch-mode power supply for use with high-power

audio amplifiers.

The EMI filter must be designed to prevent

common-mode signals that are generated in the power

supply from being conducted to the external supply

mains while at the same time offering minimum series

impedance to the differential 60 Hz ac voltage of the

supply mains. This is accomplished by having balanced

inductors in each of the supply lines with positive

mutual inductance between the inductors in upper and

lower lines. This arrangement maximizes the inductance

for common-mode currents that flow in the same direc-

tion in both conductors. For the oppositely directed

differential currents of the 60 Hz main supply, the

mutual inductance is negative, thus forcing the overall

series inductance to a small value. A typical EMI filter

circuit is shown in Fig. 20-32.

The line operated rectifier and capacitor filter imme-

diately follows the EMI filter. Such an arrangement

appears in Fig. 20-33.

The circuit of Fig. 20-33 features an inrush current

limiter in the form of a thermistor that presents a large

Figure 20-30. Basic class H output stage.

Positve

class H

output

Negative

class H

output

Phase splitter

Drive

audio

in

S+

+2V

+V

–2V

S–

Power amp out

• V

Figure 20-31. Block diagram of switch mode power supply.

EMI Filter

Line-operated

rectifier and

capacitor filter

dc to dc

converter

Startup,

control and

regulator

120–240 V 60 Hz In Rail supplies out

Low-voltage

supply for control

and startup

circuits