Handbook for Sound Engineers

Power Supplies 683

that each switch operates at one-half the ramp frequency
or 20 kHz.
The addition of a triac preregulator and associated
control circuit improves regulation and ripple. The triac
is a bidirectional device and is usually connected in
series with one side of the input primary. Whenever a
gating pulse is received, the triac conducts current in a
direction that is dependent on the polarity of the voltage
across it. The goal is to control the triac so that the
bridge rectifier output (dc input to the switches) is held
relatively constant. This is accomplished by a control
circuit that issues a phase-adjusted firing pulse to the
triac once during each half-cycle of the input ac. The
control circuit compares a ramp function to a rectified ac
sine wave to compute the proper firing time for the triac.
Although the addition of the preregulator circuitry
increases complexity, it provides three important
benefits:

1. By keeping the dc input to the switches constant, it
   permits the use of more readily available lower
   voltage switching transistors.
   2. The coarse preregulation it provides allows the main
   regulator to achieve a finer regulation.
   3. Through the use of slow-start circuits, the initial
   conduction of the triac is controlled, providing an
   effective means of limiting input surge current.

19.4.4 Phase-Controlled Regulated Power Supplies

In the phase-controlled supply, the pass element is

switched on and off at line frequency and controls the

output voltage by a varying pulse width. This is most

often accomplished by using an SCR as the pass ele-

ment. By delaying the firing point of the SCR 1 in each

cycle, the output voltage can be varied, Fig. 19-15.

SCR 1 is fired by applying a voltage to the gate. The

voltage is obtained by C 1 charging through R 2 and the

ballast lamp. When the gate firing voltage is reached

across C 1 , SCR 1 fires. Once the SCR 1 is on, it remains

on until its anode voltage goes to zero, which is during

the second half of the cycle. When SCR 1 is on, C 1 dis-

charges and remains discharged until the phase of the

Figure 19-14. Switching supply with push-pull transistors and feedback for regulation.

D

Input

rectifier

Regulator

switches

Output rectifier/filter

Voltage

comparison

amplifier

Turn-on

comparator

Steering

logic

40 kHz

ramp

generator

Pulse width modulator (IC chip)

Mod drive Q 1

Mod drive Q 2

Vref

Q 1

Q 2

Reference

and bias

supplies

Vac

Vdc

+

Rtx

Rtx

Vcontrol

Ramp Ramp

(40 kHz)

Turn-on comp

output

Mod drive, Q 1

(20 kHz)

Mod drive, Q 2

(20 kHz)

Vcontrol

A

B

C

D

E

Q 1 on

Q 2 on

A

C

B

Switch outputs,

transformer

primary

Vref

RP

Sample

E

Vref