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SECTION IV
PRINCIPLES OF OPERATION
Figure 4-1. Overall Block Diagram
4-1
OVERALL BLOCK DIAGRAM
DISCUSSION
4-2
The major circuits of the power supply are shown
on the overall block diagram of Figure 4-1. The ac input
to the power transformer is preregulated by a triac, which
forms a feedback loop in conjunction with the preregulator
control circuit. This feedback loop maintains a low and
constant voltage drop across the series regulator in order
to minimize dissipation in the series regulator transistors.
4-3
To accomplish this, the preregulator control
circuit issues a phase-adjusted firing pulse to the triac once
during each half cycle of the ac input. The control circuit
continuously samples the input line voltage, the dc input
to the series regulator, and the voltage across the series
regulator. On the basis of these inputs it controls the time
at which each firing pulse is generated.
4-4
The output of the triac preregulator is stepped down
by the power transformer, full-wave rectified, and filtered.
The resulting preregulated dc voltage is applied to the
series regulator, which varies its conduction to provide a
regulated voltage or current at the output terminals.
4-5
The series regulator is part of another feedback loop
which consists of the error and driver amplifiers, the
constant-voltage comparator, and the constant-current
comparator. The series regulator feedback loop makes
rapid, low magnitude adjustments to the output while the
preregulator feedback loop handles large, relatively slow
regulation demands.
4-6
The feedback signals that control the conduction of
the series regulator originate in the constant-voltage or
constant-current comparator. During constant-voltage
operation the constant-voltage comparator compares the
4-1
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