Chapter 4 Theory of Operation
Power Mesh and Control
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Power Mesh and Control
For the ±25V power mesh and control circuit, a preregulator is added ahead of the
series pass transistor to minimize the power dissipated in the series pass transistor by
controlling the dc level across the input filter capacitor, depending on the output
voltage.
To achieve this, tap switching is accomplished by a TRIAC and one bridge diode and
the TRIAC control circuit in each power circuit; Q10 and CR29 on the top board for
+25V power circuit, Q3 and CR7 on the top board for -25V power circuit. By turning
on or off the TRIAC, these circuits allow the input capacitors (C54, C55, and C56 for
+25V power circuit and C34, C35, and C36 for -25V power circuit) to charge to one
of two discrete voltage levels, depending on the output voltage required. When the
TRIAC is not fired, the bridge diode conducts and the low voltage of two discrete
voltage levels is developed across the input filter capacitors.
The TRIAC control circuit determines whether TRIAC is to be fired by monitoring
the output voltage and comparing this value against internally derived reference
levels.
The series pass transistor is part of a feedback loop which consists of the driver and
the Constant Voltage/Constant Current error amplifier. The feedback loop provides
"fine and fast" regulation of the output while the preregulator feedback loop handles
large, relatively slow, regulation demands.
The series pass transistor is made to alter its conduction to maintain a constant output
voltage or current. The voltage developed across the current sampling resistors is the
input to the constant current error amplifier. The constant voltage error amplifier
obtains its input by sampling the output voltage of the supply. Any changes in output
voltage or current are detected and amplified by the constant voltage or constant
current error circuit and applied to the series pass transistor in the correct phase and
amplitude to counteract the change in output voltage or current.
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