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Chapter 4 Theory of Operation
Power Mesh and Control
86
Power Mesh and Control
Refer to the schematics shown on page 121.
For the 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 SCR and one bridge diode
and the SCR control circuit in each power circuit; CR14, CR17, CR18, CR19,
and CR11. By turning on or off the SCR, these circuits allow the input
capacitors (C36, C65, and C46) to charge to one of two discrete voltage levels,
depending on the output voltage required. When the SCR is not fired, the bridge
diode conducts and the low voltage of two discrete voltage levels is developed
across the input filter capacitors.
The SCR control circuit determines whether SCR 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 from differential amplifier which
senses the output voltage. 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.