User`s guide

Chapter 4 Theory of Operation

Power Mesh and Control

For the power mesh and control circuit, the preregulator which is controlled by the

phase control circuits is added ahead of the series pass transistor to minimize the

power dissipated at the series pass transistor by controlling the dc level across the

input filter capacitor, depending on the output voltage.

For the dual range of output, a controlled transformer tap switching is used. It is

accomplished by two SCR and one bridge diode and the SCR control circuit in the

power circuit; CR17, CR19, and CR44. By turning on or off the SCR, these circuits

allow the input capacitors (C39, C46, and C74) to charge to one of two discrete voltage

levels, depending on the output voltage required. When all SCR’s are not fired, the

bridge diode conducts and the lowest 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 the feedback loop which consists of the driver and

the Constant Voltage/Constant Current error amplifier. This feedback loop provides

‘’fine and fast’’ regulation of the output while the feedback loop which is controlled

by transformer tap switching handles large, relatively slow, and 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.