Theory of Operation 4
Keysight E3633A and E3634A Service Guide 107
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.
Two error amplifiers are included in a CV/CC supply, one for controlling output
voltage, the other for controlling output current. Since the constant voltage
amplifier tends to achieve zero output impedance and alters the output current
whenever the load resistance changes, while the constant current amplifier
causes the output impedance to be infinite and changes the output voltage in
response to any load resistance change, it is obvious that the two amplifiers
cannot operate simultaneously. For any given value of load resistance, the power
supply must act either as a constant voltage source or as a constant current
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