Keysight E3631A, Service Manual

Page: 107 / 131

Theory of Operation 4
Keysight E3631A Service Guide 103
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.
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