Agilent Technologies Welding System Agilent Technologies Welding System User Manual

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Chapter 4 Theory of Operation
Power Mesh and Control
88
Power Mesh and Control
Refer to the schematics shown on page 130 and page 131.
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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