Agilent Technologies E3634A, E3633A manual Theory of Operation Power Mesh and Control

Chapter 4 Theory of Operation

Power Mesh and Control

Power Mesh and Control

Refer to the schematics shown on page 129.

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.

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