Output Power And Control Circuits - Agilent Technologies 6631B Service Manual

4 – Principles of Operation
The primary bias circuits are referenced to chassis (earth) ground. They provide the bias for the GPIB,
RS232 and RI/DFI interfaces, the interface micro-processor circuits and the front panel.
The secondary bias circuits are referenced to the secondary (output) common and are isolated from chassis
ground. They provide the bias for the amplifier and output circuits located on the A1 pc board. They also
provide the bias for the Logic Array, EEPROM, DAC and ADC circuits and the secondary side of the Opto-
isolators on A2.

Output Power and Control Circuits

As shown in Figure 6-2, the power circuits consist of: input power rectifiers and filter, current-monitoring
resistors, an output stage, a voltage gain stage, an overvoltage SCR, and an output filter.
The ac input rectifier and filter converts ac input to a dc level. The output stage regulates this dc level at the
output of the power supply. The output stage has up to four parallel NPN transistors mounted on a heatsink
and connected between the +Rail and the +Output. These transistors are driven to conduct by a positive-
going signal from driver Q303 (located in the voltage gain stage). The output stage also has up to four
parallel PNP transistors mounted on a heatsink and connected between the +Rail and the -Rail. These
transistors are driven to conduct by a negative-going signal from driver Q304 (located in the voltage gain
stage).
The voltage gain stage is controlled by a signal from the control circuits. A positive-going signal to the
voltage gain stage makes the output more positive. A negative-going signal to the voltage gain stage makes
the output more negative. The Turn-on control signal to the voltage gain stage simply keeps the output of
the unit turned off for about 100 milliseconds at power turn-on while the microprocessor is initializing the
unit.
Two current shunt resistors monitor the output current. RmHi (R473) monitors the high current range;
RmLo (R403) monitors the low current range. Shunt clamps, connected in parallel across RmLo, turn on at
approximately 25 mA to limit the voltage drop at high currents. The Range_Select signal sets the level at
which switching occurs. The output of the current monitor drives the level.
The SCR, connected across the output, will fire and short the output when an overvoltage condition is
detected. The SCR is controlled by the OV_SCR* signal from the crowbar control circuit (described in the
next section).
The output filter capacitor provides additional filtering of the dc output.
Control Circuits
As shown in Figure 6-2, the control circuits consist of the CV/CC controls, output voltage/current monitor,
bias supplies, and SCR control.
The CV/CC control circuits provide a CV control loop, a positive CC control loop, and a negative CC
control loop. For any value of load resistance, the supply must act either as a constant voltage (CV) or as a
constant current (CC) supply. Transfer between these modes is accomplished automatically by the CV/CC
control circuit at a value of load resistance equal to the ratio of the programmed voltage value to the
programmed current value. The negative CC control circuit is activated when a current source such as
another power supply is connected across the output terminals and its voltage is greater than the
programmed voltage. A low level CV_Detect*, CC_Detect*, or CCN_Detect* signal is returned to the
secondary interface to indicate that the corresponding mode is in effect.
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