Agilent Technologies 6612B, Service Manual, Page: 50 / 74

4 - Principles of Operation
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The EEPROM (electrically erasable programmable read-only memory) chip on the A2 interface board
stores a variety of data and configuration information. This information includes calibration constants,
GPIB address, present programming language, and model-dependent data, such as the minimum and
maximum values of voltage and current. One of the EEPROM storage locations holds a checksum value
which is used to verify the integrity of the EEPROM data. Access to the calibration data in the EEPROM
is controlled by the combination of a password and switch settings on A2S201, located on A2 interface
board (see Chapter 3 "Inhibit Calibration Switch").
The Dual 12-bit DAC converts the programmed value of voltage and current on the bus into the CV_Prog
and CC_Prog signals, which are sent to the CV control circuits in order to control the magnitude of the
output voltage in the CV mode and output current in CC mode. The CV_Prog and CC_Prog signals are in
the 0 to -5 V range, which corresponds to the zero to full-scale output ratings of the dc power supply.
The Quad 8-bit DAC converts programmed information for the following circuits into analog format:
overvoltage setting (OV_Prog), and fan speed programming (Fan_Prog). The OV_Prog signal is applied to
the OV detect circuit, which compares the programmed overvoltage setting with the actual output voltage.
The Fan_Prog signal is applied to the fan speed control circuit in order to speed up the fan as temperature
increases, and to slow the fan speed down as temperature decreases.
The 16-bit ADC in conjunction with a 4x1 multiplexer returns data from the following measurement
signals to the logic array: monitored output voltage (VMon), monitored high-range current (Imon_H),
monitored low-range current (Imon_L), and monitored peak current (Imon_P). All measurement signals are
in the range of 0 to +5V, which corresponds to the zero to full-scale readback capability of the dc power
supply.
The 8-channel, 8-bit ADC returns the following signals to the logic array: high-range output current
(Imon_H), overvoltage (V_Mon), ambient temperature (Temp_Amb), heatsink temperature (HS_Therm),
and output fuse state (Fuse). Four of these signals are for fan control. The logic array varies the Fan_Prog
signal depending upon the ambient temperature, the heatsink temperature, and the present output voltage
and current. The Fuse signal informs the logic array if the output fuse (F309) is open.
A1 Main Board Circuits
Power Circuits
As shown in Figure 4-2, the power circuits consist of: input power rectifiers and filter, an output regulator,
a downprogrammer circuit, current-monitoring resistors, an overvoltage SCR, and an output filter.
The ac input rectifier and filter converts ac input to a dc level. The output regulator regulates this dc level at
the output of the power supply. The output regulator stage consists of two parallel NPN series regulators
mounted on a heatsink and connected between the +Rail and the +Ouput. The conduction of these series
regulators is increased or decreased by the Control signal from the CV/CC control circuits in order to
regulate the output voltage (in CV mode), or output current (in CC mode).
An NPN downprogramming transistor is connected between the +Output and the -Rail. The conduction of
the downprogramming transistor is controlled by the DP_Control signal from the CV/CC control circuits.
Whenever the output voltage is greater than the programmed voltage setting, the downprogramming
transistor conducts and shunts current away from the load until the output voltage equals the programmed
setting.