Power Mesh And Control Circuits - Agilent Technologies 6622A Service Manual

is high, the ON/OFF signal is low and turns off the control
circuit thus preventing power from reaching the output
terminals.
If the line voltage drops below a minimum level, the
comparator described above will shut-down the output
(remove power from the output terminals) until normal line
voltage is restored. This resets the microcomputer and sets
the output to the turn-on state.
2-33 Bias Supplies and Precision Reference Voltage. The
bias supplies (U300-U303) generate the voltages required to
operate the circuits on the output board. The precision
reference voltage circuit (U318, U319A/B) operates from the
+ 15 V bias and generates the VREF outputs (10 V ± 0.5%)
which is used by the DAC's and the control circuit.
2-34 Power Mesh and Control Circuits (Figure 2-5)
The power mesh circuit in the upper half of Figure 2-5
converts the AC from the power transformer to regulated
DC output power. The primary power control element is the
power module hybrid. Note that the main difference
between the 40W and 80W output boards is, the 40W output
boards have one power module (U338), while the 80W
output boards have two power modules (U338, U339)
connected in parallel. The power mesh circuit generates a
constant voltage or constant current output under control of
the control circuits shown in the lower half of Figure 2-5. In
addition to controlling the power mesh circuit, the control
circuits send measurement and status data back to the GPIB
controller and/or front panel via the interface circuits on the
output board and the GPIB board.
2-35 Rectifier and Filter. These circuits consist of two full-
wave bridge rectifier circuits with filter capacitors connected
across the rectifier outputs. The proper ac voltage levels are
applied to the rectifiers via secondary windings of chassis
power transformer T1 (see Figure 2-1). The rectifiers provide
raw dc to the power module at three different levels (high,
medium, and low power rails). The return line for the
rectifier circuits is fuse protected. If this fuse opens, the
power supply will fail self test, all outputs will be disabled,
and the error message "FUSE CH < n > " will be displayed
(where n specifies the particular output board, 1-4).
2-36 Power-On Circuit and Current Sources. The power-on
circuit (U341A and Q319) is used to turn on the current
source transistors and the bleed circuit (see paragraph 2-40)
which is connected across the output of the supply. The
power-on circuit is activated when it receives the ON/OFF
signal (2 V level) from the signal processor (U327).
The current sources (U336 on 40W boards, Q326-329 on 80W
boards) are a series of transistors connected to the high rail.
When activated by the power-on circuit, the current sources
supply a few milliamps to the +BASE DRIVE of the power
module and to the power module reference voltage circuit.
The +BASE DRIVE, in conjunction with the -DRIVE signal
(see base drive circuit description below), controls the
conduction of the series pass elements in the power module.
2-37 Power Module Reference Voltage. When the current
sources have been turned on, this circuit (P/O U340 and
U337) provides a reference voltage (about 2V above +V) to
the power module REF input. The REF input is used by an
internal control circuit that allows switching between the
low, medium, and high rails. The reference circuit includes a
transistor (P/O U340) that turns on when the current source
apply power, a programmable reference (U337) which
provides the reference voltage, and bypass capacitors C366
and 0367.
2-38 Power Module. The power module hybrid U338 (80W
output boards have two hybrids, U338 and U339, connected
in parallel) receives three unregulated DC voltage levels on
its high, medium, and low voltage input rails. The power
module contains series regulator stages, an SCR overvoltage
circuit, a down programmer, a built-in overtemperature
thermistor, and a reverse output voltage protection diode.
Series Regulators - The series regulator stages consist of series
pass transistors which regulate the voltage received from
the selected power rail. The power module automatically
selects the proper input rail depending upon the output
voltage required, For example, if the low rail is supplying
current and the output voltage exceeds the low rail minus
about 2.5 V, the medium rail begins to raise the voltage on
the BYPASS input and supply current. Finally, if the output
voltage exceeds the medium rail minus about 2.5 V, the high
rail will begin to supply current.
As stated previously, the conduction of the series pass
transistors is controlled by the +BASE DRIVE and -DRIVE
inputs. Normally there is about a diode drop between these
two input pins. The current sources drive the series
regulator into conduction via the +BASE DRIVE input. The -
DRIVE input from the base drive circuit (see paragraph 2-
42) controls the amount of +BASE DRIVE current that
drives the series regulators in order to maintain a regulated
output. Any +BASE DRIVE current from the current source
that is not required by the series pass transistor to regulate
the output is drawn away by the control circuit through the
- DRIVE input via Q335. A level of current through Q335
that exceeds the +BASE DRIVE current can turn on the
power module current sink transistors to sink output
current up to the negative current limit value.
SCR Overvoltage Circuit - The power module has an internal
SCR whose gate input is capacitively coupled to the OV
GATE pin. The OV GATE signal can fire the SCR for a
number of reasons which are descibed later under the
"Overvoltage Protection Circuit" paragraph. In addition to
shorting the output, the fired SCR will cause the OV SENSE
signal to go low signaling the microcomputer to program
the output to zero. The output will remain shorted and
programmed to zero until the circuit is reset. The SCR
circuit is reset when the POV DISABLE signal (OVRST
command) is received by the OV reset circuit (Q320). The
condition that caused the overvoltage must be removed in
order for the circuit to remain reset. If the condition is not
removed, the OV GATE signal will again fire the SCR and
disable the output. Note that in addition to resetting the
2-10
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