Agilent Technologies 6622A Service Manual page 21

SCR, the OVRST command will program the output to the
settings that existed before the OV occurred.
Down Programmer - Separate transistors in the power
module are used to sink output current and are capable of
rapidly downprogramming the output voltage to about 2 V.
An external FET down programmer circuit (see paragraph
2-46), is connected across the output to continue down
programming the output voltage below 2 V.
Overtemperature Protection - The power module also
contains an overtemperature circuit that consists of a
negative temperature coefficient thermistor that senses the
power module's temperature. When the power module's
temperature rises enough to reduce the THERM input
resistance to about 8 K ohms, the thermistor drops below 2.5
V(approximately) notifying the signal processor that an
overtemperature (OT) condition has occurred. The signal
processor then relays this information to the microprocessor
which will shutdown the particular output with the
overtemperature condition. The output will be restored 30
seconds after a safe operating temperature is reached.
Reverse Output Voltage Protection Diode - The power module
contains a diode with its cathode connected to the
COLLECTOR output and its anode connected to the power
module COMMON. This diode is essentially connected
across the power supply's output terminals to protect the
output from having reverse voltages applied.
2-39 Peak Current Limit. This circuit Q321, P/O U340, and
P/O U341 (plus P/O U341 and Q325 on 80W output boards)
quickly limits the amount of current through the series
regulator elements in the power module. It is activated
when the output current exceeds the full scale value + about
75% in either the sourcing or the sinking direction.
The series pass regulator in power module U338 is
connected in series with an external resistor (R407). For 80W
output boards, the series regulator in the additional power
module U339 is connected in series with external resistor
R416. (see Functional Schematic, Figure 6-3 for details). The
peak current limit circuit for 80W boards includes addtional
transistors P/O U341 and Q325 to monitor the current
through R416. When the voltage across R407 (or R416 for 80
W boards) exceeds a diode drop in either direction, the peak
current limit circuit is activated and limits the conduction of
the series pass transistor element or current sink transistor.
This circuit reacts much faster than the + or- current control
circuits (see paragraph 2-44).
When the peak current limit circuit is activated in the
current source direction, not only will the conduction of the
series regulator be limited, but the current control circuit
(U346) will be quickly activated through P/O U340 and
R405 to take control of the current limiting action.
2-40 Bleed Circuit. This circuit (Q341, R456, etc.), connected
from + V to - 5.75 V, provides a fixed current of about 15
mA (30mA in 80W boards) through the series pass elements
in the power module so that they are never completely
turned off. The bleed circuit is activated via the power-on
circuit when the ON/OFF signal is high. The bleed circuit
maintains stability with large output capacitors under light
loading conditions and helps to keep the output impedance
constant.
2-41 Sense Protect Circuit. This circuit (P/O U340 and P/O
U341) monitors the voltage from +V to +S and from -S to –V.
If either of these voltages exceeds 1.5 V, the sense protect
circuit will generate the SENSE PROTECT signal which will
fire the overvoltage protection circuits and shut down the
output (see paragraph 2-47). This circuit prevents the output
voltage from being regulated at a value higher than the
maximum value for which it was designed.
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Notice also that the series combination of R509 and R487
comprise the "positive sense protect resistor" and the series
combination of R512 and R483 comprises the "negative
sense protect resistor". In the case of open sense leads, these
resistors allow the power supply to effectively switch to the
local sensing mode.
2-42 Base Drive Circuit. When activated (ON/OFF is at
approximately 2 volts), this circuit (Q335 and U348)
provides the - DRIVE input to the series regulator and
current sink transistors in the power module. The - DRIVE
input determines how much drive current (+BASE DRIVE)
the power module will receive. The - DRIVE input is
controlled by either the voltage control (CV), current control
(+ CL), or negative current limit circuits ( - CL).
The CV or + CL signal controls the base drive circuit via OR
gate diodes CR351 or CR348 to generate the - DRIVE signal
in order to control the conduction of the series regulators in
the power module and provide a regulated output. If the
output is less than the programmed value, the -DRIVE
signal will allow more + BASE DRIVE current causing the
series regulators to conduct more and raise the output. If the
output exceeds the programmed value, the - DRIVE signal
will divert current through Q335 and U348 of the base drive
circuit and away from the + BASE DRIVE power module
input causing the series regulators to conduct less thereby
reducing the output. The voltage control (CV) circuit and
the current control ( + CL) circuit is described in paragraphs
2-43 and 2-44, respectively.
When the output is operating in negative current limit, the -
CL signal controls the base drive circuit via diode CR354 so
that the - DRIVE signal controls the conduction of the
current sink transistors in the power module. The negative
current limit circuit which generates the - CL signal is
described in paragraph 2-45.
24-3 Voltage Control Circuit. When the output is operating
in the constant voltage mode, this circuit generates the CV
control and CV LOOP signals. The CV control signal is
applied through OR gate diode CR351 to control the base
drive circuit in order to regulate the output voltage. The CV
LOOP signal is sent back to the secondary interface circuit
to indicate that the output is in the constant voltage mode of
operation. The ON/OFF signal, received from the secondary
2-11
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