Agilent Technologies 6621A, Service Manual

Page: 24 / 156

will conduct less allowing more current to flow into the +
BASE DRIVE input. This will cause the power module's
series regulators to conduct more and thus increase the
output voltage.
2-44 Current Control Circuit. When the output is operating
in the constant current mode, this circuit generates the + CL
control and the + CL LOOP signals. The + CL control signal
is applied through OR gate diode CR348 to control the base
drive circuit in order to regulate the output current. The +
CL LOOP signal is sent back to the secondary interface
circuits to indicate that the output is in the constant current
mode of operation. The ON/OFF signal, received from the
secondary interface circuit, must be on (about + 2 V) in
order to activate the current control circuit.
The current control circuit compares the output current to a
programmable reference voltage (+ CL REF) that represents
the programmed current value. This comparison produces
the + CL control signal. In order to make this comparison,
the circuit monitors the voltage (I-MON) across current
monitoring resistor R408. This voltage drop represents the
amount of output current. The I-MON and + CL REF signals
are connected through scaling resistors to summing point S2
for application to U346 (CC Error Amplifier) as shown in
Figure 2-6. Based on this summing action, U346 generates
the + CL control signal which is applied to the base drive
circuit via OR gate diode CR348 to control conduction of the
series regulators in the power module in the same way as
described above for the voltage control circuit. The I-MON
signal is also amplified and sent back to the secondary
interface to indicate the magnitude of the output current.
The current control circuit receives an input from the peak
current limit circuit (in the current sourcing mode only) as
shown in Figure 2-5. When the peak current limit circuit (see
paragraph 2-39) is activated, it immediately limits the
conduction of the series regulators in the power module and
also notifies the current control circuit to take control of the
current limiting action.
2-45 Negative Current Limit Circuit. This circuit provides a
limit to the amount of current that the supply can sink. The
circuit may be activated if a current source such as another
power supply (or energy storage capacitor) is connected
across the output terminals and its voltage is greater than
the programmed output voltage.
When the output is in negative current limit, this circuit
generates the - CL control and the - CL LOOP signals. The -
CL control signal is applied through diode CR354 to the
base drive circuit. The - CL LOOP signal is sent back to the
secondary interface to indicate that the output is in the
negative current limit mode.
As shown in the simplified schematic of Figure 2-6, the
negative current limit circuit consist mainly of open
collector toggle comparator (part of U351) and - CL error
amplifier (U350).
The voltage drop (I-MON) across the current monitoring
resistor R408 is applied to summing junction S3 along with a
reference voltage. Based on this summing action, error
amplifier U350 generates the - CL control signal which is
applied through diode CR354 to control the base drive
circuit.
Comparator U351 toggles the reference voltage between two
different levels. This is required because the output board
has two fixed ranges: a high voltage/low current range and
a low voltage/high current range. Figure 2-7 illustrates the
output range characteristics for the various output board
types. As you can see in the figure, a 40W low voltage board
can sink up to 2.2 A when its output voltage is above 10 V
and up to 5.5 A when the output is between approximately
2 V and 10 V.
U351 constantly monitors the output voltage in order to
provide the proper reference voltage to the summing
junction of U350. If the output voltage is in the high range,
the open collector output of U351 will be near ground;
thereby dividing down the VREF voltage to summing
junction S3 resulting in a lower sink current limit (- 2.2 A for
a 40 W low voltage board). If the output voltage is in the
low range, the collector output of U351 will be open,
resulting in a higher sink current limit (about - 5.5 A for a 40
W low voltage board). R476 provides a small amount of
positive feedback (hysteresis) to prevent ''jitter" at the switch
point.
2-46 FET Downprogrammer. When the output is sinking
current and the output voltage drops below 2.0 V
(approximately), the down programming characteristics
(current sinking characteristic) are as shown in Figure 2-8
for each type of output board. The FET Downprogrammer
circuit (Part of U351), connected across the output, senses
when the output falls below 2.5 V (approximately) and
connects a resistor across the output to aid
downprogramming. Notice that in Figure 2-8 on the 40W
low voltage graph, the 0.2 ohm slope relates to the saturated
impedance of the current sink transistors, the 7.5 ohm slope
is due to the resistor R457 in series with FET Q342, and the -
0.015 amps at VOUT equals zero volts represents the bleed
current in Q341.
2-47 Overvoltage Protection Circuits. These circuits
generate the OV GATE signal which fires the SCR in the
power module and shuts down the output. Figure 2-9 is a
simplified schematic of the overvoltage protection circuits
which are comprised mainly of a fixed overvoltage sensing
circuit (U354), signal processor U327, diodes CR356-CR360,
and pulse transformer (T301) that couples the remote trip
signals that are sent/received via the + OV and - OV
terminals.
As shown in Figure 2-9, the main input to the overvoltage
protection circuits is the OV DRIVE signal which is received
from the overvoltage detector (P/O U327, see paragraph 2-
32). The OV DRIVE signal goes high to activate the OV
GATE signal which is sent via diode CR357 to fire the SCR
in the power module. The conditions which activate OV
DRIVE are described in the following paragraphs.
2-14
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