HP 6268B Operating And Service Manual

4-45 Voltage Clamp Circuit
4-46 The voltage clamp circuit keeps the constant-
voltage programming current relatively constant when the
power supply is operating in the constant-current mode.
This is accomplished by clamping terminal A2, the voltage
summing point, to a fixed bias voltage. During constant-
current operation the constant-voltage programming resis¬
tors are a shunt load across the output terminals of the
supply. When the output voltage changes, the current
through these resistors also tends to change. Since this
programming current flows through the current sampling
resistor, it is erroneously interpreted as a load change by
the current comparator circuit. The voltage clamp
eliminates this undesirable effect by maintaining the
programming current at a constant level.
4-47 The voltage divider consisting of Z2A, Z2B, and
VR1 back biases CR2 and Q1 during constant-voltage
operation. When the power supply goes into constant-
current operation, CR2 becomes forward biased by the
voltage at pin 12 of Z1. This results in the conduction of
Q1 and the clamping of the summing point at a potential
only slightly more negative than the normal constant-
voltage potential. Clamping this voltage at approximately
the same potential that exists in constant-voltage operation
results in a constant voltage across, and consequently a
constant current through, current pullout resistors R3, R4,
and R5.
4-48 Mixer and Error Amplifiers
4-49 The mixer and error amplifiers amplify the error
signal received from the constant-voltage or constant-
current input circuit to a level sufficient to drive the series
regulator transistors. Mixer amplifier Q41 receives the error
voltage input from either the constant-voltage or constant-
current comparator via the OR-gate diode (CR1 or CR20)
that is conducting at the time. Diode CR1 is forward biased
and CR20 reverse biased during constant-voltage operation.
The reverse is true during constant-current operation.
4-50 Transistor Q40 provides a constant current to
the collector of Q41 and also generates a negative-going
turn-off signal for the series regulator when the unit is
first turned off. Feedback network C41, R47, and R53
shapes the high frequency rolloff in the loop gain response in
order to stabilize the series regulator feedback loop.
4-51 Error amplifiers Q42 and A4Q101 serve as the pre¬
driver elements for the series regulator. A4Q101 also provides
a discharge path for the output capacitors in order to allow
faster down-programming and conducts a bleed current for
the series regulator to keep it in its active region when the
supply is set for zero output current. Diode CR44 prevents
A4Ql01's base from going more negative than -3 volts
in order to limit the bleed current through R57 and protect
A4Q101 from damage if a voltage higher than the program¬
med output voltage is applied across the output terminals.
4-52 Overvoltage Protection Crowbar
4-53 The overvoltage protection crowbar circuit protects
sensitive loads against the application of an excessively high
voltage, as might result from a series regulator transistor
failure. It accomplishes this by immediately shorting the
output of the supply as soon as a preset threshold voltage
is exceeded. Until silicon controlled rectifier A4CR110has
been triggered, it acts as an open circuit and has no effect
on the output voltage. Transistors Q91 and Q92 detect an
overvoltage condition and trigger the SCR to fire. When
the SCR fires, it shorts the supply's output.
4-54 During normal operation, Q92 is biased on by
current through R99, Q91 is kept turned off by Q92, and
CR91 is reverse biased by the voltage divider formed by
resistors R90, R95, and A5R125. Zener diode VR90
provides a stable reference voltage with which the —S
potential is compared. Potentiometer A5R125 (OVER¬
VOLTAGE ADJUST} establishes the output voltage at which
CR91 becomes forward biased and turns 092 off. When Q92
turns off, 091 begins to conduct, sending a positive-going
trigger pulse to A4CR110 and causing it to create a near
short circuit across the output. When A4CR11 0 fires,
overvoltage lamp A5DS2 turns on, completing a path for
a +11V unregulated holding current through A5DS2. (R92
supplies the holding current if the lamp should open.) This
current holds A4CR110 on even after the output voltage
has fallen. A4CR110 remains in conduction until the
supply is turned off. A4 R106 protects A4CR108 and
A4CR110 from the large surge current that occurs when
A4CR110 is first fired. CR93 damps out negative over¬
shoot in the trigger pulse.
4-55 The firing of A4CR110 biases Q90 into conduction.
This places approximately +11 volts on the cathode of
CR74 in the preregulator control circuit to reverse bias
CR74 and CR75. By preventing transistor Q72 from
turning off, this prevents the generation of any trigger pulses
and turns off the preregulator.
4-56 The crowbar circuit creates an extra current path
during normal operation of the supply, thus changing the
current that flows through the sampling resistor. Diode
CR92 keeps this extra current at a fixed level for which
compensation can then be made in the constant-current
comparator circuit.
4-57 A slaving arrangement for the crowbar circuits in
more than one supply >s made possible by an extra
4-6

HP 6268B Specifications

General

Brand	HP
Model	6268B
Category	Power Supply
Language	English