HP A Series Operating And Service Manual page 14

the voltage comparison amplifier, during constant volt¬
age operation, or the current limiting amplifier during
current limit operation.
4-5 The reference supply provides regulated bias
voltages which are used by the internal circuitry
throughout the main power supply.
4^6 DETAILED DESCRIPTION
4-7 The following paragraphs describe in detailed
terms, the operation of each of the supply's major
circuits. Throughout this discussion, refer to both the
simplified schematic of Figure 4-1 and the appropriate
schematic diagram at the rear of the manual.
4-8 Feedback Loop
4-9 The voltage comparison amplifier, together with
the driver and series regulator, comprises the constant
voltage feedback loop. This feedback loop maintains
a constant output voltage during normal (constant
voltage) operation. To accomplish this, the voltage
comparison amplifier continuously monitors the out¬
put voltage of the supply. It detects instantaneous
changes in the output and applies an amplified error
signal to the series regulator (via the driver) which is
of the correct phase and amplitude to counteract the
change.
4-10 The feedback loop also prevents the output
current from exceeding a preset current limit value.
Current limiting operation is initiated by the current
limit comparison amplifier which conducts if the out¬
put current that flows through the current sampling
resistance exceeds a value established by the current
limit-adjust potentiometer. When conducting, this am¬
plifier sends a turn-down signal to the series regulator,
via the driver.
4-11 An output capacitor, Cq , connected across the
output of the supply, helps to stabilize the feedback
loop regardless of the type of load that is imposed.
4-12 Constant Voltage Comparison Amplifier
4-13 This circuit consists of the VOLTAGE ADJUST
potentiometer R7 and a differential amplifier stage
(U2 and associated components). An integrated circuit
is used for the differential amplifier for increased
reliability and to minimize differential voltages due to
mis-matched transistors and thermal differentials.
4-14 The voltage comparison amplifier continuously
compares the output voltage of the supply with the
voltage drop between the —S terminal and the circuit s
summing point (junction at voltage divider R5, R7, and
R8). if these two voltages are momentarily unequal,
an error voltage is generated whose amplitude is propor¬
tional to the difference. The error signal ultimately
alters the conduction of the series regulator which, in
turn, alters the output current so that the output volt¬
age becomes equal to the voltage at the summing point.
Hence, through feedback action, the difference between
the two inputs to U2 is held at nearly zero volts.
4-15 The inverting input of the comparison amplifier
(pin 2 of U2) is connected, through jumper W1 to the
constant voltage summing point. (As indicated on
NOTE 7 of each schematic, W1 is connected either to
the junction of R5, R7 for supplies below 10 V or to
the junction of R7, R8 for supplies 10 V and above.)
The non-inverting input to the comparison amplifier
(pin 3 of U2) is connected directly to the output volt¬
age sensing terminal (+S) of the supply. Instantaneous
changes in the output voltage, or changes at the sum¬
ming point due to rotation of the voltage pot, produce
a difference voltage between the two inputs of the
comparison amplifier. This difference voltage is am¬
plified and appears at the output of the amplifier, at
pin 6.
4-16 Capacitor C12 and resistor RIO couple rapid out
put voltage variations to the input of U2 while slower
(dc) changes are coupled to U2 via resistor R8. Diodes
CR11 and CR12 prevent excessive voltage excursions
from over-driving the comparison amplifier; RC net¬
work R12,C13 provide degenerative feedback to help
stabilize the feedback loop.
4-17 Driver
4-18 The driver amplifies the error signal from the
constant voltage or current limit comparison amplifiers
to a level sufficient to drive the series regulating tran-
sistor(s). Inverting stage Q2 receives its input signal
from the OR-gate diode (CRI3 or CR14) that is con¬
ducting at the time. During normal (constant voltage)
operation, CR13 is forward biased and CR14 is reverse
biased. The reverse is true during current limiting
operation.
4-2