Detailed Circuit Analysis; Feedback Loop; Series Regulator; Constant Voltage Input Circuit - HP BENCH Series Operating And Service Manual

amplifier and driver, and applied to the series
regulator intiie correct phase and amplitude to
counteract the change in output voltage. The ref
erence regulator circuit provides stable reference
voltages which are used by the constant voltage
input circuit for comparison purposes. The meter
circuit provides indications of output voltage or
current in either operating mode.
4-4 Diode CR14, connected across the output
terminals of the power supply, is a protective de
vice which prevents internal damage that might
occur if a reverse voltage were applied across the
output terminals.
4-5 DETAILED CIRCUIT ANALYSIS (Refer to overall
schematic diagram at rear of manual)
4-6 FEEDBACK LOOP
4-7 The feedback loop functions continuously to
keep the output voltage constant, during constant
voltage operation, and the output current at a safe
limit during current limit operation. For purposes
of this discussion, assume that the unit is in con
stant voltage operation and that the programming
resistors RIO and Rll have been adjusted so that
the supply is yielding the desired output voltage.
Further assume that the output voltage instantane
ously rises (goes positive) due to a variation in
the external load circuit.
4-8 Note that the change may be in the form of a
slow rise in the output voltage or a positive going
AC signal. An AC signal is coupled to the voltage
input circuit through capacitor C1 and a DC volt
age is coupled through RIO and Rll.
4-9 The rise in output voltage causes the voltage
at the base of Q1 to decrease (go negative). Q1
now decreases its conduction and its collector
voltage rises. The positive going error voltage is
amplified and inverted by Q4 and fed to the base
of series transistor Q7 via emitter follower Q5.
The negative going input causes Q7 to decrease
its conduction so that it drops more of the line
voltage, and reduces the output voltage to its
original level.
4-10 When the external load resistance decreases,
the output current increases until the current limit
is reached. The positive voltage developed at the
wiper of R50 causes Q3 to conduct. CR4 becomes
forward biased and controls the conduction of Q5
and Q7. Any further decreases in load resistance
increase the negative voltage on the base of Q5
which decreases the conduction of Q7. Thus,
through feedback action the output current is limi
ted to the value at which CR4 conducts.
4-11 SERIES REGULATOR
4-12 The series regulator consists of transistor
stage Q7 (see schematic at rear of manual). The
regulator serves as a series control element by
altering its conduction so that the output voltage
is kept constant and the current limit is never ex
ceeded. The conduction of Q5 is controlled by the
feedback voltage obtained from driver Q4. Diode
CR7, connected across the regulator circuit, pro
tects the series transistor against reverse voltages
that could develop across it during parallel or auto-
parallel operation if one supply is turned on before
the other.
4-13 CONSTANT VOLTAGE INPUT CIRCUIT (Refer
to overall schematic at rear of manual)
4-14 The circuit consists of the coarse and fine
programming resistors (RIO and Rll), and a differ
ential amplifier stage (Ql, Q2, and associated
components). Drift due to thermal differentials is
minimized, since both transistors operate at es
sentially the same temperature.
4-15 The constant voltage input circuit continu
ously compares a fixed reference voltage with a
portion of the output voltage and, if a difference
exists, produces an error voltage whose amplitude
and phase is proportional to the difference. The
error output is fed back to the series regulator,
through the error and driver amplifiers. The error
voltage changes the conduction of the series reg
ulator which, in turn, alters the output voltage so
that the difference between the two input voltages
applied to the differential amplifier is reduced to
zero. The above action maintains the output volt
age constant.
4-16 Stage Q2 of the differential amplifier is con
nected to a common (+S) potential through imped
ance equalizing resistor R6. Resistors RS and R7
are used to zero bias the input stage, offsetting
minor base-to-emitter voltage differences in Ql
and Q2. The base of Ql is connected to a sum
ming point at the junction of the programming
resistors and the current pullout resistor, R12.
Instantaneous changes in output voltage result in
an increase or decrease in the summing point
potential. Ql is then made to conduct more or
less, in accordance with the summing point volt
age change. The resultant output error voltage is
fed back to the series regulator via the remaining
components of the feedback loop. Resistor Rl, in
series with the base Ql, limits the current through
the programming resistors during rapid voltage
turn-down. Diodes CRl and CR2 form a limiting
network which prevents excessive voltage excur
sions'from over driving stage Ql. Capacitor Cl,
4-2