Series Regulator - HP A Series Operating And Service Manual

4-19 Stage Q2 provides mainly voltage amplification
of the feedback signal while emitter follower Q3 pro¬
vides most of the current gain. Some models; e.g.
62048A, contain only one driver stage (Q2) instead of
two.
4-20 Feedback capacitor CIS provides degenerative
feedback to Q3 to prevent oscillation and Cl 9 helps
shape the high frequency rolloff of the feedback loop
response curve. R28 provides a leakage current path for
transistor Q3.
4-21 Series Regulator
4-22 The series regulator, or series control element,
acts as a variable resistance connected in series with
the load. Its conduction is controlled by the feedback
signals from the driver so that the output voltage is
maintained constant or the current limit is not ex¬
ceeded. A diode (CRs on Figure 4-1) connected across
the regulator, protects the series eiement(s) from
possible damage by a reverse current flow.
4-23 Many modular power supplies employ one series
transistor as indicated on Figure 4-1. However, higher
power units (such as Models 62003E thru 62048E) use
two regulating transistors, connected in parallel to
minimize power dissipation. Approximately half of
the output current flows through each transistor.
4-24 Current Limit Comparison Amplifier
4-25 This circuit consists of current limit adjust po¬
tentiometer R18 and a differential amplifier stage
(U3and associated components). Stage U3 is similar
in appearance and operation to the constant voltage
comparison amplifer, U2.
4-26 The current limit comparison amplifier effect¬
ively monitors the output current of the supply by
monitoring the IR drop across current sampling resis¬
tor R15. The voltage drop across R15 is compared with
a reference voltage established by the setting of current
limit potentiometer R18. During constant voltage
operation, the output of U3 is not positive enough to
forward bias OR gate diode CR14. However, if the out¬
put current increases to approximately 104% of the
rated value, the input to pin 2 of U3 is driven momen¬
tarily less positive. If this occurs, a positive going out¬
put signal from U3 forward biases CR14and reverse
biases CR13. This feedback signal then decreases the
conduction of the series regulator.
4-27 The current limiting circuit contains a current
"cutback" feature which protects the series regulating
transistor(s) against excessive overloads. As the load
resistance decreases, the output current is cut back
linearly, from the initial current limit point, to approxi¬
mately 10% of the rated current under short circuit
conditions (see Figure 4-2). Cutback action is made
possible by R18and R19, which are connected across
the output of the supply, if the load resistance de¬
creases after the initial current limiting point, the
resultant drop in output voltage causes a decrease in
the current flowing through RISand R19. This, in
turn, causes U3 to conduct even harder further reduc¬
ing the conduction of the series regulator.
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Figure 4-2. Typical Current Limiting Characteristics
4-28 As described in Section III, the cutback point
for the supply can vary from approximately 3% to
15% of the rated output current. Resistors R20 and
R21 determine the actual cutback point.
4-3