EG&G ORTEC 402D Руководство по эксплуатации и обслуживанию - Страница 7
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Печатать страницуWhen it is necessary to rack mount several Bins and
Power Supplies, especially when other heat-generating
equipment is located within the rack, the term "ambient
temperature" becomes less clearly defined. A better guide
to maximum power loading capability is to monitor the
heat sink temperature. In no case allow the heat sink
temperature to continuously run above 85° C. Although
this is not the maximum operation temperature, any addi
tional temperature rise due to other conditions of the sys
tem may force the Supply out of tolerance and may cause
it to automatically shut down operation. Should your
operation produce a temperature of 85° C, a blower to
remove the dissipated heat is recommended.
5.
CIRCUIT DESCRIPTION
The 402A Power Supply produces four dc output volt
ages. A power transformer transforms the input ac line
voltage into four separate low-voltage sources. The
sources or windings are full-wave-rectified, capacitor-
filtered, and regulated by electronic series regulator cir
cuits. The regulator circuits provide short circuit, current
limiting, and reverse current protection.
Each of the four series regulator circuits is identical in
operation: they are physically different only in component
values for each Supply. The regulator essentially operates
in two modes: First and normal is the voltage regulation
mode: second is the constant-current or current-limiting
protection mode.
The regulation will operate in the voltage regulation mode
at any current output up to and including the full rated
output of a particular supply. When current output beyond
the rated output is required, which includes a direct short
across the output terminals, the regulator automatically
shifts into a constant-current mode. This provides current
limiting and protection of the regulator's circuitry and
components. When excessive current demands are re
moved, the regulator resumes the voltage regulation
mode.
For operation of the regulator, please refer to Circuit
Drawing 402A-1100-S1. For convenience, only the +24-V
regulator will be discussed, and the following is an
explanation of the regulation in the normal voltage regu
lation mode.
Transistors Q6 and Q7 operate as a differential amplifier
pair, comparing the reference voltage of 04 at the base of
Q6 with a portion of the output voltage divided down
through R16, R17, and R18. Trim potentiometer R17 is
used to adjust the output voltage to the specified level. A
difference voltage at the collector of Q7 is dc amplified by
Q4 and Q2. The collector of Q2 drives emitter-follower
Q1, which supplies the necessary current to drive the
remotely located series power transistor. This transistor
is heat-sink-mounted to dissipate the power consumed in
the regulation process.
In the constant-current or limiting mode, remotely located
resistor R1, in series with the output, senses the output
current level and produces a proportional voltage rise.
The sense voltage is compared to the output voltage at
the base of Q5. For output current levels less than or
equal to the rated output, Q5 remains back-biased and
will have no effect on the regulator performance. How
ever, when the output current exceeds the rated output,
Q5 becomes forward-biased and conducts, causing Q2
to conduct harder, thereby reducing the available base
drive current to emitter-follower Q1 and the series pass
regulator transistor. As a result, the output voltage is
reduced until the output current is within the required
limits. Upon removal of the short circuit or excessive cur
rent demand, the regulator resumes the normal voltage
regulation mode.