KEPCO JQE 150-1.5MVPY-26956 Half Rack - Manual (Page 43)

JQE SPECIAL SVC 081111 5-3
Since the primary coil of the AUXILIARY POWER TRANSFORMER (T202) is in parallel with the
secondary for the overvoltage protection circuit, any failure in the auxiliary circuits, reflecting
back to the primary of the auxiliary power transformer (T202) will also activate the sensing coil
of CB101 and shut down the power supply. A thermal sensing element (S301) is mounted to the
heatsink assembly (A3) to protect the series regulator transistors from overtemperature. If the
preset temperature on S301 is exceeded, its contact closes and energizes the sensing coil of
CB101, resulting once more In the tripping of the a-c power circuit breaker and subsequent
removal of the a-c input power from the power supply.
5.2.2 MAIN D-C SUPPLY
The main d-c power is derived from a center-tapped secondary winding on T201. A full-wave
rectifier circuit with silicon diodes (CR201, CR202) operates into a capacitor input filter (C201 or
C201/C202) which is paralleled by a bleeder resistor (R202). The main d-c supply delivers the
output current via the series regulator or pass-element.
5.2.3 PASS-ELEMENT SECTION
The pass element section varies in complexity according to the output voltage and current rating
of the particular model. The NPN silicon pass-transistors of the JQE are located on high-effi-
ciency heatsinks and are cooled by a fan (B201). These models have a single heatsink assem-
bly (A3) with NPN transistors. The number of transistors depends on the individual model. While
in the low voltage/high output current models (E
O
< 36 Volts) all pass-transistors are arranged in
a parallel circuit, higher voltage models (E
O
> 36 Volts) use a series/parallel configuration in the
arrangement of their pass-transistors. However complex the pass-element section, for analytical
purposes it can be reduced to a single pass-transistor. Electrically, this pass-element is part of a
series circuit consisting of the unregulated main d-c power supply, the external load and the
pass-transistor. The output voltage of the power supply is kept constant, regardless of variations
in the unregulated supply, by changing the series resistance of the control element (the pass-
transistor) in the described series circuit. The necessary base-drive for the pass-transistors to
affect the change in series resistance is supplied by the “Darlington” connected driver stages.
The main driver stages, in turn, receive their base signals from the pre-driver stage (A1Q2).
5.2.4 VOLTAGE ERROR AMPLIFIER (VEA)
The main function of the voltage error amplifier A1IC1 (see Figure 7-9) is to amplify the d-c error
signal, derived from the comparison bridge, to a level suitable to pass the diode gate circuit and
drive the pre-driver stage A1Q2. The voltage error amplifier is a d-c coupled, high-gain opera-
tional amplifier. The amplifier input may be disconnected from the internal comparison bridge
circuit, and programmed externally. Although the amplifier is used in the non-inverting configu-
ration, a positive input signal will produce a negative power supply output with respect to the
“common” (plus sensing terminal +S), since the pass-elements provide another signal inversion.
If a link is installed between TB501, terminals 6 (RVC) and 7 (RPVC1) (see Figure 7-8) and the
E
OUT
LOCAL - E
OUT
REMOTE switch is set to E
OUT
LOCAL, the output voltage is controlled by
front panel VOLTAGE control R102. When the link between between TB501, terminals 6 (RVC)
and 7 (RPVC1) is removed and the E
OUT
LOCAL - E
OUT
REMOTE switch is set to E
OUT
REMOTE, output voltage can be controlled by an external resistance connected across TB501,
terminals 7 (RPVC1) and 8 (RPVC2).
5.2.5 CURRENT LIMIT CIRCUIT
The differential input of the current sensing amplifier, A1IC2A, is connected to the current sens-
ing resistor (R
S
= R203). The amplified voltage drop across the current sensing resistor is con-
tinuously compared to the reference voltage, set by the current limit control. As long as the