If the output voltage from +V (R359) exceeds the
programmed overvoltage setting (derived from OV REF
through R357), the overvoltage comparator signal (OV
COMP) goes positive and activates the OV DRIVE and will
fire the SCR provided that the POV DISALE signal is low.
The CURRENT COMP signal is included in the comparison
to compensate for the voltage drop across the current
monitoring resistor and permit an accurate comparison. The
POV DISABLE signal is high only during power on and for
a brief time during an overvoltage reset.
Note that the OV DRIVE signal is also sent to the ± OV
terminals via diode CR356 and transformer T301 to either
notify a remote circuit that the overvoltage circuit was
tripped or alternatively to fire other output boards (up to
eight) by paralleling the external OV lines. The OV TRIP
signal can activate the OV DRIVE and shut down the supply
regardless of the state of the POV DISABLE signal. As
shown in Figure 2-9, OV TRIP is the output of a wired OR
gate and can be activated by either the SENSE PROTECT
signal (as described in paragraph 2-44) or by the REMOTE
OV TRIP signal. The REMOTE OV TRIP signal can be
generated by the fixed OV sensing circuit or by a remote
signal connected to the output’s + OV and – OV terminals.
Fixed Overvoltage Sensing Circuit –
The fixed overvoltage sensing
circuit (U354) continually monitors the voltage across the
output terminals. Because it is biased by the voltage at the
output terminals, it can be activated and provide protection
even when the supply is not connected to the ac power line.
The fixed overvoltage sensing circuit will activate when it
senses a voltage that is approximately 120% of the
maximum rated output voltage for the associated output. If
the output voltage exceeds this threshold, the OV GATE
signal is generated via diode CR358 and fires the SCR. Note
that the fixed overvoltage sensing circuit will also activate
the OV DRIVE signal via diode CR359 (REMOTE OV TRIP).
The OV DRIVE signal then transmits the overvoltage
condition to the ± OV terminals via diode CR356, and
transformer T301 as previously described.
As shown in Figure 2-9, the trip signal enters at the ±OV
terminals and is coupled through pulse transformer T301,
diode CR360, and the overvoltage detector circuit to
generate OV GATE and shut down the supply.
2-51 Guard Bands. Guard Bands are employed on the +
Shunt track, the – Sense track, and Common track (pin 3 of
R408) on the PC board. The guard bands shield these traces
ensuring they are not affected by DC leakage currents from
adjacent tracks.
2-16
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