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TM 11-6130-429-40
feedback and control loops.
d. Application of input voltage also activates the
i. The converter incorporates electronic current
blower which provides cooling air for the converter
limiting and short circuit protection which limits the
and, to some extent, for the host system. Input
maximum current that the converter provides to ap-
power is controlled by a front panel ON-OFF switch
which activates the control circuits (when closed) to
proximately 85A so that overload will not cause any
initiate operation. If the ON-OFF switch is in the
damage. If the converter is short circuited, the cur-
OFF position, a front panel FAULT light is automati-
rent limit circuit "folds back" and limits the short
circuit current to approximately 35A until the short
cally activated.
is removed, at which time the converter automati-
e. The 20 kHz switching converter utilizes two
cally recovers.
"half-wave forward" converters, each operating at 20
kHz but 180 out of phase. The pulse width drive to
j. Other fault circuits include undervoltage detec-
the individual converters is automatically changed
tion which activates the front panel FAULT light if
to control the duty cycle and hence, the average
output voltage is below 4.5V, and overvoltage pro-
power delivered through the isolation transformer to
tection which turns off the power supply automatic-
the output rectifier filter. Since the switching con-
ally if the output voltage exceeds 6.25V. Overvol-
verter is, in effect, on and off, the converter and
tage shutdown results in an undervoltage condition
which automatically activates the front panel
regulator mechanism is highly efficient, with losses
FAULT light.
primarily assigned to the switching transistors in the
k. Front panel controls and indicators include the
primary, and the rectifiers in the secondary.
f. The output of the rectifiers is a 40 kHz rectangu-
ON-OFF switch, FAULT lamp, voltage adjust, test
lar waveform (in effect, full wave rectification of the
points, output voltage (P1 and P2 connectors) after
20 kHz square wave). The rectangular (pulsed) wave-
the isolation diodes, and output voltage before the
form from the rectifiers is averaged by an output
isolation diodes. All test points are isolated with a
470 ohm resistor which prevents damage in the event
filter to obtain smoothed DC which, in turn, is deliv-
the test point is shorted.
ered through the isolation diodes to the specified
loads. As the output load increases, the pulse width
1-14. Circuit Description
and hence, the average power is increased automati-
(fig. FO-2)
cally by the regulator loop to maintain the output
voltage constant. The output pulse width is also au-
a. EMI Filter and Related Components (fig. 1-3)
tomatically increased as the output voltage is re-
(1) Input power is applied to P1A1 with the re-
duced in such a manner as to maintain the output
turn on P1A2 and/or P2A1 and P2A2. Filter choke
voltage constant.
L1, in conjunction with filter capacitors C4 and C5,
prevent any differential EMI noise generated by the
g. The output is monitored both before and after
converter from reaching the input bus and similarly,
the isolation diodes. The monitor circuit after the
prevent any noise on the input bus from reaching the
regulation diodes regulates the load voltage as previ-
converter and/or the output.
ously described. When the converters operate in re-
(2) L2 is a common mode choke which, in con-
dundancy, the converter set to the higher output
voltage (as little as a few MV differential) will, in
junction with the capacitors C1 and C2, filter any
effect, be on line, and the converter set at a slightly
common mode noise generated by the converter from
lower voltage will be "on standby".
reaching the input line and similarly, prevent com-
mon mode noise on the AC line from reaching the
h. The sense lines which regulate the voltage after
converter and/or the output.
the diodes will accordingly sense that the output
voltage is "too high" and attempt to shut down the
Common mode noise is defined as noise
converter to achieve the set voltage. The sense line
voltage variations between the two lines
before the diodes, however, insists that the output
voltage prior to the diodes is maintained approxi-
and ground. Differential noise is defined
mately 0.5V higher that the set voltage. This ensures
as noise between the input lines. Differen-
that the standby converter will be instantly ready to
tial noise is typically generated by load
take over if the on line converter fails. Accordingly,
variations or switching on the power line
whereas common mode noise is generated
the sense line before the isolation diodes regulates in
by noise on the line and stray capacitance
the standby mode and the sense lines after the diodes
to case ground which, in effect, sets up
regulate in the operate mode. The output circuits are
potential ground circuits.
isolated from the input circuits both by the isolation
transformer and by light-coupled transistors in the

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