Circuit Description

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

NOTE

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

1-4