HP A Series User Manual

APPENDIX A
OPTION 011, Overvoltage Protection Crowbar
INTRODUCTION
This option is instailed and tested at the factory and
applies to the following Hewlett-Packard modular power
supplies:
62003A through 62048A
62003C through 620480
62003E through 62048E
62003G through 62048G
The Option 011 crowbar circuits for the above sup¬
plies are similar in design with many supplies utilizing
identical crowbar circuits. Some variations in circuit op¬
eration and in component values do; however, exist so
that two crowbar schematics are presented as part of this
appendix. Also, all significant differences in circuit oper¬
ation among the crowbar circuits are completely described.
In general, the crowbar circuits consist of compon¬
ents mounted on a separate printed wiring board. For the
A- suffix models however, the trip level adjustment (R45)
is mounted on the power supply main printed wiring board.
For the G-suffix models, this resistor is mounted on the
power supply main chassis.
CIRCUIT DESCRIPTION
The overvoltage protection crowbar circuit protects
delicate loads from high voltage conditions that might result
from a power supply failure such as a shorted series regula¬
tor. The crowbar circuit monitors the output of the power
supply such that if the output exceeds an adjustable thres¬
hold, a virtual short circuit is placed across the output of
the supply. The short circuit causes the output current
to attempt to rise at which time the current limit com¬
parison amplifier responds to reduce output voltage to¬
wards zero. Due to differences in voitage/current ratings
among the modular power supplies, some employ a triac
as the shorting element while others utilize an SCR. In add¬
ition, because of possible circuit delays in the low-voltage/
high-current supplies as well as due to the high current dis¬
sipation requirements of some of the intermediate-voltage
supplies, an alternate holding current path for the shorting
element in these supplies is provided to prevent the supply
from turning completely off during crowbar operation.
Figures A-1 and A-2 illustrate the two basic crowbar
circuits provided for the 011 Option. As shown in the illus¬
trations, the crowbar circuits are similar in design. Resistor
R45, (CROWBAR ADJUST) adjusts the bias of Q1 with
respect to —S and zener diode VR1 provides a stable reference
voltage which is compared with the -S potential. During
normal operation, is conducting and the next stage (Q2
or Q3) is cut-off. If the output voltage exceeds the voltage
at which R45 is set, Q1 turns off which turns on the next
stage to trigger the triac (CR5) or SCR (CR6) depending on
the particular model. The triac or SCR thus places a short
circuit across the output. The choice between using either
the triac or SCR is merely a function of the current rating
of the supplies with the higher current supplies employing
the SCR circuit. Notice that all crowbar circuits employing
the SCR require a pulse transformer (Tl) which produces a
positive trigger pulse to fire the SCR. The triac does not re¬
quire a positive trigger pulse so the pulse transformer is not
employed.
Notice that the higher current rated models (using
either the triac or SCR) have a holding current path (through
R9) returned to the negative side of C9 in the main power
supply. This holding path keeps the SCR or triac conducting
if the series regulator is turned off by the current limit cir¬
cuit. For supplies with the highest current ratings (Figure
A-2), transistor Q4'is included to immediately initiate turn¬
off when the crowbar fires. Turning down the supply in this
manner, limits current dissipation in the crowbar circuit.
Temperature switch TS1 (Figure A-2 only) is provided
to shut off the supply in the event of certain types of power
supply failures where the current limit circuit cannot turn
down the output and the line fuse does not blow. For ex¬
ample, if the series regulator became shorted, a turn-down
signal from transistor Q4 would have no effect and the con¬
tinuous current flow through the CR6 crowbar path could
cause component damage. In this case, the current flow
through the TS1 "heater” leads causes the device to open
the positive lead in the main supply. Notice that once a
temperature switch has been activated, it must be replaced
as outlined on the next page. For those supplies with
lower current ratings (Figure A-1), a temperature switch is
not required because the line fuse will blow in the event of
a series regulator short or any other failure where the supply
cannot be turned-down automatically.
OPERATION
The crowbar can be set to trip anywhere in the range
from -fO.BVdc above the minimum rated output voltage of
the supply to +2V above the maximum rated output voltage,
For example, the 62003A crowbar trip point can be set any¬
where between 3.OVdc to B.BVdc.
A-1

HP A Series Specifications

General

Brand	HP
Model	A Series
Category	Power Supply
Language	English