HP 3562A Service Manual page 314

C I RC U I T D E SC R I PT I O N S
Pulse Width Modulator
T h e vo ltage output of the power supply is control led by the pulse width m od u lator (PWM).
I t d rives the chopper switches in a m a n ner wh i c h m ai nta i n s a constant vo ltage output.
The PWM com pares the 5 Vdc voltage to its own i nternal reference using the error ampl ifier.
T h i s comparison determ ines the duty cycle of the FET chopper switches. The PWM contains
its own internal osc i l l ator and + 5V reference. The PWM can be tu rned off by the protection
m o n itor, power down, and cu rrent l i m it c i rcu its .
Protection Circuits
T h e re are five sections of protection c i rcu its: the p r i mary c u rrent l i m it, the power down,
the cu rre nt mon itor, the over temperatu re c ircu it, and the protection mon itor. The primary
c u r rent l i m it c i rc u it q u ickly s huts down the powe r su pply when the c u rrent i n the prim ary
exceeds about 8A by s hutting off the vo ltage s u pply of the PWM. I f this fau lt occurs,
the power m u st be cyc led for the i nstrument to operate. When switc h S1 is in the 1 1 5V
pos ition and the l i ne voltage d rops to less than 84 Vrms, the power fai l sign (PW R D N L)
becomes active. I f the l i ne voltage d rops to less than 1 60 Vrms, PWRD N L becomes active
(when switch S1 is i n the 220V switc h pos ition). T h i s signal goes to the p rotection and
c u r rent mon itors and the PWM is shut off. The power down s ignal also goes to the A30
A n alog Sou rce and the A2 System C P U to signal a power down is occ u rr i ng.
T h e cu rre nt m o n itor senses an over-c u rrent cond ition in the 5V s u p p l y. When this fau l t
occurs, it red u ces the contro l voltage to the PWM, w h i c h resu lts i n a red u ction o f the
o utput power of the p r i m ary transfo rm er. The protection mon itor sen ses two th i ngs: over
vol tage i n the pos itive s u p p l ies and if the voltages from the 'negative s u p p l ies rise above
g ro u nd. When a voltage fau lt, cu rrent fau lt, power fai l u re, or over tem peratu re (OT E M P)
o c c u rs the protection m o n itor sh uts off the PWM and l ights the correspon d i ng fau l t L E D .
T h e L E D i n d icates w h i c h fau lt-detecti ng c i rc u it was activated. A l l the L E D s a re off d u ring
n o r m a l ope ration.
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MO D E L 3562A