HP 3562A, Service Manual

Page: 314 / 570

C I RC U I T D E SCR I PTIONS MOD E L 3562A
6-96
Pulse Width Modulator
The voltage 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 manner wh ich m aintains a constant vo ltage output.
The PWM compares the 5 Vdc voltage to its own internal reference using the error ampl ifier.
This comparison determ ines the duty cycle of the FET chopper switches. The PWM contains
its own internal oscillator and + 5V reference. The PWM can be turned off by the protection
m o n itor, power down, and cu rrent l i m it circu its.
Protection Circuits
There are five sections of protection c i rcu its: the primary cu rrent l i m it, the power down,
the current monitor, the over temperatu re circu it, and the protection monitor. The primary
c u r rent l i m it c i rc u it q u ickly shuts down the power su pply when the cu rrent i n the prim ary
exceeds about 8A by shutting off the voltage supply of the PWM. I f this fau lt occurs,
the power must be cyc led for the instrument to operate. When switch S1 is in the 1 1 5V
position and the l ine voltage d rops to less than 84 Vrms, the power fai l sign (PWRDNL)
becomes active. If the l ine voltage drops to less than 1 60 Vrms, PWRDN L becomes active
(when switch S1 is in the 220V switch position). This signal goes to the protection and
c u r rent mon itors and the PWM is shut off. The power down s ignal also goes to the A30
Analog Sou rce and the A2 System C P U to signal a power down is occ u rring.
The cu rrent monitor senses an over-cu rrent cond ition in the 5V supply. When this fau lt
occurs, it red uces the contro l voltage to the PWM, which resu lts in a reduction of the
output power of the primary transform er. The protection mon itor senses two th ings: over
voltage in the positive suppl ies and if the voltages from the 'negative suppl ies rise above
g round. When a voltage fau lt, cu rrent fau lt, power fai l u re, or over temperatu re (OTEMP)
occurs the protection mon itor shuts off the PWM and l ights the corresponding fau lt L E D .
T h e L E D indicates which fau lt-detecting circ u it was activated. A l l the LEDs are off du ring
normal operation.