HP 3562A Service Manual page 313

MO D E L 3562A
6-1 2 A 1 8 POWER SUPPLY
The H P 3562A power su pply is a h a l f-bridge switch i n g power s u p p ly. It supp l i es 5 vo l ts
d c at 30 amps regu l ated power. It also su ppl ies low power, l i near reg u l ated ± 30 Vdc,
± 1 5 Vdc, + 8 Vdc, and + 2.6 Vdc. The power s u pply operates with a l in e voltage from
84 Vac to 1 29 Vac and from 1 87 Vac to 255 Vac. The l i ne freq uency can vary from
47 Hz to 66 H z .
Refer to f i g u re 6-A1 8 for this d i scussion. T h e p r i mary rectifier converts the a c l i ne voltage
to a high dc voltage with ripple. This dc voltage is fed i nto FET chopper switches to produce
positive and negative p u l ses. The power output of the power su pply increases and decreases
w ith the pu lse width. The pu lse width is contro l l ed by the p u l se width modu lator (PWM)
compensati n g for l i ne voltage and l oad variations. The control voltage u sed by the PWM
to determ i n e the pu lse width is d e rived f rom the 5V supp ly. As the 5V s u pply deviates
f rom the 5V reference voltage, the PWM red u ces or in creases the d uty cyc le of the FET
chopper switc hes c hanging the output vo ltage. The output of the prim ary transformer
is a square wave at 1 28 kHz which is rectified and fi ltered to m ake a dc output w ith ripple.
The 5V s u pply and the second ary s u p p l ies then reg u l ate the output to prod u ce the d c
voltages n eeded b y t h e H P 3562A.
Turn On
At the mom ent the power supply is tu rned on, the prim ary capacitors and the b i as power
s u pply capac itors beg i n charg i ng. The su pply is not switc h i n g and the entire i n stru ment
i s i n the reset mode. The power down c i rc u i t m o n itors the bias s u p p l y's transformer to
determ i ne when the l i ne voltage i s at the corre ct leve l . When the voltage reaches the
correct level, the power down circuit signals the PWM and the power su pply starts switch ing.
T he s low start c i rc u it is then triggered and the pu l se width of the FET chopper switches
i ncreases s lowly (0.1 s) to its normal w idth. The s l ow start c i rcu it is then triggered and the
PWM s l owly (0.1 s) i n creases the pu l se w idth to the correct leve l . As soon as the 5 vo l t
output comes up to 4.75 volts, the PWR U P s ignal goes high a n d u nlocks the A2 System C P U .
Primary Rectifier and RC Filter
This f u l l-wave bridge rectifier and R-C f i l ter network converts the ac power i n put to a '
h igh d c voltage between 236 Vdc and 364 Vdc with ,a 1 0V to 1 5V rippl e. When switc h
,
a f u l l-wave bridge c i rcu it.
Bias Power Supply
The b ias power su pply is a l i near, l ow power su pply that p rovides power fo r the PWM,
control c i rc u its, and protection c i rcu its. T h i s s u p ply takes its i n put d i rectly from the ac
l i ne through its own transfo rmer. The b ias power supply p rovides ± 1 2 Vdc w h i c h rises
a nd fal l s with the l i ne voltage.
IS
C I RC U I T D E SC R I PT I O N S
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