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HP 811 OA Se rvice Guide
Over load
P
rotect i on
Thi s secti on is on sch e m ati c 4Β
'Soft-Start' at Power-up
Thi s section is on sche matic 4Α
Te mp e r ature Control
Thi s section i s on sch e matic 4Α
7-8
Fun ctio nal Descr iption
Duty
c
y cle is co n trolle d by t he PWM control uni t
U
401 .
A dd i ti onall y, th e PWM control uni t
p
rotects t he
p
ow er supply us in g
integrated
p
rotecti on
c
ircuitr y, e .g .: 'Soft-start',
F
oldback beh a viour,
T
emperatu re control ,
m
o nitoring short-circuits in t he secon dary
sectio n , and s wi tc hing off t he
p
ower supply in cases of und er- vol tage
and overv oltage at t he
p
ower input (pin 6 of U401) .
Th e 'on-time' i s controll ed, ensuring t hat t he secondary
o
u t pu t
voltages are con sta nt, and independent from the conn ected
l
oad and
t he AC p o w er input .
Α
n
ew s witchin g cycl e
b
egins wh en the power stored in the
tran sfor mer
p
rim ary win din gs is completely transferred to the
secon dar y windin gs .
Th e PWM co ntrol unit internally reproduces t h e drain current of t he
s wi tching tran si stors, using C409. Α
v
oltage comp arable to the drain
c
urre n t i s applied to pin 2 o f U 401 .
A
t α certain le vel α controlled
s hu tdown
o
f th e
p
ower supply is
p
er formed,
p
roviding overload
p
rotecti on.
Ca p acitor C411 connected to pin 7 of U401
p
rovi des
f
or α gradual
increase of th e o ut pu t voltages wh en the in str um ent is powered-up,
d u e to red u ced 'on-time' of the switc hin g transistors while C411 is
charg in g .
In addi t i on to t h e built-in temp erature control
o
f t he PWM control
uni t, an exter n al temp erature control is also available . Th e RT403
(kn own as ΜΟΧΙΕ ) is placed at α crit ical temperature
p
oint . As
temperat u re in creases, the resistan ce of RT 403 decreases, taking
Q402 base to GND . Thi s cau ses th e
p
ower input, pin 6, of U 401 to
drop b elow t h e minimum
o
peratin g voltage . 1η
t
his way U401 causes
t he compl ete s hutdown
o
f t he power supply. LED DS401 illumin ates
to in d i cate thi s condition.
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