Franc Zabkar wrote in message
...
On Thu, 4 Oct 2007 09:32:41 +0100, "N Cook" put
finger to keyboard and composed:
I know this is just a pc ps but I'd like to understand what is going on,
or
not going on, even if not repaird.
There are 3 HF transformers and an opto-isolator bridging the HV and LV
sides.
The startup oscillator, on the HV side, is a KA1H0165R whose o/p crosses
the
divide to give a Vcc of 12V on the main SMPS controller a KA3511DS, both
datasheets downloaded.
This controller o/p should then cross the divide to drive the HV side
main
oscillator to drive the main transformer back to the LV side supply
rails.
The SMPS "flag" output pin goes high meaning no-function and no o/p
pulses
for either push-pull output of this KA3511.
As the KA3511 monitors over-voltage, and more to this point,
under-voltage
of the supply rails , how should it ever get started if it never outputs
any
main drive pulses, so no main supply rails.
If i find time today I will run the KA3511 from 12V bench supply and see
what happens .
If it's a typical ATX PSU, then one transformer is part of the +5VSB
supply, a second drives the oscillator/chopper, and the third supplies
the secondaries. The optoisolator may provide voltage feedback for the
+5VSB rail, or it may be part of a protection circuit that shuts down
the oscillator.
See http://www.pavouk.org/hw/en_atxps.html.
Otherwise, if you have an AT PSU, then AFAIK the PWM controller on the
secondary side gets its supply rail from the very first "kick" of the
main oscillator and then "bootstraps" itself into regulation.
- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.
Didn't get around to powering from 12V dc but did trace a bit of the tracks.
The opto would seem to link, DC fashion, from the 3.3V rail back to the
feedback pin of the low power startup IC to shut that one down.
This IC, with the reservoir caps, will keep the main smps IC running for 10
seconds after removing the mains supply (in the present unfunctional as a ps
state)
--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/