Switching PSU Theory of Operation / Repair Tips & Techniques

From: Geoff Roberts <geoffrob_at_stmarks.pp.catholic.edu.au>
Date: Tue Jul 25 20:32:15 2000

----- Original Message -----
From: "Tony Duell" <ard_at_p850ug1.demon.co.uk>
To: <classiccmp_at_classiccmp.org>
Sent: Wednesday, July 26, 2000 8:17 AM
Subject: Re: Switching PSU Theory of Operation / Repair Tips &

> > > > No, if I measure across the positive and negative, I'm seeing
> > 600vdc.
> > >
> > > Eeek!. That's double what I'd have expected. Does this thing run
off the
> > > normal 115V mains, or 220V? How does it (effectively) quadruple
the mains
> > > input?

This is starting to sound like it has either a separate + and - supplies
with a common ground, or
it's some kind of voltage doubler. Since I can't imagine why they'd
need to do a doubling act
for the primary side of a SMPS I'm inclined to suspect the former.

> > > > : switcher, which runs the 280V DC through the torroidal
transformers to
> >
> > > > : produce the other voltages. The Big Bridge is to convert the
output of
> > the
> > >
> > > That's more what I'd have expected. About 300V supply to the
> > > stage, not 600V.

Yes, sounds like a classic description of the mains rectifier/filter
side of an average SMPS.

> > You may not have yet seen the correction I posted, but I was seeing
> > only when I measured the + terminal of one of the blue
electrolytics, and
> > the - terminal of the other. Why would I do that? I'd just gotten
out of
> Yes, but aren't the 2 capactiors in series, something like :
> ------------+-------------------- +ve
> |
> =====
> -----
> |
> ------------+
> |
> =====
> -----
> |
> ------------+--------------------- -ve

Well they could be if they were across a potential higher than the
invidual voltage rating of the caps.
That gives you an effective voltage rating equal to the sum of the
ratings of the caps. The penalty is
that it divides the effective capacitance by the number of caps in the
series circuit (assuming all are the
same value - it's a bit more complex if they have different values)
Have you got a + where there should be -?
The above diagram looks wrong to me. If this was a +/- supply, then the
lower cap should have it's anode connected
to the common (centre) ground point, or it's effectively reverse
It would be more common to find caps in parallel to increase the
capacitance (opposite of series - the total
c of the circuit is equal to the sum of the values of all the caps in

> And you're seeing 600V between the 2 points I've marked '+ve' and
> right? That's still twice what I'd have expected -- I'd expect peak
> voltage (i.e. 115*sqrt(2)) across each capacitor, or about 320V
> +ve and -ve in that diagram.

The Prime techie mentioned a bridge, so I assume it's not a doubler
that leaves some kind of +/- supply but I'm blowed if I know how they
achieve that
out of a bridge. I'd definitely like to see a circuit for this one.

> I am still puzzled as to how it gets 600V without either other large
> capacitors, or a mains-frequency transformer.


> I will just ask again -- this thing does run off 115V, right?

I keep wondering the same thing, if it was multiphase it might make
sense, hmmm,
I *wonder* if this was originally 3 phase and been converted to single?
2 mains rectifiers
in series? With a common rail? Sounds ugly but possible I suppose.

"When you have eliminated the impossible, whatever remains, however
unlikely, must be the truth."

Sherlock Holmes
(Sir Arthur Conan Doyle)

> Hmmm.. Well, whatever you do, don't work on it _after_ a hard day's
> > > > > ****PAGING TONY DUELL****
> > >
> > > Did you honestly think I'd not reply to a question like this :-).

No, not really....;^)

> > > PALs? Are you sure? It's not at all common to have significant
amount of
> > > digital electronics in an SMPSU. Most of the time it's either
> > > analogue circuitry either as discrete components, or one of the
> > > SMPUS control chips.

How old is this box of tricks anyway? I must agree, if there is a lot
of logic there,
from the description of all the led monitoring etc, it might be more to
do with that than
part of the supply proper. Possibly control logic to ensure the system
won't power
up if the supplies are out of spec, or shutdown the whole thing if one
fails etc...

> > > In which case look for dried-up (high ESR) electrolytic
capacitors. This
> > > is a very common heat-related problem.

Indeed. Probably the commonest fault in PSU's, which tend to get
pretty hot
anyway. The startup resistor is still a potential culprit here too.


Geoff Roberts
Computer Systems Manager
Saint Mark's College
Port Pirie,
South Australia
ICQ: 1970476
Received on Tue Jul 25 2000 - 20:32:15 BST

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