Switching PSU Theory of Operation / Repair Tips & Techniques

From: Douglas Quebbeman <dhquebbeman_at_theestopinalgroup.com>
Date: Wed Jul 26 09:43:25 2000

Geoff Roberts stated thusly:

> 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.

Here's a quote from a former Prime engineer (who's been as helpful as his
memory permits):

: These things are much more complicated than that. Functionally the Line is

: directly connected to a special rectifier configuration called a voltage
: doubler. It produces rougly 280 volts DC. There is no isolation
transformer,
: which makes these things potentially lethal. The 280V DC is input to
: 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

: switch from high frequency (probably about 25Khz) AC to DC.

Howwver, your former remark would also seem to be true, as the two rectifer
outputs are +280vdc and -280vdc, respectively.

[..snippage..]

> > 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
polarised. 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 it.)
>
> > And you're seeing 600V between the 2 points I've marked '+ve' and '-ve',
> > right? That's still twice what I'd have expected -- I'd expect peak
mains
> > voltage (i.e. 115*sqrt(2)) across each capacitor, or about 320V between
> > +ve and -ve in that diagram.
>
> The Prime techie mentioned a bridge, so I assume it's not a doubler
circuit,
> 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.
>
> Ditto.


                                                    --------------------- +
to BBU
                                                    |
                                                    |
             +---------{ MCI 307138 }---------------+--------------------
+ve
            / \ | + |
           / \ ===== / 12k ohm
          / \ ----- \ 5%
         / \ | - /
    ----* *---------------------------------+----------*
    | \ /| | + \
    | \ / | ===== / 12k ohm
    | \ / | ----- \ 5%
    | \ / | | - |
    | ----------{ MCI 307138 }---------------+---------------------
-ve
    | | |
    | | |
    | | ---------------------- -
to BBU
    | |
    | |
    | |
115v Load 115v Neutral

> > 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.

There is an adhesive label on the front of the PSU that says "115 V". Since
it's
a removable label, that there is some way to change it over for 230v
operation.
With that in mind, I can report that there is a connector on the main PCB
that
has wires that loop back to itself. I'm wondering if it's replaced with a
plug
either without wires or with wires in a different configuration that it
changes
the PSU from accepting 115v input to 230v input. But about phases, I am very
ignorant.

[..snip-o-rooney..]
 
> > > > 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 simple
> > > > analogue circuitry either as discrete components, or one of the
well-know
> > > > 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...

I think this is correct.

> > > > 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, see my other post which is a reply to the same message to which you
were replying in this message to which I was replying. [hope that's clear!]

regards,
-doug quebbeman
Received on Wed Jul 26 2000 - 09:43:25 BST