I've been given a number of switchers to look at, by a company that I do
other work for. The one that concerns me here, is an open frame type, single
12 volt output at, I would guess, 3 - 4 amps. It appears to be a very simple
design, in that the chopper drive circuit is discrete, employing two bipolar
transistors as an astable. The output of this is fed pretty much directly to
the gate of a single FET. In the drain of the FET, is a single primary
winding up to the raw rail from the input bridge.

Across that primary, is a network comprising a 330pF 2 Kv disc ceramic cap,
and two 3 watt cement-body resistors, all in series. On every one of the
examples sent to me, the two resistors are chalky and very discoloured to
the point where you can't read the bands. On some of them, one of the
resistors is open. Of the remaining resistors, they all seem to go around
150 ohms, so I'm taking that to be the original value, based on the fact
that this type of resistor doesn't usually go low, and some of them have
gone open. Make no mistake, these resistors look like they run very hot
normally, to the point where the solder on their joints has crystalised, and
on some, scorch damage has been done to the print, and the substrate.

Now here's the bit that I am finding puzzling. If you take one where one or
both of the resistors has gone open, the supply works just fine. Loaded up
to a couple of amps, it runs cool and the regulation is good. The switching
FET is barely breaking a sweat, as you would expect. So I went ahead and
replaced the resistors with a pair of 150 ohm 3 watt types that look pretty
much identical to the originals - even down to the blue body colour. The cap
checks ok for value and leakage. With the resistors in place, the supply
still works just fine, except that it now runs pretty hot, even when
unloaded. The FET is a lot hotter than it was before. The resistors get well
hot, as I was expecting, given the condition of the originals, but with the
supply loaded up to a couple of amps, they get very hot, and the FET becomes
uncomfortably hot as well.

As far as I can make out, doing some on-line reading about SMPS design, this
network across the transformer primary, is a simple snubber (as opposed to a
clamp or combination clamp and snubber as is also sometimes found in this
position). Texts suggest that its purpose is to limit the level of voltage
spikes at the switching point, to keep the switching device operating within
its SOA and reduce dissipation, which seems a fair enough comment. However,
quite the reverse appears to be true. The whole supply seems a lot happier
with that network 'not there' as it effectively is, when the resistors are
open.

So has anyone got any good thoughts as to what is going on here ? I've done
a great deal of repairs to switchers over the years, and am well versed with
the principles of operation and repair, but I freely admit that I am not a
designer in this field, so I'm at a bit of a loss as to whether it's just
generally a poor design, or whether there's something else wrong that I'm
missing. As those components were originally designed in, and are clearly
faulty now, they need to be replaced, but the fact that the supply seems to
run less efficiently when they are in place, feels altogether
counter-intuitive

Arfa