"Arfa Daily" wrote in message
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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


All the snubbers of this type I've seen were a fast diode feeding the tops
of the flyback pulses to a capacitor that was shunted by a bleed resistor -
if the diode in such a snubber were to fail S/C I would expect the chopper
transistor to warm up a bit.