"Ian Field" wrote in message
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"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.


According to the write-ups, that is a combinational snubber and clamping
circuit, Some designs have just a snubber - like the one I'm working on
here - some have just clamping diodes, and some have a network of R, C and
D, as you say.

Arfa