Re: failure of the low-ESR electrolytic capacitor in second winding

On Tue, 27 Nov 2007 06:37:38 -0800 (PST), Simon <Simon.jwfg@xxxxxxxxx>
wrote:

I met a big problem that the DC-DC converter circuit failure happend
frequently

I use UC3843B to control the Planar TRf 101 transformer to provide
multiply DC output;

and almost every failure has the same feature:

The failure on the low-ESR electrolytic capacitor at the second wiring
of the transformer;

it's kind of Low-ESR electrolytic capacitor, 1000uf/10V (ESR=0.07
ohm).

The UC3843B working at 100KHZ.


Is there anybody met that problem before?

What's the possible reason to lead to the failure?

If these things aren't burned in or the product of carefully evaluated
and tested design, the caps could be inserted reverse polarity. Even
the artwork could be wrong, so everything looks ok 'per dwg'.

Electrolytics don't always fail immediately or catastrophically when
mis-applied in low voltage circuitry. With your double section filter,
there need be no immediate indication of misbehavior on the outputs,
if you're measuring ripple/regulation at final test. Cross-regulation
might show an exageration or shifted centering.

I've seen batches of capacitors fail because they were mislabelled.
There's no defence against this other than burn-in and supplier
vetting

On multiple output flybacks, you are at the mercy of the end-user when
it comes to operating stress - end-use loading can grossly exceed the
(rapparent) 2A ouput rating of this circuit. Assuming a flyback as
it's a capacitive input filter and a single ended switch controller.

I'm surprised to see the relative distribution of the components - the
bulk of the capacitance budget should be in the first stage of a
flyback output.

RL
.

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