[SI-LIST] : Flaming story

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From: Michael Nudelman (mnudelman@tellium.com)
Date: Wed Jan 24 2001 - 16:25:51 PST


OK.

I have a situation.

I have a rather large board (3'x1.5')

It'a telecom board. I have 48V DCDC on it (big one, 200W). The EMI filter is
a common mode choke with a ceramic 0.01uF cap 100V in parallel with 10uF
100V cap before and after the choke. In front of it is a slow-start module,
which is a mosfet controlled by an RC in its gate. It ramps in-rush current
ove couple of ms.

It is a module, once suggested by Lucent for its DCDCs in their appnote and
it is a standard solution on all our boards.

Now, here what happens.

Before i/o this filter we used to have a Filter module by the Mfr who does
the DCDC, and few times it blew up on us.
We designed it out, and temprarily installed wires shorting out the Filter.

Boards worked normally, until we had to pass some tests and started
installing a daughterboard, containing the filter, described in the
beginning.

Soon after large batch of boards was assebmled the front end ceramic cap
(facing the 48V righ after the slow start module , the 0.01uF one)started
blowing up in a large percentage of the boards (10-20%).

Sometimes the cap that is the rear end (after the CM choke) blows up. But
not as often as the front end one.

Notice, that after the slow start, in parallel to this EMI filter and DCDC,
there is another EMI filter, exactly same, with CM choke of about twice the
inductance of the first, and same set of capacitors. Neither capacitor in
this filter blows.

The only difference here I see is that the first filter is on the
daughterboard, and the second is on the main board. The second power supply
however is small (10W)

Now about the board: for almost 3 feet the feeding 48V line runs from
connector to the slow start, after which it connects to the EMI filters
within an inch and then the filters goes to the DCDCs.

----------

Now my theories:

1. Wrong capacitor is being installed. This was checked - the spool has
right part # on it, but who knows what's in there.
2. The daughterboard is being stressed with the caps already mounted, which
cracks the caps, and they flame.
These two theories made me do an experiment: I took 30 boards and exposed
each to 100V for 1 minute. Nothing happened.

Theory #3.

The slow start module creates an amlifier (common gate one), and a noise
created during plugging connector in, amplifies times R(load)/Rsource,
where Rload may be significantly larger than R source. R source is
essentially output impedance of the humongous power supply, and R load is
DCDC (it does not draw much current until it is turned on ) plus CM choke.
Of course, at first moment it is shorted by a capacitor.
The noise due to charging pre-slowstart board capacitance, though short, is
very significant. I saw the 80Mhz oscillaton with current up to 80A for few
microseconds.

The theory is far-fetched, but....

Anyone has a similar experience? Is it my paranoia and it is just
mechanically mishandled/wrong capacitors?

---------------------

Sorry for long text

Mike Nudleman.

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