RE: [SI-LIST] : RE: Flaming story

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From: Mendelsohn, Joseph P (Joseph) (jmendelsohn@lucent.com)
Date: Thu Jan 25 2001 - 05:26:37 PST


Cracked Capacitor theory.
The fact that the filter components for the 10W supply may steer you away
from a cracked capacitor problem. If the 200W supply is bolted to the board
and the mounting hardware is very close to the caps, you could be cracking
them. You should also check to ensure the caps are not located near the
salvage edge of the board(I doubt you have one such a large board). If you
suspect cracked capacitors, you need to look at "good" ones. To remove them
from the board, you should use two soldering irons(or those tweezers) or you
will crack the cap by removing it. Send a few out to have cross sections
done.

Over current/voltage theory.
You may want to look at the startup characteristics of the 200W supply. It
may have a built in threshold(either by design or by accident). The Mosfet
Inrush limit circuit will charge the input capacitors to some level. If the
supply starts at a very low voltage, you would normally blow the fuse or fry
the choke since the current would be extremely high. After the 200W supply
hits the threshold and starts, there will be a huge inrush current. This
will excite the tank resonance of the filter and blow your caps. You didn't
mention an over-voltage/under-voltage circuit so you may be very susceptible
to this. The 10 Watt supply should have a much lower inrush current and
therefore not have this problem. The good news is that you can measure this
to see if it is the problem.

Joseph

-----Original Message-----
From: Larry Miller [mailto:ldmiller@rhapsodynetworks.com]
Sent: Wednesday, January 24, 2001 8:56 PM
To: 'Michael Nudelman'; brent.dewitt@us.datex-ohmeda.com;
si-list@silab.eng.sun.com
Subject: [SI-LIST] : RE: Flaming story

1. Does the cap blow up at the instant when you insert or remove the board?

2. Have you monitored the voltage across the cap that is blowing up?

3. 10 uF sounds too small. The CM chokes have very tight coupling, which
means that they can ring up pretty strongly. For a power supply voltage E it
is not uncommon to get transients that are 3E or more. In the IEEE 802.3af
committee we were always talking at least 47 uF to eat the transients.

4. Have you tried increasing the turn-on time (RC) of your soft-start
circuit?

5. Perhaps the reason that the small cap is blowing up is that the ESR or
inductance of the bulk cap is forcing the small cap to eat the whole
transient; the big cap is not taking its share. You might try a MOV or other
voltage clamp as a brute force experiment.

6. I have seen ceramic surface mount caps get cracked during manufacture. If
the temperature profile of the process is set for big, thermally massive
components [like the big cap or CM choke], you may be damaging the small
caps. In this case, an nice old-fashioned thru-hole mounting beastie
(suitable for 200W Weller soldering guns!) might have better survival.

The trick is you have to measure the conditions on the cap at the time it
blows. If the voltage and current are all right, then you have to suspect
damage during assembly.

Larry Miller

-----Original Message-----
From: Michael Nudelman [mailto:mnudelman@tellium.com]
Sent: Wednesday, January 24, 2001 4:26 PM
To: 'Larry Miller'; brent.dewitt@us.datex-ohmeda.com;
si-list@silab.eng.sun.com
Subject: Flaming story

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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