[volt-nuts] Curious overvoltage event

[Chuck Harris]

I have seen plenty of 900V FET's pop on such supplies.  A couple
of reasons were usually the cause.  In one instrument, there was
obvious signs of water infiltration that allowed a water short
between the FET's terminals and the chassis... POP!  Not likely
your problem...

In most others, there is a copper colored flash on the bottom of
the circuit board, characteristic of a high joule discharge that
has sputtered copper vapor against the bottom of the board... POP!

Look at the bottom of your board.  There are several pinch points
where the layout has been designed to absorb power line transients.
They are right near where the AC comes onto the board.  If there
is copper flash on the board around them, that would indicate a
bad line surge caused the problem... if not, it was something else.

I have found spider remains to be the cause of some such failures...
and the odd Brown Chinese Marmorated Stink bug.  They are at the
wrong place at the wrong time, and... POP!

After the surge has killed the FET's, the supply's next attempt
at starting up will usually kill the bootstrap resistors.

9 times out of 10, just replacing the FET's, the bootstrap resistors,
and cleaning the copper flash off of the circuit card will restore
operation.  That 10th time will usually be a real bear to fix.

Pay careful note to the position of the little copper foil gizmo
that is between the FET's and the heatsink.  It is important,
and must be put back correctly for proper operation.  It is supposed
to be isolated from the heatsink, and the FET's.

-Chuck Harris

Andrea Baldoni wrote:
> On Mon, Aug 10, 2015 at 10:22:47AM -0400, Chuck Harris wrote:
>
>> If the mains supply hangs around in certain brown out voltage ranges, it
>> can fool the start up control circuitry into exceeding the ratings on
>> the bootstrap circuitry, or the inrush limiting circuitry, and toast parts.
>
> Hello Chuck. Thank you for your idea.
> As I wrote later, I was wrong and the involved circuitry is another one; even
> if at this point it could be not a mains-related failure, it's still a curious
> thing that two PS failed in exactly the same way in such little time.
>
> Best regards,
> Andrea Baldoni
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