[volt-nuts] Curious overvoltage event

[Andrea Baldoni]

On Mon, Aug 10, 2015 at 03:34:59PM +0200, Andrea Baldoni wrote:

> interactive" UPS burned out. I opened it, and the chain of resistors and
> diodes giving the startup power to the control IC UC3843 arched and the IC
> itself exploded (more details on the circuit later on request, I still didn't
> reverse engineer it).

I was wrong, it's not the chain of resistors for startup. There are two 900V
MOSFETs, S&D paralleled, each one with a 1N4148 and a 330 ohm resistor
(SMD) in parallel to get gate drive from a common line, coming from the
exploded UC3843. Obviously the transistors actually are SD shorted (one is
also G shorted, the others have various resistances).
The sources are connected to GND through a 0.17 ohm power resistor, now ashes.

It seems that the starting event could have been a transistor drain-to-gate
short, that itself, assisted by a back EMF from the transformer, made gate
arching to source pads on the PCB (connected to GND) and all the rest.
At this point, despite the fact that another customer (other 20-something Km
the other side) just called because his router power supply burned yesterday,
probably it was just a coincidence.
What do you think about?

Best regards,
Andrea Baldoni