I have an automotive battery charger like this one:

http://www.carstel.com/index/product...fo/id/100.html

I have not been able to find schematics, and the manufacturer ignores
me.

There are two boards, one looks like the power supply section, while
the other one contains buttons and LED display. The SMPS board looks
like an ordinary flyback arrangement, with a transformer and opto
isolator for feedback.

The symptom is that the 5 A input fuse blows as soon as power is
connected. The fuse blows with a magnificent blue flash and produces
an audible pop, so the overcurrent is considerable.

The SMPS is based on an ST 3845B, driving a 9N90C MOSFET. Across the
MOSFET's D/S, there is a 470 pF / 1 kV capacitor, located very close
to the MOSFET. The capacitor is split open and has spewed its guts
onto the MOSFET. The capacitor measures open circuit with an ohmmeter.
I do not have a megger available, so I cannot check the capacitor at
high voltage.

I have unsoldered the MOSFET and the capacitor. Now the fuse does not
blow. Between the MOSFET's G and S pads on the PCB, I can see a nice
27 kHz square wave with declining amplitude for 7 ms, and then nothing
for 17 ms, before another burst of 27 kHz, so it looks like the
controller is trying to start from its bootstrap supply.

The MOSFET tests OK on one of those cheap ATMEGA-based component
testers.

What is the most likely fault scenario?

What should I check next?

Could the blown capacitor be the only problem, or is it only a result
of the actual fault?