I have an automotive battery charger like this one:
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?