What would cause a charger FET to blow?

[jonnybump]

Hello all, I have a 48V 4A charger that I get my money's worth out of - it's used daily to charge my commuter bike. Earlier this week I plugged it in as usual, there was a spark from inside the charger and it tripped the RCD that it was plugged into, and the main house MCB too.

I checked inside and found the small glass fuse had blown, waited for some more to arrive, replaced it, and the same thing happened again.

I took the charger apart today, nothing looks out of the ordinary, then slid the circuit board out of the charger case and can see a burn mark on the case right next to where one of the MOSFETs were - took the cover off the MOSFET and can see there's a little hole in it - and it stinks like it's been burning.

Anyone know what would have caused this? I can't find any replacement FETs for sale in the UK - it's showing "J13009-2", so I'll probably end up replacing the charger, but it'd be handy knowing why this happened in the first place (and that I won't blow up the new charger!).

Cheers
Jon

 

[spinningmagnets]

I when I saw the title, I was about to post something clever that I read, but never have done myself (that's kind of "my thing" around here). Such as: the FETs in a controller are supposed to be bonded to some larger aluminum component as a heat sink. The cheapest type press them against the aluminum case, and better models screw them onto an aluminum bar, and after inserting the guts into the case, a couple of screws hold that bar against the inside of the aluminum housing.

Between each aluminum component that is pressed/screwed against another, the factory should squirting a tiny dab of thermal paste, to ensure the best possible heat transfer. Sometimes, the assembly line guy misses getting one of the dabs in the right spot, or maybe the screw that holds the two pieces together hit resistance, and the assembler mistakenly believes they are connected. Either way, that particular FET gets way too hot and dies.

In your case, I suspect it was just a bad FET from the FET factory. Never replace a cheap FET with another one, they are very inexpensive. Find the same size and style, then purchase a quality unit from a trusted vendor.