SMPSU IC explosion... (UC3844)

[Tony Duell]

> > I have never had a chopper transistor fail open-circuit.
> 
> casualties so far are both choppers (IRF740's), the UC3844, and a couple of 
> resistors (they were hidden by the heatsink before; rather obviously cremated 
> now I've removed it!)
> 
> The UC3844's current sense line got a blast of mains down it which is what 
> killed it...
> 
> I'll test other components in the area though in case they've failed with no 
> visible effects.

In particular check any snubber capacitors and diodes connected to the 
primary of the chopper transformer. If these fail, the result is often 
shorted chopper transistors. Also, in some designs, a shorted rectifier 
diode on the secondary side will kill the choppers.

-tony