SMPSU IC explosion... (UC3844)

[Jules Richardson]

Tony Duell wrote:
>>
>> Random failure of the weekend was a UC3844 IC in a switchmode power supply - 
>> with a very loud bang, the entire top of the chip blew off, but there's no 
>> *obvious* sign of any other failure within the supply.
> 
> This deos not mean there are no other component failurs. This sort of 
> thing is very common actially.

I must have been lucky... all the ones I've had to fix before have had failed 
in other ways...

> I would be very suprised if an IC failed that violently without there 
> being other component failures.

Yes, so would I to be honest.

>> Shame Sun don't publish schematics!
> 
> It wouldn't do you a lot of good if they did, the supply is almost 
> certainly bought-in and there wouldn't be a schematic in the service 
> manual anyway. But it can't be that hard to draw out, can it?

It's not *that* simple, because this is such a compact unit. There are a 
couple of vertically-mounted boards soldered on the main PCB. One contains the 
dead UC3844 and holds the circuitry that sits between the high voltage input 
side and low voltage output. The other board seems to be purely on the HV DC 
side and has a few surface-mount resistors on it, a small transistor, and a 16 
pin IC labelled UC385x (there's a splodge of dye or something on it so I can't 
read the last number - doubtless it'll be easy to find via google when I come 
to look it up anyway).

Then there are three large L-shaped heatsinks on the board which wrap over the 
top of the board components, and then another large slab heatsink is screwed 
to those.

The slab heatsink comes off easily enough, but method of PSU construction is 
to bolt heatsinked components to the L-shaped heatsinks *before* soldering 
them in place. There's no access to undo the transistor / diode heatsink 
clamps without desoldering half the board. Component level repair was 
obviously not on Sun's list :)

I can trace stuff with a continuity meter, common sense, and a visual on the 
PCB traces on the board underside (where luckily 95% of them are) - but a 
visual conformation of the PCB's topside is out without some major 
dismantling... (which of course I'm going to have to do if the chopper 
transistor's dead anyway in order to change it...)

cheers

Jules