SMPSU IC explosion... (UC3844)
ard at p850ug1.demon.co.uk
Mon Dec 5 16:37:37 CST 2005
> 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...
You have been very lucky. I have never had a chopper transistor fail
SMPSU problems tend to come in 4 categories for me :
Startup resistor open-circuit
Chopper transistor shorted, many other components burnt out as a result
High ESR capacitors
> >> 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
Generally (for example the Zneith supply in my ACW), I desolder all
daughterboards and treat them seperately.
I must admit it's a bit nerve-wracking when you take a working PSU apart
(i.e. desolder daughterboards and coils, which will test as dead short on
an ohmmeter and confuse me when tracing connections), then put the bits
back in and power it up for the first time. I'm always worried I'll have
forgotten to solder some connection and the result will be the thing
blowing apart. TOuch wood, this hasn't happeend yet :-)
> 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
Can't you desolder all the transistor and diode connectuons and pull the
heatsink out? That's what I would do (and how it was presumably
> obviously not on Sun's list :)
> I can trace stuff with a continuity meter, common sense, and a visual on the
I find the continuity test a lot better than my eye at following a track
(particularly on a board with many find tracks covered with soldermask).
But watch out for anything that will appear as a short on your meter --
inductors, transformer windings, low value resistors (often used for
current sensing in SMPSUs), etc. Unoslder these (at least at one end)
before you start or you will go insane.
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