SMPSU IC explosion... (UC3844)

[Tony Duell]

> 
> 
> 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.

A typical SMSPU circuit contains a section something like this

                                       HT+
                                       --- 
                                        |
                                        )
                                        ) Chopper transformer primary
    COntrol IC                          )
      (Part)                            )
                                        |
   ------------+   base resistor       /
               |   (~10 Ohms)_       |/
           Out |----/\/\/------------|    Chopper transistor (NPN)
               |                     |\ 
               |                       >
               |                        |  
       I Sense |------------------------+
               |                        |
               |                        /
     Gnd       |                        \ Sense resistor (~0.5 Ohm)
  -------------+                        /
      |                                 \
      |                                 |
      +---------------------------------+
                                        |
                                     ---------
                                     HT common 

There are likely to be more diodes, snubber networks, etc, but this is a 
start. HT+ and HT common are the 2 ends of the mains smoothing capacitor 
and have about 400V between them

What happens is that the chopper trnasistor goes short-circuit all ways 
round. The HT line is now effectivel shorted out via the transformer 
primary (very low DC resistance), the shorted transistor, and the sense 
resistor. The latter normally burns out, it is likely to be a 'safety' 
flameproof resistor so you may not see any signs of failure. If you are 
unlucky, some of the PCB tacks fail as well. The chopper control IC now 
sees about 400V between its ground and its output/current sense pins. It 
generally expires esplosively.

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

  
> 
> This was one of two such supplies in a Sun RAID disk controller; the two do 
> current-sharing under normal conditions - but the unit will run on one supply 
> if the other fails, which is exactly what happened here. Leads me to 
> *tentatively* say that the RAID unit itself is 100% and the failure was 
> totally in the PSU.

Very likely. Most SMPUS failures result in no output, not over-high 
outputs (although watch out for 2-stage supplies if the regulator chopper 
fails!).

> 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?

-tony