At 02:34 PM 6/11/2010, Philip Pemberton wrote:

>I'm now in the situation where when I power the thing up, it kicks in
>for about a second, then the output Schottky blows dead-short, and the
>Philips chip goes BANG. Literally -- as in, sparks flying, and its
>output power-FET blows open.

I've successfully fixed problems like that but its painful.

Your going to need an isolation transformer to safely troubleshoot 
this.  Also disconnect the PSU from the remainder of the DVD recorder 
if possible (pull the connector if one exists).

Start by disconnecting the bottom end of the primary winding from the 
chip.  Connect the Drain pin on the chip to Vunreg with a high-value 
resistor (100k or so).  Also connect a variable supply to the Vdd pin 
on the chip and another supply to whichever output rail is monitored 
by the voltage feedback network.

With both variable supplies turned to 0V, apply primary 
power.  Monitor the waveform on the Drain pin of the chip.

The chip should be operating in burp mode at this time - the voltage 
on Vdd rises slowly until it reaches the start threshold, then you 
should see PWM on the Drain pin.  That will last until the charge 
stored on the Vdd capacitor drops too low, then PWM stops and the 
cycle repeats.

Now bring up the variable supply on the Vdd line.  You will reach a 
point where chip stays working, rather than burping.  PWM duty-cycle 
should be at max (whatever that happens to be for this particular chip).

Now start to bring up the supply on the output rail so as to provide 
feedback for the chip.  You should reach a point where the PWM 
duty-cycle starts to decrease.  That point should be about the rated 
output voltage for that rail.

This should allow you to get close to the area that isn't working.


Couple of other suggestions:

1) check the resistor on the Source pin on the chip.  Is it open or shorted?

2) When you think that you've found the problem, re-connect the 
bottom of the primary winding back to the Drain pin on the chip via a 
resistor.  Start with 1k 1/4W and see if it burns up.  If it does, 
you haven't found the problem.

Whatever you do find to be the problem, do let us know what it 
was.  Stuff like that is always useful to hear about and to get added 
to the 'how to fix smps problems file.

dwayne

-- 
Dwayne Reid   <dwayner@planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax
www.trinity-electronics.com
Custom Electronics Design and Manufacturing

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