Author Topic: SMPS short  (Read 1281 times)

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Offline Rachie5272

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SMPS short
« on: July 20, 2015, 05:52:34 pm »
I'm trying to fix the power supply of an LCD television.  When I first plugged it in, it went *poof*, letting some magic purple smoke out, and blowing the fuse.  I replaced some FETs, diodes, the bridge rectifier, and the series NTC resistor.  There's still a short somewhere though, and I can't find it.

A light bulb in series with the mains glows at full power, but even when passing 100 Watts through the shorted component, nothing heats up save the NTC.  The voltage on the main caps reads only a couple volts at this point.

I'm not sure what to try next.  The rectifiers seem fine, along with everything else I test.  I know a couple secondary caps are bulging, but they aren't likely to cause these problems.  Any ideas?

Offline T3sl4co1l

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Re: SMPS short
« Reply #1 on: July 20, 2015, 06:08:06 pm »
If nothing else, you can use the voltmeter while it's sinking current, try to find the point with the least AC/DC voltage drop (as the case may be).  Or track the voltage drop along traces (in the <1mV range, so you need a sensitive meter to do this).

Which one's the NTC, the black thing?  What's under heatshrink near the input?  MOV?

There shouldn't be very many things connected to the DC link, so keep checking.  Guessing, besides the switching transistor and transformer, there's a smaller DC-DC chip in there, providing standby power.  And the line filter stuff.  It's also possible it tried to start up, then failed again, for the same reason it did originally (but without whatever emitted the "purple" smoke last time).

Heh... the "Pb Free" symbol looks like undies... ;)

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Offline djQUAN

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Re: SMPS short
« Reply #2 on: July 20, 2015, 06:28:34 pm »
It looks like a power factor corrected SMPS.

I usually start by removing the diode bridge. This lets me know right away if the short is in the AC side or DC side.

It will be pretty easy to find in the AC side but in the DC side, you start removing the main switching mosfets to see if it is one of those that is causing the problem and so on.

Once you clear them out, then you look at the gate drivers and then the secondary side.

Offline Rachie5272

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Re: SMPS short
« Reply #3 on: July 21, 2015, 05:26:47 am »
Yes, the NTC is the black disk in front of the bridge rectifier, and the greenish heat shrinked item is a MOV.

I've made some progress.  I removed the secondary fuse for the high voltage CCFL driver, and the light went out.  I'm mapping out the circuit now, but the problem is almost certainly in this section.

One of the small ICs on the bottom was causing the trouble.  It nearly unsoldered itself from the heat.
« Last Edit: July 22, 2015, 01:27:37 am by Rachie5272 »

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