How to repair SMPS

[Shagul]

Hello all how to repair SMPS I need tutorial pls help me

[firewalker]

http://www.catb.org/~esr/faqs/smart-questions.html

Alexander.

[5282]

you need oscilloscope  . oscilloscope  is every thing in electronic
my dream to have one oscilloscope   

[kripton2035]

one good tutorial here : http://www.repairfaq.org/sam/smpsfaq.htm

be *very* careful, this is not something for beginners, you're dealing with high voltages that can kill you.

[james_s]

An oscilloscope is a useful instrument, but I've repaired many SMPS's and it's rare that I get out the scope when I work on one. About 80% of the time the problem is simply worn out electrolytic capacitors and most of the remaining failures have something obvious like a crater in the chopper transistor.

[kripton2035]

and if you put the scopes probes on an smps without a differential probe, you blow up the mains fuses, and/or the scope ...

[james_s]

I plug the device under test into an isolation transformer, that is usually sufficient, although you still do need to be careful given the voltages involved.

[Ian.M]

There's also probe derating with frequency to consider.  Lets suppose you have a probe rated for 600V DC + peak AC like [this] one.  Your line voltage is 120V AC so you are expecting 170V on the reservoir cap +ve terminal (assuming you have used an isolating transformer and grounded the -ve rail).   The snubber network and the voltage rating of the chopper transistor lead you to believe that the turnoff transient wont exceed double the rail voltage - 340V, so you go ahead and probe the chopper transistor drain with your 600V probe on x10 with the scope set to 20V/cm. 

BANG!

Your probe just explosively dismantled itself and there is smoke coming out of your scope.

What went wrong?  Well the chopper circuit was running at 100KHz, and the waveform had a fast rising edge with a lot of energy in the higher odd harmonics.
At 100KHz (sine wave) your probe was only rated for just over 300V peak, falling to a bit under 150V at 1MHz and the spike looks a lot like a bit less than a half cycle of a 340V 1MHz wave.  Your probe flashed over and discharged the reservoir cap through the scobe input attenuator. 

*GAME* *OVER*

Hopefully you are only out one scope, probe + the power board of the D.U.T. but if you were holding the probe, or adjusting the scope controls, you could be in need of an ambulance or worse.

Probing high power high voltage nodes is a high risk operation and should *NEVER* be done 'hands on', and you should always be 100% certain the probe has an adequate rating.  If there's a 600V rated transistor in there, I wouldn't go anywhere near it with a probe with less than a 1KV rating at the operating frequency.   Anyway the collector or drain waveform of the chopper transistor rarely tells you anything useful that cant be got from the base or gate drive waveform or a feedback or secondary winding waveform - if its wrong the primary side usually destroys itself before you can fix anything.

[netdudeuk]

Power Supply Troubleshooting and Repair Tips
https://youtu.be/pffOJdCQ7kw