Hello everyone here .

I got some old high current welder's SMPS , i tried to repair them.

So after finding some shorted IGBTs , i replaced them by new ones , the unit has started , and after 6 minutes of welding the chopper again shorted out and fried a resistor that was connecte to the main inputs .



What could be causing this ?

Here is a service manual attached :

 

Years of designing and working on switchers taught me unfortunate truths: whenever you find a power converter with blown transistors.... you can usually be close to 100% that they did not blow simply because the transistors were bad (they were murdered by something else). The fix usually requires figuring out what killed them.

Experience has taught me to always assume electrolytic caps are bad.......

 

Maybe the transformer has fried ?

 

+1 Transformers also do not age well. basically, suspect any component that operates hot and has higher voltages applied to it.

 

How do i check the SMPS transformer ??

 

Failed snubber caps?

 

I see , thanks for all your inputs ,
By the way is the ESR meter able to test with both electrolytic and non polarized caps ??

 

High voltage polyester snubber caps often fail to open circuit. You can test that with any capacitance meter.

 

High voltage polyester snubber caps often fail to open circuit. You can test that with any capacitance meter.

Thanks RB , i have checked two interferon suppression caps ( 2 * 100 nf ) connected with the mains , i measured 3 nf , and 1.5 nf on the second one .

After replacement the unit is working nicely , i but i really don't know how they caused the transistor to short if they were open ?

 

The job of the snubber cap is to absorb the energy of the switching spike.

The caps take a bit of a beating and often fail (go open circuit), so after that there is nothing to absorb the spike, so the spike starts killing semiconductors like FETs or IGBTs etc.

Congrats on a successful repair.