What could be causing This SMPS's chopper to short ??

Discussion in 'General Electronics Chat' started by LETITROLL, Feb 22, 2014.

  1. LETITROLL

    Thread Starter Member

    Oct 9, 2013
    218
    2
    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 :
     
  2. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    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).:eek: The fix usually requires figuring out what killed them.

    Experience has taught me to always assume electrolytic caps are bad.......
     
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  3. nigelwright7557

    Senior Member

    May 10, 2008
    487
    71
    Maybe the transformer has fried ?
     
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  4. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    +1 Transformers also do not age well. basically, suspect any component that operates hot and has higher voltages applied to it.
     
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  5. LETITROLL

    Thread Starter Member

    Oct 9, 2013
    218
    2
    How do i check the SMPS transformer ??
     
    Last edited: Feb 22, 2014
  6. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    Failed snubber caps?
     
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  7. LETITROLL

    Thread Starter Member

    Oct 9, 2013
    218
    2
    I see , thanks for all your inputs ,
    By the way is the ESR meter able to test with both electrolytic and non polarized caps ??
     
  8. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    High voltage polyester snubber caps often fail to open circuit. You can test that with any capacitance meter.
     
  9. LETITROLL

    Thread Starter Member

    Oct 9, 2013
    218
    2
    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 ?


    [​IMG]
     
    Last edited: Mar 9, 2014
  10. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    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. :)
     
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