Inverter MOSFET (IRF740) get short after some time !

Discussion in 'Technical Repair' started by Ahmed Abbas 1, Oct 3, 2016.

  1. Ahmed Abbas 1

    Thread Starter New Member

    Oct 3, 2016
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    Dear All ,

    I have inverter (600W 1000VA) with builtin charger and short circuit protection and all this thinks . it was work perfectly i attached it with 12V 250AH Battery , few days ago it was off with beep and show error then i check and found one MOSFET short and change it with MOSFET (IRF740) but after few days it short's again and again. last time i change all MOSFET's but still same problume i check all other IC's and found it in good. Here i post the images can any one tell me why it shorts rapidly , or if there is any other MOSFET of higher value which can work perfectly with this inverter then please tell me .

    Thanks & Regards.
     
  2. Ahmed Abbas 1

    Thread Starter New Member

    Oct 3, 2016
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    Still No replies !
     
  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    1. Check the traces on the pcb, make sure there are no nicks or damage to the Gate Pin of that third MOSFET.
    2. Check if any diodes or Zener diodes connected to (near) that third MOSFET are looking normal (the odd looking one will be the issue).
    3. If they all look normal, then you'll have to measure them.
     
  4. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Maybe the FETs are passing excessive current because of failure of some other component?
     
  5. ci139

    Member

    Jul 11, 2016
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    your Fig1. shows a lot of dust accumulated (high voltage spikes like to use such for random discharge path - get soft brush and vacum - and or - blow the "extra" conducting layer off your device ? use air filters at air inputs)

    the PCB looks riveted to place ? how you replace your MOSFETS and confirm they get connected and to possible inner copper sheets on PCB

    what it seems there's a load share problem e.g. the 3-rd mosfet gets the most power to pass ---- as you already replaced it the problem might be at drive(rs) circuitry . . . it also is quite centered on the heat-sink (the warmest spot)
     
    Last edited: Oct 7, 2016
  6. Ahmed Abbas 1

    Thread Starter New Member

    Oct 3, 2016
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    Thanks for your replies . I already check PCB traces there is no damages and also check the drive rs and replace the IC s . the one think is that load share problem . so if i replace IRF740 to IRFP350 its Continuous Drain Current is 16 A on 25C and 10A on 100C . however IRF740 is 10A on 25C and 6A on 100C
    is this can solve the problem ?
     
  7. Ahmed Abbas 1

    Thread Starter New Member

    Oct 3, 2016
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    If the MOSFET is not connected properly then it shows error on start and give beep sound continues . after change the MOSFET the unit works fine .
     
  8. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Maybe.

    It depends if you only have a heat and current issue and you need a higher current MOSFET (lower ON resistance), or if this is related to switching speed and heat generated as the gate charges and the mosfet may spend too much time in the linear region as it switches (slow switching speed).

    The IRFP350 has a higher gate capacitance than the IRF740 and may switch more slowly and make your problem worse.

    On the other hand, it has lower ON resistance and a BETTTER PACKAGE (TO-247) instead of TO-220. The TO-247 package has in integrated insulated screw hole while the TO-220 needs a little insulator ring that can deform (creep) on repeated heat/cool cycles and eventually allow a short.

    It is hard to diagnose without having the board in hand but these are the things you have to look for.
     
  9. ci139

    Member

    Jul 11, 2016
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    the reverse side of a PCB the burnt marks -- if they are burnt marks by MOSFET -- then they likely go inside the board surface and may have some conductance

    anyway your mosfets seem to be driven by same line/timing -- if there is load sharing problem -- it's something i can't imagine right now

    it also might be - the indication of transformer developing problems - if the PCB current path is most likely through that MOSFET - actually i can't suggest anything for shure
     
  10. ci139

    Member

    Jul 11, 2016
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