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N type Mosfet blowing up! Please help

Discussion in 'Troubleshooting and Repair' started by mattg, May 28, 2012.

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  1. mattg


    May 28, 2012

    I have an inverter which converts 36v into 240v (Grid tie inverter)

    The mosfets keep blowing up after a week of use I had 18n50 (500v) chips in and changed to 14n60 (600v)

    There is an arc on the back of the mosfet where it looks like it has grounded what can I test or do to permanently fix this?

    And suggest would be appreciated, I have circuit diagrams if anyone wants to take a look
  2. clove333


    May 21, 2012
    mosfet failure

    Are there movistors in the circuit?
    These are commonly used to protect against voltage spikes. Most power strips
    that have them say "surge protected".
    Metal Oxide Varistor I believe is the technical name for the devices though they
    are known by a number of different trade names of which Movistor is one.

    It is difficult to tell when one has failed as they usually fail 'open' rather than
    shorted. At any rate if this protection was no longer working then it might explain
    the seemingly random failures you are having.

    By the way. How hot are the mosfets running? If the circuit is reasonably well executed then you should be able to hold the side of your pinky against the
    semiconductor for ten seconds without getting burned. If you can then it passes
    'the ten second rule'. Yeah, I know mosfets are supposed to be intrinsically
    protected against thermal runaway because their Rsat goes up as die temp
    That's only true up to a point.
    Dont electrocute yourself doing this test?!?!

    You could also check to see if the power devices have parasitic oscillation on
    their waveforms. That'll kill a mosfet right quick.

    I hope that helps.
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Anything with an output voltage of 240V is not going to get my pinkie anywhere near it. :)

    Is this a circuit of your own design? Is it a commercial device? Do you have a schematic?
  4. mattg


    May 28, 2012
    Thanks for your suggestions

    It is a sun grid tie inverter 500w made by masspower

    I have had good luck with the unit I have put in a 25amp 600v mosfet and the unit had been working fine for the past few months

    Fingers crossed
    Last edited by a moderator: Jul 13, 2012
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Excellent information.

    So you replaced failed 18N50 with 18N60?

    And it's a MassPower 500w grid tie in inverter?

    Like this?

    If it's a design fault, it would be nice to get all of that in a message to help people googling for answers.

    edit: other than the fact you said it was 240V (and sorry for editing your post (oops))
    Last edited: Jul 13, 2012
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