What destroyed this MOSFET?

Discussion in 'General Electronics Chat' started by Roderick Young, Jan 7, 2016.

  1. Roderick Young

    Thread Starter Member

    Feb 22, 2015
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    HowDestroyed.png
    First of all, I fully acknowledge that I did something stupid, so I'm not looking for "user error" as the root cause.;)

    I was bringing up the circuit on the left. It's part of a buck-boost converter, but I've simplified the schematic for illustration. Q2/D2 (and Q1/D1) are actually one part, an FDPF085N10, an N-channel MOSFET with body diode. Vdd is 24 volts. When a button is pressed, a single 10-volt, 10 uS positive-going pulse is driven into the MOSFET by a driver (not shown).

    I tested the MOSFET before using it, and it was good. Then I soldered it onto the PC board, turned on the power, and hit the button a few times, expecting the voltage to jump at least a little. Nothing happened. In fact, I found that my 24-volt supply had gone into shutdown.

    Ok, my fault, I had inserted the MOSFET backwards, and in fact, gate and source were swapped, resulting in the circuit on the right. I removed the MOSFET and found gate, source, and drain fused together. I replaced the MOSFET with one oriented in the correct direction, and everything worked fine.

    So the question: in technical terms, what was the mechanism of the failure?
     
    kesarraghav likes this.
  2. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    What is the part number of the MOSFET?
     
  3. Brownout

    Well-Known Member

    Jan 10, 2012
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    Is it just me, or does the answer seem painfully obvious?

    The diode is probably not a separate component in the package, but rather integrated with the FET silicon. When it goes, so goes the FET.
     
  4. RichardO

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    May 4, 2013
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    It is not obvious to me. Can you give more details?
     
  5. Brownout

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    Jan 10, 2012
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    Hmmm.... I misinterpreted what he meant by "backwards." The clue might be the supply shutting down. I have to wonder if in fact, the Drain/Source were reversed. Otherwise, we need to see what the driver was doing.
     
  6. RichardO

    Well-Known Member

    May 4, 2013
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    This part has the pinout, left to right: Gate, Drain, Source. I could have wired it wrong too, expecting the gate to be in the center. :(
     
  7. dl324

    Distinguished Member

    Mar 30, 2015
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    What is VDD and coil resistance? You don't appear to have a snubber diode (don't know what output is driving), so you're counting on the body diode to protect the transistor; which it, apparently, isn't able to do. What is the MOSFET part number?
     
  8. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Unless D1/C1 manage to clamp the output voltage to something less than the Vds max of the FET it's goodbye FET. Drain and source fused/shorted would account for the supply tripping.
    For many FETs the maximum Vgs is +- 10V or more, so you may have got away with confusing source and gate.
     
    cmartinez likes this.
  9. tcmtech

    Well-Known Member

    Nov 4, 2013
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    Most likely simple over current overheated the die. The resistance of the inductor or the current limit of the power supply was just within the limits of what could cause the die melt down without blowing up the devices case.
     
  10. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    The idea of a tiny EMP crossed my mind. Depends on where the inductor is compared to the FET. A sudden burst of 24 volts through a coil might be able to generate an electro-magnetic-pulse that could destroy the thin layer of silicon oxide, thus destroying the MOSFET.

    I'm going way out on a limb here, though. This probably wasn't the cause.
     
  11. Picbuster

    Member

    Dec 2, 2013
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    could have two reasons
    a:coil has no diode negative pulse could destroy s d junction
    b:gate is blown up voltage to high also missing a small resistor to gate >=100Ohm)
    The input cap gate is at high freq a low impedance absorbing current resistor will limit this.
     
  12. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    The inductive kick from the coil blew the gate oxide while the gate and source were interchanged. The max voltage from Drain to Gate is like 20V. YOur circuit produces hundreds of volts.
     
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  13. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    Would that mean that the circuit needs a flyback diode? Because I'm not sure how the pair C1-D1 are supposed to work
     
  14. ronv

    AAC Fanatic!

    Nov 12, 2008
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    The maximum gate to source voltage spec is +/- 20 volts. So the way it was hooked up would violate that.
     
  15. Roderick Young

    Thread Starter Member

    Feb 22, 2015
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  16. Roderick Young

    Thread Starter Member

    Feb 22, 2015
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    I'm 99% sure that the wiring is ok, as when I replaced the part with a correctly-oriented one, the circuit worked fine. It was an insertion error, my bad, and the MOSFET fused. But I'm at a loss to explain the theoretical mechanism of why it failed.
     
  17. DerStrom8

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  18. Roderick Young

    Thread Starter Member

    Feb 22, 2015
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    Vdd is 24 volts, from a switching power supply.
    The resistance of L1 is negligible. It's about 4 feet of #12 solid copper wire on a torroid.
    The MOSFET part number is FDPF085N10A.
    This is a buck-boost converter, so the inductive kick in normal operation is rectified through D1 and stored in C1.
     
  19. Roderick Young

    Thread Starter Member

    Feb 22, 2015
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    Under normal operation, D1/C1 rectify the output voltage, nominally 24 volts also, putting 48 volts across the drain and source of the transistor. And in fact, the circuit works like a champ with a properly-inserted transistor.
    The MOSFET used has as +-20 volt max Vgs, and it seems to me that by hooking it up backwards, I was applying a pulse of -10 volts to the gate. I would understand why the circuit didn't work, but not why the transistor was destroyed.
     
  20. Brownout

    Well-Known Member

    Jan 10, 2012
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    Just don't do it again.
     
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