Re: Root Cause of FDC6303N FET drain and gate short circuit?

Discussion in 'Electronic Design' started by Winfield Hill, Jun 30, 2010.

  1. Myauk wrote...
    >
    > In a typical PCBA design which consists of relay driver circuit, we
    > found the FET drain and gate short circuit problem causing damage to
    > MCU DIOs due to excessive sink current in production on 5 or 6 failed
    > out of 100 PCBA units.


    > 2. Or is there any possible fault condition which can cause a good
    > working FET to have gate and drain shorted?


    Drain-to-gate shorts is a typical failure mode for power mosfets.

    Amusingly, the mosfet is still probably operating properly, that
    is it's operating the same as any other mosfet with its drain and
    gate externally connected together. But I digress.

    You want to know why the part failed. SFAIK, any of the various
    overstress conditions can cause this type of failure. E.g.,
    overheating, in any of the ways Rds(on), switching losses,
    avalanche heating, or overvoltage. Including gate overvoltage,
    which can happen if a high current is switched too fast, causing
    high dI/dt, and a substantial source-wiring inductance, causing
    high V = L dI/dt, which can be a short damaging gate-voltage spike.

    Lot's of handwaving there, but the standard remedies apply, a diode
    across the coil, a gate resistor to slow down switching speed, etc.


    --
    Thanks,
    - Win
     
    Winfield Hill, Jun 30, 2010
    #1
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  2. Winfield Hill

    tm Guest

    "Myauk" <> wrote in message
    news:...
    On Jun 30, 9:38 pm, Winfield Hill <>
    wrote:
    > Myauk wrote...
    >
    > > In a typical PCBA design which consists of relay driver circuit, we
    > > found the FET drain and gate short circuit problem causing damage to
    > > MCU DIOs due to excessive sink current in production on 5 or 6 failed
    > > out of 100 PCBA units.
    > > 2. Or is there any possible fault condition which can cause a good
    > > working FET to have gate and drain shorted?

    >


    ..But I think it is not really a design issue. We have already added
    ..freewheeling diode verifying the back emf is already eliminated by
    ..checking the waveforms. And 10k Gate resistor is there. So there is no
    ..way possibly to damage the FET.
    ..
    ..Does anyone have similar experience?
    ..
    ..Thanks and Best Regards

    If you have a 10k gate resistor, how is it killing the DIO?
     
    tm, Jun 30, 2010
    #2
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  3. Winfield Hill

    tm Guest

    "Myauk" <> wrote in message
    news:...
    On Jun 30, 9:58 pm, "tm" <> wrote:
    > "Myauk" <> wrote in message
    >
    > news:...
    > On Jun 30, 9:38 pm, Winfield Hill <>
    > wrote:
    >
    > > Myauk wrote...

    >
    > > > In a typical PCBA design which consists of relay driver circuit, we
    > > > found the FET drain and gate short circuit problem causing damage to
    > > > MCU DIOs due to excessive sink current in production on 5 or 6 failed
    > > > out of 100 PCBA units.
    > > > 2. Or is there any possible fault condition which can cause a good
    > > > working FET to have gate and drain shorted?

    >
    > .But I think it is not really a design issue. We have already added
    > .freewheeling diode verifying the back emf is already eliminated by
    > .checking the waveforms. And 10k Gate resistor is there. So there is no
    > .way possibly to damage the FET.
    > .
    > .Does anyone have similar experience?
    > .
    > .Thanks and Best Regards
    >
    > If you have a 10k gate resistor, how is it killing the DIO?


    ..Current from 5 V supply through relay is approximately 60mA sinking
    ..into the LOW Dio pin rated at 15mA. (4 times the rated value).

    Then there is something you are not telling us. From your two comments, you
    say the
    MCU DIO pin connects to the fet through a 10k resistor. If the fet shorts to
    5 volts
    (g to d), then the max current can be 0.5 mA. I=E/R= 5/10000=0.0005

    Maybe you need to draw us a picture.


    T
     
    tm, Jun 30, 2010
    #3
  4. Winfield Hill

    SoothSayer Guest

    On 30 Jun 2010 06:38:25 -0700, Winfield Hill
    <> wrote:

    >Myauk wrote...
    >>
    >> In a typical PCBA design which consists of relay driver circuit, we
    >> found the FET drain and gate short circuit problem causing damage to
    >> MCU DIOs due to excessive sink current in production on 5 or 6 failed
    >> out of 100 PCBA units.

    >
    >> 2. Or is there any possible fault condition which can cause a good
    >> working FET to have gate and drain shorted?

    >
    > Drain-to-gate shorts is a typical failure mode for power mosfets.
    >
    > Amusingly, the mosfet is still probably operating properly, that
    > is it's operating the same as any other mosfet with its drain and
    > gate externally connected together. But I digress.
    >
    > You want to know why the part failed. SFAIK, any of the various
    > overstress conditions can cause this type of failure. E.g.,
    > overheating, in any of the ways Rds(on), switching losses,
    > avalanche heating, or overvoltage. Including gate overvoltage,
    > which can happen if a high current is switched too fast, causing
    > high dI/dt, and a substantial source-wiring inductance, causing
    > high V = L dI/dt, which can be a short damaging gate-voltage spike.
    >
    > Lot's of handwaving there, but the standard remedies apply, a diode
    > across the coil, a gate resistor to slow down switching speed, etc.



    I always noticed on year, when doing a huge amplifier design, which
    contained a row of huge IGBTs and FETs on a 20 inch long sink, that we
    had some odd failures early on in the design that had no reason for
    being...

    Early failure modes were tied to coplanarity issues on the sink. It
    seems our assembler did not understand the concept of coplanarity until I
    showed him the proper assembly sequence. Without it, it was as if any
    part that did get attached right was not attached at all.

    Anyway, we experienced plenty of what appeared to be "immediate"
    failures due to the lack of a part being sinked properly. It was really
    a matter of a few seconds. and went away once assembly efficacy was
    achieved.

    It seems that those thermal spikes were fast enough to take the part to
    critical failure temperatures. Good heat sinking is REQUIRED, and that
    right where it belongs... on the back of that tab!
     
    SoothSayer, Jun 30, 2010
    #4
  5. Winfield Hill

    SoothSayer Guest

    On Wed, 30 Jun 2010 06:49:37 -0700 (PDT), Myauk <>
    wrote:

    >On Jun 30, 9:38 pm, Winfield Hill <>
    >wrote:
    >> Myauk wrote...
    >>
    >> > In a typical PCBA design which consists of relay driver circuit, we
    >> > found the FET drain and gate short circuit problem causing damage to
    >> > MCU DIOs due to excessive sink current in production on 5 or 6 failed
    >> > out of 100 PCBA units.
    >> > 2. Or is there any possible fault condition which can cause a good
    >> > working FET to have gate and drain shorted?

    >>
    >>  Drain-to-gate shorts is a typical failure mode for power mosfets.
    >>
    >>  Amusingly, the mosfet is still probably operating properly, that
    >>  is it's operating the same as any other mosfet with its drain and
    >>  gate externally connected together.  But I digress.
    >>
    >>  You want to know why the part failed.  SFAIK, any of the various
    >>  overstress conditions can cause this type of failure.  E.g.,
    >>  overheating, in any of the ways Rds(on), switching losses,
    >>  avalanche heating, or overvoltage.  Including gate overvoltage,
    >>  which can happen if a high current is switched too fast, causing
    >>  high dI/dt, and a substantial source-wiring inductance, causing
    >>  high V = L dI/dt, which can be a short damaging gate-voltage spike.
    >>
    >>  Lot's of handwaving there, but the standard remedies apply, a diode
    >>  across the coil, a gate resistor to slow down switching speed, etc.
    >>
    >> --
    >>  Thanks,
    >>     - Win

    >
    >
    >Thanks Win,
    >
    >But I think it is not really a design issue. We have already added
    >freewheeling diode verifying the back emf is already eliminated by
    >checking the waveforms. And 10k Gate resistor is there. So there is no
    >way possibly to damage the FET.
    >
    >Does anyone have similar experience?
    >
    >Thanks and Best Regards


    Do you have a thermal imager?

    If not, you could rent one. Examine the device as you utilize it.
    Watch the failure mode as it occurs and examine the waveforms during that
    period to determine if it is circuit design related of heat management
    related.

    Make sure that your heat sink attachment has a high integrity thermal
    pathway. Do not simply use a dry, "air" attachment. Make sure that it is
    well sinked.

    Once you rule out that, then get back to probing the circuit.

    Have you ever used 'transzorbs', or put ferrite beads on the gate lead
    during assembly? As close to the part as possible.
     
    SoothSayer, Jun 30, 2010
    #5
  6. Winfield Hill

    tm Guest

    "Myauk" <> wrote in message
    news:...
    On Jun 30, 10:20 pm, "tm" <> wrote:
    > "Myauk" <> wrote in message
    >
    > news:...
    > On Jun 30, 9:58 pm, "tm" <> wrote:
    >
    >
    >
    > > "Myauk" <> wrote in message

    >
    > >news:...
    > > On Jun 30, 9:38 pm, Winfield Hill <>
    > > wrote:

    >
    > > > Myauk wrote...

    >
    > > > > In a typical PCBA design which consists of relay driver circuit, we
    > > > > found the FET drain and gate short circuit problem causing damage to
    > > > > MCU DIOs due to excessive sink current in production on 5 or 6
    > > > > failed
    > > > > out of 100 PCBA units.
    > > > > 2. Or is there any possible fault condition which can cause a good
    > > > > working FET to have gate and drain shorted?

    >
    > > .But I think it is not really a design issue. We have already added
    > > .freewheeling diode verifying the back emf is already eliminated by
    > > .checking the waveforms. And 10k Gate resistor is there. So there is no
    > > .way possibly to damage the FET.
    > > .
    > > .Does anyone have similar experience?
    > > .
    > > .Thanks and Best Regards

    >
    > > If you have a 10k gate resistor, how is it killing the DIO?

    >
    > .Current from 5 V supply through relay is approximately 60mA sinking
    > .into the LOW Dio pin rated at 15mA. (4 times the rated value).
    >
    > Then there is something you are not telling us. From your two comments,
    > you
    > say the
    > MCU DIO pin connects to the fet through a 10k resistor. If the fet shorts
    > to
    > 5 volts
    > (g to d), then the max current can be 0.5 mA. I=E/R= 5/10000=0.0005
    >
    > Maybe you need to draw us a picture.
    >
    > T


    ..10k resistor is the parallel pull down resistor between gate and
    ..source of FET.
    ..
    ..There is no resistance, the DIO directly drives the FET Gate.
    ..
    ..Regards

    I would at least put a small resistance, say 100 ohms or so, in series
    with the gate to PIO pin. I'm not sure why you need the pull down as the
    MCU should drive it ok. Unless the MCU three states the pin prior to
    completing the reset. What bypassing are you using in the circuit? MCU?
    Fet?


    Regards,
    T
     
    tm, Jun 30, 2010
    #6
  7. Winfield Hill

    Hammy Guest

    On Wed, 30 Jun 2010 14:31:45 -0400, "tm" <> wrote:


    >
    >I would at least put a small resistance, say 100 ohms or so, in series
    >with the gate to PIO pin. I'm not sure why you need the pull down as the
    >MCU should drive it ok. Unless the MCU three states the pin prior to
    >completing the reset. What bypassing are you using in the circuit? MCU?
    >Fet?
    >
    >
    >Regards,
    >T
    >
    >
    >

    I use a pull down when driving FET's directly off a PIC as well. Most
    uC's on start-up default to input so a pull down ensures the FET stays
    off. I usually use 220k Bleeder gate to source or higher though.

    Its a pretty low threshold FET are you sure your Vgs is going low
    enough to completely turn it off especially with a 10k pull down?

    I don't know what uC your using but the worst case LOW on an I/O for a
    PIC can be as high as 0.6V At as little as 1.2mA. I've never seen it
    that high but it is the worst case.

    You would figure a rupture of the gate oxide from over voltage would
    cause it to fail open. I've never had a FET fail that way usually when
    I blow one its pretty spectacular and obvious. ;-)

    You really want to keep the loop for the gate source signal small as
    well as the drain source loop to reduce trace inductance. You should
    be using a gate resistor 4.7 to 22 ohm or a fusible resistor may be
    better in this case;-)

    Oh and just because your not seeing ringing doesnt mean it isnt, you
    may just need a higher BW scope to see it. The gate oxide of such a
    low threshold fet wouldnt be to tolerant to overvoltage transients.
     
    Hammy, Jul 1, 2010
    #7
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