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Re: FET G-S voltage limits

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On 2017-03-24 14:34, Tim Wescott wrote:
> A customer of mine is repurposing a board that I designed for them
> several years ago, and we're running into some interesting issues.
>
> One of the issues is that there's a master turn-on to the power section
> of the board that's operated by pulling the gate of a P-channel FET low.
> I just noticed that the thing is rated for a maximum G-S voltage of 20V
> -- we had been putting 24V on it (oops), and now we're putting about 29V
> on it (bigger oops).
>
> We're now seeing that part get hot at high current draw, but we've
> identified two other possible mechanisms for that happening.  I'm trying
> to sort through the suspects to find a culprit.
>
> How much does the maximum G-S voltage rating on a FET matter?  Could it
> make a FET overheat at high currents, or would it just cause the whole
> thing to pop somehow?
>

Many FETs can actually take in excess of 50V on the gate. Of course, 
that is not a legit design because abs max is usually 20V. There are a 
few FETs where Vgs is rated abs max at 30V.

The failure mechanism would be a sudden breakdown of the oxide layer and 
then you have a conductive path from whatever is driving the gate to the 
power path (the channel). A FET getting hot at such high drive level has 
other causes such as very high current and consequential ohmic losses, 
or too frequenct and sluggish switching which causes transistion losses.

If you still can, add a 12-15V zener from gate to source and a series 
resistor towards whatever drives the gate.

-- 
Regards, Joerg

http://www.analogconsultants.com/

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FET G-S voltage limits Tim Wescott <seemywebsite@myfooter.really> - 2017-03-24 16:34 -0500
  Re: FET G-S voltage limits Joerg <news@analogconsultants.com> - 2017-03-24 14:41 -0700
    Re: FET G-S voltage limits Tim Wescott <seemywebsite@myfooter.really> - 2017-03-24 17:02 -0500
      Re: FET G-S voltage limits Joerg <news@analogconsultants.com> - 2017-03-24 15:42 -0700
  Re: FET G-S voltage limits dagmargoodboat@yahoo.com - 2017-03-24 14:42 -0700
  Re: FET G-S voltage limits John Larkin <jjlarkinxyxy@highlandtechnology.com> - 2017-03-24 15:33 -0700
    Re: FET G-S voltage limits Joerg <news@analogconsultants.com> - 2017-03-24 16:08 -0700
      Re: FET G-S voltage limits John Larkin <jjlarkinxyxy@highlandtechnology.com> - 2017-03-24 16:24 -0700
  Re: FET G-S voltage limits "Tim Williams" <tiwill@seventransistorlabs.com> - 2017-03-25 03:01 -0500
  Re: FET G-S voltage limits Robert Baer <robertbaer@localnet.com> - 2017-03-30 23:37 -0800

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