Capacitively driving a mosfet gate...

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Externet
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Capacitively driving a mosfet gate...

Post by Externet » Fri May 01, 2009 6:02 pm

Hello all.
Switching power supplies which stop oscillating by whatever reason may fry the mosfet in conduction state if the gate gets stuck 'hi' .
If the gate can be driven trough a capacitor, that should not happen, but have not seen such approach. Have you? Does it work equally well ?
Miguel
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MrAl
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Re: Capacitively driving a mosfet gate...

Post by MrAl » Sat May 02, 2009 9:14 am

Hi there,

It is possible i guess, but i would say usually if something goes wrong with the drive circuit then it
doesnt matter that much if the MOSFET burns out, and there should be a fuse somewhere too.

Also, it is hard to characterize a drive circuit failure anyway...it might still be able to turn the
MOSFET on improperly for certain types of failures.

Also, what if the MOSFET burns out without the gate drive burning out? :smile:
LEDs vs Bulbs, LEDs are winning.

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Joseph
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Re: Capacitively driving a mosfet gate...

Post by Joseph » Sun May 03, 2009 1:44 am

Since MOSFETs turn on by gate charging, dumping a capacitor charge into it can keep it on indefinitely, unlike biplar transistors which need flowing current from the base to the emitter to stay on.

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dtief
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Re: Capacitively driving a mosfet gate...

Post by dtief » Sun May 03, 2009 4:43 pm

I suspect that by the time you have a cap big enough to keep the drive undistorted, it would be too big to prevent the failure. If the FET is on too long (10s of microseconds - depends greatly on the switcher frequency), the inductor saturates and the current spikes way up since there is only parasitic resistance of the circuit to limit it. This happens way too fast for a fuse to be of any use.

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MrAl
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Re: Capacitively driving a mosfet gate...

Post by MrAl » Mon May 04, 2009 7:38 am

Hi,


Good points Joseph and dtief. The MOSFET stays on too long anyway.
LEDs vs Bulbs, LEDs are winning.

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