MOSFET gate discharge circuit

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eem2am

Joined: 22 Jun 2008
Posts: 753
Helped: 13

09 Nov 2008 15:18 mosfet gate discharge

hello

i am making an LED driver and i wonder if my mosfet driver will turn the fet off quickly enough?

as you see below if you want........do you know what resistance the mosfet sees looking back into the emitter follower stage?......am i right to saying that it sees 500 Ohms/hfe ....ie 500/100 = 5 Ohms ?

MOSFET gate discharge circuit

thanking for any thoughts

FvM

Joined: 22 Jan 2008
Posts: 5160
Helped: 767
Location: Bochum, Germany

09 Nov 2008 17:36 mosfet gate follower

The circuit has more serious flaws than slow turn off. First of all, it doesn't turn the FET on completely. Depending on the threshold voltage, it most likely stays permanently off. The maximum gate voltage from this circuit is about 1.8 V, with no drain current, reduced by the sense resistor voltage drop.

You should at least supply undivided 5 V to the gate, if it's a logic level FET with a low threshold voltage.
Faster turn off may be achieved by a push-pull gate driver, may be advisable for a high switching frequency.

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max0412

Joined: 26 Oct 2003
Posts: 312
Helped: 31

10 Nov 2008 1:08 gate discharge

These are proper discrete drivers.

MOSFET gate discharge circuit

Or use a mosfet gate driver IC.

* EDIT You have twelve volts available put the collector of the NPN (fig 23) to the twelve volt rail. As long as your FET can take the Vgs.