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mosfet gate drive impedance

I have a situation where I need to use a pretty stiff pull-up resistor on a mosfet gate because I'm going to use a comparator to turn it off, and I plan to limit the current the comparator sinks to about 5 mA. The N-mosfet source will be sitting at 15 volts and the gate drive comes from a voltage doubler. That's a bit less than 30 volts so the pull-up resistor needs to be in the 6k ballpark. I'm going to use the NTP90N02:

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Speed doesn't matter -- this is basically an on-off switch, so I have only to consider the dc behavior of the mosfet. The mosfet has to conduct ten amps. My question is, how much resistance can you put in front of the gate before it gets too weak to turn the mosfet on hard? Mosfet models don't address this, so far as I can tell. I haven't ordered the NTP90N02 yet so I can't simply experiment on it, but I remember other mosfets having problems turning on when the gate resistance gets into the tens of kilohms.

Reply to
gearhead
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Attention: Keep in mind that the abs max for Vgs is usually no more than

+/-20V, including the FET you have selected.

It can be very high but two things can cause some grief:

a. Slow turn-on, on account of a high Cgs and Cdg. In your case Cgs is over 2000pF so a 6K would slow that down to 10usec or more. During this time the FET transitions through its linear range, briefly dissipating lots of power and possibly going kablouie. Then there is Cgd which works "against" turning it on, further slowing it down.

b. Depending on what is connected Cgd might cause a "dent" in the turn-on phase and possibly oscillation around that point. With a stiff load the latter is usually accompanied by a bang, molten solder splattering about and so on.

--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

I design my own MOSFET amplifiers, up to 600 watts RMS. I usually use a 330R in series with the gate. The drive to the 330R is very low impedance.

Reply to
Marra

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