mosfet gate input resistor

[green]

I see some PI circuits have a mosfet gate input resistor(R1). Why and how to select the correct value?

[kt315]

mosfet gate has some input capacitance. value is related to transistor power. than more power then more the cap value.
for irf approx 1000pF. so issue is to recharge this cap in time of TURN ON and TURN OFF states that the resistor does limiting
the current charge-recharge. set 10-20Ohm and forget about it.

[eclipse]

Typical values 10-20 ohms. It can reduce ringing effect if trace to the gate is long, unwanted oscillations may occur otherwise. Also dissapates some heat, too.

[kt315]

question of 2001 year old, so oldest http://www.findmall.com/read.php?34,129460
why you are so not curious? english is not my matter, its YOUR. findmall is yours forum, not mine.

[Teleno]

The gate resistor on a MOSFET is really there to protect whatever is sourcing the current. Much like a discharged capacitor, the gate will initially look like a short to ground when voltage is first applied. A MOSFET with a very large gate capacitance can sink a very large amount of current for a short period of time. If you're driving the gate with, say, a MCU pin, it's usually a good idea to put a small resistor to reduce that current surge to a value the MCU can handle.

[Davor]

These are called "gate stoppers" with respect to the same thing used at tube grids, and no surprise, called "grid stoppers".

[kt315]

ufff... an answer is more difficult than I might guess (( I found a text in russian but I am not able to translate all. anotherway mosfet structure has some inside parasitic bipolar structure and the resistor does not give it to open in some TURN ON and OFF moment.
http://radiokot.ru/forum/viewtopic.php?f=21&t=28574
also you can set special small ring on gate pin with this matter and do not set the resistor.

[kt315]

[Old cart]

kt315 those are ferrite bead. They will make the gate feed look like small inductor and will slow the signal turn on and off I would think. They may be useful for reducing electromagnetic interference since when combined with the large FET gate capacitance will reduce the resonance frequency.