# MOSFET gate resistor value

#### lokeycmos

Joined Apr 3, 2009
431
quick question, i am substituting IRFP460 MOSFETS for this inverter circuit:

http://electroschematics.com/220/12v-dc-220v-ac-converter/

could someone please show me how to calculate the ideal gate resistor for this application? i know that the gates are voltage driven, so do i even need resistors? please show me how you calculate so i can learn. One other thing, i know i can put MOSFETS in parallel to increase the current handling. Does this affect the value of the gate drive resistor? thank you!!

datasheet is here:
http://www.vishay.com/docs/91237/91237.pdf

#### lokeycmos

Joined Apr 3, 2009
431
ok thanks. a little more help is needed. the total rise and fall time is 117nS. so i multiply that by 5 and i get 585. the input capacitance is 4200pF. with this what formula do i use to get resistance in ohms?

#### tracecom

Joined Apr 16, 2010
3,944
I didn't mean to imply that I knew the answer. I am hoping someone else will explain it to both of us. #### lokeycmos

Joined Apr 3, 2009
431
I didn't mean to imply that I knew the answer. I am hoping someone else will explain it to both of us. so if i have 2 mosfets in parallel the gate capacitance would double. then i just use the same formula that i hope someone will post?

#### lokeycmos

Joined Apr 3, 2009
431
let me know if i got this right.

charge time = 5*R*C

117nS = 5*R*4200pF

117nS=R*21000pF

R=.000000117/.000000021= 5.57 ohms

please verify this for me and correct any mistakes. TY

#### lokeycmos

Joined Apr 3, 2009
431

#### lokeycmos

Joined Apr 3, 2009
431
could anyone verify post #6 for me? im basing these caculations from information in the link in post number 2

#### GopherT

Joined Nov 23, 2012
8,012
That would be the maxium resistance you could use. Since virtually no current flows through the gate once the gate capacitance is charged, you don't need to limit the current flow like in a bipolar transistor.

Your intuition was correct in your first post, you really don't need the gate resistors unless you are possibly pulling the gate voltage ABOVE the Drain voltage. note that those are general recommendations, I have not idea if your simple replacement will still work int his circuit.

#### vrainom

Joined Sep 8, 2011
126
The gate driving resistor is only critical in very high frequency circuits, where Rgate * Cgate becomes a significant part of the period in the operation frequency. For tens of kilohertz I think it's quite common to use a Rgate of anywhere between 10 and 47 ohms.