Can anyone explain how to calculate MOSFET Turn off time?

I just calculating by multiplying RG (external resistor) and Capacitance (CGS). Is it fine or any other method ?

 

depends on how you define "turn off".

 

That gives a rough estimate, but for a more accurate calculation you need to determine the time it takes for the gate voltage to go from its on to its off (threshold) value.

 

Can anyone explain how to calculate MOSFET Turn off time?

I just calculating by multiplying RG (external resistor) and Capacitance (CGS). Is it fine or any other method ?

Most data sheets give you a spec for gate charge (Qg). You can multiply this number by gate current to get turn on time in nano seconds. In the example below if you had 1 amp of gate current it would switch in 72 ns.

 

by multiplying RG (external resistor) and Capacitance (CGS).

(Just sharpening up the numbers...)
That gives you the RC amount for Cgs, but look at Qg on this datasheet. More than double Qgs. This particular mosfet can survive a 30 volt Vgs. If you need to get the gate down from 20 volts to 4 volts, that's more than one RC time constant. The datasheet will get you right where you need to go, even if they present the number in different ways than one would expect.

Calculating RC gives you 1 time constant, but it might take 2 or 3 RC's depending on the voltage change. If you need to get down hard on the exact number, you can use this nasty formula: change in capacitor voltage = change in stimulus voltage x e^-(time/RC)

Yeah, running an RC in your head will tell you whether you're in the KHz range or the MHz range, but the math will get you down to small uncertainties, like the difference between guaranteed specs and, "typical" specs.

 

You can multiply this number by gate current to get turn on time in nano seconds. In the example below if you had 1 amp of gate current it would switch in 72 ns.

Are you sure about this multiplication ? What is the unit of amps times charge ? What about charge divided by amps ?

 

Are you sure about this multiplication ? What is the unit of amps times charge ? What about charge divided by amps ?

OOh,
Thank you Jony. Absolutely, divided by current.

 

It is a mistake to assume that you can calculate turn off time. You have to look at the method used to turn the device off and many methods have the visible characteristic of dropping the voltage to a plateau, sitting on that plateau for a while, before dropping to the threshold value and below. It would be nice if it was simple and straightforward, but it is not. This characteristic is especially fatal in motor drive and H-bridge circuits.