Hi all,
Hopefully somebody can enlighten me here.
I'm trying to calculate approximate Mosfet gate current.
Using the formula I = Q/t, what time does 't' refer to? Most datasheets include
Turn-On Delay time
Rise Time
Turn-Off Delay Time
Fall Time
If you were charging a capacitor then t= the time to move the voltage on a cap from one voltage to another voltage. (0 to 10V)
A MOSFET is more complex than that.
The turn-On delay is related to G-S capacitance and turn on voltage.
The turn-Off delay is related to G-S capacitance and the on voltage back down to 'gate turn on voltage'.
There is G to D capacitance that you must overcome during rise/fall times.
In the data sheet you will find a spec. called Qg (total gate charge). This is the time it will take the FET to switch given 1 amp of gate current. From this you can select a gate resistor or driver to get the speed you want. Lets say Qg is 100. Then it should turn on in 100 ns with 1 amp of gate drive or 1 us with 100 ma of gate drive. I have never gotten them to switch this fast - I think because of other layout parameters - but it gets you close.
If you are concerned about power dissipation during switching (as with a switching regulator or PMW switch for example) you want the transistor to switch as fast as possible. You would then want t to be less then the rise and fall times specified in the data sheet so that the actual rise and fall times are close to the minimum. This can require a high-current push-pull type MOSFET driver to drive the gate.