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I know that there have been many posts on this topic, but I'm confused about calculating the gate current of a MOSFET.

I have calculated the gate current as Igs = Qg/t. For example, I want to drive IRF540n with PWM at 100 kHz. It has Qg = 94 nC and 100 kHz = 10000 ns.

If I use Igs = Qg/t then Igs = 94/10000 = 0.0094 A. I think it's quite a small current.

Are my calculations correct or not?

SamGibson
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Sahasrar
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  • No Your calculation is wrong. If u r driving PWM first find lowest duty cycle you want to drive. if Lowest duty cycle is D, findout ON time for D as tON choose the rise time as 10%-30% of tON like 0.1tON - that should be the time to get MOSFET drive current. – user19579 Jun 04 '20 at 10:23
  • @user19579 the duty does not influence the gate drive current (unless it is 0% or 100%). The frequency, the capacitance and the desired rise/fall time does. A 1% duty waveform as the same number of edges as a 99% duty waveform. The gatedrive needs are edge related –  Jun 04 '20 at 11:48
  • @JonRB: You are correct gate drive current depends on rise/fall time. How to arrive at probable rise/fall time spec - for that only i took min duty cycle from that derived rise/fall times. – user19579 Jun 04 '20 at 14:31

1 Answers1

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the method you are persuing is calculating the average current needed to switch a MOSFET. This is one part of the calculation as one of the key need is the peak current to ensure you are switching fast enough.

what the best way to calculate Rg gate driver for Mosfet

As far as your calculations are concerned, this is only half of the calculation.

Every switching period has two switching edges

  • Turn on
  • Turn off

Each edge requires the transfer of charge (in your case 94nC). If you half the period you will be closer to the average current flowing.

The other approach is to calculate the rms current

\$I_{rms} = \frac{1}{R_g} \sqrt{ \frac{\int_{0}^{period}(V\cdot e^{-t/R_g \cdot C_g})^2{} }{period}} \$

  • Thanks, you help me a lot... In this case I need to know ammount of current for choose right type of mosfet driver... – Sahasrar Jun 04 '20 at 10:46
  • yes. There are two sides to this 1) the rms current as this drives the power need (especially on the gate resistor) 2) the peak current. This is to ensure the MOSFET turns on quick enough. you need to determine the peak current capability to correctly select the driver. at the very least consider one that can output 1-2A –  Jun 04 '20 at 10:59