## Calculating MOSFET gate charge

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I've been trying to better my understanding of MOSFET characteristics. I figure that's a fairly important skill to have.

So - I'm trying to see if I can derive and understand some values in MOSFET datasheets. Specifically, I'm interested in the charge parameters - total gate charge, gate to source charge, gate to drain (miller) charge, and total gate charge sync. I'm looking at this data sheet: http://www.irf.com/product-info/...

So - I figure I should calculate out the charge that I typically care about - the charge it takes to turn a MOSFET on from VGS = 0. In other words - the total gate charge. So, my calculations gave me:

Q = Cgs * Vgs + Cgd * (Vgs + Vsupply - VDSon)

This is assuming an N-FET switching the low side of a resistive load. VDSon is Vds at whatever current will be flowing with that load at that Vgs.
Does this look right? It gives me a value of 13.5nC for the linked FET, while the FET specs 22nC. So it seems like I'm off by quite a bit.

My math:
Cgs = Ciss - Crss = 1080pF

Vgs = 10V, Vsupply = 30V, Vdson = 0.25V (from page 3, figure 1)

I used similar math for finding the miller capacitance, and I was completely off for that too! I'd probably be off for the gate to source charge, and I don't have a clue what the total gate charge sync is!

My guess is that FET capacitances don't act like nice capacitors. And that I can't use traditional math on them. But that's just a guess!