[SOLVED] MOSFET's total gate charge does not sum up to individual gate charges, why?

[mrinalmani]

The total gate charge of a MOSFET always turns out to be more than the sum of gate-source and gate-drain charge. What is the reason for this? Where does the additional charge go?

 

[alexan_e]

I can't provide a theoretical explanation but here is a graphic representation, hope it helps



It is from page 9 of https://www.fairchildsemi.com/an/AN/AN-9010.pdf

 

[FvM]

Because total gate charge is the sum of three rather than two terms.

See IRF an-1084, Power MOSFET Basics

The gate charge (QGS + QGD) corresponding to time t3 is the bare minimum charge required to switch the device on. Good circuit design practice dictates the use of a higher gate voltage than the bare minimum required for switching and therefore the gate charge used in the calculations is QG corresponding to t4.

 

[mrinalmani]

Thanks everyone!
I got it, the extra charge is needed to over-dose the gate-source capacitance in order to reduce Rds(on)