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RE: Gatedrive:charge vs. capacitance vs. potential



Original poster: "Mccauley, Daniel H" <daniel.h.mccauley-at-lmco-dot-com> 


The simplest way to use this parameter is using the following equation:

GateCapacitance = Qgate / Vgate

Qgate (gate charge) can usually be derived from a graph in the datasheet
showing gate charge vs. operating voltage (Vds)
And yes, the gate charge does vary with voltage and current across the
FET.

For example, the IXFN44N50 FET lists about 270nQ for gate charge under
some set of parameters.

270nQ / 12V = 22nF.

This number is usually good enough for most simulations and power
dissipation calculations.
Of course, in reality the gate capacitance is quite a complex beast.
There are lot of capacitive junctions which
are being charged/discharged and all act non-linear.

For best bets, the above calculation will do quite well.

Dan


 > All,
 >
 > Can someone explain, how to arrive at the equivalent gate capacitance,
 > knowing the Qg(on) and Vgs.
 >
 > Is it correct that this gate charge varies with voltage across FET as
 > well as current trough it?
 >
 > Cheers, Finn Hammer
 >
 >
 >