# Is MOSFET gate charge affected by drain current?

Discussion in 'General Electronics Chat' started by strantor, Sep 25, 2011.

1. ### strantor Thread Starter AAC Fanatic!

Oct 3, 2010
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Looking at the data sheet for my IRFP4110PbF, I see under the total Gate charge (Qg) is 210nC, if the drain current is 75A and the voltage on the gate is 10V. since 10V is a pretty normal Gate voltage and 50V is close enough to my 48V, I need to know how the drain current will affect the gate charge. For example, what if Id was 120A? What would Qg be?

these documents:
http://www.irf.com/technical-info/appnotes/mosfet.pdf
http://www.irf.com/technical-info/appnotes/an-944.pdf
address gate charge and indicate that a different Vds will affect the gate charge but fail to mention anything about how the drain current will effect gate charge.

...

It seems rather silly now that I think about it, because in order for there to be any drain current, the device has to be already switched, so drain current shouldn't matter I suspect, then why do they specify Id of 75A in the datasheet under Gate charge?

2. ### ifixit Distinguished Member

Nov 20, 2008
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110
Hi Strantor,

Refer to figure 13 in mosfet.pdf. Read the paragraphs beside it.

A changing drain voltage effects how the gate turns on in a complex way through the capacitance Cgd, which in turn would make it difficult to determine the correct gate drive current from simply knowing the gate cap between gate and drain, or gate and source. Therefore, manufactures also supply a gate charge (nC) number, which they have determined to be a good value to use to calculate gate drive current using this formula: Gate current(Ig) = Gate Charge(Cg) / required switching time(t).

e.g. for t=150nS. Ig = 150nC / 150nS = 1 Amp.

The actual drain current is not part of this calculation.

Regards,
Ifixit

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