Gate capacitance

Thread Starter

Jaguarjoe

Joined Apr 7, 2010
767
I read somewhere that the gate capacitance between the gate and the drain is more critical than the capacitance between the gate and the source. Why is that so, or isn't it? How can you affect one without the other, there's only one value for gate capacitance (or charge)
 

mik3

Joined Feb 4, 2008
4,843
The gate charge is the charge required to charge both the gate-drain capacitance and the gate-source capacitance.
 

Thread Starter

Jaguarjoe

Joined Apr 7, 2010
767
I found it. Cdg is important because it is the Miller capacitance. It is feedback from the drain to the gate, like C-B in a BJT and some vacuum tubes had/have. I think that's one reason for screen grids.
 
Last edited:

timrobbins

Joined Aug 29, 2009
318
Cdg can also contribute to gate failure from overvoltage due to transient voltage on the drain (if the gate-source shunt resistance is too high). Depending on fet technology, Cdg can be quite non-linear, and a significant contributor to Coss (which may be a power loss mechanism in certain switchmode topologies).
 
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