mosfet gate voltage.

Crazymechanic

A simple and quick question please clear this up.
Let's say I have a 350v rated power n channel mosfet , how much of positive voltage do i have to apply to the gate to fully open the mosfet?

I know this varies from type of transistor and so but the average?
i am just getting mixed up some things like the voltage drop let's say i need to apply 20 volts does that mean that on the Drain there will be my let's say 300v dc and then on the gate i should apply like 290 or something or i just should apply 20volts.
I guess I am saying is the gate voltage measured from the drain reference or from zero volts reference?

Thanks.

Enthalpy

The gate voltage is measured versus the source.

How much depends on each transistor and is given by the datasheet; 15V gate-to-source is a common value, and destruction at 30V an other common value.