Does current flow to gate of mosfet to charge it ?? Or is it just the Electric field?

[shabazsyed]

Does current flow to gate of mosfet to charge it ?? Or is it just the Electric field?

 

[KlausST]

Hi,

There are two gate capacitors inside a FET.
One is from gate to source. It often is the larger one. This capacitor needs to be charged and discharged like an ordinary capacitor.
The ammount of charge is: Ugate x Cgs

The other - usually smaller one - if from gate to drain. On first sight one may imagine that the smaller capacitance may cause smaller ammount of gate charge charge. This is true as long as the voltage on drain is constant. But using a FET as a switch causes the drain voltage to change.
Imagine a FET as low side switch for a 24V valve. During switching the voltage on drain falls from 24V down to 0V. This charge (voltage multiplied with capacitance) you need to add to the usual calculation Ugate x Cgd. Now you get total gate charge:

Qt = Ugate x Cgs + Ugate x Cgd + Uds x Cgd.

The additional gate charge is exately in the time where the drain voltage moves (switching). And this slows down switching. Switching time increases and in the same ammount switching loss increases.

The additional gate charge is called "Miller effect". It is a main parameter with switching FETs, especially with high voltage switching.

Klaus

 

[juicedry]

Yes. The MOSFET gate is like a capacitor and when the MOSFET is off it has no charge on the gate. To turn it on, you apply a voltage between gate-source and this charges the gate capacitance. Current flows into gate capacitor until it has reached the voltage potential. This enhances the channel and turns on the MOSFET. When turning on and off current flows in and out of the MOSFET gate capacitance. In general, a lower Rds(on) MOSFET has a higher gate capacitance and it takes more charge to turn on and off.

This is in reference to the more common enhancement mode MOSFET. Depletion mode MOSFETs you need to apply a voltage to turn them off!