How many mA to switch on a n channel Mosfet.

Hi there.

The voltage can be 20V to switch the gate on but how do we calculate the mA needed. If you have to switch on ten IRF 510s it could be important.

Thanks.

I know this is a stupide question but does it not say in the Dsheet?

it depends on the charging current of 'mos gate capacitor'.
must give sufficient value and at a fast rate as dictated by the switching frequency.

srizbf
4thmay2010

Bazoid said:

I know this is a stupide question but does it not say in the Dsheet?

Click to expand...

Of course not. Their is no fixed gate current required to switch a MOSFET on. The gate is a capacitance so the current required depends on how quickly you want to turn the MOSFET on. That depends on what the circuit is doing. Some more information would result in a more specific answer.

Keith

Hello,

A mosfet is a voltage driven device, the voltage between gate and source determines whether it conducts or not. To compare: a bipolar junction transistor is a current driven device.

There is capacitance inside the mosfet (one between gate and source and a smaller one between drain and gate). To get voltage across gate source, you need to charge that capacitance. The charge required is mostly given in the datasheet. They also mention the capacitances (as function of voltage).

As with a normal capacitor, when you want to charge it rapidly (so high dV/dt), you need to apply a large current pulse for a short time. It is not strange to apply Amps during some ns.

When you want to make the voltage zero (mosfet off), you need to remove the charge (that is charge from Cgs and charge from Cgd).

The so-called gate leakage current is generally very low, typically nA range.

Just for a while look at switching of 10 gates of power mosfets from different perspective ..
Most of integrated drivers can give you at max 6A pulse current, so if you try to drive ten gates it means (roughly) 600mA current pulse per mosfet ..
To ensure that the driver is not overloaded you add resistors in series with each gate ..
For example, if the voltage used for switching is 20V then the combined resistance (@6A) is 3.3Ω and that means 33Ω gate resistor per mosfet ..

What I wanted to show you is that not only you have to estimate with what gate current you can switch mosfets but how are you going to do it in practice ..
In some applications people use only one driver per mosfet – you can consider driving several gates out of one driver, all depends on your design requirements ..

IanP

Right on.


You can't get any clearer than that.