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When you look at the datasheet for a MOSFET, in the gate charge characteristic you will see a flat, horizontal portion. That is the so-called Miller plateau. When the device switches, the gate voltage is actually clamped to the plateau voltage and stays there until sufficient charge has been added/ removed for the device to switch.
It is useful in estimating the driving requirements, because it tells you the voltage of the plateau and the required charge to switch the device. Thus, you can calculate the actual gate drive resistor, for a given switching time.
The charge injected is:
Qgate=Igate*tsw
But
Igate=(Vcc-Vplateau)/Rgate
where Vcc is the supply voltage of your driver (actually it should be its peak output voltage, but often that is what we use for a quick estimate).
Thus, you can select Rgate to obtain the required tsw, for a given device. Qgate is the difference between the charges at the ends of the plateau.
Look at the picture. The Miller plateau is in the middle. The plateau voltage is about 1.8V, and the required gate charge is 2.2-0.7≈1.5nC.
This is a very fast, low power device. Other devices, like power MOSFETs, have gate charges in the tens and hundreds of nanocoulombs.
If i know the POWER MOSFET dimensions, Gate resistor and Output capacitor value can i calculate the Miller plateau voltage for different values of output capacitor?[/b]
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