question : power mosfet turn ON waveforms

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cmos_ajay

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Hi,
Attached is a waveform of a power MOSFET turn on procedure.
* I would like to know why the Cgs ( gate source capacitance) has a constant voltage across it during time interval T2 to T3 ?
* Why does Cgs 'not' charge during T2 to T3 ?
This is called as a Miller plateau region, but the text book explanation is not clear to me.
Can someone explain ??
 

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  • mosfet turn on.jpg
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Miller capacitance is another name for Cgd. Between T2 and T3, all gate current will go into Cgs. This happens under the assumption that the load is drawing a constant current, which is an almost realistic asumption in case of inverter circuits with output inductor.
 

Cgs swing comes from converting the depletion region to
inversion, now you "hook up" the area under the gate
(Cox) to the source via the channel. There is only so
much charge to be put, for a fixed Vgs, and it all happens
before to shortly above threshold.

Then the drain begins to swing, and that high drain
drain voltage across drain-gate capacitances is the
plateau charge. Vgate/RgateDrive = I = Cds*dV/dt,
generally the fixed gate drive creates a linear ramp
on the FET drain, and this is why low output impedance
gate drivers are needed for high frequency & efficiency.
 

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