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Not able to understand basic gate charge vs Vgs in MOSFET during switching

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VarunKumar89

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Hello,
I have confusion about MOSFET switching on characteristics. Why the voltage Vgs becomes constant as charge goes on increasing. My docs says that Id becomes constant and Vds also becomes constant . But i am not understanding its basics. Please help me

Curve is attached

Regards
 

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When the device is still in its active region, Vgs will be clamped since all the gate current flows through the gate to drain capacitance. When the on state Rds-on value is reached, the gate to source voltage becomes unclamped and continues to rise again.
 
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It's like capacitor charging up. The capacitance between gate and source ( as you may have known that this junction is insulated), starts charging up through the resistor Rgs that you may have attached to maintain Vgs (a zener is usually attached parallel to it too). The time constant of charging is given by the product Cgs* Rgs. As time passes, charge starts to accumulate in the junction and slows down further increase as it does in usual capacitor.
 

When Vgs reaches the point that makes Vds swing, the
Miller current (Cgd*dVds/dt) pushes back into the gate
and if the drain has a large compliance range, plateaus
the gate drive (against its source resistance). So you
need a low driving resistance to make it swing fast, or
you can add resistance to get a controlled slew.

But the nut of it is, Qgg contains both Qgs and Qgd
terms and one voltage is not static.
 

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