MOSFET question: Drive voltage vs. Vgs(th)

We (the engineers where I work) never drive MOSFETs at their max threshold, because there would be instances where they might not turn on (temperature, process differences, connecting component variability, etc.). We always "overdrive" them. When possible we overdrive by a factor of 2 (Vgs,th,max * 2), but always by a several volt margin. There would have to be other factors in the circuit design to use the exact Vth-specified.

The "specified" Vth is simply a measuring point to obtain the other component parameters so they, too, can be "specified".

the threshold is simply the critical point of operation and you normally don't want to rely on that.. normally it's good practice to go above that threshold to insure reliable switching. it's best to drive them with an acceptable level well above the threshold. in this case of your choice, i would say 2 volts is good.. i noticed that it will handle up to 8.

V(gate,source) is the external applied drive voltage. The internal die of the MOSFET is seeing that in another way as of the parasitic devices (mostly of the package), e.g. serial impedances of the pins Gate AND Source. Another thing is the gate capacitance, which defines the Vgs "plateau" to charge the internal gate capacitance (composite of C(gate,source), and C(gate,drain) multiplied with linear gain of the MOSFET). In the datasheets of MOSFET you can see this plateau on a diagram. Try to locate design/application notes with "Miller capacitance" or

Hope this clarifies - Henry

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