MOSFET question: Drive voltage vs. Vgs(th)

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Can anyone tell me the difference between the drive voltage and the
gate-to-source threshold voltage (Vgs,th) of a MOSFET, if any?


I was under the impression that these refer to the same voltage - the
voltage that is needed to turn the MOSFET 'on'.
However, if you look at the following datasheet for a Fairchild
MOSFET...


http://www.fairchildsemi.com/ds/FD/FDG311N.pdf


you'll see that the title says "2.5V Specified", which I understand
refers to the drive voltage. However, under the "On Characteristics,"
you'll see Vgs(th) min. = 0.4V and Vgs(th) max. = 1.5V. Why isn't
Vgs(th) max. = 2.5V, as the title suggests? Is there a difference
between Vgs(th) and the "specified voltage"?


Thanks!

 

[tlbs]

Can anyone tell me the difference between the drive voltage and the
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".

 

[Jamie]

Can anyone tell me the difference between the drive voltage and the
gate-to-source threshold voltage (Vgs,th) of a MOSFET, if any?


I was under the impression that these refer to the same voltage - the
voltage that is needed to turn the MOSFET 'on'.
However, if you look at the following datasheet for a Fairchild
MOSFET...


http://www.fairchildsemi.com/ds/FD/FDG311N.pdf


you'll see that the title says "2.5V Specified", which I understand
refers to the drive voltage. However, under the "On Characteristics,"
you'll see Vgs(th) min. = 0.4V and Vgs(th) max. = 1.5V. Why isn't
Vgs(th) max. = 2.5V, as the title suggests? Is there a difference
between Vgs(th) and the "specified voltage"?


Thanks!

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.

 

[Henry Kiefer]

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 www.wikipedia.org

Hope this clarifies -
Henry