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MOSFET question: Drive voltage vs. Vgs(th)

N

Navraj

Jan 1, 1970
0
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!
 
T

Tim Williams

Jan 1, 1970
0
Navraj said:
Can anyone tell me the difference between the drive voltage and the
gate-to-source threshold voltage (Vgs,th) of a MOSFET, if any?

Threshold is where it begins to conduct. This varies a hell of a lot
between FETs so a much better idea is to "turn on" to a few extra volts,
which also affords you a lower ON resistance if you'll inspect the Ids vs.
Vds, Vgs graph (if printed), or R(on) vs. Vg graph.

Tim
 
J

John Larkin

Jan 1, 1970
0
Can anyone tell me the difference between the drive voltage and the
gate-to-source threshold voltage (Vgs,th) of a MOSFET, if any?

Threshold is the voltage where it barely begins to conduct. Drive is
the voltage you elect to use in a given situation. If you want it to
conduct hard, drive it way past threshold.

John
 
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