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MOSFET question: Drive voltage vs. Vgs(th)

Discussion in 'Electronic Basics' started by Navraj, Nov 15, 2005.

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  1. Navraj

    Navraj Guest

    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!
     
  2. Tim Williams

    Tim Williams Guest

    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
     
  3. John Larkin

    John Larkin Guest

    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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