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small high frequency mosfets

Discussion in 'Electronic Design' started by mook johnson, Oct 22, 2012.

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  1. mook johnson

    mook johnson Guest


    I'm looking for a high sped, low capacitance mosfet in a package the
    size of a to-92 or smaller than a SOIC-8 with the following characteristics.


    Attached to a 1:1 push pull transformer with 5V on the CT to generate ~
    10V p-p on the output. This means the switching transistors will see at
    least 10V when the other one is one + leakage inductance spike.

    Each transistor will turn on between 25 and 50nS each. The peek current
    during each pulse will be ~ 300mA

    When both transistors are off, it is desired that the reflected
    impedance from the secondary is high hence the reason for low
    capacitance mosfets.

    The MOSFETS I've seen that fit this description usually have very
    voltage. (Vds ~7V)

    Wondering if you guys know of a higher voltage "RF" mosfet for an
    application like this one.

  2. mook johnson

    mook johnson Guest


    If you know of a few more. I'm all ears.
  3. Tim Williams

    Tim Williams Guest

    What kind of transformer? Most circuits like that, I'm running, say,
    200kHz (2500ns turn-on is a lot more than 25-50, of course), and the
    transformer probably has 20-30 turns of fine wire on it, having about as
    much capacitance as the transistors themselves.

    As far as creative approaches, I suppose J105 is still an option, but
    using JFETs for switching is probably suboptimal, and not common at least.

    2N4401 can be made to switch that fast. Use a small base-emitter
    resistor, and a fair amount of drive (or better still, active base drive
    in both directions). That'll have lower saturation and capacitance, but
    you make up for it in switching time and losses.

    Otherwise, like John said, 2N7000 (or 2N7002, or BS170, or a zillion
    others) is hard to beat for generic purposes. There are a few similar
    parts in the same range, 0.5-2A, 30-60V, "logic" level, Digikey or Mouser
    can help find them. "Complement" is BSS84 or so.

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