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How can I compare MOSFET transconductance specs when test currents are different?

Discussion in 'Electronic Basics' started by John, Jul 4, 2007.

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  1. John

    John Guest

    I was looking at the specs for two MOSFETs, the IRFP2907 and
    IRFP90N20D, and was wondering how I could compare their

    The IRFP2907 is spec'd at 130S at 25Vds, 125A

    The IRFP90N20D is spec'd at 39S at 50Vds, 56A

    How can I tell which one has the lower transconductance in the 0A-25A
    range at 0Vds-55Vds?

    It seems that I'd use one of the drain-source voltage vs. drain
    current curves for the Vgs I'd be using? I could derive a couple of
    di/dv numbers to get the transconductance? But, those curves are such
    low resolution that the numbers aren't very useful.

    And since IR's datasheets don't have transconductance vs. drain
    current curves, how can I find out the lower drain current
    transconductance values for these FETs?

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  2. leo123

    leo123 Guest

  3. John Larkin

    John Larkin Guest

    Look at the transfer curve, the curve of Id versus Vg. The derivative
    (slope) of this curve is the transconductance.

  4. John

    John Guest

    Look at the transfer curve, the curve of Id versus Vg. The derivative
    OK, cool....
    To get the most accurate data, I'm guessing I should try to find the
    smallest "slice" of the transfer curve (in the region for my target
    drain current) when calculating the slope.

    IR has some really lousy, low-res graphics for their transfer curves
    though. <grrrrr> Why can't they just post the transconductance curves
    like so many other companies. :)

    Thanks again for your help John!

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