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Reading MOSFET SOA graph?

Discussion in 'Electronic Basics' started by John, May 17, 2007.

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  1. John

    John Guest

    I need a reality check here. :)

    Am I reading the FBSOA graph (Fig. 11) correctly for this IXYS MOSFET?
    http://ixdev.ixys.com/DataSheet/99022.pdf

    It looks if I wanted to use it as a 100% duty-cycle switch (on for
    seconds to minutes at a time), I'm limited to a Vds of about 25V?
    Even if my current level is low, say 1A? I've never seen the Vds
    rating drop with increasing pulse width before but I've always used
    Intl. Rect. FETs and, so far, each one's Vds max has been the same for
    various pulse widths in their SOA graphs.

    When running a MOSFET as a basic power switch (essentially 100%
    duty-cycle), I've always run it without regard to the SOA as long as
    my Vds was comfortably below the rated max, I keep the junction temp
    low enough, etc. I'm wondering now if that's not a real good idea.

    How can I extrapolate a DC curve for something like an IRF1405?
    http://www.irf.com/product-info/datasheets/data/irf1405.pdf

    Or do I not have to worry worry about 100% duty cycle operation for
    the IRF1405 because, unlike the IXYS FET, the IR FET's Vds rating
    doesn't change with pulse width?

    Thanks!
    John
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  2. Phil Allison

    Phil Allison Guest

    "John"

    ** Only the wise ever imagine this.


    ** Not true at all.

    The graph shows that at 24 volts, a current level up to 30 amp is
    llowable - since that equates to the 720 watts. Such a condition implies
    the device is NOT switched hard on - ie the gate drive voltage is circa
    4 volts instead of the needed 10 volts.

    All the " DC " curve on fig 11 shows is the combined effects of "on"
    resistance and the max allowable dissipation of the device.

    Note the heading says "Forward Bias "

    - not " Saturated Switching".




    ......... Phil
     
  3. John

    John Guest

    Am I reading the FBSOA graph (Fig. 11) correctly for this IXYS MOSFET?

    Aha! So the DC curve is really just a visualization of the Pd rating
    (730W for that FET) for when the FET is operated linearly, plotting
    the Id and Vds that result in the 730W Pd rating.

    If I'm hard-switching the gate (e.g., Vgs = 10V), then those curves
    aren't applicable and I can use the other graphs to make sure I can
    pass the current I need to with the available Vds, how much power I
    have to dissipate (using Id and Rds(on)), etc.

    That explains a lot, thanks Phil.

    John
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  4. John

    John Guest

    The graph shows that at 24 volts, a current level up to 30 amp is

    Is it just me or does the DC curve not really offer very much? With
    the Pd rating, and knowing my Vds and Id, I can easily calculate
    whether I might exceed the device's rating. I guess it can save me a
    few seconds time if the graph is handy.

    But without something like submerged ice-water cooling of the FET and
    its heat sink, I'm never going to get even remotely close to the
    device's 730W rating when operated linearly at 100% duty cycle!

    Might explain why IR doesn't publish DC curves?

    John
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