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  1. #1
    Advanced Member level 5
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    Sep 2008
    473 / 473

    Failure rate for FET at 125degC junction temperature

    We are using the IPP90R1K2C3 FET in our offline SMPS.
    Its junction temperature is 105 degC.

    We wish to know how long it will last.

    Infineon sent us the equations which we need to use to calculate the probable number of failures that will occur over a given amount of time.
    This document is attached.

    Do you know what we should put in the equation for the “confidence level, PA”.
    Presumably the equation supplied doesn’t tell us what failure rate we’ll get, but just gives us parameters which we then insert in to the chi^2 distribution?

    IPP90R1K2C3 FET

    All we want is a figure of how many failures we’ll get over an inserted period of time for a quantity of n FETs under investigation.
    Am I right in saying that a full knowledge of the chi^2 distribution is going to be needed here, and a dusting off of the old probability books is going to be required? I remember doing the “normal” distribution, but its all many years ago now.

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  2. #2
    Advanced Member level 5
    Points: 38,213, Level: 47

    Join Date
    Mar 2008
    1792 / 1792

    Re: Failure rate for FET at 125degC junction temperature

    You can't connect -a- transistor's service life to a
    -population's- statistics directly. You can only get
    a "confidence interval" (range in which the failure
    will probably fall, the confidence level widens that
    the more confidence you demand). Without peering
    into the HTOL qual statistics (and hopefully this
    includes ongoing product surveillance, not just the
    first two lots out the pipe and believe nothing ever
    changes subsequently) you won't know how much
    wearout life varies lot-lot, quarter to quarter. And
    it must - how much, is the question.

    You also would not want to advertise the 125C life
    without couching it in terms of a worst case, and
    might like to also put out a "use model based" life
    (how many systems truly spend their entire service
    life at max rated temp? Usually there are swings in
    ambient temp and power dissipation, and you need
    a sophisticated analysis to get within a factor of 10
    if your temp swing is 30C (/2 per 10C is a rule of
    thumb, but who knows where that thumb has been?).

    If you were a significant customer, putting this to the
    vendor's applications engineering org with a request
    for a reliability assessment of you r use case, might
    get some attention. But not if you are trying to buy
    cheapo commercial and apply outside the datasheet
    limits, instead of paying the industrial or automotive
    grade price.

    I tend to see 90% or 95% in HiRel applications, but
    that's for stuff that sees 100% burnin and lot level
    HTOL sample qual. If you're buying floor sweepings
    by the sackful, might want a 110% confidence. Heh.

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