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Power FETs

Discussion in 'General Electronics Discussion' started by Aggiemundo, Oct 14, 2008.

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  1. Aggiemundo


    Feb 7, 2008
    Reading through the datasheets of most Power FETs I see that most are rated to be capable of max values such as 60V and 27.5A with a maximum power dissipation of 120W or so. (see OnSemi datasheet for such a part:

    I have an application where I'd like to pass a voltage between 6V to 24V at currents up to 15A through the FET. Gate voltage would be 10V. My calcs from the datasheet (assuming Rdson of 65mohms) tell me that I would dissipate 14.625W across the package at this current (well under 120W), but with a thermal resistance of ~40C/W I'd see junction temps of ~300C!!!

    It seems odd that a part rated to handle 27.5A would never be able to get even close to that. I guess those current ratings are assuming an infinite heatsink. Am I missing something, or should I just assume that FETs aren't capable of such high currents?
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