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Mosfet - conneciton question

Discussion in 'Electronic Basics' started by Andrew, Sep 20, 2003.

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  1. Andrew

    Andrew Guest

    For the IRLM6802 which is a micro6 package.

    The chip has 4 drain leads. 1gate, 1source

    I was wondering, is it necessary to connect all of these to the
    output, or can i just connect one and leave the other three open
    circuit.

    I'm guessing connecting all of them to output helps reduce Ron?

    Surely it would still work with just one connected though. How would
    this effect the operation?

    Ta for the input.
     
  2. Gareth

    Gareth Guest

    The main reason for having four leads is probably to reduce the thermal
    resistance from the die to the outside world, so if you only connect one
    lead the MOSFET will not be able to dissipate as much heat, and
    therefore not be able to achieve its rated power.

    The datasheet should give figures for thermal resistance from junction
    to case, and junction to ambient as well as the maximum junction
    temperature. If you know how much power you need to dissipate and the
    maximum ambient temperature you can calculate the maximum junction
    temperature.

    Now repeat the calculation but multiply the thermal resistance by four
    (because you are only using one of four drain leads[1]) and see if you
    are still below the maximum allowed junction temperature.

    Or, if you have some FETs to spare, and prefer experiment to theory,
    connect it up with one drain lead and see how hot it gets.

    [1]The thermal resistance won't actually be 4 times higher since you
    also have the source and gate leads, and the surface area of the
    package, but this gives a worse case estimate.

    Gareth.
     
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