MOSFET Switch Thermal Considerations...

Discussion in 'General Electronics Chat' started by liquidair, Nov 10, 2011.

  1. liquidair

    Thread Starter Active Member

    Oct 1, 2009
    89
    5
    Hi guys-

    Using MOSFETs as a switch, I'd just like to make sure my thinking is correct so I don't blow anything up. I am switching off and on relays, and the MOSFET will never see more than 500mA. I want to make sure I protect the part thermally but I don't see how the MOSFET will actually dissapate any power other than I^2 * Rds(on). Am I missing something and is a heatsink advisable if this stays well below 1W?

    Thank you in advance!

    Brian
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Why don't you post a schematic of your proposed circuit?

    It will help a good deal if you include part numbers.

    This page is well worth the time spent reading:
    http://mcmanis.com/chuck/robotics/projects/esc2/FET-power.html

    Don't know what you're planning on using for MOSFETs, but check out these 4-pin DIP's rated for 2.4A, 60V, Rds(on)=100m @ 5v when Id=1.5a
    [eta]
    Forgot the link! here it is:
    http://search.digikey.com/us/en/products/IRLD024PBF/IRLD024PBF-ND/812492
    Datasheet:
    http://www.vishay.com/docs/91308/sihld24.pdf
     
    Last edited: Nov 10, 2011
  3. liquidair

    Thread Starter Active Member

    Oct 1, 2009
    89
    5
    I thought I posted a reply, but it never showed up.

    The switching circuit is super generic like that found here under P Channel MOSFET switch:
    http://www.electronics-tutorials.ws/transistor/tran_7.html

    The MOSFET I am looking at is here:
    http://www.fairchildsemi.com/ds/FQ/FQD11P06.pdf

    I have no perticular fondness for this MOSFET just that it is IPAK. I read and did the calculations on the link you provided and it looks like I can switch 1A without issue (assuming a 40 deg. C ambient temp, which I doubt it will see) without a heatsink. Great link Sgt Wookie!
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    OK; I don't know what your supply voltage is. If it's <25v, you can probably get away with that switching scheme. If you're closer to 30v than that, you will really need some way to limit Vdss so that it doesn't approach ±30V; as the MOSFET will be destroyed if that happens.

    I don't know for certain how you plan on switching the MOSFET on and off. The gate must never be allowed to "float" (that is, not connected to anything) even for a short period of time. If you are going to be using a hardware switch, then connect a 10k resistor from the source terminal to the gate. This will keep the MOSFET turned off by default.
     
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