Is my math corecct? Please help.

Discussion in 'Math' started by 1213brett, Feb 9, 2015.

  1. 1213brett

    Thread Starter New Member

    Feb 1, 2015
    13
    0
    I'm trying to find out if my mosfet needs a heat sink. my load draw 1.20 A and im triggering the mosfet with ardino pin 7 5V

    P= current squared * RDS
    1.20 A * .16 Ohms squared = .2302

    TJmax - Tambient / ROJA
    150 - 25 / 62 = 2.02

    2.02 is larger then .2302 so I need a heat sink
     
    Last edited: Feb 9, 2015
  2. ScottWang

    Moderator

    Aug 23, 2012
    4,853
    767
    How is your load V/I, do you have a circuit?
    You can choosing some other mosfets has lower Rds then it could reducing the power consumption.
     
  3. Magician

    New Member

    Aug 18, 2014
    13
    6
    No, you don't. MOSFET is o'k up to 2W w/o heatsink, and it's needs to dissipate 0.23W. Are you using PWM? Losses may be more in case PWM
     
  4. 1213brett

    Thread Starter New Member

    Feb 1, 2015
    13
    0
    No I'm not. And thank you!
     
  5. WBahn

    Moderator

    Mar 31, 2012
    17,716
    4,788
    Note that
    150 - 25 / 62 = 149.60

    Remember order of operations!

    If you mean (150 - 25) / 62, then say so.

    Also, use your units and your answer becomes much more informative:

    Pmax = (TJmax - Tambient)/ROJA = (150 °C - 25 °C)/(62 °C/W) = 2.02 W

    Pactual = I²·Rds = (1.2 A)²·(0.16 Ω) = 0.23 W

    Since Pactual < Pmax, no heat sink needed.
     
  6. uwed

    Member

    Mar 16, 2015
    64
    17
    Rds=0.16 ohm is usually normalized (room temperature). If you operate at Tj=150°C we would have Rds=2.2*0.16=0.35ohm (see Fig.4 in the datasheet).
     
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