MOSFET power dissipation

Discussion in 'General Electronics Chat' started by JawadWalker, Feb 19, 2016.

  1. JawadWalker

    Thread Starter New Member

    Feb 19, 2016
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    Hi, I'm making a brushless motor driver like http://ww1.microchip.com/downloads/en/AppNotes/01175A.pdf (page 2). I need to be able to control 44.4 volts, 80 amps. I chose http://www.infineon.com/dgdl/IPB019...e0e77&fileId=db3a30431ddc9372011e2600a49e467b MOSFET. Here are my calculations.
    I=80 amps
    R=0.0019 ohms
    P=I^2*R=12.16 watts
    That MOSFET has a maximum power dissipation of 250 watts. I tried to calculate the temperature (probably doing it wrong though).
    Temperature=room temperature+thermal resistance*P=20+62*12.16=773.92 degrees celsius
    That is extremely high. Am I calculating it right, or do I need a different MOSFET/multiple MOSFETs in parallel. I will be mounting 6*number of MOSFETS in parallel (1 right now) on an aluminium square tube that has a volume of 76 square centimeters (I may need a longer one, meaning more volume), so I guess you could halve the temperature (just a guess) because it will act as a heat sink. Thanjs,
     
  2. Bordodynov

    Active Member

    May 20, 2015
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    You calculated the temperature of the transistor die without a radiator. In this case, the transistor may dissipate about 2 watts.
    Put the radiator and you will be only one transistor.
     
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  3. JawadWalker

    Thread Starter New Member

    Feb 19, 2016
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    What's a radiator?
     
  4. Bordodynov

    Active Member

    May 20, 2015
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    Excuse me. This heatsink. 76 square centimeters quite enough.
     
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  5. JawadWalker

    Thread Starter New Member

    Feb 19, 2016
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    76 cubic centimeters sorry. So I just have to use six (the circuit uses six in a H-bridge-ish way) of those mosfets? No mosfets in parallel needed? Can I use ones with lower amp rating (because there are six of them and I don't know if the whole 80 amps is split into six) (they are cheaper)?
     
  6. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    The RthJA in the datasheet is without any heatsink (62 K/W) or with extra copper area on the PCB (40 K/W)

    To make it fairly simple you need to know the thermal resistance of your "heatsink (radiator)"
    The hardest part about using a chunk of metal vs a specified/tested heatsink is finding out the thermal resistance..

    If its 3 deg C/W (its not) then you are looking at something like a
    3+.62 = 4 deg C/W (added some for thermal compound resistance)
    Then thats Trise = 12.16 x 4 = 50 deg C junction temp rise over ambient..

    Some good beginner reading for you..
    http://sound.westhost.com/heatsinks.htm
     
  7. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The package looks like a Dpack.

    Mosfet_package.png

    How would you attach the heatsink?

    Bertus
     
  8. JawadWalker

    Thread Starter New Member

    Feb 19, 2016
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    I would probably clamp it onto the metal with some thermal paste in between. Or I could use a different TO-220 MOSFET that I can screw on. Can I use a say 30 amp or even 15 amp MOSFET because it has three pairs of one p and one n channel MOSFETs.
     
  9. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Maybe you could tell us what you are trying to do. 80 amps at 44 volts is like a golf cart motor. Is it something like that?
    I haven't studied the schematic, but the way the gates are driven the gate to source voltage would be 22 volts, but the fets are only rated for 20. It looks like that design is for a much smaller motor.
     
  10. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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  11. JawadWalker

    Thread Starter New Member

    Feb 19, 2016
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  12. matzeri

    New Member

    Feb 15, 2007
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    You are missing some points, and you are misleaded by datasheet cosmetic numbers:
    - Use graph 9 of datasheet. Assume Tj ~ 140 C Rdson = 3. mOhm max
    - Conduction losses of system = 3 mOhm * 80A^2 *2 = 38W ( in a brushless the current pass in 2 mosfets at any time)
    - Include switching losses, plus conduction losses on the trace : Total Losses ~ 50W (can be more)
    - Heatsink needed assuming ambient 45C and board 125C Rth-case_to_amb = (125-45)/50 = 1.6 C/W
    so you need a big hetsink and you need a good thermal contact board to heatsink.
    A fan will help to dissipate the power.
    If the ambient is hotter eg 85C you need (125-85)/50 = 0.8 C/W


    You can consider to use mosfets in parallel to reduce the power losses, but driving them will became more tricky
     
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