Voltage Drop in a PMOSFET?

Discussion in 'General Electronics Chat' started by joe426, Jan 27, 2012.

  1. joe426

    Thread Starter Active Member

    Aug 11, 2009
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    Quick question: If a P-Channel MOSFET is completely open, is there any voltage drop from Source to Drain? Specifically, I want to use FQP27P06 to switch 50v at 25A. When I say "switch" I mean just once when the vehicle is turned on.

    Thanks!
     
  2. crutschow

    Expert

    Mar 14, 2008
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    The maximum current rating of that MOSFET is 27A which is too close to 25A for decent reliability. And it has a maximum on resistance of .07 ohms which would drop 1.8V when conducting 25A and dissipate 45W which would require a large heat sink for the transistor.

    You should derate at least 75%, so find a MOSFET that has at max current rating of 33A or more. The larger the transistor the lower the ON resistance which means lower ON voltage and power dissipation, which also reduces the heat sink requirements.
     
  3. joe426

    Thread Starter Active Member

    Aug 11, 2009
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    0
    Good to know, thank you. I found the FQP47P06 which can handle 47A.

    So, in your last post, you're saying that when the FET is fully "on," there WILL be a voltage drop from source to drain, correct? I looked up the Rds(on) for the 47A one and it's .026 ohms! Less voltage drop! :).

    So, in conclusion, you're saying that ALL fets have this Rds(on) resistance and that there will ALWAYS be a voltage drop from source to drain and that I just need to find ones with higher current to reduce the actual voltage drop and power dissipation, correct?

    I mean, I could go crazy, and get this fet which has an RDS(on) of < .012?
     
  4. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Yes.

    Yeah, it doesn't get much better than this for that voltage/current rating.
     
  5. crutschow

    Expert

    Mar 14, 2008
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    MOSFETS aren't like mechanical relays which may have only a few milliohms of contact resistance. Even with that low MOSFET resistance of 12 milliohms you will still be dissipating 7.5 watts at 25A so be sure to mount the MOSFET on a decent heat-sink.
     
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