Mosfet switch question

Discussion in 'General Electronics Chat' started by curry87, May 30, 2010.

  1. curry87

    Thread Starter Member

    May 30, 2010
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    Hi i have a STP36NF06 MOSFET N 60V 30A which i wish to use to switch a headlight on and off but is getting very hot even though the drain current is within limits .

    Atm the circuit consists of a pull down resistor 22k on the gate to ground and 4.5v being applied to the gate pin.The drain-source part is 12v + to the bulb(5w) then to the drain pin and finally the source is connected to ground.The Mosfet switches the buld on but gets very hot after a few seconds why could this be ?


    Also a few questions:

    How do i know when the Mosfet is fully open can i apply a varied voltage to the gate and measure the resistance between drain and source ?

    Can i have a different voltage on the gate-source to the drain-source ?
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    A FET is fully on when the gate is at least 10 volts above the source. It's in the ohmic region when that voltage is only 4.5.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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  4. curry87

    Thread Starter Member

    May 30, 2010
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    What does gate- source voltage ± 20 mean ?

    Do Mosfets get hot to the touch anyway during normal operation with heavy loads on the drain and source or is mainly because the Fet isiant driven enough on its gate?
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Vgs means the voltage on the gate, using the source terminal as the reference point. If Vgs > 20v or Vgs < -20v, the MOSFET will be destroyed.

    They get hot if they do not have adequate heat sinking to remove the heat generated.

    Let's say that your headlamps are rated for 60 Watts each.
    That is 60W/14v = 4.286 Amperes.

    If your MOSFET is fully turned on, it's Rds(on) is less than 40 milliOhms. The power dissipated in the MOSFET should be a bit less than 172mW; you would not need a heat sink. However, you are only raising the MOSFET gate to 4.5v, so it will get VERY hot.
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
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    Those are the limits of applied voltage. Beyond that, the device will be destroyed.

    They get hot when the device is not completely in conduction (not enough gate voltage).
     
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