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N channel mosfet confusion

Discussion in 'General Electronics Discussion' started by shumifan50, Jul 11, 2014.

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  1. shumifan50


    Jan 16, 2014
    I have a circuit that is powered from 2 plugs from the same battery. The one plug powers the logic including a micro controller. The ground of the other plug gets switched by a N=channel mosfet. The only PCB connection between the logic and the mosfet is the gate. When both plugs are connecyed the ground is connected externally through the battery (this I am sure is the problem). The logic switches the mosfet fine while both plugs are connected. If I unplug the power to the logic, which also means there is no longer a resistor holding the gate to ground, the mosfet slowly starts turning on until it is fully on, still with nothing connected to the gate. It is like the voltage on the gate is slowly creaping to the source voltage.
    The problem is easily fixed by either connecting the logic ground to the ground of the mofet, or place a pull-down resistor on the mosfet to hold the gate low when the logic power is removed.
    My confusion is this: As the transition is quite quick from off to full on, it seems there is quite a bit of current flowing causing the gate voltage rise. The source is connected to 12V.

    Does this mean that if I have a pull down resistor on the gate, there will always be current flowing through the mosfet?

    Note: MOSFET is ST95N2LH5
    Last edited: Jul 11, 2014
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    You can't control a mosfet with only a single wire connected between the control circuit and the mosfet.. If this is due to the ground lead becoming disconnected then, yes, this will cause problems.

    A resistor between gate and source may eliminate the problem by keeping the mosfet turned off when it would otherwise float (leaving it floating is a bad thing).
  3. shumifan50


    Jan 16, 2014
    Thanks Steve. I appreciate that, it was unintended.

    However, the question is really: does that mean there will always be current flowing, even when the gate is held low? Is that the Igss in the data sheet?
  4. BobK


    Jan 5, 2010
    The gate charge for MOSFETS is measured in nanocoulombs. Which means that nanoAmps will charge it in one second. I don't think you need to worry about the current.

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