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Peculiar MOSFET operation ??

Discussion in 'General Electronics Discussion' started by Pessimist, Jan 5, 2016.

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  1. Pessimist

    Pessimist

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    Jan 5, 2016
    I have a P-channel enhancement mode MOSFET which I am trying to use for high-side power switching. (specifically, the MOSFET is the IRF9Z24N). I have an 18K resistor between gate and source and a switch from gate to 'ground'. There is a 100 ohm load resistor between drain and 'ground'. Applying a positive voltage to the source gives 'normal' operation where if the switch is open-circuit (Vg = Vs, so Vgs = 0), the drain voltage is 0 volts (FET is off), and if the switch is set to ground the gate, the drain voltage equals the source voltage (FET is on). So that's all as expected.
    Where I get confused, is if no voltage is applied to the source terminal, but IS applied across the 100 ohm load instead.
    By my reckoning, the Vgs is still 0 volts and the FET should be off. However, the voltage applied across the load appears at the source terminal implying that the FET must be ON !!
    I've no doubt that there is a rational explanation to this, but it defeats me !!

    Any advice will be welcomed !!
     
  2. duke37

    duke37

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    Jan 9, 2011
    Have you got the fet the right way round? It has a diode in it which will pass current if Source and drain are interchanged.
    Also, the load should be on the drain Not the source as you say.
     
  3. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    He said it's a P channel. In that case the Drain Source orientation would be the same as a PNP when used for high side switching. IE, the Emitter (Source) would be connected to the positive rail and the base (Gate) forward biased by bringing it closer to GND just as he's doing. His load is where it should be ..in the Drain leg.

    You're correct about the internal Diode though. It's connected opposite of an N channel. This puts the Diode's anode on the Drain and the Cathode on the Source. It explains his measurements but not why he did it. :confused:

    Chris

    Duke, never mind. I misread your post. I'm leaving this as is because I don't delete my misteaks! I eat them.:p
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

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    This is called the "body diode". This diode is the reason why two MOSFETs are connected back to back in AC-switching configurations, e.g. solid state relays (example).
     
  5. Pessimist

    Pessimist

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    Jan 5, 2016
    Hello everybody,
    Since my original posting, I have revisited the datasheet and seen that "body diode" - the voltage drop that I measure between drain and source would confirm that this is the path that the current is taking, as the diode is forward biased under this condition.
    Chris, the reason why I'm interested in this situation is that, as part of a larger project, I need a battery backup facility, and am therefore using two FET's with their drains dot-OR'ed. When the main supply fails, the battery FET switches in and raises the voltage at the dot-OR (commoned drains). The problem then arises that the other FET's body diode is then forward biased and allows that FET's source to rise in voltage. This "upsets" the control circuitry upstream of the FET.
    I hope that I've explained this clearly enough ! My current solution is to include an extra diode in series with the source terminal to block this "reverse" condition, but I would prefer NOT to lose the 0.7 volt drop across the external diode. What is the reason for the body diode (or is it an inevitable consequence of P-channel device construction ) ? Are any p-channel devices available without this diode ? (I need a max current spec of about 1 amp and would prefer a TO92 package (although I haven't found any so far at this current)
    Thanks to everybody for their interest !
     
    Last edited: Jan 6, 2016
  6. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Use a second MOSFET back to back, cf. my post #4.

    This diode is not P-channel MOSFET specific. N-channel MOSFETS have the same diode, reverse polarity. Yes, this is a by-product of the manufacturing process.

    I don't know any, see above answer.
     
  7. Pessimist

    Pessimist

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    Jan 5, 2016
    Thanks Harald ! I think that I may have to live with the external diode as I'm a little pressed for physical space. Incidently, DO you know of a TO92 p-channel device that will handle 1 amp (or thereabouts) ?
    Thanks again !
     
  8. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Not by heart, I'd have to search, too.
     
  9. Pessimist

    Pessimist

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    Jan 5, 2016
    OK ! Worth asking nevertheless ! Thanks again !
     
  10. Harald Kapp

    Harald Kapp Moderator Moderator

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    Use a Schottky diode to minimize voltage drop across the diode.
     
  11. Pessimist

    Pessimist

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    Jan 5, 2016
    Good thought ! Thanks for that Harald !
     
  12. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Check out the FDN358P. It's not a TO92 but it is a small SST-3 package.

    Harald, there's always more to learn. I had no idea that the body diode was a manufacturing byproduct. All these years I've been laboring under the mis-impression that it's an integral protection diode that shouldn't be relied upon. :)

    Chris
     
  13. Pessimist

    Pessimist

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    Jan 5, 2016
    I TOO have learnt a lot from this, Chris ! ( I must admit that I haven't a great deal of experience with MOSFET's)
    Thanks for the pointer to FDN358P !
     
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