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Stuck on MOSFETS

  1. Nov 18, 2015 #1
    I am new to using MOSFET transistors as load switches. Here, I am trying to trigger the P-channel MOSFET to run a current through the 180 ohm resistor. I can not figure out what I am doing wrong. It is supposed to turn on for a small moment (at the zero-crossing moment of the AC phase), then turn back off. Opto with MOSFET.jpg
     
  2. jcsd
  3. Nov 18, 2015 #2

    Svein

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    Your circuit turns on the PMOS at all times except for a small moment (at the zero-crossing moment of the AC phase).
     
  4. Nov 18, 2015 #3
    I know it does, but why is it doing that? It is a p-channel MOSFET, why doesn't it turn on when a positive voltage is present? Also, when it does turn on, it has a very small current, (240uA in this LTSpice simulation).
     
  5. Nov 18, 2015 #4

    Svein

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    A p-channel MOSFET turns on when the gate is negative relative to the source. You need to invert the driving signal.
     
  6. Nov 18, 2015 #5
    OK, that's confusing. My next question is, why is the current so small? What do I have to do to increase the current?
     
  7. Nov 18, 2015 #6

    Svein

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    I do not think the optocoupler is able to drive the PMOS gate as far down as necessary. You need to amplify and invert the output of the optocoupler.
     
  8. Nov 18, 2015 #7
    Hmmm, I tied the gate directly to ground, and that didn't change the current through the 180 ohm resistor. I finally get the proper current (27.8mA) when I set the gate to negative 1 million volts!
     
  9. Nov 19, 2015 #8

    Svein

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  10. Nov 19, 2015 #9

    meBigGuy

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    It could be the gate threshold for the PMOS is too high. Try setting V2 to 10V and ground the gate.
     
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