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LTSPICE bug?

Discussion in 'General Electronics Chat' started by Mosaic, Nov 2, 2017.

  1. Mosaic

    Mosaic Well-Known Member

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  2. alec_t

    alec_t Well-Known Member Most Helpful Member

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    The body diode shown passes current regardless of the gate voltage. Reverse the FET if you need to shut it off (but then the circuit won't function as per the linked article).
     
    Last edited: Nov 2, 2017
  3. Daniel Wood

    Daniel Wood Member

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    Looks fine to me..
    Its probably best to use a "real" device instead of the default unassigned PFET. I've had issues simulating with generic transistors and fets.
     

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  4. dave

    Dave New Member

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  5. crutschow

    crutschow Well-Known Member Most Helpful Member

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    The simulation is fine.
    The bug is in your test.
    The circuit is not designed to turn off under gate control.
    It's designed to block a reverse voltage to the load, as Daniel's simulation shows.
     
  6. Mosaic

    Mosaic Well-Known Member

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    Ok, I see, in order to achieve gate control I'd need back to back PFets, one for polarity blocking and one for gate. Cuz of the body diode, not shown in the LTspice schematic.
     
  7. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    P.png
     
  8. Mosaic

    Mosaic Well-Known Member

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    That indicates if it is a PFET or an NFET... that's not the body diode dude. The body diode sits across the drain/source.

    But then maybe I'm wrong about that.
     
  9. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    The multitasking arrow also indicates the direction that the device conducts when the Voltage across the Drain-Source is reversed biased (compared to normal NMOS or PMOS polarities), so the arrow implies current flow from Drain to Source when the drain is positive and source is negative in a PMOS.
     
  10. throbscottle

    throbscottle Well-Known Member

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    That's a really good tip about the arrow, Mike! I'd finally settled on "substrate connected to source" as my mnemonic for remembering which way up these symbols go ('cuz they both begins with "S"), but looking at it as the body diode is much better! Thanks :)
     

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