hey crutschow, is this a dumb idea?

Discussion in 'Digital Circuit Design' started by joewales44, Nov 25, 2017.

  1. joewales44

    Thread Starter Member

    Oct 8, 2017
    143
    4
    is there a way to replace the schottky diode on the switch high side with a mosfet?
    it seems this would eliminate the schottky voltage drop so i wouldn't need to reverse the M1 pfet action.
    maybe i don't have it all connected right but i thought it would give you my general idea.
    does it matter which way a mosfet is connected as long as i take into account the body diode direction?
    otherwise, will they conduct in either direction?
    thanks
    upload_2017-11-25_21-13-19.png
     
  2. crutschow

    Expert

    Mar 14, 2008
    20,067
    5,651
    Unfortunately that circuit will always conduct in either direction, independent of the MOSFET direction since one side of the MOSFET is always more positive than the gate, which is at ground.
    The side that's more positive acts as the source terminal, so the MOSFET will never turn off.

    One way to use a MOSFET as an ideal diode is the circuit below, which uses a dual transistor to turn on the MOSFET when the circuit flows in one direction (same as the substrate diode) and turns it off when current tries to flow in the other direction.
    Its operation is described here.
    (Note the circuit shows two separate transistors but it requires a matched (but inexpensive) dual-transistor in a single package for proper operation.)

    Edited to show high voltage version of circuit.

    upload_2017-11-26_10-1-25.png
     
    Last edited: Nov 26, 2017
    be80be likes this.
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