MOSFET as a low-drop power-diode

Discussion in 'General Electronics Chat' started by Robin66, Jan 19, 2016.

  1. Robin66

    Thread Starter Member

    Jan 5, 2016
    102
    3
    I have a low voltage, high peak power device that I want to protect from reverse polarity. I was going to bias a MOSFET in the usual direction (as in pic) but discussions online suggest biasing so that the body diode (not shown) is forward biased. Can anyone give any indication of the merits of the alternative? To keep my pic simple I tidied the Gate to +Vcc, but in reality I'll have a ~10V source available to ensure the FET is hard on and can handle 30A+ with ~0.3V drop across D-S.
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,016
    3,235
    If you look at the direction of the body diode you will see why the circuit won't work as you drew.
    Connecting the N-MOSFET with source to ground will allow a reverse voltage to forward bias the body diode and thus appear across the power device.
    That's why you need to connect the transistor with the drain to ground.
    A MOSFET conducts equally well in either direction when ON, but with the drain to ground, the body diode is now reversed biased with the application of a reverse voltage thus blocking the reverse voltage to the device.
     
    #12 likes this.
  3. Robin66

    Thread Starter Member

    Jan 5, 2016
    102
    3
  4. ramancini8

    Member

    Jul 18, 2012
    442
    118
    Sometimes yes and sometimes no. Rds on can be different for different transistors when reversed.
     
    #12 likes this.
  5. crutschow

    Expert

    Mar 14, 2008
    13,016
    3,235
    It basically follows the same Ids versus Vgs curve below the point where the substrate diode starts to significantly turn on (below a Vds of about 0.6V).
    Above that point the transistor can't block the voltage, of course, since the diode starts conducting.
     
    Last edited: Jan 19, 2016
  6. crutschow

    Expert

    Mar 14, 2008
    13,016
    3,235
    Okay, you dropped a teaser. :rolleyes:
    What types of FETs don't have the same Rds(on) in either direction when fully on?
     
Loading...