MOSFETs with built in reverse current diodes

Discussion in 'General Electronics Chat' started by stube40, Feb 1, 2011.

  1. stube40

    Thread Starter Member

    Feb 3, 2010
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    Alot of MOSFETs come with a built-in diode across the source and drain. The attached diagram outlines what I mean. I would like to confirm that the purpose of this diode is to allow the MOSFET to handle reverse current to flow.

    I know this question might sound really dumb, but I'm not an electronics expert.

    Further, if this is indeed the case, is it possible to exploit this diode to enable the MOSFET to allow reverse current in an application where we expect regular reverse currents?

    Finally, which particular parameters in the data sheet for a MOSFET describe the diode's maximum reverse current and how long it can maintain this current for?
     
  2. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    The body diode is "built in" to the MOSFET structure. It is often drawn as a Zener diode to indicate the reverse breakdown voltage of the MOSFET.

    Is is the datasheet parameter for the forward current.
    Vsd is the drop of the body diode
    Trr, Qrr and Ton are other aspects of the body diode for turn on time.

    Example Datasheet for IRF540 A common N-Channel MOSFET explains the different parameters fairly well.

    The body diode doesn't turn on as fast as an external Schottky diode, so when driving an inductive loads, Schottky diodes are added across the source and drain to prevent reverse breakdown.
     
  3. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    Reverse conduction in the MOSFET channel is also possible. This effect can be utilised, e.g. when MOSFETS are used as rectifiers.

    The FET can block current in the normal forward direction if biased OFF. In the reverse current direction the body diode would tend to conduct, but if the FETs are biased ON during the conducting phase the channel also conducts. Provided that the product of Rds(on) and the current is small enough, little of the current will pass in the body diode, and a lower voltage drop will be obtained.

    Edit: I think my assignment of "forward" and "reverse" directions may cause confusion. I was referring to the FET. What is reverse conduction for the FET is forward conduction for the body diode.
     
    Last edited: Feb 1, 2011
  4. Jaguarjoe

    Active Member

    Apr 7, 2010
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    Actually, the MOSFET has a body transistor that is a byproduct of the MOSFET's design. The B-E junction of that transistor is shorted during manufacture so all that is left is the body diode.
     
  5. garydeck

    New Member

    Feb 17, 2011
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    What was the problem you found with the Hbridge circuit you were working on in April of 2010? Were you able to get the circuit working?
     
  6. JMac3108

    Active Member

    Aug 16, 2010
    349
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    Stube44,

    Everyone else answered you question correctly, but to summarize...

    No, the purpose of the diode is NOT to allow reverse conduction.

    Mosfets conduct in current in both directions anyway.

    The diode is intrinsic to the construction of the mosfet. In other words its just there, and you need to be aware of it ensure that it does not affect your circuit.
     
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