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Understanding Mosfet modules (parallel and series diode)

Discussion in 'General Electronics Discussion' started by SACM, Nov 6, 2012.

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  1. SACM


    Nov 6, 2012

    I've seen Power MOSFETs modules that may consist of a number of MOSFETs in differents arrangement configurations for specific applications (e.g. motor control or switched mode power supplies). However, i've seen a configuration with a single MOSFET, a series diode and a paralell diode which I don't understand, the diagram is attached.

    This is the Microsemi APTM120U10SAG for SMPS, Motor control and Welding Converters. Mainly, i can`t figure out the function of the series diode as it seems to 'cancel' the intrinsic body diode. Maybe i'm not understanding the behavior of the intrinsic diode when the MOSFET is switching off and there is a reverse current.

    As for the parallel diode, i've seen that there is a configuration for half-bridge for DC-DC converters where there is a parallel diode (not the intrinsic diode) where a high value of current goes through it as seen in the next figure:


    So, does this paralell diode can work as VD3 and VD4 meaning that there is no need of buying these separately?

    Thanks for you help understanding this.

    Attached Files:

  2. duke37


    Jan 9, 2011
    As I understand, the intrinsic diode in the fet is not very good so a fast or low voltage drop diode may be connected across the fet to help.

    The circuit you show seems to be a resonant circuit which could produce reverse voltages across the fet.
  3. SACM


    Nov 6, 2012
    You're right. So, in this case the parallel diode will substitute the intrinsic diode. But what can be the function of the series diode? I really can't see it's pupuose: when the MOSFET is turning off, there will be a reverse current due to the reverse recovery time and charge.

    In the datasheet for this MOSFET, for the series diode, it's 500 nC and 60 ns. If i=(dq/dt), i'm expecting about 8 A of reverse current, which i think isn't desirable for many circuits topologies, right?

  4. duke37


    Jan 9, 2011
    This is getting well outside my knowledge.
    If you are switching a tuned circuit it is possible to get a reverse voltage due to overswing. A series diode will stop reverse current.
    You could of course use a valve (tube) which will not conduct in reverse.:)
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Looking at the datasheet might help.

    I might go looking for any application notes on these devices. If they're anywhere, they should be near here. (can't find any though)
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