no body diode mosfet

Discussion in 'General Electronics Chat' started by ElectronicMotor, Oct 16, 2016.

  1. ElectronicMotor

    Thread Starter Member

    May 1, 2016
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    full h.jpg

    I need a MOSFET without a body diode to switch the flyback current on a full h bridge without using the bog standard flyback diode but the body diode will short on the other phases
     
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    I don´t think that such a thing exists. You could use two mosfets back to back but you will have more trouble driving them.
     
  3. BR-549

    Well-Known Member

    Sep 22, 2013
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  4. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    The body diode is an "artifact" of the mosfet manufacturing process, not something that can be removed. If you need something like a mosfet, without the body diode, maybe IGBT is what your looking for. Same driver as a mosfet is used with an IGBT.
     
    Last edited: Oct 16, 2016
    cmartinez likes this.
  5. ScottWang

    Moderator

    Aug 23, 2012
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    The internal structure of IGBT as two BJT combined with a mosfet, so it still has the problem of body diode, isn't it?
     
  6. shortbus

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    But in that way of looking at an IGBT, isn't that "body diode" on the gate and not the collector and emitter? In the case of a mosfet the "body diode" is on the drain and source, in the path of the power flow through the device. Or at least that's how I understand it. Been wrong many times though.:)
     
  7. ScottWang

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    Aug 23, 2012
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    If the current not draw that much and still want to use IGBT then it's too expensive, if doesn't care about the price then using IGBT seems no problem for body diode, because the leader in the IGBT is bjt not the mosfet ... :D

    [​IMG]

    The source of circuit.
     
  8. kubeek

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    shortbus and ScottWang like this.
  9. shortbus

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    IGBT's and mosfets in the same "package", are not that different in price.
     
  10. ScottWang

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    Aug 23, 2012
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    What I mean is two components, the one is mosfet and another is IGBT, and the price of IGBT is more expensive.
     
  11. shortbus

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    Thats what I meant too. When I said "package" I didn't mean two devices in one "package". I meant the type of "package", like "D-Pac", "TO-220", etc. Is price really that different, when doing a one off? Maybe for a commercial product, but not for a personal use thing.
     
  12. fuk

    New Member

    Oct 19, 2016
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    Sorry I've taken so long to reply to my thread
    I have heard of a four terminal MOSFET where the gate drain source substrate are brought out on separate pins
    Normally the substrate is connected to the source for grounding and I think this is where the body diode comes from
    I was hoping the diode could be eliminated using a four pin mosfet and leaving the substrate disconnected
    I need to do this because I think a MOSFET is better at driving high current electronically commutated motors
    The diode one normally uses across the coils to protect the MOSFET is also meant to allow the current to continue to flow through the coil and basically harness wasted current/energy
    The problem is the diode voltage drop wastes energy also and catch fire
    I want to switch this current using a MOSFET instead but the body diode seems to be a real problem (don't confuse this with the flyback/protection diode) because after a bit of analysis it seems introduce a short when in a full h bridge driver
    I havnt built the circuit yet though but have a look at cct I originally posted and u need to have some understanding of how the cct works
    It took me yonks to figure out
     
  13. ScottWang

    Moderator

    Aug 23, 2012
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    @fuk,
    So you have two ID, it is not allowed in our forum, now you have to decide which ID that you want to keep and the other one will be deleted?
     
  14. MrAl

    Well-Known Member

    Jun 17, 2014
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    Hello,

    My question would be why do you need a device with no body diode in the first place when nobody else in the power converter industry needs this in order to drive a 3 phase bridge.

    If this issue is all just about power dissipation in the diode, then the standard technique to get rid of that is to use an external Schottkey in parallel to the internal body diode, which because of it's lower voltage drop passes all of the reverse current so the internal diode gets little or none. Thus no more extra internal diode power dissipation as it all is then moved to a location somewhere outside of the transistor case.

    Another question would be if you did have such a device, how would you expect it to handle the reverse voltage inductive kickback that could be produced during the transistor 'off' times. That diode is a necessary evil. Internal or external, it must be present.
     
    Last edited: Oct 28, 2016
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