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  1. #1
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    mosfet without diode

    I have some specific needs for which it would be most suitable for me to find some MOSFETs that doesn't have internal diodes.


    Any ideas?

    •   Alt23rd June 2005, 21:00

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    mosfet without internal diode

    I asked this question a year ago. No one could give a model number. You may have to put a diode in series with the drain to prevent current in the reverse direction.


    1 members found this post helpful.

    •   Alt23rd June 2005, 22:23

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    mosfet internal diode

    Hm, I rather detest putting extra components into circuits if I can help it but it seems I'll have no other choice.

    Oh well, thx for the info though. ;)



    •   Alt23rd June 2005, 23:19

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    fet diode

    Sorry, but that diode is intrinsic to the MOSFET's semiconductor design. Too bad it's a sluggish diode. You have to work around it by using external components.



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    fet without diode

    Actually I always use parallel auxiliary fast diode but in this case I need to nullify effect of that diode entirely. No alternative to another diode I am afraid- the flatulent is correct.



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    fet with diode

    This internal diode in discrete FETs is from internally connecting the source to the back gate or bulk material. This makes the threshold voltage more controlled.

    In the CD4000 series of logic gates, the analog multiplexer uses floating N and P channel FETs without the back gate connected to either.

    In SPICE simulations the FET models have a back gate connection you have to connect to some point in the circuit.

    I guess that there is not much market for a four terminal discrete FET.



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    Re: mosfet question

    I am working on a motherboard, with two mosfets which got very hot. I removed them, but I could not find replacements. They are P-channel mosfets with the same pinout as a 4407, so I tried putting two 4407s in place, and they just hissed at me and got hot.
    I had once observed a motherboard which had two mosfets "jumped" - from source to drain. There was a tantalum cap separating the two. Presumably, those mosfets where bad. It seemed like a good idea. I removed the 4407s and connected where source would be to where drain would be - with no success.
    Is jumping mosfets like this a reliable way of working around a failed pair of hard to find mosfets? If so, why have my attempts been unsuccessful?

    ... More information:
    The original mosfets were "tpc8107" - manufactured by toshiba. They are P-Channel Mosfets. The board is from an Asus notebook, G1S.
    With my DMM, when connecting source to drain caused no short with 19V detected. During on experiment, I connect the source to drain, the other way around - and no voltage was detected. However, when I connected S-D both ways around at the same time, a short was created. I probably shouldn't have tried this, but am curious about the behavior of this circuit and want to gain some more insight.



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    Is there any MOSFET without internal diode?

    You always have the body, hence body-drain diode, except
    in the case of fully depleted SOI. Which you will not be using
    for high voltage, high current switching.

    The function of DMOS works because of how the body
    pinches the "neck" (VDMOS) or drift region (LDMOS) to stand
    off the drain field. Since you need the body, you get the body
    diode.

    Sometimes in integrated circuits you will see the body
    diode-blocked (I have done analog muxes with this scheme).
    A 4-terminal MOSFET would let you do this, but a standard
    power MOSFET ties body to source, hard, for best ruggedness.



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