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need a MOSFET without a body diode

Discussion in 'General Electronics Discussion' started by Vavat, Aug 15, 2011.

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  1. Vavat

    Vavat

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    Aug 15, 2011
    I need a symmetrical MOSFET, i.e. the kind that would not conduct Source-Drain. Are there any? Or do all MOSFETs have body diode?
    The circuit in question is attached. Transistor Q1 works as a load switch and will only be used in Drain-Source direction. However, transistor Q2 is a memory key. it either loads up gate of Q1 with charge or sinks charge to ground. Q2 is meant to separate Q1 from the ON/OFF circuit and is only activated when SYNC is positive.
     

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  2. duke37

    duke37

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    Jan 9, 2011
    Look at the cmos 4066 quad bidirectional switch.
     
  3. Vavat

    Vavat

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    Aug 15, 2011
    thanks, but this seems like an overkill. a simple symmetrical transistor should do the job here.

    Also, the solution is very price and space sensitive. would rather like to have transistor there.
     
  4. duke37

    duke37

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    Jan 9, 2011
    I am under the impression that mosfets have the diode as an inherent part of the structure but I may be wrong (it has been known).
    jfets can be used as variable resistors but voltages will be critical. A single jfet will be more expensive than a 4066.
    I think that the 4066 will be no more expensive than a mosfet transistor and you get four switches in a packet. If you are very short of space you could go to surface mount components.

    Some means of defining the base voltage of Q1 is needed when Q1 is open circuit.

    RS do the 4066 for 20p and Cricklewood for 30p (plus VAT)
     
    Last edited: Aug 15, 2011
  5. Vavat

    Vavat

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    Aug 15, 2011
    you, sir, seem to be correct. I spoke to my manufacturer and they say that manufacturing cost of placing single ssop is far smaller than 4 transistors.

    Do you know of any 4066 equivalents that have Open Drain output? this will eliminate switched transistor too.
     
  6. Vavat

    Vavat

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    Aug 15, 2011
    I think body diode is a property of double diffused vertical MOSFETs, not planar ones. This is a side effect of extra doping done to improve their power handling capability.
    http://en.wikipedia.org/wiki/Power_MOSFET
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Take a look at the CD4007.

    I think that's as close as you're going to get to individual mosfets.
     
  8. Vavat

    Vavat

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    Aug 15, 2011
    hmmm, this seems good. do you know if anyone makes them in small packages?
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,491
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    Jan 21, 2010
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