Reverse Polarity Protection, or, Why Is This MOSFET Smoking?

Discussion in 'General Electronics Chat' started by BLUESHIFT, Mar 7, 2014.

  1. BLUESHIFT

    Thread Starter New Member

    Aug 23, 2013
    24
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    Hey team,

    I'm about 95% of the way done with the design on the (open-source) Helium supercapacitor-powered portable speaker. It works great if you plug it in the right way...

    But the reverse polarity protection I put in seems to not work, i.e. when connected in reverse the N-CH MOSFET that should block the negative side smokes.

    (12V common cathode zener should limit Vgs to 12V ish.)

    I'm pretty sure I'm just being dumb...does anyone see how?

    [​IMG]

    [​IMG]

    https://github.com/blueshiftPDX/Helium/releases/tag/1-2

    THANKS A TON.
     
  2. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
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    What about just using a connector that only goes together one way ??
     
  3. BLUESHIFT

    Thread Starter New Member

    Aug 23, 2013
    24
    4
    That is a good point.

    But given that I'm selling a DIY kit version (the Capacitizer) I think it's going to be worth making it impossible to mess that up.

    I think there's got to be a mistake in my design...
     
  4. crutschow

    Expert

    Mar 14, 2008
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    Sounds like the MOSFET parasitic substrate diode may be conducting. Are you sure the MOSFET drain and source are connected correctly on the PCB?
     
  5. #12

    Expert

    Nov 30, 2010
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    There are simpler ways to do this. A diode in series with the input voltage or a fuse followed by a diode to short he supply if it's backwards.
     
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  6. ScottWang

    Moderator

    Aug 23, 2012
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    The symbols are not the zener diodes, those are Schottky diodes, what are the real diodes you are using?
    And why not just put one zener diode?
     
  7. tindel

    Active Member

    Sep 16, 2012
    568
    193
    hmmm. Several things...

    I think you're using the nfet as a low side switch as power is applied...

    I don't think your schottkey diodes are doing anything... look at the reverse breakdown voltage... They are going to be pretty high... the diode will only work as a negative voltage protection if you use a single diode with the anode connected to ground. You will also want to connect it directly across the input...

    Also with a 24 volt input you will exceed most fet rated voltage (usually +/-20V) and you don't have a voltage divider that I can make out.
     
  8. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Why not use polarized plugs to insure the user connects it correctly?
     
  9. AfdhalAtiffTan

    Active Member

    Nov 20, 2010
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  10. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    You really want a P channel device there.
     
  11. BLUESHIFT

    Thread Starter New Member

    Aug 23, 2013
    24
    4
    Thanks everybody.

    #12 - Fuse + diode is a great idea, accomplishes the goals more transparently plus adding a fuse is a good idea anyway. I'm trying to avoid the drop of a diode in series, because it seems avoidable.

    The part is a dual Zener, just used the dual Schottkey symbol out if laziness. Part number is on the PCB - I'll revise the schematic.

    Is the transistor soldered correctly? It looks good but I wouldn't be shocked if something is wrong. Actually it could even be on there backward, I'll check that out.

    Super helpful, I appreciate it.
     
  12. BLUESHIFT

    Thread Starter New Member

    Aug 23, 2013
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    Also - is p-channel actually a better solution? Blocking imaginary positive charges vs. blocking electrons, seems symmetrical, right? I can't think of a counterexample...
     
  13. bertus

    Administrator

    Apr 5, 2008
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  14. crutschow

    Expert

    Mar 14, 2008
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    You can use a P--channel in the positive lead or an N-channel in the negative lead. Both perform the same function. Has nothing to do with the polarity of the charge. But an N-channel has a lower ON resistance for the same chip size so is normally preferred, all else being equal.
     
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  15. BLUESHIFT

    Thread Starter New Member

    Aug 23, 2013
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    Right - symmetrical but the voltage drop across n vs. p channel thing from previous post. Thanks everybody.
     
  16. AfdhalAtiffTan

    Active Member

    Nov 20, 2010
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    Have you checked the PCB trace? It seems to me that it is mirrored...
     
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  17. BLUESHIFT

    Thread Starter New Member

    Aug 23, 2013
    24
    4
    Not mirrored, not on backward. Solder joints appear solid, but that seems to be the most likely problem...

    I'm going to stop thinking about this and switch over to a fuse / diode for the next revision anyway.
     
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