Reverse voltage and pullup/pulldowns

Discussion in 'General Electronics Chat' started by cl10Greg, Jul 16, 2015.

  1. cl10Greg

    Thread Starter Active Member

    Jan 28, 2010
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    I have a general question about reverse voltage protection. For most of my designs that involve some moderate power, I tend to use the PMOS on the high side with a zener to clamp and protect it. My question is about the other transistors in the circuit and how the pullups and pulldowns are affected by reverse battery. If I have a NMOS turning on a load and use the traditional pull down resistor, the reverse orientation would tie it to the input voltage. Now there is no ground since the PMOS never is turned on to protect against this but I just wanted to make sure I am right with that assumption. There is no way that a pullup and pulldown can activate the transistor with a PMOS reverse protection scheme correct?
     
  2. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Without a schematic or even a block diagram your description is very difficult to follow. While the idea of a pullup resistor iw well known, it's location in a circuit can be anywhere.

    ak
     
  3. #12

    Expert

    Nov 30, 2010
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    If no power supply is connected, it can't be backwards. The answer lies in how perfectly the PMOS refuses battery current. If the load (circuit) can't access enough voltage to turn on a diode junction, you are safe.
     
  4. cl10Greg

    Thread Starter Active Member

    Jan 28, 2010
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    upload_2015-7-16_11-28-38.png
    This is a basic example in the normal orientation. Q2 would be driven by a microcontroller that would be after the PMOS. So if this circuit was hooked up backwards the ground would be floating because the PMOS would never connect it to ground. Is this correct?
     
  5. #12

    Expert

    Nov 30, 2010
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    Your example is one of extremes, no defined load. Microcontrollers do not have information about their current draw under very small voltages. If you look up the reverse leakage of the PMOS at the highest temperature expected, you can add a load resistor to be sure some few microamps will not develop enough voltage to do any harm. For instance, 50k ohms will dissipate 10 ua to common when 0.5 volts are available.

    As usual, one experimental result is worth a dozen theories. When in doubt, measure it.
     
  6. AnalogKid

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    Aug 1, 2013
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    Q1 looks backwards. A MOSFET is controlled by Vgs, not Vgd. You are relying on the forward biasing of the internal zener diode to establish a voltage relationship from Source to Drain (GND). I guess it will work, but it just doesn't look right somehow.

    ak
     
  7. Ancel UnfetteredOne

    Member

    Jul 3, 2015
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    this will help.
     
    #12 likes this.
  8. crutschow

    Expert

    Mar 14, 2008
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    The connection of the P-MOSFET (Q1) is correctly orientated for reverse bias protection.
    MOSFETs conduct equally well in both directions so when a positive voltage is applied to the P-MOSFET's drain with the gate grounded it will fully turn on and conduct the load current. The substrate diode is also forward biased under this condition but conducts no current (except perhaps during turn on) since its forward voltage is greater then the MOSFET ON voltage from drain to source.
    If a reverse (negative) input voltage is applied to Q1's drain, the transistor will stay off since the drain-gate voltage is zero and the substrate diode is reverse biased.
     
  9. AnalogKid

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    Aug 1, 2013
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    Yeah, I know all that; I wasn't saying it is backwards, just looks backwards. And while I prefer N-channel devices in the return leg for this function, I've designed them in in both directions based on other circuit requirements. Still, if you think of a PNP device acting as a switch device in the positive rail, you grow up thinking that the emitter (source) is the input and the collector (drain) is the output. A MOSFETs on-state bidirectionality expands the rules, but still...

    ak
     
  10. crutschow

    Expert

    Mar 14, 2008
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    Well, you seemed uncertain as to whether it would work in your post #6, hence the explanation.
    My apologies if I misunderstood. :oops:
     
  11. AnalogKid

    Distinguished Member

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    No prob. But I am disappointed that you didn't whip up six LTSpice simulations.
     
  12. crutschow

    Expert

    Mar 14, 2008
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    I was feeling lazy.
    Anyway my limit is usually one simulation per post. ;)
     
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