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DC Reverse polarity protection using P-MOSFET

  1. Oct 1, 2012 #1
    Hello All,

    I'm designing a circuit board that needs to run on a 12VDC supply and will need to supply a fair amount of power to drive a bunch of 12V relay coils, in addition to some electronics. Since I want to keep the voltage drop on the supply a small as possible, I thought about using a P-CH MOSFET to make an active diode on the input of the board to prevent damage if the user plugs the power in backwards.

    Using this method, I would have the circuit setup like the attached picture, however, the limitation here is that the input voltage is limited to 20V because of the MOSFET gate. Under normal operating conditions, going over 20V should never be a problem for this particular design, but I'm curious to know if there are any clever ways to allow the reverse voltage to go above 20V without harming the gate (using zener diodes and resistors or something). I've tried to come up with a few ideas but so far nothing obvious sticks out. Does anyone know of any simple solutions to this problem that don't involve using any fancy ICs?

    Thank you,
    Jason O

    Attached Files:

  2. jcsd
  3. Oct 2, 2012 #2


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    Something like this should work.

    Attached Files:

  4. Oct 2, 2012 #3
    Why make it more complicated than it needs to be. Just put a full wave rectifier on the input.
  5. Oct 2, 2012 #4


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    Hi Skeptic. The OP stated that the object of the mosfet was to avoid the diode drop and increase efficiency. A full wave rectifier would add two diode drops, the exact opposite of OPs stated purpose.

    Of course the op could have used a schottky diode to make some improvement in efficiency, but the mosfet used as an an "active" diode can be even more efficient, and can actually have a forward bias volt drop very close to zero.
  6. Oct 2, 2012 #5
    It seems like it would work without the zener and 10 K resistor.
  7. Oct 2, 2012 #6
    Hi uart,

    Thank you for your reply, I actually drew a circuit very similar to this one but missed one small detail. This is exactly what I was looking for though.

    Thank you!
    Jason O
  8. Oct 2, 2012 #7
    The point of adding the zener diode is to keep the magnitude of the gate source voltage from exceeding 20V if a higher supply voltage is desired for the load.

    - Jason O
  9. Oct 2, 2012 #8
    But w/o the 10k resistor & Zener diode, the P-FET may never turn on, & the conduction would be via the body diode, which has a forward voltage drop as large as a p-n junction device. The 2 parts in the above diagram insure that the P-FET gate is biased on, the Zener clamps Vgs, & the resistor limits Zener current to a safe value. Conduction is via the drain-source channel of the P-FET, & the forward voltage drop is typically well below that of a diode, even below that of a Schottky. I've measured well below 0.100 volts, & in some cases even as low as tens of millivolts depending on Rdson & forward current. This approach is well proven, & I've used it many times.

  10. Oct 2, 2012 #9
    I ran it in LTSpice both ways, by putting the gate-source across the load and also putting it across the supply. I like putting it across the supply better but it works both ways. Good idea about using the zener to protect the gate-source voltage. The voltage drop across the PMOS FET was about 0.78 or about the same as a single diode. However I used a generic FET and it should be possible to find one with lower on resistance.
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