I saw this kind of MOS used as this :


simulate this circuit – Schematic created using CircuitLab

I don't think it can work & I m not sure a mos used like this can be used as a reverse power connection...so I don't realy understand why is this for any idea ?

  • \$\begingroup\$ What is the intended use of this? Also, remember the fet body diode. \$\endgroup\$ – Stiddily Feb 8 at 19:55
  • \$\begingroup\$ Where did you see this circuit? Please provide a link so we can see the context for the circuit. \$\endgroup\$ – Elliot Alderson Feb 8 at 20:00
  • \$\begingroup\$ This looks similar to a reverse-polarity protection circuit. \$\endgroup\$ – Hearth Feb 8 at 20:11
  • \$\begingroup\$ Sorry I have no link to provide, I saw it on simple paper board yesturday & just remeber it ... Thanks to Platytude and matzeri & all of you for the answers \$\endgroup\$ – francois P Feb 8 at 21:41

Assuming you meant to say reverse polarity protection, then yes, the circuit that you posted will not work. Though that's simply because it's been drawn using a P-channel FET rather than an N-channel one. A correctly drawn low-side reverse polarity circuit would look something like this:

enter image description here

The circuit works as follows. When Vin is positive, the gate of the FET is pulled up through the voltage divider formed by R2 and R1 (R2 must be significantly smaller than R1 to ensure that the gate-source voltage of the FET is held well over the conduction threshold). When Vin is negative, the reverse happens, and the gate-source voltage of the FET is now negative, causing it to turn off. D1 serves to limit the gate voltage to a safe level even if the input voltage is above the safe threshold (most FETs have a maximum VGS of ± 20V)

Edit: An additional little tidbit. The reason that you may see this circuit used over a traditional high-side P-channel reverse polarity protection circuit is that typically N-channel devices have better performance than an equivalent P-channel device (lower RdsON, etc).

  • 2
    \$\begingroup\$ I'm afraid it is your circuit that is wrong. As others have noted you need to take account of the body diode so the usual way is to operate the FET in reverse mode so that the drain is positive in the fault scenario. In normal operation the drain is negative with respect to the source and so the body diode would conduct if the gate was not biased. However the channel is made conducting by the positive bias on the gate such that the voltage across the device is less than that required for any significant current to flow through the body-diode. Your comments about gate protection are correct. \$\endgroup\$ – Kevin White Feb 8 at 20:34
  • \$\begingroup\$ Good catch, LTspice doesn't included the body diode in their FET symbol so I frequently get these backwards... I've corrected the diagram above. \$\endgroup\$ – Platytude Feb 8 at 20:48

The circuit is correct, but with the battery in that polarity the current is blocked. With the battery polarity reversed the current can flow.

In the App Note of Infineon

you can find the the proper schematic including the Mosfet P channel body diode

enter image description here


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