It was suggested in "Select power supply voltage using MOSFETs" that a second MOSFET can be added in series to prevent reverse current flow.

My question is, can I just reverse Drain and Source on the switching MOSFETs and not need extra MOS or diode?

voltage selection circuit .


1 Answer 1


If you just reverse Drain and Source then in normal operation the body diode will conduct, even when the FET is switched off!

The solution is to wire two FETs 'back-to-back' so that their body diodes are in opposition, like this:-

enter image description here

  • 1
    \$\begingroup\$ Is there a difference between "body diodes pointing at each other" and "body diodes pointing away from each other"? I searched online and have seen both in use in reverse-protection circuit. \$\endgroup\$
    – daniel
    Jul 10, 2015 at 0:54
  • 1
    \$\begingroup\$ To control both FETs with the same Gate voltage the body diodes must point towards each other (or away from each other when using N-channel FETs). If each FET has a separate Gate drive circuit then the diodes may point the other way. \$\endgroup\$ Jul 10, 2015 at 5:56
  • \$\begingroup\$ @BruceAbbott How do you choose between 2 N-channel or 2 P-channel if they're both being used in the same place in the circuit? \$\endgroup\$
    – endolith
    Jul 5, 2016 at 19:01
  • \$\begingroup\$ Depends on whether you want to switch them on with positive or negative voltage. Usually that means using P-channel for switching a positive supply rail, because the Gate is pulled down to Ground so it gets negative voltage (You could use N-channel, but then you would have to drive them with a Gate voltage higher than the output voltage of the LM317). \$\endgroup\$ Jul 5, 2016 at 19:40

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