I'm looking to build a bidirectional power switch for a negative power rail. I have a +12V, +5V, +3V3 and -5V supply to switch. For the positive supplies, I'm using two P-channel mosfets in a common source configuration, which can be controlled directly from a logic level (0-5V) microcontroller: Bidirectional Power switch for positive supply rails

I am struggling what is the easiest (low component count) solution for the -5V supply rail. My first thought was to use the same circuit, but then I need a negative gate source voltage to turn the fets on, meaning I would need to put roughly -7V on the gates (-5V + Vgs,th), which makes it complicated.

Another thought was to switch to two N-channel fets connected as common source, but then I would need -5V on the gates to keep them off, which I could do using a pulldown, but the ESD diodes of the IO pin of the MCU would pull the gate signal back up to 0V, turning the fets on.

My initial conclusion is therefore that I would probably need another FET to make it work, but I can't yet figure out how. Any ideas?

  • \$\begingroup\$ At -5 volts, ground and +3.3V are very positive to an N-channel MOSFET. Depending on if it is logic type or not, Vgs only needs to be 5 to 10 volts for it to saturate ON. With no gate drive voltage, a 10K resistor to source pin would shut it off. A zener from gate to source limits max gate voltage. \$\endgroup\$
    – user105652
    Mar 9 '19 at 19:48
  • 2
    \$\begingroup\$ Can you explain why you need a bi-directional switch? \$\endgroup\$ Mar 9 '19 at 20:29
  • \$\begingroup\$ you have drawn N channel mosfets but written a P channel part number. \$\endgroup\$
    – Jasen
    Mar 10 '19 at 6:34

something like this perhaps.


simulate this circuit – Schematic created using CircuitLab

  • 2
    \$\begingroup\$ I would personally then opt for a P-channel mosfet instead of Q2 a bipolar transistor. The reason being that you get charged for how many different components you have and TS has indicated he needs the P-channel fets already. This argument is less valid if you're using physically big P-fets for the high side switches. Then you'll want a smaller one in this circuit. I'm using SOT23 P-fets for 2A. \$\endgroup\$
    – rew
    Oct 12 '20 at 17:53

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.