# How to make a bidirectional power switch for negative supply rail controlled from logic level MCU

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:

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?

• 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.
– user105652
Mar 9, 2019 at 19:48
• Can you explain why you need a bi-directional switch? Mar 9, 2019 at 20:29
• you have drawn N channel mosfets but written a P channel part number. Mar 10, 2019 at 6:34

something like this perhaps.

simulate this circuit – Schematic created using CircuitLab

• 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.
– rew
Oct 12, 2020 at 17:53