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?
Best Answer
something like this perhaps.
simulate this circuit – Schematic created using CircuitLab