dual mosfet AC switches

or bidirectional power switch.

i want to be able to control an AC signal using two mosfets back-to-back both common drain and common source. I'm trying to understand the difference between each approach common drain and source as shown in the 2 right hand circuits in the image below.

as far as i can tell, common drain requires separate resistors between the source and gates to guarantee that the fet is off while a common resistor could be used for the common source case

Most people would use a triac.

i'm trying to control DCC power and possibly reverse the polarity to section of a model railroad when a short occurs at the boundary of a reversing section of turnout frog

You should use common source, two n-FETs, and a PV opto-isolator to drive the gate/source voltage. That provides an AC- and DC-compatible low on-resistance solid state relay behaviour.

p-FETs are just inferior in performance, only use if you have to.

common-drain requires two different gate drive circuits.

So n-FET common-source is the only topology its sensible to use.


common-drain requires two different gate drive circuits.

why? can you explain