Context: A digital phase-controlled light dimmer system.
- 120V AC mains
- I use an isolating transformer (and rectifying circuit) to generate +12V and +5V DC rails.
I want to use reverse phase control (I've read that this produces less EM noise), so I'm looking to switch the AC mains using 2 Power MOSFETS (e.g. FDB14N30) with the source pins connected together.
In order to put the MOSFETs into the "on" state, I need to raise Vgs to 10V-30V, but in order to do this, I think I need to hook up the MOSFET source pins to my isolated ground rail, and the gate pins to my +12V switching signal.
By doing this, have I defeated the whole purpose of an isolated power supply, in terms of danger of death and damage to components?
Best Answer
If you want to maintain isolation (I assume you have some legitimate reason for it beyond it being "good", i.e. user-accessible low-voltage electronics) and still drive your floating MOSFETs, you could generate a gate voltage with a full-wave rectifier and a shunt regulator, then switch it using a standard optoisolator. The drive current will be incredibly low if you're only switching at 60 Hz, so your regulator's Iq can also be quite low.