What transistor or combination of transistors should I use to make a fully controllable AC switch?

Question

Suppose I want to switch AC. I want to control (allow, prevent, or interrupt) current flow in either direction. SCRs won't do because I want to interrupt current, and I don't recall seeing many IGBTs or FETs that lack an antiparallel diode. Those few I have seen have been low-power and didn't have much of a spec for reverse-blocking.

Are there transistors or other solid-state devices that will do what I want? Or should I perhaps use two IGBTs in series with the directions reversed?

Voltages I'm interested in are on the order of 600VAC. Currents are 10-200 amps. Frequencies are 1-20 kHz.

Answer 1

Yes, hefty back-to-back MOSFETs (maybe) or IGBTs should do what you want. Driving them would require isolated gate drive circuits. The complexity will vary a bit depending on how quickly you want them to switch. IGBTs tend to switch a lot more lethargically, but are more suited for the ~1200V PIV type operation required in this application.

SCRs (and triacs) as, you point out, have to wait for a (current) zero crossing, and the high frequency operation you speak of (20kHz) means that they would tend to stay on due to the high dv/dt at the zero crossing.

Answer 2

Try two fets like this: -

The opto is a bonus but getting the opto to switch at 20kHz might be a problem.

The way it works is that both MOSFETs can be activated by the common gate voltage (relative to the common source) and this switches the switch on. When the gate voltage is reduced to zero, both FETs are off and their respective parasitic diodes are in opposition therefore the circuit turns off.

The switch contact shown in the diagram is superfluous to the understanding and practical realization of this circuit.

I haven't checked but I'm presuming that FETs capable of withstanding voltages in excess of 850V pk are possible. Getting the current and the voltage may be trickier so you could opt to make parallel banks of FETs.