A lot of folks have said that leaving a transistor in avalanche mode is bad for its long term health, but I'm not convinced. The metrics of that health that continuous avalanche impact are important for the normal use of the transistor, but in this case the transistor has no other function.

Still, if nothing else it seems a waste of power to run the 20v boost converter and avalanche circuit continuously when they're needed only for a few ms every once in a while.

To that end, I've decided future hardware will include a logic output from the controller to the boost converter's !SHDN pin to allow the avalanche supply to be turned on and off.

But there's a wrinkle there: you can turn off a boost controller, but there will still be a conduction path from the boost input supply through the inductor and catch diode to the output. Without taking extra steps, you can never turn a boost supply completely off.

Fortunately, there's an easy solution to this, and it's apparently a classic one. You connect a P channel MOSFET up on the output of the boost converter and connect the gate to the input power supply. P channel MOSFETs are high impedance ("off") when the gate voltage is (nearly) equal to the source voltage, which will be the case when the boost converter isn't switching. Usually you turn on a P channel MOSFET by dropping the gate voltage, but in this case it will turn on because the source voltage will rise relative to the gate. The result is a true on-off controlled "high" voltage power supply. Exactly what we want. This doesn't switch the inverter chip on and off, but that's ok. It should wind up in a stable state without any input and take relatively little power on its own.