To prove that the 312V charging voltage can be switched, I used an IRFP460 MOSFET for shorting the charge voltage to ground, which turns the charging off.
The charging voltage is already current-limited by a 100k ohm resistor on its output, so shorting it to ground is not a problem.
Now, I can try to apply some signal to the charge electrode and try to measure something on the phase detector next.
While with a single MOSFET, it's only possible to send a square wave to the charge electrode, I will ultimately need a sine wave that has the same frequency as the signal that is sent to the piezo.
Next, I will try to measure a signal with the square wave and if that doesn't work I will try to build a "sine wave driver circuit" and try again.
I figured out that the phase testing signal is not a sine wave, but a burst of 16 pulses which cover 0 to 360 degree in 22,5 degree steps of the piezo signal's sine wave...
More about it in the next build log.