LTspice 2 grounds for AC PWM control by mosfets
You need to use only one GND in LTSpice.
This makes it sound like a limitation, but it isn't. In the real world the grounds will be connected, just via a very high impedance link.
I have often done LTSpice simulations for isolated DC-DC converters, where you have the same issue: two isolated grounds.
The fix for this is to remember that LTSpice simulates the real world with models. In the real world the two side are linked via a very high resistance, and a very small capacitance. So, for the lack of any concrete numbers to use, I use a 1Meg resistor in parallel with a 1nF capacitor to link the two grounds (number are open to some debate, as I've been given some, I've updated these numbers used here). Then I call one ground &quot;iso_gnd&quot; or something so that I can use that as the ground reference for my probes.
So I have a V_IN and GND_IN, then a V_OUT and GND_OUT. I connect the proper ground to GND_IN, connect GND_OUT to GND_IN via a 1Meg resistor in parallel with a 1nF capacitor. And simulate. To get the output voltage, I plot the voltage of V_OUT minus GND_OUT.