I'm curious to learn more about the principle of operation of Suntie ST2500 inverter. Not sure if you, Solar Guppy, are in position to discuss this in detail, but if you can would you shed some light to my assumptions/speculation?

When I was repairing mine, I noticed two main blocks in there:
1) Isolating Switchmode DC/DC converter, controlled by UCC3806 (if I recall right... anyway it is the newer BiCMOS version of the old UC3846 Push-Pull controller.)

2) H-Bridge motor controller (don't remember the IC type...). This is to convert the high voltage DC back to AC and into the grid.

The first stage needs to take care of the power harvesting from PV panels. This could have been set to work in peak current mode. Input (peak current of each switching pulse) current from the PV array can be adjusted by the microcontroller DA converter. In this case the stage works more or less as constant power load as seen by the PV array.

Second possibility is to have it working in voltage mode: An artifical ramp (can be taken from the PWM controller clock signal) is fed to the PWM comparator input and it is compared to the constant control voltage (generated by the microcontroller DA converter). With this method the converter runs at constant programmed duty cycle regardless of input or output voltage. The PV array see it more like constant resistance load.

The second stage, H-bridge, is running at "full speed" all the time. Depending on the voltage on the intermediate DC bus (which have those 400V electrolytic capacitor) defines how much power is being pushed to the AC output line. This does not need any feedback or control at all.

Well... that is what I'm speculating... Did any of this get even close?