Hi,
There are a lot of issues in the schematic.
At least we miss the transformer connection and the output filter. (Besides decoupling capacitors.,)
--> show transformer and filter schematic
You say it is "SPWM" but there is nothing about SPWM in the scope picture.
--> Show the driver signals.
--> why do you use optocouplers when you connect input and output side (GND)?
--> to measure the output voltage use a self made resistive divider 100k/1k. Should work for low frequency signals.
Klaus