thank you for using the E2E forum.
When targeting in output power of 1kW you need to go with a multiphase design.
Below reference designs might be a good starting point:
The TL431 might work with an optocoupler at 300kHz to 1MHz but it would require a very aggressive compensation network to maximize the TL431's bandwidth. If the TL431's bandwidth is not sufficient, a better solution would be to use a discrete…