If you are trying to get 800W out of that converter switching at 200 Hz, that transformer will need to be huge, massive and heavy.
It so happens I designed a 12V - 300V converter (600W) for a portable missile launcher. We used FETs switching at 40 kHz. NOTE: if you are trying to get 800W off a 12V source, the peak currents through the FETs will be about 80A or more. The FET you show will not handle anywhere near that. We had to use 3 FETs in parallel on each side. Efficiency will be poor, given the high promary side currents (85% would be excellent) so your converter will have about 150W of internal dissipation. I suspect you have not thought this through very completely.