UC3843: Device recommendation

Nison,

1.2 kV input and 500 W output is too much for a single switch topology like a buck converter. You should consider a phase shifted full bridge which can easily handle the 3:1 input voltage range, would not require the use of SR on the output and would reduce the VDS stress on the primary to VIN for each MOSFET. TI has many controllers supporting the phase shifted full bridge topology and I have filtered them parametrically here. You can also download TI Power Stage Designer and run simulations for all mainstream power topologies to see which might be the best fit for your requirements.

Regards,

Steve M