How to reduce MOSFET heating in Buck Converter
If it is confirmed the MOSFET needs a better drive then the best is to advise a suitable circuit. A simple bootstrap circuit described by Monsieur Balogh in a TI application note can do the job nicely in a cost-sensitive application. As noted in some of the comments, the UC384x was not really meant for hi-side drive - unless you make it entirely float and tie its GND pin to the MOSFET source and supply the IC via the rectified buck output - but this little circuit does the job fine:



Below is the circuit I tested with component values:



This is excerpted from a seminar I taught in an APEC seminar in 2019.