I want to design buck converter for 385VDC to 48VDC for that I calculated the values of component and simulated it with proteus but I am not getting the desired output. What is wrong? PWM has a 20v amplitude as given in IRF740 data sheet. And 20kHz frequency and 12% duty cycle.
To turn on Q1 you need to drive the gate about 10 volts positive with respect to the source. Given that you expect the source to rise to around 385 volts, this means that gate has to rise to 395 volts to adequately and efficiently turn-on the MOSFET.
I’d strongly consider using a proper boot strapped driver circuit to do the job. Additionally, you shouldn’t drive it more than +15 to +18 volts above the source because the absolute max limit is stated as 20 volts in the IRF740 data sheet.