Buck Converter

kahei

Hi. I would like to design a buck converter which step down the voltage from 40V(d.c.) to 12V (d.c.) and the current range is 1.2-0.2A. Although I found the information from websites, they don't provide the right information or equations. I simulate them in Spice, but the output voltage and current don't match my requirements.

Would anyone please provide some information or website for me? Thanks a lot.