Hi Azhar,
I hope u are still alive when u receive this message. To use a '51 seems to be a bit of a overkill, there are some excellent dedicated IC's specifically designed for SMPS apps. Eg; TL494, UC3842 MC34063. Linear Technology has some great reading material. The Trafo is ALWAYS a bitch of a thing to design. Too much parameters where the elusive parameter is nearly ALWAYS the Permeability constant. Only the Core manufacturer can specify these values in their spec sheets. I know about fireworks-had my own show for a couple of months! And the maths involved is pretty hairy to say the least! In the end, I opted for "off the shelf" SMPS units instead. Much cheaper compared to dev time. But if u insist on designing SMPS, I wish u my deepest sympathy :-)

Good luck
Alexz