I just got a question on a project which I'm working on in my university. I am designing a switching power supply using flyback topology. I have calculated all the parameters which are necessary but somehow my professor tells me that the duty cycle is wrong. I assumed a duty cycle of 0.5 and he keeps telling me that the duty cycle changes. I understand that but I can not regulate that, I think an IC can do that. He wants me to calculate using a variable duty cycle. Does anybody here have experience in designing and implementing power supplies and can help me? Following are the requirements:

input voltage: 12V-24V

output voltage: 4.95V-5.05V

Output power: 20W

Assumptions:

duty cycle: 0.5

Efficiency: 80%

switching frequency: 100kHz

Voltage drop Schottkey diode: 0.3V

Calculations:

Prim. winding: L1= 8.5 uH

Sec. winding: L2= 1.9 uH

Transformer core: EF20

N1= 13 windings

N2= 6 windings

I will appreciate so much if someone helps me since I am working on this problem for more than a month.