I'm trying to design a step down buck converter. I have couple of issues here to discuss.

Let me give you a brief idea on what I'm working on:

- The input voltage is 15 to 30 DC volts, and the output is 5 volts.

- The output current is 5 Amps.

I can figure the duty cycle, I ripple, and all that stuff EXCEPT! I was trying to find inductance, but couldn't figure out how to determine frequency!

I wanna know what determines frequency and how to calculate it in order to find the value of components (inductance and capacitance).

another issue is the variable input. If I have a variable input then the duty cycle will be an interval of numbers!

Please I need head ups on finding frequency

Thanks