The old 2006 Elektor design....


SillyChip recently had a similar circuit using the MC34063 and an IRF1405 MOSFET (SiliconChip, May 2011 pp78-84).
Very similar though the FET may be cheaper and easier to get (it has double the current rating of the 2505), and the MC34063 is also very common (many phone chargers etc as well as stuff that improves your fuel economy (LOL) though limited to ~100kHz).
The sillychip circuit is also adjustable from 15-35V (100W with current sensing...) though it is only {therefore} upwards of 80% efficient.


The datasheets or ANs (Application Notes) often give good design guides. There are even some web calculators that do the design. The trick is probably knowing what choke/inductor (or transformer if isolation is needed) is suited to the current & frequency....