Burngate wrote:Why buy off-the-shelf when you can build your own, which does what you want rather than what someone else thinks you should want?
Personally, because off-the shelf is cheaper quicker and easier. I can buy a BEC from ebay that will be in my hands as fast as if I buy the parts from Farnell, be significantly cheaper than the parts, and the BEC will be working five minutes later. If I'm just blindly plugging parts together following a specimen design on a datasheet (as I would be in the case of something like a DC-DC buck switcher) there's basically no interest in it, and I'd rather have it quicker easier and cheaper by buying it assembled. There's little satisfaction in assembling such a thing - I'd rather be assembling something that I've actually worked out.
But I agree that to meet the application described a buck-boost stage is needed, not just a buck. Personally, I'd have one power supply module for the 24V cases and a different one for the 12V ones.