Another thread has discussed controlling a cheap DC-DC converters such as this one: http://www.ebay.co.uk/itm/NEW-LM2596-DC-DC-adjustable-power-step-down-module-/130704327784?pt=LH_DefaultDomain_0&hash=item1e6e95c468...This ad specs <=24V inputs and outputs. The input capacitor is, however, 220 mfd, 35V and the output capacitor is 220mfd, 15V.
I would never select power components that had to operate closer then say 75-80% of their maximum rating to allow for some 'headroom' and possible future increase loads.
Device maximum specifications are almost always stated optimistically...
As one says here in Italy "mi raccomando".
I have one pcb (constant regulator one) that I use to drive 2 3W leds. I have it setup so it gives about 600mA, has been working for more then 6 months.
It is all about disclosure.
This one: http://www.ebay.co.uk/itm/LM2596S-Power-Supply-DC-Converter-Module-Constant-Current-5V-35V-to-1-25V-30V-/261075646112?pt=LH_DefaultDomain_0&hash=item3cc95236a0