Buck boost converter
The goal is to convert the wrong DC voltage into the desired DC current for a limited voltage range of an array of LED's.

They are connected in series and loading must be satisfied on both sides.

Some topologies include Cepic, buck/boost, flyback.

That being said, you will find many obsolete CV power supplies with bad ripple being sold as LED drivers cheap on Ebay. Since they have a simple pot to adjust the voltage over a small range, they can work OK with some high power  LED arrays, but you have to measure the current yourself.

Here is one you will find on Ebay with specs in Mouser for 10 cents per watt.

http://www.ebay.com/itm/350W-LED-Power-Supply-Regulated-for-High-Power-LED-Driver-33V-34V-35V-36V-37V/251313308892?rt=nc&amp;_trksid=p2047675.m1851&amp;_trkparms=aid%3D222002%26algo%3DSIC.FIT%26ao%3D1%26asc%3D163%26meid%3D2845582659162210208%26pid%3D100005%26prg%3D1088%26rk%3D1%26rkt%3D4%26sd%3D251211196069%26

http://www.mouser.com/search/ProductDetail.aspx?qs=vtueAHjjgCNkwPrFEZ/9pA==

All I can say is Buyer Beware...