You may want to consider a regular AC switcher. The first thing that these switchers do, is to rectify the AC into a DC then convert it to the required DC voltage. Using a DC at their inputs will do the same effect. The DC will just flow through the diodes up to the DC filters. The issue then becomes to find an AC-DC switcher that does work over the full range of 120 to 220VAC, as your DC source fits right in between these two. Check at places like Condor or PowerOne if their switchers operate over the whole range, or if they auto-switch between the two ranges. You need one that does operate within the full input range. That would make a reliable power supply, inexpensive, without the need to build one. Building a high-power switcher, quantity one is a hasardous task that can do a lot of damage and harm.