I built this DC-to-DC converter for my 48V electric bike because I wanted to be able to plug-in some common 12V accessories, e.g. my cell-phone charger, or a GPS unit.
Step 1: Schematic
Here's the schematic. The cap values are somewhat flexible depending on how much ripple you can tolerate. Be sure to observe the correct polarity for the electrolytic caps. I do not have a list of Digikey part numbers because I found many of the parts I needed at a local electronics surplus store. But all these parts (or something close) are available from Digikey.
Since I found many of the parts I needed at a local electronics surplus store, I don't have Digikey part numbers for everything, but I have attempted to find Digikey part numbers or substitutes where possible. The connectors are up to the builder's discretion, broken PC power supplies are a good source for connectors and wires.
Step 3: Layout
This is a hypothetical layout for a single-sided circuit board. I did not follow this exactly when I built my protoytpe.
Step 4: Prototype Component Placement
This shows the components placed on a perf board (from Radio Shack). I just used point-to-point wiring on the back of the perf board to connect the circuit. The case is from a discarded cell phone charger. It's not shown in this picture, but I later bolted on a small copper heat sink to U1 to help keep it cool. For my purposes (cell-phone and GPS battery charging) I do not expect any heat problems from the converter. Be sure to use some thermal grease when attaching any heat sink.
Step 5: Ready to Charge That Cell Phone
This shows the finished converter with a 12V cigarette-lighter dongle that is suitable for plugging-in a cell-phone charger or other 12V car accessory. I purchased the dongle at a local auto-parts store. The orange connectors are one-off connector types I found at a local electronics surplus store, but almost any 2-pin electrical connector will work. Salvaging connectors from a broken PC power supply is a good source of connectors and wire. I went a little connector crazy here; I don't really need any connectors on the 12V side since the cigarette-lighter dongle could be wired directly to the converter.
Note that the case does not close completely and I used 4 nylon standoffs (epoxied in place) to secure the top. I see this as a feature because it allows for airflow to the regulator :-)