Looking around for appropriate devices for a PCB if poss, to convert 12v DC to 5.1v 3a so as to power a raspberry pi. I know I can get the wired stuff from Amazon, but I'd really like to have a go at designing one in a circuit.

Simon

 

Look at DC-DC converter ICs. They have design examples in the datasheets usually.

Edit: also known as switching regulators.

 

You could use a linear regulator, though at max load you'll be dumping over 20W to heat so you'll definitely need a heat sink and likely a fan to dissipate that. Not to mention what that will do to your battery life if you're running on battery power.

Even better would be a DC-DC switcher which will be massively more efficient. I would suggest looking for a "module", which will be a complete switching regulator circuit in a black box. Check digikey, mouser, even amazon. If you're really adventurous you can try building your own switching power supply. TI has an application called TI Switcher Pro that will help you, BUT be aware that layout will be as important as the parts themselves so only go this route if you really want to learn a lot.

 

Did I hear somebody say "switching power supply"?

If you are up for it, I recommend the MC4063 (also other brands such as KA4063) which is a very simple type of switching regulator chip. It is only good for 1.5A peak current and you will need more like 6 amps (no, I did not calculate that -its a guess) but luckily it can drive an external switching transistor as shown below.

Datasheet for the chip:
https://www.onsemi.com/pub/Collateral/MC34063A-D.PDF

Application note for the chip:
https://www.onsemi.com/pub/Collateral/AN920-D.PDF

Only a few parts, seems to work every time, and I would guess the efficiency will be near of 80%, maybe a little higher, as opposed to about 40% for a linear regulator. I've bought what may be only very good copies via eBay vendors for as little as US $0.05 each. Can't beat it for a bargain.

If you want to make a linear regulator you might want to try the LM350, but you will need a heatsink, a pretty good size one.
ttps://www.onsemi.com/pub/Collateral/LM350-D.PDF