1-5V 20A adjustable power supply

I'm wanting to find, or build, a power supply that can supply up to 20A at 5V but is also adjustable from 1-5V.

I haven't managed to find anything off-the-shelf that can do this. I have found a board that can supply 20A at 5V and my initial think was to then build a circuit with an adjustable voltage regulator.

Here is the PSU: http://uk.farnell.com/jsp/displayProduct.jsp?sku=1441235&CMP=e-2072-00001000&gross_price=true&mckv=YKMwxwd8|pcrid|14164337469|plid|%7bplacement%7d

I'm looking for any advice on how to make this work, the voltage can either be adjusted with a pot or a voltage signal.

Thanks in advance!

For high current a voltage regulator chip that drives an external pass transistor would be needed, but here the voltage is v. small and current really high - bit of a problem as a Darlington pass transistor would waste too much voltage and an NPN capable of 20A is going to have low gain.

I think a variable output SMPS would be much to be prefered if it can be found.

I'd be willing to bet that these guys could custom build something for you:

http://www.acopian.com/

Just realize that what you are asking for is a bit special, and will likely cost a pretty penny (whether you can find it off the shelf, or custom designed).

Thanks for your help,

Finally managed to find a bench PSU that matches the spec I need: http://www.ebay.co.uk/itm/30V-20A-LED-High-Power-Switching-Variable-DC-Power-Supply-220V-New-/261179472046?pt=UK_BOI_Electrical_Test_Measurement_Equipment_ET&hash=item3ccf8278ae

Would still prefer to have a smaller unit that I could integrate into my device and hook a pot up too myself but this might do. It has constant current control which is a plus.

Next step is to build an H-bridge board to flip the polarity of the supply. My initial thought was to use one of these: http://www.ficrelay.com.hk/details/FRA9T.pdf, just a DPDT relay that I'll control from a separate supply (probably 24v or 12v). Is it as simple as this or is there anything else I should be thinking about?