Building a better 12V supply

I've recently been on a bit of an efficiency drive, replacing more light bulbs with LEDs in an attempt to reduce the ever-growing electricity bill, and it occurred to me that one of the most important things to focus on is appliances which are on 24/7.

We're generally pretty good at switching stuff off, as I've put "kill switches" on all the main groups of stuff like the telly and computers, but the cable modem and router tend to be left on all the time.  I'd never bothered to check what kind of power they were using, but saw that they were linear-style "wall warts" and as a result probably not very efficient.

Measuring the two together with the little "Energy Monitor" block brought a figure of 12W (16VA, 0.61PF)... which isn't awful although could probably be a lot better.  As they both use 12V,  I thought I'd try a few of the switching supplies I had lying around to see if any could do better, only to find they had a horrid power factor and were not much better at around 9W.

So I decided it was time to have a go at building my own.  A fully switched supply can be efficient and operate at a near 1.0 Power Factor with suitable correction, but they are not commonly (yet) efficient at lowish powers, as the cheap adapters I tried demonstrated, though are likely to become so in the next few years.  A good compromise is to use a fat toroid (a house staple!) and high efficiency rectifier bridge followed by a DC/DC converter.

Looking on eBay, I chanced across this unit here.  It's a wide input range (16-40V) buck converter board with good efficiency that looked ideal for the job... so for less than eleven quid, I bought one.  When it arrived, I was slightly disappointed at the lack of the shown Oscons but nevertheless worked fine when I added a wodge of input decoupling.  With a test load of 726mA and trimmed to 12V, I calculated an efficiency of 96.4% for the unit for an input of around 17V, which is very impressive!

The unit is based around a TI TPS40057 synchronous buck controller driving a pair of Toshiba TPCA8016 60V 25A N MOSFETs... good quality parts... you'd struggle to build it yourself for the UK parts cost alone.

Building the supply

So with the converter in place, I needed to pull the other items together.  Main thing I was missing was a case, which was dutifully purchased so it could be put into something!  First thing to be installed was the toroid which is an encapsulated 2x6V 50VA unit... ideally go as big as you reasonably can for this kind of thing as power as wasted in the copper resistance, but for the low drain on the supply, this wasn't a major issue.

For the rectifier bridge, I went with a favourite choice of mine, NXP PMEG3050 - these are very efficient, very small 30V 5A rated Schottky diodes which have a commendably low voltage drop at an amp or two.  They were mounted to a little piece of vero board using copper tape, and screwed to the top.

As I wanted the unit to be able to power at least four 12V items, I went for four sockets on the back, all individually polyfused.  The appropriate LEDs on the front will dim if a fuse trips, indicating a problem - there are far more sophisticated things you can do but this would be enough to stop one device turning off the whole lot.  Even though the LEDs only run at ~1mA, they are plenty bright!

I added a few ferrite clamps to suppress some of the inevitable switching noise, and with some additional decoupling I was pretty much there, short of a few hours with the hand drill cutting out the holes!

And ended up with this...

The module looks so small in comparison to everything else!  In retrospect, I really should have put the toroid further over to the right as it would have freed up some more space for decoupling but too much on the input will hurt the power factor.

Here it is in situ, next to the cable modem and router..

The LED lights are a lot brighter than it looks here!  I think blue LEDs are a bit noughties now, but I had a bunch doing nothing, so why not... to prevent them from being eye-piercingly bright and to not stick out, they were sawn down and sanded to near-flat to give a more diffuse effect. 

Any good?

So... the point was to save some power - did all this work pay off?  Well, the consumption of the new 12V supply running the same gear came out at 7W (9VA, 0.76PF) which is a saving of 5 watts.  Not bad!  I could have increased the PF to around 0.8 by fitting a choke but couldn't find a satisfactory way to mount it... maybe at a later point.

Bigger gains will be had when more equipment is added to the supply - in particular a NAS is likely to feature at some point... have yet to decide which one best suits our requirements although the Silverstone DC01 is looking strong for good performance and low power consumption.

Probably time to consider a Gb router too... :)