Power supplies go digital

Just about everything else in IT has gone digital and now it's the turn of power, as power supplies - which are traditionally analogue - increasingly go digital instead.

Technologists claim this could mean cost savings for end users thanks to improved energy efficiency and reliability, plus cost savings for manufacturers thanks to cheaper and more repeatable designs.

The initial pull for digital power comes from telecomms and datacomms. Driving it is the widespread concern over how much power is being consumed - and heat generated - by high performance networking gear, huge server farms and so on.

Of course, power itself is inherently analogue, but it's the control side that is amenable to digitisation. Modern switch-mode power supplies already create one voltage by rapidly chopping another, and chip designers say managing this process digitally is less complicated.

Texas Instruments (TI) is one of the companies working in this field. TI's digital power product manager Steve Bakota says that for some years now the company has sold its general-purpose digital signal processor (DSP) chips into areas such as power rectification for telecomms equipment.

He adds though that the key for future growth is power-optimised controllers - reprogrammable chips specifically designed for power supplies, capable not only of delivering the desired output, but of monitoring it and adjusting themselves as necessary.

Analogue needs wide tolerances

With typical analogue control circuits, the power supply must be designed and built for a specific output or range of outputs. It must be tested on the bench and then 'programmed' by using the resistors, capacitors and so on that give the desired result.

And because the load on the power supply will vary - according to how busy is the computer it's supplying, for example, or because the properties of its components will vary as they age - it must be designed with plenty of tolerance. That in turn makes the power supply less efficient, because it is almost never running at its optimum.

The advantage of digital control is that its programmability and feedback can enable a simpler design to do a whole lot more.

"If you have a controller that happens to be digital, you program it by putting numbers into a GUI," Bakota adds. "If the system designer has more information about power consumption and dissipation, there's a lot you can do to optimise your power use.

"For example, if a server runs hard to 10pm, then light to 3am, it would be good if you could power it down - with digital control, you can adjust the power supply according to the load. Most power supplies are designed for maximum efficiency at a specific load, but with digital control you can have multiple modes, or even linear functions, to optimise according to the system's needs."

As well as potentially reducing energy costs, TI says digital power supplies could help detect system failures earlier by detecting shifts in power consumption, as well as reconfiguring themselves as the system's passive components age.

Digital adapts and evolves

"For example, capacitors can change over time, but by adjusting or reprogramming you can compensate for that," Bakota explains. "The designer has to build margin into the power supply to allow for the worst case scenario - a digital design can self-adapt to the system's real characteristics. That means less need for margin in the design, which means lower cost and greater reliability."

He adds that digital controllers are a lot easier to program, and because they are programmable, the same chip can be used with different software for many different power applications. That assumes you have the right software tools, of course, which is why TI has developed wizard-type programs to help the designer convert their power parameters into code for the DSP.

"The proportion of digital power supplies is very low today," he says. "Conceptually it's very attractive, but it's a big change because it requires different competency. The people skilled in analogue design are not skilled in DSP software design, so products which allow parameterisation will be key."