Power Conversion Gets Digital

The public profile of the digital domain is far more prominent than its analogue cousin; it has become the ‘poster child’ for an increasingly tech-savvy audience coming to terms with how or why things ‘just work’. Invariably, when the term ‘digital’ is used in an advertising campaign or product brief which is aimed at a non-technical audience, it is intended to promote a feeling a quality and modernity, without necessarily explaining why.

Currently the drive is behind digital TV and (in the UK, at least,) DAB (digital radio). Just why these things are being presented as automatically ‘better’ just because they’re digital doesn’t seem to be a valid point for exploration. Of course, the real reason behind media providers making a wholesale move away from analogue transmissions has more to do with bandwidth allocation, but that too seems to be in the sidebar.

To an engineering audience the digital domain has been around a lot longer than its public image would suggest, and so is already well known and well understood to provide a level of functionality, portability, predictability and even serviceability that is difficult to replicate in the analogue domain. There will always be a case for analogue (even DAB modulates sound using an analogue signal) but it’s difficult to argue that digital doesn’t deliver on these fronts.

Furthermore, there is one area that even the most hardened analogue devotee would probably concede is improved by the advent of a digital aspect; control. The ability to quantise any value without discrepancy means that control is really digital’s trump card and is, fundamentally, what lies behind its present day success.

Because of this, digital control has now moved firmly in to the analogue domain and most notably in to power conversion. The argument here is that digital control provides greater efficiency in power conversion, in a number of ways. There are now numerous companies focusing on digital power conversion, spanning AC-DC and DC-DC, using various techniques but ultimately with similar objectives.

Efficiency Gains

One of those objectives is, of course, efficiency. Improving AC-DC conversion efficiency could have a massive impact on the environment due to their proliferation; almost all modern day devices operate from a DC voltage, stepped down using an AC-DC converter. While governments are keen to adopt greener technologies, it seems the industry isn’t always prepared to accept greater efficiency at a cost; the solutions on offer must also present a cost saving along with efficiency gains.

This has implications on the solutions being developed; any additional silicon area has a commensurate impact on manufacturing costs and this has precluded the use of what really represents the best technical solution to adding digital control to power conversion; using a digital signal processor (DSP).

One company that believes it has overcome this hurdle — and has filed around 120 patents to protect it — is iWatt. Its core technology is called PrimAccurate, which focuses on the primary side of AC/DC conversion and it underpins most of what it does. Not only does it avoid the need for a DSP but it also removes the use of opto-isolators as used in the feedback path. While a number of iWatt’s competitors have eliminated opto-isolators in the feedback path, it claims none have moved away from a DSP based solution, which the company maintains increases the overall cost.

The solution iWatt has developed uses PID (proportional, integral, differential) control implemented using a state machine. The waveform analysis has limitations, however; it only works when the secondary stage operates in continuos flyback mode, and the technology can’t scale beyond 40W. The most recent product launched by iWatt to use the technology is the iW1760, which is optimised for up to 40W operation while achieving less than 50mW standby current consumption.

Shifting Complexity

Another company using ‘hidden’ digital power conversion is Exar with its digital PWM/PFM programmable power management system — the most recent example being the XRP7724; the company’s third generation of programmable power conversion but the first to implement PFM (pulse frequency modulation).

Again, the key to this technology is the implementation of digital control in the form of PID using a state machine and therefore avoiding the expense of a DSP core.


Exar is targeting the Industrial and Communications markets with its 3rd Generation of programmable power conversion devices

According to James Lougheed, Vice President of the Power Management Division within Exar, this means the solution can address low current applications at a price equivalent to purely analogue-based controllers — while most of Exar’s competitors must target higher current applications which can support the higher cost of a processor-based solution.

While Exar has seen success with its 2nd Generation power conversion devices in consumer applications — probably most notably in set-top boxes — Lougheed stressed that the XRP7724 is aimed at the industrial and communications markets: “ARM-based servers will be a big market for the 7724,” predicted Lougheed.

Using a state machine is much simpler — both for Exar and its customers — than a ‘fully programmable’ solution, as well as being lower cost. However, with other device manufacturers entering this market on an almost daily basis, with product offerings that offer a greater level of programmability (such as Microchip), target increasingly specific use-cases, it seems likely that choosing the most appropriate power conversion solution could rapidly become more complex than actually deigning with them.