Until relatively recently, the idea of trusting system power supply control to software was viewed as potentially foolhardy. When a single bug in a line of software could result in thermal meltdown or a catastrophic voltage being dumped on the system, it is no wonder that the use of digital power controllers was largely ignored.
Analogue controllers, by contrast, could be trusted to protect the system – and were typically a third of the price – and so were the standard option. However, as prices of digital componentry have come down to virtual parity with their analogue equivalents, their advantages are now being looked at more closely. Shane Callanan, director of applications engineering for Excelsys Technologies, commented on the limitation of closing the control loop with analogue components. "It is a slow, but stable, process. We can make control loops underdamped, overdamped or critically damped – but if you have an analogue solution, you have a fixed reaction. You can't change the parameters of your control system from day one. "Up to maybe seven years ago, all control systems were analogue, but now there is the potential to use a digital control loop and the beauty is this can be dynamic; you can alter sensitivity (or reaction times). The other joy is that you can 'talk' to it through the software – find out if it is behaving itself. What voltages are there? What current is it delivering? What is the temperature? How many times was it turned off or on in the last 48 hours? You can do a whole load of stuff under the label of digital power management, or DPM." A number of companies have started introducing options for DPM and DPC (digital power control) but, according to Callanan: "A lot of customers in areas such as medical do not want their power systems controlled by software," he claimed. "Their concern is 'what happens if a DPC locks up and suddenly there is 70V on my load?'. So, while they are insisting the control loop continues to be analogue, we are permitted to add some digital power management – where we would take the information, feed it into the microcontroller and that way we can create a history of how the power has been used. For the system designer, it opens up a whole host of options for monitoring." An enabling technology here is the Power Management Bus (PMBus). Implemented over the SMBus, this open standard power management protocol facilitates communication between power devices in a power system. Combined with the virtues of DPC, this opens the door to system control, programming and real time (or very close to real time) monitoring. "You can control it, read back information, set flags. You can tell it how to respond if it starts to fail – it makes if very flexible for the systems integrator," commented Callanan. "Digital controllers are becoming more standard in power systems. Obviously, they are used a lot at the moment in microprocessors, but while that is at the point of load, we can see the methodology working its way to the front end of the system, to where the ac mains comes in. Digital controllers are getting more traction. There is no doubt that, over the next five or ten years, there will be more convergence on DPM and DPC. It may be the loop is closed using analogue, but there is more integration with the DPM through microcontrollers. That doesn't need to happen real time; if you are looking for temperature readings every few seconds on a power supply switching at 200kHz, then that is a lifetime! If I need to know what is happening 200,000 times a second, then that is getting beyond the microcontroller and I would have to go to a dsp core, something more expensive. But I think we will see a balance between the two in the next few years." One company that has made huge strides in digital power control is TDK-Lambda, which has recently opened an advanced technology lab in Bristol to looking at the development of digital power supplies. The choice of analogue or digital control is down to more practical reasons than faith in the technology, says the company's chief technology officer Andrew Skinner. "Most of our customers use dsps and fpgas in their products and are comfortable with digital technology. The most important thing for most of our customers is that the power supply performs its job reliably. How it is controlled or what is inside it is largely irrelevant." There are cases, purely for practical reasons, when analogue control is the preferred option. "In my experience, most older engineers are comfortable with analogue because they grew up with it. In terms of power conversion, it tends to be that younger engineers are working in the digital domain," commented Skinner. "But most people who have been shown the benefits of a digital solution are happier with the flexibility it provides over an analogue solution." Some end user nervousness about software reliability may have resulted from the recently introduced Third Edition of the medical standard EN60601, which refers to the integrity of the software in terms of a risk assessment of the equipment. However, as this is applicable to the function of the end product, it should not impact on the embedded systems, such as the power supplies. Skinner points out that commonly used hardware safety solutions, like crowbar circuits, obviate the need for having to write safety critical software for the power supplies. Other considerations include variations in programming languages from different manufacturers, but the advantage of using an industry standard solution like the PMBus removes this as a problem. In fact, because this bus sits on an i2c bus, it fits into the overall requirements of most systems, and an analogue control loop for the power supply could be the only analogue component in the system. Digital isn't always necessary Skinner believes the switch to digital is not always necessary. "You need to look at things like reuse. If someone has a design that works with an analogue interface and they are happy with it, then they may want to stick with that solution, not necessarily because it is better, but because they have already got it, the development costs are lower and development time is shorter. If someone is looking to extend their range with cost reduced options, they are more likely to look at reuse, rather than redevelopment. Starting from scratch tends to happen when companies bring out a new platform." Digital control loops therefore appear to be the way forward, particularly in equipment requiring power system control or reporting, where flexibility to be incorporated across a range of products and an appropriate communications bus is available, or where space is at a premium. However, if these are not issues, but saving on development time and costs is, then reusing a legacy analogue solution may still tick all the boxes.