Economic and environmental factors are playing an ever increasing role in the drive to reduce the power consumption of electronic equipment.
Today, the biggest demand for high efficiency power supplies comes from the operators of large data centres, as part of their efforts to improve the power use effectiveness (PUE) of their facilities, driven by the huge annual cost of electricity they consume as well as legislation for the environment. Demand has also come from consumer electronics responding to Energy Star (USA) and ErP Eco-design Directive (EU) requirements. However, in the industrial and medical equipment markets, more and more manufacturers of end systems are starting to ask power supply manufacturers not only for the headline efficiency of products, but also for high efficiency levels over the typical operating load range of their equipment, as well as for low standby power consumption. Even when their own equipment does not fall within the scope of any standards or Directives today, there is a real desire to produce environmentally responsible products on a voluntary basis. Put simply, the efficiency of a power supply is measured by comparing the power going in to the dc power coming out. If, for example, a power supply consumes 1kW in order for it to provide 900W of dc output power; the efficiency of the supply is 90% and 100W is dissipated. By comparison, an 80% efficient power supply would dissipate 225W. The direct savings in kWh of electricity usage and the indirect cost of cooling systems to deal with the wasted energy, in the form of heat, are significant. Since there are no efficiency standards for embedded industrial and medical power supplies, manufacturers are now looking at ways to benchmark their products in a way that can be understood by their customers, voluntarily following standards derived for power supplies used in applications such as computing. One such standard is the Climate Savers Computing Initiative. Started by Google and Intel in 2007, the Climate Savers Computing Initiative aims to improve the energy efficiency of computing equipment and reduce the energy consumed when the computer is inactive or in standby mode. Working hand in hand with the ENERGY STAR program, which acts as a technical baseline, and the 80 PLUS program, which uses the same naming conventions, the Climate Savers categorisation of power supply efficiency stretches from Bronze though to Platinum levels. In November 2010, Climate Savers announced plans to also develop efficiency targets for networking technologies. In order for computing equipment manufacturers to meet the obligations laid out in Climate Savers, they need to use power supplies conforming to relevant specifications. • Non redundant capable power supply units. Generally lower power devices (300 to 800W) typically used in pcs and workstations • Redundant capable power supply units. Generally 1kW and more, typically 'hot swap' style used in server racks. For the non redundant standard, only Bronze to Gold levels are specified and the minimum efficiency levels are slightly lower than those for redundant power supplies, which also include the higher Platinum standard. For embedded medium power (300 to 400W) industrial/medical power supplies, it is appropriate to choose the Climate Savers non redundant Gold standard as a convenient benchmark. Power supplies for information technology applications have reasonably benign operating profiles – they are rarely at more than 50% load and experience moderate load step changes – such that the efficiency can be optimised around a relatively narrow operating band using analogue techniques to minimise the switching duty cycle as much as possible. However, power supplies for industrial and medical applications have a much broader range of operating conditions, including significant load step changes in some cases, and this requires a more robust design approach to achieve the same efficiency performance without compromising characteristics such as dynamic load response. If considered early in the design stage, it is possible to reach Gold status, although significant challenges have to be overcome. Climate Savers efficiency targets do not differentiate according to the output power ratings of power supplies. At low loads, this presents real challenges for 300 to 400W power supplies, compared to 1000W units, because the residual load independent losses, due to factors such as housekeeping and inrush control, become a much lower proportion of total losses in higher power units. Recent advances in power supply design involve digital power conversion technologies. By replacing analogue circuitry with a microcontroller for housekeeping routines, such as controlling the timing to turn power on and off and controlling the PFC, the maximum efficiency can be improved to more than 92%. Where digital power conversion has a real impact (and a significant advantage over analogue control circuits) is in opening up opportunities for design optimisation to improve efficiency levels further, especially at loads of 50% load and less – in particular, to meet the stringent Climate Savers requirements at 20% load. The use of an interleaved boundary mode boost converter, where the two converters operate at 180° to each other, is a useful technique. At less than 50% load, one of the converters can be programmed to disable, which reduces losses and increases efficiency. An added benefit of this approach is that the current through each PFC choke is halved and thus reduces the ripple current in the boost capacitor – this in turn reduces the temperature of the boost capacitor and increases its life. At really low loads (approaching 20% and less), a PFC burst mode function can be implemented, which switches the converter on and off in short bursts to improve the low load efficiency of the unit. The beauty of a digital control implementation is that a multivariable control law algorithm is used, allowing continuous optimisation of the boost voltage and duty cycle according to the load level, allowing optimum efficiency to be maintained, even at very low loads. An analogue control approach has to be optimised to a specific load point since multivariable control cannot be implemented in a practical manner. TDK-Lambda UK continues to maintain high levels of R&D and is committed to developing higher efficiency power supplies. Efficiency levels of 94% and higher for 300/400W power supplies can be expected in the near future and a key benefit of digital power conversion techniques will be that infinite real time efficiency optimisation will become much more common. Further into the future, advanced component technologies will have a significant benefit to further improving efficiency. GaN diodes, for example, could replace the costly SiC diode in the PFC circuit and GaN fets could provide significant speed and performance benefits. With these new devices, the choice facing designers will be whether to return to more simple lower cost power supply circuits that can achieve the same levels of efficiency as now or to push efficiency even higher – and this decision could be made for them by legislators continuing to raise the bar. Andrew Skinner is chief technology officer for TDK-Lambda UK.