Testing for standby power consumption

External power supplies are, as of 1 April 2008, subject to minimum energy performance standards (MEPS) as specified by AS/NZS 4665. The standard encompasses devices such as adaptors, plug packs and battery chargers. The standard calls for maximum permissible standby power consumption levels, which are demanding, for example, less than 1 W for 300 W outputs and 500 mW for supplies with a 10 W rating or lower. The new standards reflect the increasing attention being paid to the double-edged sword of the rising cost of energy and greenhouse gas levels. In Australia, standby losses overall are estimated as being in excess of 5%. Add this to transmission losses of the order of 7% and it becomes obvious that effecting decreases in energy waste is a pressing matter.

The industry drive towards smaller, lighter and more efficient electronics has led to the development of switching-mode power conversion technology. Switch-mode power supplies (SMPS) incorporate power handling electronic components which are continuously switching on and off with high frequency. Linear power supplies can only step down an input voltage. There are always associated ‘headroom losses’ occasioned by the voltage drop across the device passing the load current. In addition the use of mains frequency input transformers in linear power supplies militates magnetising current losses that make it very difficult, if not impossible, to meet the stringent standby loss maxima that are now being imposed. The use of switch-mode topology, on the other hand, allows for the switching device to be either hard on or hard off, thus limiting the headroom losses due to very low saturation voltages. SMPS are therefore able to reach very high efficiencies in the vicinity of 70–90%. They effectively connect and disconnect energy-storing inductors to and from the input source or the output. By varying duty cycle, frequency or phase shift of these commutations, an output parameter (such as output voltage) is controlled. Typical frequency range of SMPS is 50 kHz to several MHz. The high operating frequency results in the smaller size of switch-mode power supplies since generally the size of power transformers, inductors and filter capacitors is inversely proportional to the frequency.

Headroom losses and the cost of heat sinks make linear power supplies uneconomical for many applications above output powers of 10 W. An unhelpful feature of SMPS is that their input current has very high harmonic content, peak current being drawn at voltage peak, and only for a very short conduction angle, thus making the accurate measurement of low level power consumption (for example, during standby mode) problematic.

The requirements to measure input power sufficiently accurate as demanded by the MEPS regulations make many power analysers unsuitable for the task. Securing a stable, very low harmonic distortion and source impedance voltage supply is also important. In practice, this means that mains supplies cannot be relied on. Furthermore, the VA rating of the AC source used should exceed by a large factor (x 10 being suggested in testing procedures) that of the power supply being tested. As far as the power analysers to be employed is concerned, the instruments need to perform accurately under conditions of very high crest factor input currents. Furthermore, the sampling frequency needs to be such as to provide a bandwidth far exceeding the basic switching frequency of the power supply being analysed.

The new standby energy measurement requirements specified in the current AS/NZS 4665 standards make the selection of good measurement protocols as well as appropriate power and energy analysis instrumentation an extremely important matter. With regard to analysers, it has to be faced that many used for standard polyphase measurement tasks are simply not of practical value when it comes to milliwatt levels.

*John Thompson, managing director, Westek Electronics.