THINKING Differentiating between operating efficiency and average efficiency in power supplies

Words by David Buck, market development manager, TDK-Lambda B

efore the energy efficiency standards were introduced, the power supply

product datasheet and specifications referred only to the operating efficiency. As the actual efficiency varies with the output load, input voltage and component tolerance, this is stated as either a typical or minimum figure. The conditions are typically written as 93 per cent efficient at 230Vac input, 100 per cent load, and a stated ambient temperature: this is likely to be a best-case scenario. Efficiency changes with

input voltage, which is primarily due to wound component and MOSFET semiconductor resistance losses (power = current x resistance). Efficiency also reduces at lighter output loads, due to the internal fixed losses from the control circuitry or internal fan, and the effect of the switch-mode converter running at a narrower (sub-optimum) pulse width. The output voltage of the power supply can also impact efficiency. Different output rectification techniques, such as low loss synchronous rectification, may be used on lower output voltage models. A 12V model will have a higher current rating than a 24V or 48V model, hence higher resistance losses. Most power supplies operate at 115V / 230Vac and are seldom run at full load. Without an efficiency curve, system designers cannot correctly predict the amount of heat that the power supply will dissipate. As the internal temperatures rise, component reliability reduces, resulting in a shorter equipment lifetime. To compensate for this, a


fan-cooled power supply, or a system cooling fan, may be used. The figure below shows the effect of load, input and output voltage on TDK- Lambda’s CUS150M AC-DC power supply series. Armed with this information, a more informed decision can be made in regards to system cooling.

rated between 49-250W, has to be at least 89 per cent.

Although designed for internal use and not classified as an external power supply, the average efficiency of the 24V output CUS150M, operating at 230Vac, is calculated to be 92.8 per cent. The product datasheet states that the operating efficiency is up to 94 per cent, with an average efficiency of >91 per cent, which is better than the >89 per cent legislation requirement.

The term ‘average efficiency’ was introduced when energy efficiency legislation was passed for external power supplies, used with consumer devices. A power supply for a laptop, tablet or phone charger will operate at low loads for the majority of the time, once the device’s battery is charged. As millions of these power supplies are in use, daily and worldwide, the legislation was intented to minimise energy losses. Average efficiency is calculated

by measuring the power supply’s efficiency at 25, 50, 75 and 100 per cent output loads. The sum of these four measurements are added together and divided by four to obtain an average efficiency figure. To comply with the latest DoE Level VI and EU Tier two efficiency standards, the average efficiency for an external power supply,

In summary, operating efficiency and average efficiency are not measured and/or calculated using the same methodology. Both values are useful, however, and can provide a quick indication of the product’s overall performance. In the anticipation that the scope does extend to other product types, system manufacturers are starting to specify internal power supplies that meet this legislation. An added benefit is that it also enables them to differentiate their end equipment by reducing operating costs.


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