Standby for New Power-Supply Metrics

Oct. 1, 2007
Performance metrics are constantly evolving to suit changing user requirements. That certainly seems to be the case with power electronics. For instance,

Performance metrics are constantly evolving to suit changing user requirements. That certainly seems to be the case with power electronics. For instance, there was a time when most power supplies were described in terms of their power density in watts per cubic inch. Although that metric is still frequently used to rate ac-dc supplies, many dc-dc converters are now measured by their current density. That change reflected an emphasis on current delivery, expressed as amps per cubic inch or square inch, as supply voltages fell and current levels rose.

Some metrics, like power density, do not normally show up on a datasheet. Instead, they'll appear in a vendor's promotional materials, or in articles or reports that discuss technologies in broader terms without reference to specific products. However, efficiency is a common metric of power-supply performance, and it's certainly one you'll expect to see on power-supply datasheets.

There's a difference between the way efficiency is expressed as a stand-alone metric of power-supply performance and how it appears on the datasheet. In promotional materials, power-supply vendors will routinely quote a single value of efficiency as a measure of their products' performance. But naturally, this just reflects performance for a specific set of operating conditions. Hopefully, for the user's sake, the datasheet goes into much more detail about how efficiency varies over line, load and temperature.

Efficiency as a metric of power-supply performance is just a snapshot or summary of a product's overall or typical performance. Often, it's a measure of performance at the rated load. But as demand for energy efficiency grows in countless applications, the use of a single-value metric to describe power-supply efficiency is becoming insufficient.

As a result, new metrics are emerging. For example, if you look at this month's Product Innovation (page 47), you'll read about a power-supply controller that's been tweaked to improve light-load efficiency of ac adapters and other ac-dc supplies used in consumer products. Here's a case where the need to measure power-supply efficiency under light loads or in standby operation is largely driven by regulatory standards established by the Environmental Protection Agency, the California Energy Commission and other agencies.

A similar trend is happening in power supplies developed for use in data centers. A recent announcement of a new server power supply heralds the product's light-load efficiency, noting how this supply exceeds the light-load efficiency goal established by the Climate Savers Computing Initiative (see New Products, page 49).

As these stories suggest, light-load efficiency is becoming another popular metric for power-supply performance. So, expect to hear it quoted routinely by power-supply vendors and chipmakers alongside the “typical” efficiency values that are given when touting new products. Also, expect to see the term used in the broader discussions of power-supply technology and where it is headed.

Given the ever-evolving nature of power supplies and applications, perhaps other metrics might be needed to assess power performance. Consider another emerging trend: power supplies with ultrawide input-voltage range. Think universal input, which typically signifies 85 Vac to 265 Vac, but then extend that range even further.

For example, one of the features in this issue discusses the design of an ac-dc power supply that operates over the range of both residential and industrial ac line voltages (see “SMPS Design Extends Universal Input to 690 Vac,” page 20).

That particular design targets energy meters. Nevertheless, there are other examples like DIN-rail power supplies that are being designed to run off any available single- or three-phase ac source (see New Products, page 49).

Will old metrics like power density, typical efficiency and cost per watt be sufficient to describe such versatile power products and to rate how they are improving and how they live up to application requirements? Or are new metrics needed, ones that reflect new capabilities and changing needs?

Feel free to share your thoughts on how we might better judge different aspects of power-supply performance by e-mailing me at [email protected].


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