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MIT makes strong advances in power conversion research

14 Aug 2014

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Global electricity consumption is pushing into a two-fold increase—creating an enormous market opportunity for the power electronics industry, in which innovations deliver new functionality, improved performance and efficiency, as well as reduced footprints, according to David Perreault, an MIT professor of electrical engineering and head of the Power Electronics Research Group.

"We have a set of interrelated technologies that enable size reductions and efficiency improvements in a broad range of applications, and the techniques we've developed can be applied, or modified and improved for use, in a wide range of other applications," Perreault said. "We often use highly specialised circuit techniques, because we can make the application so much smaller and higher-performance."

Much of Perreault's group's work focuses on ultra high-frequency conversion and ultraminiaturized converters. Its recent work also includes ways to eliminate electrolytic capacitors in power supplies, including those that convert between single-phase AC power and DC devices.

Driving from the AC grid to DC devices

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The front (top) and back (bottom) of a prototype 30W LED driver. Source: David Perreault, MIT

A prototype LED lamp driver gives an example of three of the Perreault group's closely aligned research initiatives in AC/DC power conversion.

"When you buy a cheap fluorescent lamp and it dies, usually it's the power electronics that have failed," he said. "They may have failed because people have tried to do it very cheaply, so they've combined components in a manner that is not particularly good, and they may have failed because the components used aren't particularly good."

The most likely suspects among inadequate components are electrolytic capacitors—passive components that help to buffer energy between single-phase AC wall outlets and the LED lamp.

Manufacturers use electrolytic capacitors because they are cheap and can store the energy in a relatively small volume. "But they are inherently unreliable—they have a liquid electrolyte that dries out—and they have low temperature limits," Perreault said. "So one of our thrusts is developing energy-buffering techniques that let us eliminate the electrolytic capacitors while maintaining small size. In the LED lamp prototype design, the architecture is set up so that it can use ceramic capacitors to buffer the energy. The trick is to do this without making the device bigger or more expensive."

The LED driver exploits another closely related line of research, on very high-frequency (30MHz to 300MHz) switching that helps to miniaturise energy storage and filtering needs in the converter.

Perreault uses an analogy of transferring water in buckets to explain the approach. "A power converter scoops some energy from the input, transfers it, and then throws it to the output," he saiid. "Here the energy-source elements—inductors and capacitors—are my buckets for moving around energy. If I can run this converter much faster, I can use a smaller bucket and still transfer the same amount."

"That approach does a few things for you," he explains. "First, you can change your buckets for thimbles. Secondly, you can physically manufacture thimbles in ways that aren't practical for buckets, so you can do a better job of integrating the various pieces in a power supply. Third is bandwidth: If you want to suddenly change what you're doing, you're only stuck with a thimbleful of energy instead of a bucketful of energy that you've got to redirect, so you can get more speed by going to these frequencies."

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