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September 2, 2013

VHF Power: The SMPS Revolution

Tony Sagneri

Power Channels: Batteries and Portable Power, Communications Power, Digital Power, Packaging and Cooling, Power Components, Switch-Mode Power

Year-on-year consumers expect greater functionality and increased performance in ever-smaller packages. Whether cell phones, laptops, all-in-one PCs, or the latest flatscreen TV, some dimension is shrinking while features and performance explode. This creates an interesting challenge in the field of power electronics. Efficiency improvements aside, the weight of consumer expectations has created a mismatch between the capabilities of the existing SMPS and the penchant we all seem to share for compact and powerful consumer devices.

Nowhere is this trend more obvious than in the current class of high-performance, ultra-portable laptops, “ultrabooks.” In this space, it’s common to find the ultrabook weighing in at a trim 2-plus pounds and the companion power adapter adding another 25% to the mass and volume, a virtual boat anchor—and those are best-in-class numbers. Look a little harder and you’ll find examples where the adapter exceeds a shoulder-cramping 40% of the laptop’s weight and volume—talk about the elephant in the room!

One solution is to turn up frequency…a lot. Of course, you say, switching frequency has been climbing since the advent of the SMPS. What we’re aiming for here is a discontinuous change—10 to multi-hundred-watt, AC/DC (off-line) converters with switching frequencies in the neighborhood of 30 Mhz to well over 100 MHz. That’s on the order of 1000 times current practice. That last happened in the 1970s when off-line converters went from rectifiers switching at 2x line frequency to the multi-kHz regime of the SMPS.

While it may seem like a lot of trouble to increase frequency in one big jump, there are a few compelling reasons to do so. One is pretty clear: In the SMPS size, weight, and cost tend to be dominated by the passive components and a dramatic increase in switching frequency leads to significant wins. At 30 MHz the required inductance and capacitance for an off-line converter around the 100-W level is measured in the 100 nH and 100 pF ranges. At these levels small, air-core inductors and transformers that are printed in-PCB and surface-mount capacitors are used throughout much of the system. With no magnetic materials and batch assembly, this translates directly into cost and manufacturing benefits.

Some of the other reasons to make such a large jump are subtler. Modern magnetic materials create a frequency no-man’s land. Flux derating necessary to reach 1 –2 MHz in an off-line converter can actually yield larger magnetics in an effort to stem peak AC flux swing and peak flux density—definitely not the desired result. While, throwing away the magnetic core in favor of air-core designs also doesn’t work at a few MHz, as these implementations don’t win in size until frequencies reach 20 – 30 MHz.

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