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Wireless charging metrics arguments heat up

Posted: 06 Aug 2015     Print Version  Bookmark and Share

Keywords:WPC  Qi  wireless charging  A4WP  PMA 

In a 5W cellphone battery, Qi and resonant systems take about the same amount of time to charge despite having different efficiencies. A 7.5W resonant system with 40 per cent efficiency will deliver 3W of power to the battery, while a 5W Qi system that's 70 per cent efficient will also deliver nearly 3W. Perzow said upcoming Qi standards will support faster charge times.

Samsung may have solved the transmitter problem in its Galaxy S6, which has integrated wireless charging and will work with Qi and PMA standards. However, Moore of CSU noted, the chip giant had to compromise its antenna design by de-tuning to accommodate both standards. CSU and WPC did not study multi-modal receivers or antennas, though Perzow pointed to antenna design, switching frequency, and impedance of the receiver coil or interface between receiver coil and output regulator as the three biggest factors in system efficiency.

Switching losses are proportional to higher frequencies, Cameron said, but the extent to which this matters depends on operational specifics. The WPC/CSU study found that the 6.78MHz system produced less more than 200mW of wasted power, even though it used gallium nitride power transistors, which are optimised for lowest possible switching loss.

"When transferred power is smaller, then switch losses represent a larger portion of total energy consumption. For larger transferred power its likely not as much of a concern," he said.

Industry acceptance

The ultimate results of the study, should it get published by the IEEE, remain unknown. While Perzow and Moore hope it will lead to a standard method for comparing wireless power systems, both were sceptical about industry acceptance.

"I don't think everyone's suddenly going to say, 'This is how we're going to measure,'" Perzow said. "I expect that, at best, those that need to know will be able to get to what they need, and will be able to ask more informed questions."

Green said the study will be a conversation starter, but standardisation takes a long time and is a political process.

"Clearly there's the potential for a generic wireless charging industry standard to be introduced in the future, but it's not about to happen in the next few months, and that's not to say that everyone would then adopt it into their certification in the future either," he said.

- Jessica Lipsky
  EE Times

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