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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 

WPC officials compared their power evaluation method to the way the mobile phone industry measures antenna performance. "It has taken a long time and is perhaps contentious, but it is necessary for the industry to have done do," WPC said.

Does it measure up?

The WPC commissioned study found that the high-frequency, loosely-coupled system used 50 per cent more energy than the Qi system (67kJ versus 44kJ). They also observed that the resonant system was about 66 per cent efficient into a load at the output of the receiver whereas the Qi system was 80 per cent efficient. Measurements were taken at the optimized spatial location for each system at 5W.

High-frequency resonant system

'It does not make sense to use a less efficient, high-frequency resonant system when a close-coupled approach will do the job,' WPC said in its study. Source: WPC

WPC's efficiency definition is generic and doesn't acknowledge that all wireless power systems exhibit low conversion efficiency, said Michael de Rooij, executive director of applications engineering at gallium nitride power device developer Efficient Power Conversion Corp. (EPC) and author of a wireless power primer. EPC found that Qi systems exhibit around 70 per cent efficiency and A4WP between 50-80 per cent, though A4WP's systems are strongly dependent on operating conditions.

PMA and A4WP officials balked at the results, citing a lack of attention paid to charging distance. de Rooj added that the WPC study used a resonant EPC demo kit that is not A4WP compliant and designed for much higher power, then "varied the distance to further place the A4WP system at a disadvantage."

"Positioning of the receiving coil matters, as the Qi-based system uses a fixed alignment and distance between the coil sets. Increasing the distance to 20mm for the A4WP system, which does not require a fixed alignment of the receiving device, effectively places it outside of the A4WP standard," de Rooij noted. "Thus, the measurement at this distance should no longer be a relevant data point for comparison."

WPC and CSU used an EPC demo kit in its study, which de Rooij said was not completely A4WP compliant. Measurements were made with a 16W rated coil set operating at one third the rated power, de Rooij noted. If the comparison were made using an A4WP class 2 source coil (10W rated), the results would be more comparable.

Analyst Green of IHS questioned the study's measurement techniques because of power losses from the wall plug to the transmitter as well as transmitter variability. Every mobile phone has a different efficiency because of its transmitter architecture, Green said, adding, "the only consistent way to measure would be if every device had the same receiver design and implementation, and clearly that is never going to happen."

Basic efficiency definitions aside, the WPC/CSU study found that the two charging methods had nearly identical charge times.

"Most interesting I believe was how small differences in load-efficiency behavior could have considerable impact on charging cycle efficiency," Moore said. "We obtained results that, at a glance, most would say looked similar or comparable."

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