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The truth about wireless power transfer (Part 3)

Posted: 14 Apr 2015     Print Version  Bookmark and Share

Keywords:Wireless power transfer  WPT  electromagnetic interference  EMI  <i>Rezence 

On Feb 11, 2014, A4WP and PMA announced that their two associations had signed an agreement aimed at establishing global interoperability of their heretofore distinct standards. Lest this lead to a lot of excitement about convergence of the standards, the reality check is best stated in The Verge:

"Technically, it's a little bit more complex than that. According to the presidents of both alliances, the PMA and A4WP aren't completely merging their organisations, and they also won't necessarily be combining their two different charging methods into a single brand like Rezence quite yet. They've simply come to the conclusion that each group had a piece of the puzzle that the other needed. The PMA gets the A4WP's so-called resonant wireless charging spec, which can wirelessly charge several gadgets at once, and without requiring them to be oriented in any particular direction on a charging pad. The A4WP gets the PMA's legacy inductive charging tech, plus cloud-based software infrastructure for managing and monetizing charging points, which could direct smartphone users to the nearest charging hotspot when they're low on juice, and allow Starbucks to let its customers top off those devices with the purchase of a latte.

What both parties get, though, is a leg up on Qi. Originally, the Wireless Power Consortium's Qi standard was the front-runner in wireless charging—attracting device manufacturers like HTC, LG, and Samsung to build phones and tablets with the tech—but over the past couple years the PMA and A4WP have attracted a host of supporters, including Intel, AT&T, Starbucks, McDonalds (via PowerKiss), and The Coffee Bean and Tea Leaf. Qualcomm and Samsung have been playing the field, presumably hedging their bets, but both are founding members of the A4WP. WiTricity, the MIT spinoff that invented resonant wireless charging, is now an A4WP member as well.

"The stars are beginning to align for wireless charging tech"

The competing Wireless Power Consortium hasn't been sitting idly, mind you, inking deals with PowerbyProxi and Texas Instruments to build its own resonant charging solution, and attracting Verizon Wireless and Qualcomm with seats on the WPC board. But even if we're left with two competing standards bodies, the idea has the potential to snowball now that so many companies are aligned. With upcoming dual-mode devices that support both PMA inductive and A4WP resonant wireless charging, the two cooperating alliances believe the wireless charging market can open up. "Because of the collaboration, you've future-proofed yourself going forward," says A4WP president Kamil Grajski. "There's no reason for anyone to hesitate any longer."

The entry of magnetic resonance into the Qi-standard
Knowing that Microwave Power Transfer (MPT) offers signification spatial freedom, albeit at potentially significant environmental cost, that technique was largely ignored in this article. In contrast, both the MI-based standards, offer x and y freedom (off-centre spatial freedom) of typically 5 mm, whereas the z-freedom (vertical separation) is only about 2 to 3 mm (measured from the transmitter coil surface). In contrast, MR-based units (e.g. Rezence) potentially offer greater spatial freedom (even more z-freedom), and that is the main reason for the increasing attention in this relatively still immature technology. But some recent reported experiments offering 30 to 40 mm of z-freedom in Qi-compliant systems have spurred interest further. Back in June 2011, Fulton Innovation had already demonstrated up to 40 mm "z-freedom" by using WPC-compatible magnetic resonance ("MR") technology. The new draft standard v1.2 of Qi is expected to address MR.

So, MI has finally entered the MR domain, albeit at a much lower frequency than Rezence. It could run into EMI problems though, because it does not use CISPR 11's "intentional" EMI bands mentioned previously. But special techniques are being developed to overcome that.

This takes us to an interesting statement which was included in the April 2012 newsletter of WPC while announcing the Qi 1.1 specification (italics below added by this author):

"At this moment 6 additional transmitter designs are under review. One of these 6 transmitter designs uses magnetic resonance techniques to transfer 5 Watt into a Qi receiver through a 40 mm table top". That was indeed interesting: a resonant transmitter powering a Qi receiver, producing 40mm of "z-freedom".

This implies a certain degree of backwards compatibility and interoperability too, though it was not fully clarified at the time.

In June 2014, at Computex, a New Zealand company, PowerbyProxi, currently being funded by Samsung among others, demonstrated a resonant charging bowl for 7.5 W and also, a 15 W transmitter unit offering 30mm z-freedom. This was Qi-compatible. PowerbyProxi claims over 200 patents and was formed out of IP developed for the past 25 years at the University of Auckland.

In Oct 2014 this was on the wire:

"ConvenientPower unveiled today the Tx R2 and Tx R3 resonance charger series, the Tx R2 being the world's first resonant platform enhancing the recently approved Qi wireless charging A27 specification. The new generation of resonance wireless charging solutions include the first commercialized intelligent continuous charging through varying distance of up to 30mm coil-to-coil which is enabled through a Qi compliant charging platform, the newly approved Qi v1.1 A27....The first to be designed to wirelessly charge Qi devices or receivers in the market, the Tx R2 is Qi compliant up to 5mm and Qi compatible up to a charging distance of 20 mm. The newly approved Qi Addendum A27 underpins the Tx R2 extensions enabling resonance charging. ConvenientPower is the first to launch resonance charging through the Qi Addendum A27, the new power transmitter description offered in the Qi v 1.1 specification. The Tx R3 resonance charger features uniform charging across the charging surface and distance up to 30 mm, with the device placed in any direction and location. Wireless charging at up to 35 degree angle while delivering competitive efficiency levels are also key features of the resonance series".

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