Wireless power for charging mobile devices

Wireless power has become a hot topic as wireless charging of mobile devices is get getting some popularity. Wireless charging isn’t something new; the technology exists since 1981 and Nikola Tesla has made first wireless power experiments over 100 years ago. Wireless charging for Qi technology is becoming the industry standard on smartphones (pushed by Wireless Power Consortium) as Nokia, HTC and some other companies use that. There is a competing AW4P wireless charging standard pushed by Samsung ja Qualcomm. And there is more standards coming. Power Matters Alliance is heavily pushing their own wireless charging standard. It seems there is going to be fight on wireless charging in near future. It seems that right now we’re in the midst of a battle between two standards for wireless charging – Qi from the Wireless Power Consortium and Power 2.0 from the Power Matters Alliance. It seems that a common Wireless Power Standard Years Off as Battle Heats Up.

As obviously useful as wireless charging is, it suffers from a Tower of Babel problem with incompatible standards and competing interests keeping it from truly going mainstream. Wireless charging continues to be a niche category until there’s a common standard. Heavyweights are backing the idea of wireless charging capabilities embedded in phones, and public charging stations are beginning to pop up. Differing standards, however, still make for a rocky adoption. Realistically there probably isn’t room for two or more standards, which do essentially the same to end user but are incompatible, so expect some technologies to disappear in the near future. Charging portable devices without needing to carry a power adapter sounds handy when we can agree on one standard. “Wireless charging continues to be a niche category until there’s a common standard,” said Daniel Hays, a consultant with PricewaterhouseCoopers. “The hassle factor is still high.”

Qi seems to be at the moment standard that gets most attention. The news that Nokia to join Qi party with wireless-charging Lumia 920 have given lots of publicity to it. Even if the Lumia isn’t a big seller, the publicity and visibility it will provide for Qi should be enough to make everyone forget there was ever an alternative, if indeed there ever was. Also some HTC phones and Nexus 4 phone use this standard. Toyota launches the world’s first wireless charging of mobile phones in the car. Toyota’s car will get wireless mobile phone charger using Qi standard.

Qi has been here for some years. Qi has been around for a while, gaining the name and logo back in 2009. The Qi standard came out of water filtration units, which needed wireless power, and has been widely endorsed but devices are still quite rare. Under the Qi specification, “low power” for inductive transfer means a draw of 0 to 5 W, and that’s where mobile device charging solutions most probably go. The system used inductive coupling between two planar coils to transfer power from the power transmitter to the power receiver. The distance between the two coils is typically 5 mm, but can be expanded to 40mm.

The Qi system uses a digital control loop where the power receiver communicates with the power transmitter and requests more or less power via backscatter modulation. Besides low-power specification up to 5 watts, there is also a medium-power specification will deliver up to 120 watts. The frequency used for Qi chargers is located between about 110 and 205 kHz for the low power Qi chargers up to 5 watts and 80-300 kHz for the medium power Qi chargers.

Method: inductive coupling between two planar coils
Frequency: 110-205 kHz (80-300 KHz)
Communication: backscatter modulation

WiPower was a technology start-up company that used the principles of inductive coupling to develop a near-field wireless energy transfer system. Qualcomm bought WiPower in 2010 and started quietly negotiating with manufacturers to get the technology embedded in their kit. Qualcomm argues that the additional range of WiPower (which can charge devices up to 45mm away) allows new possibilities. WiPower system is based on modified coreless inductive technology and dynamically adjusts power supplied by the transmitter to power demanded by the receiver without the need for control systems or communication. WiPower chargers are claimed to operate at about 60-75 percent efficiency.

Method: inductive coupling
Communication: no need for specific communication

Samsung and Qualcomm’s Alliance for Wireless Power (A4WP) promises more flexibility in wireless charging. Instead of induction, this standard will use loosely-coupled (LC) wireless power transfer (a series resonance-tuned pair of magnetically-coupled coils) to transmit power. This construction allows that the transmitter and receiver don’t have to be in direct contact, which gives more flexibility to industrial designers. This designs will support simultaneous charging of multiple devices with different power requirements. A4WP specification takes advantage of Bluetooth 4.0. The biggest downside in this design is that currently there are no products with this technology are yet on the market.

Method: series resonance-tuned pair of magnetically-coupled coils (loosely coupled)
Frequency: 6.78 MHz
Communications: Bluetooth 4.0

The Power Matters Alliance (PMA) is working on an open standard for wireless charging. A group of companies back up this initiative (including Google, AT&T, ZTE, Starbucks, ,McDonalds, PowerKiss). PMA uses inductive charging method used in Duracell’s Powermat product. It requires the transmitter and receiver be close together, placing the mobile device on the charging pad.

This is quite new alliance but it seems to get lots of backers: over the last few months, the PMA has seen a tenfold increase in membership. One very big thing is that AT&T is seeking from its handset vendors a commitment to one standard of wireless charging.

The PMA is working to advance the widespread acceptance of the wireless power paradigm in multiple sectors. PMA is intent on leading and organizing the Power 2.0 agenda to commercial realization, while working under the umbrella of the most trusted name in standards: the IEEE. Powermat is capable of delivering 5-to-50 watts of power. Powermat allows a built-in check for alignment via light and voice signals based on RFiD Handshake feature. When you place a Powermat-enabled device on one of its mats, the two exchange a “handshake” using RFID: The mat identifies the device, determines how much power it needs and transfers energy to it. Powermat operates at 277-357 kHz frequency. Once a device is fully charged, Powermat stops the electricity from flowing. But as much momentum as the PMA has achieved, it is far from clear whether it will be that bandwagon.

Power Matters Alliance (PMA)
Method: inductive charging
Frequency: 277-357 kHz
Communication: RFID

As obviously useful as wireless charging is, it suffers from a Tower of Babel problem with incompatible standards and competing interests keeping it from truly going mainstream. There are also attempts to support several standards on one product. Samsung Galaxy SIII wireless power supports both Qualcomm’s WiPower and Wireless Power Consortium Qi. The Samsung Galaxy S4 will support both PMA and Qi standards. NXP has developed a charging station, which allows you to use both the general mobile phone charging standards (as well as one rare third standard).

The technologies I mentioned are not the only ones trying to push to the market in the near future. Apple is trying to patent wireless charging, claiming its magnetic resonance tech is new and that it can do it better than anyone else. Digitoday writes that Finnish research organization VTT is planning to combine wireless power and NFC technologies. The reasearchers believe that in the future NFC devices could be made to work as way to get power into device and send power to other device cheaply. Technology is not ready yet, because today’s NFC antenna circuits are not optimized for power transfer and there is no standard that covers this kind of use yet. NFC operates within the globally available and unlicensed radio frequency ISM band of 13.56 MHz.

Wireless Power: Convenient, But Its Shortcomings Are Somewhat Sour article tells that close-proximity inductive coupling is commonly estimated to deliver 50 to 70% efficiency. That’s considerably worse efficiency that what you get with a well designed wired charger. Intel increases consumer-product power consumption 50% blog post says that a system that is 50% efficient on top of the ac-dc conversion, and pumps RF energy all over the place is far from ideal in world where some other parties try to conserve every single watt. In a world with 15 billion chargers, energy efficiency is a big deal. Based in that is makes me a little bit hard to believe the Power Matter Alliance claims that wireless charging could save a lots of power in the future. How Wireless Charging Will Keep Toxic Waste Out of Landfills article tries to describe how wireless power could be more eco-friendly, but it is hard to believe all those claims without good data. I can believe that wireless chargers can have better energy efficiency than some old chargers supplied with consumer devices, but I given the limitations wireless charging it is very hard to believe that wireless charger could ever be more efficient than well designed wired charger. But wireless charger could be well “good enough” to be acceptable.


  1. Tomi Engdahl says:

    Partnership to Bring Over-the-Air Wireless Charging to Smartphones

    Energous, the developer of WattUp wireless charging 2.0 technology, is teaming up with vivo Global, a Chinese smartphone manufacturer, to explore integrating WattUp into smartphone designs that will be capable of charging wirelessly over-the-air. The two companies expect the collaboration to bring WattUp distance charging to consumers and drive forward second-generation wireless charging.

    A scalable RF-based technology, WattUp differs from older wireless charging systems because it supports power at-contact, as well as at-a-distance, enabling a charging ecosystem that frees users from always having to actively charge or manage batteries.


  2. Tomi Engdahl says:

    Craig Lloyd / iFixit:
    A theory on Apple’s AirPower: its overlapping multi-coil design worked in the lab but complying with FCC’s rules for safe wireless emissions was too challenging

    What Finally Killed AirPower

    In a surprising turn of events, Apple has cancelled its long-awaited AirPower wireless charging mat. The company says the product didn’t meet its “high standards,” but wasn’t specific as to why. We’ve been watching this space carefully and have an informed guess as to what happened.

    AirPower was first introduced to the public in September of 2017

    Apple planned on releasing AirPower the year after the iPhone X, in 2018.

    Starting in 2019, though, a glimmer of hope emerged

    Overheating and Interference

    Managing these overlapping harmonic frequencies is incredibly challenging, and gets harder the more coils that you are integrating. From patent filings, it looks like Apple’s ambitious plan was to use considerably more coils than other charging pads on the market.

    Other multi-device wireless chargers place two or three coils side-by-side

    ”Well, so what always happens is you get it functional first. No one looks at [Electro-Magnetic Interference] until the end.” The FCC rules for wireless charging devices like AirPower are quite strict, and limit exposure at 20 cm (8 in) above the device to 50 mW/cm^2.

    Rumors have circulated for months about AirPower’s overheating issues, which would click nicely into place with this theory.

  3. Tomi Engdahl says:

    Wireless charging: The state of disunion

    I’d thought that Qi (from the Wireless Power Consortium) was still officially specified as only working up to 5W, and that the higher-power “fast” charging approaches from Apple, LG, Samsung, and others were all proprietary … this ends up not being the case, at least not exactly. Then there’s the dueling dual-standard alternative approach offered up by the merged-in-2015 Alliance for Wireless Power (A4WP) and Power Matters Alliance (PMA), now known as the AirFuel Alliance. And what about medium-power applications like home appliances and power tools … and high power applications like electric vehicles? I’ll touch on all of these topics in coverage to come.”

    Inductive coupled wireless charging

    Capacitive coupled wireless charging

    One potentially obvious advantage of the inductive and capacitive coupled approaches in particular, thereby explaining the technology’s frequent use with appliances found in bathrooms and other environments where water-induced electrocution is a possibility, is that both the energy source and battery can be completely embedded within the (respective) charger and destination device, electronically insulated not only from each other but also more generally from all other environmental factors.

    Inefficiency versus traditional corded charging schemes is one key shortcoming of any wireless charging approach, with the degree of inefficiency being to some degree implementation-dependent.

    As mentioned earlier, the Wireless Power Consortium’s Qi is the predominant wireless charging approach in the market today, courtesy of its widespread (and growing) adoption in smartphones, tablets, smartwatches, and the like. Also as mentioned earlier, Qi was initially specified as supporting “Basic Power Profile” transmission and reception up to 5W (and often less, for small-capacity battery applications such as wearables).

    beginning with spec v1.2 (albeit not absent controversy), the “Industrial and Consumer Extended Power Profile” (EPP) expanded power transfer capabilities up to 15W

    Samsung officially specs its “fast charge” at 9W (thereby explaining the 9V@1A specification of the charging pad I tore down). Competitor LG followed with “10W” one-upmanship. Always-conservative Apple eventually launched its first wireless charging-capable phone, the iPhone X, at 7.5W in late 2017.

    AirFuel Alliance competitor to Qi is still fighting the good fight publicly, behind the scenes it’s probably retooling for those medium- and high-power alternative applications

  4. Tomi Engdahl says:

    Resonant wireless charging is more user-friendly than inductive wireless charging, as the receiver only needs to be placed somewhere in the vicinity of the transmitter (typically up to 30 mm of vertical freedom). And multiple devices of different size and power can be charged in parallel.

    AirFuel offers a resonant technology that operates at 6.78 MHz. To develop resonant solutions, two main topologies can be used: Class D (full-bridge or half-bridge) and Class E (single-ended or differential) amplifier.



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