IEEE.org
`
` IEEE Xplore
`
` IEEE Standards
`
` IEEE Spectrum
`
` More Sites
`
`
`
`IEEESTANDARDS UNIVERSITY Innovation · Compatibility · Success
`
`E-Magazine
`
`Game
`
`Videos
`
`Courses Workshops
`
`Library
`
`Grants
`
`&&&
`
`$
`
`%
`
`FIND A STANDARD
`
`CHOOSING A STANDARD FOR PORTABLE
`WIRELESS CHARGING SYSTEMS DESIGN
`
`31 May 2016 | Alan Li, David Law
`
`! Back to E-Magazine
`" Airfuel | Wireless Power | Wireless Power Transfer
`
`Which Standard Should You Consider When Designing Portable
`Wireless Charging Systems?
`Introduction
`Smartphone technologies are the key indicators of consumer electronic trends. New technologies and features continue to emerge such as USB type C, 4K video, and
`immersive VR gaming. Wireless charging is one of these technologies that has been in use since late 2000, but is still not used by the masses in smartphones. The industry
`generally agrees that the reason behind it is the eco-system; the standard. There are multiple standards in the world, but their technologies are incompatible at present.
`
`The Standards
`Currently there are two global organizations, but three standards for mainstream consumer electronics. In 2008, the Wireless Power Consortium (WPC) established the Qi
`wireless power standard based on Magnetic-Induction (M.I.) power transfer technology. The Power Matter Alliance (PMA) established the PMA standard based on a very similar
`M.I. technology. All For Wireless Power (A4WP) established a standard based on a di!erent Magnetic-Resonant (M.R.) power transfer technology.
`
`PMA and A4WP merged in 2015 and was named the Airfuel Alliance in early 2016. Despite the merger, the Airfuel Alliance continues to have two incompatible standards:
`Airfuel-Inductive and Airfuel-Resonant.
`
`As of April 2016, WPC has 226 members with 844 certi"ed products. Over 30 countries have deployed Qi products including China, Taiwan, Hong Kong, North America, Europe,
`South Korea, and Japan. The Airfuel Alliance has 149 members and 62 certi"ed products. The main players in the Airfuel Alliance are located in North America and South
`Korea.
`
`The Technologies
`The Qi and Airfuel-Inductive standards both use M.I. technology that relies on tight coupling coils of magnetic "elds transfer. They are very similar except for the operating
`frequencies and communication protocols for wireless power transfer operations. The Qi operating frequency is between 110 kHz and 205 kHz and the Airfuel-Inductive
`frequency is between 235 kHz and 275 kHz. The Airfuel-Resonant standard employs a di!erent M.R. technology that is characterized as loosely coupled coils of magnetic "elds
`transfer operated at 6.78 MHz.
`
`Despite the name di!erence, M.I. and M.R. technologies are similar in many ways, which should become clearer later in this paper. Both technologies use transmitter and
`receiver antennas called Tx and Rx coils, and rely on an alternating magnetic "eld generated in the Tx coil coupling into the Rx coil for wireless power transfer in close
`proximity. Both technologies fall under the inductive coupling and near-"eld power transfer categories.
`
`As shown in Fig. 1, the alternating current "rst conducts in the transmitter coil and in"nite imaginary magnetic "elds loop around it. With a receiver coil in close proximity,
`some of these magnetic "elds are captured into the Rx coil. This action induces the alternating current in the receiver coil, which is known as the wireless power transfer.
`
`Fig. 1. Conceptual wireless power transfer.
`
`Magnetic #ux is a measurement of how much magnetic "eld passes through the Tx and Rx coils. The higher the magnetic "eld passing through the Tx and Rx coils, the higher
`the #ux that enhances the power transfer. Near-"eld wireless power transfer technologies apply the same principle of the ideal transformer for power transfer without the
`metal core, which con"nes the magnetic "eld transfer from the primary to the secondary side winding. It is therefore the challenge for the wireless power technology to
`capture as much magnetic "eld in the air between the Tx and Rx coils as possible in order to increase the magnetic #ux for optimizing the power transfer.
`
`Wireless power is a complex subject; the numbers of variables that the designers need to deal with can be overwhelming. Some variables are inversely related and some have
`nonlinear e!ects. The variables that a!ect wireless power transfer e$ciency are the size of the Tx and Rx coils, their positions and gap, the coil material, diameter, length,
`number of turns, number of layers, the tuning capacitor value and the capacitance coe$cient, the resonant frequency, skin e!ect, proximity e!ect, magnetic shielding
`materials and con"gurations, temperature, and more. These variables relate to the coils’ DC and AC resistances, which are the self and mutual inductances that dictate the
`power transfer performance. Fortunately, all of these variables can be summed up by two important factors, which are the Coupling factor k and the Quality factor Q. It can be
`proven that near "eld inductive coupling power transfer e$ciency [1] is
`
`(1)
`
`where k is the coupling factor between the Tx and Rx coils, and Q and Q are the Quality factor of the Tx and Rx coils, respectively.
`1
`2
`
`2
`The k and Q factors are "gure-of-merit because if we can make the term kQQ much larger than 1, then the power transfer e$ciency approaches 1. It is virtually impossible
`1 2
`to achieve 100% due to the numerous variables described. The k and Q factors apply to both M.I. and M.R. technologies with di!erent degrees of in#uence on power transfer
`e$ciencies. An example of measured k factors is presented in [2]. The Q factor can be measured in certain LCR meters. In practice, k and Q are usually estimated.
`
`The Coupling factor k is unit-less, with the greatest number of 1 when all the magnetic "elds pass through the Tx and Rx coils in maximum strength. As seen in Fig. 1, some of
`the magnetic "elds cannot be captured by e!ects such as coil alignment and gap. Some "elds travel longer distances than others because their magnetic "eld strengths are
`relatively weak. In M.I. technology, the Q factor, which will be explained later, is essential, but the k factor is far more important than the Q factor. In general, the coupling
`factor is best when the Tx and Rx coils’ shapes and sizes are matched, coils are aligned, and are in a close gap within 10 mm in the x, y and z directions or tighter. Currently,
`M.I. technology for smartphone applications reaches about 80% e$ciency from AC input to DC output.
`
`The Quality factor Q is also dimension-less. High Q indicates a low rate of energy loss relative to its stored energy in the power coil. Typical Q in M.I. designs is between 30 and
`50, and typical Q in M.R. designs is between 50 and 100. The Q factor increases in frequency, peaks at the resonant frequency, then falls sharply and equals to 0 when it
`reaches the self-resonant frequency. Further increases in frequency alters the coils’ inductive characteristic to capacitive. Optimum frequency for the maximum Q occurs at the
`resonant frequency. Since the coil is inductive, it is best use a tuning capacitor to tune the LC to the resonant frequency. Also, Tx and Rx coils are rarely identical nor perfectly
`aligned, and the Q factor is unique to each coil.
`
`In M.R. technology, the k factor is generally not at its optimum since the coils’ alignment and gap requirements are looser. On the other hand, the Q factor is critical. M.R. coils
`generally have few windings and also require a gap between adjacent coils, as shown in Fig. 2, to reduce the proximity e!ect. The proximity e!ect is a phenomenon in high
`frequency operation such that the in#uence of the adjacent conductor can reduce the e!ective cross-sectional area of the measuring conductor for the #ow of the current; it
`therefore increases the AC resistance that a!ects the power transfer. In addition, it can be proven that the optimum Tx coil radius R is
`
`R = D
`
`(2)
`
`where D is the distance between the transmitter and receiver coils for optimum near "eld inductive coupling power transfer [1]. Since the goal of M.R. technology is to achieve
`spatial freedom with looser coil alignment and proximity, M.R. coil size is generally larger than the M.I. coils. Figure 2 shows tri-mode coils where the inner and outer coils are
`for M.I. and M.R. technologies, respectively. Since M.I. relies on tight coupling coils in close distance, equation (2) does not a!ect M.I. technology as much as it a!ects M.R.
`technology.
`
`M.R. coupling also allows a single transmitter to conduct power transfer to multiple receivers. In principle, M.R. technology can yield satisfactory power transfer, but the design
`challenge is very high because of the narrow range of optimum Qs.
`
`Development
`In M.I. and M.R. technologies, semiconductor makers are mainly from the US, Japan, Europe, Taiwan, and Hong Kong. Since di!erent and incompatible wireless power
`standards exist, few semiconductor companies develop multimode transmitter and receiver ICs. Dual-mode devices include Qi + Airfuel-Inductive transmitters and receivers.
`Currently, Tri-mode devices are Qi + Airfuel-Inductive + Airfuel-Resonant receivers only. Multimode wireless power solutions address the dilemma of multiple standards. On
`the other hand, M.I. and M.R. technologies employ unique coil designs where the coils cannot be shared. [refer to the Tri-Mode Wireless Power Coils in Fig. 2 (courtesy of TDK).]
`The center coil is used for both Qi and Airfuel-Inductive standards for 5 W operations. The outer coil is dedicated to the Airfuel-Magnetic standard for 6.5 W operation. The coil
`module dimension is 75 mm x 60 mm. The multimode operation increases the complexity and cost of the coil and system designs.
`
`Fig. 2. Tri-mode wireless power coils, where the inner coil is for M.I. Qi and
`Airfuel-Inductive operations and the outer coil is for M.R. Airfuel-Magnetic
`operation (courtesy of TDK who provided the tri-mode wireless power coils
`picture).
`
`Other new developments in wireless power technology include phase-shift control. Driving Tx and Rx coils involves inductive switching with the AC voltage leading the AC
`current to a maximum of 90 degrees. Researchers are working on phase-shift control in order to align the AC voltage and current waveforms for optimizing the power transfer
`e$ciency.
`
`WPC also has several new developments. It has a task force working on publishing the resonant extension in the Qi speci"cation. Their goal is to use the same M.I. Tx and Rx
`coils for M.R. applications at low frequencies to leverage system designs. WPC also adds a Shared Mode extension to allow multiple receivers to be powered by a single
`transmitter. In addition, WPC is also working on a 60 W–200 W version of wireless power transfer for power tools, drones, and laptop computer applications.
`
`In Airfuel-Resonant technology, at least one company has been proposing GaN FET to replace MOSFET in the transmitter inverter design to reduce the loss in the switching
`circuit.
`
`In the future, a wireless power transceiver (transmitter and receiver combined) IC is a possibility. Applications include a battery bank where the battery is a receiver when it is
`being charged from a transmitter base. The battery becomes a transmitter when it is used to charge a receiver device.
`
`Reliance by employers on complying with standards for introducing their products to the marketplace
`Companies who embrace wireless charging usually adhere to standards. Standards enable the expansion of the eco-system that can lead to the mass adoption of the
`technology.
`
`In addition, researchers, semiconductor companies, and coil makers designing wireless power solutions and contributing to the standards have spent years understanding the
`dynamics of the technology. As a result, companies who follow the standards and apply reference designs not only avoid reinventing the wheel, but also increase the chance of
`design success.
`
`Toshiba semiconductor has been providing wireless power solutions since 2012. The company follows the Qi standard and has just launched its fourth-generation chipset, the
`TC7718FTG transmitter and TC7766WBG receiver, for Qi 15W wireless power applications.
`
`Summary
`Standards enable the development of the eco-system. It is imperative that users be able to charge their phones in places like airports or co!ee shops seamlessly without
`understanding the standards issue. Given time, the market should self-regulate such that wireless power standards should converge.
`
`Any wireless power technologies o!er convenient wireless charging possibility. However, there is no one-size-"ts-all wireless power technology exists that meets all the system
`cost, size, and e$ciency requirements and yet o!ers the ultra-convenient feature of wireless charging from a distance. As a result, the standard selection depends on the
`application requirement. For applications that demand maximum e$ciency and a tighter design footprint, Qi and Airfuel-Inductive are the standards. For applications that
`demand charging without strict Tx and Rx coil alignment and allow for a few centimeters coil-to-coil gap freedom, the Airfuel-Resonant coupling is the standard at present and
`perhaps Qi resonant in the future.
`
`References
`[1] T. Sun, X. Xie, and Z. Wang, Wireless Power Transfer for Medical Microsystems. New York: Springer, 2013.
`
`[2] Eberhard Wa!enschmidt, Qi Coupling Factor. https://www.wirelesspowerconsortium.com/technology/coupling-factor.html
`
`Alan Li alan.li@taec.toshiba.com Toshiba Semiconductors
`
`Alan Li graduated from Florida International University with a BSEE degree. He started his applications engineer career at National
`Semiconductor, which was later spun o! as Fairchild Semiconductor. Alan has also worked at Analog Devices and Intersil in applications and
`marketing roles. Currently Alan is a senior business development manager at Toshiba Semiconductors where he is responsible for wireless
`power, motor control, and a variety of analog products for the American market. Alan has over 20 years of analog semiconductor experience
`in which he de"ned numbers of analog products such as DAC, digital potentiometers, LED drivers, and power management ICs for the
`industry. Alan has also published a number of articles and application notes in digital-to-analog converter and digital potentiometers
`applications.
`
`Related E-magazine Articles
`
`#
`
`#
`
`#
`
`#
`
`2nd Quarter 2016 eMagazine
`Issue Focusing on Wireless
`Power Transfer Now
`Available
`13 May 2016
`
`News and Further Reading
`19 May 2016
`
`Student Application Papers
`20 May 2016
`
`Developments in Wireless
`Power Transfer Standards
`and Regulations
`31 May 2016
`
`Standards Education Program
`
`Standards Education Professional Sta!
`
`Standards Education Committee
`
`About Other Organizations’ Standards Education Programs
`
`IEEE Standards in the Classroom
`
`E-Magazine Editorial Board
`
`Sitemap
`
`Contact & Support
`
`Accessibility Nondiscrimination Policy
`
`Privacy & Cookies
`
`Feedback
`
`Connect with IEEE-SA Here
`
`$
`
`%
`
`© Copyright 2019 IEEE – All rights reserved. Use of this Web site signi"es your agreement to the IEEE Terms & Conditions.
`A not-for-pro"t organization, IEEE is the world's largest technical professional organization dedicated to advancing technology for the bene"t of humanity.
`
`Apple v. GUI Global Products
`IPR2021-00472
`GUI Ex. 2006 Page 1
`
`