Wireless power: Accommodating new industry standards
Introduction
Wireless power technology is a significant growth area in the electronics industry and enhances a wide variety of applications. Volume growth is driven by a relentless consumer appetite for battery powered portable electronic devices and the inconvenience of keeping them charged. The rollout of portable devices capable of wireless charging has grown dramatically and as this growth trend continues, wireless power will become a part of our daily lives and portable device usage habits.
Magnetic Induction (MI) and Magnetic Resonance (MR) are the two predominant wireless power transfer technologies in the consumer market due to developing standards, solution miniaturization, and cost reductions. Due to tighter magnetic coupling, MI currently offers higher power transfer, ease of design, and better efficiency while MR offers more spatial freedom, multiple receiving devices per transmitter pad, and less heat accumulation into near field metallic objects.
Within the MI sphere there are two prevailing standards: the Wireless Power Consortium (WPC) with its ‘Qi’ standard (the current prevailing wireless charging standard), and the Power Matters Alliance (PMA). For designers of today’s MI-based portable devices this creates an issue of whether to ensure compatibility with Qi, or PMA, or both standards.
MR standards are dominated by the Alliance for Wireless Power (A4WP). Products compatible with Qi or PMA standards are on the shelves today and A4WP-based products are expected next year. This article reviews the wireless power sector and the relevance of MI and MR technologies. It also discusses how the chip manufacturers responsible for making today’s MI wireless power receiver ICs can help overcome the dual standards issue for MI technology.
A rapidly developing wireless power world
Wired connections for power and data have been an unavoidable inconvenience in many areas of our daily personal and business lives. Wireless data access has been made ubiquitous, but a similar ease of power access has not. Carrying around bulky wall warts, cables and adapters and the hassle of then finding working ‘public’ power points is a constant irritation for travelers.
Mobile phones will benefit most when a widespread wireless power ecosystem is built out for several reasons. Large and bright displays, powerful multi-core processors, an array of onboard radios, apps demanding real time data, as well as new biometric apps have resulted in the need to recharge several times a day. This is compounded by consumer demand for lighter and thinner devices. Unfortunately the power densities of Li-ion batteries have not increased commensurately.
Most major portable device manufacturers are already shipping Qi-compatible mobile phones in response to customer demand. The mobile phone carriers also have motivation to keep mobile devices charged and consuming wireless data, and therefore are encouraging the build-out of wireless power delivery ecosystems.
Charging wirelessly anywhere and everywhere by simply placing the device on a wireless power-enabled surface requires the ubiquitous availability of transmitter pads. That build-out is happening on several fronts. Qi and PMA transmitter pads are currently available for the home and office in a wide variety of form factors. Qi transmitters are available in new car models from companies such as Toyota and Chrysler with more makes/models coming, as well as numerous aftermarket automotive solutions available now.
PMA is rapidly signing on partners who find value in deploying wireless power ecosystems and smart networks in their restaurants, retail outlets, hotels, and other venues to attract customers and potentially another revenue stream. For example, Starbucks’ "Never Powerless" program started with installations in Boston, MA last year, and have moved to new installations in the Silicon Valley region with mid-term plans to provide over one million charging spots in the US alone.[1]
Mobile devices designed to support multiple wireless power standards will benefit most as the transmitter station ecosystems and standards are built out. This fast developing analog and digital technology being introduced by companies such as IDT and the growing demand for wireless power is underscored by a recent IMS Research report stating wireless power shipments are expected to experience a high growth rate in the next few years, passing 300 million units in 2016 and reaching 1 billion units in 2018. This represents a remarkable rate of growth from a market that was almost non-existent as recently as 2011. [1]
Figure 1: Wireless charging will make the use of portable devices such as mobile phones much more convenient and alleviate the need to carry multiple device specific chargers and cables.
Magnetic Induction (MI) and Magnetic Resonance (MR)
Magnetic Induction technology (Qi and PMA) was first to market and dominates in the nascent wireless power market. However Magnetic Resonance (A4WP) has some real advantages over MI, along with additional challenges.
The A4WP fixed higher operating frequency of 6.78 MHz compared to Qi’s 110 to 205-kHz operating range allows more efficient power transfer at looser coupling factors (more positional flexibility), apparent through Faraday’s law of inductance. The higher frequencies and higher coil voltages also allow smaller and thinner receive coils, making the mechanical fit into mobile devices easier. Another benefit of the higher operating frequency is lower heat buildup in metallic foreign objects in proximity of the transmit pad due to lower surface eddy currents. This also means parasitic metals in the device under charge (like the battery) are less likely to accumulate heat.
The A4WP standard uses bidirectional Bluetooth Low Energy (BLE) out-of-band signaling to communicate and regulate the power needs of device(s) under charge. In contrast, Qi and PMA use a unidirectional in-band communication method of load modulation to communicate power regulation information back to the transmitter. The Qi method is simple and inexpensive, but can only handle one receiver, is limited to a low communications rate, and can be susceptible to system generated EMI.
MR has some implementation challenges that are being worked through and production solutions further optimized for the mass market. The MR receiver uses an LC tank circuit with a high Q factor operating directly at the resonance frequency. The challenge is keeping the tank circuit tuned to a fixed resonant frequency over temperature and voltage. When it drifts, efficiency drops off.
MI standards are simpler to implement because they always operate above the resonant frequency and therefore do not require a high Q circuit or precise passive components. However, the cost of higher-tolerance components for MR’s high-Q circuit is offset by a lower coil cost. The unshielded MR receive coil is also smaller and uses thinner wire than the MI coil, and therefore the cost should be less for that critical component.
Electromagnetic radiation generated from a wireless power system will be a consumer concern but is beyond the scope of this article to discuss in full. Mechanically, MI is a closely coupled system meaning the transmit and receive coils are placed directly on top of each other enabling MI power transfer, and it is this arrangement that allows ferrite shielding to be employed directly above and below the coils (Figure 2).
Figure 2: The magnetic field is mostly contained using ferrite shielding material.
These ferrite shields help in two ways: first, they improve the circular flow of the magnetic flux lines by keeping them closer to the coils for better coupling, and second, the shields reduce the amount of electromagnetic radiation emanating from the system. MR is a loosely coupled system, meaning the receiver can be 10s of centimeters away from the transmitter (Figure 3) and therefore ferrite shields will not achieve the advantages found in MI implementations. Keeping the radiation levels well within safe limits is part of the technology advances enabling wireless power transmission in the consumer market.
Figure 3: Magnetic resonance wireless charging allows loose coupling and spatial freedom.
The near-term MI dual standards challenge
In the short term, a potential problem that could inhibit the effective proliferation of public access wireless power charging mats, hot-spot tables, countertops, and other surfaces in locations such as airports, coffee shops, and entertainment venues is the existence of two competing MI standards: Qi and PMA. Both are viable and offer reliable performance, and therefore both seem likely to be specified into publicly deployed wireless power hot-spot locations. In theory, this could result in handset owners having to seek out specific wireless power access points compatible with their particular handset. This undoubtedly will lead to consumer frustration and possibly lower adoption rates.
IDT is one company developing wireless power solutions that has recognized this issue and has addressed it by developing wireless power receiver ICs such as the that eliminate compatibility barriers between transmission standards with a single-chip, dual-mode receiver solution.[2] These solutions enable mobile device and accessory OEMs to sell into a broader market and save money by avoiding the need for multiple product variants. OEMs can now use a single bill-of-materials (BOM) and minimize the application footprint through the use of a single universal circuit layout.
Conclusion
The wireless power market is an exciting new technology that is further enabling the mobile phone revolution by allowing more "ON" time. Battery charging via product-specific charging adapters and cumbersome wires has proved to be an inconvenience and perhaps the weakest link in portable communications and computing. Now, with a rapidly expanding wireless power ecosystem taking shape, we can look forward to the day when we leave our adapters and wires at home and eventually dispose of them entirely.
MR and MI technologies both have a future because each has unique characteristics and can address different wireless power applications. Within each of these approaches competing standards are healthy because they help drive innovation, however confusion and frustration by consumers is a risk. With the availability of dual-mode receiver technology for MI that can switch portable devices seamlessly and automatically between wireless power standards, that risk of complicated and undesirable scenarios can be avoided.
References:
1. Power Matters Alliance Website Article: "Duracell Powermat and Starbucks Expand Wireless Charging in Silicon Valley Area"
2. IDT, IDTP9021 Dual-mode Wireless Power Receiver IC
About the author
Jack Deans is a Field Applications Manager at Integrated Device Technology. Diverse background consisting of FPGA design, embedded processor architectures, packet processing and routing, timing circuits, integrated power managemet solutions, and wireless power technologies. BSEE from UC San Diego and has been with IDT since 2000.
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