Inductive Versus Resonant Wireless Charging: A Truce May Be a Designer’s Best Choice

The advantages to the consumer are equally simple to grasp. Widespread adoption of wireless charging brings convenience akin to ubiquitous wireless connectivity. Smartphone and tablet owners will be able to drop their mobiles down on a desk, workbench or coffee shop table for a quick recharge without worrying about carrying a voltage adapter or searching for a power outlet.

However, from a designer’s perspective, wireless charging is murkier. There are two alternative technologies and two (formerly three) opposing standards bodies. Those opposing standards bodies each embrace both technologies. The merits of the competing standards are for others to debate (several key consumer electronics manufacturers have hedged their bets by joining both alliances) and, besides, based on previous standards conflicts, consolidation is the likely outcome. However, the alternative technologies are well worth deeper analysis. It turns out that they are complementary rather than competitive and therefore, designers need to be familiar with both to ensure they make the best choice for their application. This article aims to instill that familiarity.

Need efficiency? Choose inductive

In his pioneering work on wireless power transfer, Nikola Tesla, the celebrated Serbian-American engineer, worked out the basic principles that apply today. Tesla ascertained that an alternating current in a wire loop generated an alternating magnetic field which in turn induced an alternating current in a nearby secondary coil. By attaching a load to the secondary coil, the induced AC current could be made to do useful work (for example, charge a battery).

The magnetic field generated by the primary coil radiates (approximately equally) in all directions, hence the flux drops rapidly with distance (obeying an inverse square law). Consequently, the secondary coil must be placed as close as possible to the primary to intercept the most flux. In addition, the amount of energy the secondary coil captures is proportional to the cross section it presents to the magnetic field. The optimum cross section is offered by a secondary coil of identical dimensions to the primary, aligned parallel and with a vertical separation of just tens of millimeters. The separation, alignment and sizes of the respective coils determines the “coupling factor” which has a significant influence on the efficiency of the energy transfer. Perfect coupling, whereby all the flux generated by the primary coil is captured, has a coupling factor of 1. Practical close-coupled systems typically have a coupling factor of 0.3 to 0.6 (see Figure 1).

 

Figure 1: Closely coupled inductive wireless chargers employ well aligned, similarly sized coils in close proximity, maximizing efficiency. (Image courtesy of the Wireless Power Consortium)

Engineers with a passing interest in wireless charging are likely to be most familiar with the Qi (pronounced “Chee”) specification, championed by the Wireless Power Consortium (WPC). Qi has stolen a march on competing standards by initially focusing its efforts on the more mature inductive wireless charging technology and promoting its adoption in smartphones.

A number of mobiles are on the market incorporating Qi and several manufacturers have introduced compatible wireless charging pads. The Alliance for Wireless Power (A4WP) and the Power Matters Alliance (PMA) –– which merged in June 2015 to form the AirFuel Alliance –– have also released an inductive wireless charging specification. The only real difference between the WPC and the AirFuel Alliance’s inductive wireless power specifications are the transmission frequencies and connection protocols used to communicate with devices and control power management.

The latest Qi specification (v1.2.2) calls for two types of devices: a base station for charging and the device to be charged (see Figure 2). Base stations typically have a flat surface onto which the user places one or more mobiles. To maximize efficiency, the user either employs an “alignment aid” (which can be as simple as markings on the base station or more complex such as the use of magnets) or positions the mobile somewhere on a multicoiled base station in the hope that one of the coils lines up fairly well with the one in the handset. In either case, the mobile must be flat on the base station surface such that the coils are parallel and the gap between them is less than 10 mm.

 

Figure 2: Basic system configuration of a Qi wireless charging system. The power transmitter (housed in the base station) includes two main functional units - a power conversion unit and a communications and control unit. The primary coil is part of the power conversion unit. The control and communications unit regulates the transferred power to the level that the power receiver requests. (Image courtesy of Menno WPC at English Wikipedia, CC BY 3.0, Commons at Wikipedia.org)

The Qi specification calls for an AC frequency in the primary coil of between 110 and 205 kHz for the “low power” Qi chargers (up to 5 W) and 80 and 300 kHz (up to 120 W) for the “medium power” chargers. The technology also includes some nice extras such as foreign object detection (FOD) such that the charger does expend energy heating up objects that have been accidentally placed in the field.

The key benefit of a closely coupled inductive wireless charging system is its relatively high efficiency. A carefully designed system can transmit 30 to 60 percent of the power (depending on where the measurement is made) driving the primary coil to the secondary coil. Because of this relatively high efficiency, heat build-up is kept down, allowing the transfer of significant power, speeding up charging cycles.

Because the Qi specification was first published back in August 2010, chip makers have had time to introduce chips that integrate much of the control and compensation functionality demanded by the standard. The use of such devices makes it a little easier for a non-expert engineer to design a Qi-compliant wireless charging base station. NXP Semiconductors, for example, offers its NXQ1TXH5 5 V wireless charging-controller and driver IC for a 5 V Qi-certified low-power wireless charger.

When a Qi-compliant receiver is placed on a charging pad, the NXQ1TXH5 safely initiates wireless power transfer from the transmitter to the receiver, while monitoring for fault conditions such as overheating or interference from metal objects. The device is optimized to operate from a 5 V USB power supply and uses “smart power limiting” to adjust the output power automatically to compensate for power-limited supplies.

Need convenience? Choose resonant

Early in the last decade, Massachusetts Institute of Technology (MIT)[2], among others, explored ways of improving the efficiency of wireless charging systems. The institute focused on the fact that magnetic field flux rapidly falls off as the coils in an inductive wireless charging system are moved apart. Beyond a few centimeters, the flux becomes so weak that power transfer all but stops. MIT researchers realized that a “non-radiating” wireless charging technique was needed to free power transfer from the inverse square law that governed inductive techniques.

MIT came up with a system that transferred power between coils operating at (identical) resonant frequencies (determined by the coils’ distributed capacitance, resistance and inductance). The technique is still “inductive” in that the oscillating magnetic field generated by the primary coil induces a current in the secondary but it takes advantage of the strong coupling that occurs between resonant coils – even when separated by tens of centimeters.

The physics of resonant power transfer is complex, but the basic premise is that the energy “tunnels” from one coil to the other instead of spreading Omni directionally from the primary coil. The result is that, although energy still attenuates to some degree with distance, the primary source of attenuation is the Q factor (gain bandwidth) of the coils. Engineers can improve Q factor with good design. Better yet, resonant energy transfer is not so reliant on the coils being in the same orientation (providing that the secondary coil presents a large enough cross section to the primary coil so that in each cycle it absorbs more energy than is lost by the primary). A further advantage of the technology is its ability to transfer power between a single primary coil and multiple secondary coils.

Resonant wireless charging addresses the main drawback of inductive wireless charging; the requirement to closely couple the coils which demands precise alignment from the user. Resonant wireless charging is not without its own drawbacks, though. Chief among these are a relatively low efficiency due to flux leakage (even at close range a well-designed system might demonstrate an efficiency of 30 percent at 2 cm, dropping to 15 percent at 75 cm coil separation (again, depending on where the measurement is made)), greater circuit complexity and, because of the (typically) high operating frequencies, potential electromagnetic interference (EMI) challenges.

Nonetheless, the technique offers sufficient promise that both standards bodies have included a form of resonant wireless power technology into their specifications. The 1.2 version of the Qi standard, for example, introduced resonant charging to the specification. To ensure compatibility with existing Qi transmission frequencies, the technology is limited to a maximum coil specification of 45 mm. Resonant charging has also been championed for some years by A4WP.

Unfortunately, the technology is taking time to make its mark and commercial solutions are thin on the ground. Integrated Device Technology (IDT) is one of a few manufacturers that has released details of a portfolio of wireless charging chips for inductive and resonant wireless charging that will conform with both WPC (Qi) and the AirFuel Alliance’s specifications. For now though, IDT’s commercial products predominantly support the WPC’s inductive wireless charging specification.

An example of the company’s current range is the P9038 5 V Wireless Power Transmitter which meets the WPC 1.1 specification. The device is suitable for charging pads and offers up to 8 W power transfer (at 1.6 A) and can be powered from either a power adapter or USB connector across a voltage range of 4.5 to 6.9 V. The device includes integrated current sense and FOD. IDT supports the chip with a wireless power evaluation kit.

Linear Technology does offer a resonant wireless power transmitter, the LTC4125. However, the chip isn’t designed to comply with any of the standard specifications; rather it automatically adjusts its driving frequency to allow maximum power delivery from a low voltage input supply (3 to 5.5 V) to a tuned receiver. To optimize system efficiency, the LTC4125 employs a periodic transmit power search and adjusts the transmission power based on the receiver load requirements. The device stops delivering power during a fault condition, or if a foreign object is detected. The transmitter works in partnership with the LTC4120 wireless power receiver.

AirFuel’s resonant wireless charging technology (which is based on the Rezence specification) uses a system comprising a single power transmitter unit (PTU) and one or more power receiver units (PRUs). A Bluetooth low energy link is specified to control power levels, identify loads and protect non-compliant devices (Figure 3).

 

Figure 3: The Rezence wireless charging system uses high-frequency resonant coupling and Bluetooth low energy communications for power level control. (Image generated using Digi-Key’s Scheme-it online schematic and diagramming tool, based on original image, courtesy of Reference 3.)

The standard supports power transfer up to 50 W at distances up to 50 mm. The power transmission frequency is 6.78 MHz (chosen because it offers good energy transfer but also falls in a license-free part of the radio spectrum). Up to eight devices can be powered from a single PTU depending on transmitter and receiver geometry and power levels (Figure 4).

 

Figure 4: The Rezence resonant wireless charging architecture allows for up to eight PRUs to be charged from one PTU. (Image courtesy of Reference 3)

An engineer could be excused for thinking that closely coupled coils both operating at their resonant frequencies would overcome the efficiency constraints of resonant wireless charging. However, this is not the case because there is a minimum distance at which both coils can maintain resonant operation. This distance depends on the size of the coils and operating frequency, but is greater than the typical coil separation of a typical closely coupled system. If the resonating coils are moved too close, their mutual inductance causes the oscillating magnetic fields to “collapse” and power transfer ceases. It turns out that the most efficient wireless power transfer occurs with closely coupled coils operating near to (rather than exactly at) their resonant frequencies in an inductive topology.

Conclusion

What it boils down to is designers have a choice of two wireless charging technologies. Inductive wireless charging relies on a relatively low-frequency oscillating field transferring power between non-resonant but closely coupled coils at medium-to-high power levels with good efficiency. The technology is relatively simple, rapidly maturing, supported by both standards organization, is already (primarily in the form of Qi) incorporated into a good selection of mobile devices and supported by several chip vendors. The downside is the finicky requirement to keep the charger and the device-under-charge carefully aligned.

Inductive wireless charging is a good choice if the requirement is to be able to charge a single device quickly and efficiently on a dedicated charging mat that incorporates alignment aids or multiple coils.

Resonant wireless charging relies on a high-frequency oscillating magnetic field transferring energy between two coils operating at the same resonant frequency. The coils can be loosely coupled but demand a high Q factor if energy transfer is to be maintained over several centimeters. Several devices can be charged from a single primary coil. The technology is more complex than inductive and is less efficient (see Figure 5). While resonant wireless charging is supported by both standards organizations, it has yet to make its mark and it is difficult to source components that adhere to the specifications.

Figure 5: A closely-coupled inductive wireless charging system (such as one adhering to the Qi specification) exhibits higher power transfer efficiency than a resonant system (such as one adhering to the Rezence specification). (Image courtesy of Reference 3)

Resonant wireless charging will be a good choice once chips supporting the technology become more widely available. However, the designer must be prepared to trade efficiency for convenience, such as being able to charge several devices simultaneously, without the need for accurate alignment. The technology is also a good choice for fitting under desks and coffee-shop tables where the thickness of the table has a minimal impact on energy transfer.

References:

1. “Wireless Power Transfer through Inductive Coupling,” Mohamed A. Hassan, A. Elzawawi, Recent Advances in Circuits
2. “Power Transfer Through Strongly Coupled Resonances,” Andre Kurs, MIT, September 2007.
3. “Rezence – Wireless Charging Standard based on Magnetic Resonance,” Pratik Dubal, Somaiya College of Engineering, India, International Journal of Advanced Research in Computer and Communication Engineering, December 2015.