Wireless power transfer (WPT), or simply ‘wireless charging,’ has become a headline feature of many consumer tech devices. Moreover, its usefulness does not end there: it’s also attractive for the automotive industry because there’s no direct link – and therefore no opportunity for a spark that could trigger an explosion. Moreover, a wireless charging system can be sealed to keep out grease, grime and other troublesome substances.
The two main near-field charging technologies
Wireless charging systems are typical of the ‘near-field charging’ variety. This technique uses two coils: a transmitting coil to create an oscillating magnetic field, and a receiving coil where energy is transferred through induction.
Typically, the charging station/pad contains the transmitter and primary coil, while the device being charged has the receiver and secondary coil. The transmitter creates magnetic flux, part of which penetrates the coil in the receiver to transfer power, resulting in the two coils to act as a transformer. The transfer efficiency of this transformer is a function of the coils’ quality factor (Q) and their coupling (k).
To authenticate the receiving device and manage the transfer of power, a bidirectional communication channel is superimposed onto the power waveform. Typically, a foreign object detection (FOD) is built into the wireless charging system. This is to ensure any unwanted metal objects placed between the receiver and transmitter don’t pose an issue to safety, as they can absorb energy, reduce efficiency, or increase the system’s temperature.
There are two broad forms of near-field charging technology. The first is inductive charging, where the two coils are close together and tightly coupled. The second is resonant charging, where the coils are tuned to the same resonant frequency. While both use inductive coupling, ‘inductive charging’ typically refers to the former.
Inductive charging: At a glance
Inductive charging is the only wireless power transfer technique currently benefiting from mass-production. Despite inductive charging being efficient, it’s quite sensitive to coil misalignment. Because of the inverse square law relationship, the further you move two coils apart, the quicker power transfer drops. For high efficiency in consumer applications, manufacturers have no more than 7mm between the coils.
There are two competing standards for inductive charging. The first is the Qi (pronounced ‘Chee’) standard, which is controlled by the Wireless Power Consortium (WPC). Qi operates at frequencies between 100 and 200 kHz. Competing with it is the AirFuel Alliance’s AirFuel Inductive standard, which operates between 100 and 350 kHz.
Both offer efficiency more than 70 percent, and while no wireless charging system can deliver the same level of power as a wired setup, things are improving. For example, WPC version 1.2 enables up to 15W – a significant increase compared to the 5W capability of earlier versions.
Many device manufacturers are supporting both standards.
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Inductive charging: How the transmitter works
For an inductive charging setup, the power transmitter remains in a low-power mode most of the time, waking periodically to ascertain whether there’s a receiver within range. If so, the transmitter authenticates it and starts transmitting power. Note that the transmitter doesn’t control the transfer process: the receiving device does.
Should the receiver not authenticate correctly, the transmitter returns to low-power mode.
An example of a Qi-compliant wireless transmitter is the Toshiba TB6865AFG. This transmitter is known as a complex “system on a chip” (SoC) device, and includes pre-drivers that enable the implementation of a full-bridge inverter to drive the power coil.
Inductive charging: A look at receivers
Wireless receivers, such as the BD57015GWL from Rohm, are also SoC-based devices. This model includes a fully synchronous rectifier circuit with low-impedance field-effect transistors (FETs). Qi and AirFuel communication between the receiver and transmitter are handled by a packet controller, using amplitude modulation. Voltage output can be regulated and complied with both the Qi medium-power and AirFuel standard.
Rohm also sells a Qi low- and medium-power-compliant BD57020MWV transmitter.
The coils
The Qi standard sets out detailed requirements for the coils in the transmitter, covering materials, geometries and electrical characteristics. This is to ensure products from different manufacturers are interoperable.
To give an idea of the level of detail, the A11 coil specification sets out a single-layer circular coil, made up of 10 turns of wire on a base of ferrite. The input operating voltage is specified at 5V, inductance at 6.3µH and a top DC resistance of 60mΩ.
The three-coil A6 specification increases the charging area by cutting the sensitivity to coil misalignment. However, because each coil requires its driver, this increases the overall cost.
Both receiver and transmitter coils are usually made from Litz wire, which decreases the skin effect and proximity losses. Both coils should be about the same size, to ensure the best-possible energy coupling.
A receiver coil must strike a balance between being thin enough to fit into a portable or wearable device, and being sufficiently strong to cope with the bumps and shakes such devices will endure.
Since receiver coils are used in many consumer electronics, the coil tends to have a custom design. Texas Instruments has some useful guidelines on how to create Qi-compliant receiver coils. That being said, there are also other standard products on the market as well. Lastly, note that you can get modules that include both the coil and the transmitter or receiver. A good example of this would be the TDK WTM505090.
Inductive wireless charging development kits
For anyone designing products who want to incorporate inductive wireless charging, there are various development kits and tools available. IDT’s WP3W-RK kit, for example, comes with a transmitter, receiver and three sizes of the coil. The kit also has a ready-to-use reference design, layout module and bill of materials.
Resonant wireless charging: the next step?
While current wireless charging is nearly all about near-field induction, two other technologies look set to make their mark in the medium and longer term: resonant charging and far-field charging.
Like inductive charging, resonant charging is a near-field approach but isn’t as efficient. However, resonant charging can work over a longer range and is more tolerant of misaligned coils, because the coils are loosely coupled and tuned to the same resonant frequency. Resonant charging can also deliver power to several devices at the same time and doesn’t require the coils to be of similar size.
These greater tolerances make resonant charging ideal for industrial or automotive applications, where it is impractical or even impossible to achieve the precision and proximity required by near-field induction.
Indeed, the Society of Automotive Engineers (SAE), which sets the standards for the automotive industry, is creating a resonant charging standard for vehicles (SAE J2954). Expected to be released next year, it provides three levels of charging for light- and heavy-duty scenarios, ranging from the 3.7kW residential-focused WPT1 to the 11kW fast-charging WPT3.
There’s also a resonant option for the Qi standard, aimed at consumer use and with a maximum range of 45mm. AirFuel also has a resonant charging specification. However, despite the standards existing, there are currently few commercially available products that take advantage of resonant charging.
Further ahead: Far-field charging
The utopia of wireless power transfer, of course, would be to get rid of the need for the receiver to be close to the transmitter. In other words, your portable or wearable device would continually trickle-charge, no matter where you are. This type of charging is known as ‘far-field,’ and would use radio waves, as opposed to near-field magnetic induction.
Now while this ‘charge anywhere’ utopia is not going to happen anytime soon, we are moving towards it. The first target is to enable wireless charging with a range of up to several meters. This could, for example, be of use in an airport or coffee shop, where lots of people congregate in a relatively small area.
Ossia is one of the companies working in this space and demonstrated the latest iteration of its Cota system at CES in 2017. This system uses 256 antennas and beamforming to send power to multiple devices within a 30-foot (just over 9 meters) range.
Others are investigating alternative technologies, such as ultrasonic waves. Ultrasconic waves have various benefits over electromagnetic energy, such that it does not cause electromagnetic interference nor does it interfere with communications. uBeam, for example, is working on a system that operates at a sonic range above what humans and animals can hear. It delivered a successful pilot in early 2017. However, we are still some years away from seeing commercially available products using this type of technology.
Opportunities for product designers
These cutting-edge projects showcase what the future of wireless charging could look like. However, even in the shorter term, with two well-established standards, a growing number of commercially available products and ready-made development kits, device designers can start to reap the benefits of wireless charging today.
