While consumers like the wireless aspect, they are looking for wireless solutions that can charge as fast as wired charging, with more spatial freedom. Mark Patrick says wireless charging is now moving rapidly and there is far more successful deployment than ever before.

 

The charge indicator on mobile devices almost serves double duty as an ‘anxiety meter’ – the lower it falls, the more anxious the owner gets at the prospect of becoming disconnected from their online world. The anxiety is all the greater if they are not carrying a battery pack, or there is no immediately available charging point – and anxiety turns to frustration if they don't have the required charging cable with them. Given the reliance we have on mobile devices these days, one could be surprised that there are few wall outlets available in public spaces such as hospitals, airports, shopping malls, concert venues and the like. 

 

In an ideal world, charging would be accomplished by simply being able to place the device down on a convenient surface and waiting a short while. While this may seem a utopian dream, modern wireless charging technologies have made it a near-term reality – or, at least, a possibility.

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Wireless charging has been around for over a century, as evidenced by a 1902 patent application by the famous electrical 

engineer, Nikola Tesla.

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Convenience is one of the big drivers behind this new technology, by eliminating wires from the equation, users can charge their devices wherever a charging pad exists. Increasingly, coffee shops such as Starbucks are implementing the technology as are several major hotel chains. Ikea is also offering retrofit kits for DIYers to adapt existing furniture to include the technology. The technology is spawning a significant business, estimated to be growing at 13% CAGR by research firm Global Market Insights.

 

While wireless charging is being discussed as though it is a new technology, it has been around for over a century, as evidenced by a 1902 patent application by the famous electrical engineer, Nikola Tesla. His concept was revolutionary for the time and consisted of transmitting electrical energy between two coils that were separated by an air gap. His concept remains valid today and takes three different forms:

 

As well as the method of charging, applications are also segmented by the amount of power needed, which tends to correspond to the type of device being charged.

 

Standardisation is necessary

Interoperability is key to wireless charging as the receiver and transmitter must form a pair to allow power to be transferred. As with many emerging sectors, several competing standards have emerged to define this relationship, much like the early years of video with VHS and Betamax. In the early days of wireless charging, there were three standards bodies - Alliance for Wireless Power (A4WP), Power Matters Alliance (PMA) and Wireless Power Consortium (WPC) – each promoting their approach. In 2015, the new Airfuel Alliance (AFA) was born as a merger of A4WP and PMA, reducing the competing standards to two.

 

The WPC has been promoting and licensing the Qi standard since 2009, and this received a significant boost in 2017 when Apple announced that wireless charging in their iPhones would be based on this standard. Many of the technology leaders (including Google, Samsung, Apple & Bosch) support the Qi standard – along with around 200 other companies.

 

The Qi standard is squarely aimed at smartphones and other lower power handheld consumer devices as the current specification (version 1.2.3) is focussed on power levels in the range 5W to 30W. The primary part of the standard addresses close proximity (<7mm) inductive charging with frequencies in the range 87kHz to 205kHz. For greater distances, potentially covering distances close to 45mm, a further version of the Qi standard is being developed by WPC, based on magnetic resonance technology. 

 

The decision of Apple to follow the Qi standard drove many other industry changes. Powermat was an early entrant to the wireless charging space with their charging stations, which had been based on the approach preferred by PMA. Just after Apple made its announcement, Powermat announced (in January 2018) that they were moving to WPC. Starbucks had been a significant customer for the PMA-based Powermat stations, but as a result of Apple's decision, they began to update their charging stations to be iPhone compatible.

 

As WPC is beginning to dominate the close-contact charging with the Qi standard, the new(er) AFA is concentrating on non-contact charging technologies. The first is Airfuel Resonant that can bridge gaps of 50 mm, giving greater positioning latitude for devices. The second (Airfuel RF) can handle a wide distance range, from just a few centimeters to around a meter, albeit it at low power. As the industry has now settled on two different approaches (WPC/Qi and Airfuel), there is sufficient commonality that many believe this is a tipping point and, as a result, wireless charging will gain significant momentum.

 

Developing solutions in a new technology space

As wireless charging is relatively new, at least in its current incarnation, almost everyone entering the space has a learning curve to climb. To support this, many leading component suppliers are sharing their knowledge and know-how in the form of development tools. 

STMicroelectronics has its STEVAL-ISB045V1 wireless battery charger transmitter evaluation board that can provide 2.5W of charging power. Its STWBC-WA digital controller drives the board and manages intelligent features such as active presence sensing and foreign object detection (FOD).

 

Another developer’s tool comes from IDT. Its IDT WP3W-RK wireless power reference kit is a WPC-compliant wireless power transmitter (P9235A-R-EVK) and receiver (P9027LP-R-EVK) for prototyping and evaluation of 0.5W to 3W solutions. At the heart of the transmitter is a 32-bit ARM Cortex-M0 processor that has on-chip voltage and current demodulation as well as fully integrated full bridge power stage drivers. The development kit includes three different coil sizes for different power levels, allowing designers to get up-and-running quickly. 

Despite this functionality, the IDT boards are very compact, with the transmitter measuring just 22.7 mm x 21.2 mm and the receiver measuring 5.7mm x 5.7mm.

 

One driver for the growth of wireless charging is the growth in wearable devices – as these devices are so small, often there is little space for a wired charging port. Semtech has developed its TSWITX-G4-EVM (refer Figure 1) specifically for the low power sector, as it delivers around 1.25W. The system comprises a TS80002 wireless power transmitter controller and a TS51231 driver that form a complete system when matched with a receiver. 

 

As geometries are so small in the wireless space, compromises are necessary. For example, the small coils used are not Qi-compliant and there is no need for FOD technology as, with the low power operation, there is no risk of overheating nearby metallic objects.

 

Key system elements

As well as transferring power, the receiver and transmitter ‘talk’ to enable the wireless charging to work in an optimal way. Key to this is the ability to modulate the transmitter power and chipsets are required to include this technology. One Qi-compliant wireless power transmitter controller that does this is the BQ500210RGZT from Texas Instruments (refer Figure 2). This device is suitable for single-channel WPC-compliant contactless charging as all the necessary logic functions are integrated. These functions continuously monitor communications from the receiver and change the power levels delivered to the coil to meet the needs of the device being charged. 

 

To ensure safe and correct operation, the controller includes over-temperature protection and the ability to manage a number of fault conditions as well as parasitic metal object detection (PMOD). The ability to work across a wide temperature range (-40°C to 110°C) and fit in a small space (7mm x 7mm) make it ideal for a range of consumer applications including digital cameras, MP3 players and smartphones as well as other devices such as tools.

 

The heart of any wireless charging system is the coils that transmit and receive the electrical energy across the air gap. While it is possible to build these in-house (and some applications may make it necessary to do so), there are an increasing number of off-the-shelf coils in various diameters, thicknesses, and inductances. 

 

On the transmitting side is Vishay’s IWTX47R0BEEB6R3J11 6.3 µH coil that includes high permeability shielding and measures 47 mm diameter and 4.9mm thick. The AWCCA-36R36H08-C51-B from Abracon is a 12 µH receiver coil that is particularly suited to consumer electronics applications. Other similar receiver coils include Wurth Electronics’ 760308101303 (12µH, 26.3mm diameter, 1.31 mm thick). TDK’s WT252512-8F2-SM (2.76 µH DC resistance of 150 mOhms, 25 mm diameter, 2.05 mm thick) is another example of a transmitter coil.

Wireless charging is now moving rapidly and there is far more successful deployment than ever before. However, the technology is far from mature and is still constrained by needing a close distance between the transmitter and receiver as well as being slow to charge and not able to charge at all with metallic cases.

 

While consumers like the wireless aspect, they are looking for wireless solutions that can charge as fast as wired charging, with more spatial freedom. Component manufacturers and designers continue to innovate and are now focussing on faster charging over larger distances, to deliver the next generation of this exciting technology.

 

About the author:

Mark Patrick is the Supplier & Technical Marketing Manager EMEA of Mouser Electronics. Mark joined Mouser Electronics in July 2014 having previously held senior marketing roles at RS Components. Prior to RS, Mark spent 8 years at Texas Instruments in Applications Support and Technical Sales roles and holds a first class Honours Degree in Electronic Engineering from Coventry University. For details, contact Helen Chung, Asia PR Specialist of Publitek, on email: helen.chung@publitek.com