NFC charging standard emerges for wearable devices

Wireless charging continues to grow in its popularity and is now featured in billions of devices around the world. The largest portion of this wave has come in the form of smartphones, with leading global manufacturers standardising on the Wireless Power Consortium’s Qi (pronounced ‘Chi’) standard (inductive charging).

Meanwhile, the wearable devices industry continues to grow rapidly due to the popularity of innovative consumer products, industrial and enterprise class wearables, and medical devices. From smart watches and fitness trackers, to augmented reality (AR) glasses, as well as access and security wearable devices and hearables, the market has grown 100% since 2014 and is expected to be a US$57bn industry by 2022.

Influenced by the broad adoption of wireless charging in the smartphone industry, wearables manufacturers are exploring wireless charging in their products as a means of enhancing user experience and new form factors, eliminating of charging contacts which are leading points of failure and encapsulating devices to improve durability and sealability.

For product designers, the current Qi standards have left smaller wearable devices without a viable, standards-based option. The Qi standard is based on closely-coupled inductive charging technology, which requires relatively similar coil sizes and extremely close proximity of coils for effective power transfer. As form factors for wearable devices continue to shrink, standard Qi antenna technology cannot be integrated directly into the device.

 Alternative standard

 Near field communication (NFC) is a communications standard that allows devices like phones, tablets, handheld industrial computers, credit cards and other products to pass data back and forth. Credit card tap-to-pay, device pairing, product authentication and security are just a few of the many applications enabled by NFC.

Figure 1: Leading companies are integrating WLC into wearable devices

Now, the communications standard is not just for data. In an effort to enable new features and functionality, product developers are taking advantage of a recent update to the NFC standard.

This update - which was announced in May of 2020 - approved the Wireless Charging (WLC) specification, making it possible to wirelessly charge small, battery-powered consumer and IoT devices with a smartphone or other NFC charging device. Power transfer is specified of up to 1W transmitted (500mW received), bringing wireless power and data to smaller devices like ear buds, wearables, active stylus, smart glasses and a host of IoT applications with smaller battery and power requirements.

NFC charging technology is already appearing in several applications. Samsung integrated NFC charging into the Galaxy Fit line of wrist-worn fitness tracking devices. Earlier versions of the Galaxy Fit use a pogo pin, copper pad and plastic snap construction, whereas the update means place-and-go NFC charging.

Huawei has also integrated the technology into its newly released Huawei+Gentle Monster smart glasses. This implementation eliminated the need for a bulky connector that constrained design and could become a failure point with rough use.

 In addition to smart glasses and fitness trackers or smart watches, other applications that are well suited for NFC charging include hearables (charging in a case or free-charging), styluses, industrial and automotive devices (sensors, access devices, key fobs), medical devices and contactless connectors.

Examining NFC technology 

NFC charging operates on the same 13.56MHz band as NFC. Using this loosely coupled wireless power technology creates a host of advantages. Higher frequency means that devices do not need to be within millimeters in order to charge (as in Qi), allowing for greater spatial freedom both in coil-to-coil distance and orientation. For developers, this equates to greater latitude for industrial design. Irregularly shaped surfaces and products that do not lie flat will not see the efficiency drop that is characteristic of a tightly-coupled system. Ultimately this equates to a better user experience because consumers will not have to be as conscious of product alignment while charging.

Figure 2: The power / data ranges for NuCurrent’s proprietary extensions

Given its resonant inductive nature, NFC charging technology also enables multiple receiver coils to charge from a single transmitter coil. This decreases the total number of coils in the system, reducing complexity and total cost. For example, a consumer can simply drop two wireless ear buds and a pair of smart glasses on the same pad and have the confidence that each device is receiving power.

NFC charging opens up opportunities for wireless charging in a variety of small, space-constrained devices like smart watches, fitness bands, smart glasses, and styluses. This technology can be used for a variety of applications, but to reach the full market potential, the power transferred needs to be increased, while advancing NFC’s data transfer rates in the 108 to 848kbits/s range.

One advance that is underway is a proprietary NFC extension that enables up to 3W of power received (with plans for 5W received) while driving data transfer rates at 106kbit/s and above. Figure 2 shows the power / data ranges using NuCurrent’s extensions.

Balancing power and data

One of the important dynamics of designing NFC charging into a product includes finding the right balance of power and data transfer via the NFC antenna.

Power and data are competing optimisation factors. For example, increasing quality factor (Q) in a design will increase power efficiency but data integrity will suffer, and vice versa.

If system efficiency is important, but spatial range is not as critical, designers can constrain the transmitter/receiver in an over-coupled configuration in which the coupling efficiency is so high, that a low Q (good for data transfer) can be realised without affecting system efficiency.

If spatial range is important, more so than efficiency, designers can include sacrificial resistance to force the lowering of Q, thereby enhancing data rate while maintaining a high spatial range (at the expense of lower efficiencies.)

Eliminating connectors

In addition to delivering wireless power using the NFC antenna, these NFC charging solutions also have the ability to deliver a meaningful level of bi-directional data to/from the device wirelessly. In some cases, mechanical debug ports - basically connectors that serve as a UART, USB or I2C communications links - are being rethought, since NFC can deliver both power and data via the NFC antenna.

Figure 3: Flow diagram of the NFC charging system

Wireless data ports have significant benefits for manufacturers. 

The first is touchless, high volume testing. Instead of having to mate a connector to a device for testing, NFC power and data solutions speed up the testing process while eliminating the likelihood of viral connector damage in high volume testing.

Another is that during manufacturing runs, devices sometimes need to be reprogrammed with software/firmware updates. Using NFC charging, the devices can be powered up and reprogrammed without having to be taken out of the packaging, saving considerable time and expense.

Finally, the assemblies and manufacturing processes are simplified by eliminating a connector. There is no need to find complex moulding, compounds or processes to seal around 3D interconnects. More standardised adhesives and sealing methods can be used.

For mobile phones, Qi has been a very important technological advancement, providing a global standard that delivers interoperability and safety across thousands of products. However, the standard was not designed for smaller, space-constrained devices with lower power needs. While the WPC is exploring lower power options in upcoming versions of the Qi standard, the NFC WLC standard - and extensions of it by industry innovators - are already bringing the promise of wireless charging to a new group of product categories.

Michael Harmon is director of marketing and communications at NuCurrent