Wirelessly Charging the IoT

­As the prevalence and diversity of IoT systems grow, the need to ensure these devices are continuously powered has never been so important. This isn’t just for remote ultra-low-power sensors that need to run for years on miniscule energy budgets, but also for more powerful and more critical systems. And for these, wireless power is an important option.

Wireless charging is a well-established technology with proprietary and open standards. However, the majority of its implementations have been for smartphones and tablets, and the technology’s limitations in relation to size, bill of materials (BOM) and efficiency at reduced loads have limited its use for IoT and small smart devices. Here Yaser Abuzarifa, a field application engineer with Eggtronic, examines a new approach to wireless charging designed to specifically meet the needs of such low-power systems.

Benefits of IoT Wireless Charging

Wireless power transfer has become a well-established technology for charging everyday items from smartphones to earphones, and electric toothbrushes. In addition to a range of proprietary standards, the Wireless Power Consortium’s open Qi standard has been in use since 2008 and, as of August 2024, the WPC listed over 300 developers and manufacturers – including Apple, Google, Samsung and Sony – on its members page. Eggtronic is also a WPC member. Similarly, the Airfuel wireless charging standards alliance has been in existence since 2015 following the merger of the Alliance for Wireless Power and Power Matters Alliance (both founded in 2012).

The growth of wireless charging and the associated standards has principally been driven by the smartphone market. OEMs and city planners are also looking to higher power systems for improved charging in public services (such as ebike and car-share fleets), but, arguably, one of the most important growth sectors should be small devicessuch asearphones, hand controllers for gaming, wearables (smart watches, smart glasses), and other small devices (up to 6W). And at this power level, this makes wireless charging particularly attractive to the rapidly growing number of small, low-power IoT products across a variety of sectors.

Take connected medical devices and health monitors such as pacemakers, orthopaedic implants, insulin pumps, smart plasters and glucose monitors. These have a wide range of sensor functions, and the data transmitted will vary from system to system. The IoT functionality might be as simple as alerting hospitals to problems and/or battery status (pacemakers) or might require readings from multiple accelerometers to understand motion range during post-operative rehabilitation (hip implants). For all of these, especially for implanted devices where battery changes would require invasive surgery with serious risks, it is critical that systems run continuously without battery failures, and wireless charging offers a lower-risk alternative.

But medical devices are just one example - wireless charging can benefit a range of gaming, smart-home, smart-agriculture and smart-city IoT systems where the feasibility of a direct wired connection is low and/or replacements are not desirable.

Challenges of Wireless Power for IoT

Currently, most wireless charging is bulky and expensive, and not particularly convenient with very precise positioning of the device being charged needed to ensure effective coupling.In the main, designs are also relatively complex (one significant factor that hampers efficiency). For example, the Qi architecture for smartphones requires five stages for the complete conversion process as shown in Figure 1. It is also noteworthy that efficiency falls rapidly when not operating at full load, with typical efficiency for such a system being in the region of 70% and sometimes less than 60% for low power devices.

This lower efficiency leads to thermal problems in small devices and/or reduced charging speeds. Additional issues come from the large size of the coil, which is simply not compatible with the size of IoT and other small devices. On this note, conventional wireless charging not only requires a large coil, but a thick coil, which must be planar and therefore creates problems when it comes to the small / non-planar surfaces found in many IoT products.

The final (and arguably key) issue that needs to be overcome if wireless charging is to be used more widely for IoT charging is component count. A traditional wireless charging system has between 12 and 16 active devices, at least two magnetic components, two coils, an AC cable and a DC cable. IoT devices tend to be small and low-cost systems and the large bill of materials can lead to form factors and costs that put it out of reach for many manufacturers.

An IoT-Specific Wireless Charging Architecture

It is, however, possible to simplify this design to create a more efficient wireless charger specifically for the needs of IoT and similar low-power systems.

If we first look at the transmitter, it is possible to base the topology on a single FET (used to excite the system) and coil (used to transmit energy and as a resonant component), which reduces BoM by eliminating the second FET that would be used in a Class-D amplifier, and by adopting the Tx coil as a hybrid inductor that acts as a quasi-choke and as a quasi-resonator, which would be both required in a Class-E amplifier. Taking such an approach to create a high-frequency wireless power architecture would also use fewer components than a standard class E, class F, class Phi, and other resonant wireless power transfer solutions including Qi and Airfuel-based solutions.

Moving to the receiver side, here we can base the charger on a single coil with either an Rx FET or diode as the rectifier, with a resonant architecture that is a mirror image of the transmitter. A diode can be selected for an easier control algorithm, while an Rx FET allows greater efficiency and bidirectional functionality. Performance with either component would be extremely high, and ZVS-ZCS (zero voltage switching, zero current switching) conditions would be maintained in a broad range of input and output conditions, significantly reducing the dynamic losses of the system. In comparison, if the receiver were instead to use an active or full-bridge rectifier it would require the use of four diodes or FETs, and the performance would be lower because of the dynamic losses, in particular when high frequencies are required to reduce the size of the system.

Finally, a battery charger controller can also be eliminated through the use of a step-down converter implementing a CC/CV mode algorithm.

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Figure 2: WaveEgg LP offers much simplified, more efficient end-to-end conversion for IoT applications (Tx on the left, Rx on the right)

These adaptations can be seen in the above schematic for the WaveEgg LP wireless charger developed by Eggtronic and based on the company’s EPICⓇ(Eggtronic Power Integrated Controller) ICs. EPIC ICs are designed to optimise performance and low-load to full-load efficiency, minimise standby power consumption and reduce component count and form factor in high-performance power converter and wireless power transmission systems. Built around a 32-bit RISC-V core and high-performance digital and analog peripherals, the ICs feature a flexible internal structure that supports control of both standard and proprietary power conversion architectures.

In the case of WaveEgg, which was formally announced in September 2024 with the launch of an evaluation board (EVB), the EPIC ICs have allowed Eggtronic to develop the first architecture of its kind for a power range of between 0.5 W and 6 W – ideal for IoT systems and similar low-power devices such as games controllers and wireless headphones. WaveEgg can work at extremely high frequencies (from some MHz to tens of MHz, including ISM 6.78 - 13.56 - 27.12 MHz) and exhibits extremely high efficiency over  the whole load range. This is thanks to ZVS and quasi-ZCS on the transmission low side FET and ZVS+ZCS on the receiver low side FET. At the same time, the low number of devices reduces losses coming from the non-ideal behaviours of each component.

The end-to-end efficiency for the new architecture is significantly higher than for traditional systems, with WaveEgg achieving an efficiency of 85% at switching frequency of 2 MHz and with an output of 6 W. It should also be noted that, by making even higher frequencies possible, the implementation of a synchronous rectifier on the secondary side would further improve efficiencies without stressing the diode. This, in turn, enables the implementation of smaller coils or allows flex coils - flexible printed circuits containing integral inductive coils - to be used.

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Figure 3: The wave shape of the WaveEgg LP is similar in pattern to that of a Class-E amplifier

Thanks to the capabilities of the EPIC controller, WaveEgg supports a PD/PPS input voltage as well as standard DC voltages. The receiver is also capable of charging a battery through a step-down converter implementing a CC/CV mode algorithm, eliminating the need for a battery charger controller, or can be compatible with fixed voltage output or other proprietary algorithms. Furthermore, the receiver can act as a PMIC, handling multiple inputs - including power delivery or fixed voltage - when wireless power is not used, and can offer a bidirectional battery charger that acts as a boost converter when no input is detected and the load should be supplied from a battery. 

Summary

The deployment of IoT systems has spread rapidly in recent years and with this comes a need to improve the way we power such systems. While well-established, wireless charging solutions have focused on the smartphones and tablet market, the technology’s efficiency, BoM and size made its use for IoT limited at best. New wireless charging architectures, which use a simpler design with fewer components are enabling significant leaps in size and efficiency and bringing efficient, rapid and small form factor charging to low-power IoT devices.

Eggtronic