Inductive fast charging of IoT devices : An in-depth analysis of short-range wireless charging technologies based on induction

Sammanfattning: In the era of Internet of things (IoT), sensor-equipped devices exchange data over networks. In battery powered IoT devices, the lifespan of the devices is often much longer than the battery life, leading to multiple costly and environmentally hazardous battery replacements during the operational life of the devices. As a result, there is a growing interest in using rechargeable batteries that can be wirelessly fast charged to prolong the lifespan of IoT devices and their batteries. In wireless power transfer based on induction, the transmitter and receiver antennas can be accurately modeled as two coils in separate circuits. The transmitter coil, energized by alternating current, generates an oscillating magnetic field that induces an electric field in the nearby receiver coil, following Faraday's law of induction. By connecting a resistive load to the receiver coil, it is then possible to extract energy from the induced electric field. This project investigates inductive fast charging for IoT devices with a focus on the electromagnetic power transfer. Two different types of coil antennas were simulated in a solver based on the finite element method and tested in lab for verification purpose. One was a transformer-like ETD coil and the other a flat spiral coil. Both the transmitter and receiver coils were compensated with a capacitor in series to allow for increased efficiency and power transfer at the designated frequency of 100 kHz. The compensating capacitors were tuned such that frequency bifurcation or frequency splitting was avoided. Due to the higher quality factor of the ETD coil compared to the spiral coil they were compensated differently to operate at the resonance peak. The simulation and the experimental tests agreed well, and the findings indicate that both types of coils demonstrate the ability to transfer high power with high efficiency. Theoretically there is no limit in the power transfer for both types of coils since it is proportional to the square of the excitation voltage. All tested coils exhibited the ability to transfer a power of at least 30 W with an 86 to 92 % efficiency without experiencing any significant temperature elevation. The advantages of each coil depend on the design of the systems surrounding the power transfer unit and the nature of the built charging system. For scenarios where the equivalent load resistance of the battery charger unit on the receiver remains relatively constant throughout the charging process, the spiral coil proves to be a suitable choice due to its inherent capacity for easy dimensioning, allowing optimal efficiency for a specific load resistance. Conversely, if the equivalent load resistance fluctuates significantly during the charging process, the ETD coil would be a better alternative, since it exhibits small load dependence and high efficiency. Finally, to further increase the validity of the simulation model, the full magnetization curve of the ferrite core and a more general core loss model should be implemented to enhance the accuracy in studying the effects of higher harmonics and when operating closer to saturation.