Optimized NFC Circuit and Coil Design for Wireless Power Transfer with 2D Free-Positioning and Low Load Sensibility ^†

This paper proposes a method for optimizing and designing a wireless power transfer system operating at $13.56\mathrm{MHz}$. It can be used as guidelines for designing coils for the new-trending technology that enables NFC devices to not only to communicate but also to charge. Since NFC wireless charging is an emerging technology, it is of interest to propose optimizations and a dedicated circuit design for such systems. This work proposes an optimization procedure to calculate the dimensions of a transmitter and receiver pair that assures the highest efficiency while considering all possible positions of a receiver that is placed on a desired surface. This procedure seeks to facilitate and automate the design of rectangular-shaped coils, whereas the literature proposes mainly square-shaped coils. Afterwards, a circuit analysis was conducted, and the series-parallel compensation network is proposed as the most promising topology of the receiver to assure a low efficiency sensibility to load variations for $13.56\mathrm{MHz}$ wireless power transfer systems. A pair of optimized transmitter and receiver coils is prototyped, and the experimental results are tested against the theory. The transmitter of $7\mathrm{cm}\times 11.4\mathrm{cm}$ and receiver of $4\mathrm{cm}\times 4\mathrm{cm}$ are separated by $10\mathrm{mm}$. The receiver can move on a surface of $8\mathrm{cm}\times 12\mathrm{cm}$ and the load can vary from $36\mathsf{\Omega }$ to $300\mathsf{\Omega }$ while assuring a minimum and maximum efficiency of $80\%$ and $88.3\%$, respectively.