Equation-Based Optimization for Inductive Power Transfer to a Miniature CMOS Rectenna

This work presents an analytical approach for optimizing the single-ended (SE) and differential (DF) reader coils and miniature rectenna designs in near-field inductive power transfer (IPT) systems. In this design context, the appropriate operation frequency is in the gigahertz range, and the nonuniform current distribution on the reader coil and the radiation resistance must be considered and are included in this paper. Utilizing the rectenna miniaturization, the reader coil and the rectenna designs can be decoupled and their figure of merits (FOMs) are identified. The equation-based approach optimizes the IPT design rapidly, and the optimized reader-coil FOM and the optimized rectenna FOM, both plotted versus the IPT frequency, co-decide the optimal IPT frequency and the coil geometries for the highest RF-dc power transfer efficiency. The analytical method is verified by EM-simulation and measurement, and the DF reader coil topology is demonstrated to be more efficient than the commonly used SE coils. An optimized 2.2-mm IPT with a DF coil is realized at 4.8 GHz and significantly outperforms the previous works. Under a total RF power of 33.1 dBm, the CMOS rectenna, with a coil size of only 0.01 mm2, harvests the designed dc conditions of 1 V/0.1 mA.