Optimizing power transfer efficiency in biomedical implants: a comparative analysis of SS and SP inductive link topologies

Inductive link intended to deliver energy to implantable devices is recently becoming a popular method. To improve link efficiency, the compensated capacitors are adopted to resonate at the same frequency with the power transfer frequency on both sides. However, there is limited research on link efficiency and output power concerning quality factors, the selection between serial–parallel (SP) and serial–serial (SS) topologies, as well as coil design. This study delves into the efficiency of SP and SS topologies using rigorous mathematical derivations with reflected impedance models in terms of quality factors. It introduces a selection factor to help choose topology based on link efficiency. Results show that SS is better when the load quality factor (QL) is lower than the selection factor, and vice versa. The study also examines the performance limits of both topologies concerning frequency, inductance, and load impedance. It proposes a specific QL for maximum efficiency and a coil design procedure considering specific absorption rate limitations. By leveraging simulations and the design procedure, the study manufactures a pair of flexible coils for biomedical use. The designed link achieves a 62.99% efficiency under a 500 Ω load, with an overall efficiency of 46.96% including a passive rectifier.