Analysis and Design of a 3-Coil Wireless Power Transmission System for Biomedical Applications

Wireless power transfer is a practical and widely used method to power various implantable devices. Commonly, the implanted receiver (RX) must be small, often on the order of millimeter, which poses significant design challenges. In this paper, a technique to improve the performance of systems with a size (inductance)-limited implanted RX coil is explored. Conventionally, only mutual coupling between coils is used to optimize the performance, which constrains the layout/geometry and choice of coils. In the proposed system, mutual coupling, mutual capacitance, and relative polarity can all be used, thereby reducing the constraints on coil layouts. Performance is further enhanced by two additional techniques that maximize the reflected impedance between the RX coils. Three designs (two-coil, conventional three-coil, and the proposed three-coil) are implemented with a 35 mm diameter RX coil, and their performance is measured at 5 MHz with a 1 $\text{k}\Omega $ load resistance. The efficiency of these designs is measured at varying distances (20–60 mm) between the transmitter and RX. The efficiency of the proposed three-coil system at 50 mm separation is 40%, while the implemented conventional three-coil system and the two-coil system are each less than 10%. Finally, the advantage of the proposed strategy in terms of RX coil current and load tolerance is discussed.