An investigation on the characteristics of conical coils for wireless power transfer systems

Historically, there have been many efforts to transfer power over the air medium and eliminate the necessity of conducting wires. Recent success stories on this direction has empowered unforeseen applications in many fields ranging from biomedical, wearable, food industry, and consumer products to electric vehicles. In this work, conical coil structure is presented as a viable solution to improve the efficiency of magnetically coupled wireless power transfer (WPT) systems. It is shown, both qualitatively and quantitatively, that the use of conical inductors in place of traditional planar coils increases the self-resonance frequency of transmitter resonator while maintaining the flux linkage to the receiver side. Electromagnetic (EM) and circuit simulations predict an efficiency increase using conical coils in wireless power transfer link. The measurement results of a prototype 3-coil structure built based on the conical structure confirm the validity of simulation results. The analysis, design, and characterization comparison of WPT systems that use both planar and conical coils is also presented. A power transfer efficiency of up to 53.9% is achieved for a 4-coil WPT system employing conical coil resonators that are 50 cm apart (which translates into the separation distance of ~1.5× the diameter of the resonators). In contrast, the same system using planar resonators achieves a power transfer efficiency of 32.8%. Thus, employing conical coils improves the power transfer efficiency by a factor of up to 1.6×. Finally, an adaptive control mechanism to improve the efficiency of magnetically-coupled resonators (MCRs) is presented. To minimize the degradation in power transfer efficiency, the proposed system dynamically adjusts the capacitance of MCRs as the distance between the transmitter (TX) and receiver (RX) coils changes. The control unit operates in a self-sufficient manner through rectifying a portion of the AC signal present on TX and RX coils. A proof-of-concept circuit operating at 13.56 MHz is designed in a 0.13 μm CMOS technology and the simulation results confirm the validity of the proposed scheme.Applied Science, Faculty ofElectrical and Computer Engineering, Department ofGraduat