Design of Self-Resonant Spiral Coils for Mid-Range, High-frequency Wireless Power Transfer Systems

This paper presents a design of self-resonant coils for mid-range, high-frequency (HF) wireless power transfer (WPT) systems. In HF, WPT systems, a critical coupling coefficient between two resonators determines the power transmission distance. However, because the critical coupling coefficient is inversely proportional to the inductance value of the coupling coils, the longer transmission distance in the WPT systems requires a high inductance value, which causes two major challenges: (1) the small capacitance (Cs) in the compensation network at the fixed frequency, and (2) the low parasitic-resonant frequency due to the parasitic capacitance (Cp) in each coupling coil. To overcome these issues, first, we designed and fabricated the optimized self-resonant coils to achieve a longer transmission distance without any external capacitors at 10's of MHz frequencies. The self-resonant coil consists of two identical spiral coils, and the distance between them creates the series capacitance (Cs) without any additional components. The proposed self-resonance coil design addresses the capacitor breakdown issue and allows us to tune the coil easily. The Cs value was selected using the coil's equivalent model such R, L, and Cp for the resonance in the WPT condition. In addition, the proposed self-resonant coil design provides the alternative design to maintain the zero-phase angle (ZPA) for the specific over-coupled condition by adjusting the Cs value. We designed the three types of self-resonant coils with different critical coupling coefficients in 160 cm diameter and 13.56 MHz. The conversion efficiencies of the three types of coils were around 96% in these systems under the 180 W AC-to-AC WPT condition. The designed self-resonant coil successfully operated without any additional capacitor by tuning the Cs value considering Cp value and increasing the transmission distance while keeping the inductance value high.