High-frequency, mid-range wireless power transfer system using critical coupling coefficient adjustment

This paper presents a 200 W 13.56 MHz wireless power transfer (WPT) system for mid-range distance applications using the adjustment of the critical coupling coefficient (kcrit) of the two-coil resonators. The critical coupling coefficient is a coupling coefficient where the scattering parameter (S21) has a maximum value or induces a maximum voltage gain between resonators. If the coupling coefficient is higher or lower than the critical coupling coefficient (k 6= kcrit), the S21 parameter decreases and reduces the efficiency of the WPT system at the resonant frequency. Also, the critical coupling coefficient is inversely proportional to the inductance value of the coupling coils and the resonant frequency. In this paper, we propose a design method of a high-frequency WPT system to achieve longer transmission distance by controlling the critical coupling coefficient in the same size of the coils. We demonstrate three types of spiral coupling coils to adjust the critical coupling coefficient: Type-A with 2.3 μH, Type-B with 4.6 μH, and Type-C with 7.6 μH. A 200 W DC-to-DC converter, consisting of a class E inverter and a synchronized class E rectifier with the matching circuits, was implemented and connected to the coupling coils. The measured optimum transmitting distances of Type-A, Type-B, and Type-C coils were 5 cm, 7 cm, and 12 cm, respectively, which corresponds to the maximum S21 parameter of each resonator. Each coil provides maximum efficiency at the designed transmission distance by controlling the critical coupling coefficient. The critical coupling adjustment by changing the inductance value with the same size coil shows allows us to increase the transmission distance while maintaining high efficiency in the high-frequency WPT system.