With the development and availability of various electronic devices such as smart phones, tablet PCs, and laptops, people can use such electronic devices without being restricted by time and space in a fast and convenient manner. Most electronic devices are being charged through wires. As the wired charging method uses cables, such devices that use this charging method cannot move during charging and are restricted by space. In addition, if contact failure or damage occurs at charging terminals, charging may not be performed or electric shock may occur. Interest in wireless power transmission(WPT) has increased rapidly to address such problems of the wired charging method, and related studies have been conducted all over the world. WPT is a technology that can transmit power without using cables. As cables are not used, it is possible to move the devices during charging. Furthermore, there is no risk of contact failure or electric shock because charging terminals are not used.
WPT can be largely classified into three types: A magnetic induction type, a magnetic resonance type, and a microwave type. The most popular type at the moment is the magnetic resonance type, which was proposed for the first time by the Marin Soljacic research team of MIT in the United States in 2007. This type can transmit several kW of power up to several meters and enable multiple charging by using the resonance phenomenon of transmitting and receiving coils. It is also not harmful for human bodies. However, power loss is large, and the values of the inductance(L) and the capacitance(C) vary significantly depending on the design of the coils, which may dramatically affect the efficiency. To address this problem, research on the efficiency improvement is essential.
In this paper, superconducting coils were used to improve the efficiency of the magnetic resonance type WPT and the optimal design of the superconducting coils was investigated. Resonance is a phenomenon in which a specific frequency is selectively blocked or passed when L and C repeat the process of accumulating and discharging energy in electric and magnetic fields. When resonance occurs, if either L or C is stronger, energy is deflected and loss occurs. When L and C are equal in force, the force equilibrium prevents loss. In other words, when the imaginary part(X) of the impedance(Z) is zero, the force equilibrium is reached and no loss occurs. For this reason, the values of L and C are very important factors in the magnetic resonance type.
The values of L and C were calculated using high frequency structure simulation(HFSS) according to the diameters and the turns of the superconducting resonance coils. The optimal values of the superconducting resonance coils were derived by analyzing their S-parameters. Furthermore, actual coils were fabricated using the optimal design values of the superconducting transmitting and receiving resonance coils found through simulation, and their S-parameters were analyzed. As a result, S11 was approximately ?15 dB and S21 was approximately ?2.5 dB on average when the coils were designed using arbitrary design values. In the case of the superconducting transmitting and receiving resonance coils designed using the optimal values, S11 decreased by ?9.06 dB to ?24.06 dB and S21 increased by 1.22 dB to ?1.28 dB. If optimal superconducting resonance coils are designed considering the values of L and C, it is possible to implement a WPT system with a high power transmission characteristic value.