Gunbok Lee

Optimal coil size ratios for wireless power transfer applications

Optimization of coil sizes for wireless power transfer applications has a significant impact on the range and efficiency of the wireless power system. Often it is difficult to accurately analyze how a set of coils will perform before they are constructed due to the complexity of approximating the parasitic components and coupling coefficients associated with the coils. Also, for certain wireless power applications such as consumer electronic devices, implanted medical devices and industrial equipment that have physical constraints on the size and shape of the coils, it can be a difficult and time-consuming process to design and evaluate several different coil configurations for optimal range and efficiency. This paper provides simplified design equations to accurately calculate the self inductance, capacitance, resistance, quality factor and coupling coefficient in terms of coil geometries for flat, spiral coils. Experimental results validate the analysis and a design example is provided to optimize the size of a transmit coil for maximum range and wireless power efficiency to a 5.8cm receive coil used in a four-element wireless power transfer system.

Empirical formulas for self resonance frequency of archimedean spiral coils and helical coils

This paper suggests empirical formulas for self resonance frequency of an Archimedean spiral coil and a helical coil for wireless power transfer. The suggested empirical formulas can be used as design guidelines for SRF of helical and spiral coils for wireless power transfer, enhancing our intuitive understanding of these structures.

A Reconfigurable Resonant Coil for Range Adaptation Wireless Power Transfer

To widen the range of a magnetically coupled wireless power transfer system, we propose a novel reconfigurable resonant coil. This device consists of a series of subcoils that use switches to control the number of turned-on subcoils. With this design, both the coupling coefficient between coils and the quality factor of the proposed coil can be dynamically tuned when the distance between the transmit and receive coils changes. Thus, the proposed coil system increases the efficient power transfer range compared to a conventional resonant system. In this work, we replace the first coil in a four-coil system with the proposed coil. This coil uses six switches to change the number of series subcoils from six to one as the distance between the transmit and receive coils increases. A theoretical analysis demonstrates how to properly design and configure the proposed coil. Experimental results show the range at which the system achieves 70% efficiency or higher increases by 120% compared to the conventional system without the proposed method. The proposed system keeps high efficiency when the load varies from 40 to 300 Ω.

Simplified Vector Potential and Circuit Equivalent Model for a Normal-Mode Helical Antenna

A normal-mode helical antenna with open-ended and center-fed configurations is investigated. The investigation is twofold. First, explicit formulas are derived by employing the vector potential with the triangular current distribution for a normal-mode helical antenna. These give straightforward results when compared to previous numerical analysis. Second, the equivalent circuit model is proposed to obtain the input impedance. The proposed equivalent circuit model consists of a parasitic capacitance, an effective self-capacitance, an effective inductance, and a radiation resistance in a series combination. The radiation characteristics are compared to the commercial simulation to verify whether the obtained formulas and the equivalent circuit model achieve a good agreement.

An investigation of cross-coupling for magnetically coupled wireless power transfer

The effects of parasitic cross-couplings in 4-coil wireless power transfer (WPT) systems are investigated. In WPT applications, the available space for the receive coil is limited in comparison to that of the transmit coil. This physical limitation, therefore, results in different sizes for transmit and receive coils. However, asymmetry in the dimensions of the coupled coils can result in significant parasitic effects. Coupling between coil₁ and coil₃ (k₁₃), and coupling between coil₂ and coil₄ (k₂₄) cause deviation of optimal frequency and degradation of efficiency. Coupling between coil₁ and coil₄ affects only the efficiency degradation, with independence of k₁₃ and k₂₄. To reduce parasitic effects, a capacitive compensation method is proposed. This method can compensate for the unwanted shift in peak efficiency due to parasitic effects by adding capacitance to coil₁. In our work, the proposed method increased the efficiency from 69.5% to 85.3% at the desired operating frequency.

A Sample Reduction Technique by Aliasing Channel Response for Fast Equalizing Transceiver Design

This paper proposes a technique to reduce the sample number of channels pulse response required to optimize equalization coefficients. In our method, channels pulse response is aliased in calculating equalization coefficients reducing the number of samples of the channels frequency response. As a result, the computation time to acquire the channels frequency response is greatly reduced. To demonstrate our method, equalization coefficients are calculated using the conventional and our methods and then compared. Since the necessary number of samples is reduced by 8 times at most in our method, the computation time is reduced up to by 7.01 times. For accuracy verification, we also calculated the equalized eye sizes using both methods. The calculated eye sizes are almost identical. These results show that using our method, engineers can accurately optimize equalizing transceiver design with reduced efforts to simulate channels frequency response.

Analysis and Comparison for a 4-Coil Magnetic Resonance Wireless Power Transfer System

The critical point analysis(CA) and impedance matching analysis(IA) are performed and compared for a 4-coil magnetic resonance wireless power transfer system. Because the operating frequency splits at short distance while the efficiency drops drastically at long distance in this system, the optimization technique is needed for either a specific distance or efficiency at the fixed frequency. CA uses the critical point where shows maximum efficiency in the entire range and IA uses the impedance matching technique to achieve maximum efficiency at the specific distance. Comparison result shows that IA is more efficient than CA. Also, it shows one side matching has a tradeoff relationship comparing to both side matching. By using four spiral resonant coils, the analysis was experimentally verified. The measured data agreed well with the calculated data.

Efficiency comparison between resonant coil and metamaterial sheet for wireless power transfer

Resonant coils and metamaterials are useful to increase the effective range of a wireless power transfer system. We experimentally compared the characteristics of a resonant coil and a metamaterial with similar sizes. The resonant coil had higher Q and higher coupling ability with a loop than did the metamaterial. The transmission coefficient with the resonant coil system averaged 3.6 times higher than that with the metamaterial system.

Analysis of RF Inductive Couplers for Power Line Communication

전력선 통신에서 사용되는 비접촉식 커플러의 신호 전달 특성을 향상시키기 위한 비접촉식 커플러의 구조와 자 기장 결합에 대한 연구를 수행하였다. 비접촉식 커플러의 결합 구조를 분석하였고, Rogowski 코일을 이용한 커플러와 자성체 코어를 이용한 커플러를 제작하여 구조에 따른 신호 전달 특성을 분석하였다. 집중 정수 소자와 임피던스 트 랜스포머를 이용한 임피던스 매칭법을 제안하여 협대역과 광대역에서 신호 전달 특성을 향상시켰고 전달 신호의 크 기는 대역폭과 트레이드오프가 있음을 보였다. 본 연구는 추후 부하 변동에 따른 임피던스 매칭 연구에 도움을 줄 수 있다. We investigated the structure of inductive coupler and its magnetic coupling to increase the transmission coefficient for power line communication. A Rogowski coil, which is an air-cored inductive coupler, and a magnetic cored coupler were fabricated to analyze the transmission coefficient for different coupler parameters. This paper proposes the impedance matching method using lumped elements and an impedance transformer to increase the transmission coefficient. In the experiment, the transmission coefficient of the proposed system was increased in both narrowband and broadband cases, and a trade-off between the transmission coefficient and the bandwidth was shown. This method will be useful for the further study of impedance matching with the load variation.

A Novel Impedance Matching Topology for Magnetically Coupled Wireless Power Transfer

A modified 4-coil magnetic resonance wireless power transfer (MRWPT) system is proposed. Four coils based on 2-coil system with additional two matching coils were used in this topology. When Tx-Rx distance is changed, the input impedance is changed. However, it can be adjusted by coil parameters of matching coils to maintain impedance matching for maximum efficiency. The equivalent circuit of MRWPT system was analyzed for both transmission function and optimum coupling coefficient of the matching coils. By using four spiral resonant coils, these design considerations was experimentally verified. The measured data agreed well with the calculated data and the transmission function of the proposed system was more efficient than that of conventional 2-coil system.