Kainan Chen

Frequency Decrease Analysis of Resonant Wireless Power Transfer

Recent years have witnessed the booming development of resonant wireless power transfer (RWPT). Compared with conventional inductive power transfer, the frequency of RWPT is usually much higher. To reduce the resonant frequency while maintaining the transfer efficiency constant at the same transfer distances, two solutions are proposed and realized in this letter. Two fundamental structures for RWPT are analyzed and the expressions of the transfer efficiency are deduced. It is pointed out that the transfer quality factor and the load matching factor are two important factors of achieving high transfer efficiency. The larger the transfer quality factor, the higher the transfer efficiency. There is an optimal load matching factor to reach the highest transfer efficiency. The transfer efficiency can remain constant at the same distances if the transfer quality factor is kept at the same level and the load is matched. Theoretical calculations and experimental results provide a sound basis for decreasing frequency.

Analysis of the Double-Layer Printed Spiral Coil for Wireless Power Transfer

As a critical part of the wireless power transfer system via strongly coupled magnetic resonances, the resonant coils must be cautiously designed for the specific resonant frequency and high quality factor. There are some issues needed to be considered and studied in the coil design, such as the coil structure, parasitic parameter extraction, and optimizing. In this paper, the double-layer printed spiral coil is used, which could fully take advantage of the limited space and make larger parasitic capacitance for lower resonant frequency. Using the simplified partial element equivalent circuit method and finite element method, the circuit model with consideration of parasitic parameters and high-frequency losses is built, and the impedance characteristic of coil is simulated, which coincides well with the measurement result. In addition, several elements affecting the high-frequency loss, including the skin effect, proximity effect, and dielectric loss, are discussed for reaching higher quality factor, which is critical for the power transfer system.

Frequency-Splitting Analysis of Four-Coil Resonant Wireless Power Transfer

Based on four-coil magnetically coupled resonant wireless power transfer, this paper demonstrates, explains, and analyzes the frequency-splitting phenomenon by using the circuit theory. First, the complete and simplified models are built to describe the whole system. The numerical formulas of the system efficiency both at and away from the resonant frequency are presented. Then the frequency-splitting phenomenon is displayed and further explained mathematically and physically. An important factor for frequency splitting is introduced. The boundary condition for frequency splitting and its splitting frequency points are deduced. The amplitude-frequency and phase-frequency characteristics of the input impedance are measured and described to verify the explanation. Three related factors are studied, and the sensitivity analysis is conducted. A solution is proposed that helps improve the efficiency when frequency splitting occurs. The theoretical calculations and experimental results provide a sound basis.

Selective Wireless Power Transfer to Multiple Loads Using Receivers of Different Resonant Frequencies

In multiple receivers of resonant wireless power transfer, selective power flow among the loads is an important issue. This paper proposes a new method to control power division. The two-coil structure with different resonant frequencies of the sending and receiving loops is modeled and analyzed. The efficiency is proved to peak at the resonant frequency of the receiving loop, regardless of the resonant frequency of the sending loop. Using this feature, selective power transfer can be achieved by setting the receiving loops at different resonant frequencies. The efficiency of a particular load is greatly influenced by the driving frequency. The multiple-load system with different resonant frequencies is modeled and the efficiency expression of each load is deduced. The mutual inductances of the receiving coils have a small impact on the efficiency distribution. The closer the resonant frequencies of the receiving loops, the less isolated the related loads. The calculations and the experiments confirm the analysis.

Closed-Form Oriented Modeling and Analysis of Wireless Power Transfer System With Constant-Voltage Source and Load

In many practical applications of wireless power transfer, the battery, which can be modeled as a voltage source, is charged wirelessly from the voltage-source inverter via the transmitter and the receiver. Therefore, it is crucial to analyze such a wireless power transfer system. In this paper, the closed-form oriented modeling and analysis of the wireless power transfer system with the constant-voltage source and load are conducted. Two cases are studied: both the transmitter and the receiver are under resonance, and only the receiver is under resonance. In the latter case, the transmitter is set to be inductive for the implementation of zero voltage switching. The battery current, the output power, and the transfer efficiency of both cases are analyzed and compared in detail. The voltage gain, the power factor, and the output power of the latter case are studied to offer physical insights and design guidelines. An experimental prototype is implemented to verify the analysis. The experiments agree with the calculations.

Frequency splitting analysis of magnetically-coupled resonant wireless power transfer

Based on four-coil magnetically-coupled resonant wireless power transfer, this paper demonstrates, explains, and analyzes the frequency splitting phenomena with the circuit theory. Firstly, we build the complete and simplified models to describe the whole system. Then the frequency splitting phenomena are displayed and further explained by measuring and describing the amplitude-frequency and phase-frequency characteristics of the input impedance. Three related factors, i.e. the source internal resistance, the mutual inductance of the source coil and the sending coil, and the mutual inductance of the load coil and the receiving coil, are studied. Theoretical calculations and experimental results provide a sound basis for the explanation.

Wireless Power Transfer to Multiple Loads Over Various Distances Using Relay Resonators

Resonant wireless power transfer has attracted much attention in recent decades. In some practical applications such as wireless sensor networks, multiple-load transfer over various distances is required. In this letter, the intermediate-coil structure is utilized to transfer the same power to multiple loads over various distances, which indicates that the intermediate coils work both as relay resonators and as power receivers. The mathematical model is built and in-depth analysis is conducted. Four important factors, namely the source matching factor, the load matching factor, the transfer quality factor, and the reflected impedance factor, are employed to build the mathematical model of n-load transfer. The conditions to transmit the same power to all the loads attached in each relay resonator are investigated. The optimal load resistance and the highest efficiency with the same load resistance are derived. The theoretical calculations and the experimental results of double-load and three-load transfer confirm the analysis.

Employing Load Coils for Multiple Loads of Resonant Wireless Power Transfer

The load coils are employed for multiple loads of resonant wireless power transfer in this paper. With the addition of the load coil, this three-coil structure has easy access to transferring power to multiple loads with the advantages of a compact structure and controllable power flow. Both single-load transfer and multiple-load transfer are modeled and analyzed by means of the circuit theory. The transfer quality factor and the load matching factor are utilized in the analysis of efficiency. In the single-load transfer, the load matching condition is fully explored. Based on the single-load transfer, the multiple-load transfer is researched. The double-load transfer, acting as an illustration, is studied with the uncoupled and coupled load coils. Equivalent reflected resistances are introduced to decouple the model of the double-load transfer with coupled load coils mathematically. An experimental prototype is implemented to verify the aforementioned analysis. The experimental results agree with the theoretical calculations.

Maximum efficiency point tracking of the wireless power transfer system for the battery charging in electric vehicles

Recently the technology of WPT (Wireless Power Transfer) has received much attention on the new charging way of electric vehicle (EV). In the combination of the WPT and EV application, one of the key points is to keep the whole system operating under the high efficiency conditions. Considering the request of battery priority, this paper proposes a strategy of the maximum efficiency point tracking. The perturbation and observation method is used in this strategy to improve the efficiency of the whole system. Based on an actual WPT system in EV application, the strategy of maximum efficiency point tracking is implemented and tested in in Matlab/ Simulink, and the validity of this strategy is proved.

Transmitter-Side Control of Both the CC and CV Modes for the Wireless EV Charging System With the Weak Communication

Due to the convenience, safety, and aesthetics, the wireless charging is promising for the electric vehicles. For the charging system, the real-time wireless communication between the transmitter and receiver sides is conventionally used for the closed-loop control of the transmitter-side inverter. However, since the real-time wireless communication depends on the data transmission with the high speed and the low latency, it is sensitive to the interference. In order to improve the reliability and reduce the cost, the concept of the weak communication is introduced, and a transmitter-side control scheme is proposed to realize both the constant current (CC) and constant voltage (CV) charging modes as well as the smooth transition between the two modes. In the proposed control scheme, the mutual inductance is estimated without using the structural parameters in the receiver side. The online recursive least square filter is adopted to reduce the influence of the sensor noise for the estimation. Based on the estimated mutual inductance and the information obtained from the weak communication: 1) for the CC mode, the control command for the target charging current of the battery management system is calculated out directly and 2) for the CV mode, the reference transmitter current for the target charging voltage is calculated and a closed-loop control is used to make the transmitter current trace the reference value. The proposed control scheme is robust and can be easily implemented. Experimental results have confirmed the feasibility and the validity of the proposed scheme.