Hybrid Impedance Matching Strategy for Wireless Charging System

Hybrid Impedance Matching Strategy for Wireless Charging System Ting-En Lee Automotive Research and Testing Center Research and Development Division Changhua County, Taiwan(R.O.C) leetn@artc.org.tw Tzyy-Haw Huang Automotive Research and Testing Center Research and Development Division Changhua County, Taiwan(R.O.C) thhuang@artc.org.tw Abstract With the rapid and mature development of wireless power transfer technology, many wireless charging systems (WCS) have been proposed and applied for various electric and electronic products. However, when the transmitting distance is changed, the efficiency of the WCS would be getting worse because of the impedance of system is non-matching. Obviously, the transmitting distance is an influence factor for the WCS. In order to overcome this problem, a hybrid impedance matching strategy is proposed in this paper for increasing the robustness of WCS. The key feature is to do the integration of capacitance tuning and frequency tracking techniques. To verify the effectiveness of the proposed method, we practically realize the hybrid impedance matching strategy for a 3kW WCS of electric vehicle (EV) which are all designed by ourselves. The experimental results indicate that the developed WCS with hybrid impedance matching strategy could keep the system efficiency over 80% when the transmitting distance is adjusted within 10-20cm. The developed innovative system is potential for the application of commercial electronic products and electric vehicles. Keywords wireless charging system; hybrid impedance matching, capacitance tuning; frequency tracking; electric vehicle I. INTRODUCTION In recent years, the environmental problem is one of the biggest issues around the world. A variety of green technologies are drastically developed to improve the air quality and reduce the fossil fuel consumption [1]. Accordingly, numerous investigations aim at low pollution, less emission, and energy saving in the field of transportation. Not only the hybrid vehicles are commercially sold by several vehicles manufactures presently, but also many research and development of electric vehicles have been widely executed [2]. However, there are a few safety concerns for conductive charging of EV. Moisture is the main factor for causing these problems. The wet environment is dangerous for the conductive charging because it is much possible to get an electric shock. The contact point of charger is also easily getting rusty, and then the charging efficiency is accordingly getting worse. For overcome the disadvantage of traditional charging way, wireless power transfer is one of the most promising technologies in the decade. As researches for wireless power transfer technology around the world, there are four technologies to be categorized, i.e. magnetic resonant coupling, inductive coupling, microwave power transfer and laser power transfer [3]. Among these technologies, the magnetic resonant coupling technology possessing the property of high power and long transmitting distances is much applicable for EV. In 2007, the wireless power transfer based on magnetic resonance coupling for transmitting 60W over a distance of 2m is presented [4]. Many subsequent theoretical and experimental studies have been proposed to broaden potential applications and further analyze this phenomenon [5]-[6]. For high efficiency power transmission via a wireless resonant power link, especially for a mid-range distance with a strong coupling regime, determining the optimum frequency for the power transfer is significant [7]-[8]. During the wireless power transfer process, resonant frequency may be changed because of the resonant inductance is changed with obstacles, parasitical parameters, impacts of receiving loop, temperature rising in circuit and so on. Once the detuning happens, the efficiency of wireless power transfer will drop drastically. Hence, it is necessary to design an impedance matching system for achieving efficient power transfer in the near-field. In the cases of varying displacements between two antennas, some methods have been proposed to improve power transfer efficiency in literature [9]-[10]. However, these proposed methods are difficult to accomplish practically. In this paper, a hybrid impedance matching technology for WCS is developed based on the practical consideration of EV. The proposed system mainly consists of capacitance matching technique and automatic frequency tracking technique. Accordingly, the WCS can accomplish wide ranging high transmitting efficiency under a certain extent of transmitting displacement. The following contents illustrate the design procedure and implementation of proposed system. II. HYBRID IMPEDANCE MATCHING STRATEGY A typical structure of wireless power transfer system is depicted in Fig 1, where the presented system is a full bridge type inverter. The resonant LC circuit is series in primary side and parallel in secondary side (SP type). The transmitting

antenna and receiving antenna are equipped with the tunable capacitors for adjusting the resonant frequency of the antennas. The resonant frequency is described by (1)-(2). Fig. 3. The control strategy of hybrid impedance matching. (1) o L L )( C C ) ( m S (2) e L L )( C C ) ( m S If the tunable capacitors are tuned into the same value, the resonant frequency will be translated to left or right simultaneously. On the other hand, the curve of power transmission ( ) will be shifted to left or right and the shape 21 of the curve is not changed, as illustrated in Fig. 2. Therefore, once the displacement is changed, one of the resonant frequencies can be shifted as consistent with the operating frequency so that high transmitting efficiency always can be maintained with different transmitting displacements. Even though the above mentioned capacitance tuning technique is available to achieve impedance matching, but it requires a large number of capacitors to achieve exactness. For the consideration of cost and realizable, the frequency tracking technique is incorporated into the developed system to achieve a more accurate impedance matching. Consequently, a hybrid impedance matching technology is proposed and its control strategy is illustrated in Fig. 3. As be shown in Fig. 3(a), the capacitance tuning is performed firstly to satisfy the substantial impedance matching. After that, the frequency tracking is executed to attain high accuracy of impedance matching. The algorithm of frequency tracking is depicted in Fig. 3(b). With the integration of capacitance tuning and frequency tracking techniques, the proposed method can maintain the high transmitting efficiency within a certain extent of transmitting displacement. Compared with the conventional WCS which only has a pinpoint optimal transmitting efficiency under a specific transmitting displacement, the WCS with hybrid impedance matching can accomplish wide-ranging high transmitting efficiency. Fig. 1. Wireless power transfer system with SP topology. Fig. 2. Tuning the resonant frequency by capacitance matching. III. (a) Capacitance tuning procedure (b) Frequency tracking procedure DESIGN OF WIRELESS CHARGING SYSTEM A. Coil Design As to the wireless power transfer coils, it is designed in circular type in this work. The design parameters and simulation results are illustrated in the table I. In the beginning, the transmitting distance of coils is designed in 14cm, and the operating frequency is assigned to 85kHz. For this case, the S- parameter is represented by Fig. 4, where the coil efficiency is around 99%. The intensity distribution of magnetic field is shown in Fig. 5, and we can observe that all the magnetic fluxes are concentrated between the Tx and Rx coils. When the transmitting distance of coils is less than or larger than 14cm, the efficiency of coil will be decreasing. For examples, the S- parameters of Gap=10 and Gap=20 are respectively represented in Fig. 6 and Fig. 7, where the coil efficiencies are just around 82% and 80%. For these worse cases, the proposed hybrid impedance matching method is able to increasing the coil efficiency from 80% to 90%. Furthermore, the charging power and efficiency could be maintained at an optimum condition.

TABLE I. The design parameters and simulation results of coils Gap Lp Ls Cp Cs η@ k (cm) (uh) (uh) (nf) (nf) 85kHz 10 446.2 23.4 0.354 8.91 161.4 82.1% 12 428.2 22.6 0.296 8.91 161.4 93.1% 14 417.1 22.2 0.248 8.91 161.4 99.2% 16 409.9 21.8 0.206 8.91 161.4 98.7% 18 405.2 21.7 0.172 8.91 161.4 91.9% 20 402.1 21.6 0.144 8.91 161.4 80.8% Fig. 7. The S-parameter of designed coil with Gap=20cm. Fig. 4. The S-parameter of designed coil with Gap=14cm. Fig. 5. The intensity distribution of magnetic field for designed coil. B. Realization method To realize the hybrid impedance matching technology, we have done lots of testing to collect the experimental data. Based on these data, we can establish a look-up table to analysis the system property in advance. Accordingly, the design diagram of capacitance tuning is represented in Fig. 8, where the resonant capacitors are switched in series. After the switching of capacitors, the frequency tracking algorithm will be executed by MCU (Infineon 2387C is used in this case) for calculating the transmitting efficiency. Accordingly, the optimal operation frequency could be caught to approach the best impedance matching. Fig. 8. Design diagram of capacitance matching technique. Fig. 6. The S-parameter of designed coil with Gap=10cm. IV. EXPERIMENTAL RESULTS In order to verify the effectiveness of proposed method, we practically develop a platform as Fig. 9 to do the testing. Based on the platform, we can stably fix the coils and related circuit module, and also can accurately adjust the transmitting distance during 0~30cm. Accordingly, the experimental results of impedance matching would be much significance. First, we pre-test the wireless power transfer function of the platform. With the input voltage 100V, electrical load 35Ohm, and operation frequency 85kHz, the testing results are as illustrated in Table II. The best condition for wireless power transferring is at Gap=14~15, where the system efficiency is about 88%. Once the transmitting distance is increased or decreased, the system efficiency is getting worse. Especially for Gap=20cm, the system efficiency is decay to only 77.5%. It is obviously that how the effectiveness of transmitting distance is for system efficiency. Then we perform the functional test of the proposed hybrid impedance matching method for the wireless charging system. The comparison results are illustrated in Fig. 10. With the aid of hybrid impedance matching method, the system efficiency

could be improved up to 7%. During Gap=10-16cm, the coil efficiency is at least 93.5%, and the system efficiency is at least 87.9%. Even though the transmitting distance is changed to 8cm, the coil and system efficiencies are at least 90% and 80%, respectively. Fig. 9. The wireless charging system flatform Finally, we practically apply the designed WCS to an EV which is developed by Automotive Research & Testing Center (ARTC). The photos of whole system are depicted in Fig. 11. With the assistance of impedance matching, we can find that the best condition for wireless power transfer is illustrated in Table III. To assume the Gap=15cm, and then the frequency is optimally adjusted to 90kHz. The input power is 350V/11.12A, the output power is 347.8V/9.93A, so that the system efficiency is 88.7%. Consequently, the developed hybrid impedance matching technology seems to be implementable to validate the feasibility. Fig. 11. The EV wireless charging system developed by ARTC. TABLE II. The specification of wireless charging system for EV Gap Input Power Output System Power Efficiency 10cm 100V/1.4A 63V/1.8A 83.7% 11cm 100V/1.5A 68V/1.9A 85.7% 12cm 100V/1.7A 73V/2.0A 87.1% 13cm 100V/1.9A 78V/2.2A 88.5% 14cm 100V/2.2A 83V/2.4A 88.4% 15cm 100V/2.5A 87V/2.5A 87.5% 16cm 100V/2.6A 89V/2.5A 86.4% 17cm 100V/2.7A 89V/2.5A 84.6% 18cm 100V/2.8A 90V/2.6A 82.8% 19cm 100V/2.8A 89V/2.5A 80.3% 20cm 100V/2.6A 85V/2.4A 77.5% Fig. 10. The comparison results of impedance matching testing TABLE III. The specification of wireless charging system for EV Gap Freq. Input Power Output Power System Efficiency 15cm 90kHz 350V/11.12A 347.8V/9.93A 88.70% V. CONCLUSIONS Due to its convenience and commonality, WCS has been an innovative and effective technology in recent years. According to international industrial survey, the annual exponential growth rate of WCS for EV will be 126% in 2014~2020, and the overall market will increase to $1.75 billion in 5~7 years. This shows that the WCS is going to be a mainstream for EV. To improve the WCS efficiency, a hybrid impedance matching strategy consists of capacitance tuning and frequency tracking techniques is proposed to overcome the transmitting problem. With the practical realization and testing, we can conclude that the input power of developed WCS could be up to 3.7kW, the best system efficiency could be over 88%, and the efficiency could be keep over 80% once the transmitting distance is within 10-20cm. The proposed innovative technology of WCS seems to be useful for the EV and also be potential for the application of commercial electronic products. ACKNOWLEDGMENT This work was supported by the Technology Development Program (no.105-ec-17-a-25-0843) from Ministry of Economic Affairs of Taiwan.

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