General rights This document is made available in accordance with publisher policies. Please cite only the published version using the reference above. Full terms of use are available: Abstract: Increasing the resonant frequency of a wireless power transfer (WPT) system effectively improves the power transfer efficiency between the transmit and the receive coils but significantly limits the power transfer capacity with the same coils. Therefore, this paper proposes a coil design method for a

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... ies-series (SS) compensated WPT system which can power up the same load with the same DC input voltage & current but with increased resonant frequency. For WPT systems with higher resonant frequencies, a new method of realizing soft-switching by tuning driving frequency is proposed which does not need to change any hardware in the WPT system and can effectively reduce switching losses generated in the inverter. Eighty-five kHz, 200 kHz and 500 kHz WPT systems are built up to validate the proposed methods. Experimental results show that all these three WPT systems can deliver around 3.3 kW power to the same load (15 Ω) with 200 V input voltage and 20 A input current as expected and achieve more than 85% coil-system efficiency and 79% system overall efficiency. With the soft-switching technique, inverter efficiency can be improved from 81.91% to 98.60% in the 500 kHz WPT system. Energies 2018, 11, 7 2 of 17 capacitances of the coils. There are four basic topologies of the compensation circuit, according to how the compensation capacitances are added to the transmit and receive coils: series-series (SS), series-parallel (SP), parallel-series (PS), and parallel-parallel (PP) topologies [3] . In this paper, the basic SS compensation topology is adopted for its simple structure, but the analysis and the method can also be applied to the other compensation topologies. Though there have been many papers studying the WPT characteristics [4, 5] , various compensation topologies [6, 7] and coil structures, few papers give a detailed account of their coil design. The coil design can be very challenging, as multiple design parameters and requirements must be considered simultaneously to build up a WPT system, including input/output parameter requirements, inverter efficiency, coil-to-coil efficiency, voltage-ampere (VA) rating, device stress and stability. Increasing the resonant frequency of the WPT system effectively improves the power transfer efficiency but significantly limits the power transfer capacity with the same coils. Therefore, in this paper, a design method for a loosely coupled SS resonant WPT system of given resonant frequency, input and load conditions is proposed which can help to increase the system resonant frequency without impacting the system power level. As the single-phase H-bridge DC-AC inverter is widely used to generate the high frequency power to excite the transmit coil in WPT systems [6-11], a reliable high-frequency DC-AC inverter is critical to output high-frequency power. To increase the resonant/driving frequency and reduce inverter losses, wide bandgap semiconductor devices such as SiC MOSFETs [9] and soft switching techniques are adopted in high-power WPT systems. Different compensation topologies are studied in-depth to achieve soft switching. In [6], the author proposes a double-sided LCC compensation network and its tuning method to realize zero voltage switching (ZVS) for the primary-side switches in the WPT system. A zero-current switching (ZCS) condition could also be achieved by tuning the parameters of the LCC compensation network [7], but those topologies need additional passive components in the circuit which will increase losses and component count. Others seek to realize soft switching through inverter control strategies. In [10], a polyphase current-fed push-pull resonant converter with full-autonomous operation based on mutual magnetic coupling among the phases for polyphase WPT applications has been presented: full ZVS of all three converters has been achieved with accurate 120 • phase balancing at 73.5 kHz. A DC-voltage-controlled variable capacitor for stabilizing the ZVS frequency of a resonant converter for WPT system is proposed in [11] . It can be seen that those inverter-side control methods and those specially designed structures are too complex. Besides this, soft-switching is desired under different operating conditions in a WPT system; however, the system parameters in [6, 7, 10, 11] are fixed once designed and set, and adjusting the values of those additional components to realize full load-condition soft switching is impractical for variable load-resistance systems such as a battery charging system. Therefore, in this paper, a phase shift strategy is proposed to realize soft switching by simply changing the input impedance of the inverter through tuning the driving frequency. Detailed analysis and accurate calculation methods are both presented. Compared to the existing soft-switching methods, the components in our WPT system do not need to be tuned corresponding to the system operating conditions, and are hence more suitable for practical applications where the WPT system is used for charging batteries or powering up different electrical equipment. Besides, soft-switching on and soft-switching off can be chosen arbitrarily according to the requirements. To validate the proposed coil-system design method and soft-switching method, 85 kHz, 200 kHz and 500 kHz WPT systems are all built up. Designed with the proposed coil-system design method, all these three WPT systems can deliver around 3.3 kW power to the same load (15 Ω) with the same input voltage (200 V) and the expected 20 A DC input current. With the soft-switching strategy presented in this paper, the inverter efficiency can be effectively improved, particularly in WPT system with higher resonant frequencies. For example, the inverter efficiency can be improved from 89.81% to 98% in the 500 kHz system. Even though the frequency tuning method would lead to a slight decrease in the coil-to-coil efficiency, the overall efficiency can still be maintained at the similar level compared to that of the resonant conditions. With the greatly improved inverter efficiency, the volume of the Energies 2018, 11, 7 3 of 17