The pursuit of battery charging technology for electric vehicle (EV) has led to extensive research on the inductive-based wireless power transfer (WPT) systems. In this paper, the compensation component (including coils) stresses will be studied in two commonly adopted compensation topologies, namely S-S and LCC-S compensations. Due to the peak voltage calculation inaccuracy for certain components based on conventional fundamental frequency analysis, an improved peak voltage calculation method is introduced in closed form, which is proved to be more accurate by both simulation and experiments.@en

This article presents a parameter recognition-based impedance tuning method for the impedance mismatch caused by capacitance drift and coil misalignment in series-series-compensated wireless power transfer (WPT) systems. First, a parameter recognition method is proposed to identify the unknown parameters of the resonant circuits by only measuring the rms values of the coil currents. No phase detection circuits and auxiliary measurement coils are required. Furthermore, according to the recognized parameters, the reactance on both sides are minimized simultaneously by regulating the system frequency and the phase shift angles of the active rectifier. Compared with the existing methods, the proposed parameter recognition method adopts a dynamic frequency approaching strategy to avoid severe system detuning due to the bifurcation phenomenon. Moreover, based on the recognized parameters, the proposed impedance tuning method can simultaneously cope with the parameter deviations caused by capacitance drift and coil misalignment on both sides without using extra circuits and switches. Experimental results show that the unknown parameters of the resonant circuits are recognized accurately, with the average relative errors all less than 3%. Additionally, by implementing the impedance tuning method, the dc to dc efficiency of the WPT prototype is improved by 4.3%-15% in the experiments.

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Wireless charging has the potential to speed up the transition to electric vehicles (EVs) because it is intrinsically a user-friendly technology. Furthermore, it is essential when charging completely autonomous EVs, and it enables the charging of EVs in motion without using overhead cables. The most common technology used in EV wireless charging is inductive power transfer (IPT) with magnetic resonance coupling. This is based on the magnetic field exchange between coupled coils connected to compensation networks to minimize the circulating reactive power. IPT systems have two main variables influencing their operation: the coupling factor between the coils depending on their alignment, and the equivalent load based on the battery charging profile. The coils' alignment and load operating conditions might vary when considering different applications. Nevertheless, all IPT systems share the same challenges: ensuring a highly efficient power transfer, guaranteeing that the intentionally radiated electromagnetic field (EMF) is both safe for the living beings in the surroundings and lower than the recommended electromagnetic compatibility (EMC) limits, and providing interoperability between IPT charging stations and EVs produced by different manufacturers. This thesis explores these matters. For instance, the content is divided into three main parts: conventional inductive power transfer systems, variable compensation, and voltage/current doubler (V/I-D) converter.@en

Light-duty electric vehicles (EVs) typically have a rated voltage of either 400 or 800 V. Especially when considering public parking infrastructures or owners with multiple EVs, e.g., car rental companies, EV wireless chargers must efficiently deliver electric power to both battery options. For this purpose, this article proposes an advanced and compact version of the previously defined voltage/current doubler (V/I-D) converter, here comprising two coupled series-compensated bipolar pads (BPPs). The presented system can efficiently charge EVs with both battery voltage classes at the same power level without affecting the current rating of the converter's circuit components. The control scheme is implemented at the power source side in terms of switching frequency and input voltage, and only passive semiconductor devices are employed on board the EV. The equivalent circuit is analyzed, focusing on the BPPs' undesired cross-coupling and its effect on the power transfer. Methods to compensate for the cross-coupling are proposed regarding the BPP design and operating strategy. At 7.2 kW and aligned BPPs, the dc-to-dc efficiency of 96.34% and 96.53% have been measured at 400 and 800 V, respectively. The proposed method has been experimentally validated at different misalignment profiles while considering battery voltages 300-400 V and 600-800 V, which proves that the V/I -D converter is a universal charging solution for EV batteries.

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The lithium-ion battery of an electric vehicle (EV) is typically rated at either 400 or 800 V. When considering public parking infrastructures, EV wireless chargers must efficiently deliver electric power to both battery options. This can be normally achieved by regulating the output voltage through a dc-dc converter at the cost of higher onboard circuit complexity and lower overall efficiency. This article proposes a wireless charging system that maintains a high power transfer efficiency when charging EVs with either 400- or 800-V nominal battery voltage at the same power level. The control scheme is implemented at the power source side, and only passive semiconductor devices are employed on board the EV. The presented system, called voltage/current doubler (V/I-D), comprises two sets of series-compensated coupled coils, each of them connected to a dedicated H-bridge converter. The equivalent circuit has been analyzed while explaining the parameters' selection. The analytical power transfer efficiency has been compared to the one resulting from the conventional one-to-one coil system at 7.2 kW. For the same power level, the dc-to-dc efficiency of 97.11% and 97.52% have been measured at 400-V and 800-V voltage output, respectively. Finally, the functionality of the V/I-D converter has been proved at both the even and uneven misalignments of the two sets of coupled coils.

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This paper investigates the interoperability of the proposed voltage/current doubler (V/I-D) converter used for wireless charging of electric vehicles (EVs), which achieves high efficiency when charging both 400V and 800V batteries at the same power. Nominally, the V/I-D converter employs bipolar pads (BPP) at both the primary and the secondary circuits. In this study, the functionality of the converter is assessed when the primary BPP is coupled with a standard secondary coil, here being the VA test station WPT2/Z2 from SAE J2954. First, the intended operation of the V/I- D converter is explained. After that, the equivalent circuit of the BPP primary coupled with the standardized secondary coil is modeled analytically. The operation based on the misalignment is discussed. Then, the interoperability is verified through experimental results for the entire constant current charging mode for a rated output power of 7.2kW. Even though the functionality of the V/I-D converter is not optimal during the interoperability, the measured DC-to-DC power transfer efficiency in the considered operating range reaches the maximum at 95.22%, while the minimum is 92.86%.@en

Due to the urgent desire for a fast, convenient, and efficient battery charging technology for electric vehicle (EV) users, extensive research has been conducted into the design of high-power inductive power transfer (IPT) systems. However, there are few studies that formulate the design as a multiobjective optimization (MOO) research question considering both the aligned and misaligned performances and validate the optimal results in a full-scale prototype. This article presents a comprehensive MOO design guideline for highly efficient IPT systems and demonstrates it by a highly efficient 20-kW IPT system with the dc-dc efficiency of 97.2% at the aligned condition and 94.1% at 150-mm lateral misalignment. This achievement is a leading power conversion efficiency metric compared to IPT EV charging systems disseminated in today's literature. Herein, a general analytical method is proposed to compare the performances of different compensation circuits in terms of the maximum efficiency, voltage/current stresses, and misalignment tolerance. An MOO method is proposed to find the optimal design of the charging pads, taking the aligned/misaligned efficiency and area/gravimetric power density as the objectives. Finally, a prototype is built according to the MOO results. The charging pad dimension and total weight, including the housing material, are 516∗552∗60 mm3/25 kg for the transmitter and 514∗562∗60 mm3/21 kg for the receiver. Correspondingly, the gravimetric, volumetric, and area power density are 0.435 kW/kg, 581 kW/m3, and 69.1 kW/m2, respectively. The measured efficiency agrees with the anticipated value derived from the given analytical models.

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Wireless charging must be highly efficient throughout the entire battery charging profile to compete in the electric vehicle (EV) industry. Thus, optimum load matching is commonly used: it operates at the equivalent load that maximizes the efficiency, which depends on the coil's alignment. In this article, the optimum load is made independent of the coils' position by changing the system's resonant frequency through switch-controlled capacitors (SCCs). This eliminates the need for load-side voltage control. The output current follows the battery voltage rise during the battery charging cycle to always match the optimum load, which can be achieved by regulating the input voltage via the power factor correction (PFC) converter. This method is called here constant optimum load (COL). Two SCC topologies have been implemented in a 3.7-kW hardware demonstrator. The one implementing the half-wave modulation achieves higher efficiency than the one employing full-wave modulation, with 96.30% at 3.2 kW and aligned coils. When misalignment occurs, the half-wave modulation technique results in higher efficiency than the conventional-fixed compensation, where the efficiency is lower by up to 0.68% at partial load. Based on these results, the proposed COL method is proven suitable for 3.7-kW EV-static wireless charging achieving one of the highest peak efficiencies listed in today's literature for the same power class.

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When considering EV wireless charging that uses inductive power transfer with magnetic resonance, the coils’ current distortion must be minimized to guarantee compliance with the electromagnetic compatibility limits on the radiated magnetic field set by the relevant industrial standards. This paper analyzes the current distortion caused by switch-controlled capacitors (SCCs) used as series compensation to achieve constant optimum load (COL) matching at different coils’ alignments. First, the proposed COL charging method is explained where the SCCs have either the half-wave or the full-wave modulation. Their impact on the measured coils’ current distortion has been analyzed up to 30MHz by computing the fast Fourier transform (FFT). Additionally, the currents’ FFT from the half-wave modulation has been compared to those resulting from the conventional series-series compensation with fixed capacitance. The SCCs using the half-wave modulation result in the highest total-lumped distortion. However, the individual amplitudes corresponding to the critical frequencies of the radiated magnetic field’s limit from SAE J2954 are comparable or lower than those resulting from the other implementations. Finally, the radiated magnetic field resulting from each strategy has been evaluated using the finite element method. All results are well within the SAE J2954 recommended limits at 10 m. Moreover, a minimum distance of 25 cm from the outer sides of the coupled coils ensures a safe exposure to both the general public and implanted medical devices according to the ICNIRP reference levels.@en

Nowadays, inductive power transfer (IPT) with magnetic resonance is the most used method for high-power wireless battery charging applications. Once the topology of the compensation network and the operating frequency are selected, there are infinite combinations of the circuit equivalent inductance and compensation capacitance values resonating at that frequency. Choosing an appropriate ratio between these passive devices is essential to meet the target output power while ensuring that the required DC input and output voltages are found within the permitted range limited by the power source and the battery load. This paper proposes design trade-offs for selecting the optimum ratio between the inductance and capacitance in IPT systems with series-series compensation applicable to any power level. First, the target mutual inductance must be computed. Based on that, the coupled coils are designed depending on the physical constraints. An example is provided considering a 3.7 kW wireless charging system for electric vehicles (EVs) where different coils’ combinations are analyzed through the finite element method. The most suitable design is implemented, achieving or the application a relatively high measured peak DC-to-DC efficiency of about 96.24% at 3.28kW while the coils are aligned with 11cm distance. The required power is delivered at different battery voltages and coils’ alignments by regulating the DC input voltage.@en

The increase in popularity of electric vehicles (EVs) and the pursuit of user convenience makes wireless power transfer (WPT) an attractive technology for the charging of batteries. The usage of WPT in e-transportation is not straightforward because the current standardization limits the allowed operating frequency range and magnitude of the irradiated magnetic field. Although, to safeguard the zero voltage switching (ZVS) of the intrinsic inverter switches, their operating frequency needs to be slightly adapted at all time such that the circuit functions in the equivalent inductive region of the passive network. Besides the semiconductors’ soft switching, another control objective is limiting the inverter current to restrain the irradiated magnetic field. The start-up of the WPT system can be particularly challenging because uncertainties on the loading condition and coils’ misalignment can complicate these control objectives. This paper benchmarks three start-up modulation strategies for the H-bridge inverter which aim to reduce the amplitude of the transient currents and to ensure ZVS operation for the S-S compensation and double-sided LCC compensation. In addition two soft shut-down strategies are compared for the S-S compensation. The results show that the symmetrical phase-shift (SPS) control with self-oscillating feedback control, also known as Dual Control gives the best performance for S-S compensation at start-up and shut-down. The combination of frequency and SPS control starting below resonance gives the best results for the soft start-up of the double-sided LCC compensation.@en

This paper aims to investigate the dynamic charging performance of an 11 kW dynamic inductive power transfer (DIPT) system. First, a multi-objective optimization (MOO) method is proposed to find the Pareto front of the DD charging pad. Then, the optimal design with a 96.82% efficiency is selected as the target design for the DIPT system. Based on the coupler mutual inductance at different misalignment, the orientation of the transmitter (Tx) and receiver (Rx) pads and the distance between Tx pads are studied and optimized. To obtain the dynamic characteristics of the DIPT system, the impact of mutual inductance variation is investigated, and a dynamic model using Laplace phasor transformation is built to solve the waveform amplitude of electrical variables. Finally, a time-variant circuit model is built. Based on the simulations, the dynamic model is proved to be accurate, and the proposed DIPT system displays a good dynamic charging performance.

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In wireless charging systems, the H-bridge converter's switching frequency is set close to the system's natural resonance for achieving optimized zero voltage switching (ZVS). Variations to the system's natural resonance are commonly tracked by following the changes in the resonant current's polarity, i.e., current zero-crossings. The main implementation challenge is accounting for the time delay between the real monitored current and the final resulting switches’ commutations. This becomes critical at high switching frequencies, particularly when the magnetic coupling and loading vary widely. This paper proposes an auto-resonant detection method that continuously ensures optimized ZVS turn-on with the minimal circulating current over the operable range of magnetic coupling and load. The suggested implementation provides two split variable references for the resonant frequency detection, which adaptatively compensate for the propagation delay based on the resonant current slope. The auto-resonant scheme is benchmarked against the commonly employed method with fixed current detection references. The results highlight the auto-resonant strategy's advantages, namely extended operable range, wider ZVS turn-on region, ease start-up, and improved DC-to-DC efficiency. The auto-resonant features and functionality are verified experimentally with a 200 W low-voltage e-bike wireless charger. Finally, the benefits of the presented method are analytically explored for high-power applications by considering the H-bridge semiconductor losses of a state-of-art 50 kW wireless charging system.@en

Industrial wireless charging systems use standardized coils to guarantee interoperability between different manufacturers. In combination with these coils, the compensation network can still be designed and optimized. This paper explains the step-by-step design of the compensation network for a 7.7 kW wireless charging system (power class WPT2), which is composed of standardized coils. The compensation network must satisfy the output power and voltage requirements, the soft-switching of the inverter, and the limit of voltage and current stress on the components. The S-S compensation network is found to be unfeasible for those coils, and an optimized double-sided LCC compensation network is designed. The 3-phase grid connection is selected despite the 1-phase one because it gives the lowest total conduction losses. Finally, two parallel SiC MOSFETs C3M0075120K are chosen as inverter's switch because of their low conduction losses. This solution can achieve a payback time within a year with respect to the cheapest one.@en

This paper proposes a new method of electric vehicles detection (EVD) and foreign objects detection (FOD) for dynamic inductive power transfer (DIPT) systems. The proposed detection method applies both passive coil sets (PCSs) and active coil sets (ACSs) to achieve both EVD and FOD with a high detection sensitivity. The operation mechanisms and design of the detection coil sets topology and resonant circuits are elaborated. Finally, both circuit and magnetic field simulation are carried out. The results verify the feasibility and sensitivity of the proposed detection method.

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In high-power wireless battery charging that uses inductive power transfer, a considerable amount of power losses are located in the transmitter and receiver coils because they carry high resonant currents and typically have a loose coupling between them which increases eddy current losses. Therefore, the nominal operation needs to be chosen such that the coils' losses are minimized. Additionally, the inverter's semiconductors soft-switching improves both the power conversion efficiency and the electromagnetic compatibility of the system, thus it needs to be safeguarded for a wide operating range. However, depending on the chosen quality factor of the coils, it might happen that the minimum coils' losses and soft-switching are not satisfied at the same time. This paper defines a guideline on the parametric selection of the coils' quality factor such that the optimum operation of both the coils and the resonant converter can be achieved simultaneously. This parametric guideline is proposed for resonant converters implementing the four basic compensation networks: series-series, series-parallel, parallel-series, and parallel-parallel. Finally, circuit examples are provided for an 11 kW wireless battery charging system.

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In inductive power transfer applications that use resonant compensation networks, the commonly employed H-bridge inverter should be kept operating in soft-switching to ensure high power efficiency and low irradiated electromagnetic noise. To achieve so, the zero-crossing detection circuit for the resonant current or voltage must be fast and accurate in any operating condition. This paper researches the concept of an auto-resonant control for the typical H-bridge resonant converter used in wireless charging systems. In the method proposed here, the reference levels for the zero-crossing detection of the inverter's current are automatically adapted depending on the slope of the current itself at the zero-crossing. In this way, it is possible to compensate for the circuit delay even in the presence of parameters' variation and to ensure that the soft-switching is always maintained. The functionality of this control method is proven first mathematically, and then with circuit simulations. The core steps for the implementation are described with the support of functional blocks. Finally, the system start-up strategy is explained, which uses an auxiliary timed oscillator to modulate the inverter with a fixed 50% duty cycle at a higher frequency than the nominal. This guarantees that the start-up is in the inductive region and, thus, the zero-voltage switching turn-on. Once the detection circuits sense the current flow, the oscillator is automatically disabled, and the nominal power transfer starts.

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If electric vehicles have to be truly sustainable, it is essential to charge them from sustainable sources of electricity, such as solar or wind energy. In this paper, the design of solar powered e-bike charging station that provides AC, DC and wireless charging of e-bikes is investigated. The charging station has integrated battery storage that enables for both grid-connected and off-grid operation. The DC charging uses the DC power from the photovoltaic panels directly for charging the e-bike battery without the use of an AC charging adapter. For the wireless charging, the e-bike can be charged through inductive power transfer via the bike kickstand (receiver) and a specially designed tile (transmitter) at the charging station, which provides maximum convenience to the user.

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This paper aims to investigate the radiated magnetic field by 11 kW inductive power transfer (IPT) systems used for the charging of electric vehicles. Two reference designs suggested by SAE J2954 are studied. Both designs are analysed to obtain the coils winding currents, and 3D FEM models are built in COMSOL without considering the car chassis, which constitutes a conservative approach. The magnetic field intensity at specific distances from the IPT coupler are calculated. Finally, the simulation results are compared with the respective magnetic field limits defined in the international standards SAE J2954, IEC 61980-1 and ICNIRP. The results show that the magnetic field radiations at 10 meters points are significantly lower than the limits established in the SAE J2954, while the emissions at 0.9 meters points are only slightly below the limits defined by ICNIRP.@en