Highly Efficient Inductive Power Transfer: Variable Compensation for Misalignment Tolerance and Voltage/Current Doubler for Battery Interoperability

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.DC systems, Energy conversion & Storag

Voltage/Current Doubler Converter for an Efficient Wireless Charging of Electric Vehicles With 400V and 800V Battery Voltages

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.Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.DC systems, Energy conversion & Storag

Voltage/Current Doubler Converter for Electric Vehicle Wireless Charging Employing Bipolar Pads

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.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Electrical Sustainable EnergyDC systems, Energy conversion & Storag

Interoperability of the Voltage/Current Doubler Converter Employing Bipolar Pads with the SAE J2954 VA WPT2/Z2 for EV Wireless Charging

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%.Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.DC systems, Energy conversion & Storag

Inductive Power Transfer based on Variable Compensation Capacitance to Achieve an EV Charging Profile with Constant Optimum Load

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.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.DC systems, Energy conversion & Storag

Design Trade-Offs Between the Coupled Coils’ Inductance and the Series-Series Compensation Capacitance for EV Wireless Charging Systems

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.Accepted author manuscriptDC systems, Energy conversion & Storag

Advantages and Tuning of Zero Voltage Switching in a Wireless Power Transfer System

In charging applications, wireless power transfer (WPT) is mostly used in the form of inductive power transfer with magnetic resonant coupling. Therefore, both the transmitter and the receiver coils are combined with capacitors, such that only active power is transferred. To evaluate the operation of the WPT charging system, its equivalent circuit can be analyzed in the frequency domain. However, this is limiting since the H-bridge inverter operation is not intrinsically considered. As an example, the operating points of both zero current switching (ZCS) and zero voltage switching (ZVS) operations might be still analyzed, but it is not possible to assess their performance in terms of efficiency. In this paper, the advantage of ZVS over the ZCS is evaluated in terms of the efficiency and the delivered output power. To enable the full potential of ZVS, this is tuned considering the switch capacitance and the dead time.DC systems, Energy conversion & Storag

Study on Soft Start-Up and Shut-Down Methods for Wireless Power Transfer Systems for the Charging of Electric Vehicles

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.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.DC systems, Energy conversion & Storag

Detection of Metallic Foreign Objects and Electric Vehicles Using Auxiliary Coil Sets for Dynamic Inductive Power Transfer Systems

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.Virtual/online event due to COVID-19 Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.DC systems, Energy conversion & Storag

Electric Vehicle Charging Based on Inductive Power Transfer Employing Variable Compensation Capacitance for Optimum Load Matching

In inductive power transfer applications, it is possible to ensure high efficiency of the main coils by operating at the optimum load. Since the optimum load depends on the coupling between the main coils, the operation needs to be adapted to match this case at different alignment conditions. This paper proposes a method to keep the optimum load constant by varying the natural resonant frequency of both the primary and secondary circuits of a S-S compensation network. This is possible by changing the value of the compensation capacitors at different alignments. This strategy differs from the ones found in the literature, where the input and the output voltage are changed to always match the optimum load. The proposed concept is proven through circuit simulations of an 11 kW EV battery charging system, and several strategies for the implementation of the variable capacitance are discussed.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.DC systems, Energy conversion & Storag