Bidirectional Wireless Power Transfer Research Articles

A New Bidirectional Wireless Power Transfer Topology

There is a growing demand in the marketplace for wirelessly powered consumer applications. Some of these low-power applications require wireless power transfer with bi-directional power flow. For such consumer applications, this paper proposes a new bi-directional wireless power transfer (BD-WPT) topology that facilitates wireless power transfers in both directions using fewer components. The paper describes the operating principles of the topology in detail and presents a mathematical model to predict and investigate its performance. Besides, the the control method is proposed for this topology including the power transfer control and synchronization. Simulations and theoretical results are presented in comparison to the measured results of a 60W prototype BD-WPT to demonstrate the viability of the proposed converter topology. Results are convincing and indicate that the proposed converter is suitable for low power applications being low cost, reliable, and efficient.

Bidirectional LCC–LCC-Compensated 20-kW Wireless Power Transfer System for Medium-Duty Vehicle Charging

This article presents the design and demonstration of a bidirectional 20-kW wireless charging system (WCS) with a significantly large air gap (11 in) and asymmetrical input-output voltage levels. Analytical and experimental sensitivity analyses of the WCS resonant tank were conducted to verify the optimal operating region under the load and frequency variation. The inverter and rectifier were designed with switching components, the charging pads were designed with double-D (DD) coils, and the tuning networks were designed with an LCC - LCC tuning circuit. The grid and vehicle side tuning circuits were designed separately to achieve a ~1:2 gain for the asymmetrical input (800 V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dc</sub> ) and output (350 V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dc</sub> ) voltages. The proposed WCS was designed, simulated, and tested to verify the efficiency, power transfer capacity, and sensitivity under load variation. The experimental results show that, at 20-kW output power, the achieved grid-to-vehicle dc - dc efficiency was 96.1%, and the vehicle-to-grid dc - dc efficiency was 96.2%. The proposed system is the largest-air gap bidirectional WCS with the highest efficiency and power density.

Design Consideration of Bidirectional Wireless Power Transfer and Full-Duplex Communication System via a Shared Inductive Channel

Communication between the primary and secondary sides is pivotal to the wireless power transfer (WPT) system. The system control commands and feedback information need simultaneous wireless information and power transfer (SWIPT). In this paper, a FSK-based SWIPT system with full-duplex communication via a shared channel is provided. Considering the complexity of the coupling relationship in this kind of full-duplex SWIPT system, this paper proposes an analysis method based on the transmission channel, studies the crosstalk between the power channel and the information channel, and between the forward and reverse transfer of information. A design method of full-duplex communication SWIPT system based on shared coupling channels is provided. A 60 W SWIPT prototype with a full-duplex communication rate of 20 kbps is built to verify the proposed method.

Design of a Bidirectional Wireless Power Transfer System for Vehicle-to-Home Applications

Energy storage plays a fundamental role in balancing the power fluctuations induced by the distributed generation of renewable energy sources. In this scenario, electric vehicles can strongly contribute to exchange power with the grid through their on-board batteries. When the vehicle is parked, the battery can be discharged, injecting active power into the grid, provided that its state of charge will be restored before vehicle utilization. This paper presents a comprehensive step-by-step design of a wireless charger for a Vehicle-to-Home application. The design procedure begins from the constraints disposed by the Italian reference technical rules for Low Voltage utilities and by the standard SAE J2954 for Wireless Power Transfer for electric vehicles. The selection of the output power of the battery is followed by the power sizing of each stage of the bidirectional wireless charger.

Current-Decomposition-Based Digital Phase Synchronization Method for BWPT System

Phase synchronization between the primary and secondary power converters is indispensable to achieve bidirectional wireless power transfer (BWPT). For sharing the phase information without any physical contact, traditionally, some wireless communication devices or auxiliary windings are proposed, yet with the disadvantages of low reliability and high cost caused by the electromagnetic interference and extra devices. This letter proposes a digital phase synchronization method for the BWPT system. Based on the active and reactive components of the ac current flowing into/out of the secondary high-frequency converter (HFC), the voltage phase of secondary HFC can be controlled to synchronize with that of primary HFC by adjusting the reactive current to be zero. The advantages of the proposed method are the fast response and high reliability for phase synchronization. A 1-kW prototype controlled by a field-programmable gate array is built to verify the proposed method. The experimental results prove that the system can quickly switch the power transmission direction in 2 ms with a maximum efficiency of 96.48% and can operate stably when the parameters perturbation occur. This letter is accompanied by an experimental video to prove the feasibility of the proposed method.

Stable power transfer method for bidirectional wireless power transfer in change of transmission distance

Stable power transfer method for bidirectional wireless power transfer in change of transmission distance

An Energy Injection Method to Improve Power Transfer Capability of Bidirectional WPT System With Multiple Pickups

Track wireless power transfer (WPT) systems are increasingly used for monorail robots, industrial transportation, etc. Conventionally, the track WPT system's primary power supply is designed to be equal or higher than the total running pickups’ rated power. However, in practice, some of the pickups may transiently require more power than their normal operating state. It is not cost effective to increase the power rating of the primary source for the transient case. Hence, this article proposes a method to strengthen the track's power transfer capability by reversely injecting energy from available online pickups to the transiently over-rated one. A bioperation mode is proposed, which includes both the normal operation and the strengthening operation. A bidirectional pickup topology is used, and the equivalent circuit is analyzed accounting the bioperation mode. In addition, a synchronization method of the primary and reverse injection pickups is proposed. A criterion to determine the boundary of the reverse energy injection is also presented. By detecting the peak current, this energy injection method can be practically implemented and the proposed method is verified through the experimental results.

A Coupling Mechanism With Multidegree Freedom for Bidirectional Multistage WPT System

For the application of wireless power transfer (WPT) technology in robot arm, it is necessary to provide flexible power supply for each arm device across multiple arm joints. However, rotation of the joints and the variation of the loads will lead to the fluctuation of the output voltage at each stage. To guarantee stable output voltage, a bidirectional multistage WPT (BM-WPT) method is proposed to provide energy for multiple loads. A bowl-coupling structure and three-dimensional orthogonal ring coil arrangement are proposed to reduce coupling variation against joint rotation. With a special resonant topology design for each stage, energy can be injected or output freely on each stage regardless of energy transfer direction. This topology can also reduce the sensitivity of the output voltages with the variation of the loads. The simulation and experimental results have verified the effectiveness of the proposed method and the coupling mechanism.

Constant Output-Voltage Design for Bi-Directional Wireless Power Transfer System with Multiple Stages

For the application of wireless power transfer (WPT) technology in a robot (like the snake robot), the power is supposed to be transferred to each device across multiple robot joints. This paper proposes a multi-stage bi-directional WPT (MB-WPT) system that not only provides power to multiple loads but also increases the power transfer distance. Besides, the last stage can charge for the preceding stages by reverse power transfer. The constant output voltage can be achieved whether the power is transmitted in a forward or reverse direction, and different output voltages for each stage can be achieved to satisfy the respective voltage requirement through the parameter design method. The validity of the theoretical analysis and the feasibility of the system are verified by experiments.

Bidirectional DC–DC Wireless Power Transfer Based on LCC-C Resonant Compensation

A bidirectional dc-dc wireless power transfer (WPT) based on LCC-C resonance compensation is proposed in this article. For the power flowing from the primary side to the secondary side (P2S), the voltage source is converted to a current source (V-C) on the primary side and the current source is converted to a voltage source (C-V) on the secondary side. For the power flowing from the secondary side to the primary side (S2P), the V-C conversion occurs on the secondary side and the C-V conversion occurs on the primary side. The proposed bidirectional dc-dc WPT using LCC-C resonance compensation exhibits several advantages: 1) it operates in bidirectional power flow; 2) it prevents burn out of the coil when there is an open circuit on the load side or the coils are misaligned; and 3) it outputs a constant current (CC) or a constant voltage (CV). A hardware prototype is designed using the specification SAE-J2954, the operating frequency is 85 kHz, the maximum output power is 3.7 kW and the vertical distance for the coil gap is 15 cm. The experimental results verify that the proposed bidirectional dc-dc WPT performs as expected.

A Direct Three-Phase AC–AC Matrix Converter-Based Wireless Power Transfer System for Electric Vehicles

For the bidirectional wireless power transfer system of electric vehicles, the topology proposed in this paper includes a direct three-phase AC–AC matrix converter as the pre-stage main circuit, a bilateral inductor–capacitor–capacitor–inductor (LCCL) as the resonance compensation network, and a full-bridge converter as the latter stage circuit. According to the characteristics of the system topology, a new control strategy is proposed based on the de-re-coupling method. The control principle of the coordination work of the scheme is expounded, and the corresponding switch combination logic is designed. According to the three-phase voltage amplitude relationship at different stages, combined with the resonant frequency, the switch arms of the matrix converter are alternately controlled separately. The number of switching operations is reduced, the system efficiency and safety are improved, and the full range of soft switching operations of the converter is realized. The theoretical analysis of the bilateral LCCL resonance compensation network is carried out, and its constant voltage/current output characteristic and high power factor transmission characteristic are obtained. Finally, the effectiveness and feasibility of the bidirectional wireless power transfer system for electric vehicles proposed in this paper are verified by simulation analysis.

A Bidirectional Wireless Power Transfer System Control Strategy Independent of Real-Time Wireless Communication

For the bidirectional wireless power transfer (BWPT) technology, the phase synchronization between the transmitting and receiving converters is very essential and difficult to achieve by wireless communication between two sides because the delay and data transfer interval of wireless communication severely influence the control performance. This article proposes a novel control strategy for BWPT, which can achieve the phase synchronization and maximum efficiency point tracking independent of the real-time wireless communication. To solve the overshoot and oscillation problem incurred by the proposed control strategy and the nature of the system itself, the dynamics of a series-series compensated BWPT system is then analyzed and a small-signal model of this system is derived based on the generalized state space averaging and extended describing function methods. Using the small-signal model, a BWPT control system based on the proposed control strategy is designed and optimized. The validity of the proposed strategy and the performance of the control system are verified experimentally using a 3.3-kW prototype system.

Simulation-Assisted Design of a Bidirectional Wireless Power Transfer With Circular Sandwich Coils for E-Bike Sharing System

A wireless power transfer (WPT) system with bidirectional power flow control for charging batteries and drawing electric power from batteries is presented in this paper for e-bike sharing applications. The proposed WPT consists of a bidirectional DC-DC converter and a sandwiched coil set, which includes primary winding with ferrite pad in charging stations and a movable secondary coil without ferrite pad, which is inserted into the primary coils to receive or transmit energy, on the e-bike. Hence, the proposed coil set could provide high coupling factor, better tolerance of misalignment, and a compact size that enables easy installation in the current mechanism of bike-sharing systems. The Maxwell 3D is adopted to design coil sets, which are based on the known resistance of Litz wire, and the effect of the ferrite pad and misalignment are analyzed to achieve good performance. To select a suitable converter, the characteristics of predesigned coils are simulated using Maxwell 3D, MATLAB and PSIM. Then, a full-bridge CLLC converter with the same resonant frequency for charging and discharging modes is chosen to provide minimal loss by using the synchronous rectifier on the 48-V side and voltage gain without the loading effect. Finally, a digital signal processor-based digitally controlled CLLC converter is constructed to verify the performance of the proposed WPT, which can provide 200 to 48 V/500 W charging and discharging functions with constant current/constant voltage modes and fast-response current control. In addition, the maximum efficiency is 96% at 52 V and 75% rated load in charging mode.

A Phase Synchronization Technique Based on Perturbation and Observation for Bidirectional Wireless Power Transfer System

For the bidirectional wireless electric vehicle charging system, when both the primary and secondary converters are active, the phase synchronization between the two converters are necessary to control the power flow direction. This article proposes a new phase synchronization method by tracking the maximum (or minimum, depending on the power flow direction) value of the output current, without auxiliary hardware or the real-time communication between the primary and secondary sides. First, the relationship between the output current and the phase difference of the control signals is derived. It is found that the maximum output current is determined by the relative phase-shift angle (time interval between the middle points of the primary and secondary voltages) and the internal phase-shift angles of the two converters (time interval when the output voltage equals to the dc-link voltage). Second, a new scheme for generating the control signals is proposed to ensure that the relative phase-shift angle is not influenced by the internal phase-shift angles. Third, based on the perturbation and observation method, a procedure is proposed to track the extreme value of the output current and obtain the target relative phase-shift angle for the phase synchronization state. Moreover, the relationship between the internal phase-shift angle and the transfer power is derived to regulate the magnitude of the transfer power, with the dead-time effect taken into consideration. Finally, experimental results verify the validity of the proposed method with different air gaps and power levels. The method is easy to be implemented, helpful for the optimal operation of the static and quasi-static wireless charging system and suitable for the bidirectional power flow.

Zero-Phase-Angle Controlled Bidirectional Wireless EV Charging Systems for Large Coil Misalignments

Misalignment between charging coils is an unavoidable problem in all electric vehicle (EV) charging systems that are based on wireless power transfer (WPT) technology. This is because variations in both self- and mutual inductances of charging coils due to coil misalignments significantly affect the charging rate and efficiency of the EV charger. As a solution, this article proposes a new control technique that maintains the rated charging power level at optimum efficiency in spite of large coil misalignments. To compensate for the variations in inductances, the technique uses the information extracted from input impedance to operate the system essentially at unique converter voltages with zero phase angle, ensuring that charging will take place at rated power with optimal efficiency despite any pad misalignments. The article presents a comprehensive mathematical model, describing the theoretical basis of the proposed control philosophy. To demonstrate the validity of the proposed control concept, a 1-kW bidirectional WPT EV charging system is built and results are presented under misaligned coil conditions. Results convincingly indicate that the proposed controller is efficient and capable of maintaining the rated charging rate despite large coil misalignments and without any dedicated wireless communication.

Finite Element Modeling and Analysis of High Power, Low-loss Flux-Pipe Resonant Coils for Static Bidirectional Wireless Power Transfer

This paper presents the optimal modeling and finite element analysis of strong-coupled, high-power and low-loss flux-pipe resonant coils for bidirectional wireless power transfer (WPT), applicable to electric vehicles (EVs) using series-series compensation topology. The initial design involves the modeling of strong-coupled flux-pipe coils with a fixed number of wire-turns. The ohmic and core loss reduction for the optimized coil model was implemented by creating two separate coils that are electrically parallel but magnetically coupled in order to achieve maximum flux linkage between the secondary and primary coils. Reduction in the magnitude of eddy current losses was realized by design modification of the ferrite core geometry and optimized selection of shielding material. The ferrite core geometry was modified to create a C-shape that enabled the boosting and linkage of useful magnetic flux. In addition, an alternative copper shielding methodology was selected with the advantage of having fewer eddy current power losses per unit mass when compared with aluminum of the same physical dimension. From the simulation results obtained, the proposed flux-pipe model offers higher coil-to-coil efficiency and a significant increase in power level when compared with equivalent circular, rectangular and traditional flux-pipe models over a range of load resistance. The proposed model design is capable of transferring over 11 kW of power across an airgap of 200 mm with a coil-to-coil efficiency of over 99% at a load resistance of 60 Ω.

A Novel Bidirectional Wireless Power Transfer System for Mobile Power Application

This paper presents a bidirectional wireless power transfer system for mobile power applications. A novel 2-switch bidirectional wireless power transfer system with dual-side control is proposed for mobile power applications. Although only two switches are adopted, the energy can be transferred from the transmitter side to the receiver side and vice versa. The term bidirectional means that the power-flow is bidirectional and also that the transmitter is also a receiver and the receiver is also a transmitter. The output energy can be easily controlled by the duty ratios of the two switches. Thus, the proposed bidirectional power transfer system uses only one circuit to achieve bidirectional power transfer. Hence, the system cost and volume can be reduced so that the system is small and convenient for mobile power systems, portable and/or wearable electronic devices. A prototype system is constructed and the experimental results verify the validity of the proposed bidirectional wireless power transfer system.

A Reconfigurable Bidirectional Wireless Power Transceiver for Battery-to-Battery Wireless Charging

Battery-to-battery (B2B) wireless charging can take place in many scenarios, such as using a mobile phone to charge another mobile phone, wearable devices, or low-power sensor nodes. To facilitate this wireless power transfer (WPT) function with the minimum additional cost, we propose a monolithic reconfigurable bidirectional WPT transceiver designed for the first time in CMOS, which can be reconfigured between a differential class-D power amplifier (PA) and a full-wave rectifier. Meanwhile, we employed a maximum current charging mode to maximize the B2B charging efficiency, by directly charging the loading battery with the rectifier, and by powering the PA with the sourcing battery. Then, we reduced the number of cascaded WPT stages from five in the conventional design to three. This bidirectional WPT transceiver fabricated in 0.35 μ m CMOS occupies 3.9 mm2 of silicon area. The bidirectional WPT function, verified at 6.78 MHz with only one off-chip capacitor, exhibits peak efficiencies of 91.5% and 58.6% for the receiver and the overall system, respectively, when the output power is 1.55 W.

Dynamic WPT system for EV charging with integrated energy storage

Wireless power transfer (WPT) can be used to charge electric vehicles (EVs) safely and efficiently. Dynamic wireless EV charging systems to charge EVs while moving has been developed but this technology is not yet widespread. One of the key challenges of dynamic charging is the pulsed nature of the transferred power, which may negatively impact battery life and the utility grid. Hybrid energy storage systems have been demonstrated as a potential solution, at the expense of a dedicated converter to interface with the energy storage element. This study presents a possible solution to the problem of adsorption and conditioning of high-power pulses, in the form of a novel converter topology that combines inductive WPT and super capacitor energy storage without the need for an additional converter stage. A suitable switching pattern is presented along with a steady-state mathematical model based on inductive power transfer, providing insight into the operation of the system. Experimental results of a prototype bi-directional WPT system are shown to validate the model and demonstrate the applicability of the proposed converter topology.

Three‐phase bi‐directional wireless EV charging system with high tolerance to pad misalignment

Electric vehicles (EVs), which are becoming increasingly popular, can be charged by either wired or wireless means. The latter, based on wireless power transfer (WPT) technology, is convenient and safe, but pad misalignment is one of the major concerns as it causes a reduction in charging power level, prolonging the charging process. Therefore, wireless charging systems with high tolerance to pad misalignment are necessary. This study proposes a three-phase bi-directional wireless power transfer (BD-WPT) system with a control scheme that offers high tolerance to pad misalignment in EV charging applications. The system uses simple coil designs for pads and utilises the magnetic coupling among pads to compensate for the power variation. A comprehensive mathematical model, incorporating all magnetic cross-coupling effects, is presented to investigate the charging performance of the system under pad misalignment and different pad spacings. To demonstrate the validity of the proposed system, theoretical and simulated results are presented, benchmarking against a single phase WPT system, and in comparison to a prototype 1.3 kW three-phase BD-WPT system. Results are convincing and indicate that the proposed three-phase BD-WPT system with small pad spacing is more tolerant to wide pad misalignment in EV charging applications.