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This article presents a systematic frequency-sweeping based method for fast and accurate determination of mutual coupling coefficient in a series–series wireless power transfer system. The novel contributions involve a sensitivity analysis to evaluate the robustness of the proposed method and an evaluation of several adaptive frequency step functions to minimize detection time with an accuracy of less than 1% error. It is found that the proposed method is the most sensitive when the frequency is swept from the lower frequency boundary in a weakly-coupled system. Several adaptive frequency step functions are evaluated and compared. A power factor reference and an adaptive frequency step function are designed in a systematic way to achieve the dual purposes of minimum detection time and high accuracy. The experimental results obtained from the hardware setups show that the mutual coupling coefficients under a range of conditions can be estimated within 7 ms and with errors typically less than 1.0%. [ABSTRACT FROM AUTHOR]

Free positioning wireless charging for consumer electronics allows the devices to be charged at arbitrary positions and angles to improve user experience. However, a user-initiated sudden movement of the device during charging can cause hazards due to the abrupt variation of the coupling coefficient. To solve this issue, the coupling coefficient variation at different positions should be mitigated, which is also good for the design and high-efficiency operation of power electronics converters. This article proposes a design methodology to employ antiparallel windings to smooth the coupling coefficient variation over different positions by reducing the coupling coefficient at central positions and enhancing it at boundary positions. Two optimization methods are proposed: turn-by-turn optimization and winding-by-winding optimization. A design flow is offered. The hexagonal coil is compared with the square coil and proved to achieve better performance than the latter. An experimental prototype is implemented to validate the effectiveness of the proposed design methodology. [ABSTRACT FROM AUTHOR]

The isolated feature between primary and secondary sides in wireless power transfer requires dual-side communication, phase synchronization, and two controllers on both sides. The phase difference between dual-side ac voltages can be changed for output regulation or bidirectional power flow. Zero crossing detection of the secondary ac voltage or current is normally conducted for frequency locking and phase synchronization. However, it is inaccurate at light-load conditions for high-power applications due to a large sensing scale and much noise; also, it can generate undesired results with discontinuous or distorted waveforms having more than two zero crossing points in one cycle. To solve these issues, this letter proposes a dual-side phase-shift implementation method using driving windings to regulate the dual-side phase difference conducted only on the primary side. By controlling the phase difference between the primary-side power and driving windings, the phase difference between the dual-side power windings can be regulated. In this way, the dual-side regulation is independent from power transfer to get rid of the issues of the conventional phase synchronization method, and bidirectional power flow can be more easily realized. [ABSTRACT FROM AUTHOR]

The closed-loop control of wireless power transfer (WPT) systems requires a communication link between the power transmitter and the power receiver for dual-side cooperation. Near-field communication techniques that utilize the power link are preferred for this purpose due to high reliability and security. In this letter, we propose a frequency-modulated phase shift keying (FMPSK) communication technique for the closed-loop maximum efficiency point tracking (MEPT) control of WPT. The technique uses very shallow phase modulation depth to minimize the disturbance on power flow and reduce the efficiency drop caused by the PSK. Meanwhile, it ensures the signal accuracy and precision by multiple methods and precise frequency modulation and demodulation. In experiment, we obtained 82%–85% system efficiency with a 25–125 W load over a distance equal to the coil diameter, and 93% efficiency over a distance equal to the coil radius. The efficiency drop caused by FMPSK was less than 1%. The dynamic processes of voltage regulation and MEPT took only about 1 ms and 10 ms, respectively. [ABSTRACT FROM AUTHOR]

A wide operating region of inductive power transfer systems indicates a stable output against large load and coupling changes. In most of the published works, output regulation is achieved by single-side regulation that utilizes either primary-side or secondary-side power converters for load regulation. However, secondary-side regulation tends to have a relatively narrow operating region due to space and cost considerations. Meanwhile, although primary-side regulation may provide a larger operating region, system performance can be degraded due to the needs of secondary-side information. To overcome the drawbacks of single-side regulation, this article proposes a dual-side cooperative output regulation method implemented by burst-mode pulse density modulation. In the proposed method, dual-side cooperative control is established by sensing primary-side current drops caused by secondary-side burst operations and, hence, requires no communication devices. With the proposed method, the system operating region can be effectively extended without sacrificing steady-state performance. Experimental results are provided in this article to validate the proposed method. [ABSTRACT FROM AUTHOR]

In inductive power transfer systems, impedance matching is critical in obtaining high efficiency. However, since the load impedance is subject to variations in most applications, especially wireless charging, maintaining impedance matching is a challenge. One valid solution is to maintain the equivalent load impedance at the optimal value against load variations by controlling a load-side power converter. Nonetheless, the system overall cost, size, and complexity can be increased. In this article, based on a proper selection between full-bridge and half-bridge modes of the inverter and rectifier, an operation mode selection method is proposed that can achieve semi-impedance-matching with large load variations. In general, with the proposed method, high overall efficiency can be obtained while neither additional power converters nor complicated control is required. A 1-kW prototype has been developed and the experimental results have verified the proposed method. [ABSTRACT FROM AUTHOR]

The pulse-density-modulation (PDM) zero-voltage-switching (ZVS) full-bridge converter is a promising power converter for wireless power transfer (WPT) systems. The converter has the advantages of both direct conversion ratio control and load-independent soft switching. These advantages reduce the overall system complexity and power loss. However, the converter suffers from the limitations of large low-frequency subharmonics, a narrowed modulation range, and a large modulation delay. These limitations are caused by the existing PDM strategy, which was designed to generate a symmetric ZVS current to ensure the ideal ZVS for minimizing the switching loss. This paper finds that even with an asymmetric ZVS current, the ideal ZVS can still be ensured by the negative feedback effect of the dead-time voltage. Based on this finding, a PDM strategy that allows asymmetric ZVS currents is proposed to overcome the aforementioned limitations. In experiments, a ZVS full-bridge converter was modulated by the existing and the proposed PDM strategies, respectively. The converter performances and responses were compared and the results showed that the proposed PDM strategy can overcome the limitations while achieving ZVS. The proposed PDM strategy was also tested in a WPT system for verification. [ABSTRACT FROM AUTHOR]

In wireless power transfer (WPT) systems, the relative position between the transmitting and receiving coils could change suddenly, which is one of the most significant distinctions between wireless and wired power transfer systems. To the best of the authors’ knowledge, no theoretical analysis has been reported on the dynamic behavior of the WPT systems under the relative position transient (RPT). This article proposes the concept of “coupling-coefficient susceptibility” to describe this dynamic behavior. The extended describing function method is applied to extract the mathematical model of the coupling-coefficient susceptibility. Finally, an RPT testing platform, composed of a series-series compensated WPT system and a coil motion mechanism is designed and fabricated to verify the theoretical analysis. The experimental results exhibit a good agreement with the proposed model. [ABSTRACT FROM AUTHOR]

To increase the power density of a wireless power transfer (WPT) system, the coupling of coils can be strong. However, in a strongly coupled series–series WPT system with a diode rectifier, currents can be distorted and discontinuous, and first harmonic approximation is no longer valid. The inverter is likely to work in hard switching, leading to extra noise, decreased efficiency, and a potential hazard for a safe operation. Through time-domain analysis, this article reveals that with a dual-side 90° phase shift, the hard switching of the inverter can be avoided, the power level can be enhanced, and the discontinuous conduction mode can be avoided to eliminate noises. The factors impacting the soft switching operations of the inverter and the rectifier are analyzed. To achieve 90° dual-side phase shift, the conventional approach of using the receiver voltage/current, which can be distorted and discontinuous in strongly coupled WPT systems, for phase synchronization is no longer suitable. A new set of coils, namely driving coils, is used for phase synchronization so that dual-side phase shift can be implemented independent from power transfer. Experimental results have validated the effectiveness of the analysis. [ABSTRACT FROM AUTHOR]

The extended describing function (EDF) method is widely used in modeling the resonant converters. This article finds that the model of the phase-shift controlled resonant converter deduced by this method might be inaccurate especially when the duty-cycle is close to 1. The reason behind it is investigated, showing that derivation of the inverter bridge model based on its fundamental harmonic will cause error. A novel model of the inverter bridge by Fourier analysis is proposed. Incorporating the novel model into the EDF-based resonant model, the accuracy of the phase-shift controlled resonant converter is greatly improved. A series–series compensated inductive power transfer system is fabricated in the lab to verify the validity of the proposed model. [ABSTRACT FROM AUTHOR]

Duty-cycle control is a widely used control method of resonant converters. In this article, the existing model of duty-cycle controlled resonant converters is found to have discrepancy compared with the simulation and experimental results. The reason behind is investigated, showing the phase change in the output voltage of the inverter bridge caused by the duty-cycle perturbation was ignored in the existing model. The phase change law is highly dependent on the duty-cycle scheme used, which is further determined by the forms of the carriers and the switching sequences of the resonant converters. With this consideration, improved models of both full-bridge and half-bridge resonant converters with different duty-cycle control schemes are rederived. It is revealed that different duty-cycle control schemes lead to different phase curves of the duty-cycle-to-output-voltage transfer functions, therefore care should be taken when selecting the appropriate duty-cycle control scheme. In the end, a series-series compensated wireless power transfer system is built to verify the validity of the proposed models. [ABSTRACT FROM AUTHOR]

Maximum efficiency point tracking (MEPT) control has been adopted in state-of-the-art wireless power transfer (WPT) systems to meet the power demands with the highest efficiency against coupling and load variations. Conventional MEPT implementations use dc/dc converters on both transmitting and receiving sides to regulate the output voltage and maximize the system efficiency at the expense of increased overall complexity and power losses on the dc/dc converters. Other implementations use phase-shift control or on–off control of the transmitting side inverter and the receiving side active rectifier instead of dc/dc converters but cause new problems, e.g., hard switching, low average efficiency, and large dc voltage ripples. This paper proposes a pulse density modulation (PDM) based implementation for MEPT to eliminate all the mentioned disadvantages of existing implementations. Delta-sigma modulators are used as an example to realize the PDM. A dual-side soft switching technique is proposed for the PDM. The ripple factor of the output voltage with PDM is derived. A 50 W WPT system is built to validate the proposed method. The system efficiency is maintained higher than 70% for various load resistances when the power transfer distance is 0.5 m, which is 1.67 times the diameter of the coils. [ABSTRACT FROM AUTHOR]

To balance the phase output currents of a multiphase inverter, negatively coupled inductors (NCIs) can be used to achieve a larger differential mode (DM) loop impedance than the common mode (CM) loop impedance. For a good current sharing effect, the inductances of the NCIs should be large, leading to bulky size and cost ineffectiveness, which should be avoided in high-power applications. This article proposes a novel current balancing method based on parallel resonance. A parallel resonant network (PRN) is inserted between any two phases. With $\Delta $ -Y transform, the inductor common point and the capacitor common point are generated, either of which can be used as the CM output node. For a series–series compensated wireless power transfer system, part of the existing compensating capacitance can be employed to form PRNs with the added parallel inductors. The DM currents are suppressed by the large impedance of the PRNs. Compared with the existing method, the inductances of the NCIs can be greatly reduced to achieve better current balancing performance. The added parallel inductors are of small current ratings, which are only visible to DM currents. A prototype is implemented to validate the proposed method. [ABSTRACT FROM AUTHOR]

Load-independent output characteristics of inductive power transfer (IPT) systems are increasingly popular in battery charging. This paper proposes a novel variable-parameter T-circuit (VT) for an IPT system charging a battery with constant current (CC) or constant voltage (CV) output. The VT can transfer a CC/CV input to a CC or CV output by using an ac switch and a passive component (inductor or capacitor). An IPT system with a VT for CC–CV charging can reduce the number of passive components and ac switches. Besides, the proposed VT merits more design freedom of charge current/voltage with the constraints imposed by the loosely coupled transformer parameters compared to that of the traditional one. In addition, there are three kinds of VTs for various IPT charging systems with different requirements. A 400 W laboratory-scale prototype with a 150 mm air gap was built to verify the theoretical analyses. Both electronic load and lead-acid battery are utilized to verify the charging profile of the proposed method. The experimental results of the IPT system indicate that the fluctuation of the charging current in CC mode is less than 2%, and the change rate of charging voltage in CV mode is within 2.9%. The maximum overall efficiency 93.93% of the charging system is achieved from a dc 110 V input to a dc 100 V output. [ABSTRACT FROM AUTHOR]