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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]

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WPT system with a diode rectifier, currents can be distorted and discontinuous, and first harmonic approximation is no longer valid.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 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]

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]