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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]

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WPT system with a diode rectifier, currents can be distorted and discontinuous, and first harmonic approximation is no longer valid.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

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]

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]