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.

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.

It is well-known that the low-frequency negative input impedance of the constant power load (CPL) is the major cause of the cascaded system instability, and the heavier the power, the worse the stability. In this article, a power adaptive load-side parallel virtual impedance (PALPVI) control strategy is proposed to improve the stability of the cascaded dc system with buck-type CPL. First, a parallel impedance with power adaptability is derived followed up with the derivation of the corresponding compensation controller transfer function to realize it virtually. Considering that the compensation controller is highly dependent on the circuit parameters and needs extra current sensors to acquire the power information, a simplification of the compensation controller is made based on the open-loop characteristics of the buck-type CPL. The final PALPVI control strategy does not require the circuit parameters or any current sensor and has almost no side effect on the dynamic performance. Finally, a 48–24–12 V cascaded dc system is fabricated to verify the feasibility and effectiveness of the proposed PALPVI control strategy.

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.

This paper proposes a distributed fractional-order predefined-time sliding mode controller (DFOPTSMC) to adjust the external energy storage to improve the transient stability of the power system with time delay and input saturation. The time delay is able to worsen the performance of the power system, and even bring about instability. Based on the Artstein transform theorem, the model of the power system with communication delay is transformed into a model without delay. Meanwhile, the output of the external energy storage should be limited in practical power systems. The saturation compensation system is proposed to remedy the influence of input saturation of the external energy storage. Therefore, a DFOPTSMC considering communication delay and input saturation is proposed to adjust the energy storage device, which has fast response ability and strong regulation capability, to increase transient stability of the power system. The predefined-time stability of the power system under the DFOPTSMC is analyzed through the Lyapunov function. Finally, simulations are taken to verify that the proposed DFOPTSMC is able to raise the transient stability of the power system as well as ensure that the power system returns to stability region under a large delay margin.

The unipolar and bipolar coils are two most common coil structures used in inductive power transfer systems. However, they are decoupled from each other when fully aligned, indicating that they are not interoperable with each other. In order to solve the interoperability problem, an interoperable transmitter composed of two adjacent unipolar coils and a three-switch dual-output inverter is proposed in this letter. It can achieve compatibility with unipolar or bipolar coils through circuit configuration by alternating the current directions with efficient wireless power transmission. Two different operating modes are presented, and the mathematical models are established. An experimental prototype is established. Besides, the anti-offset has been tested. The calculated and experimental results have revealed that the proposed system can effectively improve interoperability and has great potential for development.