A major challenge for practical wireless power transfer (WPT) applications is to attain stable power transfer with high and constant transfer efficiency under a dynamic change of coupling condition. In order to address the issue, this paper proposes a novel nonlinear parity-time-symmetric (PT) model, wherein the nonlinear saturable gain is provided by a self-oscillating controlled inverter. In this paper, the transfer performance and stability criterion of the nonlinear PT-based WPT system are analyzed based on the coupled-mode theory (CMT). The theoretical analysis shows that the proposed system automatically achieves constant output power with constant transfer efficiency against the variation of coupling coefficient. Moreover, based on the gain saturation mechanism, the control strategy for the inverter only need detect the current in the transmitter, which eliminates auxiliary circuits of wireless communication for feedback control from the receiver. As a case study of dynamic charging, a drone-in-flight wireless charging platform is improved by applying the nonlinear PT-symmetric model. Experimental results show that, when the flying drone hovers in a confined 3-D volume of space above the WPT platform, a stable output power is maintained with approximately constant transfer efficiency of 93.6%.

A high step-up DC-DC converter with active switched LC-network is proposed in this paper, which can be used in the photovoltaic (PV) system as the front-end stage to generate the required dc bus voltage. Based on the transformerless DC-DC converter with an active switched inductor (ASL) network, this proposed converter introduces an active switched capacitor (ASC) network, only one capacitor and one diode are added, while the voltage gain is efficiently increased by a compound of ASL and ASC network. The structure of the proposed converter is simple and voltage stresses on the additional diode and capacitor are low. In addition, unlike other voltage-boosting structures that utilized switched capacitor, the proposed converter avoids the extreme instantaneous currents of capacitor. The detailed analysis of the proposed converter and performance comparison with recently published ASL network-based converters are presented. Finally, a 100 W, 20/200 V prototype circuit is operated to verify its performance.

This paper proposes a new topology for an enhanced-boost quasi-Z-source inverter with an active switched Z-network. Compared to enhanced-boost quasi-Z-source inverters (EB-QZSI) with two switched impedance networks, this proposed inverter has the same high boost factor while using two fewer LC pairs and one fewer diode and only adds one switching device. In addition, current stress across switches is reduced by half, which leads to significant reduction in the conduction loss of switches and improves efficiency of the overall inverter. Moreover, the proposed inverter also draws continuous input current and maintains common ground between the source and the inverter bridge. Operation principles and comparisons with EB-QZSI are presented. Both simulation and experimental results validate theoretical analysis of the proposed inverter.

For a multiload wireless power transfer (WPT) system, it has always been a challenge to maintain constant output power with constant efficiency for each load when the coupling conditions change. Besides, the unbalanced power distribution and cross-coupling among loads are also the key issues. In order to solve these problems, a novel multiload WPT system combining the parity-time (PT) symmetry principle and time-sharing control strategy is proposed. First, the multiload PT-symmetric WPT (PT-WPT) system with single natural resonant frequency is modeled and analyzed based on the circuit model. Then, a multiload PT-WPT system with time-sharing control strategy is constructed by adding a controllable resonant capacitor array at the transmitter. The time-sharing control strategy is realized by periodically turning on/off a controllable resonant capacitor selectively, which enables a single transmitter to supply power to multiple loads with different natural resonant frequencies, and each load realizes PT transmission characteristics. Further, using the time-sharing control strategy, the power distribution among multiple loads is realized by only controlling the duty cycle of each load. Finally, two dual-load PT-WPT experiments are designed and carried out. Experimental results show that each load can maintain constant output power with constant transfer efficiency about 90% in a wide coupling range, and realize power distribution at desired values simultaneously.

Fast and accurate estimation of state of charge (SOC) of supercapacitors is essential for the safe and reliable operation of energy storage systems. However, existing SOC estimation methods are based on the integer-order model of supercapacitors and do not take into account the fractional-order characteristics of supercapacitors. Hence, the accuracy of SOC estimation results usually ignores the impact of modeling and relies on the correction effect of the algorithm. In this paper, a supercapacitor energy storage system is firstly established for fractional-order modeling and fractional-order SOC estimation of supercapacitors. Next, a Kalman filter is devised to track the SOC of supercapacitors in real time, and it can correct the estimation results online. Finally, experiments under different working conditions are conducted to demonstrate the effectiveness of the proposed fractional-order modeling and SOC estimation scheme. Compared with the data obtained by the traditional integer-order method, the accuracy of the proposed fractional-order method is higher, and the mean relative errors are less than 5%. With the online correction of the Kalman filter, the maximum errors under different working conditions are limited to 1.3%.

In existing capacitive wireless power transfer (CPT) systems, the coupling capacitance of the traditional couplers is usually small, which will lead to a significant decrease in the power transfer capability when the transfer distance increases. Thus, it is a big challenge to achieve stable output power and constant transfer efficiency under different coupling capacitance. In this work, the circuit model of the CPT system with double-side LC compensation network is established firstly. Then, based on the parity-time (PT) symmetric theory, a primary-side control strategy is presented to achieve stable output power and constant high transfer efficiency when the coupling capacitance changes. Finally, the experimental results show that when the insertion depth of the stack array coupler changes from 60 to 180 mm, the output power of the proposed CPT prototype is almost stable at 25W, and the transfer efficiency is kept at around 88%, which verifies the feasibility and effectiveness of the proposed design.