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 deals with a new single-stage high boost quasi-Z-source inverter based on the active switched Z-impedance network. The proposed inverter provides higher voltage boost factor, draws continuous input current, shares the same ground between the input source and the bridge inverter. Compared with the traditional enhanced boost (quasi-)Z-source inverters (EB-ZSI/EB- qZSI), for two less inductors, one less capacitor and one additional active switch used at the impedance network, the output voltage boost factor of the proposed inverter is twice as much as the EB-ZSI and EB-qZSI, which implies that it can use a very higher modulation index to provide improved quality output waveforms. Besides, for obtaining the same dc-ac output voltage gain, the proposed method has lower active switching voltage stress, lower passive component voltage ratings and lower shoot-through current stress with lower input current ripple. The operating principle analysis, power loss calculation, simulation results and performance comparison with other high boost impedance source inverters are presented. To verify the operating theory of the proposed inverter, a laboratory prototype based on the TMS320F28335 DSP was constructed and tested with 60V dc input and ac 110Vrms phase output. Finally, both simulations and the experimental results confirmed that the proposed inverter has high boost voltage inversion capability.

Implanted medical device (IMD) wirelessly powered by magnetic resonant coupling has attracted wide attention recently. In this study, an optimized 2-coil wireless power transfer (WPT) system is adopted to eliminate the requirement of embedded battery in an IMD. In order to deliver stable power to implantable devices with wide coverage range and high efficiency, coil optimization is investigated, including the consideration of the coil structure, pitch and number of turns. By using finite element analysis (FEA), both the transmitting and receiving coils have been optimized at 6.78MHz. A 200 mm $\times$ 300 mm rounded rectangular transmitting coil and a novel double-layer circular receiving coil with an outer diameter of 24 mm were developed, and the transmitting coil was segmented by multiple resonant capacitors to significantly reduce the coil voltage to a safe level. Experiment results show that stable power transfer efficiency over 40% can be achieved at a distance of 5 cm with the optimized transmitting and receiving coils.

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%.