Owing to the flexibility of coupling structure, low standing losses and low Electromagnetic Interference (EMI), capacitive power transfer (CPT) has drawn much attention as one of the new wireless power transfer (WPT) technologies. However, to date, the load effect on the system performance has not been analyzed comprehensively, and the CPT system is usually fully tuned (tank network resonant frequency fully tuned to the fundamental components of the nominal switching frequency), which is challenging to maintain practically. Based on these considerations, this paper first provides a detailed analysis of how the load can affect the performance of a CPT system. An optimal load resistance is shown to exist for a CPT system with a non-fully/partially tuned LCC tank (tank network resonant frequency is designed to be lower than the fundamental components of the switching frequency). By dynamically transforming the load to its optimal value, the system can transfer maximum power to the load while maintaining zero voltage switching (ZVS) operation. Then, a controlled buck-boost converter based on perturbation and observation algorithm is introduced to the system to obtain the optimal equivalent resistance against the actual load variations via tracking the maximum power transfer point. The final prototype has demonstrated that a maximum power of 10 W is obtained, with an end-to-end power efficiency of about 70% over a wide range of load variations from 5 Ω to 1 kΩ .

It is well known that a DC or fully tuned inductive power transfer (IPT) system operating at its maximum power output point results in a low power efficiency being equal or below 50%. Based on the analyses of the maximum power transfer condition against the detuning factor of an LC resonant circuit, this paper proposes a Capacitive Power Transfer (CPT) system which can track the maximum power output using a DC-DC converter, at the same time achieve a high-power efficiency when the circuit is not fully tuned. The method is applied to a voltage-fed inverter at series tuned off-resonance operation for reduced power losses. A prototype system is built with a half-bridge inverter to drive the capacitively coupling circuit, and a buck-boost converter is designed for impedance matching. A perturb and observe algorithm is developed for controlling the duty cycle of the DC-DC converter to track the maximum power against load variations. It is found that under a fixed electric field coupling condition, the system can maintain the maximum power transfer operation at 10W and achieve a power efficiency of about 70% over a wide range of load variation from 5Ω to 500Ω.

Hybrid tuning topologies had been proposed with polarized magnetic couplers to improve the alignment tolerance of inductive power transfer (IPT) systems. However, the technologies are mainly applicable to IPT systems with directional coupling and relatively low inductance variations. This paper proposes an IPT system with a dual concentric coil Tx and single coil Rx coupling configuration with hybrid tuning networks for improving the output voltage characteristics over a wide range of coupling and load variations. The concentric coils allow free rotation while integrated hybrid tuning networks help reduce the output voltage fluctuation under varying horizontal and spatial misalignments. A design procedure is proposed using Finite Element Method (FEM) and circuit simulations to select tuning parameters accounting for varying self and mutual inductances. Simulation and experimental results show that the self-inductance of the proposed coil can change by 48%, and the coupling coefficient varies between 0.43 and 0.74 under misalignments, leading to over 100% change of the unregulated output voltage if the system is series-series tuned. In contrast, a laboratory prototype of the proposed system demonstrates a stable output voltage within ± 9% of fluctuation around 13.25 VDC.

This paper introduces a DC–DC buck converter on the secondary side of the capacitive power transfer system to reduce the voltage and electric field across the interface, and to reduce the circuit Q, and thus the system sensitivity. The system is mathematically analyzed to study the improvement in sensitivity and voltage stress. The leakage electric field emissions around the plates are investigated by simulation. The analytical and simulation results show that by reducing the duty cycle of the buck converter at a constant output power, the voltage across the plates can be significantly reduced and the circuit becomes less sensitive to the variations in parameters. Experimental results demonstrated that Q and the voltage stress over the capacitive interface are reduced by changing the duty cycle of the buck converter. For delivering 10 W of power, the maximum voltage stress across one pair of the coupling plates is reduced from 211 V in the conventional system without using a DC–DC converter, to 65 V and 44 V at duty cycles of 30% and 20%, respectively. The system achieves an end-to-end power efficiency of 80% at an output power of 10 W and a duty cycle of 30%.

This paper introduces the analysis and optimization of capacitive power transfer system with a CL compensation circuit on the secondary for reduced voltage stress across the electric field coupling interface and less system sensitivity. The system has been mathematically analyzed to understand the effect of the CL circuit. And simulation and experimental studies are also carried out for verifications. It is found by increasing C _p (and reducing L _p accordingly to be tuned to the nominal frequency), the quality factor (Q) can be reduced to make the system less sensitive to circuit parameter and load variations, thus easier to tune. However, the CL network should be carefully designed to reduce the voltage stress and electric field leakage around the interface. A practical circuit is built which has demonstrated 5 W of power transfer across a combined interface capacitance of ~1 nF at an operating frequency of 1 MHz, and the system power efficiency is above 65%. The voltage stress and sensitivity reduction have been confirmed practically.

This paper proposes a high-frequency bidirectional Wireless Power Transfer (WPT) system that achieves soft-switching, synchronous rectification, and a constant voltage (CV) output over a wide range of operating conditions. The system employs autonomous current-fed push-pull converters as the inverter on the transmitter (Tx) side and the rectifier on the receiver (Rx) side. Tx and Rx circuits are configured symmetrically across the magnetic couplers, enabling a reversible power flow by changing the DC source and load so that the system can facilitate a bi-directional power transfer. Theoretical analysis demonstrates that the proposed resonant system can achieve a unity voltage gain and deliver a constant output voltage, even when the load, coupling, and coil inductance vary simultaneously. The inverter gate-drive circuits are designed to enable dynamic zero-voltage soft-switching at high frequencies. Circuit simulations and experimental results show that the system can maintain soft-switching while providing a stable output voltage close to the input of 20V, with load, coupling, and Tx-Rx coil inductance ranges between 40Ω-2kΩ, 0.4-0.8, and 1.4μH-1.8μH, respectively.