Capacitive Power Transfer Research Articles

Load Effect Analysis and Maximum Power Transfer Tracking of CPT System

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~\Omega $ to 1 $\text{k}\Omega $ .

Design of capacitive coupling structure for position‐insensitive wireless charging

The Internet of Things (IoT) connects billions of smart devices through wireless sensor networks (WSNs). However, to ensure the power supply for numerous passive devices in a WSN, it is necessary to replace batteries periodically, which will undoubtedly increase maintenance costs. This study proposes a capacitive coupling structure that is different from the previous capacitive power transfer (CPT) structures. The proposed structure can allow the positional variation of the receiver, and multiple receivers to be charged at the same time. Both theoretical and experimental results demonstrate that the power transmission is less affected by the positional variation of the pickup plates in the vertical and horizontal directions. In this study, all coupling capacitors between the plates are considered and the equivalent circuit model is derived. Further output characteristics of the circuit are derived and analysed. The dimensions of the structure are designed and simulated by using the EM full wave simulation. A prototype is designed to verify that a battery can be charged based on capacitive coupling. A 60 mAh rechargeable lithium-ion button battery is fully charged in 250 min, and the output power is ∼36 mW during charging.

Preliminary study of 50 W Class-E GaN FET amplifier for 6.78 MHz capacitive wireless power transfer

A preliminary study of Class-E radio frequency power amplifier for wireless capacitive power transfer (CPT) system is presented in this paper. Due to a limitation in coupling capacitance value, a high frequency operation of switching power inverter is necessary for the CPT system. A GaN MOSFET offers reliability and performance in a high frequency operation with an improved efficiency over a silicon device. Design specification related to the parallel load parameter, LC impedance matching and experimental analysis of the amplifier is explored. An experimental setup for the proposed inverter and its integration with the CPT system is provided, and the power efficiency is investigated. As a result, by utilizing a 6.78 MHz resonant frequency and a 50 Ω resistive load, 50 W of power has been transmitted successfully with an end to end system efficiency over 81 %. Additionally, above 17 W wireless power transfer was demonstrated successfully in the CPT system under 6 pF coupling with the efficiency over 70 %.

A Filter Theory Approach to the Synthesis of Capacitive Power Transfer Systems

Resonant-coupled wireless power transfer (WPT) systems are typically modeled as circuits with inductively or capacitively coupled resonators. In a two-resonator system, the transmit and receive resonators are linked by a coupling coefficient, and the coupling can be adjusted for critical coupling, overcoupling, and undercoupling. In this work, we show that the coupled-resonator structure is equivalent to a filter structure and provide a demonstration of how filter theory can be used to design a capacitive WPT system. An advantage of this approach is that it provides access to a wide range of canonical structures and methods of impedance scaling to realize matched links. We show a maximally flat filter has a critically coupled response and an equiripple Chebychev filter has an overcoupled response. There is a relationship between frequency bandwidth in a filter design and spatial bandwidth in a WPT link. Therefore, the filter context can be used to synthesize wideband filters that are more robust to spatial variation than narrowband filters.

Misalignment tolerance wireless power transfer system combining inductive and capacitive coupling

To resonate with the capacitive power transfer (CPT) system, inductors are usually needed, while additional capacitors are required for the inductive power transfer (IPT) system. To take advantage of the inductance of the IPT system, this study proposes a misalignment tolerance wireless power transfer system that employs the IPT coupler to compensate part of CPT's capacitance for railway applications. In the proposed system, two power transfer paths are formed by the IPT and CPT couplers. Additionally, the system merits a good anti-misalignment ability by carefully designing the parameters of the proposed system. In a well-alignment case, only the CPT coupler is utilised to transfer power. While in misalignment case, the IPT coupler is activated to transfer power to improve the misalignment tolerance. Besides, the transfer capability of the combined system can be improved when the misalignment happens. The system coupler, connection method and output power characteristics are presented in detail. Then, the general design procedure is provided with an example system at 1 kW. Experimental results show that the maximum variation of the output is 5.6% when misalignment is up to 250 mm (50%). While the power variation of the system involving single CPT can be as large as 52%.

Three-Phase Resonant Capacitive Power Transfer for Rotary Applications

This article proposes a three-phase resonant capacitive power transfer (RCPT) system for rotating applications requiring loose coupling. Existing plate designs for rotational CPT, such as the stacked four-plate and concentric structures, are found to exhibit imbalances that give rise to ground return or common-mode currents. These are particularly harmful at the high switching frequencies employed by compact and low-cost RCPT systems. Inherently balanced multiphase electrode designs are proposed to address this at the expense of permitting some coupling variation over rotation. Simulations show that three or more phases are required to avoid coupling nulls. While four or more phases may be preferred for applications requiring very stable coupling, three-phase is found to be the most practical solution in the context of cost, complexity, and inductor size. The proposed electrode structure offers high balance, a competitive average coupling coefficient, and a low per-plate resonating inductance. A highly integrated 27.12-MHz RCPT system was designed and implemented to validate these findings while accounting for practical limitations, such as manufacturability and cost. Using primarily commercial-off-the-shelf components, the system transferred 50 W at 25 mm under full rotation with an end-to-end efficiency of 73%.

Capacitive Wireless Power Transfer with Multiple Transmitters: Efficiency Optimization

Wireless power transfer with multiple transmitters can have several advantages, including more robustness against misalignment and extending the mobility and range of the receiver(s). In this work, the efficiency maximization problem is analytically solved for a capacitive wireless power transfer system with multiple coupled transmitters and a single receiver. It is found that the system efficiency can be increased by adding more transmitters. Moreover, it is proven that the cross-coupling between the transmitters can be eliminated by adding shunt susceptances at the input ports. Optimal values for the input currents and receiver load are determined to achieve maximum efficiency. As well the optimal load, the optimal input currents and the maximum efficiency are independent on the cross-coupling. By impedance-matching the internal conductances of the generators, the maximum-efficiency solution also becomes the one that provides the maximum output power. Finally, by expressing each transmitter–receiver link with its kQ-product, the maximum system efficiency can be calculated. The analytical results are verified by circuital simulation.

An NFC-CPT-Combined Coupler With Series- None Compensation for Metal-Cover Smartphone Applications

A near-field communication capacitive power transfer (NFC-CPT)-combined coupler with a series-none compensation topology is proposed for metal-cover smartphone applications. To achieve capacitive power transfer through metal cover, the metal cover itself is used as the receiving metal plates. To realize NFC through metal cover, the cross slot including a slim horizontal slot and a rectangular slot is embedded on the metal cover, which mitigates the eddy current loss and increases the inductive coupling for the NFC antenna. Besides, the cross slot divides the metal cover into two separate metal plates, which is necessary for capacitive power transfer. Considering the space limitation and safety requirement, no compensation components are integrated inside the smartphone and only a series inductance is integrated on the transmitter side. A prototype of the proposed NFC-CPT-combined coupler has been built. The wireless power transfer and NFC through metal cover have been validated through experiment. The experimental results show that the prototype achieves 5-W output power with a 93.5% circuit efficiency at 6.78 MHz, and the detection range of the NFC antenna is 61 mm.

Hybrid control of capacitive power transfer system for coupling capacitance variation

Focusing on the output voltage reduction caused by variation of the equivalent coupling capacitance in capacitive power transfer (CPT) system, this paper proposes a hybrid control strategy to achieve constant output voltage with the variation. The hybrid automata model is introduced for the cascaded CPT system considering the characteristics of higher order, nonlinear, multi-mode, continuous time and discrete states. The control process of the system is transformed into a selection of boundary transition conditions, which simplify the controller design and implementation. Then, a hybrid automata model of the system is established based on the theory of hybrid system, and the boundary transition of each mode of the system is given. The simulation and experimental results show that the proposed hybrid control strategy can keep the output voltage constant when the coupling capacitance of CPT system varies in a certain range.

Class-E2 DC-DC Converter With Basic Class-E Inverter and Class-E ZCS Rectifier for Capacitive Power Transfer

A Class-E2 dc-dc converter with basic Class-E inverter and Class-E ZCS rectifier for capacitive power transfer (CPT) is proposed. The proposed circuit partially absorbs the secondary-side compensation resonance inductance into the equivalent inductance of the Class-E ZCS rectifier. The ZVS operation of the inverter is maintained in wide range of the plates distance without any control owing to the ${{\pi }}1\text{a}$ impedance matching topology. The observed waveforms in circuit experiment were agreed well with the theoretical waveforms. The dc-dc power conversion efficiency was maintained about the same level of the maximum efficiency in wide range of the plates distance by keeping soft-switching on both the inverter and the rectifier stages.

Capacitive Coupling Wireless Power Transfer with Quasi-LLC Resonant Converter Using Electric Vehicles’ Windows

This paper proposes a new capacitive coupling wireless power transfer method for charging electric vehicles. Capacitive coupling wireless power transfer can replace conventional inductive coupling wireless power transfer because it has negligible eddy-current loss, relatively low cost and weight, and good misalignment performance. However, capacitive coupling wireless power transfer has a limitation in charging electric vehicles due to too small coupling capacitance via air with a very high frequency operation. The new capacitive wireless power transfer uses glass as a dielectric layer in a vehicle. The area and dielectric permittivity of a vehicle’s glass is large; hence, a high capacity coupling capacitor can be obtained. In addition, switching losses of a power conversion circuit are reduced by quasi-LLC resonant operation with two transformers. As a result, the proposed system can transfer large power and has high efficiency. A 1.6 kW prototype was designed to verify the operation and features of the proposed system, and it has a high efficiency of 96%.

Realizing Constant Current and Constant Voltage Outputs and Input Zero Phase Angle of Wireless Power Transfer Systems With Minimum Component Counts

In both normal and fast wireless electric vehicle charging systems, constant current/constant voltage (CC/CV) charging profile, regardless of the variation of the battery state of charge, is one of the most essential characteristics to ensure the battery performance and reliability. The input zero phase angle (ZPA) is able to minimize the system volt-ampere rating, enhance the power transfer capability, and make it easy to achieve soft-switching operation over the full range of battery charging profile. Therefore, the load-independent CC and CV output characteristics with ZPA conditions are necessary for wireless charging systems. However, the existing methods that can achieve these functions either add power switches or need a large number of compensation components, which make the system inefficient, uneconomical, and bulky. In this paper, a unified resonant tuning configuration with minimum passive component counts is proposed to achieve CC and CV outputs at two ZPA operating frequencies. According to the proposed configuration, all the possible inductive power transfer (IPT) and capacitive power transfer (CPT) topologies are analogized. With these topologies, both CC and CV outputs with ZPA are achieved using a minimum number of compensation components and no additional power switches. Among all the simplest IPT topologies, a primary Series-secondary Series and Parallel (S-SP) compensation topology is illustrated to demonstrate the analysis.

Analysis and Design Capacitive Power Transfer (CPT) System for Low Application Using Class-E LCCL Inverter by Investigate Distance between Plates Capacitive.

This study presents the analysis and design of a Capacitive Power Transfer (CPT) system for low power application using a Class-E LCCL inverter based on varying the distance between capacitance plates. The Class-E LCCL inverter can produce a high-frequency alternate current and reduce the size of the capacitance plate to minimise power losses during energy transfer besides yielding low switching losses. In specific, the performance of the LCCL CPT system at 1 MHz operating frequency and 24 V DC supply voltage was analysed via simulation and experimental works. Finally, a prototype of the CPT system was successfully designed, producing 10 W output power through a capacitive plate size of 0.0327 m2 and a 0.1 cm air gap at 96.68% efficiency. Based on the performance of the LCCL CPT System design, an efficiency analysis of the LCCL CPT System was performed; the capacitance plate distance was varied from 1 cm to 20 cm and produced a change in total impedance calculation from 57.69+j198.42 Ohm to −2.05j Ohm. Meanwhile, efficiency decreased from 96.68% to 1.25% but power was still transmitted in that range. These findings could be beneficial for hazardous electrical environments, portable applications, consumer electronics, and medical implants.

Multiobjective Parameter Optimization of a Four-Plate Capacitive Power Transfer System

For a four-plate capacitive power transfer (CPT) system, the existing coupling and compensation design methods can only provide an approximate local parameter optimization solution, instead of a globally optimized solution. This article proposes an accurate calculation method for cross-coupling capacitance and a multiobjective optimization algorithm combining nondominated sorting and Kent map for an SS-type CPT system. Mathematical models of the cross-coupling capacitances and port capacitances are established based on a two-port capacitive coupling model of the four plates. Besides, the constraints considering the reasonable region of the parameter values are given. Taking the output power, efficiency, and total harmonic distortion (THD) as objective functions, with the power transfer distance, operating frequency, load resistance, and input voltage as decision variables, the Pareto-optimal fronts are obtained by the proposed algorithm. A group of global optimal parameters is chosen from the solution sets, and a simulation model and a prototype of the CPT system are constructed. The efficiency of 87.1% at 252.3-W output power has been achieved. The simulation and experimental results verified the feasibility and effectiveness of the proposed calculating method for the cross-coupling capacitance and optimization method.

Analysis and Design of Inductive and Capacitive Hybrid Wireless Power Transfer System for Railway Application

Inductive power transfer (IPT) and capacitive power transfer (CPT) are mainly two effective ways to achieve wireless power transfer (WPT). IPT system needs capacitor to compensate the system, while the CPT system requires inductor to tune the system. Therefore, IPT coupler can be used to compensate the CPT coupler and vice versa. In this article, an inductive and capacitive hybrid wireless power transfer (HWPT) system is proposed to improve the system coupler antimisalignment ability. The couplers of IPT and CPT are employed together to compensate each other and transfer power together. Superposition theory is used to analyze the system working principle in detail. With the analysis results, a scaled-down system is built to validate the performance of the proposed approach. Experimental results show that the proposed HWPT system can achieve 653 W output power with 87.7% dc–dc efficiency at the well-aligned condition, and the maximum variation of the output power is 8.3% with the coupler misalignment from 0 to 270 mm (halfwidth of the coupler), which agree well with the analysis results.

Efficiency comparison of capacitive wireless power transfer for different materials

<p>This paper describes the application of several types of materials to act as capacitive plates in a Capacitive Power Transfer (CPT) system. In general, the efficiency of CPT system is greatly influenced by the coupling capacitance which is varied by distances and permittivity values. Thus this paper intended to compare the performance of CPT system in terms of the output efficiency for several types of capacitive plates. To be specific, copper plate, zinc, and aluminium are used in this work to act as coupling plates to the CPT system. The CPT system in this work applied class E inverter as it has lowest switching losses among its competitors, i.e. class D and class F. The work is validated through experimental setup of CPT system in which copper material provides the best efficiency of the system.</p>

An Improved LCL-L Compensation Topology for Capacitive Power Transfer in Electric Vehicle Charging

This paper proposes an LCL-L compensation circuit for high power capacitive power transfer (CPT) aiming at minimizing number of resonant components and improving system performance. The proposed topology adopts only four external resonant components at both sides of the capacitive coupler. The output power is proportional to the capacitive coupling coefficient, therefore, it simplifies the design procedure and abolishes protection circuit requirements under coupler’s misalignment. Moreover, optimizing efficiency at full-load conditions of compensation network can be easily achieved in this system by designing resonant components at the highest value of the mutual capacitance. Theoretical analysis of the proposed system is conducted alongside comparison to lasted CPT compensation circuits. Simulation and experimental results of a 1.5-kW CPT prototype with an air gap distance of 150 mm are provided to verify the feasibility and the effectiveness of the proposed system. System performances under different coupler’s misalignment conditions and output power levels are also examined and discussed as well.

A Sleeve-Type Capacitive Power Transfer System With Different Coupling Arrangements for Rotary Application

This paper presents a sleeve-type capacitive power transfer (CPT) system for contactless rotary applications. Sleeve-type capacitive couplers with different coupling arrangements between primary and secondary metal sleeves are proposed in this research, which lead to different coupling characteristics with positive, zero, and negative coupling coefficients. The effect of coupling coefficient variations caused by both the power transfer distance and the ratio between the overlapping length and the height of outer sleeves is analyzed. Combining the sleeve-type coupler, the CPT system is developed with a double-sided LC-compensated network and half-bridge inverter. For the CPT systems with positive and negative coupling coefficients, parameter design methods are presented to determine system parameters to achieve zero phase angle (ZPA) condition under the same output power. Then overall system performances of the CPT systems with local maximum positive and local minimum negative coupling coefficients are analyzed and compared by simulation in MATLAB/Simulink and Maxwell. Finally, an experimental prototype is constructed, which has demonstrated a power efficiency of 85.3% at 100.8 W. Simulation and experimental results show that the CPT system with local minimum negative coupling coefficient has clear advantages in increasing transfer distance, reducing voltage on sleeves, suppressing fringing electric field, and improving power density.

Maximum efficiency solution for capacitive wireless power transfer with <i>N</i> receivers

AbstractTypical wireless power transfer (WPT) systems on the market charge only a single receiver at a time. However, it can be expected that the need will arise to charge multiple devices at once by a single transmitter. Unfortunately, adding extra receivers influences the system efficiency. By impedance matching, the loads of the system can be adjusted to maximize the efficiency, regardless of the number of receivers. In this work, we present the analytical solution for achieving maximum system efficiency with any number of receivers for capacitive WPT. Among others, we determine the optimal loads and the maximum system efficiency. We express the efficiency as a function of a single variable, the system kQ-product and demonstrate that load capacitors can be inserted to compensate for any cross-coupling between the receivers.

A 1–10-MHz Frequency-Aware CMOS Active Rectifier With Dual-Loop Adaptive Delay Compensation and >230-mW Output Power for Capacitively Powered Biomedical Implants

This article presents a novel CMOS active rectifier for the emerging modality of capacitive wireless power transfer to biomedical implants with high power budgets. For operation versatility in terms of the input frequency, output power level, input voltage range, and load value, dual-loop adaptive delay compensation is utilized to provide both high resolution and high dynamic range in switched-offset currents of comparators that can be reconfigured for low- and high-speed operation modes for automatic adaptation to the input frequency. Fabricated in 0.18 $\mu \text{m}$ 1P/6M CMOS, the rectifier features power conversion efficiency (PCE) of >84.4% (peak of 91.5%) and voltage conversion ratio (VCR) of >88.6% (peak of 95.1%) when operating in 1–10 MHz and driving a load of 300 $\Omega $ . Moreover, for a load range of 20 $\Omega $ –1 $\text{k}\Omega $ at 5 MHz, PCE of 81%–91.8% and VCR of 78.4%–95.4% are achieved, with a maximum output power of ~232 mW at 50 $\Omega $ with a PCE of 90.1%. The active rectifier is also interfaced with a series-resonant capacitive link formed with coated flexible copper plates around a 3-mm-thick layer of muscle tissue, demonstrating an end-to-end power transfer efficiency of >40% in 2–10 MHz and with a load of 300 $\Omega $ .