Two-coil Wireless Power Transfer Research Articles

Analysis of Simultaneous WPT in Ultra-Low-Power Systems with Multiple Resonating Planar Coils

This paper analyses the conceptual application of a wireless power transfer (WPT) system with multiple resonators supplying outdoor sensors using a mobile charger. The solution is based on the idea of using sensors, located in open space, to monitor environmental parameters. Instead of the typical two-coil WPT with a single charger, energy transfer is realized simultaneously, using a group of identical planar coils as transmitters and receivers connected to the independent power supply circuits of each sensor and microcontroller. By isolating these charged circuits, a higher reliability and powering flexibility of the weather station can be achieved. The concept of the proposed system was discussed, and it was proposed to include the main devices in it. A theoretical analysis was performed considering all mutual couplings and the skin effect; hence, the system is characterized by a matrix equation and sufficient formulae are given. The calculations were verified experimentally for different frequencies, two possible distances between the transmitters and receivers, and equivalent loads. Both the efficiency and load power are compared and discussed, showing that this solution can provide power to ultra-low-power devices, yet the efficiency must still be improved. At the small distance between the transmitting and receiving coils (5 mm), the maximum efficiency value was about 40%, with a load resistance of 10 Ω. By doubling the distance between the coils, the efficiency of the WPT system decreased by three times.

Spiral Resonator Arrays for Misalignment Compensation in Wireless Power Transfer Systems

In this contribution, the authors focus on the use of a metasurface (physically implemented as a 2D array of spiral resonators) as an additional component of a two-coil Wireless Power Transfer (WPT) system, with the aim of increasing the robustness to misalignment between the transmitter and the receiver coils. Resonator arrays have been proven to have a positive effect on WPT systems’ performance since they produce a focusing effect on the magnetic field; at the same time, they contribute to the reduction of the electric near field. In addition, we herein demonstrate how proper control over the metasurface’s unit cells can contribute to making a WPT system more tolerant to misalignment. In particular, the comparison between metasurfaces of different sizes (keeping the same transmitting and receiving coils) and their optimization performed to improve misalignment robustness is proved by numerical simulations.

Analysis of the Magnetic Field Magnetoinductive Wave Characteristics of a Wireless Power Transfer System.

This study analyzes the magnetic field wave characteristics of a wireless power transfer (WPT) system from a time-varying view in the nonradiative near field. Phenomena of both forward and backward traveling waves were found. These wave phenomena refer to magnetoinductive waves (MIWs) according to the findings in this study and MIW theory and characteristics. A traditional MIW only appears in the MIW waveguide, which is always constructed by many parallel coils. However, this study analyzed MIWs in a two-coil WPT system, proving that MIWs exist not only in a multi-coil system but also in a basic two-coil system. The velocity of MIWs, a kind of a phase velocity, was calculated. An approximate equation for evaluating wave velocity is proposed. Furthermore, the MIWs in the two-coil WPT system were extended into a more general situation. In this general situation, two separated standing waves were set, and a traveling wave was generated by those two standing waves. The result explains the mechanisms of MIWs in a general situation from a time-varying view. Lastly, a simulation was conducted to verify the accuracy of the study. The results demonstrated that MIWs exist, and the approximate equation is correct. This study presents a novel view on the mechanisms of the WPT system from a wave view.

A Note on the Analysis of Two-Coil Wireless Power Transfer Systems

A Note on the Analysis of Two-Coil Wireless Power Transfer Systems

Efficiency improvement of a three-coil WPT system with rectified load based on a selected compensation capacitor

ABSTRACT In the existing pieces of literature about the three-coil wireless power transfer (WPT) system, a three-coil structure is proposed to increase the efficiency compared to the two-coil WPT system. The cross-coupling among three coils, compensation capacitor of the repeating coil (RPC) and the rectified load are considered incompletely. The circuit model of a three-coil WPT system with rectified load is developed by taking the cross-coupling of all coils and the compensation capacitor of RPC into consideration in this paper. The relationship among system efficiency, compensation level, and rectifier load is deduced. The detailed mathematical analysis and compensation capacitor selection of RPC are investigated to improve the system efficiency. An experimental study demonstrates that the selected compensation capacitor of RPC achieves a higher efficiency than resonant compensation in a three-coil WPT system with different cross-coupling conditions and rectifier load conditions. System efficiency is promoted from 73.9% (resonant compensation capacitor for RPC) to 84.2% (selected compensation capacitor for RPC).

Analysis of Symmetric Two and Four-coil Magnetic Resonant Coupling Wireless Power Transfer

This study examined the efficiency of power transfer for two-coil and four-coil spiral magnetic resonant coupling wireless power transfer (WPT) using distance to coil diameter (D/dm) ratio and reflection coefficient, S21 value. Adding resonators reduced the total resistance in the two-coil WPT system while increasing the S21 values of the whole system. A same-size spiral coil was proposed for the system and simulated using computer simulation technology (CST). A prototype with similar specifications for a four-coil design was implemented for verification. The proposed method yielded an optimal efficiency of 76.3% in the four-coil system, while the two-coil WPT yielded a 23.2% efficiency with a 1.33 D/dm ratio.

Compensation Optimization of the Relay Coil in a Strong Coupled Coaxial Three-Coil Wireless Power Transfer System

With regards to wireless power transfer (WPT) systems with relay coils, the power transfer distance, efficiency, and capacity can be effectively improved by tuning the relative position of the relay coil compared with a common two-coil WPT system. However, the cross-coupling between nonadjacent coils can cause the system to no longer resonate even when the compensation capacitors resonant with the corresponding coils. This will cause an increase in reactive power and additional losses in the coils. In this article, the optimal operating conditions of the coaxial fully resonant three-coil WPT system are analyzed first for achieving higher efficiency compared with the two-coil WPT system with the same load and the same power transfer distance, by adjusting the relative location of the source coil and relay coil. An optimization design method for further improving the efficiency of the three-coil WPT system by tuning the compensation capacitor of the relay coil is proposed subsequently. The experimental results show that for a given load (10 Ω in this article), the efficiency of the three-coil WPT system can be improved from 88.8% to 92.3% compared with the two-coil WPT system, and be further improved to 93.3% when the relay compensation capacitor is optimized.

A simple method to determine the performance of two-coil wireless power transfer systems without direct output measurement

ABSTRACT Two-coil wireless power transfer systems are composed of two circuits tuned at the same resonance frequency, one containing the source and the other containing the load, where both are connected to each other by a mutual inductance. The power delivered to the load circuit is divided by the total power supplied by the source or, alternatively, by the maximum ideal amount of power that can be delivered to the load circuit, are usual figures of merit known as efficiency and power transfer capability, respectively. Additionally, a third figure of merit can be defined as the power dissipated at the source circuit divided by total power supplied by the source. It has recently been demonstrated that efficiency and power transfer capability are related to this recently defined third figure of merit. In this paper, a simple method is presented to monitor the power dissipated at the source circuit divided by total power supplied by the source, which allows a determination of the efficiency and/or power transfer capability without a direct measurement at the load circuit. The advantages and limitations of the proposed method are discussed in detail. Practical results are included to verify the proposal.

Investigation of the effects of common and separate ground systems in wireless power transfer

This article presents an investigation of the effects on a grounding system of wireless power transfer (WPT) when transmitting over relatively far distances, that is, up to 1.25 m. Conventional two-coil WPT systems are sufficiently commercialized in strong coupling range, but it is important to accomplish the long-range WPT in weak coupling range for further various applications. This system depends on the coupling effect between the two coils that the grounds of the transmitting and receiving coils should be completely separated. However, when evaluating the performance of two-coil systems with the instrument consisting of two ports and one common ground, undesirable problems occur in weak coupling ranges, for example, obtaining disagreeable transmission efficiency and degrading system stability/reliability. We investigate the problems of the leakage power from common ground systems and provide a practical solution to obtain a reliable WPT system by using an isolation transformer. The usefulness of this approach is that it is possible to achieve the stability of the system with relatively far transmitting distances and to determine the exact transmission efficiency.

Design Method of Three-Coil WPT System Based on Critical Coupling Conditions

It is well known that the power transfer efficiency (PTE) of a two-coil wireless power transfer (WPT) system is maximized at critical coupling under fixed system parameters. However, the two-coil WPT system is critical-coupled with a relatively short distance between the coils. To overcome the problem, a multicoil WPT system has been studied. In this paper, we review the impedance characteristics of the critical-coupled two-coil WPT system and reveal that the conventional three-coil WPT system cannot have the same impedance characteristics as critical coupling. Based on the impedance matching conditions with critical coupling of the two-coil WPT system, we propose a design method of a three-coil WPT system in which the free resonant frequencies of all resonators are changed. The proposed design method is successfully implemented at the operating frequency of 6.78 MHz. The experimental results are in good agreement with the simulated PTEs and show that by changing the free resonant frequencies, a higher PTE can be effectively achieved in comparison to the conventional two-coil and thee-coil WPT systems.

Maximum Output Power Improvement Using Negative Coil in Over-Coupled WPT System

Conventional two-coil wireless power transfer (WPT) systems achieve the maximum output power at the optimal distance. In the over-coupled state, in which the distance between coils is shorter than the optimal distance, the input impedance is significantly higher than that with the optimal distance. As a result, the output power at the load is dramatically decreased. In this letter, we propose a method to improve the maximum output power by applying a negative-impedance converter (NIC) to an additional coil. The proposed system can drastically improve the output power at a load even in the over-coupled state because the negative coil eliminates the impedance reflected by mutual coupling between coils. To validate the proposed method, the NIC was implemented with an operational amplifier and four identical coils. In the experiment, the proposed system transferred enhanced power, which was three times higher than the maximum output power of the conventional system.

Analysis and design of a high‐efficiency three‐coil WPT system with constant current output

The wireless power transfer (WPT) system has been widely researched due to its inherent advantages compared with the traditional plug-in charging system over security, flexibility, convenience and so on. Compared with the two-coil WPT system, the three-coil WPT system has been proven to have higher efficiency over a relatively long transmission distance. In many applications, WPT systems are required to have the load-independent constant current output (CCO) characteristic and zero phase angle (ZPA) operation. This study proposes a three-coil WPT system that can achieve the CCO and ZPA operation through flexible parameter design. First, the detailed mathematical deduction for the CCO and ZPA operation is given. Then, a comprehensive analysis of the three-coil WPT system is conducted in term of parameter design, the sensitivity of the CCO to changes in compensation parameters, implementation of zero voltage switching operation and efficiency analysis. Furthermore, a detailed efficiency comparison with the SS WPT system is performed to reflect the high efficiency of the proposed three-coil system, and the additional performance of the proposed three-coil system is compared with previous related works to verify its advantages further. Finally, the correctness of the theoretical analysis is verified by simulation and experiment.

Highly Efficient Single-Switch-Regulated Resonant Wireless Power Receiver With Hybrid Modulation

In this paper, a highly-efficient single-switch-regulated resonant wireless power receiver with hybrid modulation is proposed. To achieve both high efficiency and good output voltage regulation, phase shift and pulse width hybrid modulation are simultaneously applied. The soft switching operation in this topology is achieved by the cycle-by-cycle phase shift adjustment between the input current and the gate drive signal and also attributed to the reactive components such as the series-compensated secondary coil and the parasitic capacitor of the active switch . The soft switching operation also leads to high efficiency and low EMI. By adjusting the duty ratio of the switch, tight regulation of the output voltage can be attained. The steady-state and dynamic models of the resonant receiver with hybrid modulation are analytically derived in order to properly design the feedback controller. An experimental setup of a two-coil wireless power transfer system, including the hardware prototype of the proposed receiver, is constructed for experimental verification. The experimental results show the effectiveness of the soft-switching operation in the receiver with high efficiency while maintaining good regulation of the output voltage, regardless of line and load variations.

High-Order Parity-Time Symmetric Model for Stable Three-Coil Wireless Power Transfer

A noteworthy challenge in actual wireless power transfer (WPT) applications is to achieve stable power transfer efficiency at varying distances. It was recently proposed and demonstrated that maximum transferred efficiency was ensured when a two-coil WPT system was always tuned at the purely real eigenfrequency. However, this condition prevents optimal operation with a fixed operating frequency. To address the issue, we introduce the concept of high-order (higher than second-order) parity-time (PT) symmetry to bypass this difficulty and construct straightforward physical pictures to obtain the mechanism of stable and efficient power transfer in a three-coil WPT system. Before the PT phase transition, there is a coupling-independent entirely real eigenfrequency, despite three branched eigenmodes existing. It means that this WPT system, with high-order PT symmetry, could be highly efficient without frequency tracking at varying distances. Additionally, the nonideal high-order PT model, with asymmetric coupling for practical WPT applications, is discussed. It is demonstrated experimentally that the efficient stability of the high-order WPT system is significantly superior to that of the second-order system at a fixed frequency, which can be explained by the fact that the eigenfrequency of the nonideal high-order model used is less sensitive to the coupling strength than that of the second-order model. Furthermore, the idle power loss of this WPT system is less than that of the other at the resonant frequency (in an idle state without receiver terminals), reducing power consumption at the transmitters and benefiting wireless charging intermittently. As an example, the high-order PT symmetric model is applied to the WPT system with miniaturized receivers, showing stable transfer efficiency with a wide range of axial transfer distances and lateral misalignment. Our work provides an alternative application for the study of high-order PT physics in designing a properly multiple-coil WPT system in the long term.

Highly Efficient Wireless Power Transfer System With Single-Switch Step-Up Resonant Inverter

In this article, a highly efficient wireless power transfer (WPT) system is realized by employing the proposed single-switch step-up resonant inverter with a series–series-compensated network. This resonant inverter has the merits of a simplified circuit structure, low component count, inherent voltage boosting, zero-voltage switching (ZVS), and high efficiency. Based on the equivalent circuit model, the two-coil WPT system is analyzed in the frequency domain, which shows that a quasi-constant current (CC) in the primary side and a corresponding quasi-constant voltage (CV) in the secondary side can theoretically be achieved. A systematic top-down design methodology of the entire WPT system is also provided, which serves as a practical design guideline. The proposed solution benefits real WPT applications by generating a clean and stable dc voltage for the built-in dc–dc battery management system of the receiving device. The experimental results show that the maximum efficiency of the WPT system at the rated output power of 11 W is around 86.4%.

Optimization of the Circuit Parameter in Class-E Power Amplifier in the Two-coil Wireless Power Transfer System

In this paper, the circuit parameters of the Class-E power amplifier are investigated and optimized to achieve the maximum of output power and efficiency in a two-coil Wireless Power Transfer (WPT) system. The circuit is simulated with software named Saber. Working frequency, the value of capacitors and inductor, which has relationship with the output power and efficiency, are investigated during the simulation. With the simulation results, it is found that the frequency and the shunt capacitor need to be optimized for high performance of the wireless power transfer system.

Investigation on power optimization principles for series-configured resonant coupled wireless power transfer systems

This paper is a thorough theoretical analysis of a two-coil wireless power transfer system (WPTS) configured in series with a focus on power optimization rather than maximizing transmission (link) efficiency. Different definitions of the system efficiency in the pertaining literature are distinguished and clarified. The frequency splitting phenomenon is precisely explained, and furthermore, the analytical solutions are derived based on analysis of the input impedance. The effects of this behavior on the power transfer to a load resistance are discussed. Various aspects of the power optimization problem are explored. In particular, for the case when the system is driven at the resonance frequency, (i) the explicit expression of the optimal coupling factor between the two coils for a given load, and (ii) the optimum power with respect to the load are provided. The impedance matching methods using different circuit topologies are analytically or numerically investigated, revealing that the drive frequency can be arbitrarily chosen and not necessarily equal to the resonance frequency. This provides more options for exciting the system apart from the resonance condition, without compromising the delivered power. A comparison between optimization techniques is given in terms of the coupling factor k, showing that the bi-conjugate matching with Π-networks results in the maximum generated power and the transducer power gain (i.e., defined by the ratio between the received power and the power available from the source), which reaches 80% at k=84.4×10-3, for example.

A novel three-coil wireless power transfer system and its optimization for implantable biomedical applications

The technology of wireless power transfer (WPT) has broad applications especially in implantable biomedical devices. Because of the limitation of the size of the receiver coil, how to lengthen the power transfer distance is crucial in biomedical applications. In order to address this problem, in this paper, a novel three-coil WPT system is proposed and analyzed. In the system, there are two transmitter coils and one receiver coil. Based on the Biot–Savart’s law, the electromagnetic property of the square coil is analyzed using finite element method. Moreover, the structural design of the system is optimized by a memetic algorithm. The memetic algorithm combines features of the artificial bee colony method and the covariance matrix adaption evolutionary strategy method. The simulation and experiment results show that the receiver coil can receive about dozens of millivolt when the power transfer distance is about 15 cm. This means the proposed system is suitable for implantable biomedical devices. Compared with the two-coil WPT system, in addition, the power received in the receiver coil of three-coil WPT system can increase about 48%.

Comparative Analysis of Two- and Three-Coil WPT Systems Based on Transmission Efficiency

It is obvious that the conventional two-coil wireless power transfer (WPT) system has a short power transmission distance, which is determined by system parameters such as the loaded Q of the resonators and the coupling coefficient. There have been many attempts to improve the power transfer efficiency (PTE) or transmission distance. A typical approach is to use multiple coils like a three- or four-coil WPT system. In this paper, we propose a new method to obtain the PTE of a multi-coil WPT system based on the scattering parameter and impedance parameter. Also, we compare the two- and three-coil WPT systems in terms of their transmission efficiency rather than their system energy efficiency. For high transmission efficiency, we determined that the three-coil WPT system should have a symmetric structure and the coupling coefficient between the transmitting and receiving coils has to be as zero as possible. Additionally, we found that 1/√3 of the conventional critical coupling is the boundary coupling coefficient at which the two- and three-coil WPT systems have the same transmission efficiency. We successfully verified these theoretical analyses by implementing two- and three-coil WPT systems at the operating frequency of 6.78 MHz and measuring their transmission efficiency and spectra.

Optimized Design of Coils for Wireless Power Transfer in Implanted Medical Devices

Implanted medical devices (IMDs) wirelessly powered by magnetic resonant coupling has attracted wide attention recently. In this study, an optimized two-coil wireless power transfer system is adopted to eliminate the requirement of an 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 6.78 MHz. A 200 mm × 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.