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This article presents an interleaved buck-type rectifier with pseudo-dc-link capacitors featuring automatic current balancing, soft-switching operation, efficient ac-dc conversion, and tight output regulation. The ac-input-current-synchronized modulation scheme that hybridizes for cycle-by-cycle phase-shift and duty ratio adjustment is adopted for zero-voltage switching (ZVS) operation and output regulation. In addition, the pseudo-dc-link capacitors facilitate the convergence and current balancing between output inductors. Mathematical models are provided to quantitatively describe the mechanism of current balancing, steady-state performance, and dynamic response. A lab prototype is built, and the experimental results are provided to verify the features. [ABSTRACT FROM AUTHOR]

The energy efficiency of an inductive wireless power transfer (WPT) system will deviate from the possible peak value if the magnetic coupling of the transmitter and receiver coils or the load resistance changes. To achieve maximum-efficiency tracking regardless of coupling or load changes, on–off keying (OOK) modulation is used. However it has the issue of large surge current in the circuits during OOK modulation. The current overshoots of a series–series (SS) compensated WPT system at the startup stage might be much higher than the rated current. The current overshoots not only increase the stress on the power switch devices but also cause high voltage spikes across the windings and the resonant capacitors which is a challenge for insulation design. A soft-start scheme is proposed to address the current overshoot problem. The principle of the soft-start scheme can be essentially applied to WPT systems with different compensation topologies. The experimental results of a 3.3-kW prototype verify that the proposed soft-start scheme effectively suppress the current overshoots. [ABSTRACT FROM AUTHOR]

While detection of foreign or unintended metallic objects is an important topic for wireless power transfer (WPT), operation of WPT systems with intended metallic objects is largely an unchartered territory. This article presents a methodology for analyzing the effects of intended metallic object on a WPT system through the study of the magnetic field distribution and the use of a reduced-order equivalent model of the systems for comparing the WPT system with and without the metallic object. The methodology is applied to a metallic corona ring of low resistance used with a WPT system. The analysis quantifies how an intended metallic object could change the magnetic coupling between the transmitter and receiver coils, and alter the equivalent resonant tank parameters of the WPT system. Recompensation of the resonant tank can only enhance the power transfer capability but not the energy efficiency. The theory and practical verification of a new performance-enhancement method based on a combination of recompensating the resonant tanks and maximizing the equivalent mutual inductance of the Tx and Rx coils is presented. Such methodology provides a general approach to analyze WPT systems with or without relay resonators in the presence of intended metallic objects. [ABSTRACT FROM AUTHOR]

This paper presents a comparative study on three types of slim coil structures used as a three-dimensional (3-D) receiver in a wireless power transfer system with a planar transmitter coil. The mutual coupling values and their variations between the receiver structures and the transmitter coil are compared under different distances and angular orientations with respect to the transmitter coil. The merits of performance are related to the consistency of the mutual coupling values under different orientations in a range of distances from the transmitter coil. The practical results show that slim 3-D receiver coil structures can be compatible with a planar transmitter coil with reasonably high-mutual coupling. [ABSTRACT FROM AUTHOR]

In this paper, a nonisolated buck-boost single-inductor multiple-output (SIMO) dc–ac inverter for driving multiple independent high-frequency ac outputs of medium power is proposed. Compared with traditional bridge-type inverters, the proposed buck-boost SIMO inverter achieves 1) a smaller component count, 2) fully independent power control of its outputs, 3) better scalability in increasing the number of ac output channels, and 4) higher power efficiency. Operating in pseudo-continuous conduction mode, the rated power of each output channel of this inverter can be high while attaining zero cross regulation. The scalability factor of the proposed inverter is formally investigated and the theoretical maximum number of ac outputs is analytically derived. The targeted application of this SIMO-based inverter is for driving multiple transmitter coils to realize versatile multidevice medium-power wireless power transfer. A hardware prototype of a single-inductor three-output buck-boost inverter delivering a medium power of 8.4 W per output channel has been constructed. It is experimentally verified that precise and independent current regulation of individual transmitter coil is achievable with the proposed inverter. [ABSTRACT FROM AUTHOR]

Optimum energy efficiency in a wireless power transfer (WPT) system usually occurs only at a specific load value while the load such as a battery is normally not constant. A major challenge in WPT is, therefore, to achieve high energy efficiency over a wide load range. Previously, impedance transformation methods such as dc–dc converters and use of a relay coil have been used to transform the practical load resistance to the optimum (or near-optimum) value. In this paper, a new approach based on reconfigurable magnetic resonant structures is proposed to achieve high energy efficiency and low volt-amp ratings. This basic principle is to create more than one efficiency–load curve. The WPT system is controlled to be operated within the top regions of the energy efficiency curves of the reconfigurable circuits so that high energy efficiency can be achieved over a very wide load range. The principle of this new approach is explained with an analysis and verified with practical measurements. [ABSTRACT FROM PUBLISHER]

In this paper, a nonisolated dc−ac inverter topology, which enables a true single-stage direct conversion from a single dc input supply to multiple ac output power sources by using only one inductor in the power stage, is proposed. Compared with the conventional multiple-output parallel half-bridge or full-bridge inverters, the proposed single-stage single-inductor multiple-output (SIMO) inverter topology achieves a smaller number of inductors and power switches, high power density, scalability to multiple ac outputs, and independent power controllability for each ac output. A useful application of the proposed SIMO inverter is to drive an array of transmitting coils simultaneously for wireless power transfer applications. An experimental prototype has been developed to demonstrate the effectiveness of a single-inductor three-output boost-derived inverter without cross regulation. Experimental results show that precise and independent peak voltage regulation of each individual sinusoidal ac output is attainable with the proposed SIMO inverter. [ABSTRACT FROM AUTHOR]

Maximum energy efficiency in wireless power transfer (WPT) systems can be achieved through the use of magnetic resonance technique at a certain load resistance value. However, practical load resistance is not constant. Previously, a switched mode dc–dc converter was used in the receiver circuit to emulate an equivalent load resistance for maximum energy efficiency. In this paper, a new approach based on the On–Off Keying is proposed to achieve the high energy efficiency operation over a wide range of load power without using an impedance-matching dc–dc power converter. This simple and effective method has reduced average switching frequency and switching losses. It can be applied to any series-series resonant WPT system designed to operate at a constant output voltage. Practical measurements have confirmed the validity of the proposal. [ABSTRACT FROM PUBLISHER]

A new wireless power transfer (WPT) system based on ball-joint structure is presented in this paper. A ball-joint WPT system consists of a ball structure with a mechanical rod attached to the ball and a ball socket that accommodates the ball structure. This ball-joint structure comprises at least one winding in the ball structure and at least one winding in the ball socket structure. The ball structure can be flexibly rotated over a wide range of angle inside the ball socket, while wireless power can still be transferred from the transmitter winding to the receiver winding through magnetic resonance. The magnetic coupling coefficient between the transmitter and receiver coil over a wide rotating angular range has been analyzed and experimentally checked. Experimental results confirm that an energy efficiency of up to 81% can be achieved. [ABSTRACT FROM PUBLISHER]

High-voltage insulation (HVI) strings are commonly used to hold high-voltage electric cables and electrically isolate them from the grounded transmission tower. In this paper, a novel concept of an electric insulation string with HVI and wireless power transfer (WPT) capabilities is presented. Based on the concept of the domino-resonator WPT system, this new structure consists of coil resonators embedded inside totally sealed insulation discs, which are then connected in series to form the new insulation string structure with the simultaneous HVI and WPT functions. This structure allows energy harvested from the ac magnetic field around the high-voltage cable to be transmitted wirelessly to power an online monitoring system in high-voltage transmission tower continuously, thereby reducing the storage requirements of the battery. The design and analysis of this new WPT structure based on the dimensions of commercially available HVI rod are included. Practical measurements obtained from a hardware prototype of about 25 W have been obtained to confirm the WPT capability of the proposal. An energy efficiency of more than 60% has been achieved for a transmission distance of 1.1 m over a wide range of load. [ABSTRACT FROM PUBLISHER]

A charging time control method for wireless power transfer systems with a secondary-side hysteresis output power control is presented. It is a primary-side control method that adopts the combined use of three concepts, namely 1) an intermediate capacitor in the receiver circuit as a power flow indicator, 2) hysteresis switching actions of a shunt decoupling power switch in the receiver circuit to regulate the dc voltage of such intermediate capacitor, and 3) the turn-on and turn-off times of the decoupling switch detected on the primary side for closed-loop control. This method has the advantage of eliminating the need for 1) precise information of the mutual inductance between the transmitter and the receiver coils and 2) wireless communication system for feedback purpose. Practical results obtained from a hardware prototype are included. They confirm the proposed operating principles and indicate that the method can automatically lead to optimal energy efficiency operation. [ABSTRACT FROM PUBLISHER]

Part-II of this paper focuses on the mathematical analysis of the 3-D omnidirectional wireless power transfer (WPT) and also addresses the general principle of load detection. It provides the mathematical forms of distributions of the input power vector and output power vector, and demonstrates that the geometry of such 3-D distributed space follows the revolution of the Lemniscate of Bernoulli along its longitudinal axis. It provides the mathematical proof that the direction of energy transfer for the maximum energy efficiency is always in line with that of the maximum load power path in the 3-D space. Experimental verification is included to confirm the 3-D omnidirectional WPT theory. [ABSTRACT FROM AUTHOR]

In this paper, a new method to estimate the mutual inductance and load resistance in a series–series compensated wireless power transfer system is presented. Reasonably accurate estimations can be obtained from measurements of the input voltage and current obtained at one operating frequency only. The proposal can be used to dynamically monitor both the coupling relationship between the transmitter and receiver coils and the load conditions without any direct measurement on the receiver side. It can also be used as a simple method to measure the mutual inductance of any pair of coupled coils. A novel impedance spectrum analysis method is further presented to show that series–series compensation has special characteristics in its input impedance spectrum. Experimental results with acceptable tolerance are included to show the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

This paper presents the basic control principles of omnidirectional wireless power transfer (WPT) based on the current amplitude control. The principles involve 1) an “omnidirectional” scanning process for detecting the power requirements in a 3-D space and 2) a “directional” power flow control for focusing the wireless power toward the targeted areas. Such principles apply to any WPT system comprising three orthogonal transmitter coils and multiple receivers with coil resonators. A current amplitude control method capable of generating a magnetic vector at a set of points evenly distributed on a spherical surface is explained. Based on the voltage and the current information in the transmitter circuit, the power involved in each vector over the spherical surface can be obtained. By scanning the vector over the spherical surface, the collective power flow requirements for the targeted loads can be determined. Based on the power requirements for the vectors over the spherical surface, a weighted time-sharing scheme is adopted to focus the wireless power toward the targeted areas. This method has been successfully applied to a hardware prototype. Both theoretical and experimental results are included to confirm these principles. [ABSTRACT FROM AUTHOR]

This paper describes a method for monitoring multiple loads from the front end of a wireless power transfer system without using any wireless communication systems. A mathematical approach based on scanning the frequency around the resonant frequency has been developed for deriving the load conditions. The proposal requires only information of the input voltage and current, thereby eliminating the requirements of using wireless communication systems for feedback control. The proposal has been practically confirmed in hardware prototype with good results. [ABSTRACT FROM PUBLISHER]

This paper describes a technique for multifrequency wireless power transfer systems in which the wireless power can be transmitted through the wireless power transfer channel or channels from the transmitter to the targeted loads with receiver coils specifically tuned for energy reception. Auxiliary circuits comprising bandpass and/or bandstop circuits are proposed for incorporation into the receiver circuits and optional relay circuits so as to facilitate the selection and enhancement of the wireless power transfer to the designated load without causing significant cross interference due to the use of multifrequency wireless power flow control. A unique feature of this technique is that the nontargeted receiver will automatically act as a relay resonator to enhance 1) magnetic coupling, and thus, 2) the power transfer between the power transmitter and the targeted receiver. A second novel feature is that the chosen operating frequencies for the tuned receivers need not be widely apart because the auxiliary circuits consist of bandpass and/or bandstop filters to reduce any cross interference from the nontargeted frequency. The proposed technique has been practically verified in experimental prototype. [ABSTRACT FROM AUTHOR]

A method for automatic "maximum energy efficiency tracking" operation for wireless power transfer (WPT) systems is presented in this paper. Using the switched-mode converter in the receiver module to emulate the optimal load value, the proposed method follows the maximum energy efficiency operating points of a WPT system by searching for the minimum input power operating point for a given output power. Because the searching process is carried out on the transmitter side, the proposal does not require any wireless communication feedback from the receiver side. The control scheme has been successfully demonstrated in a two-coil system under both weak and strong magnetic coupling conditions. Experimental results are included to confirm its feasibility. [ABSTRACT FROM AUTHOR]

A new methodology for ensuring that a three-coil wireless power transfer system is more energy efficient than a two-coil counterpart is presented in this paper. The theoretical proof and the conditions for meeting the objective are derived and practically verified in a practical prototype. The key features of the magnetic design are to: 1) shift the current stress from the primary driving circuit to the relay resonator; and 2) generate a large relay current for maximizing magnetic coupling with the receiver coil for efficient power transfer. Consequently, the current rating and cost of the driving circuit can be reduced and the overall quality factor and system energy efficiency are improved. This approach utilizes the combined advantages of the maximum efficiency principle and the use of relay resonator to overcome the energy efficiency problem for applications with extended energy transfer distances. [ABSTRACT FROM AUTHOR]

In this paper, the effects of the magnetic coupling of nonadjacent resonators on the optimal frequency of wireless power transfer are addressed. A power analysis has been carried out to identify the adjacent and nonadjacent power flow components. It is found that such cross-coupling effects of nonadjacent resonators would cause the maximum efficiency operation to slightly shift away from the resonance frequency of the resonators. Theoretical reasons for such phenomena are provided and experimentally confirmed with practical measurements in a wireless power transfer system comprising several magnetically coupled resonators arranged in a straight domino form. [ABSTRACT FROM AUTHOR]