Due to limitations of low power density, high cost, heavy weight, etc., the development and application of battery-powered devices are facing with unprecedented technical challenges. As a novel pattern of energization, the wireless power transfer (WPT) offers a band new way to the energy acquisition for electric-driven devices, thus alleviating the over-dependence on the battery. This paper presents an overview of WPT techniques with emphasis on working mechanisms, technical challenges, metamaterials, and classical applications. Focusing on WPT systems, this paper elaborates on current major research topics and discusses about future development trends. This novel energy transmission mechanism shows significant meanings on the pervasive application of renewable energies in our daily life.

Wireless Power Transfer—An Overview

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Maximum efficiency tracking is an important issue for wireless power transfer (WPT) system. Traditional maximum efficiency tracking method normally focuses on load impedance matching with fixed coupling condition. However, WPT system is a loosely coupling system, the coupling coefficient varies due to relative movement between the primary and secondary sides. Unknown to this variation may result in failure of the tracking. In this paper, a novel maximum efficiency tracking method is proposed with integrated dynamic coupling coefficient estimation. This method can take almost all requirements for maximum efficiency tracking into account including adaption for coupling coefficient and load variation, and output controllability. The tracking method is easy to implement because no additional circuitry or measurement is required. Experimental results have verified the correctness of the proposed coupling coefficient estimation method. And the maximum efficiency tracking results show the system can achieve a good performance against coupling coefficient and load variation with the proposed tracking method.

Maximum Efficiency Tracking for Wireless Power Transfer Systems With Dynamic Coupling Coefficient Estimation

Inductive power transfer (IPT) converters are resonant converters that attain optimal energy efficiencies for a certain load range. To achieve maximum efficiency, it is common to cascade the IPT converter with front-side and load-side dc/dc converters. The two dc/dc converters are normally controlled cooperatively for the requirements of output regulation and maximum efficiency tracking using a control technique based on perturbation and observation, which is inevitably slow in response. In this paper, a decoupled control technique is developed. The load-side dc/dc converter is solely responsible for output regulation, while the front-side converter is responsible for impedance-matching of the IPT converter by controlling its input-to-output voltage ratio. The controls are linear and therefore fast. DC and small-signal transfer functions are derived for designing the control parameters. The performances of fast regulation and high efficiency of the IPT converter system are verified using a prototype system.

Control Design for Optimizing Efficiency in Inductive Power Transfer Systems

Maximum efficiency point tracking (MEPT) control has been adopted in state-of-the-art wireless power transfer (WPT) systems to meet the power demands with the highest efficiency against coupling and load variations. Conventional MEPT implementations use dc/dc converters on both transmitting and receiving sides to regulate the output voltage and maximize the system efficiency at the expense of increased overall complexity and power losses on the dc/dc converters. Other implementations use phase-shift control or on–off control of the transmitting side inverter and the receiving side active rectifier instead of dc/dc converters but cause new problems, e.g., hard switching, low average efficiency, and large dc voltage ripples. This paper proposes a pulse density modulation (PDM) based implementation for MEPT to eliminate all the mentioned disadvantages of existing implementations. Delta-sigma modulators are used as an example to realize the PDM. A dual-side soft switching technique is proposed for the PDM. The ripple factor of the output voltage with PDM is derived. A 50 W WPT system is built to validate the proposed method. The system efficiency is maintained higher than 70% for various load resistances when the power transfer distance is 0.5 m, which is 1.67 times the diameter of the coils.

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Pulse Density Modulation for Maximum Efficiency Point Tracking of Wireless Power Transfer Systems

This paper presents a maximum efficiency tracking control scheme for a closed-loop wireless power charging (WPC) system for wireless charging of mobile devices. Generally, wireless charging systems need a precise output voltage and current with the highest possible efficiency. In an open-loop system, output voltage and efficiency depend strongly on the coupling coefficient and load condition. Alternatively, a closed-loop WPC system has a constant output voltage against coupling and load variations. Many studies have been carried out regarding closed-loop systems. However, those previous studies have the drawback of efficiency degradation. In this paper, we propose a maximum efficiency tracking control scheme to achieve the highest possible efficiency. Therefore, the proposed WPC system satisfies both the requirements of a constant output voltage and high efficiency. The proposed control scheme determines the current of the transmitter based on the data received by the receiver via Bluetooth. For validation, the proposed WPC system was implemented at 6.78 MHz using loosely coupled series–series resonant coils, and we verified that the proposed system can track the maximum efficiency while maintaining a constant output voltage.

Design of Maximum Efficiency Tracking Control Scheme for Closed-Loop Wireless Power Charging System Employing Series Resonant Tank

In this letter, the microwave power transfer (MPT) system with the optimal number of rectenna arrays for midrange applications is proposed, theoretically analyzed, and verified. A retrodirective power transmitter is designed to overcome the degradation by the near-field effect and enhance the power transfer efficiency. The implemented transmitter is composed of 16 $\times$ 1 patch antennas with a $\lambda /2$ spacing, and each of the antennas has a circuit for simultaneously detecting and shifting the phase, and operates at a frequency of 2.45 GHz. To further improve the power transfer efficiency, the parallel dc combining rectenna array is designed since the conventional RF combining rectenna has several problems in the case of midrange. The fabricated rectenna array consists of 8 $\times$ 1 rectennas with a $\lambda /2$ spacing and a dc power management circuit. In addition, the optimal number of rectenna arrays is proposed, considering the size of the receiver. In the experiments, the optimal number of rectennas is 3 when the transfer distance is 1 m. The achieved power transfer efficiency from three-arrayed rectennas is 4.47%, while the efficiency of 5.01% is achieved from eight-arrayed rectennas. Experimental results from the implemented MPT system show good agreement with the proposed theory.

Microwave Power Transfer With Optimal Number of Rectenna Arrays for Midrange Applications

For demonstrating autonomous operation of the Internet of things (IoT) smart sensors, the compact folded dipole rectenna with radio frequency (RF) energy harvesting is proposed. The rectifying circuits in the proposed rectenna consist of the impedance matching network and the voltage multiplier for the optimised received power. It has achieved the RF-dc conversion efficiency of approximately 23% when illuminated by a radio signal of 1 mW available power from the commercial UHF RFID reader.

Compact folded dipole rectenna with RF-based energy harvesting for IoT smart sensors

A wireless power transmitter includes a signal generator for generating power signals, a modulator for modulating a phase of a power signal to be time reversed based on beacon signals received from the wireless power transmitter and modulating an amplitude of a power signal to be amplified, a controller for providing a phase and amplitude control algorithm to be performed by the modulator, and an antenna unit for transmitting the power signals and receiving the beacon signals. The wireless power transmitter controls the amplification of amplitudes by using the phase and amplitude control algorithm, thereby improving wireless power transmission efficiency. In addition, the wireless power transmitter includes a multiple polarization antenna to enable high-efficiency wireless power transmission, regardless of a direction of the power signals and a direction of polarization.

Wireless power transmitter utilizing phase and amplitude control algorithm and wireless power receiver

Based on a dual-beam switching, the wide-coverage array antenna with a 4 × 4 proposed matrix which generates three dual-beams is presented for the ultra high frequency (UHF) radio frequency identification (RFID) applications. The 4 × 4 proposed matrix generates the three dual-beams which can find the approximate location of a tag by covering the wide spatial coverage of 180°. The main beam directions can be controlled by selecting the appropriate input port of the proposed matrix. Due to the compact structure and very low reflection coefficient, the circular polarized square quadrifilar spiral antenna (QSA) array incorporated with the 4×4 proposed matrix has been configured. The fabricated array antenna operates at the 902-928 MHz band and produces three dual-beams at ±12°, ±39°, and ±68°.

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Seung-Tae Khang

Papers

Design of Maximum Efficiency Tracking Control Scheme for Closed-Loop Wireless Power Charging System Employing Series Resonant Tank

Microwave Power Transfer With Optimal Number of Rectenna Arrays for Midrange Applications

Compact folded dipole rectenna with RF-based energy harvesting for IoT smart sensors

Wireless power transmitter utilizing phase and amplitude control algorithm and wireless power receiver

Wide-coverage array antenna using a dual-beam switching for UHF RFID applications