Wireless Power Transfer—An Overview

IEEE Transactions on Industrial Electronics

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15 pages

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Due to limitations of low power density, high cost and heavy weight etc., the development and application of battery-powered devices is 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. Index Terms-Contactless charging, capacitive coupled power transfer (CCPT), dynamic charging, inductive power transfer (IPT), overview, wireless power transfer (WPT).

Figures (16)

where r is the radius of coil and a is the cross-sectional radius of copper wire. Accordingly, the load power will dramatically drop with the increasing of the transmission distance d by in IPT systems. This is the reason why IPT systems can only deal with the near-field energy transmission. Fig. 1. Inductive power transfer: (a) circuit model; (b) equivalent T-model; (c) two-port network model.

According to Eq. (5), it shows that the primary and the secondary circuits both need a capacitive compensation to eliminate the imaginary part [7], aiming to ensure the maximum V; and minimum (@L2+X,)*. Regarding to the capacitive compensation network, there are four topologies as shown in Fig. 2, namely series-series (SS), series-parallel (SP), parallel-series (PS) and parallel-parallel (PP). By using reflected impedance theory, the compensated capacitances can be calculated with respect to various network topologies [8], which are all listed in Table 1. It should be noted that the SS is the only one topology which is independent on the coupling coefficient and the load condition since the reflected reactance equals zero on the primary side. Fig. 1(b) depicts the equivalent T-model of 2-coils IPT systems, the energy efficiency 72 can be calculated as [9]:

Capacitive compensation network dramatically when d>2r. This is the reason why the 2-coils IPT system is commonly used for short-range (shorter than the diameter of the coil) transmission applications.

In [16], an auto-frequency tuning scheme was developed based on the suboptimal Class-E” converter to minimize the switching loss caused by the load variations, which aims to improve the transmission efficiency for CCPT systems. In [17], a double-sided LC-compensation circuit for long-distance transmission, which can offer both constant-voltage and constant-current working modes as well as improve the transmission efficiency under the loosely coupled effect.

Fig. 3. Difference of impact between the distance on efficiency of 2-coils and 4-coils IPT systems.

The near-field WPT technique is embracing a high-speed development period in recent years. There are increasing academic researchers and industrial engineers who focus on improving the energy transmission performance with emphasizes on the efficiency, capability, applicability, flexibility, security and so forth. In this section, this paper will overview key technical challenges for non-radiative electromagnetic WPT systems. A. Energy Efficiency

Fig. 5. A typical CCPT system: (a) block diagram; | (b) circuit. addition, the CCPT system was successfully utilized for various applications, such as EVs [21], synchronous electric machines [22], and consumer electronics [23].

Fig. 7. Optimal dipole-shaped coils for long-distance IPT systems. By comparing with the photovoltaic, acoustic, microwave, and laser energy transmissions, the IPT has the salient advantage of high power. For long-distance transmission applications, however, the IPT system has to deal with inevitable key technical issue, namely the extremely loosely Kegaraing to the switching component, an enhanced gallium nitride (eGaN) device was utilized to improve the output power capability in MHz frequency band [44]. For the circuit topology, a LCL load resonant inverter was investigated for maximum power transfer [45], which is operated in the discontinuous current mode and controlled by a variable frequency scheme. The presented work successfully predicted the inverter operating point and ensured the optimal value of the series inductance. In [46], a multiphase parallel inverter was proposed to improve the output power capability for WPT systems. In [47], a 6-kW parallel IPT power supply topology was proposed in a cost effective manner, which can minimize uneven power sharing due to component tolerance without additional reactive components for parallelization. Regarding to the control scheme, an offline-tuning scheme was proposed to ensure the WPT system to output the maximum power in instead of the online frequency regulation [48]. Accordingly, the constant operating frequency can effectively avoid the violation caused by the variation of operating frequency. The key to maximum power transfer is to ensure the impedance matching. In [49], a hybrid impedance-adjusting scheme was developed by combining the continuous conduction mode and the discontinuous conduction mode, which can effectively extend the adjusting range and thus ensure the full utilization of the power capacitor for WPT systems.

As shown in Fig. 9, the WPT system is sensitive to the relative position between the transmitting and receiving coils, which means that the transmission performance is deteriorated with respect to an angular or lateral misalignment. The output power P.,; of IPT systems can be given by [54]- [55]: In addition, due to the impact on the resonant frequency, the variation of the transmission distance is another important technical concern for IPT systems. In previous studies, consequently, there are a number attempts for the impedance matching to ensure the magnetic resonant state even if a varying distance between the transmitting and receiving coils. As shown in Fig. 8, a capacitor matrix was proposed in [52], which can offer expected compensation capacitances by using limited capacitors to deal with the frequency mismatch caused by the varying transmission distance. In [53], a multi-loops topology was utilized to reduce the variation of the input impedance.

coupled effect. According to the measurement results in [50], it shows the coupling coefficient « is mostly much less than 0.01 with respect to the transmission distance from 2m to 12m. As shown in Fig. 7, a dipole-coil-based IPT system was proposed for a long-distance energy transmission in [50]-[51], which adopts an optimized stepped core — structure for evenly-distributed magnetic field density. The experimental prototype can deliver 10.3W power up to 7m away at the frequency of 20kHz.

Fig. 10. Three-orthogonal-coil topology for omnidirectional WPT systems. [65] The key of multi-objectives WPT system is to ensure the energy supply at an arbitrary spatial position. Accordingly, the omnidirectional WPT system is moving the center of the stage, which has been reported in previous studies [62]-[64]. For smart home applications, while a compact transmitter with the ability of middle and long-range transmission is suitable for indoor utilizations, as shown in Fig. 10, a transmitter with orthogonally-assembled coils has been increasingly accepted by researchers since it can offer an enhanced flexible charging way for household appliances. Based on such an orthogonal topology, the current phase and amplitude modulation was proposed to spatially produce the electromagnetic field to cordlessly energize electric-driven devices [65]. In [66]-[67], the transmitter consisting of two vertical coils fed by current sources with adjusted amplitudes, which aims to generate a rotate electromagnetic field. In addition, the impact of the current phase was also studied for such a coil topology in [68], where a feeding phase difference of 90° was adopted to verify the capability of power transfer to a mobile receiver around the transmitter in a 2-dimentional space. In [69], a novel cubic transmitter was designed and fabricated to fulfil an omnidirectional WPT system by adopting one single power source. The presented work can simplify the control method while its capability of 3-dimensional power transfer remains to be verified.

Fig. 11. Electromagnetic filed distribution: (a) authorized receptor; (b) unauthorized receptor.

Fig. 13. Classification of previous studies on metamaterial-based WPT technologies. In [77], a hybrid metamaterial slab was proposed by combining two kinds of structures of metamaterial cells with the negative and the zero permeability for WPT systems. Along the transmission path between the transmitting and receiving coils, the proposed hybrid metamaterial slab can ensure that the inside magnetic field propagates straightly through the center due to the zero permeability while the outside magnetic field can be concentrated into the receiving coil through the edge of the proposed hybrid metamaterial slab due to the negative permeability. Accordingly, it can effectively increase the energy efficiency of the WPT system from 8.7% to 47% with the transmission distance of 20cm and the operating frequency of 6.78MHz. In [78], a double-side square spiral was developed as the unit cell of the metamaterial slab to optimize the performance. In [79], an experimental prototype was set up to carry out a comparative analysis for various metamaterial topologies, where a 40-W bulb can be wirelessly lighted with the distance of 50cm and the frequency of 27.12MHz. Based on the brightness of the bulb, it verifies the effectiveness of the metamaterial to improve the energy efficiency of WPT systems. Besides, there are increasing studies on the 3-dimentional metamaterial to further improve the transmission efficiency of WPT systems. In [80]-[81], a 6x6-array metamaterial slab was proposed to wirelessly light up a 15-W bulb, where the unit cells are periodically assembled on a 5-mm thick acrylic slab forming a 6x6 array. The experimental prototype consists of a source coil, a load coil connected with a bulb, the metamaterial slab placed between the transmitter and receiver, a transmitting coil and a receiving coil resonating at 6.78MHz. The experimental results show that the 3-dimentional metamaterial slab can increase the transmission efficiency by 24%. In [82], a 3-dimentional metamaterial structure was proposed for mid-range WPT systems, which consists of a 4x5x1l-array of three-turn spiral resonators with the negative permeability at the frequency of 6.5MHz. The corresponding experimental results show that the output power can be increased by 33% and 7.3% at distances of 1.0m and

According to the electromagnetic parameters, Fig. 12 shows the classification of materials, where the first quadrant represents a classic type of regular dielectrics possessing the positive permeability and the positive permittivity ¢. The materials in the second, third and fourth quadrants have either negative € or negative 1, or the both. As an artificial material in the third quadrant, the metamaterial has the negative permeability and the negative permittivity [74], which is commonly called as the left-handed material [75]-[76]. By means of the double-negative characteristic, the metamaterial possesses abilities of amplifying the evanescent wave and concentrating the electromagnetic energy. Accordingly, the metamaterials exhibit great potentials to improve the energy iransmission performance of WPT systems, especially for concentrating the distribution of the magnetic field and enhancing the magnetic flux density. There are increasing studies, which have focused on the combination between metamaterials and WPT technologies. Fig. 12. Classification of materials with respect to ¢ and ju. The energy efficiency is the most important concern of WPT systems. The negative permeability can collect the leaking energy of the induced electromagnetic field and enhance the mutual inductive coupling effect, thus effectively increasing the energy efficiency. Fig. 13 summarizes previous studies on the optimization of the metamaterial slabs for WPT systems, such

The key technical challenge is to increase the coupling coefficient « while reduce the magnetic flux leakage. Besides, the misalignment tolerance is another technical concern for EV wireless charging systems. In previous studies, a number of attempts have been made for aforementioned issues. From the perspective of materials, the high permeability material such as ferrites and the aluminum plate are utilized as the magnetic flux guide and the shield [93], respectively, which can effectively improve the effective magnetic field density and reduce the flux leakage. Besides, the Litz wire consisting multiple strands are commonly used to wrap the transmitting coils in both sides [94], which aims to avoid the skin effect and the proximity effect. Additionally, regarding to the similar dimension, the difference of the coil geometry and configuration results in a significant difference of magnetic coupling effect. Hence, the optimal design is necessary for the coil of EV wireless charging systems. For example, the circular- shaped and square-shaped transmitting coils are mostly used for EV static wireless charging systems [95]. In addition, a new coil topology was proposed by placing two coils shaped like “D” back to back [96], which is called as the double D (DD) structure. By comparing with a conventional single circular- shaped or square-shaped pad, the DD charging pad can effectively enhance the lateral misalignment tolerance, thus increasing the effective charging area for EV drivers. In [97], an asymmetric coil structure was investigated for EV static charging, which can also offer an enhanced misalignment tolerance.

| [72] [73] Energy is encrypted by confidentially regulating the frequency of the power source.