Contrasts between maximum power transfer and maximum efficiency

Maximum power transfer and maximum efficiency are two important design constraints in wireless power transfer applications. Several works have investigated the proper load and impedance match conditions to optimize either efficiency or power transfer. In this paper we show that the optimal load for maximum power transfer and maximum efficiency is the same (a conjugate matched load) when the source resistance is zero. This is important, as many WPT systems have a relatively low, unknown source impedance. Since the optimal load for both efficiency and power is the same as the source impedance approaches zero, the designer can use a bi-conjugate load for a near optimal design for both maximum power and efficiency. As the source impedance becomes significant, the bi-conjugate matched system provides higher power, but at the expense of lower efficiency. Maximum efficiency is achieved with a non-bi-conjugate load, when the source impedance is non-negligible.

We derive the Z-parameters for the two coupled antennas used for wireless power transfer under the assumption that the antennas are canonical minimum scattering antennas. Using the Z-parameter and the maximum power transfer efficiency formula, we determine the maximum power transfer efficiency of wireless power transfer systems. The results showed that the maximum power transfer efficiency increases as the mode number or the radiation efficiency increases. To verify the theory, we fabricate and measure two different power transfer systems: one comprises two antennas generating <TEX>$TM_{01}$</TEX> mode; the other comprises two antennas generating <TEX>$TM_{02}$</TEX> mode. When the distance between the centers of the antennas was 30 cm, the maximum power transfer efficiency of the antennas generating the <TEX>$TM_{02}$</TEX> mode increased by 62 % compared to that of the antennas generating the <TEX>$TM_{01}$</TEX> mode.

ABSTRACTA tunable optimal load circuit (OLC) for maximum wireless power transfer efficiency is presented for a magnetically coupled resonant system at 13.56 MHz. The OLC is required to transform the standardized 50 Ω receiver impedance to an optimal load to achieve maximum efficiency without using cumbersome feeding loops. The OLC based on a lumped‐element impedance converter is fabricated on a FR‐4 substrate. The size of the OLC is 25 × 50 mm. The series branch of the OLC is composed of a tunable series capacitor block and an inductor. The shunt branch of the OLC consists of a varactor and a capacitor block with switches. The theoretical and experimental results both showed that even though the loss in the inductor somewhat limits the achievable maximum efficiency, the efficiency with OLC is significantly enhanced compared with that without OLC. © 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:2619–2622, 2014

The use of resonant magneto-inductive links is an efficient technique to transfer power over midranges in the field of wireless power transfer (WPT). Power transfer efficiency (PTE) and power delivered to the load (PDL) are two important indicators of power transmission for a WPT system. A predetermined amount of PDL need be generated at maximum PTE for practical application in a WPT system; thus, the authors focus on maximising PTE and a predetermined amount of PDL for transfer in this study. First, load-matching conditions are presented for maximum PTE transfer though circuit theory-based analyses. Then, a new tuning method is proposed to transfer a predetermined amount of PDL which can be controlled by source-matching distance at maximum PTE. The relations between the loss of a load-matching resonator and the PTE (or PDL) of a correspondingly unmatched system are derived though analyses as well. Finally, the calculated results of the design examples are verified through electromagnetic simulations and experimental measurements.

Dimension effects of a magnetoelectric gyrator with FeCoSiB/Pb(Zr,Ti)O3 layered composites core for efficient power conversion

This paper presents a design methodology for RF power transmission to millimeter-sized implantable biomedical devices. The optimal operating frequency and coil geometries are found such that power transfer efficiency (PTE) and tissue-loss-constrained allowed power are maximized. We define receiver power reception susceptibility (Rx-PRS) and transmitter figure of merit (Tx-FoM) such that their multiplication yields the PTE. Rx-PRS and Tx-FoM define the roles of the Rx and Tx in the PTE, respectively. First, the optimal Rx coil geometry and operating frequency range are identified such that the Rx-PRS is maximized for given implant constraints. Since the Rx is very small and has lesser design freedom than the Tx, the overall operating frequency is restricted mainly by the Rx. Rx-PRS identifies such operating frequency constraint imposed by the Rx. Secondly, the Tx coil geometry is selected such that the Tx-FoM is maximized under the frequency constraint at which the Rx-PRS was saturated. This aligns the target frequency range of Tx optimization with the frequency range at which Rx performance is high, resulting in the maximum PTE. Finally, we have found that even in the frequency range at which the PTE is relatively flat, the tissue loss per unit delivered power can be significantly different for each frequency. The Rx-PRS can predict the frequency range at which the tissue loss per unit delivered power is minimized while PTE is maintained high. In this way, frequency adjustment for the PTE and tissue-loss-constrained allowed power is realized by characterizing the Rx-PRS. The design procedure was verified through full-wave electromagnetic field simulations and measurements using de-embedding method. A prototype implant, 1 mm in diameter, achieved PTE of 0.56% ( -22.5 dB) and power delivered to load (PDL) was 224 μW at 200 MHz with 12 mm Tx-to-Rx separation in the tissue environment.

With the extensive applications of magnetically coupled resonant wireless power transfer (WPT), improving the power transfer efficiency (PTE) of WPT systems is critical. Magnetic metamaterials (MTMs) can effectively improve the PTE. However, currently the design of MTMs is mainly based on microwave or optical theory, and their applicability in low-frequency WPT systems needs to be further explored. In this paper, a fast and effective method for the design and optimization of low-frequency MTM is provided. Firstly, based on circuit theory and mutual inductance calculation, the PTE of WPT systems is concluded in an equation that concerns the position of a 2-D MTM slab and compensation capacitance of MTM unit coils. Two WPT systems with large differences in coil size of the transmitter (Tx) and receiver (Rx) and the same size are considered. In addition, a directly separating variables method is proposed to optimize the MTM slab position and the compensation capacitance of unit coils to maximize PTE when the MTM slab position changes axially between Tx and Rx. It is found that the optimal MTM slab positions for the two systems are remarkably different, and the PTE impressively presents a capacitance splitting phenomenon with the change of compensation capacitance. Finally, two MTM-enhanced WPT systems are established to verify the theoretical calculation. The experiments show that the maximum PTE is significantly increased by 144&#x0025; and 31&#x0025; respectively by adopting optimized compensation capacitance compared with that without MTM for the two systems with different or same transceiver coil size.

Drones can be used in agriculture applications to monitor crop yield and climate conditions and to extend the communication range of wireless sensor networks in monitoring areas. However, monitoring the climate conditions in agriculture applications faces challenges and limitations, such as drone flight time, power consumption, and communication distance, which are addressed in this study. Wireless power transfer (WPT) can be used to charge drone batteries. WPT using a magnetic resonant coupling (MRC) technique was considered in this study because it allows high transfer power and efficiency with tens of centimeters, power transfers can be achieved in misalignment situations, charging several devices simultaneously, and unaffected by weather conditions. WPT was practically implemented based on a solar cell using a proposed flat spiral coil (FSC) in the transmitter circuit and multiturn coil (MTC) in a receiver circuit (drone) for the alignment and misalignment of two coils at different distances. FSC and MTC improved power transfer and efficiency to 20.46 W and 85.25%, respectively, at 0 cm with the loaded system under alignment condition. In addition, the two coils achieved appropriate transfer efficiencies and power for charging the drone battery under misaligned conditions. The maximum power transfer and efficiency were 17.1 W and 71% for the misalignment condition, at an air gap of 1 cm between two coils when the system was loaded with the drone battery. Moreover, the battery life of the drone was extended to 851 minutes based on the proposed sleep/active strategy relative to the traditional operation (i.e., 25.84 minutes). Consequently, a 96.9% battery power saving was achieved based on this strategy. Comparison results showed that the proposed system outperformed some present techniques in terms of the transfer power, transfer efficiency, and drone battery life. The proposed WPT technique developed in this study has been proven to solve the misalignment issue. Thus it offers a great opportunity as a key deployment component for the automation of farming practices toward the Internet of Farming applications.

In this paper, we show that there exists an optimal number of coils for maximum wireless power transfer efficiency through systems of identical cascaded resonators. We apply convex optimization to determine the ideal spacing of a system in which only the direct coupling between adjacent resonators is considered. We then model the high-order coupling as a perturbation to this system and solve for the optimal spacing. We show that as the number of coils in a system increases, the efficiency for a given source-to-load distance reaches a peak and then starts to drop. Beyond this point, the losses introduced by each additional resonator begin to outweigh the high efficiency achieved by increasingly tight resonator coupling.

Since the concept of wireless power transfer has been proposed and implemented by Tesla over a hundred years ago, it has obtained lots of breakthroughs and applied in many potential fields especially in implantable biomedical devices [1–2]. The implantable biomedical devices are surgically introduces into the human body to rebuild body function, achieve a better quality of life, and expand longevity. With the development of microelectronics, biotechnology, and materials, the industry of implantable biomedical devices grows up quickly. The researches show that in America 8% to 10% of the people have experienced an implantable medical device, while in industrialized countries 5% to 6% of population have consumption demand in implantable medical devices. How to provide a constant power supply, for implantable medical devices, becomes a restricted problem. In order to guarantee the implanted medical devices to work normally, the energy storage or harvesting components should be included in these devices. In the traditional implantable medical devices, there are two ways to provide power for the devices. The common method is the implantable batteries are used to be as a power supply, and the other method is to transfer the power in vitro to the devices with the lead wires through the skin. For implantable batteries, their lifetime, size and toxic composition will lead to potential hazard to the patients. The transcutaneous wires, as well, will bring infection and reliability problems. In order to address above problems, the technology of wireless power transfer is used in the biomedical devices. The system has two parts in which one part including power supply and the transmitter coil is placed outside the patients’ body, and the other part including the receiver coil is placed in the patients’ body. For this system, the receiver coil should be limited as small as possible and the power transfer efficiency should be as high as possible. Different from the traditional two-coil wireless power transfer system, a novel three-coil system is proposed in this paper. In the novel system, there are two rectangular transmitter coils and one circular receiver coil. To obtain the maximum power transfer efficiency, the system is optimal designed according to the following step as shown in Fig. 1. Firstly, the size and the geometry of the receiver coil are determined. A circular coil with a radius of 10mm is chosen as the receiver coil. Secondly, the distribution of the magnetic field generated by the rectangular coil is analyzed based on the Biot-Savart’s law. Then the relationship between the distance and the magnetic field intensity will be obtained and expressed as a formula according to the analysis result. Finally, the above formula is chosen as the optimization constrains and the power transfer efficiency is chosen as the optimization objective function. After the optimal designing, the optimal parameters of the transmitter coils are obtained and verified through the experimental system as shown in Fig. 2. The power transfer distance is about 30cm, and the power received by the receiver coil can achieve about 5W. The detailed results will be given in the full paper.

This paper presents a resonant wireless power transfer method that leverages a 90-degree voltage phase shift between the transmitting and receiving coils to enhance efficiency and maximize power transfer. When the resonant coupling is achieved, the secondary coil with an adjustable capacitor forms a tuned LC circuit. If the primary coil is driven at the resonant frequency of both the primary and secondary sides, the system can transmit 250W of power between the coils over a distance of 50 cm. Using a single power transmitting unit (PTU) board with multiple paralleled gallium nitride high-electron-mobility transistors (GaN HEMTs), the system achieves a maximum power transfer efficiency of 88%, highlighting the effectiveness of the design in high-efficiency, long-distance wireless power transmission. The key to the success of high-power, high-efficiency RWPT is in exhibiting the imaginary turn ratio presented on the air transformer. The imaginary turn ratio can realize the negative impedance conversion that converts the positive resistance on the power-receiving unit into a negative one, and thus, the damping of the resonance oscillation becomes negative and positively encourages more power to be delivered to the power-receiving unit (PRU) load. This paper derives the theory of the imaginary turn ratio and demonstrates the implementation of the RWPT system that exhibits the imaginary turn ratio effect.

In this paper, we propose an efficient wireless power transfer (WPT) system at 1050 kHz for an active capsule endoscope. The performance of the proposed WPT system is evaluated by placing a capsule receive coil in a hollow body phantom as well as that filled with colon-equivalent liquid. The power transfer efficiency has been studied with respect to the operating frequency and the load resistance. The measured results show that the maximum power transfer efficiency of 20.4% and 17.47% can be achieved in a hollow and a liquid phantom, respectively.

Unmanned Aerial Vehicles (UAVs) can charge Wireless Rechargeable Sensor Networks (WRSNs) in remote or hard to access locations. However, the charging efficiency is heavily affected by the distance between the wireless transmitter and receiver. This efficiency impacts the possible power level increase of each charged node. Most charging algorithms require full knowledge of sensor nodes' power levels to identify the nodes to charge. Collecting this power information adds overhead to the network and limits scalability. We propose and implement Charging with Power Transfer Efficiency Compensation (CPTEC), an algorithm that charges a WRSN without the need for a priori knowledge of the nodes' power levels. We show that CPTEC compensates for efficiency drops, due to landing alignments, making it practical for real-world power transfer scenarios. Our results show that CPTEC is able to perform with a median at ≈ 72% of the optimal performance of a full knowledge algorithm that assumes maximum power transfer efficiency, while other work drops to ≈ 22%. Under constant maximum efficiency CPTEC performs ≈ 90% of the optimal full knowledge case.

The efficiency of wireless power transfer (WPT) always suffers from misalignment between transmitting and receiving directional antennas. In this letter, a maximum radiation direction alignment method is proposed for WPT using the unexploited third harmonic generated from rectifiers. The widely used second harmonic is untouched, retaining the possibility of its further recycling. A hybrid coupler is used for the received power distributing and third-harmonic coupling simultaneously. Due to the nonlinearity of diodes, the third harmonic is generated during the rectification and reflected back by the rectifier filter. Its power is completely returned to the base station by the same hybrid and another antenna pair working at the third-harmonic frequency. Maximum power transfer is thus realized when both transmitting and receiving antennas are aligned. The proposed method is validated by Advanced Design System simulation and measurement.

Dynamic wireless power transfer (DWPT) systems for in-motion electric vehicle (EV) charging often suffer from unstable power delivery due to spatial variations in magnetic coupling caused by vehicle misalignment. This study presents a stabilization-oriented DWPT design methodology that prioritizes minimizing spatial variations of mutual inductance rather than maximizing peak coupling under perfect alignment. A ferrite-backed double-D coil configuration is analyzed and refined using three-dimensional finite-element electromagnetic modeling integrated with circuit-level co-simulation to evaluate coupling behavior, magnetic field homogeneity, and power transfer efficiency under realistic dynamic misalignment conditions. The proposed design achieves a coupling coefficient of 0.50–0.55 under aligned conditions and exhibits smooth, predictable degradation for lateral offsets up to 40–50 mm. Quantitative analysis demonstrates a low spatial coupling gradient of approximately 0.001 mm−1, indicating that abrupt coupling transitions are effectively suppressed during vehicle motion. The system attains a maximum power transfer efficiency of 84.37% at an 80 mm air gap, while maintaining stable performance under both lateral and vertical displacement. Comparative evaluation shows improved misalignment tolerance and coupling stability relative to conventional double-D configurations. The results demonstrate that electromagnetic field shaping focused on coupling smoothness is an effective and practical strategy for reliable dynamic wireless charging of electric vehicles.