Inductive Power Transfer Link Research Articles

Self and Mutual Inductance Behavioral Modeling of Square-Shaped IPT Coils With Air Gap and Ferrite Core Plates

The design and optimization of coils for Inductive Power Transfer (IPT) systems is an iterative process conducted in Finite Element (FE) tools that takes a lot of time and computational resources. In order to overcome such limitations in the design process, new empirical equations for the evaluation of the self-inductance and mutual inductance values are proposed in this work. By means of a multi-objective genetic programming algorithm, the self-inductance, the mutual inductance and the coupling factor values obtained from FE simulations of IPT link are accounted by analytical equations, based on the geometric parameters defining the IPT link. The behavioral modeling results are compared with both FE-based and experimental results, showing a good accuracy.

Inductive Power Transfer Link at 13.56 MHz for Leadless Cardiac Pacemakers

Inductive power links are most viable for the long-term powering of cardiac pacemakers. Designing an inductive power link without surpassing the specific absorption rate (SAR) for modern leadless cardiac pacemakers (LCPs) remains a challenging task because of its size and implantation depth. The inductive power link employed in the conventional works is either designed at a high frequency or based on the size, shape, weight, and implantation depth of conventional cardiac pacemakers. Here, a 3-coil inductive power transfer link with a circular transmitter coil and solenoidal receiver coil is designed at 13.56 MHz to provide uninterrupted power to the modern LCPs. Considering the food and drug administration approved term for implant size of modern LCP, the receiver coil is designed with 6 mm diameter and 6.5 mm length. The performance of the link has been verified through simulations and measurements under perfect alignment, lateral and/or angular misalignments, and distance variation between the coils. At a 50 mm horizontal distance between transmitter and receiver coils, the transmission coefficient is −30.9 dB. The maximum simulated average SAR at heterogeneous phantom is 0.30 W/kg, which is lower than the limit set by the Federal Communications Commission for radiation threshold exposure. Experiments conducted on pork’s heart verified the reliability of the simulated results. At a 50 mm distance between the coils, the measured transmission coefficient is −34 dB, and at an input power of 1 W, the power delivered to the load is 0.7 mW.

A novel contactless, feedbackless and sensorless power delivery link to electromagnetic levitation melting system residing in sealed compartment

The paper presents practical considerations and guidelines for designing an improved-efficiency contactless inductive power transfer link, delivering DC power in step-down mode to an electromagnetic levitation melting system residing in a sealed compartment. The proposed arrangement employs none-series compensated inductive power transfer link operating at the so-called “load-independent-voltage-output” frequency, allowing feedback-less and sensor-less design. However, the output voltage of a system operating at such frequency remains influenced by the load to some extent, residing within a certain range with boundary values (minimum and maximum) corresponding to maximum (rated) and minimum load, respectively. Consequently, the proposed contactless power delivery system is first analyzed using both first harmonic approximation (frequency-domain) based approach and differential equations (time-domain) based method to obtain corresponding output voltage bounds. Then, coil-to-coil efficiency and optimal load matching factor are determined. Comparison with both typically utilized symmetrical series-series and recently proposed series-none compensation topologies reveals the superiority of none-series compensation topology in terms of coil-to-coil efficiency for the whole range of coupling coefficients. Simulations and experiments of 400 V, 1 kW none-to-series compensated inductive wireless power transfer link, delivering contactless energy to a 250 V electromagnetic levitation melting system, demonstrate excellent matching and accurately validate the presented analysis.

Modified first harmonic approximation-based modeling of SN-compensated inductive power transfer links operating at load-independent-voltage-output frequency

Some popular applications of wireless power transfer (such as electric vehicle charging) include a transmitter residing on a static platform (charging station) and a dynamic receiver (vehicle). In such applications, compensation may be excluded from the receiver in order to minimize, simplify and lower the cost. If transmitter compensation is also maximally simplified series to none (SN) compensation topology is created. It was recently shown that classical first harmonic approximation (FHA) based analysis fails to accurately predict RMS values of system currents, thus overestimating the achievable coil-to-coil efficiency. Consequently, this paper establishes modified FHA (MFHA) equivalent circuit of SN-compensated inductive power transfer link (IPTL), utilizing analytical expressions of primary and secondary currents based on time-domain differential equations (DE) based solution. It is shown that the outcomes of proposed MFHA and DE methods are very close, justifying the proposed methodology. Simulations and experiments demonstrate excellent matching, validating the presented analysis.

A Design Technique for Optimizing Resonant Coils and the Energy Transfer of Inductive Links

Power transfer efficiency (PTE) is a key performance parameter in development work on resonant inductive power transfer (IPT) systems. Geometrically optimizing the transmitter (Tx) and receiver (Rx) coil pair is a way of improving the IPT system's efficiency. In this article, a new figure-of-merit (FoM) is proposed to find an optimum coil geometry which maximizes the PTE. The employed FoM parameter, called the “strong coupling factor” (P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">scf</sub> ), is defined such that its value indicates how strongly the Tx and Rx coils are linked together. Considering the IPT application and its physical size constraints, a proper selection method for identifying the numerical value of P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">scf</sub> is essential for optimal coil geometry design. This article presents an iterative algorithm to assist in the selection of the most favorable P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">scf</sub> value which provides maximized PTE for the designed optimum coil geometry. Design examples of two nominal IPT systems at frequencies of 415 and 0.1 MHz are used to investigate the design algorithm. Theoretical calculations show the optimum geometry designed for the IPT system operating at 415 MHz, with coupling coefficient (K) of 0.2, can achieve maximum PTE of 85.70%. Measurements presented from a practical Tx/Rx coil pair in the IPT link operating at 0.1 MHz, with K = 0.05, show a PTE of 83.10% against a calculated PTE of 84.11% validating the design process.

Dynamic Capabilities of Multi-MHz Inductive Power Transfer Systems Demonstrated With Batteryless Drones

This paper presents the design of a multi-MHz inductive power transfer (IPT) system showcasing lightweight and energy-efficient solutions for non-radiative wireless power transfer. A proof of concept is developed by powering a drone without a battery that can hover freely in proximity to an IPT transmitter. The most challenging aspect, addressed here for the first time, is the complete system-level design to efficiently provide uninterrupted power flow while allowing for variable power demand and highly variable coupling factor. The proposed solution includes the design of lightweight air-core coils that can achieve sufficient coupling without degrading the aerodynamics of the drone, and the design of newly developed resonant power converters at both ends of the system. At the transmitting-end, a load-independent Class EF inverter, which can drive a transmitting-coil with constant current amplitude and achieves zero-voltage switching for the entire range of operation, was developed; and at the receiving-end, a hybrid Class E rectifier, which allows tuning for large changes in coupling and power demand, was used. For the demo, the range of motion of the drone was limited by a 7.5 cm nylon string tether, connected between the center of the transmitting-coil and the bottom of the drone. The design of the IPT system, including all the power conversion stages and the IPT link, is explained in detail. The results on performance and specific practical considerations required for the physical implementation are provided. An average end-to-end efficiency of 60% was achieved for a coupling range of 23%-5.8%. Relevant simulations concerning human exposure to electromagnetic fields are also included to assure that the demo is safe, according to the relevant guidelines. This paper is accompanied by a video featuring the proposed IPT system.

Multi-MHz IPT Systems for Variable Coupling

In typical multi-MHz inductive power transfer (IPT) systems, a change in coupling or load resistance can significantly deteriorate the end-to-end efficiency due to a deviation from the optimal load of the IPT link and suboptimal operation of the resonant inverter due to the loss of soft switching condition. This paper proposes solutions for an IPT system to operate efficiently when large changes in coupling take place. To achieve high power efficiency independent of coupling, we utilize inherent regulation properties of resonant converters to avoid losing soft switching for any coupling value, and present the optimal load to the IPT link at the maximum energy throughput coupling. A probability-based model is introduced to assess and optimize the IPT system by analyzing coupling as a distribution in time, which depends on the dynamic behavior of the wireless charging system. The proposed circuits are a Class D rectifier with a resistance compression network in the receiving end and a load-independent Class EF inverter in the transmitting end. Experiments were performed at 6.78 and 13.56 MHz verifying high efficiency for dynamic coupling and variable load resistance. End-to-end efficiencies of up to 88% are achieved at a coil separation larger than one coil radius for a system capable of supplying 150 W to the load, and the energy efficiency was measured at 80% when performing a uniformly distributed linear misalignment of 0–12.5 cm, corresponding to a receiver moving at a constant velocity over a transmitter without power throughput control.

Variability Analysis of Efficiency and Output Power of an Inductive Power Transfer Link

The wireless power transfer (WPT) technique plays an important role in powering remote devices. The power delivered to the load (PDL) and power transfer efficiency (PTE) are both obtained by knowing the parameters of the WPT system. The knowledge of such parameters and its variability can aid the designer to establish the most important parameters for maximizing the PDL or PTE. This paper presents a variability analysis of PDL and PTE when the WPT parameters change their values. Each parameter can be measured with an associated variability range. The variations in the PDL and PTE functions are computed by using this range of variability of the input parameters. The sensitivities of PDL and PTE with respect to each parameter are also identified with the presented analysis. Experimental measurements were taken on an inductive link, which was specially designed for a recharging battery system of a wireless sensor node with a 5-mm gap between the transmitting and receiving coils. The results pointed out that only small ranges on the input rated values can be tolerated to avoid significant distortions on the output distribution and thus the deviation of the optimal operation point.

Towards Fully Integrated Low-Cost Inductive Powered CMOS Wireless Temperature Sensor

We describe the complete monolithic integration of an inductive powered wireless sensor system using commercial complementary metal–oxide–semiconductor technology. The wireless system includes: two on-chip inductors used for communication and polarization, a cross-coupled oscillator for communication ( $\text{935.6 MHz}$ ), an inductive coupled energy receiver system ( $\text{13.5 MHz}$ ), a signal converter that converts constant current signals into pulses of modulated width, an asynchronous controller for autonomous operation of the chip, and an integrated temperature sensor. The chip was wirelessly biased by an inductive power transfer link at a distance of $\text{2 mm}$ . The signal transmitted by the chip was detected by a spectral analyzer and a dipole antenna up to a distance of $\text{110 cm}$ with an intensity of $\text{-88 dBm}$ . The integrated temperature sensor shows a linear response ( $R^{2} = 0.9968$ ) and presents a sensitivity of $\text{192, 94}\text{ Hz}/^{\circ }\text{C}$ . The full system occupies $74.93\%$ of an area of $\text{1.5 mm}$ $\times$ $\text{1.5 mm}$ . This wireless system is functional out-of-the-box without the need for additional components or post-processes. This integration leads to more robust, smaller, and a potentially cheaper wireless sensor systems, and provides new opportunities for size- and cost-sensitive applications.

A novel data transfer technique for bio-implantable devices through the inductive power transfer link

A novel technique for transferring data to biomedical implantable devices through the inductive power transfer link is presented. The new modulation technique presented in this paper is based on changing the duty cycle of the switching pulse of the class E power amplifier which drives the external coil. Hence, we call it duty cycle shift keying (DCSK). Inductive link efficiency and voltage gain are analyzed for the DCSK technique. Based on the mathematical analysis of the proposed technique its bit error rate is close to that of the BFSK. However, it can achieve a data rate to carrier frequency of 100 %. The modulator and demodulator of the proposed technique are simple and make it suitable for bio-implantable devices. The proposed circuit is simulated by advanced design system simulator using the 0.18 μm CMOS technology. Moreover, in order to verify the effectiveness of the proposed technique, a test setup is implemented using off-the-shelf components. The simulation as well as measurement results will be provided in this article.

Design and Optimization of a 3-Coil Inductive Link for Efficient Wireless Power Transmission

Inductive power transmission is widely used to energize implantable microelectronic devices (IMDs), recharge batteries, and energy harvesters. Power transfer efficiency (PTE) and power delivered to the load (PDL) are two key parameters in wireless links, which affect the energy source specifications, heat dissipation, power transmission range, and interference with other devices. To improve the PTE, a 4-coil inductive link has been recently proposed. Through a comprehensive circuit based analysis that can guide a design and optimization scheme, we have shown that despite achieving high PTE at larger coil separations, the 4-coil inductive links fail to achieve a high PDL. Instead, we have proposed a 3-coil inductive power transfer link with comparable PTE over its 4-coil counterpart at large coupling distances, which can also achieve high PDL. We have also devised an iterative design methodology that provides the optimal coil geometries in a 3-coil inductive power transfer link. Design examples of 2-, 3-, and 4-coil inductive links have been presented, and optimized for 13.56 MHz carrier frequency and 12 cm coupling distance, showing PTEs of 15%, 37%, and 35%, respectively. At this distance, the PDL of the proposed 3-coil inductive link is 1.5 and 59 times higher than its equivalent 2- and 4-coil links, respectively. For short coupling distances, however, 2-coil links remain the optimal choice when a high PDL is required, while 4-coil links are preferred when the driver has large output resistance or small power is needed. These results have been verified through simulations and measurements.