Binh Duc Truong

This work studies the operating principles and performance limits of magnetoelectric transducers, which are promising compact receiver alternatives for wireless powering of biomedical implantable devices. We develop an analytical framework to investigate the maximum power that can be transferred to an electrical load. Although an electrically lossless model is widely used to explore the magnetoelectric transducer capability, we find that in this idealization, allowing a complex-valued load impedance enables the delivered power to attain the physical bound at an arbitrary operating frequency and with any electromechanical coupling strength. This behavior may not be feasible in practice. To reflect a more realistic potential of a magnetoelectric wireless power transfer system, the inevitable electrical losses are taken into account. We derive closed-form expressions for the maximum output power under a general complex load, in both series and parallel configurations, and determine the corresponding optimal conditions. The results show that the optimal power is achieved uniquely at mechanical resonance and increases monotonically with an effective figure of merit, yielding a physically attainable maximum power strictly below the theoretical bound and a finite operating bandwidth. Motivated by implant-size limitations, we formulate a power optimization subject to volume constraints. We establish an analytical condition for the optimal thickness ratio, defined as the ratio between the piezoelectric thickness and the total thickness of the composite. The conventional magnetoelectric voltage coefficient is also considered. It is shown that there exist scenarios in which reducing the thickness ratio can maximize both the power delivered and the voltage coefficient, unifying an optimal setting for wireless power transfer and sensing. These findings provide a comprehensive understanding and a general framework for realizing an efficient magnetoelectric-based system for biomedical implants.

This paper presents a comprehensive analytical investigation into the maximum output power for single-mechanism and hybrid energy harvesting systems under both displacement-unconstrained and displacement-constrained conditions. The electrically lossless single-mechanism harvester is considered an ideal model and treated as a theoretical benchmark for comparison. The electrical losses often encountered in practice are taken into account for a more rigorous and realistic assessment. Under unconstrained operation, two power optimization strategies, the impedance matching and gradient descent methods, are examined. The results reveal that both approaches yield identical optimal solutions for single-mechanism devices, whereas the gradient descent technique proves more suitable for complex hybrid systems, for which conventional impedance matching does not reach the global optimum. Explicit analytical solutions are derived for the maximum output power and associated optimal conditions for both unconstrained and motion-constrained scenarios. It is shown that, while the hybrid architecture may not be substantially beneficial under unconstrained regimes, it offers significant advantages over single-mechanism harvesters in displacement-limited operation under strong excitation. In general, the power improvement depends on the effective figures of merit for the piezoelectric and electromagnetic transducers and the input excitation strength. In particular, the output power from a hybrid energy harvesting system can be up to twice as high as that of a single-mechanism generator when the two figures of merit are comparable. These findings provide critical insights to guide the design and implementation of efficient energy harvesters, the identification and realization of optimal system parameters, and the effective deployment of hybrid approaches in challenging operational environments.

The primary objective of this work is to investigate the performance of a hybrid energy harvesting system consisting of piezoelectric and electromagnetic transducers. We first show that a single–mechanism generator with negligible electrical losses, referred to as an electrically-lossless harvester, can reach the theoretical power bound regardless of the coupling strength between the mechanical and electrical domains, which renders the use of hybrid systems unnecessary. For a more realistic analysis, the electrically parasitic losses are then taken into account. We introduce effective figures of merit for the piezoelectric and electromagnetic generators that combine transducer coupling and resistive losses. The maximum output power of single-transducer and hybrid systems are determined analytically, expressed as functions of effective figures of merit. We find that there is no benefit to utilizing a hybrid system if one of the two, or both, effective figures of merit exceeds a threshold of . We also derive the narrow conditions under which a resonant hybrid harvester system with multiple transduction mechanisms can outperform its counterpart which uses a single energy conversion. In order to provide a comprehensive assessment of the configurations considered, we analyze the relationships between optimizing system efficiency and maximizing output power. We reveal that the two problems generally yield different solutions. However, for a hybrid structure, these objectives result in a unique solution when the effective figures of merit of the two transductions are equal. This is a distinctive property of a hybrid system compared to a single-mechanism device.

Summary: We present a theoretical analysis of a MEMS electrostatic energy harvester configured as Bennet's doubler. The steady‐state operation of the circuit can be approximated by a rectangular Q‐V cycle in the ideal case or by a trapezoid diagram when the electrical losses are taken into account. A similarity between the voltage doubler and charge‐pump circuit is highlighted. The analytical solution of the saturation voltage is derived, providing a more insightful comprehension of the system performance and the influence of its parameters. The results obtained from studying the Q‐V diagram coincide with those of circuit simulations for both cases when using (i) a mathematically idealized diode or (ii) a realistic diode model that includes the electrical losses. A configuration employing a simple switch connected in series with one of the three diodes that can further increase the saturation voltage is investigated, providing an efficient alternative for boosting the output voltage of an electrostatic energy harvester.

Wireless power transfer (WPT) has received increasing attention primarily as a means of recharging batteries in the last few decades. More recently, magnetoelectric (ME) structures have been investigated as alternative receiving antennas in WPT systems. ME structures can be particularly useful for small scale devices since their optimal size is much smaller than traditional receiving coils for a given operating frequency. WPT systems using ME laminate receivers have been shown to be helpful in wirelessly powering various sensors and biomedical implants. In recent years, a large number of studies have been conducted to improve the performance of ME composites, in which various configurations have been proposed, along with the use of different magnetostrictive and piezoelectric materials. In addition, many efforts have been devoted to miniaturizing ME devices. An essential obstacle to overcome is to eliminate the need for a DC bias field that is commonly required for the operation of ME structures. In this review paper, we will discuss the basic principle of ME effects in composites, materials currently in use, various ME receiver structures, performance measures, limitations, challenges, and future perspectives for the field of WPT. Furthermore, we propose a power figure of merit which we use to compare recent ME WPT research papers.

The frequency dependence of the maximum output power and efficiency of two wireless power transfer systems (WPTSs), resonant inductive coupling (RIC) and magnetoelectric (ME), are investigated. We find that in the weak–coupling regime, the power optimization and efficiency maximization problems are equivalent and yield the same optimal load and frequency. These properties apply to both topologies under consideration. Despite the apparent difference in the energy conversion mechanisms, the two structures result in similar explicit forms of maximum power delivered to the load, and so does the optimum transfer efficiency. We discuss the essential role of a figure of merit for each configuration and show how they affect the overall performance. For a weakly–coupled inductive WPTS, both the maximum transferred power and efficiency are positively proportional to drive frequency squared. In the case of a ME–based architecture, the dependence of power and efficiency on frequency is the consequence of the transducer geometry optimization problem, subject to a volume constraint. Under a constant mechanical quality factor condition, both quantities are linearly proportional to the operating frequency. While the focus of this paper is RIC and ME mechanisms, some of the findings are also valid for relevant inductive energy harvesting or magneto–mechano–electric WPTSs.

This article presents an investigation on the optimal design of a transmit coil for a wireless power transfer system (WPTS) for biomedical implants with the goal of maximizing the magnetic flux density (B-field) at the location of the implant while not violating magnetic field exposure limits. Maximizing the B-field correlates to higher transferable electric power for certain WPTSs, such as those that use magnetoelectric receivers. While previous works have optimized the thermal efficiency, or system efficiency, to our knowledge no one yet has developed the procedure to optimize the transmitter to maximize the B-field subject to an imposed safety limit constraint for magnetic field exposure. In addition to this safety constraint, the optimal design of the transmitter is considered when the system geometry or the input current is constrained. Equations for determining the design parameters of an optimal solenoid transmitter are derived subject to constraints on either transmitter size or electric current. If a certain magnetic field strength is required, solutions to the size and current of the transmitter are presented, which allow the desired fields without violating the safety constraint. The mathematical model is experimentally validated, and a case study is described that illustrates the optimal design rules.

This article presents a thorough analysis and an equivalent circuit model of a wireless power transfer system utilizing magnetoelectric (ME) effects. Based on two-port theory, explicit analytical solutions of, (i) the ME coefficient α M E (defined by the derivative of the generated electric field with respect to the applied magnetic field), and (ii) the power transferred to a load resistance, are derived and rigorously validated by experiments. The compact closed-forms of the optimal load and its corresponding maximum output power are developed. In our particular experimental system, a power of ∼10 mW is attained at an applied magnetic flux density of 318.9 µ T with a laminated composite made by two Galfenol and one PZT layers. While α M E is widely used in the literature as a standard criterion to evaluate the performance of a ME transducer, we reveal that larger α M E does not always ensure higher optimum power delivered to the load. Instead, we quantify the essential influences of each magnetostrictive and piezoelectric phases on the maximum obtainable power. We show that the transduction factor between the magnetic and mechanical domains is often more critical for power optimization than the mechanical-electrical transduction factor as it determines and limits the maximum power available for transfer to a resistive load.

We read with interest an article by Aditya and Williamson, published in the IEEE TRANSACTION ON INDUSTRIAL ELECTRONICS (vol. 66, no. 5, pp. 3973-3982, May 2019). In this work, an efficient set of design guidelines was proposed in order to eliminate the bifurcation phenomenon occurred in a magnetic resonant wireless power transfer system (WPTS) configured in series. The central objective of the method is to restrict the minimum distance, and therefore the coupling coefficient, between the transmitter and receiver. However, we revealed that the condition used by the authors is not necessarily correct in general. This letter provides corrections and further analysis of this issue. Our results show that the bifurcation behavior is not a universal property; there exist certain regions of system parameters at which it is not present for any coupling strength. This theoretical hypothesis is validated experimentally. The fundamental dynamics of the two-coil resonant coupled WPTS are also investigated.