With the development of the Internet of Things (IoT), wireless pushbuttons are being increasingly used to control devices such as lights, fans, and air conditioners in smart homes and offices. In contrast to conventional systems, self-powered pushbuttons eliminate the inconvenience of cable arrangement and the cost of battery replacement. However, existing self-powered wireless pushbuttons can only send commands unidirectionally and lack an instant feedback mechanism. This limitation reduces system reliability and impairs the user experience. To tackle this issue, we propose ViPSN-button, a motion-powered wireless pushbutton that offers instant feedback. When the ViPSN-button is pressed, it can deliver instant feedback to the user. This feedback mechanism enables the user to confirm whether the host unit has successfully acknowledged their command. ViPSN-button is powered by a quasi-static-toggling energy harvester (QST harvester). The entire communication process of the ViPSN-button includes three steps: sending commands, receiving acknowledgments (ACK), and displaying an indication. All of these actions are carried out solely by utilizing the energy generated from a single press action. Experiments demonstrate that the ViPSN-button can achieve low-power, fast, private, and reliable bidirectional communication under the Enhanced ShockBurst (ESB) communication protocol. Field tests have been conducted, showing that the ViPSN-button can achieve reliable communication at a distance of 50 meters. ViPSN-button offers an innovative design concept for self-powered sensing nodes, facilitating bidirectional communication between host units and sensing nodes. This feature renders it highly suitable for a wide range of applications, including smart homes, smart offices, smart cities, and industrial IoT.

As an energy storage technology, supercapacitors feature a high power density. In particular, supercapacitors can be charged or discharged by a relatively large pulse current for a limited period of time. This paper takes advantage of this characteristic and develops a dual-mode fast charger for supercapacitors that employs both continuous and fine-tuned pulse currents. Based on the conventional forward converter, the proposed charger introduces an energy storage capacitor and a branch resistor to tune the rising and falling edges of the pulse current, respectively. Consequently, the transitions between the continuous and pulse current modes are significantly accelerated, which ultimately shortens the supercapacitor charging time. A prototype is built and tested using a 3 V/6 F supercapacitor. To charge the supercapacitor from 2 to 2.5 V, the proposed charger takes 0.87 and 1 s when it is configured to operate in the dual-mode and the continuous current mode only, respectively, which leads to a 13% reduction in the charging time. Moreover, compared to the forward converter, the pulse characteristics of the proposed charger are significantly improved in that the pulse rising and falling times are dramatically reduced: 2.1 versus$147~\mu $s and 7.2 versus$103~\mu $s, respectively.

Energy harvesting (EH) provides a promising solution for powering distributed Internet of Things (IoT) devices. Due to the low-level and sporadic ambient energy supply, an EH-powered device should operate in an intermittent and energy-driven mode. Commercial voltage supervisors were not optimized for the EH scenario, making it difficult to satisfy all new demands. The conventional energy management (EM) circuit has a risk of locking up during the turn-ON transient; therefore, it might fail to power the IoT load device. Previous technologies have used a relatively large circuit to solve this problem. In this article, a concise discrete three-transistor EM (3T-EM) circuit is proposed. It can track stored energy, switch ON/OFF to the load device, and provide a regulated voltage output. These key functions are realized by utilizing a minimum number of components; therefore, power consumption and manufacturing cost are largely cut. The voltage thresholds and minimum input current are theoretically derived. In experiments, the ON/OFF thresholds can be adjusted accurately, as predicted by the theory. The 3T-EM circuit can ensure the correct operation when the input current is as low as$0.4~ \mu \text{A}$. Control experiments also prove the effectiveness and performance of the 3T-EM circuit. The proposed 3T-EM circuit shows the characteristics of low cost, low power, inherent regulation, high voltage rating, and good predictability. It is a good candidate to perform the EM task in widely distributed EH-powered IoT devices.

Synchronous switch (SS) technique has been extensively studied in piezoelectric energy harvesting (PEH). The SS circuits can significantly enhance the output power under the same vibration excitation. Some SS solutions have also been developed for an inductive electromagnetic (EM) source by referring to its capacitive PEH counterpart and taking a reciprocal design. This paper proposes a synchronized current inversion and energy extraction (SCIEE) circuit for EM energy harvesting (EMEH). SCIEE utilizes two switched capacitive branches to carry out the synchronized current inversion at the electromotive voltage negative-to-positive zero-crossing instants and energy extraction at the voltage positive-to-negative zero-crossing instants. By inverting the transducer current, SCIEE increases the torque/force inside the transducer to extract more energy from the relative movement between magnets and coils. Theoretical analysis shows that the proposed circuit is suitable for use with an EM transducer, whose quality factor is relatively large. Experiments compared the output power of three harvesting schemes: SCIEE, synchronized switch energy extraction (SSEE), and conventional pulse-width modulation (PWM)-based harvesting scheme. When using the same prototyped EM harvester under the same mechanical excitation, SCIEE can harvest 38% more power, compared with the cutting-edge SSEE circuit for EMEH; and 900% more power, compared with the PWM-based harvesting scheme.

Due to the compatibility considerations, it is not attractive for the commercial wireless charger to modify the Qi-standard coils for charging multiple loads. This paper would explore the potential of a power relay module (PX) to address this issue. The multiple-coil PX would enhance the effective coupling when the standard coupler fails. In this paper, different types of PXs are developed for various applications, including a two-coil PX using series compensation, a two-coil PX using high-order compensation, and a three-coil PX using high-order compensation. Their power and efficiency characteristics are analyzed in a uniform manner, and the benefits of different PXs are justified through a planar charger and a bowl-shape charger in the experiment. The implemented bowl-shape charger is able to offer one fast-charging channel for a single device (30 W) and one multiple-load channel for at most four devices (each 10 W) simultaneously. The peak efficiency is 88% when the overall delivered power of PX is 70W.

Due to the limited size, the existing Qi standard based wireless chargers are not attractive and even fail to charge multiple devices. This paper explores the passive power relay module to extend the multiple-load capability cost-effectively without modifying the original transmitter. A power relay module constructed with three layers is proposed to rebuild an efficient power transfer path. When it is inserted between the transmitter and receivers, the power will be extracted from the transmitter by the bottom-layer auxiliary relay coil, then delivered to the top-layer relay coil through wired connection, and finally picked up by the receivers. A middle magnetic shield layer is also adopted to simplify the mutual coupling relations. The equivalent circuit model of the proposed system configuration is given based on fundamental harmonic approximation, and further to analyze the voltage gain and power transfer efficiency. Finally, a prototype system is built and measured under various load conditions. The almost load-independent output voltage is achieved, and the peak efficiency reaches 87%.

To improve the efficiency of hybrid energy storage systems composed of fuel cells and supercapacitors used in high-power applications such as electrified transportation systems and renewable energy systems, the interfacing power converters need to be carefully designed. This paper proposes an active clamping current-fed three port converter for an application scenario in which modular converters are required to aggregate distributed energy storage resources. The converter topology is conceived to implement four operation modes: single input dual output, dual input single output, and two scenarios of single input single output. These operation modes are combinations of three types of power flows: fuel cell to load, fuel cell to supercapacitor, and supercapacitor to load. Simulation results verify the functionality of the proposed converter.

A well-performed capacitive power transfer (CPT) system highly depends on its compensation to achieve load-independent (LI) output and zero phase angle (ZPA) operation. It is particularly meaningful to maintain these objectives under various misalignments. This paper is devoted to a general and comprehensive method to study and explore all the potential high-order compensations. The coupler parameter variation is evaluated under different types of misalignment, and misalignment-insensitive (MI) parameters are properly designed for MI resonance. A general and straightforward decomposition and synthesis method is then proposed to generate the high-order resonant tanks. With the help of inverse ABCD parameters, all tank candidates are further judged by their capability for LI output and ZPA operation. Finally, two example CPT systems are built to verify the LI output and ZPA operation under misalignment.

This paper proposes a bidirectional inductive power transfer(IPT) system using differential Class E converters. In order to have a good performance for high frequency applications, load-independent differential Class E converters are employed on both primary and secondary sides. This paper gives the design procedure and shows the modulation method. Finally, simulation and experiment are given to verify the system output characteristics. It is shown that the system power is modulated by the input dc voltage and the duty cycle on each side. High efficiency is achieved under a wide load range.

In this article, we introduce a vibration-powered sensing node (ViPSN), a programmable Internet-of-Things (IoT) platform for the development of vibration-powered or motion-powered sensing and transmitting systems. It leverages the exploitation and utilization of ambient vibration energy by using a piezoelectric transducer. The roles and relations of six necessary modules, including energy generation unit (EGU), energy transduction unit (ETU), energy enhancement unit (EEU), energy management unit (EMU), energy user unit (EUU), and edge demonstration unit (EDU) are discussed in detail. In particular, an enhanced EMU is proposed by making necessary complements to an extensively used off-the-shelf integrated circuit (IC) solution for piezoelectric transducers. It provides more comprehensive energy storage indicating signals, such that the sensing, computing, and transmitting tasks can be carried out more robustly by keeping a good awareness of the remaining energy. Owing to the enhanced EMU design, vibration energy in various forms, such as intermittent and transient ones, can be more effectively harvested and utilized. The performance of ViPSN is evaluated, in terms of its lifetime and Quality of Service (QoS), under different vibration scenarios. The inclusive design and affiliated opensource project of ViPSN help build a new ecosystem for the research and development of vibration- or motion-powered IoT systems.

The inductive power transfer (IPT) technique has many merits in battery charging compared to traditional charging methods. However, in low-power applications, most of the existing wireless chargers would employ redundant power conversion and isolation stages in order to be compatible with the wire-connected chargers, which leads to low overall charging efficiency. This paper proposes a novel wireless fast charger by using a two-stage configuration. An unregulated IPT stage can not only offer galvanic isolation but also provide high step-down conversion ratio, and it works like a transformer-based isolated converter. Then the whole charger only needs one more regulation stage to meet the output requirement. This paper focus on the IPT stage with high-input voltage and low-output voltage. The LCC-C compensation is selected for the voltage step-down purpose. In the analysis, the influence of layout size, resonance frequency, and the power level are discussed, based on which the circuit parameters are optimally designed. Finally, a 2MHz 25W IPT stage is used as an example to achieve 120V /12V conversion. The measured peak efficiency is 82 %, which is 9 % higher than the stage of the art product (5W, 5V/5V conversion).

Recently, omnidirectional wireless power transfer systems have been studied intensely due to their improved flexibility when compared with their planar counterparts. In this paper, a novel wireless charging bowl with multiple transmitter coils is proposed to power portable devices. The bowl-shaped transmitter coil is optimized to provide sufficiently strong and nearly uniform omnidirectional field distribution. Inside the bowl, a planar receiver coil can be charged with free-positioning and arbitrary orientation. This unique benefit is particularly attractive for low-power portable devices. In the experiment, the proposed transmitter coils are built and operated at 6.78 MHz. The magnetic field is measured to verify the uniform and omnidirectional magnetic field distribution. The coil to coil efficiency is 85% to 95%. A complete system including the inverter and rectifier stage is implemented. When charging a 5W smart phone receiver, the overall efficiency varies within a range from 68-80% for any possible position.

The spatial freedom of wireless power transfer (WPT) systems can be improved using a high operating frequency such as several megahertz (MHz). In the conventional compensations the load of the coupling coils is usually assumed to be pure resistive. However, in MHz WPT systems this assumption is not accurate anymore due to the nonneglectable rectifier input reactance. This paper discusses the impedance characteristics of the full-bridge rectifier at MHz and their influence under the series-series, parallel-series, series-parallel, and parallel-parallel compensation topologies. An undesirable nonzero phase (i.e., none unity power factor) is shown to exist at the primary input port, which leads to decreased power transfer capability. In order to minimize this negative effect, the compensation capacitors are optimally designed, and the series-series topology is found to have the smallest phase under load and coupling variations. Finally, an experimental 6.78 MHz system is built up to verify the optimized design of the compensation capacitors. The results show that the average nonzero phase is effectively reduced together with the improved power factor from 0.916 to 0.982.

Simultaneous wireless charging of multiple devices is a unique advantage of wireless power transfer (WPT). Meanwhile, the multiple-receiver configuration makes it more challenging to analyze and optimize the operation of the system. This paper aims at providing a general analysis on the multiple-receiver WPT systems and compensation for the influence of the cross coupling. A two-receiver WPT system is first investigated as an example. It shows that theoretically by having derived optimal load reactances, the important system characteristics can be preserved, such as the original system efficiency, input impedance, and power distribution when there is no cross coupling between receivers. The discussion is then extended to general multiple-receiver WPT systems with more than two receivers. Similar results are obtained that show the possibility of compensating the cross coupling by having the derived optimal load reactances. Finally, the theoretical analysis is validated by model-based calculation and final experiments using real two- and three-receiver systems.

A wireless power transfer system can increase its system frequency to several Megahertz for large spatial freedom. A MHz system usually uses the same compensation as those kHz systems. Traditionally, the rectifier is modeled as a pure resistive load for the coupling coils when designing the compensation capacitors. However, the rectifier input reactance cannot be ignored for MHz systems. This reactance can affect the system resonance and is usually not considered when designing the compensation capacitors. This paper analyzes the rectifier input impedance and discusses its undesirable effects. A non-zero phase is shown to exist seeing into the coupling coils for MHz WPT system. In order to limit the phase, the optimal compensation capacitors are discussed and found. Finally, a 6.78 MHz system is used to verify the proposed optimal compensation method. It shows the phase variation can be limited from the original range [15°-35°] to the optimal range [-5°-5°].

Wireless power transfer (WPT) has shown its potential over conventional charging systems in recent years. However, it is still challenging to determine the power distribution of a multiple-receiver WPT system for its high sensitivity, complex coupling and load relationships. This paper discusses a game theory based control approach for the power distribution of a multiple-receiver WPT system. The power receiver systems (i.e., a receiving coil, a DC-AC rectifier, a DC-DC converter, and an ultracapacitor pack in this paper.) are modelled as independent agents with different preferences under the Matlab simulation environment where the preferences of each agent are represented by utility functions. Then a non-cooperative power distribution game is set up and a generalized Nash equilibrium is found which is used as the reference solution formula to be updated at every control instant. Meanwhile, the generalized Nash equilibrium is found through finding out the variational equilibrium by Karush Kuhn-Tucker conditions (KKT) conditions. The simulation results show that the game theory based control is comparable to the highest efficiency impedance distribution approach in a three-receiver WPT system.

Wireless power transfer (WPT) has attracted an ever increasing interest from both industry and academics over the past few years. Its applications vary from small power devices such as mobile phones and tablets to high power electric vehicles and from small transfer distance of centimeters to large distance of tens of centimeters. In order to achieve a high-efficiency WPT system, each circuit should function at a high efficiency along with the proper impedance matching techniques to minimize the power reflection due to the impedance mismatch. This paper proposes an analysis on the system efficiency to determine the optimal impedance requirement for coils, rectifier, and dc–dc converter. A novel cascaded boost–buck dc–dc converter is designed to provide the optimal impedance matching in WPT system for various loads including resistive load, ultracapacitors, and batteries. The proposed 13.56-MHz WPT system can achieve a total system efficiency over 70% in experiment.