Switching Cycles Research Articles

An Explicit Discrete-Time Large- and Small-Signal Modeling of the Dual Active Bridge DC–DC Converter Based on the Time Scale Methodology

The conventional method to model a dc–dc converter is by averaging its state variable over a switching period. However, due to the dc–ac–dc structure of the dual active bridge converter (DABC), the average value of the inductor current over a switching cycle is zero. Therefore, the conventional methodology cannot be applied to the DABC. The time-scale discrete-time modeling of the DABC is proposed to mitigate the zero average state variable problem. The time-scale methodology simplifies the analysis of the DABC by segregating the fast state variable inductor current and slow state variable output voltage. The proposed model has the advantage of the explicit large signal in terms of the circuit parameter, like the average value model. The explicit steady-state circuit expression from the large-signal model helps in the study of the soft-switching range of the DABC. Moreover, the small-signal model from the proposed method results in an explicit small-signal model. The model incorporates semiconductor <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</small> -resistance, inductor equivalent series resistance, transformer resistance, and leakage inductance. The effect of the converter net equivalent resistance on the small-signal model is thoroughly studied. Finally, the model accuracy in the steady-state, open-loop, and the small-signal are verified in both the switching simulation model and experimental setup.

Cascaded Voltage and Current Control for a Dual Active Bridge Converter with Current Filters

The paper describes the cascaded voltage and current control for a bidirectional DC/DC converter in Dual Active Bridge (DAB) topology. The typical DAB converter circuit was extended by additional current filters, which allow it to operate in application fields with high requirements on current ripples. The core concept of the presented solution is usage of the modified Single Phase Shift (SPS) modulation, which allows to compensate for the DC-bias current occurring in dynamic states and provides a settling time of half switching cycle during transients. Its features were utilized to build a simplified dynamic model of the converter. The linear Proportional-Integral (PI) controllers are used in both voltage and current control loops. Based on the developed dynamic models, the tuning rules for both controllers were derived. In both cases, a number of the tuned parameters were reduced from two to one (which can present a great practical value for application engineers). The proposed solutions are validated based on a laboratory prototype. An important part of the experiments was devoted to non-linear effects occurring near the current limitation boundary of the system. The paper ends with a brief discussion regarding the future research directions.

Forming-free Pt/Al2O3/HfO2/HfAlOx/TiN memristor with controllable multilevel resistive switching and neuromorphic characteristics for artificial synapse

Controllable multilevel resistive switching (RS) and neuromorphic characteristics emerges as a promising paradigm to build power-efficient computing hardware for high density data storage memory and artificial intelligence. Nevertheless, the current nonvolatile memory still endures from reliability and variability of the memristors. In this work, Pt/Al2O3/HfO2/HfAlOx/TiN multilayer memristor was prepared by using atomic layer deposition (ALD) to examine the well-regulated multilevel RS and neuromorphic properties. The memristor was found to demonstrate admirable RS properties, including forming-free, low operating voltage (Set/Reset), high switching ratio (>100), multi-level retention time (104 s), and good durability (1000 switching cycles). Furthermore, seven and four resistance states can be accomplished by modulating CC through set-operation and stop-voltage during the reset-operation. By modulating the multi-level resistance state, the electronic synapse can simulate synaptic plasticity, such as potentiation/depression, paired pulse facilitation (PPF) and spike-timing-dependent plasticity (STDP). Results show that a multilayer memristor has potential in the application of multilevel data storage memory and bionic portable electronic devices.

Optimized energy-storage performance in Mn-doped Na0.5Bi0.5TiO3-Sr0.7Bi0.2TiO3 lead-free dielectric thin films

The 0.55Na0.5Bi0.5TiO3-0.45Sr0.7Bi0.2TiO3 (0.55NBT-0.45SBT) thin films with Mn doping were fabricated on the platinum-buffered Si substrates using the sol–gel technique. The impacts of Mn doping on the structure and properties of the thin films were systematically studied. Mn doping into the thin films can limit the valence transition of Ti cations to improve the insulation performance, and advance the ferroelectric properties. Ordered B-site cation displacement can be observed from the HADDF-STEM image, which can only be maintained in a single nano-sized crystal grain, and its order scale is limited by the grain size. The 0.55NBT-0.45SBT-0.01Mn film exhibited relatively high recoverable energy storage density (∼30.5 J/cm2) and efficiency (∼65%) at 2800 kV cm−1. Frequency stability in a wide range (0.5 ∼ 20 kHz) and long-term antifatigue stability (1 × 108 switching cycles) were also obtained, indicating their future applications in advanced dielectric capacitors for energy storage.

Gate energy efficiency and negative capacitance in ferroelectric 2D/2D TFET from cryogenic to high temperatures

We report the fabrication process and performance characterization of a fully integrated ferroelectric gate stack in a WSe2/SnSe2 Tunnel FETs (TFETs). The energy behavior of the gate stack during charging and discharging, together with the energy loss of a switching cycle and gate energy efficiency factor are experimentally extracted over a broad range of temperatures, from cryogenic temperature (77 K) up to 100 °C. The obtained results confirm that the linear polarizability is maintained over all the investigated range of temperature, being inversely proportional to the temperature T of the ferroelectric stack. We show that a lower-hysteresis behavior is a sine-qua-non condition for an improved energy efficiency, suggesting the high interest in a true NC operation regime. A pulsed measurement technique shows the possibility to achieve a hysteresis-free negative capacitance (NC) effect on ferroelectric 2D/2D TFETs. This enables sub-15 mV dec−1 point subthreshold slope, 20 mV dec−1 average swing over two decades of current, ION of the order of 100 nA µm−2 and ION/IOFF > 104 at Vd = 1 V. Moreover, an average swing smaller than 10 mV dec−1 over 1.5 decades of current is also obtained in a NC TFET with a hysteresis of 1 V. An analog current efficiency factor, up to 50 and 100 V−1, is achieved in hysteresis-free NC-TFETs. The reported results highlight that operating a ferroelectric gate stack steep slope switch in the NC may allow combined switching energy efficiency and low energy loss, in the hysteresis-free regime.

ZnO coral-like nanoplates decorated with Pd nanoparticles for enhanced VOC gas sensing

A high working temperature of the ZnO nanomaterial-based gas sensor could shorten the lifetime of the sensor and increase its power consumption. Enhancing the volatile organic compound (VOC) sensing performance of ZnO nanomaterial-based gas sensors in terms of gas response and temperature is vital for their practical application. Decoration of noble metals onto nanostructures is an effective approach for improving their sensing characteristics. Herein, hydrothermally synthesized ZnO coral-like nanoplates decorated with Pd nanoparticles are introduced to achieve the improved VOC sensing performance. The morphology, crystal structure, composition, atomic structure, and gas sensing properties of the synthesized pristine and Pd–ZnO coral-like nanoplates were investigated. The results showed a remarkable reduction of optimal working temperature from 450 °C for the pristine ZnO based sensor to 350 °C for the Pd–ZnO based sensor. The sensor response to acetone at the optimal operating temperature of 350 °C was improved three times by surface decoration with Pd nanoparticles. The response time and recovery time of the Pd–ZnO sensor were about three times faster than that of the pristine ZnO sensor. The Pd–ZnO sensor reached a theoretical detection limit of 17 ppt and a sensitivity of 3.5–2.5 ppm acetone at 350 °C. The sensor transient stability after several on/off switching cycles from air to gas revealed the effective reusability of the fabricated devices. A plausible mechanism for the VOC sensing of the porous Pd–ZnO coral-like nanoplate-based sensor is also discussed. • Enhancement of VOC sensing properties of ZnO coral-like nanoplates by Pd decoration. • The Pd–ZnO sensors show the response of 3.5–2.5 ppm acetone at 350 °C. • The Pd–ZnO sensors show the highest response of 66.7–125 ppm acetone at 350 °C. • Sensing mechanism of the Pd–ZnO sensor to acetone is discussed.

Analysis and design of four-plate capacitive wireless power transfer system for undersea applications

This paper presents a four-plate undersea capacitive wireless power transfer (CPT) system for underwater applications such as autonomous underwater vehicles (AUVs). Generally, a CPT system transfers the power based on electric fields. The complex resonant compensation networks are used to make the CPT system work in the resonant condition. The resonant voltage is always very high. It will be a big challenge to the human safety. In this paper, a virtual electrons periodic reciprocating flow theory is proposed for the CPT system. In one switching cycle, the electrons firstly flow in the forward direction through the forward path and then flow in the inverse direction through the inverse path. The CPT system has been deeply studied with the vacuum dielectric or the air dielectric. However, for the CPT system, there are few papers to show the underwater application. In this paper, an undersea four-plate CPT system is designed and studied in the underwater condition. The two coupling capacitors and other elements of the CPT system could build a closed-loop path. A small value inductor is adapted as a resonant compensation network for the four-plate CPT system. The DC voltage is inverted to the AC voltage in the primary side with the single-phase full-bridge inverter. The resonant voltage is rectified to the DC voltage in the secondary side with the single-phase full-bridge diode rectifier. A 100 W power level CPT system is constructed to verify the theory analysis and the calculation. The theory analysis is verified by the simulated and experimental results. The stable output voltage and load power are achieved in this paper.

Radio Over FSO Communication Using High Optical Alignment Robustness 2D-PDA and its Optical Path Switching Performance

We propose a 30 GHz radio over free-space optical (FSO) communication system for indoor wireless communication applications using a newly developed high-speed photodetector array (PDA) device. The new device performs the role of a photodetector with high optical alignment robustness and also acts as a 30 GHz optical-to-radio converter. Small high-speed photodetectors of 30 mm size are arranged in a 6 × 6 square-shaped alignment to achieve -4.4 dBm RF output at 30 GHz with an external amplifier. A 3-5 dB in-plane RF power distribution in the 4 × 4 PDA region was evaluated. To confirm the proposed concept, a 1 m long radio transmission over a 1.5 m long FSO transmission setup was demonstrated by switching the optical paths. We successfully confirmed a high data rate of up to 21.1 Gbps using several modulation formats under a static optical path switching condition. Additionally, stable and reproducible error vector magnitudes with a range of approximately 9% were recognized during the 10 switching cycles under a dynamic optical path switching condition.

Area-Type Electronic Bipolar Switching Al/TiO1.7/TiO2/Al Memory with Linear Potentiation and Depression Characteristics.

In electronic bipolar resistive switching (eBRS), the electron trapping and detrapping at the defect sites within the switching layer, such as the highly defective TiO1.7 in this study, constitute the switching mechanism. It is an appealing candidate solution to the nonuniformity issue of resistive switching memory. However, TiO1.7-based eBRS has suffered from a lack of endurance and retention. In this study, a 7 nm-thick stoichiometric TiO2 layer is interposed between an Al bottom electrode and a 50 nm-thick TiO1.7 layer, which is in contact with an Al top electrode. Despite the minimal structural modification, improvements in the electrical performance were substantial. The off-to-on state resistance ratio of 20 and the resistance values could be retained up to 30 000 direct current sweep cycles and 106 alternating current pulse switching cycles. Data retention also significantly improves. Moreover, the device is electroforming-free and shows fully area-type switching characteristics. Such notable improvements are attributed to the favorable energy band structure of the Al/TiO1.7/TiO2/Al structure. The device shows almost linear potentiation and depression characteristics after the repeated pulse voltage applications, which significantly improves the accuracy of the neural network, the synapses of which are composed of the Al/TiO1.7/TiO2/Al memory cells.

Highly reliable memristive devices with synaptic behavior via facilitating ion transport of the zeolitic imidazolate framework-8 embedded into a polyvinylpyrrolidone polymer matrix

Composite-based memristive devices have attracted tremendous attention as promising applications in futuristic information technologies due to their synergetic effects between the solid filler and the polymer matrix for enabling versatile information processing. Here, we demonstrate the digital data storing and the analog data processing capabilities of artificial synapses consisting of zeolitic imidazolate framework-8 (ZIF 8):polyvinylpyrrolidone (PVP) hybrid nanocomposites. The composite-based devices exhibited excellent resistive switching behaviors with reliable operating voltage (1.24 ± 0.12 and −2.75 ± 0.33 V) and a large RON/ROFF ratio (7.8 × 103) for 500 repeated resistance switching cycles. The fundamental synaptic behaviors in neuromorphic systems, such as potentiation/depression, transitions from short-term to long-term memory, and the biological learning rules of spiking time-dependent plasticity, were imitated. Finally, via structural and theoretical analyses, we demonstrated that the excellent memristive behaviors were due to the ion trapping effect of the ZIF-8 nanoparticles in the ZIF-8:PVP hybrid composite film.

DC-Link Current Harmonic Mitigation via Phase-Shifting of Carrier Waves in Paralleled Inverter Systems

DC-connected parallel inverter systems are gaining popularity in industrial applications. However, such parallel systems generate excess current ripple (harmonics) at the DC-link due to harmonic interactions between the inverters in addition to the harmonics from the PWM switching. These DC-link harmonics cause the failure of fragile components such as DC-link capacitors. This paper proposes an interleaving scheme to minimize the current harmonics induced in the DC-link of such a system. First, the optimal phase-shift angle for the carrier signal is investigated using the analytical equations, which provides maximum capacitor current ripple cancellation (i.e., at the main switching frequency harmonic component). These optimally phase-shifted switching cycles lead to variations of the output current ripples, which, when summed together at the DC-link, result in the cancellations of the DC-link current ripples. The results show that when the carrier waves of the two inverters are phase-shifted by a 90° angle, the maximum high-frequency harmonic ripple cancellation occurs, which reduces the overall root-mean-square (RMS) value of the DC-capacitor current by almost 50%. The outcome of this proposed solution is a cost-effective DC-harmonics mitigating strategy for the industrial designers to practically configure multi-inverter systems, even when most of the drives are not operating at rated power levels. The experimental and simulation results presented in this paper verify the effectiveness of the proposed carrier-based phase-shifting scheme for two different configurations of common DC connected multi-converter systems.

Long-term switching of single photochromic triads based on dithienylcyclopentene and fluorophores at cryogenic temperatures

Photochromic molecules can be reversibly converted between two bistable forms by light. These systems have been intensively studied for applications as molecular memories, sensing devices, or super-resolution optical microscopy. Here, we study the long-term switching behavior of single photochromic triads under oxygen-free conditions at 10K. The triads consist of a photochromic unit that is covalently linked to two strong fluorophores that were employed for monitoring the light-induced conversions of the switch via changes in the fluorescence intensity from the fluorophores. As dyes we use either perylene bisimide or boron-dipyrromethen, and as photochromic switch we use dithienylcyclopentene (DCP). Both types of triads showed high fatigue resistance allowing for up to 6000 switching cycles of a single triad corresponding to time durations in the order of 80min without deterioration. Long-term analysis of the switching cycles reveals that the probability that an intensity change in the emission from the dyes can be assigned to an externally stimulated conversion of the DCP (rather than to stochastic blinking of the dye molecules) amounts to 0.7 ± 0.1 for both types of triads. This number is far too low for optical data storage using single triads and implications concerning the miniaturization of optical memories based on such systems will be discussed. Yet, together with the high fatigue resistance, this number is encouraging for applications in super-resolution optical microscopy on frozen biological samples.

Bipolar resistive switching and memristive properties of sprayed deposited Bi2WO6 thin films

The present work demonstrates the development and investigation of bipolar resistive switching characteristics of the Al/Bi2WO6/FTO thin-film memristive device. The Bi2WO6 (BWO) active layer was deposited on FTO coated glass substrates using a spray pyrolysis technique and the effect of spray volume on resistive switching characteristics was investigated. X-ray diffraction analysis reveals that the thin films were polycrystalline with the orthorhombic crystal structure. The spherical grains were observed from the morphological study for an optimized sample and incomplete growth of grains was seen for other samples. The energy dispersive X-ray spectra show the presence of different constituent elements such as Bi, W, and O. The pinched hysteresis loop is the basic signature of a memristive device and was observed in all Al/BWO/FTO devices. The bipolar resistive switching characteristic was observed in all devices. The optimized device shows maximum current and the highest memristive area. Endurance test suggests that the device can be switched in consecutive 2 × 103 switching cycles and can retain the data up to 103 s without any observable degradation in the switching properties. The Al/BWO/FTO device possesses the double valued charge-flux property, suggesting the presence of memristive properties. The HRS follows the Schottky conduction mechanism while LRS govern by the Ohmic charge transport mechanism.

Effects of heavy ion irradiation on Cu/Al2O3/Pt CBRAM devices

In this work, the effects of heavy ion irradiation on atomic switches with Cu/Al2O3/Pt structure are investigated. The initial device is prone to hard breakdown after forming, while the device after irradiation exhibits good performance such as stable operating voltage, non-volatile switching (a retention characteristic of 105 s at room temperature) and good endurance (2000 switching cycles). Compared with the device after low-dose irradiation, the device under higher-dose irradiation shows more uniform off-state resistances but no relevant changes were observed on distributions of on-state resistances, set and reset voltages.

All Solid‐State Flexible Electrochromic‐Organic Light‐Emitting Diode Devices on Single‐Plastic Substrate for See‐Through Display Technologies

AbstractThe development of solid‐state electrochromics (EC) represents an emerging challenge towards effective integration with other optoelectronic units, such as the EC‐organic light‐emitting diode devices (EC‐OLEDs). By a simultaneous or independent control of optical contrast (EC function) and OLED emission upon electrical activation, these fascinating systems are capable of adaptively modulating solar radiation and display lighting. Here, we report a flexible ECOLED on a single‐plastic substrate demonstrating how the rational design of a highly efficient and durable solid‐state EC cell enables an increase of overall performances and the integration with OLED on larger area and flexible substrate. The resulting device shows high transparency and optical contrast (ΔT650nm = 60%), with lower operating voltages (0.5–3 V), faster coloring/bleaching switching times (5–10 s), and enhanced stability (1000 switching cycles) if compared to similar systems. The OLED unit reveals a luminance of 1000 cd m−2 (at 10 mA cm−2) and a current efficiency of about 10 cd A−1, making it suitable for both display and lighting applications. These results can represent a novelty in the field of next‐generation smart windows and transparent displays, including all technologies based on see‐through OLED displays, electronic devices for augmented reality, lighting panels and smart glasses.

Effect of UV irradiation on the resistive switching characteristics of low-temperature solution-processed ZrO2 RRAM

This paper presents the fabrication of a solution-processed ZrO2 resistive random access memory (RRAM) device with low-temperature UV irradiation and the effect of UV irradiation on the resistive switching (RS) characteristics. The ZrO2 switching layer was deposited by spin-coating zirconium acetylacetonate (Zr(C5H7O2)4) precursor in an ethanol solvent; the maximum process temperature was 150 °C. The RS characteristics of the fabricated device were unstable for up to 2 h UV irradiation, but they improved after 4 h or longer UV irradiation. The 4 h and 8 h UV-irradiated devices show stable RS even after 200 dc switching cycles and long retention over 104 s. The improvement of RS characteristics caused by different UV irradiation times can be attributed to the reduction of the hydroxyl group (M–OH) and the formation of enhanced metal-oxide bonds (M–O) of the ZrO2 thin films, based on field emission scanning electron microscope and x-ray photoelectron spectroscopy analysis. This research suggests a promising approach to fabricate oxide thin films with good RS characteristics at low temperatures which has high potential to be extended to future flexible RRAM devices.

Improved Modulation Strategy Based on Minimum Energy Storage Principle for Electrolytic-Capacitor-Less Six-Switch Converter

An improved modulation strategy based on minimum energy storage for DC-link capacitance reduction in a six-switch AC-AC converter is proposed. The proposed modulation strategy enables the energy on the capacitor to accumulate and release twice each in a complete switching cycle, achieving the effect of “fast charging and discharging”. Meanwhile, the inversion and rectification are modulated synchronously. Hence, there is minimum energy stored in the DC-link capacitor. Then, the time average modeling analysis is presented to take insight analysis. When there is the same voltage ripples constraint on the DC side for the conventional and improved modulation strategies, the six-switch converter under the improved modulation strategy has the much less capacitance value of the storage capacitor and even realizes non-electrolytic capacitance. Therefore, improving the system efficiency, power density, and output waveform quality and extending the system life can be achieved. The operation principle and modulation strategy are discussed in detail. Finally, the simulation model and experimental prototype are built to verify effectiveness of the topology and correctness of the proposed six-switch AC-AC converter modulation strategy.

The Influence of Mechanical Alloying and Plastic Consolidation on the Resistance to Arc Erosion of the Ag-Re Composite Contact Material.

The article presents the influence of mechanical alloying and plastic consolidation on the resistance to arc erosion of the composite Ag–Re material against the selected contact materials. The following composites were selected for the tests: Ag90Re10, Ag95Re5, Ag99Re1 (bulk chemical composition). Ag–Re materials were made using two methods. In the first, the materials were obtained by mixing powders, pressing, sintering, extrusion, drawing, and die forging, whereas, in the second, the process of mechanical alloying was additionally used. The widely available Ag(SnO2)10 and AgNi10 contact materials were used as reference materials. The reference AgNi10 material was made by powder metallurgy in the process of mixing, pressing, sintering, extrusion, drawing, and die forging, while the Ag(SnO2)10 composite was obtained by spraying AgSniBi alloy with water, and then the powder was pressed, oxidized internally, sintered, extruded into wire, and drawn and die forged. The tests of electric arc resistance were carried out for loads with direct current (DC) and alternating current (AC). For alternating current (I = 60 A, U = 230 V), 15,000 switching cycles were made, while, for constant current 50,000 (I = 10 A, U = 550 V). A positive effect of the mechanical alloying process and the addition of a small amount of rhenium (1% by mass) on the spark erosion properties of the Ag–Re contact material was found. When DC current of 10 A was used, AgRe1 composite was found to be more resistant than commonly used contact materials (AgNi10 and Ag(SnO2)10).

Resistive switching properties and conduction mechanisms of LaSmOx thin film by RF sputtering for RRAM applications

Polycrystalline LaSmOx (LSO) thin films were prepared by using RF sputtering and the bipolar resistive switching (BRS) properties in the Al/LSO/ITO structure were investigated. The impact of Ar/O2 ratio, film thickness and post metal annealing (PMA) condition on the resistive switching (RS) properties were also studied. The conductive mechanism is mainly dominated by the numbers of oxygen vacancies, which can be controlled through a different deposition atmosphere (Ar/O2) ratio and film thickness. In addition, the resistive switching characteristics can be improved by post metal annealing treatment due to film densification and the formation of AlOx interface layer in between the top electrode and the LaSmOx thin film. The optimized RRAM device revealed a profound improvement in the switching cycles, which can be as high as 4580.

Performance Improvement of a Grid-connected Voltage Source Converter Controlled by Parabolic PWM Current Control Scheme

Parabolic carrier PWM method is considered as one of the direct current control methods which has been proposed for the voltage-source converters (VSCs). This method has an excellent dynamic response. Besides, it proposes a constant switching frequency by employing a pair of parabolic PWM carriers. However, it suffers from some drawbacks and limitations. The major drawback of this method is its sensitivity to the inductance variations. In other words, in grid-connected applications the exact value of grid inductance should be exactly known to achieve a proper performance from this method. Moreover, it is essential that during each switching cycle the voltage at the point of common coupling remains constant. In grid connected applications such as active power filter these drawbacks may lead to operate at variable or non-expected frequencies. Therefore, this paper concerns the suggestions to deal with the situation. In this paper, by applying the conventional method the aforementioned problems are examined in a grid-connected active power filter. It is shown analytically that by using the proposed method, problems of sensitivity to inductance changes and also necessity to constant voltage at point of common coupling in a switching period will be solved. Finally, simulation and experimental results are presented.