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Maximizing the efficiency and power density of dc–dc converters demands parallel optimizations in design and control, especially for variable-frequency converters operating over wide frequency ranges. This letter presents the full-scale optimization of a kilowatt-range megahertz-class boost converter based on the impulse rectification. To maximize the heat extraction from the converter and increase its power density, the entire power stage is implemented on a single-layer insulated-metal substrate. For high efficiencies over wide frequency ranges, high-performance gallium nitride transistors are employed and various high-frequency materials (MnZn, NiZn, and air) with different geometries are compared to realize a wide-bandwidth inductor. Silicon carbide Schottky diodes with a zero reverse recovery are utilized for efficient high-frequency rectification, and the impact of the device's current rating on its generated reactive power and the overall system efficiency are investigated at different power levels up to 1 kW. The proposed optimum duty cycle control maximizes the conversion efficiency at different gains and powers and prevents fatal device hard switching at high frequencies. The optimized converter enables a peak efficiency of 98.6% along with an ultrahigh power density of 52 kW/l (850 W/in3). A loss breakdown summarizes major efficiency bottlenecks to be overcome by future advances in power electronics. [ABSTRACT FROM AUTHOR]

Wide-band-gap technologies enable ultra-high efficiencies in the high-frequency power conversion. However, inadequate accuracies in electrical measurements could lead to spurious efficiency measurements, even above 100%. Furthermore, bandwidth limitations, particularly of current probing, delay mismatches between current and voltage probes, loading effects, and electromagnetic interferences, make electrical techniques inappropriate for measuring extremely-low losses in switches with high dv/dt and di/dt values and magnetic components under high-frequency excitations. Calorimetric methods overcome this issue by a direct loss measurement through the generated heat. Nevertheless, limited ranges and accuracies of the existing systems hinder their application in sensitive measurements. Moreover, time-consuming calibrations with extensive data processing impede rapid design assessments. In this work, we propose a closed-type double-chamber calorimeter based on a real-time calibration between two identical chambers to eliminate extra compensation for ambient variations or changes within the system itself. By streaming the same coolant in both chambers, our system removes the need for flow measurements, enabling extremely low flow rates, which improves sensitivity and reduces complexity and system cost. The system provides an adjustable cooling and can measure losses down to 500 mW, enabling the evaluation of high-performance power converters and their components with no dependence on the geometry of the evaluated devices or systems. [ABSTRACT FROM AUTHOR]

Soft-switching power converters based on wide-band-gap (WBG) transistors offer superior efficiency and power density advantages. However, at high frequencies, loss behavior varies significantly between different WBG technologies. This includes losses related to conduction and dynamic on-state resistance (RDS(ON)) degradation, also charging/discharging of input capacitance (CISS) and output capacitance (COSS). As datasheets lack such important information, we present measurement techniques and evaluation methods for soft-switching losses in WBG transistors which enable a detailed loss-breakdown analysis. We estimate the gate loss under soft-switching conditions using a simple small-signal measurement. Next, we use Sawyer–Tower and nonlinear resonance (NR) methods to measure large-signal COSS energy losses up to 40 MHz. Finally, we investigate the dependence of dynamic RDS(ON) degradation on off-state voltage using pulsed-IV measurements. We demonstrate an insightful comparison of soft-switching losses for various normally off gallium nitride (GaN) and silicon carbide (SiC) devices. A p-GaN-gated device exhibits the most severe RDS(ON) degradation and the lowest gate loss. Cascode arrangement increases threshold voltage for GaN devices and reduces gate losses in SiC transistors; however, it leads to higher C{OSS losses. The study facilitates the evaluation of system losses and selection of efficient WBG devices based on the trade-offs between various sources of losses at high frequencies. [ABSTRACT FROM AUTHOR]

Conventional boost converters, while having a high level of simplicity and robustness, are known to have a poor efficiency at high step-up regimes. More complicated topologies have been extensively explored to increase the step-up ratio while keeping a high efficiency; however, an efficient regulated dc–dc power conversion at high step-up regimes still remains a challenge. In this letter, we demonstrate fully soft-switched operation of boost converters based on impulse rectification, which results in an efficient power conversion at very high step-up regimes. Contrary to the conventional analysis of boost converters which seeks to minimize conduction and switching losses, our approach considers reactive power as the key parameter limiting the efficiency. A theoretical basis enabled an appropriate design resulting in power conversion efficiencies above 90% for voltage gain values of 5, 10, 25, and 50, with a peak efficiency of 97%. The guidelines to design boost converters based on the proposed approach are discussed. The simplicity and high performance of this approach opens new avenues in designing regulated dc–dc converters with a high step-up. [ABSTRACT FROM AUTHOR]

Cascode topology which includes a high-voltage GaN and a low-voltage Si transistor is an attractive device concept, as it takes advantage of the low on-resistance of GaN while still being compatible with Si gate drivers. It demonstrates the highest threshold voltage among normally-off GaN devices, which is a favorable feature in power electronics. However, the observation of instability in power circuits using cascode devices highlights the need for a careful analysis to reveal the origins of this phenomenon, to fulfill the unique features of these devices in high efficiency and reliable power converters. In this letter, we demonstrate the presence of negative resistance in cascode devices, which explains the previously reported large-signal instabilities. We describe a theory to characterize this negative resistance behavior based on the subthreshold-voltage operation of the high-voltage GaN transistors, and propose guidelines to resolve this issue in cascode devices. Negative resistance behavior, although not favorable for a power transistor, is of great importance in high-frequency electronics, with applications in reflection-type power amplifiers. These results, together with the presented analysis, could guide the development of high-stability, low-loss devices for future efficient power converters. [ABSTRACT FROM AUTHOR]

Efficient dc–dc power-conversion with wide-span voltage-regulation is crucial to a sustainable and robust power electronics system. Dual-active-bridge (DAB) offers straight-forward regulation and its transformer enables voltage step-up/down required for many applications, such as battery chargers and bus converters for dc distribution systems. However, losing soft-switching at light loads or when operating at voltage gains far from the turns ratio severely degrades the efficiency of DAB, especially at high switching frequencies. In this article, we demonstrate an enhanced DAB (E-DAB) topology which employs an adjustable-tap transformer to extend the soft-switching over wider voltage gains and increase the power-transfer capability. By a proper tap adjustment and with single phase-shift modulation, the proposed gallium nitride (GaN)-based converter achieved a peak efficiency of 97.4% with an overall efficiency greater than a conventional DAB for voltage gains of up to 2.8 times higher. Employing a quasi-planar matrix transformer with integrated leakage inductance at 300 kHz allowed for an extremely high power density of 10 kW/l (7.5 kW/l with cooling). The tapped transformer did not incur extra losses to the topology. The gain versus power-transfer characteristic for soft-switching operation was derived for the E-DAB and its improvement in efficiency was experimentally verified over a wide power range. [ABSTRACT FROM AUTHOR]

The low on-resistance of wide-band-gap (WBG) transistors is a key feature for efficient power converters; however, the anomalous loss in their output capacitance (COSS) severely limits their performance at high switching frequencies. Characterizing COSS losses based on the large-signal measurement methods requires an extensive effort, as separate measurements are needed at different operation points, including voltage swing, frequency, and dv/dt. Furthermore, there is a practical tradeoff in the maximum voltage and frequency applied to the device. Here, we introduce a new circuit model, including an effective COSS and a frequency-dependent series resistance, along with a simple small-signal method to fully characterize COSS losses in WBG transistors. The method accurately predicts COSS losses at any voltage swing or frequency. Contrary to other methods, this technique directly leads to a general identification of COSS losses at different operation points, revealing new insights on COSS losses in WBG transistors, especially the dependence of EDISS on voltage and frequency. Based on the proposed approach, the issue of COSS losses in enhancement-mode GaN and SiC transistors was assigned to the limited quality factor of COSS. The precise characterization of COSS losses proposed in this letter is essential for designing efficient high-frequency power converters. [ABSTRACT FROM AUTHOR]

In this letter, we present a new measurement technique to evaluate the large-signal output capacitance (COSS) of transistors as well as the COSS energy dissipation (EDISS), based on the nonlinear resonance between a known inductor and the output capacitance of the device under test. The method is simple and robust, and only requires a single voltage measurement to extract the large-signal COSS both in charging and discharging transients. By changing the circuit parameters, it is possible to tune the resonance frequency (even above 40 MHz) and the voltage swing (even above 1 kV) with dv/dt exceeding 100 V/ns, even though the method relies only on a low-voltage dc source, without the need for high-voltage RF amplifiers. The single-pulse operation of the method enables measuring COSS and EDISS at very high frequency and dv/dt values without any thermal runaway. Using the proposed method, we extracted large-signal COSS and EDISS of power transistors based on different semiconductor technologies. The obtained results were verified by Sawyer–Tower method and data reported in the literature. In particular, dv/dt-independent COSS losses were measured in cascode GaN transistors. The precise characterization of large-signal COSS of transistors presented in this letter is essential for the design of power converters, especially those operating at high switching frequencies. [ABSTRACT FROM AUTHOR]

In this paper, we discuss and demonstrate the potential of normally-on GaN high-electron-mobility transistors (HEMTs) as opening switches in miniaturized pulsed-power circuits. The high-breakdown electric field of GaN (~3 MV/cm) makes it possible to fabricate high-voltage devices in small dimensions, resulting in smaller parasitics and faster switching times. GaN HEMTs as opening switches are compatible with inductive topologies, which offer more than one order of magnitude higher energy density than capacitive topologies, allowing further miniaturization of pulsed-power systems. In this work, we demonstrate the application of fabricated 1.5-kV normally-on GaN HEMTs on an inductive switching topology, resulting in 300-fold voltage step-up, which makes it possible to generate high-voltage pulses with a low-voltage dc source in miniaturized circuits. [ABSTRACT FROM AUTHOR]

Highlights • Tensile behaviour of different GFRP laminates under elevate temperatures has is investigated. • Laminates with continuous unidirectional fibers showed the best tensile performance. • Laminates with chopped strand fibers showed the weakest tensile performance. • Laminates with continuous woven fibers performed better than chopped strand and weaker than unidirectional laminates. Abstract This study was aimed at gaining an improved understanding of the behavior of glass fiber-reinforced polymer laminates at elevated temperatures by means of testing laminate specimens with unidirectional, woven, and randomly distributed (chopped strand mat) fibers. The testing parameters were temperature, the type of fiber, and the thickness of the laminates. The failure modes of the specimens and their elasticity moduli at ambient temperature were investigated, and analysis of variance was conducted to determine the contribution of each parameter to the behavioral test results. The findings showed that among the parameters, an increase in temperature exerted the strongest effect on the specimens. The unidirectional laminate specimens exhibited the best performance, maintaining nearly 40% of their loading capacity at 550 °C. At this temperature, the woven laminate specimens could not carry any tensile load, and at 400 °C, the laminate specimens containing randomly distributed fibers lost all their strength. [ABSTRACT FROM AUTHOR]

This paper presents the results of diagonal compression tests conducted on a series of un-reinforced and reinforced masonry specimens. Reinforced specimens were strengthened by using three types of mortar coating (with and without inner reinforcement), mesh reinforcement, and PP-band reinforcement. In particular, four different reinforcement methods with mortar coating (bare mortar coating, coating with steel mesh, coating with polymer reinforcement, and coating with GFRP mesh reinforcement), three different mesh-only reinforcement (steel mesh, polymer mesh with larger grids, and polymer mesh with smaller grids), and 2 different arrangements of PP-bands (4 × 4 and 6 × 6) were tested. Also, two different un-reinforced specimens (with and without mortar between bricks in a row) were tested to observe the effect of this matter. Each reinforcement method was tested with three wallettes and the effectiveness of each reinforcement method was observed in terms of both loading capability and ductility (especially the GFRP mesh reinforcement with mortar coating) because of the low resistance of the un-reinforced specimens. Shear stresses and strains and the elastic characteristics of each method are also presented in this paper. [ABSTRACT FROM AUTHOR]

Presents corrections to the above named paper. [ABSTRACT FROM AUTHOR]

The performance of beam-to-column connections is a key factor in the behaviour of moment-resisting frames (MRFs) under fire conditions. In this work, the behaviour and failure mechanisms of a half-scale 3D frame, composed of single-bay, single-storey MRFs and Braced Frames, in two perpendicular planes, equipped with 6 mm thick flush end-plate connections, under fire conditions was studied. A 'scaled' ISO 834 standard fire curve was followed during fire loading of the frame. The maximum reached temperature was 1100 °C. The temperatures of the elements of frames were measured at 2-minute time intervals. The rotations of the connections and the deflections of the MRFs' beams at these time points were worked out using an image processing technique. Furthermore, the effect of the ductility of connections was investigated by comparing the results of this study with those of a previous one in which less ductile connections were used. The results showed that the connections used in this study could sustain large rotations of up to 0.7 radian and endure 65 min under fire. Moreover, it was found that using flexible connections (connections with thinner end-plates) enhances the 'robustness' of moment frames in fire. • Scaled experimental models can be used to predict behaviour of structures in fire. • Image processing can be used for measuring deformations and rotations in fire tests. • Thickness-induced flexibility of thinner end-plates increases structural robustness. • Thinner end-plates reduce risk of over-stretching of bolts & structural collapse. • Simulated 'real fire' in this work triggered all forms of failure but overall collapse. [ABSTRACT FROM AUTHOR]

• The effects of UV, moisture and elevated temperature on mechanical properties of GFRP pultruded profiles have been studied. • Different cross-sections were used to study the effect of profile type. • Effect of conditioning on different failure modes were investigated. • Regression models have been developed to predict the mechanical properties retentions. The present research examines the effects of UV radiation, moisture and elevated temperature on the mechanical properties of GFRP pultruded profiles. Flexural, compressive and tensile properties of different GFRP sections were studied after they were exposed for 1000, 1500, 2000 and 3000 h to UV radiation and water vapour condensation cycles. Mechanical tests, including three-point bending, compression and tension tests, SEM analyses, and statistical studies were conducted to gather comprehensive results. The results showed that the mechanical properties of various GFRP sections generally decreased with the duration of conditioning: however, the rate of the decrease that was only slight up to 1000 h, increased rapidly during 1000–2000 h, and again it was slow during 2000–3000 h. The maximum reductions were 34%, 28% and 23% after exposure to 3000 h cycles for bending, tensile and compression tests, respectively. In the bending tests, where inter-laminar shear failure controls ultimate strength, the degradation was greater compared to the situation where fibres fracture controls the failure. Further, regarding the cross-section parameter, it was concluded that the thickness and perimeter are the effective factors; the thinner the sample and the larger the perimeter, the greater is the reduction. However, the maximum effect of the cross section in terms of thickness and perimeter did not exceed 13% among all sections. [ABSTRACT FROM AUTHOR]

This study investigates the effect of Thermal cycles on the mechanical properties of GFRP pultruded profiles with different geometries. Bending specimens consisted of I-shaped and U-channel profiles that were tested in three-point bending along their both principal weak and strong axes, whereas box profiles and laminates were used in compression and tension tests, respectively. Each specimen was exposed to a range of thermal cycles, between −20 °C and 20 °C. The failure modes of the profiles were closely investigated at both major and minor scales. Results were analyzed using ANOVA to determine the influence of each factor and a model was developed to predict the strength retention of profiles after exposure to thermal cycles. The results showed that the thermal cycles have a significant effect on the strength of the resin and a minor effect on the strength of the fibers. Bending specimens experienced a lower strength loss along their principal weak axis than that along their strong axis after conditioning. Tension specimens performed better in comparison to compression specimens in terms of strength retention after thermal exposure. These results are expected to contribute significantly toward the understanding of the influence of thermal cycles on the behavior of GFRP profiles. [ABSTRACT FROM AUTHOR]

This article presents an investigation on the durability of different glass-fibre-reinforced polymer composites when subjected to harsh outdoor conditions, including freeze/thaw cycles, ultraviolet radiation and moisture, as well as when used with seawater sea-sand concrete for construction applications. To achieve this, the effects of a number of parameters, including the environment of exposure, exposure time, profile cross-sectional configuration and orientation of fibres, on the mechanical properties of different glass-fibre-reinforced polymer composites were studied. To investigate the degradation of the mechanical properties, three-point bending, compression and tension tests were conducted on both reference and conditioned samples. Moreover, scanning electron microscopy analyses were performed to examine the contribution of microstructural deterioration to the damage mechanisms of the conditioned composites. Finally, the test results were used to develop empirical regression models to predict the level of retention of mechanical properties of different composites under different environmental conditions. The findings showed the maximum flexural, compressive and tensile strength reductions to be 35%, 48% and 37%, respectively, with regards to the pultruded profiles exposed for 3000 h to freeze/thaw cycles followed by 90 days of seawater sea-sand concrete immersion, while the flexural strength reductions recorded for the vacuum-infused samples subjected to 2000 h of freeze/thaw cycles followed by 90 days of seawater sea-sand concrete immersion were 28%, 72% and 56% for the unidirectional, woven and chopped strand mat laminates, respectively. [ABSTRACT FROM AUTHOR]

The dissipated energy (Ediss) related to the resonant charging–discharging of a transistor output capacitance becomes a dominant loss factor for power converters operating in the MHz range. A recent letter has introduced a small-signal measurement method to quantify Ediss with a frequency-dependent small-signal resistance, Rs, and an effective small-signal output capacitance, Cosseff. This letter provides further insights on the effect of Rs and Coss upon the device losses in a broader sense. In particular, the Coss loss tangent, tan (δ), is introduced as a normalized Ediss to combine the roles of Rs and Coss together with the operating frequency into a single loss parameter. By evaluating commercial device families, it is demonstrated that tan (δ) is constant for a given family, independent of device on-state resistance, RDS(on). It is shown that a minimum Ediss is achieved by having the lowest tan (δ) for a given stored energy (Eoss) in Coss. With accompanying guidelines, this letter identifies tan (δ) as a powerful figure of merit to classify field-effect transistors (FETs) for soft-switching applications, regardless of RDS(on) variations in devices within a family. The proposed concept provides a comprehensive method to characterize and benchmark field-effect transistors for efficient operation in high and very-high-frequency (VHF) applications. [ABSTRACT FROM AUTHOR]

Abstract This study investigates the flexural and impact behaviour of GFRP laminates after exposure to elevated temperatures. The effect of fibre's length and orientation, laminate's thickness, and exposure time is studied. A total number of 540 tests in terms of three-point bending and Charpy impact tests were conducted to obtain the mechanical properties. In addition, SEM analyses were carried out to investigate the degradation mechanisms. Finally, statistical study was conducted to investigate the contribution of each variable and develop probabilistic models using ANOVA and linear Bayesian regression method. The results showed that generally the flexural and impact properties of GFRP laminates decrease by increasing the temperature and time of exposure as well as decreasing the laminates' thickness. It is also observed that laminates with unidirectional fibres have the best performance under elevated temperatures, while laminates with randomly distributed fibres are the most vulnerable type. The performance of laminates with woven fibres are almost between those two other types. However, all types of the laminates lost almost all their flexural strength and impact energy absorption capacity at 300 °C. [ABSTRACT FROM AUTHOR]

• Mechanical properties of various GFRP pultruded profiles under SWSSC is investigated. • Effect of initial exposure to UV radiation and moisture before subjecting to SWSSC is studied. • Samples initially exposed to UV and moisture show higher reductions after exposure to SWSSC. • Profiles with interlaminar shear failure are more vulnerable than profiles with bending failure. In this research, the mechanical properties of various glass fibre reinforced polymer (GFRP) pultruded profiles exposed to seawater sea sand concrete (SWSSC) and its combination with UV radiation and water vapour condensation were studied. The effect of different conditions, duration of conditioning, and the profile cross-section configuration on the mechanical properties were investigated. Three-point bending, tension, and compression tests were carried out to obtain the mechanical properties, including flexural, tensile and compressive strengths and tensile elastic modulus. Furthermore, in order to scrutiny the mechanisms and extent of damage, scanning electron microscopy (SEM) was carried out on the test samples before and after the exposures. Finally, regression models were derived according to the experimental data to describe the mechanical properties of profiles after exposure to the aggressive environments used in this study. The maximum reductions of about 30%, 33% and 46% were observed in three-point bending, tensile and compressive tests, respectively amongst all samples when exposed for 90 days to simulated SWSSC. These reductions increased for the same sections to 52%, 47%, and 50% when the GFRP profiles were pre-exposed for 3000 h to UV radiation and moisture before exposing for 90 days to simulated SWSSC. [ABSTRACT FROM AUTHOR]

This study investigates the tensile properties of various GFRP laminates after exposure to elevated temperatures. Fiber configuration, exposure temperature and laminate thickness were considered as the test variables. A total number of 180 specimens were tested in tension to obtain the mechanical properties of GFRP laminates. Alongside the mechanical tests, SEM analyses were conducted on selected samples before testing to investigate the resin, fiber, and their interface damages. Regardless of the sample type, it was generally observed that the reduction rate in the tensile strength increased with an increase in the exposure time and a decrease in the laminates' thickness. The results of tensile tests after exposure to elevated temperatures showed that the laminates with continuous unidirectional fibers performed the best, laminates with chopped strand randomly distributed fibers performed the worst, and the performance of the laminates with woven continuous fibers was somewhere in between. The maximum tensile strength reductions for the unidirectional and woven laminates were about 50% after exposure to 300 °C, whereas random chopped strand laminates lost almost all of their load carrying capacity under this exposure condition. [ABSTRACT FROM AUTHOR]

The present study indicates the importance of using glass fiber reinforced polymer (GFRP) laminates with appropriate thickness and fibers orientation when exposed to harsh environmental conditions. The effect of different environmental conditions on tensile properties of different GFRP laminates is investigated. Laminates were exposed to three environmental conditions: (1) Freeze/thaw cycles without the presence of moisture, (2) freeze/thaw cycles with the presence of moisture and (3) UV radiation and water vapor condensation cycles. The effect of fiber configuration and laminate thickness were investigated by considering three types of fiber arrangement: (1) Continuous unidirectional, (2) continuous woven and (3) chopped strand mat and two thicknesses (2 and 5 mm). Microstructure and tensile properties of the laminates after exposure to different periods of conditioning (0, 750, 1250 and 2000 h) were studied using SEM and tensile tests. Statistical analyses were used to quantify the obtained results and propose prediction models. The results showed that the condition comprising UV radiation and moisture condition was the most aggressive, while dry freeze/thaw environment was the least. Furthermore, the laminates with chopped strand mat and continuous unidirectional fibers respectively experienced the highest and the lowest reductions properties in all environmental conditions. The maximum reductions in tensile strength for chopped strand mat laminates were about 7%, 32%, and 42% in the dry freeze/thaw, wet freeze/thaw and UV with moisture environments, respectively. The corresponding decreases in the tensile strength for unidirectional laminates were negligible, 17% and 23%, whereas those for the woven laminates were and 7%, 24%, and 34%. [ABSTRACT FROM AUTHOR]

Background Neuroglial cysts are rare intracranial lesions that are believed to originate from the sequestration of neural tube lining during embryogenesis. They can present anywhere within the neuraxis; however, their most common location is the frontal lobe. Cerebellopontine angle (CPA) neuroglial cysts are extremely rare and, to the best of our knowledge, there are no previous reports of a neuroglial cyst arising from cranial nerves. Case Description The current study presented a male adolescent with the reoccurrence of an intracranial cyst with symptoms of diplopia and facial numbness primarily treated as CPA arachnoid cyst with fenestration of the cyst 12 months prior to admission to our center. Different magnetic resonance imaging sequences showed a thin-walled extra-axial cyst at the right CPA without gadolinium enhancement. Direct visualization of the lesion revealed a cyst arising from the fifth cranial nerve sheath compressing the surrounding structures. The cyst was fenestrated, and a biopsy was taken from the cyst wall. The pathological analysis, along with specified histological markers, indicated the neuroglial nature of the cyst. Conclusions The rising of a neuroglial cyst from the nerve sheath is a finding that brings other possible origins of neuroglial cysts into consideration. [ABSTRACT FROM AUTHOR]

Objectives The endoscopic approach increasingly is used to treat third ventricular colloid cysts. Our objective was to assess the results of endoscopic resection of colloid cysts of the third ventricle. Methods A retrospective study was designed, and a series of 112 consecutive patients (76 male, 36 female) with third ventricular colloid cyst treated by endoscopic surgery was undertaken. Results We found that the most common clinical presentations in patients with third ventricular colloid cyst were headache, vomiting, and impaired vision. Complete resection was obtained in majority of patients (92%). Meningitis, hemiparesis, and memory deficits occurred most commonly as postoperative complications. Conclusions High rates of complete resection of third ventricular colloid cysts, with low morbidity and mortality, are possible with an endoscopic approach. The results of this study support the role of endoscopic resection in the treatment of patients with third ventricular colloid cysts as a safe and effective modality and show how endoscopic resection of third ventricular colloid cysts can produce favorable results. [ABSTRACT FROM AUTHOR]