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A three-dimensional quantum-mechanical method is used for modeling quantum tunneling and photon-assisted field emission processes in vertical and lateral vacuum nanotriodes. The electron transport through the devices is computed using a transfer-matrix technique. The possibility of using vacuum nanotriodes in current rectification is investigated at low voltages in a quasi-static and a high-frequency analyses. The obtained results show that a great enhancement in the rectification properties is achievable when a DC bias is applied between the gate and the collector. By comparing the results of the vertical and the lateral-type devices, it is shown that the former is superior in current rectification, while the latter provides higher values of current density. Furthermore, the high-frequency analysis shows that incident radiation results in photon-assisted current emission. It also demonstrates that the vacuum nanotriode can be much more efficient than the vacuum nanodiode in rectifying high-frequency signals. Published [ABSTRACT FROM AUTHOR]

Generalized Snoek's law proposed in an integral form by Acher and coauthors is a useful tool for investigation of high-frequency properties of magnetic materials. This integral law referred to as Acher's law allows for evaluating the ultimate performance of RF and microwave devices which employ magnetic materials. It may also be helpful in obtaining useful information on the structure and morphology of the materials. The key factor in practical application of Acher's law is an opportunity to employ either measured or calculated data available over a finite frequency range. The paper uses simple calculations to check the applicability of Acher's law in cases when the frequency range is limited and the magnetic loss peak is comparatively wide and has a distorted shape. The cases of large magnetic damping, pronounced skin effect, and inhomogeneity of the material are considered. It is shown that in most cases calculation of the integral through fitting of actual magnetic frequency dispersion by the Lorentzian dispersion law results in accurate estimations of the ultimate high-frequency performance of magnetic materials. [ABSTRACT FROM AUTHOR]

Recently, interest has been shown in research on the module-integrated converter (MIC) in smallscale photovoltaic (PV) generation. In an MIC, the voltage boosting high frequency transformer should be designed to be compact in size and have high efficiency. In response to the need to satisfy these requirements, this paper presents a coupled electromagnetic analysis model of a transformer connected with a high frequency switching DC-DC converter circuit while considering thermal characteristics due to the copper and core losses. A design optimization procedure for high efficiency is also presented using this design analysis method, and it is verified by the experimental result. [ABSTRACT FROM AUTHOR]

Soft-switching inverters can achieve high efficiencies and low switchdv/dtanddi/dt, but often impose additional stress on the switches due to their resonant behavior. Multicell converters, on the other hand, can process high voltages by arranging the switches to share the stress. This paper proposes a buck–boost multicell soft-switching inverter with inherent cell voltage balancing. Both the current and voltage stresses are shared by the cells. Soft-switching conditions are achieved for all switching transitions without any need for auxiliary circuitry or clamps. A multiple-winding transformer transfers energy between the terminals and provides galvanic isolation. An analytical study and experimental results on a 1200-W prototype verify the operation of the inverter. [ABSTRACT FROM AUTHOR]

This paper presents a novel single-stage soft-switched high-frequency-link three-phase dc–ac converter topology. The topology supports unidirectional dc to ac power flow and is targeted for applications like grid integration of photovoltaic sources, fuel cell, etc. The high frequency magnetic isolation results in reduction of system volume, weight, and cost. Sine-wave pulsewidth modulation is implemented in dc-side converter. Though high-frequency switched, dc-side converter is soft switched for most part of the line cycle. The ac-side converter active switches are line frequency switched incurring negligible switching loss. The line frequency switching of ac-side converter facilitates use of high voltage blocking inherently slow semiconductor devices to generate high voltage ac output. In addition, a cascaded multilevel structure is presented in this paper for direct medium-voltage ac grid integration. A detailed circuit analysis considering nonidealities like transformer leakage and switch capacitances, is presented in this paper. A 6-kW three-phase laboratory prototype is built. The presented simulation and experimental results verify the operation of the proposed topologies. [ABSTRACT FROM AUTHOR]

This paper presents a modified methodology for calculating static capacitance values between conductors with a circular cross section. Two forms of coil's stacking were considered, perfectly in 90° and each layer embedded in the previous one. Finite element simulations were carried out to determine the capacitance values for a 5 × 5 array arrangement, taking into account the two stacking forms. It was observed that depending on the position of the turn in the coil, it could present different capacitances values for the adjacent turns. A comparison between the proposed and existing methods for over isolated conductors was performed as well. The proposed method displayed good agreement with simulation and experimental results. The gain of the proposed method can be even more appreciated when comparing results for over insulated conductors. [ABSTRACT FROM AUTHOR]

A 10-year reanalysis of the PacIOOS Hawaiian Island Ocean Forecast System was produced using an incremental strong-constraint 4-D variational data assimilation with the Regional Ocean Modeling System (ROMS v3.6). Observations were assimilated from a range of sources: satellite-derived sea surface temperature (SST), salinity (SSS), and height anomalies (SSHAs); depth profiles of temperature and salinity from Argo floats, autonomous Seagliders, and shipboard conductivity–temperature–depth (CTD); and surface velocity measurements from high-frequency radar (HFR). The performance of the state estimate is examined against a forecast showing an improved representation of the observations, especially the realization of HFR surface currents. EOFs of the increments made during the assimilation to the initial conditions and atmospheric forcing components are computed, revealing the variables that are influential in producing the state-estimate solution and the spatial structure the increments form. [ABSTRACT FROM AUTHOR]

Highlights • Image decomposition using proposed optimized non-subsampled shearlet transform. • low-frequency bands are fused using scale multiplied Canny edge detector and Hessian features. • The high-frequency bands are fused using unsharp masking method. • Proposed "Variance Similarity Metric" ( Q E ), which is formulated using K-L transform. • Comparison between proposed and existing NSLP filters. Abstract Existing image fusion approaches are not so efficient to seize significant edges, texture and fine features of the source images due to ineffective and non-adaptive fusion structure. Also for objective evaluation of fusion algorithms, there is a need of a metric to measure source image features which are preserved in the fused image. To address these issues, an optimized non-subsampled shearlet transform (NSST) is developed, which is applied to decompose the source images into low- and high frequency bands. The low frequency bands are fused using proposed descriptor obtained from superposition of scale multiplied Canny edge detector features and Hessian features. The high frequency bands are fused using unsharp masking based fusion rule. Moreover, a metric Q E is formulated on the basis of Karhunen-Loeve transform (KLT). The information of image pixel variance for both source and fused images can be measured by using the proposed metric Q E , and it gives an indication of the amount of variance information transferred from the source images to the fused image. Both subjective and objective analysis show the efficacy of the proposed fusion structure and the metric Q E . [ABSTRACT FROM AUTHOR]

Incipient discharges formed in natural ester oil due to corona activity, surface discharges and particle movement were investigated using UHF technique. When the applied voltage magnitude is increased, on the inception of discharges, the peak to peak magnitude of the voltage of the UHF signal formed also increases. After certain applied voltage magnitude increase, if it is reduced, the magnitude of UHF signal formed is much higher than the magnitude of the signal formed when the voltage is increased. This phenomenon is observed with all the three types of discharges. Cross Recurrence plot (CRP) can be effectively used to determine the Time Difference Of Arrival (TDOA) of UHF signals. Self-similarity Recurrence Quantification Analysis (RQA) parameter is found to be more suitable technique to estimate the TDOA of UHF signals. CRP method is used to estimate TDOA correctly, even with low signal to noise ratio and it is found to be superior to Cross-correlation technique in estimating the TDOA. Estimated TDOA of UHF signals are validated and incipient discharge source is localized by adopting the non-iterative technique. [ABSTRACT FROM AUTHOR]

This paper deals with the design, the setup, and the operation of a rotating contactless energy transfer (CET) system. The system is used to replace the slip rings of an electrical excited synchronous machine and to transfer energy onto the rotor of the machine without mechanical contact. A compensation topology with an autoresonant circuit to generate the high frequency voltage is presented. Furthermore, the required parameters of the CET system are calculated. Based on these calculations, the system consisting of the power electronics, the compensation network, the magnetic path, the control system, and the electrical excited synchronous machine with controller, is set up. Finally, the calculated parameters are verified by measurements. [ABSTRACT FROM AUTHOR]

High frequency isolated dc–ac converters are becoming popular in on-board auxiliary inverters for railway applications. Compared to conventional fixed dc-link inverter (FDCLI), pulse dc-link inverter (PDCLI) has simpler circuit topology and higher power density, but greatly different control algorithm. This paper is going to study the application possibility of PDCLI for railway auxiliary inverter, and will be concerned with algorithm and their performance comparison between approximately independent hybrid modulation (AI-HM) and six-pulse demodulation hybrid modulation for PDCLIs, including fundamental principle and harmonic performance of output voltage under zero portion effect. Then a piecewise compensation of modulation wave and a carrier phase-shift strategy are proposed to improve the output voltage quality of AI-HM under zero portion effect. The simulation and experimental results, as well as the output filter parameter comparison are provided to verify the theoretical analysis and the feasibility of the proposed strategies. [ABSTRACT FROM PUBLISHER]

A new zero-voltage-switching (ZVS) push-pull pulse-width modulation (PWM) converter is proposed in this paper. The wide ZVS condition for all of the switches is obtained by utilizing the energy stored in the output inductor and magnetizing inductance. As a result, the switching losses can be dramatically reduced. A simple auxiliary circuit including two small diodes and one capacitor is added at the secondary side of a high frequency (HF) transformer to reset the primary current during the circulating stage and to clamp the voltage spike across the rectifier diodes, which enables the use of low-voltage and low-cost diodes to reduce the conducting and reverse recovery losses. In addition, there are no active devices or resistors in the auxiliary circuit, which can be realized easily. A detailed steady operation analysis, characteristics, design considerations, experimental results and a loss breakdown are presented for the proposed converter. A 500 W prototype has been constructed to verify the effectiveness of the proposed concept. [ABSTRACT FROM AUTHOR]

In a permanent magnet synchronous motor (PMSM) drive system, high-frequency (HF) pulsating voltage signal injection has demonstrated high accuracy to estimate the initial rotor position. However, a conventional signal demodulation method may face the problems of long convergence time and limited system stability, owing to the low-pass filter (LPF) used in signal demodulation process. Thus, a direct signal demodulation method is proposed, in which, the LPF is removed to improve system stability and dynamic property. A direct demodulation collection is generated to extract the position deviation signal from estimated q-axis HF current. Recursive discrete Fourier transform is employed to calculate the amplitude of estimated d-axis HF current, thus an amplitude normalized technique is implemented to reduce the effects of HF signal and PMSM. Meanwhile, the stability and dynamic property are compared between the conventional and proposed method, with the aid of D-partition technique and amplitude–frequency characteristics diagram. Furthermore, a novel magnetic polarity detection method is put forward based on the magnetic saturation, which has a high signal-to-noise ratio. Finally, the experimental results on three PMSM drive systems prove that the proposed method is practicable and effective. [ABSTRACT FROM PUBLISHER]

Design of duct networks is challenging because the design should consider the required flow rate, acceptable noise levels, and minimum pressure drop to achieve optimum performance. This paper presents acoustic analysis in high frequency range using sound power two-ports applied to Heating, Ventilation, and Air Conditioning (HVAC) systems. To simulate the acoustic behaviour one need to model three mechanisms; the sound power generated from sound sources (e.g. Fans), the regenerated sound power caused by the flow in different elements in the network (e.g. junctions), and the sound power loss across different elements of the network. The general approach considered here is based on two-port theory that divides the duct network into two-port elements. Each element can be described by 2 × 2 scattering matrix where the state variables are the acoustic power flow in both up and downstream directions. Junctions and branching are described by multi-port elements depending on the number of elements connected to this multiport. This algorithm is compared to measurements of HVAC system located in an academic building that shows good agreement. An advantage of this approach is the ability to use the same formalism of the two-port network theory to analyse the acoustic behaviour in both low and high frequency ranges beside the flow distribution and the pressure drop. [ABSTRACT FROM AUTHOR]

Current fed dual active bridge converters for photovoltaic generation may typically require a given leakage or extra inductance in order to provide proper control of the currents. Therefore, the many researches have been focused on the leakage inductance control of high frequency transformer to integrate an extra inductor. In this paper, an asymmetric winding arrangement to get the controlled leakage inductance for the high frequency transformer is proposed to improve the efficiency of the current fed dual active bridge converter. In order to accurate analysis, a coupled electromagnetic analysis model of transformer connected with high frequency switching circuit is used. A design optimization procedure for high efficiency is also presented using design analysis model, and it is verified by the experimental result. [ABSTRACT FROM AUTHOR]

In this paper, a class of single-phase Z-source (ZS) ac–ac converters is proposed with high-frequency transformer (HFT) isolation. The proposed HFT isolated (HFTI) ZS ac–ac converters possess all the features of their nonisolated counterparts, such as providing wide range of buck-boost output voltage with reversing or maintaining the phase angle, suppressing the in-rush and harmonic currents, and improved reliability. In addition, the proposed converters incorporate HFT for electrical isolation and safety, and therefore can save an external bulky line frequency transformer, for applications such as dynamic voltage restorers, etc. The proposed HFTI ZS converters are obtained from conventional (nonisolated) ZS ac–ac converters by adding only one extra bidirectional switch, and replacing two inductors with an HFT, thus saving one magnetic core. The switching signals for buck and boost modes are presented with safe-commutation strategy to remove the switch voltage spikes. A quasi-ZS-based HFTI ac–ac is used to discuss the operation principle and circuit analysis of the proposed class of HFTI ZS ac–ac converters. Various ZS-based HFTI proposed ac–ac converters are also presented thereafter. Moreover, a laboratory prototype of the proposed converter is constructed and experiments are conducted to produce output voltage of 110 Vrms / 60 Hz, which verify the operation of the proposed converters. [ABSTRACT FROM AUTHOR]

Analysis of lightning transients in cable systems necessitates accurate calculation of ground return impedances (GRIs) in high-frequency regions. EMT-type programs used for transient analyses incorporate various methods for calculation of GRI. Most of these methods include assumptions and approximations, the validity of which needs to be questioned at high frequencies. In particular, the common approximation of the neglecting effect of displacement currents should be reviewed. The purpose of this paper is to evaluate commonly used GRI calculation methods in terms of their accuracy and validity in high-frequency regions. In this study, GRI calculation methods are analyzed and compared, the effect of displacement currents is evaluated, and validity ranges of the basic calculation formula (Pollaczek formula) are discussed. It is shown that for high-resistivity and high-permittivity soil cases, the effect of displacement currents must be taken into account in high-frequency regions. Most default calculation routines in EMT-type programs neglect this effect. Therefore, it is very important to review and understand the calculation methods used in EMT-type programs before employing them in high-frequency transient analyses. Moreover, it is shown that in most of the practical cases the basic calculation formula (Pollaczek formula) is valid for the frequency range of interest for lightning studies. [ABSTRACT FROM AUTHOR]

This paper addresses the problem of vibration sources identification in coupled thin plates at high frequency range. An energy-based method called Simplified Energy Method (MES) is used for that purpose. It was initially developed to predict the distribution of structural energy density in mid- and high frequency ranges regarding injected power. This paper extends this energy method to solve inverse structural problems to identify vibration sources acting on coupled structural subelements. Thanks to this inverse formulation, the forces in a structure are obtained from the energy density data. Numerical simulations were performed to test the validity of the present formulation using different positions of external forces and measurement points. Numerical as well as experimental results show that the inverse MES (IMES) method has an excellent performance in estimating the input forces applied to a complex structure modeled with a set of assembled plates from the density energy measurements. [ABSTRACT FROM AUTHOR]

To meet the design requirements of high-efficiency and broadband of power amplifiers in wireless communication systems employing modulation signals with high peak to average power ratio (PAPR), an improved Doherty power amplifier (DPA) was investigated. To achieve the impedance transformations both in low power region and at saturation, a two impedance matching network design was proposed to simulate and design the main and auxiliary output matching networks, which could simplify the load modulation network of DPA and extend the bandwidth. The wideband DPA with 2.30～2.80 GHz was designed and measured, and the proposed method availability was verified. The results show that the designed DPA has the gain fluctuation less than 2 dB and the saturated output power more than 43.5 dBm over the frequency band of 2.30～2.80 GHz, while the saturated efficiency is from 66% to 71% with 6 dB back-off efficiency from 48% to 59%. The designed wideband DPA can satisfy the efficiency requirement of future wireless communication systems employing modulation signals with high PAPR. [ABSTRACT FROM AUTHOR]

This work investigates high-frequency measurements for the contribution to the understanding of different stall inception types in high-speed multistage compressors. A 3.5-stage high-speed axial multistage compressor is investigated with a 2 MW test rig in the Laboratoire de Mécanique des Fluides et d’Acoustique (LMFA) at Ecole Centrale de Lyon, France. Two different types of instabilities arise in this compressor as a function of shaft speed. At part speed, a controversy called “rotating instability” type flow field modulation is identified with the measurements. New results are the demonstration of the periodic behavior of this instability and the analogy to classical frequency modulation, periodic to one revolution of the instability. Furthermore, the amplitude of the instability is modulated by the time period of a rotor revolution. At nominal speed, the abrupt spike type stall inception is detected, taking usually less than five rotor revolutions. [ABSTRACT FROM AUTHOR]

The transportation of dry fine powders is an emerging technologic task, as in biotechnology, pharmaceutical and coatings industry the particle sizes of processed powders get smaller and smaller. Fine powders are primarily defined by the fact that adhesive and cohesive forces outweigh the weight forces, leading to mostly unwanted agglomeration (clumping) and adhesion to surfaces. Thereby it gets more difficult to use conventional conveyor systems (e.g. pneumatic or vibratory conveyors) for transport. A rather new method for transporting these fine powders is based on ultrasonic vibrations, which are used to reduce friction between powder and substrate. Within this contribution an experimental set-up consisting of a pipe, a solenoid actuator for axial vibration and an annular piezoelectric actuator for the high frequency radial vibration of the pipe is described. Since amplitudes of the radial pipe vibration should be as large as possible to get high effects of friction reduction, the pipe is excited to vibrate in resonance. To determine the optimum excitation frequency and actuator position the vibration modes and resonance frequencies of the pipe are calculated and measured. Results are in good accordance. [ABSTRACT FROM AUTHOR]

In this paper, the design procedure of a 14.4 kV output voltage, 100 kHz transformer with an isolation voltage of 115 kV using Litz wire is presented. All design models, including a generalized magnetic model for the leakage and the loss calculations as well as an electrical model for the parasitic capacitance estimation for the transformer are derived and proven by measurements. For designing the insulation, a comprehensive design method based on an analytical maximum electrical field evaluation and an electrical field conform design is used. The resulting design is verified by long and short term partial discharge measurements on a prototype transformer. [ABSTRACT FROM AUTHOR]

Parasitic capacitances of the high-frequency (HF) transformer used in dc–dc converter impact harmonic analysis of the transformer and interactions with the converter. Resonant frequencies of open- and short-circuit impedances characteristics (ICs) have been widely used to extract parasitic capacitances of HF transformers, but it was reported that some resonant frequencies were unobtainable as they were too high where the ICs fluctuated heavily, and parasitic capacitances remained unevaluated. In this paper, resonance features of ICs and voltage transfer characteristics (TCs) are investigated, and a new frequency of voltage TC is used for extracting parasitic capacitances. By analyzing inner links of known impedance resonant frequencies, why and which resonant frequencies are likely to be unobtainable is revealed. Furthermore, a new frequency corresponding to zero of TC, accounting for parallel resonance between leakage inductance and mutual capacitance of the two windings, is used to extract parasitic capacitances. With this new frequency mutual capacitance of the windings is evaluated, and missing equation due to unobtainable impedance resonant frequency is added. Besides, merits and limitations of extracting parasitic capacitances based on ICs and TCs are analyzed. These techniques are verified with several HF transformer prototypes in the laboratory. [ABSTRACT FROM AUTHOR]

The consequences of short-circuit forces for high-temperature superconductor (HTS) transformers are much more detrimental than conventional ones. That is because on one hand, HTS windings have less mechanical robustness than copper ones and on the other hand, the effects of extreme axial forces on HTS windings are irreversible and may cause irreparable damages to HTS transformers. In this paper, capability of new concentric multi-segment winding configurations in HTS transformers, proposed in Ghabeli et al. (2015), to mitigate high axial and radial electromagnetic forces due to short-circuit fault is investigated. The studied winding configurations include concentric PSP (Primary-Secondary-Primary), symmetrical and asymmetrical PSPSP (Primary-Secondary-Primary-Secondary-Primary). The study is realized with an advanced numerical method using accurate and nonlinear E-J power law via FEM-based software Flux 2D. Profiles of axial and radial electromagnetic forces in each sub-winding have been presented, analyzed and compared to each other in detail. For accurate calculation of axial and radial forces in each sub-winding, a collection of points along the longitudinal and transverse axes in the center of each sub-winding was considered. The maximum of radial and axial electromagnetic forces decrease remarkably in PSPSP winding configuration compared to PSP winding configuration. In addition, the asymmetrical winding configurations mitigate these forces more than symmetrical ones. [ABSTRACT FROM AUTHOR]

To transmit constant powers particularly at light load, the isolated dual active bridge (DAB) with conventional dual-phase-shift (DPS) control exhibits large variations of currents, losses, and efficiencies if there is a small change of outer phase shift ratio, which is caused by the change of command to adjust power transmission. This is mainly because of the narrow operation region of outer phase shift ratio if the conventional DPS modulation is used. To solve this problem, a novel DPS control with bidirectional inner phase shifts for the DAB is developed to reduce the current surge in the high-frequency transformer, which can stabilize the output power with high efficiency. From the analysis of operation modes and power characteristics of the proposed DPS control, it is shown that due to wider operation region of outer phase shift ratios compared with the conventional DPS control in which the inner phase shifts of both the primary and secondary H-bridges are in the same direction, lower surge current can be obtained in the change of phase shift command, which results in stable power transmission particularly at light load without scarifying the efficiency. Moreover, the DAB of using the proposed DPS control can transmit bidirectional energy with the inner and outer shift ratios of 0–1. For experimental verifications, the test results are also presented in this paper. [ABSTRACT FROM AUTHOR]

Spatial-saliency-tracking-based high-frequency (HF) pulsating carrier injection method is widely employed in permanent-magnet synchronous machines sensorless control. In order to avoid the problems of long convergence time, potential start-up failure and limited system stability existing in tracking observers, a strategy to inject three HF pulsating voltages with different frequencies and amplitudes into ABC frame is proposed in this paper. With the proposed method, three frequency components in the response current signals are demodulated and then combined together to calculate the rotor position directly. Different from the conventional method, the proposed solution utilizes both d- and q-axis carrier current instead of only the q-axis carrier current to enhance the acquisition of the rotor position information. Meanwhile, a new signal processing strategy is developed to completely eliminate the effects of system delay. Furthermore, the compensation of cross-saturation effect is deduced in detail to improve the accuracy of rotor position estimation. In addition, magnetic polarity detection is implemented with the same proposed solution by demodulating the second harmonic components from the response current signals, which simplifies the magnetic polarity process. Finally, the experimental results demonstrate that the proposed strategy can obtain an accurate rotor position with good steady-state and dynamic performance. [ABSTRACT FROM AUTHOR]

Recent improvements in magnetic material characteristics and switching devices have generated a possibility to replace the electrical buses with high-frequency magnetic links in microgrids. Multiwinding transformers (MWTs) as magnetic links can effectively reduce the number of conversion stages of renewable energy system by adjusting turn ratio of windings according to the source voltage level. Other advantages are galvanic isolation, bidirectional power flow capability, and simultaneous power transfer between multiple ports. Despite the benefits, design, and characterization of MWTs are relatively complex due to their structural complexity and cross-coupling effects. This paper presents all stages of numerical design, prototyping, and characterization of an MWT for microgrid application. To design the transformer for certain value of parameters, the reluctance network method is employed. Due to the iterative nature of transformer design, it presented less computation time and reasonable accuracy. A prototype of designed transformer is implemented using amorphous magnetic materials. A set of experimental tests are conducted to measure the magnetic characteristics of the core and series coupling and open-circuit tests are applied to measure the transformer parameters. A comparison between the simulation and experimental test results under different loads within the medium-frequency range validated both design and modeling procedures. [ABSTRACT FROM AUTHOR]

Traditional transformer models available in EMTP-like software packages are not capable of representing transformer behavior during a transient state, which includes high frequencies, since they usually do not adequately take into account the transformer resonant behavior caused by its highly complicated design. Therefore, more complex “Black box” models are developed. Those models can be established without any knowledge of transformer geometry, based on the fitting of the measured admittance matrix of the transformer versus frequency. Unfortunately, the measurement and exploitation of a transformer's admittance matrix are not straightforward. The existing fitting methods include solving a non-convex constrained problem. Hence, it is not always easy to find an optimal solution of the problem. The difficulties, which can arise when building a high frequency “Black box” transformer model, are described in this paper together with a comparison of the performance and fitting accuracy of different numerical packages. [ABSTRACT FROM AUTHOR]

The high-frequency transformer is an important device in the modern electrical system, and the performance requirement for this device has recently become more and more important. Thus, it is necessary to model high-frequency transformer accurately and the key is to calculate the capacitance. In this paper, the capacitance of the foil winding in the high-frequency transformer is calculated by using the static capacitance model and the finite-element method (FEM), respectively. The relative error is about 0.4% by comparing the FEM. To calculate the capacitance of solid wire accurately, a new static capacitance model is proposed considering that the actual electric field lines gradually changed in the windings. Compared with the finite-element model, the proposed model is more accurate than the traditional models. The capacitances of two wires calculated by the proposed model and the finite-element model are 115.56 and 114.76 pF, respectively. The dynamic capacitance of transformer windings is analyzed by considering the uneven charge distribution. A dynamic capacitance analytical model is present by considering the eddy current effect. The result of the dynamic capacitance is 49.81 pF. It is found that the dynamic capacitance is lower than the static capacitance according to the static capacitance and the dynamic capacitance analytical model. [ABSTRACT FROM PUBLISHER]

It is well known that there is an increasing demand for bidirectional DC–DC converters for applications that range from renewable energy sources to electric vehicles. Within this context, this work proposes novel DC–DC converter topologies that use the three-state switching cell (3SSC), whose well-known advantages over conventional existing structures are ability to operate at high current levels, while current sharing is maintained by a high frequency transformer; reduction of cost and dimensions of magnetics; improved distribution of losses, with consequent increase of global efficiency and reduction of cost associated to the need of semiconductors with lower current ratings. Three distinct topologies can be derived from the 3SSC: one DC–DC converter with reversible current characteristic able to operate in the first and second quadrants; one DC–DC converter with reversible voltage characteristic able to operate in the first and third quadrants and one DC–DC converter with reversible current and voltage characteristics able to operate in four quadrants. Only the topology with bidirectional current characteristic is analysed in detail in terms of the operating stages in both nonoverlapping and overlapping modes, while the design procedure of the power stage elements is obtained. In order to validate the theoretical assumptions, an experimental prototype is also implemented, so that relevant issues can be properly discussed. [ABSTRACT FROM PUBLISHER]

The design and development of a compact high voltage, high peak power, high frequency transformer for a converter type modulator of klystron amplifiers is presented. The transformer has been designed to operate at a frequency of 20 kHz and at a flux swing of ±0.6 T. Iron (Fe) based nanocrystalline material has been selected as a core for the construction of the transformer. The transformer employs a specially designed solid Teflon bobbin having 120 kV insulation for winding the high voltage secondary windings. The flux swing of the core has been experimentally found by plotting the hysteresis loop at actual operating conditions. Based on the design, a prototype transformer has been built which is per se a unique combination of high voltage, high frequency, and peak power specifications. The transformer was able to provide 58 kV (pk-pk) at the secondary with a peak power handling capability of 700 kVA. The transformation ratio was 1:17. The performance of the transformer is also presented and discussed. [ABSTRACT FROM AUTHOR]

As electronic devices are becoming more compact each day, the more effective and efficient active cooling technologies are needed. Microfluidic devices, such as synthetic jets, serve as a potential candidate to fulfill the thermal management needs of the next generation electronics. An experimental and computational study has been performed for circular central-orifice synthetic jets. First, a series of experiments was performed to quantify the actuator deflection, air velocity, heat transfer augmentation, and power consumption for central-orifice synthetic jets. Later, a computational study was performed utilizing the same boundary conditions in order to predict the deflection of the diaphragm. The experiments were conducted on three different types of synthetic jets, namely, low-, medium-, and high-frequency synthetic jets. Although a number of correlations were proposed for the prediction of Nu number for slot synthetic jets, no correlation was found to predict the average Nu number for a synthetic jet with a round orifice. Therefore, two correlations were developed, one for low- and medium-frequency synthetic jets and the other for high-frequency synthetic jets to predict the heat transfer coefficient as a function of the geometry, position, and operating condition for impinging flows. The proposed correlations are able to predict the impingement heat transfer of a synthetic jet with an accuracy of ±25% for a wide range of operating conditions and geometrical variables. Normalized frequency had the minimum impact on the average Nu number of a high-frequency synthetic jet compared with dimensionless distance, both have moderate impact on low- and medium-frequency jets. [ABSTRACT FROM PUBLISHER]

Inverters are required to convert the dc power from photovoltaic panels to ac. In grid connected inverters, the ac output is directly fed into the electric grid. In this paper, a high-frequency transformer based inverter topology is presented. This leads to a very compact inverter. The modulation of this inverter with a lossless snubber circuit is proposed. Proposed modulation eliminates the over-voltage spikes due to transformer leakage inductance. Commutation of the leakage inductance energy with this modulation is also discussed. The performance of the inverter with the proposed modulation is validated using simulation and experimental results. [ABSTRACT FROM AUTHOR]

With the background of power supplies for high-precision apparatuses, an isolated high-frequency dc–ac converter based on a differential output structure is proposed. It is composed of two phase-shifted full bridges (PSFBs) that share a common half-bridge as the lagging leg on the primary side. Two current-doubler synchronous rectifiers (CDRs) constitute the differential output structure on the secondary side. The ac output is synthesized by the two PSFB-CDRs generating the same dc bias and phase-reversed ac components. Compared with traditional solutions, the new circuit has the potential capability to obtain an ultralow distortion output voltage without using special control or compensations. Moreover, zero voltage switching is realized in the primary circuit within a wide load range, whereas zero current switching is achieved in the secondary circuit. Operation principles are analyzed, and a 1-kW prototype is built to validate the proposed scheme. [ABSTRACT FROM PUBLISHER]

Frequency-dependent leakage inductance is often observed. The high-frequency eddy current effects cause a reduction in leakage inductance. The proximity effect between adjacent layers is responsible for the reduction of leakage inductance. This paper gives a detailed analysis of high-frequency leakage inductance and proposes an accurate prediction methodology. High-frequency leakage inductances in several interleaved winding configurations are also discussed. Interleaved winding configurations actually give a smaller degree of reduction of leakage induction at high frequency. Finite-element analysis simulation and measurement validate the models. [ABSTRACT FROM AUTHOR]

This paper conducts modeling and controller design for a SEries-connected Output Universal Link (SEOUL) converter. The SEOUL converter was proposed to interface multiple distributed power sources to the grid with a reduced amount of reactive components. It employs a high-frequency (HF) transformer instead of a bulky line-frequency transformer like solid-state transformers (SSTs). In addition, it does not use a large dc-link capacitor at the input side and reduces the size of the input filter. This is inherently possible due to its original operating characteristic. The input phase current is not actively controlled but synthesized indirectly by HF power transfer between input and output sides of the SEOUL converter. Therefore, it is important to properly control the HF power transfer part, on which this paper focuses. An analytic model is developed, and a control strategy is proposed based on the model with a decoupling control method. The developed model and the proposed control method were verified by experiments. [ABSTRACT FROM PUBLISHER]

For evolutionary algorithms with the ability to self-adapt, linking the algorithmic operators and the problem features is one of the most interesting topics. One of the best ways to begin a study of this topic is to explore the relationship between the optimization hardness and the problem features. This paper attempts to interpret the relationship between optimization hardness and frequency features of real-parameter problems through a qualitative analysis based on an idealized model. Based on the results of a theoretically qualitative analysis, the effective high-frequency ratio (EHFR) is subsequently proposed to measure the optimization hardness of real-parameter problems. Finally, three aspects to the performance of EHFR are evaluated: stability, precision and ability to distinguish. Test results show that the EHFR is relevant not only for the results of theoretical analysis, but also for the other features related to the optimization hardness. [ABSTRACT FROM AUTHOR]

When operating higher up in frequency, the copper losses in transformer windings will significantly rise due to enhanced skin and proximity effect. This leads to a high need to develop new methods to accurately evaluate winding losses at higher frequencies. This paper investigates the effect of different geometrical parameters at a wide range of frequencies in order to propose a pseudoempirical formula for winding loss calculation in high-frequency transformers. A comprehensive analysis of the edge effect and ac resistance is done by performing more than 12 300 2-D finite element simulations on foil and round conductors. Unlike previous studies which mostly focused on specific case studies with limited applications, this model provides very high accuracy, especially where the most common analytical models drastically underestimate the winding losses, with a wide-range applicability which could be of interest for designers to avoid time consuming FEM simulation without compromising with the accuracy. Several transformers are built and the model is experimentally verified with a good agreement. [ABSTRACT FROM AUTHOR]

The increase of transmission frequency reveals more merits than low- or medium-frequency distribution among different kinds of power applications. High-frequency inverter serves as source side in high-frequency ac (HFAC) power distribution system (PDS). However, it is complicated to obtain a high-frequency inverter with both simple circuit topology and straightforward modulation strategy. A novel switched-capacitor-based cascaded multilevel inverter is proposed in this paper, which is constructed by a switched-capacitor frontend and H-Bridge backend. Through the conversion of series and parallel connections, the switched-capacitor frontend increases the number of voltage levels. The output harmonics and the component counter can be significantly reduced by the increasing number of voltage levels. A symmetrical triangular waveform modulation is proposed with a simple analog implementation and low modulation frequency comparing with traditional multicarrier modulation. The circuit topology, symmetrical modulation, operation cycles, Fourier analysis, parameter determination, and topology enhancement are examined. An experimental prototype with a rated output frequency of 25 kHz is implemented to compare with simulation results. The experimental results agreed very well with the simulation that confirms the feasibility of proposed multilevel inverter. [ABSTRACT FROM AUTHOR]

In this paper, a novel modular cascaded multi-level converter with multiwinding high-frequency (MWHF) transformer is proposed for medium- or high-voltage applications. In the proposed converter, a cascaded H-bridge rectifier (CHBR) is connected directly with the input ac source while the traditional step-down transformer is no longer necessary. Then, an isolated dc-dc converter composed by a group of H-bridge converters and a MWHF transformer with high power density is used to isolate the dc buses produced by the CHBR. The equivalent circuit and mathematical model of the MWHF transformer and the high frequency (HF) converter are obtained firstly in this paper. Then, the voltage stability under unbalanced loads is analyzed and the naturally balance ability is verified. To accelerate the dc bus voltage balancing process, a voltage balance control algorithm based on energy exchange between different transformer windings is proposed that is realized by accomplishing the phase shift adjustment of the terminal voltages on different windings. Simulations and experiments are implemented to verify the performance of the proposed converter and voltage balance algorithm. [ABSTRACT FROM AUTHOR]

This paper presents a single-stage bidirectional high-frequency transformer (HFT) link dc/ac converter topology for a three-phase adjustable magnitude and frequency PWM ac drive. This type of converters find a wide range of applications including UPS systems, drives involving renewable energy sources (Solar, Fuel cell), and energy storage systems (typically low voltage dc to high voltage PWM ac). The HFT results in reduction in cost and weight along with a considerable increase in power density. The adverse effects of common-mode voltage are well known in this kind of applications. The proposed topology along with a modulation technique reduces common-mode voltage to practically zero and generates high-quality output voltage waveform comparable to conventional space vector PWM (CSVPWM). A source-based commutation method, presented in this paper, to commute the energy stored in the leakage inductance of the HFT resulting in the following advantages 1) no need for any auxiliary circuits with passive components; 2) almost complete recovery of the leakage energy; 3) soft switching of the output side converter for all load conditions; and 4) minimization of common-mode voltage switching due to commutation. The converter along with the suggested control has been analyzed in detail. The presented simulation and experimental results confirm the operation of the proposed converter. [ABSTRACT FROM AUTHOR]

SUMMARY Two-stage isolated converters for photovoltaic (PV) applications commonly employ a high-frequency transformer (HFT) on the DC-DC side, submitting the DC-AC inverter switches to high voltages and forcing the use of insulated-gate bipolar transistors instead of low-voltage and low-loss metal-oxide-semiconductor field-effect transistors. This paper shows the modeling, control and simulation of a single-phase full-bridge inverter with HFT that can be used as part of a two-stage converter with transformerless DC-DC side or as a single-stage converter (simple DC-AC inverter) for grid-connected PV applications. The inverter is modeled to obtain a small-signal transfer function used to design the P + Resonant current control regulator, whose main advantages are the simplicity for implementation in single-phase systems and zero steady state error with sinusoidal current. A high-frequency step-up transformer results in low-voltage switches and better efficiency compared with converters in which the transformer is used on the DC-DC side. Simulations and experimental results with a 200-W prototype are shown. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Highlights: [•] A bipolar-power-transistor-(BPT)-based limiter is presented. [•] BPT-based limiter exhibit relatively good performances at high frequency operation. [•] The BPT-based limiter is good choice for high frequency ultrasound systems. [ABSTRACT FROM AUTHOR]

We use ultrafast pump-probe spectroscopy to study the mechanical vibrations in the time domain of doubly clamped silicon nitride beams. Beams with two different clamping conditions are investigated. Finite element method calculations are performed to analyse the mode spectra of both structures. By calculating the strain integral on the surface of the resonators, we are able to reproduce the effect of the detection mechanism and identify all the measured modes. We show that our spectroscopy technique combined with our modelling tools allow the investigation of several different modes in the super high frequency range (3-30 GHz) and above, bringing more information about the vibration modes of nanomechanical resonators. [ABSTRACT FROM AUTHOR]

This letter presents a new zero-voltage-switching (ZVS) isolated dc–dc converter which combines a boost half-bridge (BHB) cell and a full-bridge (FB) cell, so that two different type of power sources, i.e., both current fed and voltage fed, can be coupled effectively by the proposed converter for various applications, such as fuel cell and supercapacitor hybrid energy system. By fully using two high- frequency transformers and a shared leg of switches, number of the power devices and associated gate driver circuits can be reduced. With phase-shift control, the converter can achieve ZVS turn-on of active switches and zero-current switching (ZCS) turn-off of diodes. In this letter, derivation, analysis, and design of the proposed converter are presented. Finally, a 25–50 V input, 300–400 V output prototype with a 600 W nominal power rating is built up and tested to demonstrate the effectiveness of the proposed converter topology. [ABSTRACT FROM AUTHOR]

This paper presents a parallel-circuit Class-E amplifier that maintains it operating characteristics even when the output capacitance of the transistor is greater than the optimum shunt capacitance. The finite dc feed inductor in the parallel circuit topology is replaced by a transmission-line compensation circuit to eliminate the limitations on the operating frequency imposed by the output capacitance. Theoretical formulas for the circuit elements of the proposed network are derived in detail, and the analysis is validated by simulation and measurement. The measured maximum output power of 40.1 dBm, drain efficiency of 77.5%, and power-added efficiency of 70.8% were obtained at 2.8 GHz with a 29.4-dBm input power. These measurements show a similar or better level of efficiency and output power that is reported for other amplifiers at lower frequencies using the same transistor. [ABSTRACT FROM AUTHOR]

We show that for multiorbital quantum impurity models the noncrossing approximation and one-crossing approximation versions of the self-consistent hybridization expansions violate the sum rules relating the coefficients of the high-frequency expansion of the self-energy and the product of the self-energy and Green's function to thermodynamic expectation values. Comparison of noncrossing/one-crossing results to numerically exact quantum Monte Carlo calculations shows that the consistency with sum rules provides a useful estimate of the reliability of the approximations. The sum rule violations are more pronounced, and therefore the quality of the noncrossing/one-crossing approximation is poorer, in situations with multiple orbitals and away from particle-hole symmetry but becomes less severe as the correlation strength increases. The one-crossing approximation is markedly superior to the noncrossing approximation. [ABSTRACT FROM AUTHOR]

A new mode matching method is presented for the electromagnetic scattering from large cavity-backed apertures. The new method is based on the expansion of the field inside the cavity by the standard modes, and a periodic extension of the field on the cavity aperture to the whole ground plane. The computation cost is low by solving only the coefficients of the modes. Numerical examples are presented to show the efficiency of the approach. [ABSTRACT FROM AUTHOR]

Integration of passive magnetic devices is essential in the design of highly efficient power converters. This is due to the necessity to conserve as much space as possible for modern devices such as thin LED TVs and small bright LED illumination. This paper focuses on this multiphysics issue and presents a novel integrated magnetic system (IMS), investigated by the finite element method (FEM). Two different FEM approaches are applied to prove the accuracy of the analyzed data. Both magnetic systems have been developed with a novel capacitor layer design. This provides the system with the required resonance tank integrated in the conductor arrangement, without any interference to the transformer section or vice versa. [ABSTRACT FROM PUBLISHER]