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This paper proposes an Active Gate Current Control (AGCC) strategy for non-insulating gate WBG devices, for example, gallium nitride gate-injection-transistor (GaN-GIT) and silicon carbide super junction transistor (SiC-SJT). It provides a tool for power converter designers to further improve the converter efficiency and to extend the life time of those higher cost power transistors. By continuously...
The emerging 650 V large current rating, Enhancement-mode (E-mode) Gallium Nitride High-Electron-Mobility Transistor (GaN HEMT) is a promising device for low to medium power, high power density converters (e.g., motor drives, battery chargers), which require high robustness levels. Thus, a comprehensive study of the short circuit behavior of high power E-mode GaN HEMT is the subject of this paper...
Application areas such as electric vehicles, data centers and microgrids increasingly rely on power electronic converters and electric motor drives to control loads. These tightly regulated converters act as Constant Power Loads (CPLs) in the context of the entire system. The negative incremental impedance of a CPL causes inherent instability which may lead to a grid collapse. This paper reviews the...
Silicon Carbide (SiC) power devices with super-cascode structure provide a cost-effective solution for high performance medium voltage power switches. However, these SiC super-cascode devices are still in the early development stage, and limited information on the device characteristics is available. This paper presents the characterization and evaluation of a 4.5 kV, 40 A SiC super-cascode device...
With the rapid development of the wide bandgap (WBG) power transistor technology, the latest Silicon-Carbide and Gallium-Nitride (SiC and GaN) based power transistors can be used as the main switches in the medium power (≥10 kW) conversion systems. Electrical signal based efficiency measurement could be very challenging due to the high-frequency output signals and the high power to loss ratio. In...
This paper presents an evaluation of a newly released power stage direct drive GaN HEMT. The double pulse test is performed and the dynamic performance is characterized. PCB layout optimization is also investigated for improved switching transients. It is shown that the direct drive system provides simplified circuit design, reduced number of external components, and small gate loop parasitics. Also,...
In this paper, we study the influence from the sentiment of regular tweets on retweeting. We propose a method to calculate the sentiment score for each tweet and each Twitter user. This method enables us to place the tweets and retweets into the same time period to explore the sentiment factor. We adopt the correlation coefficient between the sentiment scores of regular tweets and those of retweets...
Many high-end power machines demand greater bearing performance in terms of running speed, precision, reliability, longevity, noise reduction, etc. An ultrasonic bearing based on Near-Field Acoustic Levitation (NFAL) effect is a kind of emerging noncontact gas bearing. Due to its self-aligning capability, the ultrasonic bearing can steadily run at a high speed under high precision. Actuated by piezoelectric...
Compared with traditional rotary-percussive drills driven by electromagnetic motors, ultrasonic drills show advantages of small size, low power, low weight on bit, and lubrication free, making them available for the extraterrestrial rock sampling, especially for the minor planet with a weak gravitational field. To solve the problem that the conventional percussive ultrasonic drills cannot remove cuttings...
Many high-end power machines demand greater bearing performance in terms of running speed, precision, reliability, longevity, noise reduction, etc. An ultrasonic bearing based on Near-Field Acoustic Levitation (NFAL) effect is a kind of emerging noncontact gas bearing. Due to its self-aligning capability, the ultrasonic bearing can steadily run at a high speed — tens of thousands of revolutions per...
Compared with traditional rotary-percussive drill driven by magnetic motors, ultrasonic drill which usually employs the high frequency longitudinal vibration of Langevin transducer to break down rocks, shows advantages of small size, low power, low axial load, and lubrication free, making it available for the extraterrestrial rock sampling, especially for the minor planet with a weak gravitational...
The nonlinear shear modulus was found to exhibit higher contrast than shear modulus for the detection of breast lesions (Bernal, Chamming's et al. 2016) and may thus serve as a potential diagnostic index of tissue pathology. Such nonlinear elastic constant of soft tissues has been demonstrated to be effectively estimated by combining ultrasound elastography and supersonic shear imaging based on acoustoelastic...
Assessing the interventricular septum (IVS) facilitates the understanding of complex ventricular-ventricular interaction and related cardiac diseases. Previous studies have revealed that the functionally bilayered IVS exhibits distinct radial and circumferential strains in its left (Ls) and right (Rs) sides. Nonetheless, the transverse shear and principal strains, which reflect the interlayer sliding...
The development of advanced dielectric materials with high thermal conductivity is of crucial importance in modern electronic and power equipment systems. Herein, multilayer graphene nanosheets/epoxy nanocomposites with enhanced thermal conductivity were fabricated by solution blended processes. It was found that the incorporation of graphene nanosheets had little impact on thermogravimetric properties...
In order to verify whether a small-scale rotorcraft can fly on Mars, this paper is proposed to establish a Martian atmosphere simulator and to design a hover test stand for coaxial rotorcraft rotor. Since the ultra-low air density of Mars is approximately 1/70 of that on the Earth, it is difficult to obtain enough thrust to lift the rotorcraft against Mars gravity. The proposed Martian Atmosphere...
This paper presents a detailed experimental approach for understanding and optimizing the hover performance of Martian UAV rotor, including a systematic discussion about several coefficients for hover characteristics and a detailed design approach for rotor blades of Martian UAV at Reynolds numbers lower than 50000. The discussed hover performance coefficients include figure of merit, thrust coefficient,...
A novel higher order scheme of current and charge integral equation (CCIE) is introduced for the accurate electromagnetic modeling in the wide frequency band. The higher order hierarchical vector bases are adopted for the current expansion, whilst the orthogonalized polynomials are utilized as scalar bases for the charge expansions. A numerical result is given to show the stable performance at wide...
The wireless sensors deployed at the highway can ensure the safety of the traveling vehicles. However, the ribbon deployed wireless sensor network in the roadside infrastructure can easily to generate energy hole. Cooperative communication between sensors and vehicles is an effective way to improve this situation and reduce the energy consumption of the sensors. A cooperative relay selection algorithm...
The Wiener index W(G) of a connected graph G, introduced by Wiener in 1947, is defined as W(G)=\sum_{u,v\in V(G)}d_G(u,v) where d_G(u,v) is the distance (length a shortest path) between the vertices u and v in G. For $S\subseteq V(G)$, the {\it Steiner distance\/} d(S) of the vertices of S, introduced by Chartrand et al. in 1989, is the minimum size of a connected subgraph of G whose vertex set contains...
Dynamic spectrum access (DSA) technique has emerged as a fundamental approach to mitigate the spectrum scarcity problem. As a key form of DSA, the government is proposing to release more federal spectrum for sharing with commercial wireless users. However, the flourish of federal-commercial sharing hinges upon how the federal privacy is managed. In current DSA proposals, the sensitive exclusion zone...
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