Thermal test method for high power three-phase grid-connected inverters

Semiconductor device junction temperatures are maintained within datasheet specified limits to avoid failure in power converters. Burn-in tests are used to ensure this. In inverters, thermal time constants can be large and burn-in tests are required to be performed over long durations of time. At higher power levels, besides increased production cost, the testing requires sources and loads that can handle high power. In this study, a novel method to test a high power three-phase grid-connected inverter is proposed. The method eliminates the need for high power sources and loads. Only energy corresponding to the losses is consumed. The test is done by circulating rated current within the three legs of the inverter. All the phase legs being loaded, the method can be used to test the inverter in both cases of a common or independent cooling arrangement for the inverter phase legs. Further, the method can be used with different inverter configurations - three- or four-wire and for different pulse width modulation (PWM) techniques. The method has been experimentally validated on a 24 kVA inverter for a four-wire configuration that uses sine-triangle PWM and a three-wire configuration that uses conventional space vector PWM.

Electroluminescence in Molecular Junctions: A Diagrammatic Approach

We compute electroluminescent signal in a current carrying single molecule junction using a superoperator formalism. Liouville space loop diagrams are used to identify all density matrix pathways that emit photons via the electroluminescence process. A frequency resolved spectrum is expressed in terms of the various Pock space states of the isolated molecule that participate in the creation and subsequent recombination of exciton. Application is made to a multilevel Coulomb blockade model system and to a gold-benzene-1,4-dithiol-gold molecular junction.

Part I: Physical Insights Into the Two-Stage Breakdown Characteristics of STI-Type Drain-Extended pMOS Device

In this paper, we study breakdown characteristics in shallow-trench isolation (STI)-type drain-extended MOSFETs (DeMOS) fabricated using a low-power 65-nm triple-well CMOS process with a thin gate oxide. Experimental data of p-type STI-DeMOS device showed distinct two-stage behavior in breakdown characteristics in both OFF-and ON-states, unlike the n-type device, causing a reduction in the breakdown voltage and safe operating area. The first-stage breakdown occurs due to punchthrough in the vertical structure formed by p-well, deep n-well, and p-substrate, whereas the second-stage breakdown occurs due to avalanche breakdown of lateral n-well/p-well junction. The breakdown characteristics are also compared with the STI-DeNMOS device structure. Using the experimental results and advanced TCAD simulations, a complete understanding of breakdown mechanisms is provided in this paper for STI-DeMOS devices in advanced CMOS processes.

Botulinum neurotoxin type-A enters a non-recycling pool of synaptic vesicles

Neuronal communication relies on synaptic vesicles undergoing regulated exocytosis and recycling for multiple rounds of fusion. Whether all synaptic vesicles have identical protein content has been challenged, suggesting that their recycling ability may differ greatly. Botulinum neurotoxin type-A (BoNT/A) is a highly potent neurotoxin that is internalized in synaptic vesicles at motor nerve terminals and induces flaccid paralysis. Recently, BoNT/A was also shown to undergo retrograde transport, suggesting it might enter a specific pool of synaptic vesicles with a retrograde trafficking fate. Using high-resolution microscopy techniques including electron microscopy and single molecule imaging, we found that the BoNT/A binding domain is internalized within a subset of vesicles that only partially co-localize with cholera toxin B-subunit and have markedly reduced VAMP2 immunoreactivity. Synaptic vesicles loaded with pHrodo-BoNT/A-Hc exhibited a significantly reduced ability to fuse with the plasma membrane in mouse hippocampal nerve terminals when compared with pHrodo-dextran-containing synaptic vesicles and pHrodo-labeled anti-GFP nanobodies bound to VAMP2-pHluorin or vGlut-pHluorin. Similar results were also obtained at the amphibian neuromuscular junction. These results reveal that BoNT/A is internalized in a subpopulation of synaptic vesicles that are not destined to recycle, highlighting the existence of significant molecular and functional heterogeneity between synaptic vesicles.

Efficient density matrix renormalization group algorithm to study Y junctions with integer and half-integer spin

An efficient density matrix renormalization group (DMRG) algorithm is presented and applied to Y junctions, systems with three arms of n sites that meet at a central site. The accuracy is comparable to DMRG of chains. As in chains, new sites are always bonded to the most recently added sites and the superblock Hamiltonian contains only new or once renormalized operators. Junctions of up to N = 3n + 1 approximate to 500 sites are studied with antiferromagnetic (AF) Heisenberg exchange J between nearest-neighbor spins S or electron transfer t between nearest neighbors in half-filled Hubbard models. Exchange or electron transfer is exclusively between sites in two sublattices with N-A not equal N-B. The ground state (GS) and spin densities rho(r) = < S-r(z)> at site r are quite different for junctions with S = 1/2, 1, 3/2, and 2. The GS has finite total spin S-G = 2S(S) for even (odd) N and for M-G = S-G in the S-G spin manifold, rho(r) > 0(< 0) at sites of the larger (smaller) sublattice. S = 1/2 junctions have delocalized states and decreasing spin densities with increasing N. S = 1 junctions have four localized S-z = 1/2 states at the end of each arm and centered on the junction, consistent with localized states in S = 1 chains with finite Haldane gap. The GS of S = 3/2 or 2 junctions of up to 500 spins is a spin density wave with increased amplitude at the ends of arms or near the junction. Quantum fluctuations completely suppress AF order in S = 1/2 or 1 junctions, as well as in half-filled Hubbard junctions, but reduce rather than suppress AF order in S = 3/2 or 2 junctions.

Statistics of an adiabatic charge pump

We investigate the effect of time-dependent cyclic-adiabatic driving on the charge transport in a quantum junction. We propose a nonequilibrium Green's function formalism to study the statistics of the charge pumped (at zero bias) through the junction. The formulation is used to demonstrate charge pumping in a single electronic level coupled to two (electronic) reservoirs with time-dependent couplings. An analytical expression for the average pumped current for a general cyclic driving is derived. It is found that for zero bias, for a certain class of driving, the Berry phase contributes only to the odd cumulants. In contrast, a quantum master equation formulation does not show a Berry-phase effect at all.

Optimizing Photovoltaic Response by Tuning Light-Harvesting Nanocrystal Shape Synthesized Using a Quick Liquid-Gas Phase Reaction

The electron recombination lifetime in a sensitized semiconductor assembly is greatly influenced by the crystal structure and geometric form of the light-harvesting semiconductor nanocrystal. When such light harvesters with varying structural characteristics are configured in a photoanode, its interface with the electrolyte becomes equally important and directly influences the photovoltaic efficiency. We have systematically probed here the influence of nanocrystal crystallographic structure and shape on the electron recombination lifetime and its eventual influence on the light to electricity conversion efficiency of a liquid junction semiconductor sensitized solar cell. The light-harvesting cadmium sulfide (CdS) nanocrystals of distinctly different and controlled shapes are obtained using a novel and simple liquid gas phase synthesis method performed at different temperatures involving very short reaction times. High resolution synchrotron X-ray diffraction and spectroscopic studies respectively exhibit different crystallographic phase content and optical properties. When assembled on a mesoscopic TiO2 film by a linker molecule, they exhibit remarkable variation in electron recombination lifetime by 1 order of magnitude, as determined by ac-impedance spectroscopy. This also drastically affects the photovoltaic efficiency of the differently shaped nanocrystal sensitized solar cells.

Optimizing Photovoltaic Response by Tuning Light-Harvesting Nanocrystal Shape Synthesized Using a Quick Liquid-Gas Phase Reaction

The electron recombination lifetime in a sensitized semiconductor assembly is greatly influenced by the crystal structure and geometric form of the light-harvesting semiconductor nanocrystal. When such light harvesters with varying structural characteristics are configured in a photoanode, its interface with the electrolyte becomes equally important and directly influences the photovoltaic efficiency. We have systematically probed here the influence of nanocrystal crystallographic structure and shape on the electron recombination lifetime and its eventual influence on the light to electricity conversion efficiency of a liquid junction semiconductor sensitized solar cell. The light-harvesting cadmium sulfide (CdS) nanocrystals of distinctly different and controlled shapes are obtained using a novel and simple liquid gas phase synthesis method performed at different temperatures involving very short reaction times. High resolution synchrotron X-ray diffraction and spectroscopic studies respectively exhibit different crystallographic phase content and optical properties. When assembled on a mesoscopic TiO2 film by a linker molecule, they exhibit remarkable variation in electron recombination lifetime by 1 order of magnitude, as determined by ac-impedance spectroscopy. This also drastically affects the photovoltaic efficiency of the differently shaped nanocrystal sensitized solar cells.

Investigations on the physical origin of lateral photovoltage in PbS-colloidal quantum dot/Si heterojunctions

Restricted area heterojunctions, an array of lead sulfide colloidal quantum dots (PbS-CQDs) and crystalline silicon, are studied with a non-destructive remote contact light beam induced current (RC-LBIC) technique. As well as getting good quality active area images we observed an anomalous unipolar signal response for the PbS-CQD/n-Si devices and a conventionally expected bipolar signal profile for the PbS-CQD/p-Si devices. Interestingly, our simulation results consistently yielded a unipolar and bipolar nature in the signals related to the PbSCQD/n-Si and PbS-CQD/p-Si heterostructures, respectively. In order to explain the physical mechanism involved in the unipolar signal response of the PbS-CQD/n-Si devices, we propose a model based on the band alignment in the heterojunctions, in addition to the distribution of photo-induced excess majority carriers across the junction. Given that the RC-LBIC technique is well suited to this context, the presence of these two distinct mechanisms (the bipolar and unipolar nature of the signals) needs to be considered in order to have a better interpretation of the data in the characterization of an array of homo/heterojunctions.

Flood Routing Models in Confluent and Dividing Channels

By introducing a water depth connecting formula, the hydraulic equations in the dividing channel system were coupled and the relation of discharge distribution between the branches of the dividing channels can be yielded. In this manner, a numerical model for the confluent channels was established to study the variation of backwater effects with the parameters in the channel junction. The meeting of flood peaks in the mainstream and tributary can be analyzed with this model. The flood peak meeting is found to be a major factor for the extremely high water level in the mainstream during the 1998 Yangtze River flood. Subsequently the variations of discharge distribution and water level with channel parameters between each branch in this system were studied as well. As a result, flood evolution caused by Jingjiang River shortcut and sediment deposition in the entrance of dividing channels of the Yangtze River may be qualitatively elucidated. It is suggested to be an effective measure for flood mitigation to enhance regulation capability of reservoirs available upstream of the tributaries and harness branch entrance channels.

Evaluation of termination techniques for 4H-SiC PiN diodes and trench JFETs

An investigation concerning suitable termination techniques for 4H-SiC trench JFETs is presented. Field plates, p+ floating rings and junction termination extension techniques are used to terminate 1.2kV class PiN diodes. The fabricated PiN diodes evaluated here have a similar design to trench JFETs. Therefore, the conclusions for PiN diodes can be applied to JFET structures as well. Numerical simulations are also used to illustrate the effect of the terminations on the diodes' blocking mode behaviour.

Switching performance of epitaxially grown normally-off 4H-SiC JFET

Static and dynamic behavior of the epitaxially grown dual gate trench 4H-SiC junction field effect transistor (JFET) is investigated. Typical on-state resistance Ron was 6-10mΩcm2 at VGS = 2.5V and the breakdown voltage between the range of 1.5-1.8kV was realized at VGS = -5V for normally-off like JFETs. It was found that the turn-on energy delivers the biggest part of the switching losses. The dependence of switching losses from gate resistor is nearly linear, suggesting that changing the gate resistor, a way similar to Si-IGBT technology, can easily control di/dt and dv/dt. Turn-on losses at 200°C are lower compared to those at 25°C, which indicates the influence of the high internal p-type gate layer resistance. Inductive switching numerical analysis suggested the strong influence of channel doping conditions on the turn-on switching performance. The fast switching normally-off JFET devices require heavily doped narrow JFET channel design. © (2009) Trans Tech Publications, Switzerland.

Novel wideband active waveguide photonic crystal polarisation selector

We have designed and fabricated a broadband and compact polarisation selector using a photonic crystal at the junction of two intersecting active waveguides. The crystal shows >8dB polarisation selectivity over a 70nm range. © 2003 Optical Society of America.

Enhanced efficiency of near-UV emitting LEDs for solid-state lighting applications

The performance of a series of near-UV (∼385 nm) emitting LEDs, consisting of high efficiency InGaN/AlInGaN QWs in the active region, was investigated. Significantly reduced roll-over of efficiency at high current density was found compared to InGaN/GaN LEDs emitting at a similar wavelength. The importance of optical cavity effects in flip-chip geometry devices has also been investigated. The light output was enhanced by more than a factor of 2 when the lightemitting region was located at an anti-node position with respect to a high reflectivity current injection mirror. A power of 0.49 mW into a numerical aperture of 0.5 was obtained for a junction area of 50μm in diameter and a current of 30 mA, corresponding to a radiance of 30 W/cm2/str.

High temperature direct double side cooled inverter module for hybrid electric vehicle application

In this paper a novel approach to the design and fabrication of a high temperature inverter module for hybrid electrical vehicles is presented. Firstly, SiC power electronic devices are considered in place of the conventional Si devices. Use of SiC raises the maximum practical operating junction temperature to well over 200°C, giving much greater thermal headroom between the chips and the coolant. In the first fabrication, a SiC Schottky barrier diode (SBD) replaces the Si pin diode and is paired with a Si-IGBT. Secondly, double-sided cooling is employed, in which the semiconductor chips are sandwiched between two substrate tiles. The tiles provide electrical connections to the top and the bottom of the chips, thus replacing the conventional wire bonded interconnect. Each tile assembly supports two IGBTs and two SBDs in a half-bridge configuration. Both sides of the assembly are cooled directly using a high-performance liquid impingement system. Specific features of the design ensure that thermo-mechanical stresses are controlled so as to achieve long thermal cycling life. A prototype 10 kW inverter module is described incorporating three half-bridge sandwich assemblies, gate drives, dc-link capacitance and two heat-exchangers. This achieves a volumetric power density of 30W/cm3.