Current Issue : April - June Volume : 2020 Issue Number : 2 Articles : 5 Articles
An abnormal star-like defect was found on the failed SiC gate turn-off thyristor (GTO)\ndevices after metal removal and KOH etching at 450 degreeC in this work. It is of extraordinary larger size\nof 210â??580 microm, even much larger than the etch pit of a micropipe in 4H-SiC. In addition, the abnormal\nstar-like defect, exhibiting the consistent orientation with the six-fold symmetry of silicon carbide, was\nfound to consist of several penetrating dislocations with the help of a LEXT OLS4000 3D laser confocal\nmicroscope. These abnormal star-like etch pits can severely reduce the forward blocking characteristic\nof GTOs, while exerting insignificant influence on the forward current-voltage characteristics between\nanode and gate electrode of the 4H-SiC GTO devices. Interestingly, the relationship between forward\nvoltage drop and dislocation density is affected by the abnormal star-like defect. A regular increase of\nforward voltage drop at 100 A/cm2 was observed with the increasing dislocation density, while this\ncorrelation disappears in the presence of an abnormal star-like defect....
Thee peak-current-mode (PCM) control strategy is widely adopted in pulse\nwidth-modulated (PWM) DC-DC converters. However, the converters always involve a sub-harmonic\noscillating state or chaotic state if the active duty ratio is beyond a certain range. Hence, an extra\nslope signal in the inductor-current loop is used to stabilize the operation of the converter. This paper\npresents a new technique for enlarging the stable range of PCM-controlled DC-DC converters,\nin which the concept of utilizing unstable period-1 orbit (UPO-1) of DC-DC converters is proposed\nand an implementation scenario based on the parameter-perturbation method is presented. With the\nproposed technique, perturbations are introduced to the reference current of the control loop, and the\nconverters operating in a chaotic state can be tracked, and thus be stabilized to the target UPO-1.\nTherefore, the stable operating range of the converters is extended. Based on an example of a\nPCM-controlled boost converter, simulations are presented as a guide to a detailed implementation\nprocess of the proposed technique, and comparisons between the proposed technique and techniques\nin terms of ramp compensation are provided to show the differentiation in the performance of the\nconverter. Experimental results in the work confirm the effectiveness of the proposed technique....
At present, optical fiber microducts are joined together by mechanical type joints.\nMechanical joints are bulky, require more space in multiple duct installations, and have poor water\nsealing capability. Optical fiber microducts are made of high-density polyethylene which is\nconsidered best for welding by remelting. Mechanical joints can be replaced with welded joints if\nthe outer surface layer of the optical fiber microduct is remelted within one second and without\nthermal damage to the inner surface of the optical fiber duct. To fulfill these requirements, an\nelectro-thermal model of Joule heat generation using a copper coil and heat propagation inside\ndifferent layers of optical fiber microducts was developed and validated. The electro-thermal model\nis based on electro-thermal analogy that uses the electrical equivalent to thermal parameters.\nDepending upon the geometric shape and material properties of the high-density polyethylene,\nlow-density polyethylene, and copper coil, the thermal resistance and thermal capacitance values\nwere calculated and connected to the Cauer RC-ladder configuration. The power input to Joule\nheating coil and thermal convection resistance to surrounding air were also calculated and\nmodelled. The calculated thermal model was then simulated in LTspice, and real measurements\nwith 50 microm K-type thermocouples were conducted to check the validity of the model. Due to the\nnon-linear transient thermal behavior of polyethylene and variations in the convection resistance\nvalues, the calculated thermal model was then optimized for best curve fitting. Optimizations were\nconducted for convection resistance and the power input model only. The calculated thermal\nparameters of the polyethylene layers were kept intact to preserve the thermal model to physical\nstructure relationship. Simulation of the optimized electro-thermal model and actual measurements\nshowed to be in good agreement....
Energy harvesters generate power only when ambient energy is available, and power loss\nis significant when the harvester does not produce energy and its power management circuit is still\nturned on. This paper proposes a new high-efficiency power management circuit for intermittent\nvibration energy harvesting. The proposed circuit is unique in terms of autonomous power supply\nswitch between harvester and storage device (battery), as well as self-start and control of the\noperation mode (between active and sleep modes). The self-start controller saves power during an\ninactive period and the impedance matching concept enables maximum power transfer to the\nstorage device. The proposed circuit is prototyped and tested with an intermittent vibration energy\nharvester. Test results found that the daily energy consumption of the proposed circuit is smaller\nthan that of the resistive matching circuit: 0.75 J less in sleep mode and 0.04 J less in active mode\nwith self-start....
Forecasting the power production from renewable energy sources (RESs) has become\nfundamental in microgrid applications to optimize scheduling and dispatching of the available assets.\nIn this article, a methodology to provide the 24 h ahead Photovoltaic (PV) power forecast based on a\nPhysical Hybrid Artificial Neural Network (PHANN) for microgrids is presented. The goal of this\npaper is to provide a robust methodology to forecast 24 h in advance the PV power production in\na microgrid, addressing the specific criticalities of this environment. The proposed approach has\nto validate measured data properly, through an effective algorithm and further refine the power\nforecast when newer data are available. The procedure is fully implemented in a facility of the\nMulti-Good Microgrid Laboratory (MG2\nLab) of the Politecnico di Milano, Milan, Italy, where new\nEnergy Management Systems (EMSs) are studied. Reported results validate the proposed approach\nas a robust and accurate procedure for microgrid applications....
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