Accurate Measurement of Dynamic on-State Resistances of GaN Devices under Reverse and Forward Conduction in High Frequency Power Converter

Because of trapped charges in GaN transistor structure, device dynamic ON-state resistance RDSon is increased when it is operated in high frequency switched power converters, in which device is possibly operated by zero voltage switching (ZVS) to reduce its turn-ON switching losses. When GaN transistor finishes ZVS during one switching period, device has been operated under both reverse and forward conduction. Therefore its dynamic RDSon under both conduction modes needs to be carefully measured to understand device power losses. For this reason, a measurement circuit with simple structure and fast dynamic response is proposed to characterise device reverse and forward RDSon. In order to improve measurement sensitivity when device switches at high frequency, a trapezoidal current mode is proposed to measure device RDSon under almost constant current, which resolves measurement sensitivity issues caused by unavoidable measurement circuit parasitic inductance and measurement probes deskew in conventional device characterisation method by triangle current mode. Proposed measurement circuit and measurement method is then validated by first characterising a SiC-MOSFET with constant RDSon. Then, the comparison on GaN-HEMT dynamic RDSon measurement results demonstrates the improved accuracy of proposed trapezoidal current mode over conventional triangle current mode when device switches at 1MHz

Barn med sosiale medier. Digitaliserte fellesskap i en nordnorsk femteklasse

Med et antropologisk blikk på barns egne perspektiver undersøker denne masteroppgaven hvordan barn i femteklasse i en nordnorsk bygd forholder seg til bruken av mobiltelefon og sosiale medier, og hvordan det påvirker deres sosiale relasjoner. Masteroppgaven skriver seg inn i et krysningsfelt mellom antropologiske studier av sosiale medier og barndomsforskning. Studien bygger på fokusgrupper og individuelle intervjuer, supplert med deltakende observasjon, og i oppgaven drøftes epistemologiske, metodiske og etiske spørsmål knyttet til studier av barn. Det argumenteres for at introduksjonen til sosiale medier representerer en ny periode i barndommen, hvor barna selv ser seg i en fase hvor de befinner seg mellom «små barn» og «ungdommer». Samtidig utgjør sosiale medier også et nytt rom i barndommen - et online rom for digital sosialitet, hvor barna har mulighet til å være i lag, på tross av fysiske avstander i bygdelivet. Ved å se på barns bruk av sosiale medier som lek vises det hvilken rolle sosiale medier spiller i barndommen. Barnas bruk av sosiale medier erstatter ikke «vanlig» offline samvær, men fungerer først og fremst som en forsterkelse av lokale sosiale relasjoner og fellesskap de allerede inngår i. I dette foregår det ulike grader av inklusjon og eksklusjon. Oppgaven drøfter også hvordan temaene kjønn og risikohåndtering spiller en rolle i barnas online fellesskap. Et sentralt funn i studien er at det finnes en diskrepans mellom barns mangefasetterte praksis med sosiale medier og voksenverdenens unyanserte diskurs. Dette er en av flere maktstrukturer som barn med sosiale medier må navigere i

A GaN-HEMT compact model including dynamic r_DSon effect for power electronics converters

In order to model GaN-HEMT switching transients and determine power losses, a compact model including dynamic RDSon effect is proposed herein. The model includes mathematical equations to represent device static and capacitance-voltage characteristics, and a behavioural voltage source, which includes multiple RC units to represent different time constants for trapping and detrapping effect from 100 ns to 100 s range. All the required parameters in the model can be obtained by fitting method using a datasheet or experimental characterisation results. The model is then implemented into our developed virtual prototyping software, where the device compact model is co-simulated with a parasitic inductance physical model to obtain the switching waveform. As model order reduction is applied in our software to resolve physical model, the device switching current and voltage waveform can be obtained in the range of minutes. By comparison with experimental measurements, the model is validated to accurately represent device switching transients as well as their spectrum in frequency domain until 100 MHz. In terms of dynamic RDSon value, the mismatch between the model and experimental results is within 10% under different power converter operation conditions in terms of switching frequencies and duty cycles, so designers can use this model to accurately obtain GaN-HEMT power losses due to trapping and detrapping effects for power electronics converters

Investigation on Single and Split Output Gate Configurations Influence on the GaN-HEMTs Switching Behaviours

This work investigates the power GaN-HEMTs switching behaviour differences resulted from usage of two gate driving configurations: single and split outputs. The analysis based on simulation and experimental results show that GaN-HEMTs could switch slower and cause higher switching losses when the split output configuration is used. This is because the output capacitance (Coss) of MOSFETs inside gate driver will be charged during the turn-on process of GaN-HEMTs, and this charging process can reduce the charging speed of input capacitance (Ciss) of GaN-HEMTs. Moreover, the gate resistance and parasitic inductance are the main parameters selected for analysis, and their distribution can amplify this effect by increasing the impedance ratio of turn-on and turn-off loop. This research provides guiding suggestions for gate driver and high-efficiency GaN-HEMTs power module design.</p

Impact of V th Instability of Schottky-type p-GaN Gate HEMTs on Switching Behaviors

Schottky-type p-GaN gate Gallium Nitride High Electron Mobility Transistors (GaN-HEMTs) suffer from threshold voltage ( Vth ) instability phenomenon. Both positive and negative Vth shifts are reported when device undertakes the voltage bias, but the impact of this Vth instability phenomenon on device switching behaviors is less investigated. In this study, the drain-source voltage ( Vds ) induced bidirectional Vth shift in hard-switching condition is characterized and decoupled by an H-bridge based double-pulse test (DPT). Subsequently, the influence of Vth shift on switching behaviors is theoretically analyzed and demonstrated through SPICE simulation and experiment, showing how a positive shifted Vth can reduce the device turn-on commutation speed and increase the switching losses, and vice versa. The results suggest that the Vth instability phenomenon should be considered in accurate switching modeling

Grensesetting og bruk av tvang overfor barn i fosterhjem

Source at https://nordlandsforskning.no/nb/om-nordlandsforskning

DMTs and Covid-19 severity in MS: a pooled analysis from Italy and France

We evaluated the effect of DMTs on Covid-19 severity in patients with MS, with a pooled-analysis of two large cohorts from Italy and France. The association of baseline characteristics and DMTs with Covid-19 severity was assessed by multivariate ordinal-logistic models and pooled by a fixed-effect meta-analysis. 1066 patients with MS from Italy and 721 from France were included. In the multivariate model, anti-CD20 therapies were significantly associated (OR&nbsp;=&nbsp;2.05, 95%CI&nbsp;=&nbsp;1.39–3.02, p&nbsp;&lt;&nbsp;0.001) with Covid-19 severity, whereas interferon indicated a decreased risk (OR&nbsp;=&nbsp;0.42, 95%CI&nbsp;=&nbsp;0.18–0.99, p&nbsp;=&nbsp;0.047). This pooled-analysis confirms an increased risk of severe Covid-19 in patients on anti-CD20 therapies and supports the protective role of interferon

Typologie des assemblages d'ostréidés (Bivalvia) et interprétations paléoenvironnementales

International audienceLes assemblages d'ostréidés fossiles présentent une grande diversité de géométries spatiales. Leurs accumulations peuvent être, soit tridimensionnelles (lumachelles), dans l'épaisseur d'un banc, soit bidimensionnelles, à la surface d'un banc (hardground). Une analyse taphonomique permet de discriminer ces différentes structures d'assemblages en fonction de la manière dont le temps a pu s'enregistrer dans les séries sédimentaires. Cette analyse préalable conditionne fortement les interprétations paléoenvironnementales. Une typologie des différents assemblages possibles est proposée à partir de plusieurs exemples d'accumulations coquillières du Crétacé supérieur nord-aquitain (Sud-Ouest de la France) et du Néogène supérieur andalou (Sud-Est de l'Espagne)

A Simple Carrier-Based Modulation for the SVM of the Matrix Converter

Today, industry has not fully embraced the matrix converter solution. One important reason is its high control complexity. It is therefore relevant to propose a simpler but efficient modulation scheme, similar as three phase voltage source inverter modulators with the well-known symmetrical carrier-based ones. The modulation presented in this paper is equivalent to a particular space vector modulation (SVM) and takes into account harmonics and unbalanced input voltages, with the same maximum voltage transfer ratio (86%). The aim of this work is to propose a simple and general pulse width-modulation method using carrier-based modulator for an easier matrix converter control. Furthermore, a simple duty cycle calculation method is used, based on a virtual matrix converter. Finally, simulations and experimentations are presented to validate this simple, original and efficient modulation concept equivalent to matrix converter SVM.TRavaux de thèses financé par le pole MEDEE, La région Nord pas de calais et STIE (Schneider Toshiba Inverter Europe