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Temperature is a critical parameter for the GaN HEMT as it sharply impacts the electrical characteristics of the device more than for SiC or Si MOSFETs. Either when designing a power converter or testing a device for reliability and robustness characterizations, it is essential to estimate the junction temperature of the device. For this aim, manufacturers provide compact models to simulate the device in SPICE-based simulators. These models provide the junction temperature, which is considered uniform along the channel. We demonstrate through two-dimensional numerical simulations that this approach is not suitable when the device undergoes high electrothermal stress, such as during short circuit (SC), when the temperature distribution along the channel is strongly not uniform. Based on numerical simulations and experimental measurements on a 650 V/4 A GaN HEMT, we derived a thermal network suitable for SPICE simulations to correctly compute the junction temperature and the SC current, even if not providing information about the possible failure of the device due to the formation of a local hot spot. For this reason, we used a second thermal network to estimate the maximum temperature reached inside the device, whose results are in good agreement with the experimental observed failures. [ABSTRACT FROM AUTHOR]

Soft switching for both primary and secondary side devices is available by using LLC converters. This resonant converter is an ideal candidate for today's high frequency, high efficiency, and high power density applications like adapters, Uninterrupted Power Supplies (UPS), Solid State Transformers (SST), electric vehicle battery chargers, renewable energy systems, servers, and telecom systems. Using Gallium-Nitride (GaN)-based power switches in this converter merits more and more switching frequency, power density, and efficiency. Therefore, the present paper focused on GaN-based LLC resonant converters. The converter structure, operation regions, design steps, and drive system are described precisely. Then its losses are discussed, and the magnets and inductance characteristics are investigated. After that, various interleaved topologies, as a solution to improve power density and decrease current ripples, have been discussed. Also, some challenges and concerns related to GaN-based LLC converters have been reviewed. Commercially available power transistors based on various technologies, i.e., GaN HEMT, Silicon (Si) MOSFET, and Silicon Carbide (SiC) have been compared. Finally, the LLC resonant converter has been simulated by taking advantage of LTspice and GaN HEMT merits, as compared with Si MOSFETs. [ABSTRACT FROM AUTHOR]

The use of a resistive shunt is one of the simplest and most used methods for measuring current in an electronic device. Many researchers use this method to measure drain current during short-circuiting of fast devices such as GaN HEMTs. However, the high switching speed of these devices together with the non-ideality of the shunt resistors produces an overestimation of the current in the initial phases of the transient. In this paper, a passive compensation network is proposed, which is formed by adding an inductor to the voltage measurement circuit and allows an accurate measurement of the current using the resistive shunt even in the presence of very fast devices. The proposed method is validated by simulations and experimental measurements. [ABSTRACT FROM AUTHOR]

This article presents the results of a 2-D finite element simulation study of the gate damages induced by heavy-ion irradiation in SiC power metal–oxide–semiconductor field-effect transistors (MOSFETs). The time evolution of the electric field in the gate oxide is studied. Two effects are investigated: the first is associated with the charge deposition in the SiC portion of the MOSFET, with the time evolution studied using the 2-D finite element simulator; the second one results from holes generated during the ion transit, trapped in the gate oxide after the fast electrons have been quickly swept away by the electric field. Two different techniques have been combined for estimating the hole concentration in the gate oxide: the well-known recombination rate was modified to consider the trapped charge yield, as was recently done to better interpret single event gate rupture (SEGR) failure of silicon power MOSFETs. Under ion irradiation test conditions at which the gate damage experimentally starts to be observed, we demonstrate that, because of the ion impact, regardless of the ion linear energy transfer (LET), the peak value of the electric field in the gate oxide becomes practically equal to the oxide breakdown field (~12–15 MV/cm). Moreover, we show that simulations can be used to predict the test conditions at which gate damage starts to appear as a function of LET and the range of heavy ions used in the irradiation experiments. [ABSTRACT FROM AUTHOR]

Insulated-gate bipolar transistors (IGBTs) operated in short circuit become instable in certain driving and load conditions. The induced oscillations can compromise robustness and reliability of the entire power converter. The stability of the device inserted in a real system can be analyzed using the theory of linear oscillators that requires the knowledge of input or output impedance of the device in real operating conditions. In this paper, we present an experimental procedure for measuring in pulsed mode the small-signal impedance of a power device biased in any test conditions. The small-signal input impedance of a 650-V 20-A IGBT operated in short circuit has been measured as a function of the frequency. This input impedance has been used to extract the stability map of the IGBT in short circuit, which allows us to easily predict the test conditions where the IGBT becomes instable. The validity of this stability map has been confirmed by a large-signal time-domain characterization of the IGBT operated in short circuit. The proposed technique is very useful to design driving circuit able to avoid instable operations. [ABSTRACT FROM AUTHOR]

The purpose of this paper is to present a detailed experimental and numerical study on the mechanisms involved in Insulated Gate Bipolar Transistor (IGBT) instability during turn-ON short circuit that can compromise its robustness in particular load and driving conditions. It is shown that the IGBT may exhibit oscillations at both low and high frequencies depending on different physical mechanisms. Furthermore, a new methodology that allows determining the stability limits of the device in relation with the parameters of the external circuit is proposed. The experimental measurements confirm the obtained results. [ABSTRACT FROM AUTHOR]

Background: Although disseminated intravascular coagulation (DIC) and other hemocoagulative abnormalities are severe complications of head injury, their effect on clinical outcome remains unclear, particularly among children. Objectives: To evaluate the frequency of hemocoagulative abnormalities and their influence on outcome among children with head injury. Study Design: We conducted a prospective observational study among 60 children with head injury, immediately evaluating severity of head injury (Glasgow Coma Scale, GCS); cerebral axial tomography; prothrombin time; activated partial thromboplastin time (aPTT); fibrinogen level; concentration of fibrin-fibrinogen degradation products (FDP), and platelet count. Two months after injury, we applied the Glasgow Outcome Score (GOS). Associations with GOS were evaluated using univariate and multivariate logistic models. Results: Among children with severe head injury, 22.2% (6/27) developed DIC, all of whom died and had shown severe brain edema. Among those with severe head injury yet without DIC, the mortality was only 14.2%. A low GOS was significantly and independently associated with a low GCS, multiple trauma, delayed aPTT, low fibrinogen level, elevated FDP and low platelet count. Brain edema was also associated with a low GOS, though not significantly. Conclusions: In addition to GCS, type of trauma, type of brain lesion and certain coagulation abnormalities are predictors of GOS.Copyright © 2001 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]

Few studies on analgesia with remifentanil (Rf) in children are available, and there are no data on the use of this drug in pediatric neurosurgery. Rf is a new μ-receptor opioid agonist, acting through the activation of pain inhibitory mechanisms. We conducted a prospective trial on the analgesic effects of Rf in 20 children less than 1 year of age undergoing a neurosurgical procedure for craniosynostosis repair. Rf was administered at doses of 0.25 μg/kg/min, by continuous infusion, 1 h after admission to the pediatric intensive care unit (PICU). The treatment was prolonged for 12 h after the operation. The postoperative pain was evaluated in our PICU, comparing the changing of behavioral (AFS and CHEOPS score) and hemodynamic (heart rate, respiratory rate, systolic and diastolic blood pressure, oxygen saturation, O[sub 2] and CO[sub 2] partial pressure) parameters, before and after treatment with Rf. This drug showed a satisfactory pain control in all the children treated. No significant side effects were noticed, except for one episode of urinary retention. In conclusion, Rf appears to be safe and effective for the treatment of acute pain in the very young child submitted to craniosynostosis repair.Copyright © 2000 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]

The results of an international multicenter study concerning the first complication of newly implanted cerebrospinal fluid shunts in nontumoral hydrocephalus are the subject of the present report. The authors have collected information on 773 cases from four continents. In particular, the following data were evaluated in relation to the general incidence of complications recorded in the first follow-up year: the patient's age at the operation, the etiology of hydrocephalus, the type of CSF shunt device used, and the modality of the surgical procedures. The overall complication rate in the series was 29%. Age and etiology of hydrocephalus appear to play a major role in influencing the complication rate; on the other hand, the choice of a specific CSF shunt device seems to be less important in this respect. [ABSTRACT FROM AUTHOR]

Tissue engineering is one of the major challenges of orthopedics and trauma surgery for bone regeneration. Biomaterials filled with mesenchymal stem cells (MSCs) are considered the most promising approach in bone tissue engineering. Furthermore, our previous study showed that the multi-phase poly [ε-caprolactone]/thermoplastic zein-hydroxyapatite (PCL/TZ-HA) biomaterials improved rabbit (r) MSCs adhesion and osteoblast differentiation, thus demonstrating high potential of this bioengineered scaffold for bone regeneration. In the recent past, CD271 has been applied as a specific selective marker for the enrichment of MSCs from bone marrow (BM-MSCs). In the present study, we aimed at establishing whether CD271-based enrichment could be an efficient method for the selection of rBM-MSCs, displaying higher ability in osteogenic differentiation than non-selected rBM-MSCs in an in vitro system. CD271(+) cells were isolated from rabbit bone marrow and were compared with rMSCs in their proliferation rate and osteogenic differentiation capability. Furthermore, rCD271(+) cells were tested in their ability to adhere, proliferate and differentiate into osteogenic lineage, while growing on PCL/TZ-HA scaffolds, in comparison to rMSCs. Our result demonstrate that rCD271(+) cells were able to adhere, proliferate and differentiate into osteoblasts when cultured on PCL/TZ-HA scaffolds in significantly higher levels as compared to rMSCs. Based on these findings, CD271 marker might serve as an optimal alternative MSCs selection method for the potential preclinical and clinical application of these cells in bone tissue regeneration.

Background: A limited number of studies have addressed the methods, indications and particular problems that may occur when programming prolonged intracranial pressure (ICP) monitoring in pediatric patients. Parenchymal fiberoptic transducers have been shown to give reliable ICP readings; moreover, they present a relatively low rate of complications, are easily placed and, as they are solid state, they are not subject to obstruction., Material/methods: A recently developed fiberoptic ICP transducer (Codman intraparenchymal sensor) was used to continuously monitor intracranial pressure in seventy children with non-traumatic neurosurgical diseases. The admitting diagnoses were hydrocephalus or shunt-related problems in 33 cases, single-suture (5 cases) or complex (16 cases) craniosynostosis in 21 patients, and sylvian scissure arachnoid cyst (SAC) in 16 cases. A software (ICP monitoring release) designed in our department was used for ICP recording storage and analysis., Results: Raised ICP values were found in six of the seventeen patients with a suspected active hydyrocephalus, 24% of children with non-syndromic craniosynostosis, 52.8% of syndromic craniosynostosis patients, 50% or the children with a Type 11 SAC and two of the three patients with Type II SAC., Conclusions: Overall, prolonged ICP monitoring proved to be extremely useful in guiding surgical indications. The fiberoptic device used in our unit was shown to be reliable and associated with a relatively low rate of complications. Finally, the software allowed easy review and analysis of the obtained data.