Power converters in WBG device technology for automotive applications and characterization setups for GaN power transistors

This PhD dissertation envisages the design of innovative power converters exploiting WBG devices to get state-of-the-art performance in products intended for industrial applications of automotive field. The collaborations with different specialized companies, provided the opportunity to access commercially-available state-of-the-art SiC and GaN technologies and the possibility to realize innovative converter prototypes. Concerning SiC technology, the complete design of a $350kW$ Battery Emulator instrument in collaboration with a company leader in the automotive testing sector, was carried out from scratch exploiting state-of-the-art SiC power-modules, planar magnetics and top-notch MCU technologies. Discrete high-voltage GaN switches were exploited in the Power Supplies design for automotive charger application to target improved performances compared to the market state-of-the-art. Specifically, two high-efficiency prototypes, an AC/DC converter and a DC/DC converter of $7.5kW$, have been realized for this purpose. To further investigate the characteristics of state-of-the-art GaN power devices two measurement set-ups have been designed. In particular, the trapping phenomenon causing the collapse of drain current during ON-state with a consequent degradation of ON-resistance has been analyzed

Characterization of the Dynamic RON of 600 V GaN Switches under Operating Conditions

High-voltage GaN switches can offer tremendous advantages over silicon counterparts for the development of high-efficiency switching-mode power converters at high commutation frequency. Nonetheless, GaN devices are prone to charge-trapping effects that can be particularly relevant in the early-stage development of new technologies. Charge-trapping mechanisms are responsible for the degradation of the dynamic ON-resistance (RON) with respect to its static value: this degradation is typically dependent on the blocking voltage, the commutation frequency and temperature, and is responsible for the reduction of power converter efficiency. The characterization of this phenomenon is very valuable for the development of a new process to compare different technological solutions or for the final assessment of performance. This characterization cannot be made with traditional static or small signal measurements since RON degradation is triggered by application-like dynamic device excitations. In this paper, we propose a technique for the characterization of the dynamic RON of high-voltage GaN switches under real operating conditions: this technique is based on the design of a half bridge switching leg in which the DUT is operated under conditions that resemble its operation in a power converter. With this setup, the characterization of a 600 V GaN switch dynamic RON is performed as a function of variable blocking voltages and commutation frequency. Additionally, this technique allows the separation of thermal and trapping effects, enabling the characterization of the dynamic RON at different temperature

Design of a 350 kW DC/DC Converter in 1200-V SiC Module Technology for Automotive Component Testing

In this paper, the design and implementation of a DC/DC converter for automotive component testing with state-of-the art performance is described. The converter is the core of a battery emulator for the characterization and development of automotive batteries, electronic chargers, traction inverters, DC-DC converters, E-motors and E-axles. Cutting edge performance, flexibility and compactness are obtained by exploiting 1200-V SiC modules, high switching frequency, planar transformer technology, suitable topology solutions and fast digital control strategies. The implemented system is a liquid-cooled, bidirectional converter with galvanic isolation capable of 350 kW continuous output power, output voltage range 48-1000 V, continuous output current up to 800 A (1600 A peak), voltage/current ramp-up time below 10/2 ms and 0.1% current/voltage accuracy. The entire instrument is implemented in a standard full-height 19-inch rack cabinet

The Role of Laparoscopy in the Management of a Diagnostic Dilemma: Jejunal Ectopic Pancreas Developing into Jejunojejunal Intussusception

Ectopic pancreas (EP) is a rare congenital anomaly defined as the presence of pancreatic tissue in topographic anomaly. It is usually silent but it may become clinically evident when complicated by acute conditions. The development of laparoscopic surgery has changed the way to manage such conditions, especially in the setting of emergency surgery, thanks to its diagnostic and therapeutic role with excellent results. We decided to perform an emergency diagnostic exploratory laparoscopy in a 29-year-old man with an acute abdomen and nonspecific radiological images for intestinal occlusion. A jejunojejunal intussusception was found, caused by a mass. We decided to carry out minilaparotomy to perform a resection of the affected jejunum. Histological examination confirmed the presence of a jejunal ectopic pancreas. Adult intussusception caused by EP represents 5% of all cases of intussusception. As CT scan, especially when performed in emergency setting for small bowel obstruction diagnosis, can usually demonstrate nondiagnostic findings suggestive of intussusception of unknown origin, laparoscopic exploration could help surgeons in order to perform a resolute diagnosis and treat the pathology

A Reconfigurable Setup for the On-Wafer Characterization of the Dynamic <i>R</i>_ON of 600 V GaN Switches at Variable Operating Regimes

Charge-trapping mechanisms observed in high-voltage GaN switches are responsible for the degradation of power converter efficiency due to modulation of the effective dynamic ON-resistance (RON) with respect to its static value. Dynamic RON degradation is typically dependent on the blocking voltage and the commutation frequency and is particularly significant in new technologies under development. The possibility to characterize this phenomenon on GaN switch samples directly on-wafer, under controlled operating conditions that resemble real operations of the DUT in a switching mode power converter is extremely valuable in the development phase of new technologies or for quality verification of production wafers. In this paper, we describe a setup that allows this characterization: dynamic RON degradation of on-wafer 600 V GaN switches is characterized as a function of the VDS blocking voltage, the VGS driving voltage, and at different temperatures. The dependency on the switching frequency is identified by measuring the current recovery of the switch after the application of blocking voltages of different durations

Combination of diagnostic laparoscopy and intraoperative indocyanine green fluorescence angiography for the early detection of intestinal ischemia not detectable at CT scan

Introduction: Acute mesenteric ischemia is the most severe gastrointestinal complication of acute aortic dissection. The timing of diagnosis is of major importance, in fact the recognition of acute mesenteric ischemia often occurs too late due to the presence of unspecific symptoms and lack of reliable exams. Recently, indocyanine green fluorescence angiography has been adopted in order to measure blood perfusion and microcirculation. Presentation of case: We decided to perform a diagnostic laparoscopy with the support of intra-operative near-infrared indocyanine green fluorescence angiography, in order to detect an initial intestinal ischemia in a 68-year-old patient previously treated with a TEVAR procedure for a type-B aortic dissection. The fluorescence system demonstrated an hypoperfused area in the ascending colon, therefore an ileocholic resection was thus performed. Opening the operatory specimen, the mucosa of the colon appeared totally ischemic, whilst the serosa was normal. Discussion: When ischemia occurs, the oxygen supply is interrupted, hence the necrosis of the enteral mucosa occurs within 3 h, whilst the necrosis of the full thickness of the bowel wall occurs within 6 h. A diagnosis during these “golden hours” is of major importance for a successful treatment. Conclusion: The combination of laparoscopy and UV light and fluorescein dye should be considered as an invaluable diagnostic procedure for the diagnosis of early stage acute bowel ischemia which is not visible at instrumental examinations nor with diagnostic laparoscopy

Bedside diagnostic laparoscopy for critically ill patients in the Intensive Care Unit: Retrospective study and review of literature

Background: Bedside diagnostic laparoscopy could be helpful in extremely critically ill patients. The aim of this retrospective study is to evaluate the safety and diagnostic accuracy of bedside diagnostic laparoscopy in the identification of intra-abdominal pathology in critically ill patients and to compare its accuracy and outcomes with the ones of laparotomy. Patients and Methods: A retrospective review was conducted on the medical records of patients admitted to the Intensive Care Unit (ICU) of Careggi University Hospital and submitted to bedside diagnostic laparoscopy between January 2006 and May 2017. This group of patients was compared with a group of patients that were admitted to the ICU and submitted directly to explorative laparotomy for suspected intra-abdominal pathologies. Results: One hundred and twenty-nine patients (M/F = 81/48, mean age = 71.64 years) underwent bedside diagnostic laparoscopy in ICU. 154 patients instead were submitted directly to explorative laparotomy in operatory room (mean age 75.70 years, M/F = 94/60). Among the 129 patients submitted to bedside laparoscopy, 53.49% were positive for intra-abdominal pathologies whereas 46.51% were negative, while among the 154 patients submitted directly to laparotomy, 76.62% were positive for intra-abdominal pathologies whereas 23.38% were negative. In 55.03% of all patients submitted to bedside laparoscopy, a non-therapeutic laparotomy was avoided, while the 33.76% of patients submitted directly to laparotomy had a non-therapeutic laparotomy that could be avoidable. Conclusions: Our results pinpoint the advantages of performing bedside diagnostic laparoscopy in the ICU setting, which can be considered an option every time there is the suspicion of an intra-abdominal pathology

Survivability of the lichen Xanthoria parietina in simulated Martian environmental conditions

Xanthoria parietina (L.) Th. Fr. is a widely spread foliose lichen showing high tolerance against UV-radiation thanks to parietin, a secondary lichen substance. We exposed samples of X. parietina under simulated Martian conditions for 30 days to explore its survivability. The lichen’s vitality was monitored via chlorophyll a fluorescence that gives an indication for active light reaction of photosynthesis, performing in situ and after-treatment analyses. Raman spectroscopy and TEM were used to evaluate carotenoid preservation and possible variations in the photobiont’s ultrastructure respectively. Significant differences in the photo-efficiency between UV irradiated samples and dark-kept samples were observed. Fluorescence values correlated with temperature and humidity day-night cycles. The photo-efficiency recovery showed that UV irradiation caused significant effects on the photosynthetic light reaction. Raman spectroscopy showed that the carotenoid signal from UV exposed samples decreased significantly after the exposure. TEM observations confirmed that UV exposed samples were the most affected by the treatment, showing chloroplastidial disorganization in photobionts’ cells. Overall, X. parietina was able to survive the simulated Mars conditions, and for this reason it may be considered as a candidate for space long-term space exposure and evaluations of the parietin photodegradability