With the rise of soft-switched converter topologies which enable high-frequency power conversion, there has been a premise that these converter topologies can help achieve loss-less switching in a power device. However, this theory is not completely true as there even with soft-switching there is some degree of loss associated in the form of output capacitance-related hysteresis loss, channel turn-off loss, and loss during the dead-time period in these converter topologies. The soft-switching converters utilize the existence of the device's output capacitance (COSS), which is charged and discharged consecutively at each switching cycle, and a hysteresis loss exists due to the difference in charging and discharging output capacitance. In order fully utilize the potential of these novel soft-switching topologies, we need to investigate further into the origins of these losses or loss mechanisms, methods to measure or compute these losses, and then devise ways to optimize the loss for a given application. This work focuses on exploring methods to quantify this loss for different operating conditions like device current, switching frequency, dV/dT, etc. In this aspect, some methods have been studied and used to quantify this hysteresis loss for a variety of power devices like SI and GaN. It is reported that only channel turn-off losses exist in devices with ZVS transition, however, we found that the charging and discharging of COSS is not loss-free and thus it is important that we account for this loss in the design process. Finally, the loss data obtained from these tests are compared with each other for five different power devices to validate their applicability, and later these test results are used to get an optimized device selection criterion for the best possible efficiency and minimal losses for a ZVS application. / Master of Science / To eliminate the switch-related losses, soft-switching or zero-voltage switching (ZVS) was introduced which provides a soft transition of voltage and current instead of a sharp transition like in hard-switched converters. Moreover, the output capacitance of the devices is charged/discharged to achieve soft switching, which also possesses a loss (hysteresis loss) due to repeated charging and discharging of output capacitance. Soft-switching converter topologies have gained quite a momentum for almost a decade, and it keeps on increasing in the distant future, to extract the maximum benefits from these topologies is to operate at high switching frequencies to minimize the size and volume of the converter. However, the hysteresis loss can severely impact the converter efficiency on even a few 100 kHz and MHz levels. This work focuses on exploring methods to measure hysteresis loss in output capacitance which has a major share in the overall switching loss. In this regard, some methods have been investigated that deal with the measurement of the output capacitance-related hysteresis loss and are implemented to get the hysteresis loss data for different power devices. Two test methods (calorimetric and non-linear resonance) are implemented on five different power devices (3 SI and 2 GaN) to get the energy loss in the output capacitance; these test results are used to explain the switching loss in an isolated boost resonance DC-DC converter, and a concept of ZVS figure of merit is used to obtain the optimized device. Moreover, a design example of a soft-switched converter is presented to highlight the impact of COSS hysteresis loss in the overall switching loss of primary side devices.

Cette thèse avait pour but de comprendre le comportement thermique de composites silicones. Pour cela, une étude bibliographique complète sur le comportement au feu des silicones comme matrice ou en tant que retardateur de flamme a tout d'abord été réalisée. Ce travail a permis de définir une stratégie permettant d'améliorer la stabilité thermique de la matrice mais également du composite silicone. Une première étude expérimentale a été consacrée à l'étude de l'influence de l'ajout de platine et de silice sur le comportement thermique d'un composite silicone modèle. Nous avons pu montrer que l'immobilisation des chaînes macromoléculaires était le facteur clef pour une céramisation efficace. Dans une deuxième partie, des composites silicones dans lesquels nous avons incorporé des charges à base soit de calcium, soit d'aluminium ont été testés selon trois différents protocoles de dégradation thermique. Nous avons d'abord mis en exergue le fait que la nature chimique (charge non hydratée, contenant de l'eau ou portant des groupements hydroxyles en surface), la taille et la morphologie des charges influençaient fortement le comportement thermique du silicone. Nous avons ensuite montré que la formation de nouvelles structures cristallines et l'absence de libération de gaz dégradant la matrice conduisaient à des résidus très cohésifs, après pyrolyse extrême. Enfin, des tests calorimétriques ont montré que l'amélioration du comportement au feu des composites était liée à la stabilité thermique et/ou à la génération d'une couche barrière. / This PhD work has been devoted to the study of the thermal behavior of silicone composites. A preliminary review on the flame retardancy of silicone reported numerous works devoted to the development of thermally-resistant silicone composites or silicone polymers used as flame retardant agents in other organic polymer matrices. The first part of our experimental work highlighted the key role of macromolecular chain immobilization, through the synergy of platinum and silica, in generating high ceramized residue content after thermal gravimetry. The second part of this work was dedicated to the study of silicone composites filled with either calcium or aluminum-based fillers. The filler nature (non hydrated, water releasing or hydroxyl groups on the surface), the morphology and the particle size strongly influenced the thermal behavior of silicone composites. The analyses on composites residues after extreme pyrolysis showed that the formation of new crystalline structures and the absence of water release favored the residue ceramization. The investigation on fire reaction of silicone composites finally granted their outstanding properties to the matrix thermal stability and/or a barrier layer formation.