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The commercialization of wide band-gap devices such as silicon carbide and gallium nitride transistors has made it possible for power electronics applications to achieve unprecedented performance in terms of efficiency and power density. However, the device characteristics which make this performance possible also create secondary consequences in these high-performance applications. One such consequence which is particularly difficult to manage in the context of power electronics applications is the occurrence of self-sustained oscillation. This problem has been recognized in the power electronics literature, but heretofore has not received an extensive analytical treatment. This dissertation provides a comprehensive analytical treatment of the self-sustained oscillation phenomenon, logically separated into two components: an initial forced cycle and the subsequent oscillatory behavior. A large-signal model has been developed in order to predict the occurrence of the initial forced cycle based on a set of estimated initial conditions derived from a user-specified operating point. The establishment of the initial forced cycle as predicted by the large-signal model creates the bias conditions necessary for the analytical treatment of the subsequent oscillatory behavior. For this purpose, a small-signal model is presented which describes this phenomenon on the basis of recognizing the wide band-gap device and a minimal set of parasitic components associated with the gate and drain circuits as an unintended negative conductance oscillator. In the context of established oscillator design theory it has been shown both analytically and with simulation that negative differential conductance exhibited by the parasitic model explains the conditions under which self-sustained oscillation is likely to occur. Both the large-signal and small-signal models are shown to demonstrate good agreement with empirical results from pulsed switching experiments obtained over a wide range of operating conditions. In addition, a catalog of known solutions to the problem of self-sustained oscillation is presented, along with a discussion of a method by which the current work can be used by application designers to preclude the occurrence of this phenomenon in practical systems by design.

Master of Science / Department of Electrical and Computer Engineering / Behrooz Mirafzal / The rise of Wide Band Gap (WBG) devices has brought excitement in the field of Power converters. The WBG based converter can operate at the very high frequency and temperature making them ideal to use in harsh environments. The commercialization of WBG devices such as SiC and GaN MOSFETs has made it interesting for power engineering professionals all over the world. The use of WBG devices capable of operating at high switching frequencies reduces the overall system size dramatically with added benefit of improved power quality at high temperature. The main goal of this thesis is to design and test an AC-DC converter based on a SiC power module. The designed rectifier can be considered an active rectifier equipped with a controller to constantly provide feedback for modification of switching signals to get the desired output voltage. The designed active rectifier converts the varying frequency input power supply into rectified DC voltage while keeping the power factor of the system to unity. This thesis covers elementary information on power supply design, switching schemes and design of the controller. System arrangement will provide more light on the use of Six Channel MOSFET Gate Driver from CREE with the overall experimental setup. The experimental analysis will summarize the behavior of the system where information on achieved rectification, effect on the line currents at the generator and concluding power factor representation is described.

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Ternary III-nitride alloys have enabled the design of various devices ranging from optoelectronics to power electronics due to their tunable band gap. BxGa1-xN is a wide band gap semiconductor with applications in detecting devices, power electronics and light-emitting diodes. The band gap can be modulated by changing the Boron concentration. It can be grown by metal-organic chemical vapor deposition as a mixed thin film of wurtzite and zincblende structures. In this work, we investigate the structural and electronic properties of BxGa1-xN (x<0.2) by first-principles calculations for both the wurtzite and zincblende phases. The formation energies of Si and Mg impurities and of a Ga vacancy are also calculated. We find that the wurtzite structure is favored over the zincblende structure. Furthermore, the Si and Mg impurities have relatively low formation energies in their neutral state, which indicates compatibility with BxGa1-xN, while a Ga vacancy has very high formation energy, hence being less likely to form spontaneously. Moreover, in the charged states, the formation energy of Mg is reasonably low for most values of the Fermi level, while the formation energy of Si depends linearly on the Fermi level, indicating challenges in achieving n-type conductivity. For a Ga vacancy in a triple acceptor state, the formation energy is reasonably low close to the conduction band, therefore, Ga vacancies interfere with n-type conductivity.

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Switch mode power supplies are widely used in different applications. High efficiency and high power density are two driving forces for power supply systems. However, high dv/dt and di/dt in switch mode power supplies will cause severe EMI noise issue. In a typical front-end converter, the EMI filter usually occupies 1/3 to 1/4 volume of total converter. Hence, reducing the EMI noise of power converter can help reduce the volume of EMI filter and improving the total power density of the converter. The EMI noise can be separated as differential mode (DM) noise and common mode (CM) noise. For off-line switch mode power supplies, DM noise is dominated by PFC converter. CM noise is a more complicated issue. It is contributed by both PFC converter and DC/DC converter. The DM noise is contributed by input current ripple. Therefore, one method to reduce DM noise is interleaving. There are three methods to reduce CM noise: symmetry, balance and shielding. The idea of symmetry concept is generating another dv/dt source to cancel the original dv/dt source. However, this method is very difficult to achieve and usually has more loss. The balance technique forms a Wheatstone bridge circuit to minimize the CM noise. However, the balance technique cannot achieve very good attenuation at high frequency due to parasitics. Shielding technique is very popular in isolated DC-DC converters to reduce CM noise. However, the previous shielding method requires precise control of parasitic capacitance and dv/dt. It is very difficult to achieve good CM noise attenuation in mass production. In this dissertation, a novel one-layer shielding method for PCB winding transformer is provided. This shielding technique can block CM noise from primary side and also cancel the CM noise from secondary side. In addition, shielding does not increase the loss of converter too much. Furthermore, this shielding technique can be applied to matrix transformer structure. For matrix transformer LLC converter, the inter-winding capacitor is very large and will cause severe CM noise problem. By adding shielding layer, CM noise has been greatly reduced. Although flyback and LLC resonant converter are used as examples to demonstrate the concept, the novel shielding technique can also be applied to other topologies that have similar transformer structure. With Wide-band-gap power devices, the switching frequency of power converter can be pushed 10 times higher than traditional Si based converters. This provides an opportunity to use PCB winding magnetics. In order to reduce the switching loss, critical conduction mode is used in PFC converter. Because of high AC current in the inductor winding, litz wire was used to build the inductor. However, with coupled inductor concept and the proposed winding structure, CRM inductor is integrate into PCB winding for the first time. Furthermore, balance technique is applied to reduce CM noise for PFC converter. With PCB winding, the balance technique has better high frequency performance. The PCB winding inductor can achieve high power density, high efficiency and automated manufacture. Traditionally, two-stage EMI filter was utilized to achieve required EMI noise attenuation. With the developed high frequency, low EMI noise converter, single-stage EMI filter can be applied. However, there are self-parasitic and mutual parasitic components to impact the filter performance on high frequency. The near-field measurement is utilized to visualize the magnetic flux near those filter components. Thus, a better filter design and layout can be achieved to have better high frequency performance. / Ph. D.

Les composants semi-conducteurs à base de matériaux à grand gap (SiC et GaN) présentent des caractéristiques intéressantes pour la réalisation de convertisseurs d’électronique de puissance toujours plus intégrés. Cependant, le packaging des composants traditionnels en silicium ne semble plus adapté pour ces nouveaux composants et apparaît même comme un facteur limitant. Le développement d’un packaging adapté aux caractéristiques des composants à grand gap est alors nécessaire. Les travaux développés dans cette thèse proposent un nouveau packaging tridimensionnel basé sur un procédé de fabrication de circuit imprimé. L’architecture du module est basé sur le concept « Power Chip On Chip » dont le principe de base permet de réduire les perturbations électromagnétiques. Le procédé de fabrication des circuits imprimés offre une grande flexibilité pour le routage en trois dimensions et permet de s’affranchir de l’interconnexion par fil de bonding entre le package et la puce. La démarche de conception du module s’appuie sur une approche multi-physique afin de qualifier le comportement électromagnétique et thermique du module puis de proposer des voies d’optimisation. Un prototype d’un module implémentant quatre cellules de commutation en parallèle, à base de MOSFET SiC, a été produit avec des moyens de production industriels. Les différents tests réalisés valident l’approche retenue dans ce projet mais soulignent également les aspects technologiques à approfondir pour la réalisation d’un module de puissance industriel. / Wide-band-gap (WBG) semiconductors (SiC and Gan) offer interesting characteristics to realize high density power electronics converters. Conventional packaging used for silicon devices is no more adapted for those now components. Development of dedicated packaging for WBG devices is absolutely required. This PhD thesis presents a new 3D package based on Printed Circuit Board (PCB) industrial process. The module architecture is based on “Power Chip On Chip” concept which allows reducing electromagnetic perturbations. PCB fabrication process offers high design flexibility in three dimensions and allows removing wire bonding to interconnect power die and package. The power module design process is buit on multi-physics design tools in the aim to quantify electromagnetic and thermal behavior of the module. Furthermore, several optimization parameters are highlighted. A power module prototype, with four commutation cells in parallel based on SiC MOSFET, has been produced thanks to industrial facilities. Tests realized on new power module confirm the validity of the concept but furthermore to highlight critical technological parameters to realize an industrial power module.

The application of nanowires (NWs) in solar cells (SCs) is of great interest due to their new promising aspects established in nanoelectronics. Semiconductors associated with plasmonic metal nanoparticles (NPs) such as Silver (Ag), Gold (Au) and Copper (Cu), show enhanced performance in solid state light absorbing SCs owing to plasmonic characteristic of noble metal NPs. Plasmonic NPs presented a significant role in development of visible light harvesting for many applications such as photocatalytic materials, photodynamic in Surface Enhanced Raman Spectroscopy (SERS) and photovoltaics (PVs). Integration of plasmonic NPs in semiconductor materials have opened the routes to expand new PV systems with high efficiency light absorption. In this project, we introduce the synthesis ZnO and TiO2 NWs used as N-type semiconducting substrates and various methods for isolating plasmonic metal NPs, which are later deposited on the semiconducting substrates. Vertically aligned ZnO and TiO2 NWs arrays were grown on the fluorine-doped tin oxide (FTO) conductive glass substrates via hydrothermal method at low temperature and the plasmonic NPs were synthesized by wet chemistry procedures and finally decorated on the NW films by using electrophoretic deposition.  The impact of metal NPs loaded on the ZnO and TiO2 NWs substrates was studied by means of UV-vis spectroscopy and Photoluminescence (PL) spectroscopy. The absorbance spectra of individual NPs were recorded. Remarkably, the reflectance spectra of produced samples presented an enhancement in light absorption of the substrates after uptake of NPs on the ZnO and TiO2 NWs. The optical properties of the as grown ZnO NWs films decorated with Ag NPs (I) in direct contact with substrate and (II) in presence of an Al2O3 insulating spacer layer have been investigated. Both systems exhibited an enhancement in the UV band-edge emission from the ZnO when excited at 325 nm. In contrast, the broad bend defect emission of the samples did not have a significant change compare to bare ZnO substrates. The observed results suggested that the ZnO and TiO2 NWs decorated with plasmonic nanoparticles can boost the optical properties of MOs NWs substrates and hence effectively enhance the separation of photoexcited electron-hole pairs and photo-conversion applications.

In this work, I studied the hybrid system based on self-assembled guanosine crystal (SAGC) conjugated to wide-bandgap semiconductor gallium nitride (GaN). Guanosine is one of the four bases of DNA and has the lowest oxidation energy, which favors carrier transport. It also has large dipole moment. Guanosine molecules self-assemble to ribbon-like structure in confined space. GaN surface can have positive or negative polarity depending on whether the surface is Ga- or N-terminated. I studied SAGC in confined space between two electrodes. The current-voltage characteristics can be explained very well with the theory of metal-semiconductor-metal (MSM) structure. I-V curves also show strong rectification effect, which can be explained by the intrinsic polarization along the axis of ribbon-like structure of SAGC. GaN substrate property influences the properties of SAGC. So SAGC has semiconductor properties within the confined space up to 458nm. When the gap distance gets up to 484nm, the structure with guanosine shows resistance characteristics. The photocurrent measurements show that the bandgap of SAGC is about 3.3-3.4eV and affected by substrate properties. The MSM structure based on SAGC can be used as photodetector in UV region. Then I show that the periodic structure based on GaN and SAGC can have photonic bandgaps. The bandgap size and the band edges can be tuned by tuning lattice parameters. Light propagation and emission can be tuned by photonic crystals. So the hybrid photonic crystal can be potentially used to detect guanosine molecules. If guanosine molecules are used as functional linker to other biomolecules which usually absorb or emit light in blue to UV region, the hybrid photonic crystal can also be used to tune the coupling of light source to guanosine molecules, then to other biomolecules.