Fuel cells provide a clean and highly efficient energy source for power generation; however, in order to efficiently utilize the energy from fuel cells, a power conditioning system is required. Typical fuel cell systems for stand-alone and utility grid-tied stationary power applications are found mostly with low nominal output voltages around 24 V and 48 V, and power levels are found to be 3 to 10 kW [1][2]. A power conditioning system for such applications generally consists of a dc-dc converter and a dc-ac inverter, and the dc-dc converter for low-voltage, high-power fuel cells must deal with a high voltage step-up conversion ratio and high input currents. Although many dc-dc converters have been proposed, most deal with high input voltage systems that focus on step-down applications, and such dc-dc converters are not suitable for low-voltage, high-power fuel cell applications. Multiphase isolated dc-dc converters offer several advantages that are very desirable in low-voltage, high-power fuel cell applications. First, a multiphase is constructed with paralleled phases, which increase power rating and current handling capability for high input current. Second, an interleaving control scheme produces a high operating frequency with a low switching frequency, and the high operating frequency reduces size of passive components. Thirdly, use of a transformer provides electrical isolation and a high conversion ratio. Lastly, several multiphase converters are capable of soft-switching operation, which increases converter efficiency. This thesis examines two highly efficient, soft-switching dc-dc converters that are targeted for fuel cell applications. The thesis also describes the converters' basic operating principles and analyzes performance for low-voltage, high-power fuel cell applications. 5-kW prototypes for each converter are built and tested with a fuel cell simulator. Experimental switching waveforms and efficiency profiles are shown to support the described basic principles and the analysis. Major features and differences between these two converters are also discussed. / Master of Science

Fuel cells typically produce low-voltage high-current output because their individual cell voltage is low, and it is nontrivial to balance for a high-voltage stack. In addition, the output voltage of fuel cells varies depending on load conditions. Due to the variable low voltage output, the energy produced by fuel cells typically requires power conditioning systems to transform the unregulated source energy into more useful energy format. When evaluating power conditioning systems, efficiency and reliability are critical. The power conditioning systems should be efficient in order to prevent excess waste of energy. Since loss is dissipated as heat, efficiency directly affects system reliability as well. High temperatures negatively affect system reliability. Components are much more likely to fail at high temperatures. In order to obtain excellent efficiency and system reliability, low-voltage high-current power conditioning systems should be carefully designed. Low-voltage high-current systems require carefully designed PCB layouts and bus bars. The bus bar and PCB trace lengths should be minimized. Therefore, each needs to be designed with the other in mind. Excessive PCB and bus bar lengths can introduce parasitic inductances and resistances which are detrimental to system performance. In addition, thermal management is critical. High power systems must have sufficient cooling in order to maintain reliable operation. Many sources of loss exist for converters. For low-voltage high-current systems, conduction loss and switching loss may be significant. Other potential non-trivial sources of loss include magnetic losses, copper losses, contact and termination losses, skin effect losses, snubber losses, capacitor equivalent series resistance (ESR) losses, and body diode related losses. Many of the losses can be avoided by carefully designing the system. Therefore, in order to optimize efficiency, the designer should be aware of which components contribute significant amounts of loss. Loss analysis may be performed in order to determine the various sources of loss. The system efficiency can be improved by optimizing components that contribute the most loss. This thesis surveys some potential topologies suitable for low-voltage high-current systems. One low-voltage high-current system in particular is analyzed in detail. The system is called the V6, which consists of six phase legs, and is arranged as a three full-bridge phase-shift modulated converter to step-up voltage for distributed generation applications. The V6 converter has current handling requirements of up to 120A. Basic operation and performance is analyzed for the V6 converter. The loss within the V6 converter is modeled and efficiency is estimated. Calculations are compared with experimental results. Efficiency improvement through parasitic loss reduction is proposed by analyzing the losses of the V6 converter. Substantial power savings are confirmed with prototypes and experimental results. Loss analysis is utilized in order to obtain high efficiency with the V6 converter. Considerations for greater current levels of up to 400A are also discussed. The greater current handling requirements create additional system issues. When considering such high current levels, parallel devices or modules are required. Power stage design, layout, and bus bar issues due to the high current nature of the system are discussed. / Master of Science

Galvanic isolation between the grid and energy storage unit is typically required for bidirectional power distribution systems. Due to the recent advancement in wide-bandgap semiconductor devices, it has become feasible to achieve the galvanic isolation using bidirectional isolated DC/DC converters instead of line-frequency transformers. A survey of the latest generation SiC MOSFET is performed. The devices were compared against each other based on their key parameters. It was determined that under the given specifications, the most suitable devices are X3M0016120K 1.2 kV 16 mohm and C3M0010090K 900 V 10 mohm SiC MOSFETs from Wolfspeed. Two of the most commonly utilized bidirectional isolated DC/DC converter topologies, dual active bridge and CLLC resonant converter are introduced. The operating principle of these converter topologies are explained. A comparative analysis between the two converter topologies, focusing on total device loss, has been performed. It was found that the CLLC converter has lower total device loss compared to the dual active bridge converter under the given specifications. Loss analysis for the isolation transformer in the CLLC resonant converter was also performed at different switching frequencies. It was determined that the total converter loss was lowest at a switching frequency of 250 kHz A prototype for the CLLC resonant converter switching at 250 kHz was then designed and built. Bidirectional power delivery for the converter was verified for power levels up to 25 kW. The converter waveforms and efficiency data were captured at different power levels. Under forward mode operation, a peak efficiency of 98.3% at 15 kW was recorded, along with a full load efficiency value of 98.1% at 25 kW. Under reverse mode operation, a peak efficiency of 98.8% was measured at 17.8 kW. The full load efficiency at 25 kW under reverse mode operation is 98.5%. / Master of Science / Electrical isolation between the grid and energy storage unit is typically required for bidirectional power distribution systems. Traditionally, this isolation is achieved via line-frequency transformers, which tend to be bulky and heavy. This imposes a limit on the overall system power density, which is a crucial performance metric for bidirectional power distribution systems. Alternatively, the required electrical isolation can be implemented through bidirectional power converters. As a result, the overall system power density can be drastically improved. However, the losses incurred by the semiconductor devices in such converters could significantly reduce the overall system efficiency, which is another important performance metric. Due to the recent advancement in semiconductor devices, it has become feasible to design the required bidirectional power converters with high efficiency and high power density. A survey of the latest generation semiconductor devices is performed. A 25 kW converter prototype was designed and built using the selected semiconductor devices. Experimental testing was conducted for the converter prototype and efficiency values exceeding 98% were captured across the entire load range. The converter prototype has a power density of 78 W/in³.

Dans l’objectif d’une augmentation en puissance des piles à combustible pour satisfaire les besoins énergétiques des applications embarquées, une solution consiste à augmenter la taille des assemblages. Dès lors, des problèmes de disparités fluidique, thermique et électrique peuvent survenir dans le cœur des piles et conduire ainsi à l’apparition de défaut. La pile à combustible, de par sa nature de source électrique basse tension – fort courant, requiert d’être couplée au réseau électrique embarqué par l’intermédiaire d’un convertisseur statique. Ce dernier peut alors être employé pour agir de façon corrective sur la pile et aussi de corriger les défaillances qui en sont liées. Dans cette perspective, le convertisseur doit avoir en permanence un retour sur l’état de santé de la pile. Pour cela, une méthode de détection et d'identification de défaut de type noyage et d’assèchement pour une pile du type PEMFC a été approfondie. Cette méthode simple, économique en capteurs, se base sur la mesure de 3 tensions de cellule judicieusement sélectionnées et localisées sur la pile. Ainsi, l’utilisation de l’information « spatiale », qui correspond à la position de la mesure de tension dans la pile permet d’identifier les défauts. Le principe de la détection localisée nous amène alors à considérer le concept de pile segmentée qui consiste à séparer électriquement la pile en 3 parties de façon à ce que des convertisseurs associés puissent agir électriquement sur chaque segment. L’action peut être du type tout ou rien, ou contrôlée. Cette dernière offre davantage de degrés de liberté, et est moins contraignante pour la pile d’un point de vue électrique. Pour choisir comment réaliser cette action, une étude comparative de plusieurs topologies de convertisseur est effectuée. Les structures alimentées en courant répondent au mieux aux contraintes électriques d’une PEMFC et sont donc privilégiées, de même que la nécessité d’une isolation galvanique imposée par la segmentation de la pile. Au final, une topologie de BOOST isolé résonant est apparue comme étant la topologie répondant au mieux à l’ensemble des critères (plage de fonctionnement, performances énergétiques, nombre de composants). L’ensemble convertisseur global intègre ainsi trois structures unitaires qui permettent d'offrir une modularité, une action indépendante sur chaque segment et de garantir une disponibilité du système grâce à un fonctionnement dégradé. Pour cela, la stratégie de commande de l’ensemble convertisseur intègre les informations issues de la méthode de détection. La thèse se termine avec le dimensionnement complet d’un pré-prototype du convertisseur avec le choix des composants actif et passifs, et du système de refroidissement associé. / In the objective of fuel cell power increase in order to satisfy energetic requirements for embedded applications, a solution consists in increasing the size of fuel cell stack assemblies. As possible consequence, fluidic or thermal disparity problems may occur in the fuel cell core and lead to the appearance of faults. The fuel cell, which is a low voltage-high current electrical source, needs to be connected to the on-board electrical network thanks to a static converter. This latter can be used in order to perform a corrective action in the aim of reducing disparities in the stack and also correcting resulting faults. In this perspective, the converter should permanently get information about fuel cell state of health. Hence, a fault detection and identification method for PEMFC has been explored. This method which is simple and requires only few sensors is based on 3 voltage measurements judiciously selected and localized over the stack. Using “spatial” information which corresponds to the position of the sensors, allows to identify some characteristic faults. The principle of the localized fault detection leads to consider the segmentation concept for the fuel cell, which in our case is electrically split into three parts and allows an independent control of each segment by the power converter. Electrical action can be “all or nothing” or moderated ones. The latter offers more degree of freedom, and is less constraining from an electrical point of view. In order to execute the action, study of multiple power converter topologies have been done. Among the candidate topologies, current structures are preferred, as well as the necessity of a galvanic isolation required by the segmentation concept. The resonant isolated boost is the adopted structure; as it meets at best the whole criteria. Thus the global converter assembly is composed of three single structures which offer modularity, independent action on each segment, and continuity of service thanks to degraded modes. The detection method is hence implemented in the converter control strategy. This Ph.D. thesis ends with the complete sizing of a power converter pre-prototype together with technological choices for the active, passive and associated cooling components.

Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(工学) / 甲第18992号 / 工博第4034号 / 新制||工||1621 / 31943 / 京都大学大学院工学研究科電気工学専攻 / (主査)教授 篠原 真毅, 教授 和田 修己, 教授 山川 宏 / 学位規則第4条第1項該当

This work builds on a semester project 2. from the winter semester of this academic year. The aim of this thesis is the design of converter using semiconductor components based on SiC technology. This converter is used in the construction of quick charger for electric vehicles. The design of this converter must be based on the requirements for compliance voltage safety. It describes the design of power components used in the construction of this facility, the determination of their losses and determines the overall efficiency of the converter. There is also proposed mathematical model of high-frequency transformer and made his simulation in Matlab-Simulink.

This thesis deals with a design, construciton and testing of a switch-mode power supply (SMPS) which is working as a LiFEPO4 battery charger with output current up to 100~A and output voltage up to 14,6~V. The output voltage and current can be regulated by the operator from zero to maximum value. For this SMPS Half-bridge asymmetrical forward converter with two transformers and common output inductor topology is chosen. The control circuits are run by the IC SG3525. Cascaded regulation of output voltage and current is implemented by two discrete operational amplifiers. Undervoltage protection of the control circuits and independent overcurrent protection on the primary side is also implemented.

This diploma thesis describes particular parts of power inverter such as gate driver, DSC, LC filter, low power supply, DC/DC converter and four-quadrant bridge and manner of its control by digital signal controller. Inverter is designated for generating of a mobile artificial electric grid (for example in a car).

The thesis deals with design and construction of low-power measurement device with power over optical fiber. This device should measure input signal in range of 50 mV to 50 V and the results of measurement sent through optical fiber into sensing module.

This work builds on a semester project 2 from the winter semester of this academic year. The aim of this thesis is the design of converter using semiconductor components based on SiC technology. This converter is used in the construction of quick charger for electric vehicles. The design of this converter must be based on the requirements for compliance voltage safety. It describes the design of power components used in the construction of this facility, the determination of their losses and determines the overall efficiency of the converter. There is also proposed mathematical model of high-frequency transformer and made his simulation in Matlab-Simulink.