Correlation between OCVD carrier lifetime vs temperature measurements and reverse recovery behavior of the body diode of SiC power MOSFETs

The reverse recovery (RR) behavior of SiC MOSFET body diode is of great importance in power application, where these devices are used in a wide range of operating temperatures. The carrier lifetime in the drift region varies with temperature, and it heavily affects the tailoring of the RR current, opening reliability issues related to the RR voltage amplitude and to possible anomalous voltage oscillations during the recovery. From the users' point of view, it would be useful to have a simple technique able to give predictive information about the body diode RR behavior of commercial devices over the whole range of working temperatures. An experimental-simulation approach is presented in this paper to correlate the carrier lifetime measured by simple OCVD measurements versus temperature with the RR behavior of the body diode, that can be useful at the design stage of power converters. Simulations of the body diode reverse-recovery are performed for a wide range of carrier lifetimes. This allows to estimate the effect of changes of carrier lifetime with temperature on the body diode switching transients. Preliminary results obtained with a 1700 V/5A commercial MOSFET are shown

A portable device for time-resolved fluorescence based on an array of CMOS SPADs with integrated microfluidics

[eng] Traditionally, molecular analysis is performed in laboratories equipped with desktop instruments operated by specialized technicians. This paradigm has been changing in recent decades, as biosensor technology has become as accurate as desktop instruments, providing results in much shorter periods and miniaturizing the instrumentation, moving the diagnostic tests gradually out of the central laboratory. However, despite the inherent advantages of time-resolved fluorescence spectroscopy applied to molecular diagnosis, it is only in the last decade that POC (Point Of Care) devices have begun to be developed based on the detection of fluorescence, due to the challenge of developing high-performance, portable and low-cost spectroscopic sensors. This thesis presents the development of a compact, robust and low-cost system for molecular diagnosis based on time-resolved fluorescence spectroscopy, which serves as a general-purpose platform for the optical detection of a variety of biomarkers, bridging the gap between the laboratory and the POC of the fluorescence lifetime based bioassays. In particular, two systems with different levels of integration have been developed that combine a one-dimensional array of SPAD (Single-Photon Avalanch Diode) pixels capable of detecting a single photon, with an interchangeable microfluidic cartridge used to insert the sample and a laser diode Pulsed low-cost UV as a source of excitation. The contact-oriented design of the binomial formed by the sensor and the microfluidic, together with the timed operation of the sensors, makes it possible to dispense with the use of lenses and filters. In turn, custom packaging of the sensor chip allows the microfluidic cartridge to be positioned directly on the sensor array without any alignment procedure. Both systems have been validated, determining the decomposition time of quantum dots in 20 nl of solution for different concentrations, emulating a molecular test in a POC device.[cat] Tradicionalment, l'anàlisi molecular es realitza en laboratoris equipats amb instruments de sobretaula operats per tècnics especialitzats. Aquest paradigma ha anat canviant en les últimes dècades, a mesura que la tecnologia de biosensor s'ha tornat tan precisa com els instruments de sobretaula, proporcionant resultats en períodes molt més curts de temps i miniaturitzant la instrumentació, permetent així, traslladar gradualment les proves de diagnòstic fora de laboratori central. No obstant això i malgrat els avantatges inherents de l'espectroscòpia de fluorescència resolta en el temps aplicada a la diagnosi molecular, no ha estat fins a l'última dècada que s'han començat a desenvolupar dispositius POC (Point Of Care) basats en la detecció de la fluorescència, degut al desafiament que suposa el desenvolupament de sensors espectroscòpics d'alt rendiment, portàtils i de baix cost. Aquesta tesi presenta el desenvolupament d'un sistema compacte, robust i de baix cost per al diagnòstic molecular basat en l'espectroscòpia de fluorescència resolta en el temps, que serveixi com a plataforma d'ús general per a la detecció òptica d'una varietat de biomarcadors, tancant la bretxa entre el laboratori i el POC dels bioassaigs basats en l'anàlisi de la pèrdua de la fluorescència. En particular, s'han desenvolupat dos sistemes amb diferents nivells d'integració que combinen una matriu unidimensional de píxels SPAD (Single-Photon Avalanch Diode) capaços de detectar un sol fotó, amb un cartutx microfluídic intercanviable emprat per inserir la mostra, així com un díode làser UV premut de baix cost com a font d'excitació. El disseny orientat a la detecció per contacte de l'binomi format pel sensor i la microfluídica, juntament amb l'operació temporitzada dels sensors, permet prescindir de l'ús de lents i filtres. Al seu torn, l'empaquetat a mida de l'xip sensor permet posicionar el cartutx microfluídic directament sobre la matriu de sensors sense cap procediment d'alineament. Tots dos sistemes han estat validats determinant el temps de descomposició de "quantum dots" en 20 nl de solució per a diferents concentracions, emulant així un assaig molecular en un dispositiu POC

Design of Energy Efficient Snubber Circuits for Protection of Switching Devices in High Power Applications

This thesis was submitted for the award of Doctor of Philosophy and was awarded by Brunel University LondonSemiconductor devices are subjected to elevated levels of dv/dt and di/dt when used at high voltage high current and elevated temperature applications. To reduce the stress from semiconductor switches, turn-on snubber circuits are used during the turn on time and turn-off snubber circuits during the turn-off time. In low power applications where the switching losses is not significant these can be ignored. Over the last few years, Voltage Controlled Voltage Source (VCVS) applications in High Voltage Direct Current (HVDC) has increased particularly with the use of Multilevel Converters (MLCs). Switching Losses in such high power applications now needs to be considered as it is no longer insignificant. Energy efficient snubber circuits (EESCs) became available only for low power applications according to the literature review. The research dealt with the design of EESCs in high power cascaded H-bridge MLCs. The main contributions made were: - (1) A critical review of present snubber circuits. (2) Design of energy efficient snubber circuits. (3) Design of Safe Operating Area (SOA) was possible by application of COMSOL thermal simulation for the power switch used in MLCs. (4) A reduction in switching power loss of 1782 MWh before EESC and 1379 MWh after the EESC (22.6%), which is an annual reduction of 403MWh, which impacts on the reduction in Global Warming. (5) Significant annual cost benefits from £125,000 to £68,612 (55%) in the reduction of wasted switching dissipated energy. (6) Additional benefit in the connection of inductors in the EESCs, resulted in a reduction of harmonic levels of 6% at V3 down to 1.5% at V7. Optimisation methods, like Particle Swarm Optimisation (PSO) and Graduated Reduction Gradient (GRG) were used to evaluate individual components in the proposed EESCs. Use of COMSOL thermal simulation software was critical in the design of the power IGBT SOA. A case study of 250 kW station, a reduced scale of a typical HVDC station of 2000 MW (for example Sellindge HVDC station), based on 7-level MLC used Isolated Gate Bipolar Transistors (IGBTs) to evaluate the annual reduction in power losses and reduction in cost. If an upward trajectory is computed, based on the number of UK HVDC Converter stations, enormous economic and energy recovery can result with significant impact towards a decrease in global warming. The results obtained validated the research goals and identified a high potential for the application of EESCs in HVDC

Contribution to improve the EMI performance of electrical drive systems in vehicles with special consideration of power semiconductor modules

Diese Arbeit dient als Beitrag zur Verbesserung des EMV-Verhaltens elektrischer Antriebssysteme in Fahrzeugen, wobei der Fokus auf dem Leistungshalbleitermodul für die Automobilanwendung liegt. Für ein besseres und tieferes Verständnis der Quelle von leitungsgebundenen Störungen werden die EMV-Mechanismen und -Effekte im Zusammenhang mit dem Leistungsmodul im Antriebssystem durch Simulationen und Messungen untersucht. Der Einfluss der Diode Reverse Recovery Effekte auf das EMV-Verhalten wird quantitativ mit verschiedenen Lastströmen sowie mit verschiedenen Diodentypen, wie z.B. SiC-Schottky-Dioden, analysiert. Durch Simulationen wird der Einfluss des Leistungsmoduls auf das System untersucht; auf dieser Basis wird die Bedeutung verschiedener Faktoren innerhalb und außerhalb des Leistungsmoduls für das EMV-Verhalten bewertet. Zur Validierung der Simulationsergebnisse wird der Messaufbau für eine konventionelle EMV-Messung für die Automobilanwendung vorgestellt. Die Messergebnisse belegen, dass die Simulationsmodelle unter bestimmten Randbedingungen für zukünftige Leistungsmodulkonstruktionen zur EMV-Vorhersage verwendbar sind. Basierend auf dem Verständnis, wie es aus den Simulationen und Messergebnissen hergeleitet wurde, werden konkrete Optimierungskonzepte für ein inhärent störungsarmes Leistungsmodul entwickelt und realisiert. Dessen EMV-Verhalten sowie der Aufwand des Musterbaus aus Sicht des Leistungsmodulherstellers werden anhand verschiedenen Kriterien verglichen und bewertet. Außerdem wird das dynamische und Kurzschlussverhalten der Prototypen einschließlich der Stromverteilung zwischen den Halbleiterchips charakterisiert. In dieser Arbeit wird ein neuartiges Testverfahren vorgestellt, mit dem es möglich ist, das leitungsgebundene EMV-Verhalten von Leistungsmodulen abzuschätzen, ohne den gesamten Testaufbau wie bei einer konventionellen EMV-Messung zu erstellen. Diese Charakterisierung kann anschließend in der Phase der Inverterentwicklung verwendet werden, um ein geeignetes Modul auszuwählen und den erwarteten Aufwand zur Einhaltung der EMV Standards zu bewerten.This work serves as a contribution to improve the EMI performance of electrical drive systems in vehicles; the focus is on the power semiconductor module for automotive application. For a better and deeper understanding of the conducted EMI source, the conducted EMI mechanisms and effects in the drive system are investigated through simulations as well as measurements with special consideration of power modules: The influence of the diode recovery effects on the EMI performance is quantitatively analyzed with different load currents, as well as with different types of diodes, e.g. SiC Schottky barrier diode. Through the simulation, the influence coming from the power module to the system is clarified; the importance of different factors inside and outside of the power module regarding EMI performance are therefore evaluated. To validate the simulation results, the setup and test bench for a conventional EMI measurement for the typical automotive application are presented. Through the measurement results it is proven that the simulation models are usable under certain boundary conditions for future power module designs with regard to the EMI prediction. Based on the understanding and the conclusions from the simulation and measurement results, concrete EMI optimization concepts for an inherently low-interference power module are developed and realized. The EMI performance as well as the feasibility of the sample modules are compared and evaluated under different criteria from the power module manufacturer’s point of view. Besides, the dynamic and short-circuit performances of the sample modules, regarding to the current distribution on the semiconductor chips, are characterized. A novel test procedure is introduced in this work, by which it is possible to estimate the conducted EMI performance of power modules without building the whole test setup like in a conventional EMI measurement. This characterization can subsequently be used in the phase of converter development to select a suitable device and evaluate the expected effort to comply with EMI standards

High Efficiency IGBTs through Novel Three-Dimensional Modelling and New Architectures

New Insulated Gate Bipolar Transistor (IGBT) designs are reliant on simulation tools, such as Sentaurus technology computer-aided design (TCAD) models, which allow for rapid device development that could not be achieved by manufacturing prototypes due to the cost and time associated with fabrication. These simulations are, though, computationally expensive and typically most design engineers develop these TCAD models only in two dimensions. This leads to inaccuracies in the model output since manufactured transistors are inherently three-dimensional (3D). Based upon a commercial IGBT, this thesis begins by outlining the development of a 3D TCAD model using design details provided by the manufacturer. Large variations between the experimental data from the manufactured device and the simulation model lead to the discovery of widespread birds-beaking within the IGBT – an uncontrollable processing defect that the manufacturer was unaware of. This thesis presents a new simulation technique to account for this processing error while minimising computational effort and investigates the consequence of this birds-beak on the reliability of the device. The verified 3D IGBT model was also used to determine an optimum cell design that considered critical 3D effects omitted from previous studies. An extensive literature review for the Reverse-Conducting IGBT (RC-IGBT) is provided. It is shown that despite the benefits of the RC-IGBT, the device suffers from many undesirable design trade-offs that have prevented its widespread use. The RC-IGBT designs that have currently been proposed in literature, either present a trade-off in performance, an inability to be manufactured, or a requirement for a custom gate drive. This thesis presents a new RC-IGBT concept, the ‘Dual Implant SuperJunction (SJ) RC-IGBT’ that addresses these concerns and is manufacturable using current state of the art techniques. The concept and proposed manufacturing method enables, for the first time, a full SuperJunction structure to be achieved in a 1.2kV device. In addition, an investigation into a coordinated switching scheme using both a silicon IGBT and silicon-carbide MOSFET was undertaken, which aimed to improve turn-off losses within the IGBT without sacrificing on-state losses. Thermal modelling of the power devices switching under inductive load was explored as the system was optimised to use a SiC MOSFET in excess of its nominal ratings, reducing the overall system cost.EPSRC Doctoral Training Partnership scheme (grant RG75686

The application of resonant-mode techniques to off-line converters for the commercial market

This thesis presents the work performed by the author on the application of resonantmode techniques to commercially-orientated off-line converters. An extensive review of resonant-mode topologies leads to the development of a method of categorisation of these topologies which allows a greater comprehension of their properties. The categories of converter thus obtained are the conventional resonant converter, the quasi-resonant converter, and the gap-resonant converter. The gap-resonant converter is selected for further investigation. An analysis reveals the limited load and input voltage capabilities of this converter, and hence leads to the introduction of a pre-regulating converter to improve reliability and commercial viability. High-frequency techniques are explored and reported, and new techniques are developed in several areas in order to extend the concept of the gap-resonant converter to a realworld practical design. Subjects explored include the high speed driving of power MOSFETs, MOSFET and diode switching losses, high frequency magnetic materials and core losses, and skin and proximity effects. The techniques developed are used in the design of a 30OW, off-line converter with an input voltage range of 165V to 380V after rectification, and a ten-to-one output load range

Light-emitting diodes from polyfluorenes: characterisation and stability of performance

This thesis deals with polymer light-emitting diodes (LEDs) containing materials from the polyfluorene family, and investigates their behaviour when employed in device structures. A study of poly(9,9’-dioctylfluorene-co-bis-N,N’-(4-butylphenyl)-bis-N,N’-phenyl-1,4-phenylenediamine) (PFB) by photothermal deflection spectroscopy (PDS) shows that the polymer undergoes a doping reaction with poly(styrene sulphonic acid). This is important because the two materials are found in intimate contact in LED structures. The conditions for reaction are investigated, and it is proposed that the reacted states are directly responsible for the drive-induced degradation of LEDs containing these two materials. LEDs are studied which contain various combinations of poly(9,9’-dioctylfluorene-co-N-(4-butylphenyl)diphenylamine) (TFB) and poly(9,9’-dioctylfluorene-co-benzothiadiazole) (F8BT), using pulsed electroluminescence. A strongly morphology-dependent spike-transient is observed in the electroluminescence at turn-on, and this is investigated by numerical modelling. Although not all features of the system can be well represented in the model, the spike transient is explicitly predicted without the need to impose any special conditions. The origin of this feature is elucidated by repeatedly running the model to a range of end-points and studying the time-evolution of space-charge distributions which result. Finally, F8BT devices are considered on their own, in order to study the evolution of device performance under low-intensity electrical excitation. A phenomenon is investigated in which the quantum efficiency is dramatically increased during the early stages of driving. Ionic motions are ruled out, and the observations are attributed to the trapping of charge in the vicinity of the anode, leading to enhanced hole injection. The reverse-bias behaviour of the effect, in which a further enhancement is seen, is also examined. The analogy is made with polymer LEDs in general which increase in performance following a period of reverse bias, and it is suggested that the causes may be related

Composite power semiconductor switches for high-power applications

It is predicted that 80 % of the world’s electricity will flow through power electronic based converters by 2030, with a growing demand for renewable technolo gies and the highest levels of efficiency at every stage from generation to load. At the heart of a power electronic converter is the power semiconductor switch which is responsible for controlling and modulating the flow of power from the input to the output. The requirements for these power semiconductor switches are vast, and include: having an extremely low level of conduction and switching losses; being a low source of electromagnetic noise, and not being susceptible to external Electromagnetic Interference (EMI); and having a good level of ruggedness and reliability. These high-performance switches must also be economically viable and not have an unnecessarily large manufacturing related carbon footprint. This thesis investigates the switching performance of the two main semiconductor switches used in high-power applications — the well-established Silicon (Si)-Insulated-Gate Bipolar Transistor (IGBT) and the state-of-the-art Wide-Bandgap (WBG) Silicon-Carbide (SiC)-Metal–Oxide–Semiconductor Field-Effect Transistor (MOSFET). The SiC-MOSFET is ostensibly a better device than the Si-IGBT due to the lower level of losses, however the cost of the device is far greater and there are characteristics which can be troublesome, such as the high levels of oscillatory behaviour at the switching edges which can cause serious Electromagnetic Compatibility (EMC) issues. The operating mechanism of these devices, the materials which are used to make them, and their auxiliary components are critically analysed and discussed. This includes a head-to-head comparison of the two high-capacity devices in terms of their losses and switching characteristics. The design of a high-power Double-Pulse Test Rig (DPTR) and the associated high-bandwidth measurement platform is presented. This test rig is then extensively used throughout this thesis to experimentally characterise the switching performance of the aforementioned high-capacity power semiconductor devices. A hybrid switch concept — termed “The Diverter” — is investigated, with the motivation of achieving improved switching performance without the high-cost of a full SiC solution. This comprises a fully rated Si-IGBT as the main conduction device and a part-rated SiC-MOSFET which is used at the turn-off. The coordinated switching scheme for the Si/SiC-Diverter is experimentally examined to determine the required timings which yield the lowest turn-off loss and the lowest level of oscillatory behaviour and other EMI precursors. The thermal stress imposed on the part-rated SiC-MOSFET is considered in a junction temperature simulation and determined to be negligible. This concept is then analysed in a grid-tied converter simulation and compared to a fully rated SiC-MOSFET and Si-IGBT. A conduction assistance operating mode, which solely uses the part-rated SiC-MOSFET when within its rating, is also investigated. Results show that the Diverter achieves a significantly lower level of losses compared to a Si-IGBT and only marginally higher than a full SiC solution. This is achieved at a much lower cost than a full SiC solution and may also provide a better method of achieving high-current SiC switche