Recent emerging gallium nitride (GaN) high electron mobility transistor (HEMT) is expected to be a promising candidate for high frequency power conversion techniques. Due to the advantages of the material, the GaN HEMT has a better figure of merit (FOM) compared to the state-of-the-art silicon (Si) power metal oxide silicon field effect transistor (MOSFET), which allows the GaN HEMT to switch with faster transition and lower switching loss. By applying the GaN HEMT in a circuit design, it is possible to achieve high frequency, high efficiency, and high density power conversion at the same time. To characterize the switching performance of the GaN HEMT, an accurate behavior-level simulation model is developed in this thesis. The packaging related parasitic inductance, including both self-inductance and mutual-inductance, are extracted based on finite element analysis (FEA) methods. Then the accuracy of the simulation model is verified by a double-pulse tester, and the simulation results match well with experiment in terms of both device switching waveform and switching energy. Based on the simulation model, detailed loss breakdown and loss mechanism analysis are made. The cascode GaN HEMT has high turn-on loss due to the body diode reverse recovery of the low voltage Si MOSFET and the common source inductance (CSI) of the package; while the turn-off loss is extremely small attributing to the cascode structure. With this unique feature, the critical conduction mode (CRM) soft switching technique are applied to reduce the dominant turn on loss and increase converter efficiency significantly. The switching frequency is successfully pushed to 5MHz while maintaining high efficiency and good thermal performance. Traditional packaging method is becoming a bottle neck to fully utilize the advantages of GaN HEMT. So an investigation of the package influence on the cascode GaN HEMT is also conducted. Several critical parasitic inductors are identified, which cause high turn on loss and high parasitic ringing which may lead to device failure. To solve the issue, the stack-die package is proposed to eliminate all critical parasitic inductors, and as a result, reducing turn on loss by half and avoiding potential failure mode of the cascode GaN device effectively. Utilizing the proposed stack-die package and ZVS soft switching, the GaN HEMT high frequency, high efficiency, and high density power conversion capability can be further extended to a higher level. / Master of Science

The Class-E topology was presented as a single-switch power amplifier with high efficiency at the optimum condition, where the switch enjoys zero-voltage switching (ZVS) and zero-voltage-derivative switching (ZDS). It is also used in MHz dc-dc converters, and in inverters for wireless power transfer, induction heating, and plasma pulsing. The load current in these applications usually varies over a range. Efficiency of a conventional Class-E design degrades dramatically due to the hard switching beyond the optimum conditions. Keeping ZVS with load change in a Class-E topology is preferred within the load range. Soft switching with load variation is realized by duty cycle modulation with additional transformer, matching network, or resistance compression network. Since two ZVS requirements need to be satisfied in a conventional Class-E design, at least two parameters are tuned under load variation. Thus, changing switching frequency, duty cycle, and component values were used. Impressively, a load-independent Class-E inverter design was presented in 1990 for maintaining ZVS and output voltage under a given load change without tuning any parameters, and it was validated with experimental results recently. The operating principle of this special design (inconsistent with the conventional design) is not elucidated in the published literatures. Load-independency illucidation by a Thevenin Model – A Thevenin model is then established (although Class-E is a nonliear circuit) to explain the load-independency with fixed switching frequency and duty cycle. The input block of a Class-E inverter (Vin, Lin, Cin, and S) behaves as a fixed voltage source vth1 and a fixed capacitive impedance Xth1 in series at switching frequency. When the output block (Lo and Co) is designed to compensate Xth1, the output current phase is always equal to the phase of vth1 with resistive load (satisfies the ZVS requirement of a load-independent design). Thus, soft switching is maintained within load variation. Output voltage is equal to vth1 since Xth1 is canceled, so that the output voltage is constant regardless of output resistance. Load-independency is achieved without adding any components or tuning any parameters. Sequential design and tuning of a load-independent ZVS Class-E inverter with constant voltage based on Thevenin Model - Based on the model, it's found that each circuit parameter is linked to only one of the targeted performance (ZVS, fixed voltage gain, and load range). Thus, the sequential design equations and steps are derived and presented. In each step, the desired performance (e.g. ZVS) now could be used to check and tune component values so that ZVS and fixed voltage gain in the desired load range is guaranteed in the final Class-E inverter, even when component values vary from the expectations. The Thevenin model and the load-independent design is then extended to any duty cycles. A prototype switched at 6.78 MHz with 10-V input, 11.3-V output, and 22.5-W maximum output power was fabricated and tested to validate the theory. Soft switching is maintained with 3% output voltage variation while the output power is reduced tenfold. A load-independent ZVS Class-E inverter with constant current by combining constant voltage design and a trans-susceptance network - A load-independent ZVS Class-E inverter with constant current under load variation is then presented, by combining the presented design (generating a constant voltage) and a trans-susceptance network (transferring the voltage to current). The impact of different types and the positions of the networks are discussed, and LCL network is selected so that both constant current and soft switching are maintained within the load variation. The operation principle, design, and tuning procedures are illustrated. The trade-off between input current ripple, output current amplitude, and the working load range is discussed. The expectations were validated by a design switched at 6.78 MHz with 10-V input, 1.4-A output, and 12.6-W maximum output power. Soft switching is maintained with 16% output current varying over a 10:1 output power range. A "ZVS" Class-E dc-dc converter by adding a diode rectifier bridge and compensate the induced varying capacitance at full-load condition - The load-independent Class-E design is extended to dc-dc converter by adding a diode rectifier bridge followed by the Class-E inverter. The equivalent impedance seen by the inverter consists of a varying capacitance and a varying resistance when the output changes. As illustrated before, ZVS and constant output can only be maintained with resistive load. Since the varying capacitance cannot be compensated for the whole load range, performance with using different compensation is discussed. With the selected full-load compensation, ZVS is achieved at full load condition and slight non-ZVS occurs for the other load conditions. The expectation was validated by a dc-dc converter switched at 6.78 MHz with 11 V input, 12 V output, and 22 W maximum output power. ZVS (including slight non-ZVS) is maintained with 16% output voltage variation over 20:1 output power range. Design of variable Capacitor by connecting two voltage-sensitive capacitors in series and controlling the bias voltage of them - The equivalent varying capacitance in the Class-E dc-dc converter can be compensated in the whole load range only with variable component. The sensitivity of a Class-E power conversion can also be improved by using variable capacitors. Thus, a Voltage Controlled Capacitor (VCC) is presented, based on the intrinsic property of Class II dielectric materials that permittivity changing much with electric field. Its equivalent circuit consists of two identical Class II capacitors in series. By changing the voltage of the common point of the two capacitors (named as control voltage), the two capacitance and the total capacitance are both changed. Its operation principle, measured characteristic, and the SPICE model are illustrated. The capacitance changes from 1 μF to 0.2 μF with a control voltage from 0 V to 25 V, resulting a 440% capacitance range. Since the voltage across the two capacitors (named as output voltage) also affects one of the capacitance when control voltage is applied, the capacitance range drops to only 40% with higher bias in the output voltage. Thus, a Linear Variable Capacitor (LVC) is presented. The equivalent circuit is the same as VCC, while one of the capacitance is designed much higher to mitigate the effect of output voltage. The structure, operational principle, required specifications, design procedures, and component selection were validated by a design example, with 380% maximum capacitance range and less than 20% drop in the designed capacitor voltage range. This work contributes to • Analytical analysis and Thevenin Model in load-independent Class-E power conversion • Variable capacitance with wide range / Doctor of Philosophy / The Class-E topology was presented as a single-switch power amplifier with high efficiency at the optimum condition. Efficiency of a conventional Class-E design degrades with load variation dramatically due to the hard switching beyond the optimum conditions. Since two requirements need to be satisfied for soft switching in a conventional Class-E design, at least two parameters are tuned under load variation. Impressively, a load-independent Class-E inverter design was presented for maintaining Zero-Voltage-Switching (ZVS) and output voltage under a given load change without tuning any parameters, and it was validated with experimental results recently. A Thevenin model is established in this work to explain the realization of load-independency with fixed switching frequency and duty cycle. Based on that, a sequential design and tuning process is presented. A prototype switched at 6.78 MHz with 10-V input, 11.3-V output, and 22.5-W maximum output power was fabricated and tested to validate the theory. Soft switching is maintained with 3% output voltage variation while the output power is reduced tenfold. A load-independent ZVS Class-E inverter with constant current under load variation is then presented, by combining the presented design and a trans-susceptance network. The expectations were validated by a design switched at 6.78 MHz with 10-V input, 1.4-A output, and 12.6-W maximum output power. Soft switching is maintained with 16% output current varying over a 10:1 output power range. The load-independent Class-E design is extended to dc-dc converter by adding a diode rectifier bridge, inducing a varying capacitance. With the selected full-load compensation, ZVS is achieved at full load condition and slight non-ZVS occurs for the other load conditions. The expectation was validated by a dc-dc converter switched at 6.78 MHz with 11 V input, 12 V output, and 22 W maximum output power. ZVS (including slight non-ZVS) is maintained with 16% output voltage variation over 20:1 output power range. The varying capacitance in the Class-E dc-dc converter needs variable component to compensate. Thus, a Voltage Controlled Capacitor (VCC) is presented. The capacitance changes from 1 μF to 0.2 μF with a control voltage from 0 V to 25 V, resulting a 440% capacitance range. The capacitance range drops to only 40% with higher bias in the output voltage. Thus, a Linear Variable Capacitor (LVC) is presented, with 380% maximum capacitance range and less than 20% drop in the designed capacitor voltage range.

Soft-switching techniques can significantly reduce the switching loss and switching stresses of the power semiconductor devices in a power converter. This work presents several soft-switching topologies for high power PWM converters. These new topologies achieve soft-switching functions with minimum increase of device voltage/current stresses and converter circulating energy, and thus have advantages over conventional techniques in efficiency, power density, reliability, and cost of power converters. The improved zero-current transition (ZCT) converters achieve zero-current switching at both turn-on and turn-off for all main switches and auxiliary switches. These converters significantly reduce the switching loss and stress of the power semiconductor devices, while have a voltage/current stress and circulating energy similar to a PWM converter’s. The analysis, design, and experimental verification are presented. The three-phase zero-voltage transition (ZVT) boost rectifiers/voltage source inverters are developed with simple auxiliary circuits. Unlike most existing three-phase soft-switching techniques, these new topologies achieve soft-switching functions without overcharging the resonant inductors, and realize the benefits of soft-switching operation with minimum extra main switch turn-offs and fixed auxiliary circuit control timing. The operation principles of the developed techniques are experimentally verified, and their efficiency performances are evaluated with experiments and computer simulation. The three-phase ZVT buck rectifier topologies developed in this work achieves zero-voltage turn-on for all main switches with an optimum modulation schemes and simple auxiliary circuits. The auxiliary circuits, which are connected directly to each main switch, can also absorb the parasitic resonance of the bridge arms, and keep the voltage stress of the power devices at the minimum. The analysis and simulation results are presented to verify the converter operation. New ZVT dc-link schemes for three-phase ac-dc-ac converters are investigated. With coordinated control of the ac-dc converter and the dc-ac converter, a set of simple auxiliary circuit can provide soft-switching function for all switches in both the ac-dc converter and the dc-ac converter. The power loss in the auxiliary circuit is also significantly lower than existing dc-link soft-switching schemes. Simulation with experimentally obtained device switching loss data proves that significant efficiency improvement can be achieved with the new ZVT dc-link techniques. New ZVT and ZCT techniques for three-level converters are also developed. The auxiliary circuits are not in the main power path, and allow the converters to be controlled with optimum PWM schemes. Analysis and simulation results are presented to demonstrate the operation principles and advantages of soft switching in three-level converters. / Ph. D.

This thesis examines the development of DC-DC converters that are suitable for Module Integrated Converters, (MICs), in grid interactive photovoltaic (PV) systems, and especially concentrates on the study of the half bridge dual converter, which was previously developed from the conventional half bridge converter. Both hard-switched and soft-switched half bridge dual converters are constructed, which are rated at 88W each and transform a nominal 17.6Vdc input to an output in the range from 340V to 360Vdc. An initial prototype converter operated at 100kHz and is used as a base line device to establish the operational behaviours of the converter. The second hard-switched converter operated at 250kHz and included a coaxial matrix transformer that significantly reduced the power losses related to the transformer leakage inductance. The soft-switched converter operated at 1MHz and is capable of absorbing the parasitic elements into the resonant tank. Extensive theoretical analysis, simulation and experimental results are provided for each converter. All three converters achieved conversion efficiencies around 90%. The progressive increases in the operation frequency, while maintaining the conversion efficiency, will translate into the reduced converter size and weight. Finally different operation modes for the soft-switched converter are established and the techniques for predicting the occurrence of those modes are developed. The analysis of the effects of the transformer winding capacitance also shows that soft switching condition applies for both the primary side mosfets and the output rectifier diodes.

In this thesis, a compact and light primary switched conversion system for AC-fed railway propulsion is investigated. It is characterized by soft switching of all converter stages and a source commutated primary converter comprising series connected valves. Both weight and volume of the conversion system are reduced significantly compared to a conventional system with a low frequency transformer. The conversion system is made up of N isolated AC/DC conversion cells, each comprising a cycloconverter and a voltage source converter (VSC) coupled by a medium frequency transformer. The cells are series connected on the AC side and connected to a common DC-link. Thus, 2N+1 voltage levels can be synthesized at the AC terminal and the voltage stress on the transformer and line filter is reduced compared to a one cell solution. Series connection of semiconductor valves allows independent choice of blocking voltage and number of converter cells. Choosing two converter cells is an attractive compromise. Five level output reduces the harmonic distortion and simplify transformer and line filter design while keeping the complexity of the conversion system low. The mutually commutated converter (MCC) allows a transformer frequency in the range of 4 to 8 kHz without derating the line side converter due to zero voltage switching of the VSC. Modern magnetic materials, like high silicon steel, amorphous and noncrystalline materials allow design of the transformer with high efficiency at elevated frequencies. In a 15 kV system, the peak voltage at the catenary is typically beyond 32 kV which is far beyond the voltage capability of currently available semiconductors. Therefore, several semiconductors are connected in series. Favourable commutation conditions and a new gate drive arrangement allow snubberless commutation of the primary converter stage. Thus, the primary converter can be highly integrated, reducing both weight and volume. The conversion system can be placed on the roof or in the underframe without compromising efficiency or vehicle performance. The feasibility of the conversion concept has been demonstrated by means of a down-scaled prototype. Snubberless commutation of series connected valves is demonstrated. / QC 20100723

High-frequency dc-dc converters are nowadays widely used in a diversity of power electronic applications. High operating frequencies entail a reduction in size of the passive components, such as inductors, capacitors and power transformers. By operating the converter at higher frequencies with conventional hard-switching topologies, the transistor switching losses increase at both turn-on and turn-off. High-voltage converters in the power range of 1-10MW will therefore have excessive switching losses if the switching frequency is higher than 4 kHz. In order to achieve a high-frequency operation with moderate switching losses a number of soft-switched topologies have been studied in [Dem1]. The favourable DC-DC converter was found to be the Dual-Active Bridge when a bi-directional power flow is demanded. Additionally, the Single-Active Bridge (SAB) topology was introduced for the first time. In this thesis the two topologies are thoroughly studied. The dynamic small-signal models are presented and the dynamic behaviour of the converters is discussed in deep. Different control strategies are presented concerning the two converters and the advantages and the disadvantages of the different control strategies are stated. Critical issues as efficiency and stability are presented separately for the two converters. / QC 20101005

In this dissertation the ac-link universal power converters, which are a new class of power converters, are introduced and studied in detail. The inputs and outputs of these converters may be dc, ac, single phase, or multi-phase. Therefore, they can be used in a variety of applications, including photovoltaic power generation, wind power generation, and electric vehicles. In these converters the link current and voltage are both alternating and their frequency can be high, which leads to the elimination of the dc electrolytic capacitors and the bulky low-frequency transformers. Therefore, the ac-link universal power converters are expected to have higher reliability and smaller size. Moreover, these converters are soft switching, which results in negligible switching losses and minimized current and voltage stress over devices. In the first part of the dissertation, the parallel ac-link universal power converter is studied in detail. This converter is an extension of the buck-boost converter. The series ac-link universal power converter, which is dual of the parallel ac-link universal power converter, is proposed in the second part of this dissertation. This converter is an extension of the Cuk converter. A modified configuration with fewer switches, named sparse ac-link universal power converter is proposed in the third part of this dissertation. The sparse ac-link universal power converters can appear as parallel or series. The performance of all these configurations is evaluated through simulations and experiments.

This work presented is DC-DC isolated ZVS Bidirectional Dual Active Bridge (DAB) single phase converter, based three-state switching cell is presented. The proposal is to apply it in photovoltaic systems with battery bank into smart networks. Basically the drive control is the duty cycle (D) of the switches and the Phase Shift (φ) of the fundamental tensions between the bridges. The gyrator modeling of the converter is presented, highlighting its natural operating characteristic as gyrator. Shows the qualitative and quantitative analysis of the converter, realizing the full study of the stages of operation of the topology and checking all sixteen regions of operation. To obtain the regions of soft-switching, the fundamental model is applied. The design procedure of the converter is presented, and the results of simulations. A 2kW prototype was developed, aimed at obtaining experimental results validate the theoretical analysis / Neste trabalho à apresentado o conversor CC-CC ZVS isolado bidirecional Dual Active Bridge (DAB) monofÃsico, baseado na cÃlula de comutaÃÃo de trÃs estados. A proposta à aplicÃ-lo em sistemas fotovoltaicos com banco de baterias em redes inteligentes. Basicamente o controle do conversor consiste na razÃo cÃclica (D) dos interruptores e o Phase Shift (φ) entre as componentes fundamentais das tensÃes entre as pontes. A modelagem por gyrator do conversor à apresentada, destacando-se sua caracterÃstica natural de funcionamento como gyrator. Mostra-se a anÃlise qualitativa e quantitativa do conversor, realizando o estudo completo das etapas de operaÃÃo da topologia e verificando todas as dezesseis regiÃes de operaÃÃo. Para obtenÃÃo das regiÃes de comutaÃÃo suave, à aplicado o modelo fundamental. O procedimento de projeto do conversor à apresentado, alÃm dos resultados de simulaÃÃes. Um protÃtipo de 2 kW foi desenvolvido, visando a obtenÃÃo dos resultados experimentais e validando a anÃlise teÃrica.

Este trabalho tem por objetivo apresentar um novo conceito para a concepÃÃo de conversores para aplicaÃÃes em energias renovÃveis e carregamento de baterias com um nÃmero reduzido de estÃgios de processamento. SÃo desenvolvidas trÃs topologias diferentes de conversores de alto ganho de tensÃo com um Ãnico estÃgio de processamento e capaz de operar tanto com um banco de baterias como com painÃis fotovoltaicos. A operaÃÃo destes conversores permite que as fontes de entradas (painÃis ou baterias) operem de forma natural, independentemente ou complementarmente, compondo um barramento CC e apresentando a comutaÃÃo suave dos interruptores. Por fim, foram desenvolvidos trÃs protÃtipos para uma potÃncia de 500 W que validam o princÃpio de funcionamento dos sistemas propostos apresentando alto rendimento, alto ganho de tensÃo e boa regulaÃÃo do barramento CC em todos os modos de operaÃÃo. / Taking into account the great technological development on renewable energy systems and their applications on power electronics, this work presents a new concept of converters applied to renewable systems and batteries charging with a reduced number of energy processing stages. This work approaches three different high voltage gain converters topologies with only one processing stage, and with bidirectional capability, with batteries, and with photovoltaic panels. The operation of these converters allow that the inputs (PVpanels or batteries), work independently or simultaneously, in order to compose a single DC bus. Also, proposed topologies present natural soft-switching operation. At last, the three prototypes were developed to work with a nominal power of 500W, validating the operation principle of the proposed system, featuring high efficiency, high voltage gain, and good voltage regulation on the DC bus.

This diploma thesis deals with design and comparison of selected DC/DC converters, where the better of them is practically realized. The first part of the diploma thesis is focused on the general analysis of DC/DC power converters. The following part is theoretical analysis focused on the first selected topology – step-up converter. The second analysed topology is forward converter with full bridge on the primary side. The theoretical analysis also includes a description of synchronous rectifier, the differences between hard and soft switching, and the types of secondary rectifiers. Another part specializes in the detailed calculation of main components of selected converters and their subsequent power dimensioning. Both designed topologies are compared according to the required aspects. The selected better topology is supplemented by the design of control circuits and an auxiliary power supply. Practical realization of converter and commissioning follows. The diploma thesis ends with verification measurements on the realized converter and their subsequent analysis.