On frequency domain analysis of dual active bridge dc-dc converters

Modern society uses electrical energy for a wide range of needs and requirements. Electrical energy is considered high value as it requires a prior conversion step from kinetic/thermal or solar energy. However, electrical power is always defined by certain properties which typically need to be adjusted in multiple stages to satisfy the specifications of electrical loads such as motors, lighting and consumer electronics. For DC (direct current) power systems, switching DC-DC power converters are the state-of-the-art solution to achieve a low-loss modification of the voltage magnitude. The Dual Active Bridge (DAB) converter is an attractive DC-DC conversion topology that can widely satisfy the future needs of DC power management and the integration of electro-chemical storage. It offers an unmatched capability to transfer energy in either direction between two DC sources while its inherent Zero Voltage Switching capability offers potential for high conversion efficiency and high power density. The current and future research activities on DAB converters mainly focus on maximising the power density through a volume reduction of the embedded passive power devices. This trend is encouraged by the market introduction of wide bandgap fast-switching semiconductor devices using Silicon Carbide (SiC) and Gallium-Nitride (GaN) to replace conventional Silicon material in many applications. The reduced parasitic capacitance and transition time of these devices allow to significantly increase the converter operating frequencies, which is the only way to increase the power density unless the material specifications of passive power devices drastically evolve. However, a higher operating frequency inevitably leads to a stronger influence of practical second-order effects, which for a DAB, particularly address the non-ideality of the switch devices, the parasitic coupling impedances in the high-frequency transformer, the peripheral connecting traces of the AC link network and the DC bus filter. Hence, all these effects have to be accommodated by a universal design framework which is yet to be found in literature. A DAB is conventionally designed using time domain analysis of the modulation sequence and device waveforms to evaluate its key performance design criteria such as active power transfer, Zero Voltage Switching (ZVS) and AC link circulating power. This analysis technique typically presumes an idealized single parameter AC link inductance to substitute for the more complex circuit model of a practical high-frequency transformer. This becomes particularly relevant as the operating frequencies increase, causing both active and passive power devices to become less ideal. More than that, advanced multi-level DAB Phase Shifted Square Wave (PSSW) modulation strategies lead to a wide solution space of control parameters that can be used to enhance the performance of a DAB by shaping the AC link current in certain ways. Within the time domain, such volatile modulation strategies require a complicated structure model analysis. This thesis now shows how to apply frequency domain harmonic analysis techniques to a DAB DC-DC converter. The approach readily accommodates the influence of complex impedance structures, practical switching effects and advanced multi-level modulation concepts, and leads to generic numerical and analytical solution expressions that significantly enhance the converter design process. The work thus establishes a new analysis strategy in the advancing field of DAB research. The thesis begins with the harmonic decomposition of the bridge output voltages and the expression of the DAB coupling network as a generic two-port impedance model. These steps establish the frequency domain analysis (FDA) framework. Next, the FDA approach is applied to derive explicit solution terms for the ZVS regions of single and three-phase DAB converters, which are crucial to minimise the power loss of the semiconductor devices during the switching transition. These expressions are used to separately investigate the impact of single impedance parameters, non-ideal switching transitions and PSSW modulation concepts on the ZVS limiting conditions, which determine the preferable operating regions to achieve minimum switching loss operation and best possible controllability of the DAB. From this work, it is shown how a single element high-frequency transformer with a reduced coupling factor is sufficient to ensure continuous and reliable ZVS operation without adding software or auxiliary hardware complexity. More than that, the strategy quantifies the impedance design parameters of the coupling network in correlation with the selected PSSW modulation concept. In this context, the benefit of adaptive 3-level DAB modulation is presented to support continuous ZVS operation and thus allow for high-efficient operation of a single phase DAB across its entire operating range. Finally, the established modeling framework is extended to identify the DC bus harmonics injected by the PSSW modulation process, for any particular DAB design and operating context. It is also shown how adaptive modulation can be used to mitigate certain harmonic frequencies that can otherwise cause severe harmonic interferences, DC bus oscillations, and thus impact on the DC side filter design process. To complete the work, experimental results are presented to verify the analytical development, using a laboratory DAB converter that can operate across a wide range of DC voltages at power levels up to 1 kW. Simultaneously, as part of an industry project with BOSCH, a customised 1.2 kW DAB prototype was built using the presented design guidelines, which achieved a conversion efficiency between 96.5% and 98.5% across the operating range

Non-inverting and Non-isolated Magnetically Coupled Buck-Boost Bidirectional DC-DC Converter

A new non-isolated DC-DC converter with non-inverting output and buck-boost operation, named Magnetically Coupled Buck-Boost Bidirectional converter (MCB³), is presented in this paper. The MCB³ passive components arrangement connects the input and output ports getting an equivalent behavior to that of the Dual Active Bridge (DAB) converter, but in a non-isolated topology. This equivalency allows applying Triple Phase Shift (TPS) modulation to MCB³. TPS is known to minimize conduction losses and to achieve soft-switching at any load in the DAB converter. Throughout the paper, the features of the DAB converter are used as a reference to show the main features of the proposed converter. Moreover, other modulation strategies based on TPS modulation are used in MCB³ to operate within the minimum losses path.The multiple operation modes found on the MCB³ under TPS modulation are identified, classified, and used to find the operating points that minimize the switching and conduction losses over the power range. The analysis is shown for the boost mode that is the worst-case design. MCB³ and DAB topologies are designed and simulated for the same specification to validate the theoretical study. Finally, experimental measurements on 460W-prototypes for both topologies corroborate the equivalent operation and the main features of the MCB³.This work was supported in part by the Ministry of Economy and Competitiveness and ERDF funds through the Research Project “Energy Storage and Management System for Hybrid Electric Cars based on Fuel Cell, Battery and Supercapacitors” ELECTRICAR-AG- (DPI2014-53685-C2-1-R), and in part by the Research Projects CONEXPOT (DPI2017-84572-C2-2-R) and EPIIOT (DPI2017-88062-R

Induction heating converter's design, control and modeling applied to continuous wire heating

Induction heating is a heating method for electrically conductive materials that takes advantage of the heat generated by the Eddy currents originated by means of a varying magnetic field. Since Michael Faraday discovered electromagnetic induction in 1831, this phenomena has been widely studied in many applications like transformers, motors or generators' design. At the turn of the 20th century, induction started to be studied as a heating method, leading to the construction of the first industrial induction melting equipment by the Electric Furnace Company in 1927. At first, the varying magnetic fields were obtained with spark-gap generators, vacuum-tube generators and low frequency motor-generator sets. With the emergence of reliable semiconductors in the late 1960's, motor-generators were replaced by solid-state converters for low frequency applications. With regard to the characterization of the inductor-workpiece system, the first models used to understand the load's behavior were based on analytical methods. These methods were useful to analyze the overall behavior of the load, but they were not accurate enough for a precise analysis and were limited to simple geometries. With the emergence of computers, numerical methods experienced a tremendous growth in the 1990's and started to be applied in the induction heating field. Nowadays, the development of commercial softwares that allow this type of analysis have started to make the use of numerical methods popular among research centers and enterprises. This type of softwares allow a great variety of complex analysis with high precision, consequently diminishing the trial and error process. The research realized in last decades, the increase in the utilization of numerical modeling and the appearance and improvement of semiconductor devices, with their corresponding cost reduction, have caused the spread of induction heating in many fields. Induction heating equipments can be found in many applications, since domestic cookers to high-power aluminum melting furnaces or automotive sealing equipments, and are becoming more and more popular thanks to their easy control, quick heating and the energy savings obtained. The present thesis focuses on the application of induction heating to wire heating. The wire heating is a continuous heating method in which the wire is continuously feeding the heating inductor. This heating method allows high production rates with reduced space requirements and is usually found in medium to high power industrial processes working 24 hours per day. The first chapters of this study introduce the induction heating phenomena, its modeling and the converters and tanks used. Afterwards, a multichannel converter for high-power and high-frequency applications is designed and implemented with the aim of providing modularity to the converter and reduce the designing time, the production cost and its maintenance. Moreover, this type of structure provides reliability to the system and enables low repairing times, which is an extremely interesting feature for 24 hours processes. Additionally, a software phase-locked loop for induction heating applications is designed and implemented to prove its flexibility and reliability. This type of control allows the use of the same hardware for different applications, which is attractive for the case of industrial applications. This phase-locked loop is afterwards used to design and implement a load-adaptative control that varies the references to have soft-switching according to load's variation, improving converter's performance. Finally, the modeling of a continuous induction wire hardening system is realized, solving the difficulty of considering the mutual influence between the thermal, electromagnetic and electric parameters. In this thesis, a continuous process is modeled and tested using numerical methods and considering converter's operation and influence in the process.Postprint (published version

Integrated DC-DC Charger Powertrain Converter Design for Electric Vehicles Using Wide Bandgap Semiconductors

Electric vehicles (EVs) adoption is growing due to environmental concerns, government subsidies, and cheaper battery packs. The main power electronics design challenges for next-generation EV power converters are power converter weight, volume, cost, and loss reduction. In conventional EVs, the traction boost and the onboard charger (OBC) have separate power modules, passives, and heat sinks. An integrated converter, combining and re-using some charging and powertrain components together, can reduce converter cost, volume, and weight. However, efficiency is often reduced to obtain the advantage of cost, volume, and weight reduction.An integrated converter topology is proposed to combine the functionality of the traction boost converter and isolated DC-DC converter of the OBC using a hybrid transformer where the same core is used for both converters. The reconfiguration between charging and traction operation is performed by the existing Battery Management System (BMS) contactors. The proposed converter is operated in both boost and dual active bridge (DAB) mode during traction operation. The loss mechanisms of the proposed integrated converter are modeled for different operating modes for design optimization. An aggregated drive cycle is considered for optimizing the integrated converter design parameters to reduce energy loss during traction operation, weight, and cost. By operating the integrated converter in DAB mode at light-load and boost mode at high-speed heavy-load, the traction efficiency is improved. An online mode transition algorithm is also developed to ensure stable output voltage and eliminate current oscillation during the mode transition. A high-power prototype is developed to verify the integrated converter functionality, validate the loss model, and demonstrate the online transition algorithm. An automated closed-loop controller is developed to implement the transition algorithm which can automatically make the transition between modes based on embedded efficiency mapping. The closed-loop control system also regulates the integrated converter output voltage to improve the overall traction efficiency of the integrated converter. Using the targeted design approach, the proposed integrated converter performs better in all three aspects including efficiency, weight, and cost than comparable discrete solutions for each converter

Design and Analysis of High Frequency Power Converters for Envelope Tracking Applications

In the field of power electronics, designers are constantly researching new methods to improve efficiency while optimizing dynamic performance. As communication technologies progress we are more often dealing with systems of increasing speed and complexity. For instance, from 1991 to 2013 we have observed the mobile broadband communication sector evolve from ~230 Kbits/s (2G) speeds to ~100 Mbits/s (4G LTE), a 430% increase in communication speed. In contrast, we have not observed the same evolutionary development in industrial power converters. Most switch-mode power supplies are still manufactured for 100 KHz to 800 KHz operating frequencies. The main reason for this is that most electrical devices only require steady-state DC power, so high speed conversion performance is largely unnecessary. But as size expectations for portable electronic devices continue to decrease, the only way to meet future demand is to realize power electronics that operate at much higher switching frequencies. Furthermore there is increasing demand to improve the transient response requirements in processor-based systems and achieve practical envelope tracking in RF communication systems. The most straightforward method of increasing the dynamic response for these systems is to increase the switching frequency of the power electronics in a sustainable and coherent manner

High power high frequency DC-DC converter topologies for use in off-line power supplies

The development of a DC-DC converter for use in a proposed range of one to ten kilowatt off-line power supplies is presented. The converter makes good use of established design practices and recent technical advances. The thesis begins with a review of traditional design practices, which are used in the design of a 3kW, 48V output DC-DC converter, as a bench-mark for evaluation of recent technical advances. Advances evaluated include new converter circuits, control techniques, components, and magnetic component designs. Converter circuits using zero voltage switching (ZVS) transitions offer significant advantages for this application. Of the published converters which have ZVS transitions the phase shift controlled full bridge converter is the most suitable, and assessments of variations on this circuit are presented. During the course of the research it was realised that the ZVS range of one leg of the phase shift controlled full bridge converter could be extended by altering the switching pattern, and this new switching pattern is proposed. A detailed analysis of phase shift controlled full bridge converter operation uncovers a number of operational findings which give a better and more complete understanding of converter operation than hitherto published. Converter design equations and guidelines are presented and the effects of the new improvement are investigated by an approximate analysis. Computer simulations using PSPICE2 are carried out to predict converter performance. A prototype converter design, construction details and test results are given. The results obtained compare well to the predicted performance and confirm the advantages of the new switching pattern