Hartmut Hinz

One of the early design decisions for DC‐DC converters is the choice of an appropriate magnetic core material. Powder cores in particular are suitable due to their saturation flux density of up to 1.5 T. However, high DC currents constitutes a significant problem since the winding cross‐section rises with the current, which consequently increases AC losses due to skin effects. Another challenge is the reduction of winding layers to decrease proximity losses. This paper presents an improved single‐layer winding design procedure for DC‐DC converter powder cores. The procedure determines the optimum number of parallel cores and stacks. This way, the procedure enables a predefined maximum wire cross‐section for copper loss optimisation and single‐layer winding. The introduced approach defines the optimum switching frequency as a trade‐off between core loss and core volume. Finally, experimental tests with two setups validate 27 kW DC‐DC converters and hence the design procedure using Si‐IGBTs and SiC‐MOSFETs. The results demonstrate that SiC‐MOSFETs reduce core volume by a factor of 3 compared to Si‐IGBTs. Further, the SiC setup reaches high efficiencies of up to 98.53% compared to 98.17% for the Si‐based converter.

Fuel cell hybrid vehicles represent an alternative to battery electric vehicles and will gain importance in the future as they do not need large battery capacities and thus require less critical raw materials. Depending on the electric architecture, the voltage of the fuel cell stack and traction battery may overlap. Accordingly, it is necessary to use a bidirectional DC–DC converter that connects the battery to the DC bus, which supports overlapping input and output voltages. Furthermore, these converters should be non-isolating in terms of compact design. Concerning complexity and controllability, the bidirectional cascaded buck and boost converter is preferable and is the subject of this study. Published literature presents the bidirectional cascaded buck and boost converter with high losses for overlapping input and output voltages, introducing two methods for this operation mode. The method selected for this study, namely buck + boost, uses two switches, whereby one switch has a fixed duty cycle. However, there is no appropriate investigation to determine the impact of this fixed duty cycle on converter efficiency to date. Furthermore, efficiency improvement is possible by switching frequency modulation, but current literature does not address this modulation method for overlapping input and output voltages. Therefore, this paper investigates a non-isolated hard-switched bidirectional cascaded buck and boost converter for fuel cell hybrid vehicles operating with up to 19.8 kW. The study focuses on determining the optimum fixed duty cycle and efficiency optimisation through a novel critical conduction mode with adapted switching frequency by utilising the load-dependent inductance of the inductor with powder cores. Measurements with an experimental setup validate the proposed modulation method with Si-IGBT half-bridge modules. The results demonstrate that a loss reduction of 39% is possible with switching frequency modulation and the optimum duty cycle compared to fixed switching frequency. As a result, the converter achieves high efficiencies of up to 99% and low device junction temperatures.

Literature compares DC-DC converters with Si and SiC power semiconductors without optimum switching frequency in terms of efficiency, volume, and costs for the respective application and operating point. Typical switching frequencies, such as 25 kHz for Si-IGBTs and 100 kHz for SiC-MOSFETs, are chosen with unconditional statements. A design with optimum switching frequency is crucial for a meaningful conclusion about the best-suited semiconductor technology for the respective application. This paper uses a novel iterative design procedure to determine the optimum switching frequency considering a trade-off between efficiency, volume, and costs for bidirectional DC-DC converters based on Si-IGBTs and SiC-MOSFETs with ferrite cores. Due to the importance of fuel cell hybrid vehicles as the long-term alternative to conventional vehicles, the application analyzed is the bidirectional DC-DC converter connecting the traction battery to the DC-bus. Both converters are evaluated and compared in the simulation environment PSIM, and their parameters are according to available fuel cell hybrid vehicles on the market. This paper proves in PSIM that the optimum switching frequency in both cases is below the standard values. Furthermore, it presents that Si-IGBTs have efficiency losses of less than 2 % for the operating point of interest than SiC-MOSFETs, reducing semiconductor costs by 68 % while increasing core volume by 344 %.

Fuel cell hybrid vehicles use an electric energy storage system such as a battery that can provide additional traction power. This is needed to improve vehicle operation during acceleration and to avoid dynamic current stress of the fuel cell stack. The battery is used for regenerative braking and is connected to the DC-bus of the drive train via a bidirectional DC-DC converter. This paper investigates non-isolated hard-switched bidirectional DC-DC converters with voltage and current ratings which are common in fuel cell hybrid vehicles. The focus is on the comparative analysis of converters regarding the current and voltage stress as well as on the passive component design. Furthermore, a general evaluation for the use in fuel cell hybrid vehicles is discussed.

Lithium-ion batteries are well known in numerous commercial applications. Using accurate and efficient models, system designers can predict the behavior of batteries and optimize the associated performance management. Model-based development comprises the investigation of electrical, electro-chemical, thermal, and aging characteristics. This paper focuses on the analysis of models describing the electrical behavior. In particular, it investigates how cell voltage and state of charge can be determined with sufficient accuracy for a given load profile. For this purpose, the Thevenin-based, the Rint, and the Shepherd’s models, as well as a generic library model of an electronic circuit simulation software package, are compared. The procedure for determining model parameters is discussed in detail. All models are evaluated for the application in the analysis of distributed power generation. The validation is carried out by comparing simulation and measurement results with the help of a case study.

Distributed generation plays an important role for reliable and flexible supply networks. They also have great potential for the integration of renewable energies. In this paper, a hybrid system consisting of a cogeneration, a photovoltaic installation and a lithium-ion battery is investigated. In particular, a methodical approach is presented that allows determining the energetic assessment criteria self-sufficiency and self-consumption rate as a function of the design parameters of the plant. These criteria are used to provide an optimization procedure which enables the installed power of the photovoltaic system and storage capacity of the battery to be specified in such a way as to nearly feasible a virtually grid-independent operation. It is shown that with the help of an average balance the distributed generation can be dimensioned in order to optimize the annual energy production and consumption with respect to the load profile of a residential area and the demand for intermediate storage of electrical energy.

Power electronics is one of the biggest economic drivers for several decades. It is a key technology in numerous industrial applications like traction, automation, electric vehicles, renewable energies or smart grids. For this reason, it is an integral part of the curriculum of the study program electrical engineering. This paper presents a didactic method, which helps students to understand the basic operation of power electronic converters in a comprehensive way. For this purpose, undergraduates are encouraged and instructed to use digital simulation during the share of self-study-time. In particular, the suggested approach promotes an interdisciplinary learning of power electronics and control theory. The proposed concept is expected to support students in learning project oriented. The results are discussed by means of one case study.

Lithium-ion batteries are well known in numerous commercial applications. Meanwhile rechargeable lithium-ion batteries are the standard electrochemical storage devices in the low power range of portable electronics. Furthermore in the last years this technology is becoming more important also in the higher power range in renewable energy generation. In particular electrochemical storage plays a critical role in the expansion of renewables in the electricity market. This paper describes the integration of a lithium-ion battery in a hybrid power generation which comprises a cogeneration and a photovoltaic plant. The focus of this study is on the investigations on the performance, modeling and validation of the used battery pack.

This paper presents a method to analyze the current ripple distribution in a multi-drive system by analyzing the DC bus impedances. By using this method the choice of the right DC link capacitor and the related switching frequency of the power electronic can be supported