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.

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.

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.