Jannik Schafer

This paper extends the Generalized Steinmetz Equation (GSE) to account for the influence of mechanical stress on ferrite core losses. Experimental measurements are used to quantify the effects of compressive and tensile stresses on the relative permeability and the core losses of different ferrite materials. Mechanical stress is found to significantly affect the core losses, depending on the relative orientation of the magnetic flux density and the applied mechanical stress. Specifically, the losses monotonically increase when the flux and compressive stress vectors are parallel, while perpendicular vectors lead to a more complex response depending on the level of mechanical stress. The measured loss characteristics are translated in an extension of the GSE (X-GSE), which is validated for different ferrite materials, and provides a useful tool for a first rough estimation of the core losses under mechanical stresses. Finally, FEM simulations demonstrate how thermally induced mechanical stresses in a ferrite core can redistribute the magnetic flux and therefore impact the resulting core losses, which underlines the importance of considering the stress dependence of core losses in ferrite, especially in the development of magnetic components to be encapsulated in incompressible materials.

This paper extends the Generalized Steinmetz Equation (GSE) to account for the influence of mechanical stress on ferrite core losses. Experimental measurements are used to quantify the effects of compressive and tensile stresses on the relative permeability and the core losses of different ferrite materials. Mechanical stress is found to significantly affect the core losses, depending on the relative orientation of the magnetic flux density and the applied mechanical stress. Specifically, the losses monotonically increase when the flux and compressive stress vectors are parallel, while perpendicular vectors lead to a more complex response depending on the level of mechanical stress. The measured loss characteristics are translated in an extension of the GSE (X-GSE), which is validated for different ferrite materials, and provides a useful tool for a first rough estimation of the core losses under mechanical stresses. Finally, FEM simulations demonstrate how thermally induced mechanical stresses in a ferrite core can redistribute the magnetic flux and therefore impact the resulting core losses, which underlines the importance of considering the stress dependence of core losses in ferrite, especially in the development of magnetic components to be encapsulated in incompressible materials.

This paper aims to advance the understanding and application of matrix transformers in contemporary power electronics based on the exemplary specifications of an electric vehicle highcurrent/low-voltage DC/DC step-down converter. First, the general applicability of matrix transformer arrangements in power electronic systems is investigated, providing a nuanced discussion on scenarios where they outperform conventional transformer designs. Subsequently, the two principal types of matrix transformers: common-voltage and common-current matrix transformers are introduced, emphasizing the distinctive features that set them apart. A comprehensive analysis of the electromagnetic behavior of the two matrix transformer types is presented, highlighting their unique characteristics and advantages. Concluding the discussion, practical design suggestions are presented to maximize the potential of matrix transformers, enhancing the efficiency and power density of power electronic systems.

Fast charging of electric vehicles (EVs) requires isolated AC/DC converters with a wide output voltage range of 200V to 1000V. Combining a three-level Vienna Rectifier (VR) with four isolated Dual-Active-Bridge DC/DC Converter (DABC) modules and latest-generation 600V GaN technology enables very high switching frequencies of 560 kHz for the VR and up to 330 kHz for the DABCs. Hence, in this paper an ultra-compact realization of a 10 kW EV charger module with a power density of 9 kW/dm ³ (about 150 W/in ³ ), not including the coldplate, is presented. In this context, a simplified DABC modulation method and straightforward yet accurate (confirmed by experiments) loss models for the DABCs and the VR are introduced, which facilitate a thorough investigation of the optimum synergetic operation of the two stages: For the considered converter, changing the VR operating mode from conventional 3/3-PWM (where the two stages operate rather independently and hence all three VR bridge-legs operate with PWM) to 1/3-PWM (where the DABCs shape the voltage of the shared intermediate DC-link such that always only one of the VR’s three bridge-legs must operate with PWM) results in an advantageous efficiency improvement of up to about 2% over a large part of the output voltage and power range, and in a peak efficiency of more than 97%. Further, the synergetic operation of the two-stage system (VR and DABCs) is experimentally verified for the first time, confirming the modeling results and the efficiency advantage of 1/3-PWM (i.e., 95.4% vs. 95.1% at the rated load of 10 kW and with 500V output voltage). Conducted EMI pre-compliance measurements indicate that the change of the operating strategy from 3/3-PWM to 1/3-PWM only requires minor changes of the EMI filter design.

The ongoing miniaturization trend of electric machines increases the demand for higher rotational speeds to provide a required power level at decreased size. The goal of this project is to push the limits of rotor miniaturization by researching new concepts for bearingless machines with ultra-high rotational speeds exceeding 25 million rotations per minute (Mrpm). Using a simple machine stator consisting of air coils limits the achievable rotor torque, which results in acceleration times of several hours until the aforementioned rotational speeds are reached. This study outlines the torque generation mechanisms of the machine and investigates the stator losses, from which improved stator designs, based on a ferrite core, are derived. The latter significantly increase the motor torque at decreased losses and facilitate fast acceleration of the rotor.