@article{59805, abstract = {{The LLC converter achieves the highest efficiency in resonant operation. Conventionally, the input DC-link voltage is controlled to operate the LLC converter at resonance for the given operating point. However, the DC-link capacitor voltage shows a low-frequency voltage ripple (typically the second harmonic of grid frequency) in cascaded converters so that the LLC has to adapt its switching frequency within the grid period. Conventionally, the LLC converter operates 50% of the time above the resonant frequency of 40 kHz and 50% below resonance. Both operating conditions cause additional losses. However, experimental measurements indicate that the below-resonance operation causes significantly higher losses than above-resonance operation due to much higher primary and secondary transformer currents. It is better to increase the DC-link voltage by 30% of the peak-to-peak low-frequency voltage ripple to mostly avoid below-resonance operation (i.e., from 650 V to 680 V in this case). With the proposed control, the LLC converter operates about 75% of time over resonance and only 25% of time below resonance. The overall efficiency increases from 97.66% to 97.7% for the average operating point with an 80% load current. This corresponds to a 2% total loss reduction. Finally, the peak resonance capacitor voltage decreases from 910 V to 790 V (−13%).}}, author = {{Unruh, Roland and Böcker, Joachim and Schafmeister, Frank}}, issn = {{2079-9292}}, journal = {{Electronics}}, keywords = {{adaptive DC-link voltage, cascaded H-bridge, resonant operation, Full-Bridge Converter, loss minimization, LLC Resonant Converter, peak capacitor voltage reduction}}, number = {{8}}, publisher = {{MDPI AG}}, title = {{{Adaptive DC-Link Voltage Control for 22 kW, 40 kHz LLC Resonant Converter Considering Low-Frequency Voltage Ripple}}}, doi = {{10.3390/electronics14081517}}, volume = {{14}}, year = {{2025}}, } @inproceedings{63157, abstract = {{Three-phase cascaded H-bridge converters (CHBs) in star configuration require reliable current controllers to evenly charge the module DC-link capacitors. Conventionally, a current control in dq-coordinates is utilized. At steady state, the resulting calculated reference arm voltages are sinusoidal, have identical amplitudes and show a phase shift of 120 degree to each other. For balanced grid inductors, the resulting grid currents also have the same amplitude. However, own simulations show that unbalanced grid inductors always lead to different grid current amplitudes (4% difference in this case). As a result, the averaged charging module powers differ and the peak DC-link capacitor voltage rises as well. In the first step, an adaptation of an existing zero-sequence voltage injection is proposed. For balanced grid inductors, it converges to the 3rd harmonic voltage injection which can reduce the peak-to-peak DC-link voltage ripple up by to 50% and balances the power between the phases. However, unbalanced grid inductors still lead to the same unbalanced grid currents of 4%. Therefore, a new method with 4 integrators based on linear regression is proposed to achieve sinusoidal grid currents for unbalanced inductors. The proposed method has a similar transient dynamic as the conventional dq control, but balances the grid currents nearly ideally. Simulation results of a 1MW cascaded H bridge and a scaled-down prototype verify the proposed method.}}, author = {{Unruh, Roland and Böcker, Joachim and Schafmeister, Frank}}, booktitle = {{2025 Energy Conversion Congress & Expo Europe (ECCE Europe)}}, keywords = {{Cascaded H-Bridge, Current Control, dq Transformation, Linear Regression, Unbalanced Inductors}}, location = {{Birmingham, United Kingdom}}, publisher = {{IEEE}}, title = {{{Three-Phase Instantaneous Current Controller for Unbalanced Grid Inductors Without DQ Transform for Cascaded H-Bridge Converters}}}, doi = {{10.1109/ecce-europe62795.2025.11238538}}, year = {{2025}}, } @inproceedings{54355, author = {{Urbaneck, Daniel and Wiegard, Jan and Schafmeister, Frank}}, booktitle = {{PCIM Europe 2024; IEEE International Exhibition and Conference for Power Electronics, Intelligent Motion, Renewable Energy and Energy Management}}, location = {{Nuremberg}}, title = {{{Analysis of Inverter Operation Modes of an IGBT-Based ZCS LLC Converter for a 2 kW Automotive On-Board DC-DC}}}, year = {{2024}}, } @inproceedings{54353, author = {{Piepenbrock, Till and Keuck, Lukas and Schachten, Sebastian and Böcker, Joachim and Schafmeister, Frank}}, booktitle = {{PCIM Europe 2024; IEEE International Exhibition and Conference for Power Electronics, Intelligent Motion, Renewable Energy and Energy Management}}, location = {{Nuremberg}}, title = {{{Study on Sample Geometries for Ferrite Characterisation in the MHz-Range}}}, year = {{2024}}, } @inproceedings{54354, author = {{Förster, Nikolas and Urbaneck, Daniel and Kohlhepp, Benedikt and Kübrich, Daniel and Schafmeister, Frank}}, booktitle = {{PCIM Europe 2024; IEEE International Exhibition and Conference for Power Electronics, Intelligent Motion, Renewable Energy and Energy Management}}, location = {{Nuremberg}}, title = {{{Pitfalls and their Avoidability in the Double-Pulse Test}}}, year = {{2024}}, } @inproceedings{54357, author = {{Piepenbrock, Till and Schafmeister, Frank and Böcker, Joachim}}, booktitle = {{SPEEDAM 2024; 27th International Symposium on Power Electronics, Electrical Drives, Automation and Motion}}, location = {{Ischia, near Naples, Italy}}, title = {{{FEM Modelling of Dimensional-Resonant Inductors for LLC Converters in MHz Range}}}, doi = {{10.1109/SPEEDAM61530.2024.10609111}}, year = {{2024}}, } @inproceedings{54356, abstract = {{Although there are numerous design and control methodologies for the LLC resonant converter, they often do not consider decentralized control strategies to operate them as isolated DC-DC converters within a cascaded H-bridge. The total output power of all LLC converters must be constant to supply a load such as a wa- ter electrolyzer. However, each individual LLC converter can vary its output power as long as the total output power remains constant. This opens new possibilities in increasing the system efficiency and robustness. Usually, the DC-link voltage of each module capacitor shows a 2nd harmonic voltage ripple. However, the total stored energy in all DC-link capacitors is constant within a grid period for a balanced three-phase system. By controlling each LLC converter’s output power locally to be proportional to the energy stored in its DC-link capacitor, modules with a lower instantaneous DC-link voltage transfer less power to the load than modules with a higher DC-link voltage. As a result, a higher efficiency, voltage gain and lower peak resonant capacitor voltage can be achieved with the same components. The 22.2kW experimental prototype of the LLC converter reaches an efficiency of over 97% at resonance which is similar to the precalculated value.}}, author = {{Unruh, Roland and Böcker, Joachim and Schafmeister, Frank}}, booktitle = {{ECCE Europe 2024; IEEE Energy Conversion Congress & Exposition Europe}}, isbn = {{979-8-3503-6444-6}}, keywords = {{Cascaded H-Bridge, Converter Losses, Decentralized Control, Full-Bridge Converter, LLC Resonant Converter}}, location = {{Darmstadt, Germany}}, publisher = {{IEEE}}, title = {{{Experimentally Verified 22 kW, 40 kHz LLC Resonant Converter Design with new Control for a 1 MW Cascaded H-Bridge Converter}}}, doi = {{https://doi.org/10.1109/ECCEEurope62508.2024.10751954}}, year = {{2024}}, } @inproceedings{58648, author = {{Unruh, Roland and Böcker, Joachim and Schafmeister, Frank}}, booktitle = {{Proceedings of the Energy Conversion Congress & Expo (ECCE Europe)}}, location = {{Darmstadt}}, publisher = {{IEEE}}, title = {{{Experimentally Verified 22 kW, 40 kHz LLC Resonant Converter Design with new Control for a 1 MW Cascaded H-Bridge Converter}}}, doi = {{10.1109/ECCEEurope62508.2024.10751954}}, year = {{2024}}, } @inproceedings{48352, abstract = {{Star-connected cascaded H-bridge Converters require large DC-link capacitors to buffer the second-order harmonic voltage ripple. First, it is analytically proven that the DC-link voltage ripple is proportional to the apparent converter power and does not depend on the power factor for nominal operation with sinusoidal reference arm voltages and currents. A third-harmonic zero-sequence voltage injection with an optimal amplitude and phase angle transforms the 2nd harmonic to a 4th harmonic DC-link voltage ripple. This reduces the voltage ripple by exactly 50% for all power factors at steady-state at balanced conditions. However, this requires 54% additional modules for unity power factor operation and even 100% for pure reactive power operation to account for the increased reference arm voltages due to the large amplitude of the optimal third-harmonic injection. If not enough modules are available, an adaptive discontinuous PWM is utilized to still minimize the voltage ripple for the given number of modules and power factor. With a very limited number of modules (modulation index is 1.15), the proposed method still reduces the DC-link voltage ripple by 24.4% for unity power factor operation. It requires the same number of modules as the commonly utilized 3rd harmonic injection with 1/6 of the grid voltage amplitude and achieves superior results. Simulations of a 10 kV/1 MVA system confirm the analysis.}}, author = {{Unruh, Roland and Böcker, Joachim and Schafmeister, Frank}}, booktitle = {{2023 25th European Conference on Power Electronics and Applications (EPE'23 ECCE Europe)}}, isbn = {{979-8-3503-1678-0}}, keywords = {{Cascaded H-Bridge, Solid-State Transformer, Capacitor voltage ripple, Zero sequence voltage, Third harmonic injection}}, location = {{Aalborg, Denmark}}, publisher = {{IEEE}}, title = {{{An Optimized Third-Harmonic Injection Reduces DC-Link Voltage Ripple in Cascaded H-Bridge Converters up to 50% for all Power Factors}}}, doi = {{10.23919/epe23ecceeurope58414.2023.10264313}}, year = {{2023}}, } @inproceedings{54352, author = {{Urbaneck, Daniel and Böcker, Joachim and Schafmeister, Frank}}, booktitle = {{PCIM Europe 2023; IEEE International Exhibition and Conference for Power Electronics, Intelligent Motion, Renewable Energy and Energy Management }}, isbn = {{978-3-8007-6091-6}}, location = {{Nuremberg}}, title = {{{Advanced Synchronous Rectification for an IGBT-Based ZCS LLC Converter with High Output Currents for a 2 kW Automotive DC-DC Stage}}}, year = {{2023}}, } @inproceedings{30347, author = {{Schafmeister, Frank}}, booktitle = {{International Conference on Electric & Electronic in Hybrid and Electric Vehicles and Electric Energy Management (EEHE),}}, location = {{Bamberg, Germany}}, title = {{{Compensation of LF Common-Mode Noise by the internal DC/DC-Stage for transformerless On-Board Chargers at Three- and Single-Phase Operation}}}, year = {{2022}}, } @inproceedings{30349, author = {{Förster, Nikolas and Rehlaender, Philipp and Wallscheid, Oliver and Schafmeister, Frank and Böcker, Joachim}}, booktitle = {{Proc. 37th IEEE Applied Power Electronics Conference (APEC)}}, location = {{Houston, TX, USA}}, publisher = {{IEEE}}, title = {{{An Open-Source Transistor Database and Toolbox as an Unified Software Engineering Tool for Managing and Evaluating Power Transistors}}}, year = {{2022}}, } @inproceedings{30350, author = {{Keuck, Lukas and Schafmeister, Frank and Böcker, Joachim}}, booktitle = {{Proc. IEEE International Exhibition and Conference for Power Electronics, Intelligent Motion, Renewable Energy and Energy Management (PCIM)}}, location = {{Nuremberg, Germany}}, publisher = {{IEEE}}, title = {{{Robust Hysteresis Control for LLC Resonant Converters Using a Fully Isolated Measurement Scheme}}}, year = {{2022}}, } @inproceedings{30844, author = {{Huber, Jonas and Wallmeier, Peter and Pieper, Ralf and Schafmeister, Frank and Kolar, Johann}}, booktitle = {{Proc. 9th IEEJ International Power Electronics Conference (IPEC)}}, location = {{Himeji, Japan}}, publisher = {{IEE Japan}}, title = {{{Comparative Evaluation of MVAC-LVDC SST and Hybrid Transformer Concepts for Future Datacenters}}}, year = {{2022}}, } @inproceedings{34176, abstract = {{Cascaded H-bridge Converters (CHBs) are a promising solution in converting power from a three-phase medium voltage of 6.6 kV...30 kV to a lower DC-voltage in the range of 100 V...1 kV to provide pure DC power to applications such as electrolyzers for hydrogen generation, data centers with a DC power distribution and DC microgrids. CHBs can be interpreted as modular multilevel converters with an isolated DC-DC output stage per module, require a large DC-link capacitor for each module to handle the second harmonic voltage ripple caused by the fluctuating input power within a fundamental grid period. Without a zero-sequence voltage injection, star-connected CHBs are operated with approximately sinusoidal arm voltages and currents. The floating star point potential enables to utilize different zero-sequence voltage injection techniques such as a third-harmonic injection with 1/6 of the grid voltage amplitude or a Min-Max voltage injection. Both well-known methods have the advantage to reduce the peak arm voltage and thereby the number of required modules by 13.4 % (to √ 3 2). This paper proves analytically that the third-harmonic injection with 1/6 of the grid voltage amplitude reduces the second harmonic voltage ripple by only 15.1 % compared to no-voltage injection for unity power factor operation and balanced grid voltages. Then it is shown, that the Min-Max injection has the often overlooked advantage of reducing the second harmonic voltage ripple by even 18.8 %. By applying the here proposed zero-sequence voltage injection in saturation modulation, the second harmonic voltage ripple of the DC-link capacitors is reduced by even 24.3 %, while still requiring the same number of modules as the Min-Max injection. For a realistic number of reserve modules, the overall energy ripple in the DC-link capacitors is reduced by 40 %.}}, author = {{Unruh, Roland and Schafmeister, Frank and Böcker, Joachim}}, booktitle = {{24th European Conference on Power Electronics and Applications (EPE'22 ECCE Europe)}}, isbn = {{978-9-0758-1539-9}}, keywords = {{Cascaded H-Bridge, Solid-State Transformer, Zero sequence voltage, Third harmonic injection, Capacitor voltage ripple}}, location = {{Hanover, Germany}}, publisher = {{IEEE}}, title = {{{Zero-Sequence Voltage Reduces DC-Link Capacitor Demand in Cascaded H-Bridge Converters for Large-Scale Electrolyzers by 40%}}}, year = {{2022}}, } @inproceedings{35126, author = {{Förster, Nikolas and Hölscher, Jonas and Piepenbrock, Till and Rehlaender, Philipp and Wallscheid, Oliver and Schafmeister, Frank and Böcker, Joachim}}, booktitle = {{2022 24th European Conference on Power Electronics and Applications (EPE’22 ECCE Europe)}}, pages = {{P.1--P.9}}, title = {{{An Open-Source FEM Magnetic Toolbox for Calculating Electric and Thermal Behavior of Power Electronic Magnetic Components}}}, year = {{2022}}, } @inproceedings{35125, author = {{Förster, Nikolas and Rehlaender, Philipp and Wallscheid, Oliver and Schafmeister, Frank and Böcker, Joachim}}, booktitle = {{2022 IEEE Applied Power Electronics Conference and Exposition (APEC)}}, publisher = {{IEEE}}, title = {{{An Open-Source Transistor Database and Toolbox as a Unified Software Engineering Tool for Managing and Evaluating Power Transistors}}}, doi = {{10.1109/apec43599.2022.9773701}}, year = {{2022}}, } @inproceedings{35127, author = {{Förster, Nikolas and Piepenbrock, Till and Rehlaender, Philipp and Wallscheid, Oliver and Schafmeister, Frank and Böcker, Joachim}}, booktitle = {{PCIM Europe 2022; International Exhibition and Conference for Power Electronics, Intelligent Motion, Renewable Energy and Energy Management}}, pages = {{1--10}}, title = {{{An Open-Source FEM Magnetics Toolbox for Power Electronic Magnetic Components}}}, doi = {{10.30420/565822103}}, year = {{2022}}, } @article{44163, author = {{Rehlaender, Philipp and Wallscheid, Oliver and Schafmeister, Frank and Böcker, Joachim}}, issn = {{0885-8993}}, journal = {{IEEE Transactions on Power Electronics}}, keywords = {{Electrical and Electronic Engineering}}, number = {{11}}, pages = {{13413--13427}}, publisher = {{Institute of Electrical and Electronics Engineers (IEEE)}}, title = {{{LLC Resonant Converter Modulations for Reduced Junction Temperatures in Half-Bridge Mode and Transformer Flux in the On-the-Fly Morphing Thereto}}}, doi = {{10.1109/tpel.2022.3180758}}, volume = {{37}}, year = {{2022}}, } @inproceedings{44164, author = {{Rehlaender, Philipp and Korthauer, Bastian and Schafmeister, Frank and Böcker, Joachim}}, booktitle = {{2022 24th European Conference on Power Electronics and Applications (EPE’22 ECCE Europe)}}, location = {{Hannover, Germany}}, pages = {{P.1--P.11}}, title = {{{Experimental Demonstration of a 2.2kW Active-Clamp Converter for High-Current Wide-Voltage-Transfer Ratio Applications}}}, year = {{2022}}, }