@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{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{29893,
  abstract     = {{Phase-shift modulated full bridge converters suffer from thermal imbalances of the inverter switches. The lagging leg switches are subject to larger commutation currents compared to those of the leading leg as the transformer current reduces in the freewheeling interval. Furthermore, after this interval, the energy in the series inductance may not be large enough to achieve zero-voltage switching (ZVS) for the leading leg. Both effects result in thermal imbalances. This paper analyzes the alternating-asymmetrical phase-shift modulation to achieve balanced conduction and switching losses for all four switches while showing that this modulation is easily implemented on standard DSPs. The modulation has been implemented to LLC converters where experimental measurement results proved its effectiveness for LLC converters by reducing the temperature deviation from 6.3 K to only 0.2 K such that the peak temperature is reduced from 95 °C to 92 °C. The paper also proves that the modulation can be utilized to improve the efficiency of LLC converters operated at very low gains while simultaneously reducing the junction temperature of all four switches compared to the conventional complementary modulation. Finally, EMI implications are analyzed, which show that the modulation may be beneficial for reducing the common-mode emissions around the operating frequency.}},
  author       = {{Rehlaender, Philipp and Unruh, Roland and Schafmeister, Frank and Böcker, Joachim}},
  booktitle    = {{2021 IEEE Applied Power Electronics Conference and Exposition (APEC)}},
  isbn         = {{978-1-7281-8950-5}},
  keywords     = {{Phase-Shifted Full Bridge, Full-Bridge Converter, Phase-Shift Control, Phase-Shift Modulation, LLC Converter, Thermal Balancing}},
  location     = {{Phoenix, AZ, USA}},
  publisher    = {{IEEE}},
  title        = {{{Alternating Asymmetrical Phase-Shift Modulation for Full-Bridge Converters with Balanced Switching Losses to Reduce Thermal Imbalances}}},
  doi          = {{10.1109/apec42165.2021.9487104}},
  year         = {{2021}},
}