Power Losses Modeling and Thermal Imbalance Suppression of Modular Multilevel Resonant DC–DC Converters

Modular multilevel resonant dc-dc converter (MMRDCs) have garnered substantial research interest within the domain of medium-voltage dc to low-voltage dc applications. Nevertheless, compared with the traditional modular multilevel converter employed in high voltage dc transmission, due to the different operating principles and modulation techniques, the electrical and thermal stress distribution in MMRDC remains yet to be fully elucidated. In this article, the power losses of submodule (SM) devices in MMRDC are investigated comprehensively. A precise calculation model of the SM conduction and switching losses is established under different operation conditions. The calculations reveal the serious thermal imbalance features among the devices inside SM, which may curtail the device lifetime and thus threaten the converter reliability. Then, to alleviate the thermal imbalance, a modulation scheme by reassigning the current path inside the SM is proposed. Without extra hardware cost and influence on the output performance, the proposed modulation can mitigate the loss imbalance between the upper switch and lower switch significantly and reduce power losses of the most highly stressed device. Finally, the accuracy of the theoretical model and the efficacy of the thermal imbalance suppression modulation are corroborated by full-scaled simulation and an MV MMRDC laboratory prototype with 7–14 kV input and 300 V output.