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The thermal stress of power electronic components is one of the most important causes of their failure. Proper thermal management plays an important role for more reliable and cost-effective energy conversion. As one of the most vulnerable and expensive components, power semiconductor components are the focus of this paper. Possible approaches to control the semiconductor junction temperature are discussed in this paper, along with the implementation in several emerging applications. The modification of the control variables at different levels (modulation, control, system) to alter the loss generation or distribution is analyzed. Some of the control solutions presented in literature, which showed experimentally that the thermal stress can be effectively reduced, are reviewed in detail. These results are often mission-profile dependent and the controller needs to be tuned to reach the desired cost-benefit trade-off. The paper analyzes also the many open questions of this research area. Among them, it is worth highlighting that a verification of the actual lifetime extension is still missing.

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Abstract-The modular multilevel converter (MMC) has become a very attractive solution for interfacing high voltages in hybrid networks. The MMC enables scalability to different power levels, full controllability provided by IGBTs and can achieve very high efficiencies by using a low switching frequency method as the nearest level modulation. However, the nearest level modulation requires a capacitor voltage balancing algorithm, which can result in unbalanced loading for the power semiconductors in the different submodules. Particularly at low power factor operation, which could occur in case of low-voltage ride through and of reactive power injection, the conventional algorithm is not effective anymore. This article provides thermal stress analysis of the MMC in operation and proposes a thermal balancing approach, which is embedded in the capacitor voltage balancing algorithm. The purpose of the thermal balancing is to achieve similar stress distribution among the different submodules to enhance the lifetime. The junction temperatures in the different submodules are studied for HVDC applications and the article proves experimentally, that the thermal balance within the submodules is significantly improved.

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Lifetime of power electronics modules can be extended with passive methods (condition monitoring) and active ones. This paper intends to give an overview in the second category, namely active thermal control or lifetime control, offering a critical comparison based on a comprehensive reference list. Mission profiles are compared to evaluate the potential of the controllers.