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This paper presents an open-circuit fault detection method for a grid-connected NeutralPoint Clamped (NPC) inverter system. Further, a fault-tolerant control method under an open-circuit fault in clamping diodes is proposed. Under the grid-connected condition, it is impossible to identify the location of a faulty switch by the conventional methods which usually use the distortion of outputs because the distortion of the outputs is the same in some fault cases. The proposed fault detection method identifies the location of the faulty switch and the faulty clamping diode of the NPC inverter without any additional hardware or complex calculations. In the case of the clamping diode faults, the NPC inverter can transfer full rated power with sinusoidal currents by the proposed fault-tolerant control. The feasibility of the proposed fault detection and fault-tolerant control methods for the gridconnected NPC inverter are verified by simulation and experimental results.

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This paper presents an apparatus and methodology for an advanced accelerated power cycling test of insulated-gate bipolar transistor (IGBT) modules. In this test, the accelerated power cycling test can be performed under more realistic electrical operating conditions with online wear-out monitoring of tested power IGBT module. The various realistic electrical operating conditions close to real three-phase converter applications can be achieved by the simple control method. Further, by the proposed concept of applying the temperature stress, it is possible to apply various magnitudes of temperature swing in a short cycle period and to change the temperature cycle period easily. Thanks to a short temperature cycle period, test results can be obtained in a reasonable test time. A detailed explanation of apparatus such as configuration and control methods for the different functions of accelerated power cycling test setup is given. Then, an improved in situ junction temperature estimation method using on-state collector-emitter voltage V C E O N and load current is proposed. In addition, a procedure of advanced accelerated power cycling test and test results with 600 V, 30 A transfer molded IGBT modules are presented in order to verify the validity and effectiveness of the proposed apparatus and methodology. Finally, physicsof-failure analysis of tested IGBT modules is provided. Index Terms-Failure mechanism, insulated-gate bipolar transistor module, lifetime model, power cycling test, physics-of-failure, reliability. I. INTRODUCTION P OWER electronic systems play an important role in a wide range of applications for power generation, distribution, and consumption in order to achieve high efficiency and also achieve high performance of the systems [1], [2]. As power electronic systems have gradually gained an important status in the power infrastructure, reliability improvement and lifetime prediction of power electronics are two important research topics [3]-[5]. The power electronic systems consist of various components. Among them, power devices are one of the reliability-critical components [6]-[8] and thus play a key role in the robustness and reliability of overall power electronic systems. In [3], the critical stressors for different components in power electronic systems have been summarized. It can be noted that the temper-Manuscript

Power cycling test is one of the important tasks to investigate the reliability performance of power device modules in respect to temperature stress. From this, it is able to predict the lifetime of a component in power converters. In this paper, representative power cycling test circuits, measurement circuits of wear-out failure indicators as well as measurement strategies for different power cycling test circuits are discussed in order to provide the current state of knowledge of this topic by organizing and evaluating current literature. In the first section of this paper, the structure of a conventional power device module and its related wear-out failure mechanisms with degradation indicators are discussed. Then, representative power cycling test circuits are introduced. Furthermore, on-state collector-emitter voltage (V C E O N ) and forward voltage (V F ) measurement circuits for wear-out condition monitoring of power device modules during power cycling test are presented. Finally, different junction temperature measurement strategies for monitoring of solder joint degradation are explained.Index Terms-Failure mechanism, insulated gate bipolar transistor (IGBT), power cycling (PC) test, power device module, reliability, wear-out failure.

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