Study On Lifetime Modeling Of IGBT Module Considering Fatigue Accumulative Effect Of Narrow Junction Temperature Cycles

Renewable power generation system installed capacity increases significantly such as wind power, PV, tidal power, and so on. It needs higher capacity, efficiency and reliability of power converters. Converters of wind turbine are operated in a harsh environment, and power electronics are long-term work in cyclic load stress profiles which causes the fatigue of the packing structure and high failure rate. Therefore the reliability which depends on an accurate lifetime model of the power electronics should be improved urgently to reduce the maintenance costs.The study on reliability and lifetime modeling have widespread attention at home and abroad in recent years. Lifetime modeling are established based on large amplitude stress cycles over ?Tj=70°C, but in an actual converter ?Tj are only 40°C. There is a large error between the experiments and actual conditions. Therefore the lifetime models can not quantify the effects of a large number of low strength stress impact to lifetime of power devices. Need to break through the traditional method of accelerated aging test, and propose a more accurate lifetime model according to the failure mechanism.This thesis focuses on in-depth theoretical analysis and systematical experimental study on the problem currently receiving wide international concern, namely how to quantify the effects of a large number of low strength stress impact to lifetime of power devices. The failure information is extracted under different conditions, and then according to the change trend of characteristic parameter and corresponding physical process the lifetime models which could quantify the effects of a large number of low strength stress impact to lifetime of power devices are established. The main research contents of this thesis are shown as follows:(1) Power cycling test rig is founded, and different levels of stress are designed to analysis the failure mechanism. After that the failure characteristic parameters are extracted for different failure styles at same time. Finally, according to the disadvantage of Coffin-Manson model where the effect of mean junction temperature is ignored the parameters of Coffin-Manson-Arrhenius model are obtained. By comparing the action strength of swing of junction temperature with mean junction temperature and combining the specific physical failure process Coffin-Manson-Arrhenius model is improved by using stretched exponential function. The model result indicates that the error of improved model is smaller. In this chapter solder fatigue is found out to be the dominant failure mode of power modules. It lays the foundation for the study of small stress cycles.(2) In this part FEA models with crack and without crack are founded to illustrate the effect of stress concentration and to obtain the stress distribution under different conditions. Small stress tests are designed based on the FEA model results to prove the effect of small stress cycles on un-aged and aged modules. The degraded rates which lays foundation for the next parts are extracted under different conditions. The effects of small stress cycles are firstly proved by experiments. The effects of small stress cycles on power module lifetime are proved by using numerical simulations and experiments.(3) The general degraded exponent model is established with large and small stress data which are extracted in the power cycling tests. A nonlinear accumulation model is founded with the degraded exponent model and Coffin-Manson model. A constant and multiple stage loads are used to prove the model. The results show that the model is effective and available. The traditional accumulation models are improved by using the effect small stress cycling, therefore the proposed model is more accurate.(4) The physical meaningful of thermal resistance is analyzed in the fracture mechanics. According to the processes of the crack to expand, a two stage nonlinear accumulation model is proposed. In the linear stage, Miner's rule and Coffin-Manson-Arrhenius model are used for linear accumulation. In the nonlinear stage, a nonlinear model is presented with swing of junction temperature, mean junction temperature and current degraded condition. A serious of results are used to prove the model. It shows that the accuracy of model is high. The lifetime model could quantify the effects of a large number of low strength stress impact to lifetime of power devices and use for large and small stress cycles and supply a new idea for reliability design.(5) Random stress profiles are designed to illustrate the disadvantage traditional model. It indicates that the traditional model has large error. The effects arrange of small stress cycles are summarized. The load profiles of doubly-fed induction generator are obtained under actual wind speed profile with different ambient temperature, and the lifetime are obtained. The results show that the ambient temperature and switching frequency have an important impact on lifetime of converter.The thesis try to solve the issue of general concern which is how to quantify the effects of a large number of low strength stress impact to lifetime of power devices. It gives a qualitative and quantitative analysis of the difference between the proposed lifetime model and traditional model. The intended outcome of this thesis will provide important theoretical and data support for security assessment and reliability monitoring of renewable generation power electronic equipment, electric vehicle, aerospace etc., and also provide scientific foundation for the design and development of converter systems, silicon carbine and power electronic devices with high reliability.