257 / 2018-09-24 22:17:57

A FEM based Estimation Method of Thermal Circuit Model for High-Voltage Press-Pack IGBT Modules

Finite-element method(FEM), insulated gate bipolar transistors(IGBTs), reliability, thermal modeling

Press pack IGBT has shown great potential in high-voltage power electronic applications due to its high reliability and the damaging characteristic of being short circuit. The temperature distribution in the press-pack IGBT device can greatly affect the reliability and the lifetime of the module [1]. Therefore, it is significant to study the thermal dynamics of high-voltage press-pack IGBT modules for the reliability analysis and the thermal design of practical application. Furthermore, aging feature parameters, such as saturation voltage drop, and gate voltage waveform, need to be normalized to the same temperature condition, which also rely on the accurate thermal analysis. FEM analysis is effective to simulate the temperature of each position inside the IGBT device through software. However, it is not efficient to conduct the FEM simulation because of the large calculation amount, especially when different dynamic processes are analyzed [2]. Thermal circuit modelling is a more efficient method to analyze the thermal dynamics of IGBT modules. In addition, the thermal characteristics of IGBT in practical application will change with the aging of the devices. Thus, the aging degree of devices can also be evaluated from thermal parameters if the thermal circuit model can be accurately established.

Traditionally, RC thermal network of IGBT device to analyze the temperature of key nodes inside the device on one dimension, which is generally got from the datasheet of the IGBT device. The accuracy of this method depends on the accuracy of thermal circuit model parameters and the accuracy of practical application conditions, such as heat dissipation and the cooling condition. Generally, these conditions are difficult to be accurately described. One of the reasons is that the thermal parameters of each layer of IGBT are difficult to be accurately obtained. There is only the value of thermal impedance between the junction and the tank of IGBT in the datasheet. And there are few studies on press pack IGBT on this problem.

In this paper, a new thermal circuit modelling method for press pack IGBT is proposed. Firstly, the IGBT finite element simulation model was established to apply heat source and cooling conditions to the model and analyze the temperature change of the key point of IGBT device in the simulation results. The temperature of different layers in each die cells in the IGBT device is focused especially. By analyzing the simulation results of the IGBT finite element model, the critical thermal parameters, such as the value of thermal resistance and heat capacity of different IGBT layers, the thermal relations among different die cells and the abstract model of the cooling conditions could be obtained. These parameters could help a 3D thermal circuit model with boundary conditions that is more in line with the actual situation to be established. In this way, the credibility of the model could be improved and the calculation amount comparing with the finite element model could be also reduced. Finally, the IGBT test platform is set up in this paper, and the thermal circuit model is verified by experiments. The difference between the tank temperature of collector and emitter got from the thermal circuit model and that got from the IGBT device in the experiment is analyzed. And it is realized that the error between the experimental results and the simulation results is very small, which proves the feasibility of this method.