Research On Thermal Characteristics Of IGBT Module For Inverter With Variable Switching Frequency

Under the national "peak and neutralization of carbon emission" targets development strategy,new energy such as wind energy and solar energy will be put into use in large quantities.Therefore,as the interface of the new energy power generation system,the reliability of the grid-connected converter plays significant role in affecting the stability and reliability of the multi-power elelctronics conveter based power system,and has become a research hot topic.The IGBT（Insulated Gate Bipolar Transistor）module as the most widely used power switching device in the converter,its reliability directly affects the overall reliability of the converter.In the new energy power generation system,the converter will work under non-stationary conditions for a long time,which will cause large fluctuations in IGBT junction temperature,thus reducing the overall life of the converter.Therefore,it is of great significance to find an appropriate method to suppress the sharp fluctuation of IGBT junction temperature caused by the random change of working conditions for improving the reliability of converter.In this thesis,the thermal characteristics of IGBT converter and the method of suppressing junction temperature fluctuation are studied.The main work is as follows:First,loss calculation and thermal network model are the basis for studying thermal characteristics.This thesis introduces the principle and method of calculating the loss of the converter in normal operation.By designing a look-up table,the speed of calculating the losses can be accelerated and the real-time loss calculation of the converter can be easily realized.For the part of thermal network model,the heat transfer process of IGBT is analyzed,the definition of thermal impedance and two commonly used thermal network models are introduced.The conversion between models and the thermal coupling effect between heating chips are also introduced.Finally,the thermal network model is reduced,which lays a foundation for subsequent IGBT module junction temperature extraction.Second,accurate acquisition of junction temperature is a prerequisite for studying thermal characteristics.In this thesis,the improvement measures are put forward according to the current challenges of IGBT junction temperature real-time acquisition.Firstly,the existing methods for obtaining junction temperature have complex computational problems.By decomposing and reducing the order of the thermal network model,the complexity of the model and the amount of calculation are reduced,which makes it suitable for real-time application.Secondly,for traditional thermal network model-based junction temperature acquisition methods,which do not consider the influence of loss calculation error and the incomplete observability of heat transfer system state based on thermal network model,a composite observer is proposed for junction temperature observation,and the thermal coupling effect between heating chips and the effect of other heat chip loss on the whole heat transfer system are also considered.The overall robustness of the junction temperature observer is improved.It lays a foundation for subsequent analysis of IGBT thermal characteristics of converter and suppression of junction temperature fluctuation.Finally,the thermal characteristics analysis of IGBT converter and the suppression of junction temperature fluctuations are the ultimate purposes of this study.Therefore,with the method of junction temperature estimation,the thesis then analyzed the junction temperature fluctuation of the converter under the switching frequency,load and ambient temperature.On this basis,an optimization strategy to suppress junction temperature fluctuation by varying switch frequency is designed for the case of abrupt load change.For the current strategies of variable cooling fan speed to suppress junction temperature fluctuation,the thesis further analyzed its internal mechanism and designed the control strategy of integral control to achieve relative suppression of junction temperature fluctuation.Finally,for the problem of easy saturation of variable cooling fan speed method,the method of combining variable switching frequency with variable cooling fan speed is used to suppress the junction temperature fluctuation.The simulation results show that this method can greatly suppress the junction temperature fluctuation caused by load mutation,and has greater advantages than using either method alone.