The features and process of three kinds of IGBT structures such as Punch Through, Non Punch Through and Field Stop are discussed in this paper. The effect of different structures on the parameters is also talked. Some principles such as punch through and transparent collector are clarified. The latest trench field stop structure and development trend are presented. The calculation method to define the current rating of IGBT ICis analyzed based on the maximum die junction temperature and thermal resistance. The total procedure to select the key parameters of IGBT in welding machine is given. The dynamic current imbalance between positive and negative thermal coefficient during the switching process is discussed and the solution is also given in the end.