The electric field concentration at the triple junction point formed by the copper clad layer

ceramic substrate and organic gel of copper clad ceramic substrate in the model of IGBT（insulation gate bipolar transistor）increases the risk of partial discharge and insulation failure of elements. With the increase of voltage class of IGBT module

the insulation level of Direct-Bond Copper（DBC）substrate becomes the key factor restricting the development and reliable operation of elements. In this paper

nonlinear field grading（NFG）composite is intended to be used as the coating to apply at the partial high electric field region at the triple junction point of DBC substrate so to improve the electric field distribution and enhance the discharge inception voltage of the triple junction point of DBC. For this purpose

the partial discharge inception voltage of the triple junction point of DBC sample encapsulated by pure silicone gel is obtained. Then

the partial discharge inception voltage is taken as the control value

the corresponding electric field distribution and initial the electric field strength of partial discharge is obtained with the help of the finite element method（FEM）simulation. The influence rule of electrical parameters of NFG coating material on the electric field at the triple junction point of DBC substrate is analyzed in sequence. Finally

the suggestion on the selection of parameters of NFG is given. The study result in this paper has provided theoretical guidance for improving the design of non-linear material of partial electric field of DBC substrate.