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The trend towards increasing blocking voltage of high power Insulated Gate Bipolar Transistor (IGBT) modules is challenging for their electric insulation system. In particular, the electric field at the edges of the substrate metallization increases accordingly and can exceed the dielectric strength of the gel commonly used as encapsulation, leading to partial discharges or even breakdown of the insulation. This paper reports on the use of nonlinear resistive field grading to mitigate the electric field within power electronic modules. Finite element simulations show how the field enhancements can be significantly reduced by applying a functional coating with suitable nonlinear resistive characteristic. Coatings made of polyimide filled with ZnO microvaristors were manufactured and tested.