Evaluation of insulated gate bipolar transistor protection with ZnO thick films varistors

To have efficient protection of insulated gate bipolar transistor (IGBT) against various electrical voltage overshoots, screen-printed zinc oxide (ZnO) varistors have been developed using a planar structure. These varistors exhibit a breakdown voltage higher than 600 V and a non-linear coefficient up to 40. Some samples have been tested in real operations conditions to protect 1200 V IGBT in the case of unclamped inductive switching operations (default on the freewheeling diode). The results have shown the high efficiency of the designed varistors by clamping the voltage overshoot appearing on the IGBT. Optimising of the fabrication process (mechanical pressure or not on ZnO layers, ZnO thickness, electrode and substrate nature), lead to dissipated energy density of 105 J/cm³.