Innovative monolithic RC-snubber for fast switching power modules

Abstract:
The combination of unavoidable parasitic inductances and steep current slopes in fast switching power modules leads to an extensive overvoltage during turn-off if no additional passive components are used for compensation. The fundamental analysis on additional pulse capacitors shows a reduced turn-off overvoltage. However, circuit simulations of a half-bridge in this scenario showed that the stimulated current amplitudes in the passives network can be multiple times larger than the actual load current, which leads to high thermal stress or destruction of the capacitors. Further analysis showed that an RC-snubber formed of a much smaller capacitance and a larger series resistance can also reduce the overvoltage sufficiently while not creating a resonant pole in the passives network. Therefore, we propose the application of monolithically integrated RC-snubbers on a silicon chip. They can be used with the same packaging techniques as active module components and profit from a low inductive design and good heat dissipation over the whole chip area. Such devices were manufactured with a design that makes them well suitable for 600 V applications at temperatures up to 200 °C and results in a 0.3 nF/mm² capacitance density per chip area. The temperature coefficient of the capacitance and series resistance is 45 ppm/K and 4000 ppm/K respectively. A life time extrapolation from time to breakdown measurements shows sufficient latitude for the realization of large capacitor areas and high temperature operation.