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This paper presents the commutating over-voltage snubber circuit design method of a thyristor converter that is applied to the International Thermonuclear Experimental Reactor poloidal field-converter module. In the course of poloidal field ac/dc converter normal operation, large transient surges called commutating over-voltages may be generated internally during the commutating process of thyristor, which poses a severe threat to system reliability and needs to be suppressed. In this paper, based on the analysis of reverse recovery transient of the parallel thyristors, the turn-off model of the PF converter thyristor is created and analytical expressions are derived, including the over-voltage crest value and the maximum over-voltage rise rate. Utilizing the analysis results, a systematic approach to the RC snubber circuit design is described.