Current Balance in Parallel Power Diodes in Three-Phase Rectifiers

A study of the current balance in parallel diodes in one leg of a three-phase rectifier is described. The rectifier has a typical rating of 600-V 23 000-A dc and has six legs in a bridge connection for six-pulse rectification; each leg has six to eight parallel, individually fused hockeypuck diodes in a stack configuration. The division of current in the parallel diodes depends upon the geometry of the diode connections, the size and spacing of the ac and dc buses, and the spread in the forward-voltage-drop characteristics of the diodes. Mathematical modeling of the stack as a ladder network and computer simulation of the conduction are explained in the paper. Significant parameters are identified and their influence on the current division is brought out. Current division during normal operation and during a dc short-circuit test are compared.