Title: Control circuit for power converter

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Abstract: A digital firing control circuit is disclosed for controlling the generation of firing pulses for the eight controlled rectifiers (thyristors) in a bridge power converter in five distinct operating modes as well as effecting a smooth continuous transition between modes. The circuit compares and combines a plurality of locally generated time related waveforms according to a predetermined control algorithm in response to an applied analog command voltage to control the d.c. output voltage as well as providing certain protection functions such as end stops or inversion limits. The locally generated time related waveforms comprise sets of square voltage signals generated in accordance with selected cross-overs of the line voltages of the a.c. sources as well as the cross-overs of six firing reference ramp voltage signals separated from each other by 60.degree. and extending 240 electrical degrees. One particular mode of interest exists at relatively low voltage levels wherein the phase leg thyristors are respectively rendered conductive for a period of 60.degree. in the range of firing angles from 90.degree. to 120.degree. with the neutral leg thyristors being fired 60.degree. after the phase thyristor firing, or at a firing angle of 150.degree. to 180.degree. .


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