Means for controlling a forced commutated hybrid a-c to d-c electric rectifying bridge to avoid reverse recovery overvoltage in the diode leg

Improved means for controlling an electric power converter used to supply direct current to an electric load circuit, said converter comprising the combination of

• an electric rectifying bridge having first and second legs connected in parallel with one another between a pair of d-c terminals, said first leg comprising at least one pair of serially connected diodes and said second leg comprising at least one pair of serially connected, alternately conducting controllable electric valves having cyclically operative valve turn off means respectively associated therewith,
• means connected between the juncture of said diodes in the first leg of said bridge and the juncture of said valves in the second leg of said bridge for applying alternating voltage from an a-c power source to said bridge,
• means for connecting the d-c terminals of said bridge to said load circuit, and
• control means for generating a set of firing signals to periodically turn on said valves in synchronism with said alternating voltage at an ignition angle that can be advanced or retarded as desired and for controlling said valve turn off means so that normally said valves are periodically turned off in synchronism with said alternating voltage at a desired extinction angle which varies within predetermined limits,
• wherein the improvement comprises valve turn on responsive means associated with said control means for temporarily blocking valve turn off operation thereof following each turn on of either one of said pair of valves in said second leg of said bridge until after the corresponding diode in said first leg of said bridge has fully recovered its reverse blocking capability.