Abstract: A hybrid phase-controlled rectifying bridge includes both controlled and uncontrolled electric valves referred to as thyristors and diodes, respectively. Each of a pair of thyristors is periodically fired at an "angle" that can be advanced or retarded as desired, whereupon load current commutates to the thyristor from the corresponding diode. Subsequently the thyristor is turned off at a desired extinction angle by the action of cyclically operative forced commutation means. To avoid reverse recovery overvoltages across a diode when the desired extinction angle is only slightly larger than the firing angle, means is provided for temporarily preventing any turnoff action during the diode-to-thyristor commutation interval and for an additional interval that gives the diode sufficient time to fully recover its reverse blocking capability.

Abstract: A phase-controlled ac-to-dc rectifying circuit includes a pair of alternately conducting main controllable electric valves and cyclically operative forced commutation means for turning off the main valves. The forced commutation means comprises a pair of auxiliary controlled rectifiers respectively connected, in circuit with a commutating capacitor, across the main valves, and the auxiliary rectifiers are alternately turned on in response to periodic trigger signals that are normally produced by associated control means. Protective means is provided for inhibiting the trigger signals when the magnitude of voltage across the commutating capacitor is lower than a critical level which depends on the magnitude of the current being conducted by the main valves, thereby avoiding a risk of commutation failure due to low capacitor voltage.