Semiconductor switching circuit

A semiconductor switching string, for use in a HVDC power converter, comprising a plurality of series-connected semiconductor switching assemblies, at least one of the semiconductor switching assemblies comprising a semiconductor switching circuit comprising:

• a main semiconductor switching element comprising a first connection terminal second connection terminal between which current flows from the first connection terminal to the second connection terminal when the main semiconductor switching element is switched on, the main semiconductor switching element having an auxiliary semiconductor switching element electrically connected between the first and second connection terminals;
• and
• a control unit operatively connected with the auxiliary semiconductor switching element, the control unit being programmed to control switching of the auxiliary semiconductor switching element in an operating cycle to selectively create an alternative current path between the first and second connection terminals associated therewith wherein current is diverted to flow through the alternative current path to reduce the voltage across the main semiconductor switching element
• wherein the switching of the auxiliary semiconductor switching element in an operating cycle to create the alternative current path comprises operation of the auxiliary semiconductor switching element in at least two of:
• a fully-on mode in which the auxiliary semiconductor switching element is operated with its maximum rated base current or gate voltage;
• a pulsed switched mode in which the auxiliary semiconductor switching element is turned on and off, and
• an active mode in which the auxiliary semiconductor switching element is operated with a continuously variable base current or gate voltage.