Abstract: The invention is directed to a neutral point clamped electric power conversion apparatus having four semiconductor switching elements connected in series. The capacity of outer free wheel diodes connected in reverse parallel with outer semiconductor switching elements are increased compared to that of inner free wheel diodes, thereby minimizing, an imbalance in losses between the inner and the outer free wheel diodes.

Abstract: A semiconductor device is provided which includes first and second semiconductor elements of different types stacked alternately. A plurality of cooling members are each disposed between the paired first and second semiconductor elements located adjacent to each other for cooling the semiconductor elements and electrically connecting the elements to each other. In other words, the cooling members will have one of the first semiconductor elements on one side and one of the second semiconductor elements on the other side. At least one connecting conductor is provided for connecting at least one of the first semiconductor elements and at least one of the second semiconductor elements in an anti-parallel connection and also for connecting a plurality of the anti-parallel connections in series to one another.

Abstract: A semiconductor stack is composed of a GTO thyristor, cooling members and wiring conductors which are stacked and tightened together. The semiconductor stack is disposed horizontally in a cooling vessel, and the vessel is filled with a Freon liquid. A condenser is disposed above the cooling vessel and is connected to the cooling vessel by a communication tube, to constitute a cooling apparatus. Each cooling member has a large number of small holes between the GTO thyristor and the wiring conductors between which the cooling member is disposed. The Freon is boiled in the small holes by the heat generated at the GTO thyristor, becomes Freon vapor, and is directed toward the corresponding wiring conductor. Each wiring conductor has a large number of longitudinal slits on the surface thereof which is in contact with the cooling member. The Freon vapor is made to flow upward along these slits by convection currents, thereby increasing the efficiency of the cooling cycle provided by the liquid Freon refrigerant.

Abstract: A GTO thyristor stack comprises a pair of GTO thyristors in parallel connection and a pair of diodes in anti-parallel connection therewith. The elements in one of the GTO thyristor pair and the diode pair are located in the middle of the stack structure and sandwiched by the elements in the other with all the elements stacked in electrical connection. The stack structure is clamped by a pair of clamper members.