The present invention relates to a protection apparatus for a transistor inverter which can automatically and electrically isolate the inverter from adjoining electrical equipment when one of the transistors in the inverter short-circuits.
FIG. 1 shows a conventional transistor inverter. In the figure, element number 1 is a DC power supply, elements numbers 2 and 3 are drive circuits which alternately supply base current to transistors 4 and 5, respectively, and elements number 6 and 7 are diodes connected across the transistors 4 and 5, respectively. In actual practice, two identical inverters like the one shown in FIG. 1 would be symmetrically disposed with respect to a load connected to both inverters. However, for the sake of simplicity, the second inverter and the load have been omitted from the figure.
In this conventional inverter, the collector currents for the transistors 4 and 5 are controlled by the base currents. Therefore, if one of the transistors in the inverter short-circuits, the current will still be limited by the other transistor. Since the current is limited, it is difficult to cut off the shorted transistor from the circuit in a short time by means of a fuse. Accordingly, when a number of inverters of this type are operated in parallel such as in an uninterruptible power supply, unless an inverter with a shorted transistor is quickly cut off, it can harmfully influence the other inverters operated in parallel with it.