PURPOSE: To surely enable setting a transistor itself to be protected, to a value lower than its ignition threshold by providing automatic commutating means between the base of the transistor and the collectors of power devices, and when the collector voltage of a power device drops from a preset value, generating currents associated with lower voltage drops between the base and collectors. CONSTITUTION: Automatic commutating means QP1 and QP2, which generate currents associated with low-voltage drops between the base of a transistor Q2 to be protected and collectors of power devices Q3 and Q4, when the collector voltage of a power device 1 drops from a preset value are provided between the base and collectors. When the collector voltage Vc of the device 1 drops from a preset value, a current having a small value and associated with a low-voltage drop flows to the collector of the device 1 from the base of the transistor Q2 through the means QP1 and QP2, so as to maintain the base/emitter voltage of the transistor Q2 at a value which is not higher than the conducting threshold of the transistor Q2. Therefore, the transistor Q2 to be protected is surely protected, so as to automatically set the transistor Q2 to a value lower than its ignition threshold.
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