| 摘要 |
PURPOSE:To surely prevent a power transistor from being damaged by an excess current due to the short-circuit of load, etc., by setting the clamping voltage of a snubber circuit below the potential of a DC power source, before the power transistor is turned off at time of generating the excess current. CONSTITUTION:When an ON signal is supplied to the power transistors Q1 and Q4, and the short-circuit of the load is generated, an ignition signal is supplied to thyristors Th1 and Th4, before the transistor Q1 is turned off. The terminal voltage of snubber capacitors CS1 and CS4 is equal to a DC power source voltage Ed until then, however, discharge to the terminal voltage Ed/2 of capacitors C1 and C2 is generated. By turning off the transistor Q1 at a time when the terminal voltage of the snubber capacitors CS1 and CS4 arrives at the Ed/2, the voltage between the collector and the emitter of the transistor Q1 is limited to a value exceeding a little the Ed/2, and the voltage duty of the power transistor Q1 at the time of turning off can be considerably reduced. Also, since the excess current continues to flow on the transistor Q4 if the state is kept as it is, the Q4 is also turned off similarly. |
