| 摘要 |
PURPOSE:To narrow the safe operation range of a power transistor (TR) in a turn-off stat, by stopping or reducing the supply of a base reverse bias current when an overcurrent flows. CONSTITUTION:When a current flowing through the power transistor TR1 is not an overcurrent, an overcurrent detecting circuit 10 outputs a signal with logic 1 to open AND gates A1 and A2. Therefore, a TR3 or TR4 turns on according to the logic 1 or 0 of a base driving signal to flow a forward bias current or base backward bias current to the TR1. If an overcurrent state occurs, the circuit 10 outputs 0 to close the gates A1 and A2, so neither the TR3 nor TR4 turns on regardless of the base driving signal. Therefore, the TR1 is supplied with neither the forward nor backward bias current. Consequently, the safe operation range of the power TR in the turn-off state is widened as much as posible to protect the TR1 against an overcurrent securely. |
