Dual-mode transistor turn-off

A method of protecting a transistor operated in a saturated emitter collector mode from the effects of a fault, such as a short circuit, occuring in a load connected in an emitter collector path of the transistor, the transistor being operated in the saturated mode in response to drive signal applied to the base electrode of the transistor, comprising:

• removing the drive signal substantially simultaneously with inserting a low impedance shunt between the emitter and base electrodes in response to detection of the fault;
• and applying a reverse bias to the base electrode while removing the low impedance shunt a few microseconds after the drive signal was removed and the shunt was inserted.

Apparatus for controlling operation of a transistor having base, emitter and collector electrodes between cut-off and saturated conditions in response to an input control signal and for protecting the transistor from the effect of a fault, such as a short circuit occurring in a load circuit for the transistor emitter collector path while the transistor is saturated, comprising means connected to be responsive to the fault for deriving a fault indicating signal in response to the fault occurring, bias means for selectively driving the base so the transistor is saturated and reverse biasing the base, means for selectively shunting the emitter base, and a controller responsive to the input command signal and the fault indicating signal for controlling the bias means and the means for shunting, the controller controlling the bias means and the means for shunting so:

• (a) with no fault occurring, the transistor is driven to saturation and the shunt is not applied in response to a first level for the input command signal and the transistor is back biased in response to a second level for the input command signal, and (b) with a fault occurring and the command signal having the first level the base drive is removed while the shunt is applied and (c) the reverse bias is applied while the shunt is removed shortly after (b).

Apparatus for protecting a transistor having emitter, base, and collector electrodes from the effects of a fault, such as a short circuit, occurring in a load circuit for the transistor emitter collector path while the transistor is saturated, comprising:

• means connected to be responsive to the fault for deriving a fault indicating signal in response to the fault occurring, bias means for selectively driving the base so the transistor is saturated, means for selectively shunting the emitter and base, and a controller responsive to the fault indicating signal for controlling the bias means and the means for shunting, the controller controlling the bias means and the means for shunting so:
• (a) with no fault occurring the transistor is driven to saturation and the shunt is not applied in response to a command for the transistor to conduct, and
• (b) the base drive is removed while the shunt is applied in response to the fault occurring while the transistor is commanded to conduct, the bias means including a second transistor having a second emitter collector path in series between a forward bias source and the base, means connected to be responsive to the fault for deriving a fault indicating signal in response to the fault occurring, the means for shunting including a third transistor having a third emitter collector path connected across the base and emitter.

Apparatus for protecting a transistor having emitter, base, and collector electrodes from the effects of a fault, such as a short circuit, occurring in a load circuit for the transistor emitter collector path while the transistor is saturated, comprising:

• means connected to be responsive to the fault for deriving a fault indicating signal in response to the fault occurring, bias means for selectively driving the base so the transistor is saturated, means for selectively shunting the emitter and base, and a controller responsive to the fault indicating signal for controlling the bias means and the means for shunting, the controller controlling the bias means and the means for shunting so:
• (a) with no fault occurring the transistor is driven to saturation and the shunt is not applied in response to a command for the transistor to conduct, and
• (b) the base drive is removed while the shunt is applied in response to the fault occurring while the transistor is commanded to conduct, the bias means including a second transistor having a second emitter collector path in series between a forward bias source and the base.