Dual mode power transistor

A dual mode Darlington-like transistor circuit having a saturation mode and an active region mode for optimizing ON state power dissipation, base drive requirements, and switching speed in applications subject to large overload currents for short periods of time, comprising:

• a bipolar power transistor biased by base drive means into saturation for normally low level load currents less than a given value therethrough between a pair of main power terminals connectable to a power source through a load, the emitter and collector of said transistor being connected to respective said main terminals, whereby to afford low forward collector-emitter voltage drop;
• a second transistor connected between one of said main terminals and the base of said bipolar transistor;
• and
• means connected to said bipolar transistor and responsive to load currents above said given value to bias said second transistor into conduction to supply additional base drive from said one main terminal through said second transistor to said base of said bipolar transistor for allowing higher current flow therethrough which in turn reduces the otherwise increased voltage level thereacross and regulates said voltage at a given threshold, said additional base drive also keeping said bipolar transistor in its active region, out of saturation, to enable fast turn-off.

A dual mode bidirectional power transistor circuit for conducting current in both directions therethrough, including AC application, and having a saturation mode and an active region mode for optimizing ON state power dissipation, base drive requirements, and switching speed in applications subject to large overload currents for short periods of time, comprising:

• a pair of bipolar power transistors connected in series opposition between a pair of main power terminals connectable to a power source through a load;
• a pair of diodes, each connected in reverse parallel with a respective one of said power transistors, such that current flow in one direction between said main terminals traverses between the collector and emitter of the first of said bipolar transistors in an ON state and then from the anode to cathode of the second of said diodes, and such that current flow in the opposite direction between said main terminals traverses between the collector and emitter of the second of said bipolar transistors in an ON state and then from the anode to cathode of the first of said diodes;
• base drive means for biasing said transistors into saturation for normally low level load currents less than a given value therethrough between said pair of main terminals, whereby to afford low forward collector-emitter voltage drop;
• and
• means responsive to currents above said given value to supply additional base drive for said bipolar power transistors, and keeping said bipolar power transistors in their active region, out of saturation, to enable fast turn-off.

A dual mode bidirectional Darlington-like transistor circuit for conducting current in both directions therethrough, including AC application, having a saturation mode and an active region mode, for optimizing ON state power dissipation, base drive requirements, and switching speed in applications subject to large overload currents for short periods of time, comprising:

• a pair of bipolar transistors connected in series opposition between a pair of main power terminals connectable to a power source through a load;
• a pair of diodes, each connected in reverse parallel with a respective one of said bipolar transistors, such that current flow in one direction between said main terminals traverses between the collector and emitter of the first of said bipolar transistors in an ON state and then from the anode to cathode of the second of said diodes, and such that current flow in the opposite direction between said main terminals traverses between the collector and emitter of the second of said bipolar transistors in an ON state and then from the anode to cathode of the first of said diodes;
• base drive means for biasing said transistors into saturation for normally low level load currents less than a given value therethrough between said main terminals, whereby to afford low forward collector-emitter voltage drop;
• third and fourth transistors, each connected between a respective one of said main terminals and the base of a respective one of said bipolar transistors;
• and
• means connected to said bipolar transistors and responsive to load currents above said given value to bias said third and fourth transistors into conduction to supply additional base driven from respective said main terminals through respective said third and fourth transistors to the bases of each respective said bipolar transistor and also keeping said bipolar transistors in their active region, out of saturation, to enable fast turn-off.