Patent GB-958792-A (GB)
Title: Improvements in trip circuit means for protecting semi-conductor switches
Inventor: WALKER ALEC HERVEY BENNETT
IPC: H03K17/082
Filed: 1959-10-15
Abstract: 958,792. Transistor switching circuits. WESTINGHOUSE ELECTRIC CORPORATION. Oct. 13, 1960 [Oct. 15, 1959; Nov. 11, 19591, Nos. 34957/59 and 38215/59. Heading H3T. [Also in Division H2] In a supply system where the value of D.C. or A.C. load current derived from an A.C. source is controlled by regulating the conductive portion of the operative half-cycle of semi-conductor switches included in the circuit, protection against excessive voltage across a switch is afforded by a circuit including a twostate arrangement having a first state to allow normal operation and a second state which cuts-off the supply. In Fig. 1, above the dotted line, there is shown a rectifier arrangement as disclosed in Specification 883,246, the value of the D.C. at terminals 5 being regulated by transistors 1, 3 in the left-hand arms of the bridge, the emitterbase circuit of these transistors being connected to driver circuits 37, 39 under the control of a direct voltage at terminals 41, 43. In the event of overload, or if insufficient base current for saturation flows, excessive voltage will appear across the transistors 1 and 3. The protective circuit shown below the dotted line includes a pair of transistors 45, 47 having their bases and emitters linked with the bases and emitters of transistors 1, 3, so that the pairs are conductive synchronously. When transistors 1 and 45 are conductive the voltage across the transistor 1 at A<SP>1</SP> is applied across a diode 71, resistor 67 and the transistor 45, and that part of this voltage across the resistor 67 is applied to the upper half of a transformer winding 63. When transistors 3 and 47 are conductive a similar voltage is applied to the lower half and an alternating output is obtained which is rectified by a bridge 75, integrated by a circuit 77, 79 and 89, and passed to a two-state circuit comprising transistors 81, 83. When the voltage at A<SP>1</SP>, B<SP>1</SP> are normal the signal to the two-state circuit does not affect the normal state in which transistor 81 conducts and transistor 83 is non-conducting. On excess voltage these conditions reverse and an output at 111, 113 is obtained. This is applied to the driver circuits 37, 39 to shut down the rectifier. The conduction may be restored by closure of a switch 121 (which again reverses the two-state circuit) and this may be done manually or automatically after a delay. In an A.C. circuit (Fig. 2) in which A.C. through a load 211 is controlled by pairs of back-to-back semi-conductor switches 203, 215 connected to different tappings on an output transformer 207 and controlled by a driver circuit 217, a protective circuit includes a current transformer 233 in the supply line the output being rectified and applied to a transistor 220. The voltage across this transistor is applied to a two-state circuit comprising transistors 243, 245 which operate in the manner described above to control the driver circuit 217 on excess current. The critical current level is predetermined by the value of currents in the emitter base circuit of transistor 220 which is set by a variable resistor 229. In low power arrangements, the current transformer may be eliminated, the line current itself being utilized.