Patent GB-1070420-A (GB)
Title: Solid state power circuits
IPC: H02M3/135
Filed: 1964-11-25
Abstract: 1,070,420. Semi-conductor firing circuits. GENERAL ELECTRIC CO. Nov. 25, 1964 [Dec. 27, 1963; Feb. 27, 1964; March 26, 1964; April 30, 1964; June 9, 1964; Aug. 3, 1964], No. 48001/64. Heading H3T. [Also in Division H2] In an arrangement for supplying power to an inductive load wherein the load is shunted by means enabling power to be fed back to the source, the load current is controlled by a bidirectionally conductive solid-state device which cannot be turned off at a gate electrode; suitable devices are the " triac " (NPNPN junction device with gate), " power disc " (NPNPN diode), and some four-layer devices. In Fig. 2 two triacs 11A, 16A are used to control the current to a motor 12, and may operate in a time-ratio control mode or a regenerative mode. Time-ratio control mode.-With the triacs non-conducting capacitor 20A charges to the potential of the D.C. source and in doing so drives inductor 19A to positive saturation. A gating signal is then applied via transformer 36 to turn on triac 11A and the potential at point 27 rises to that of the positive supply line 13. Current now flows through inductor 19A in the reverse direction, drives it to negative saturation, reverses the charge on capacitor 20A and drives 19A once more into positive saturation; 19A and 20A resonate at the desired commutation frequency. During this time the load current for the motor 12 flows through triac 11A and inductor 15. The reverse charge on capacitor 20A now makes point 27 more positive than line 13, and this potential. is applied through the secondary of transformer 36 to gate the triac 11A on in the reverse direction and shut off the load current (although some is still supplied from capacitor 20A). Transistor 41 conducts at this stage and blocks the gating pulses. When capacitor 20A has discharged, point 27 becomes less positive than line 13 and turns off the triac. The cessation of load current through inductor 15 induces an E.M.F. in it which makes point 27 more negative than the negative supply line 14 and supplies a gating pulse through resistor 26 to turn on triac 16A. Load current is now provided from the stored energy in inductor 15 flowing through triac 16A. When this current ceases, point 27 ceases to be negative with respect to line 14 and triac 16A is cut off. Regenerative mode.-Transistor 33 conducts during the commutation intervals when the commutating circuit of capacitor 22A and saturable inductor 21A make point 32 positive with respect to line 14. The motor 12 now acts as a generator and triac 16A is periodically gated on via transformer 24, the gating pulses being inhibited during the above commutation intervals by the conduction of transistor 33. During the off periods of triac 16A the E.M.F. generated in inductor 15 by the cessation of current drives point 27 more positive than line 13, turns on triac 11A and allows energy to flow back into the source. When this E.M.F. ceases, triac 11A is cut off and triac 16A can be gated on again. To prevent triac 11A from being turned on at the wrong instants by steepfronted waveforms at point 27, this point can be joined either to line 13 through capacitor 46 and resistor 45 or to the centre tap of two capacitors across the supply lines. Modifications.-In Fig. 3 (not shown) commutation is effected by two auxiliary triacs or SCR's 50, 51 which are gated on just after the main triacs 11B and 16B respectively. The commutating circuit 48, 49 is joined between the centre points of the two triac circuits, and after one half-cycle of oscillation turns off the upper two triacs 11B. 50. Triac 51 is turned on by the current pulse through triac 16B via transformer 29. Triacs 16B and 51 are turned off by the oscillation in the commutating circuit. In Fig. 4 (not shown) both halves of the circuit use two diacs, two snap-action diodes and two transistors as in Fig. 3 of Specification 1,055,030. In the regenerative mode this circuit acts in the same way as that of Fig. 2. The firing pulses of the diacs have their trailing edges steepened by the provision of saturable inductors 105, 106. The arrangement of the invention is compared with a similar circuit for performing the same function using two SCR's and a diode (Fig. 1, not shown) and it is stated that in this circuit one or more of the three semi-conductor devices may be replaced by a diac or triac.