Patent GB-1006269-A (GB)
Title: Improvements in or relating to ignition of semi-conductor controlled rectifiers
IPC: H02M5/45
Filed: 1962-12-13
Abstract: 1,006,269. Converting. F. R. SAUTER A.G., and AGIE A.G. FUR INDUSTRIELLE ELEKTRONIK. Dec. 13, 1962 [Dec. 13, 1961], No. 47195/62. Heading H2F. A series of short firing impulses rather than a single pulse are applied to semi-conductor controlled rectifiers in a converting circuit to ensure consistent firing. As shown a frequencychanging circuit comprises an input transformer 10 connected to a rectifying stage A whose output feeds an inverting circuit B. Semiconductor controlled rectifiers in the circuit A are supplied from a pulse generator circuit C; those rectifiers in the inverter are supplied from a pulse generator D; and a protection circuit E is provided to stop the converter under fault conditions such as an output short-circuit. Rectifier A.-Two uncontrolled diodes 35, 36 and two SCR's 37, 38 are connected in a bridge circuit whose output is smoothed by a capacitor 31 and passed to the inverter B through a 47. Inverter B.-A pair of SCR's 50, 51 are connected to the ends of a centre-tapped primary winding of an output transformer 42, and the supply is connected between the centre tap and a common cathode connection. A commutating capacitor 59 is provided which may be on the primary or secondary side of the output transformer. Diodes 50c, 51c in series with resistors are connected reversely across the SCR's to achieve rapid discharge of the commutation capacitor. Series diodes 52a, 53a are provided so that under heavy load conditions the capacitor 59 will retain a high charge. Resistors 55, 56 are provided to dampen oscillations which might arise under no load conditions. Pulse generator C.-High frequency pulses supplied to the rectifiers 37, 38 through resistors 21, 22 are derived from a unijunction transistor oscillator 80, 81, 82, 83, 84 fed from smoothed D.C. derived from a winding 13 on the transformer 10. The pulse frequency is determined by an R.C. circuit 84, 83 and the capacitor 83 is shunted by a transistor 89. Normally the transistor 89 is biased to be non-conductive by a voltage derived through a rectifier 90 from a transformer 24 fed with a control voltage 23. Should the supply 23 fail the transistor is rendered conductive shunting the capacitor 83 whereby pulses to the rectifiers 37, 38 cease and no D.C. is produced to feed the inverter. Pulse generator D.-This comprises two circuits essentially similar to the circuit C above, each providing firing pulses for one of the rectifiers 50, 51. However in this case the bases of transistors 107, 107<SP>1</SP> shunting the oscillator capacitors 104, 104<SP>1</SP> are connected to opposite ends of a winding 27 on the transformer 24 so that each generator is blocked for alternate halfcycles of the supply 23 and hence trains of pulses occur on the lines 30, 31 during alternate half-cycles of the supply 23 which determines the output frequency. Failure of supply 23 again stops all pulses. The supply 23 may be non-sinusoidal and asymmetrical. Safety circuit E.-This serves to protect rectifiers 50, 51 in the event for example of an output short-circuit and acts as a logical circuit. A pair of resistors 114, 119 are connected in series between the D.C. nagative terminal 15 and the junction 120 of two reversely connected diodes 111, 112 across inverter rectifier anodes. This junction 120 is also connected to the emitter of transistor 80 through a diode 116, and to the positive D.C. line of circuit C through a resistor 115. This line is connected through a capacitor 117 to the junction of 114 and 119. If under fault conditions the line 120 receives the negative potential of the point 15 the diode 116 conducts and the base of the transistor 80 receives a negative potential to render it non- conductive so that pulses to the rectifier A cease. Pulses are restored if the fault is cleared at the end of a time period dependent on the time constant of the circuit 115, 117, 119. Specifications 1,006,267 and 1,006,268 are referred to.