GB1068031A - Improvements relating to inverter circuits - Google Patents

Abstract

1,068,031. Semi-conductor oscillator circuits. HOOVER Ltd. April 2, 1964 [Feb. 12, 1963], No. 5749/63. Heading H3T. [Also in Division H2] An inverter circuit for supplying an inductive load comprises a pair of semiconductor controlled rectifiers (thyristors), connected in series between a pair of direct current supply terminals, a diode connected in parallel opposition with each controlled rectifier, a capacitor in series with the load terminals across one controlled rectifier, the controlled rectifiers being triggered alternately by signals derived from the load circuit. Fig. 1 shows a circuit for supplying power of the order of kilowatts to an induction cooking spiral 20 at 20 kc/s. Silicon controlled rectifiers 10, 11 and diodes 14, 15 are made sequentially conductive so as to generate a sinusoidal current in the oscillatory circuit formed by the capacitor 16 and the load 20. Triggering signals for the SCR's 10, 11 are ultimately derived from the voltage step at junction 36 caused by charge storage in each diode 14, 15 when the appropriate diode is turned off. The voltage steps at 36 are differentiated at 37 and 41, the negativegoing differentiated pulse being passed via diode 39 to trigger the left-hand blocking oscillator 25 which produces a positive pulse in winding 32 to trigger SCR11. The positivegoing differentiated pulse is used to trigger the right-hand blocking oscillator 25 which provides a pulse in winding 32 to trigger SCR10. Negative bias for the SCR's 10, 11 is provided by rectifying at 54 the outputs of a 10 kc/s. oscillator 51. The recovery time of each blocking oscillator 25 is reduced by capacitor 45 and diode 43. A capacitor (Fig. 2, not shown) may be connected between junctions 36 and 18 so as to form a ringing circuit at several times the output frequency which reduces the rate of rise of current when each SCR is turned on. The triggering of the SCR's may be delayed by including a differentiating circuit in each triggering lead so that the SCR's are triggered by the trailing edge of the pulse from the blocking oscillators 25. In Fig. 2 (not shown), the blocking oscillators 25. are triggered by pulses due to charge storage in diodes 14, 15 and derived by pulse transformers in series with the diodes. Starting oscillators.-The circuit shown in Fig. 1 is made self-starting by arranging the right-hand blocking oscillator 25 to be selfoscillatory. Alternatively, as shown in Fig. 4, a delay relay 70 may be used to discharge a capacitor 71 into an auxiliary winding 74 on one of the blocking oscillator transformers.