Patent GB-915853-A (GB)
Title: Inverter network utilising controlled semi-conductor devices
IPC: H02M7/515
Filed: 1960-12-14
Abstract: 915,853. Semi-conductor inverter circuits. WESTINGHOUSE ELECTRIC CORPORATION. Dec. 14, 1960 [Jan. 19, 1960], No. 43030/60. Class 40 (6). An inverter oscillator comprises at least two semi-conductor controlled rectifiers rendered conducting alternately to connect a D.C. supply in reversing polarity to a load, and a capacitor which is charged in a direction corresponding to the polarity until the capacitor voltage renders a bi-stable device conductive so as to discharge the capacitor through a transformer winding and render the non- conductive rectifier conductive. Fig. 1 shows a bridge connection of controlled semi-conductor rectifiers 22, 24, 26, 28 of PNPN or PNPM type the arrangement being such that pairs 22, 26 and 28, 24 conduct alternately to connect the source of voltage 10 in alternately opposite directions to the load 20 and provide a square voltage wave at the load 20. When 24 and 28 are conducting terminal 14 is positive and 12 negative so that the current flows through the load 20 and also through a charging resistor 58, resistor 57 and a rectifier 56 into a capacitor 52 so that the capacitor charges at a rate determined mainly by resistor 58. Charging continues until the voltage across the capacitor 52 is such that a breakdown diode 54 conducts causing the capacitor to discharge through the primary winding 34 of a transformer. This induces a voltage in secondary windings 36 and 38, causing controlled rectifiers 22 and 26 to switch on. One of these, say 22, becomes conducting first so that the input terminals 2, 4 of the bridge are now shorted through controlled rectifiers 22 and 28. Due to an inductor 102 the voltage across these terminals suddenly reduces and as the controlled rectifier 28 has no control current in its base winding it becomes non-conducting. The resulting sudden change of voltage at terminal 12 is communicated through a commutating capacitor 104 to terminal 14 so as to render semi-conductor 24 non-conducting. Accordingly, current now flows through semi-conductors 22 and 26 in the reverse direction through the load and charges capacitor 52 in the reverse direction through rectifier 60, resistor 61 and resistor 58 until a second breakdown diode 80 conducts and restores the circuit to its original condition. Resistor 58 is variable to control the frequency of operation and resistors 57 and 61 control the relative duration of conduction of the two pairs of transistors. Rectifiers 58 and 81 prevent excessive voltage across the transformer primary windings, rectifiers 53 and 79 prevent excessive reverse voltage across the semi-conductor breakdown diodes, rectifiers such as 76 prevent excessive reverse voltage at the base of the controlled rectifiers while circuits 106 and 108 prevent excessive reverse voltage across the output circuits at the controlled rectifiers. When the circuit is first switched on by switches 82 and 82a current flows into capacitors 88 and 94 through rectifiers 92 and 98 to cause the oscillator to start operation. The capacitors are shunted by potentiometers comprising rectifiers 92 and 98 operating in their reverse direction and resistors 91 and 100 so that in normal operation no voltage is transferred from the capacitor to the semi-conductor rectifiers 22 and 26 to prevent them switching off. Starting may alternatively be effected by connecting a resistance capacitance and rectifier network (Fig. 2, not shown), between terminals A and B and between terminals C and D.