Patent GB-1059409-A (GB)
Title: An electrical inverter circuit utilising semiconductor devices
IPC: H02M3/338
Filed: 1964-02-27
Abstract: 1,059,409. Converting. WESTERN ELECTRIC CO. Inc. Feb. 27, 1964 [March 5, 1963], No. 8113/64. Heading H2F. [Also in Division H3] A push-pull transistor inverter comprising transistors 12, 13 and a transformer 15 having a primary winding 14, a feedback winding and an output winding 21 supplying a load 21, is provided with means for injecting into the base of each transistor, when conducting, a reverse current of not less than the maximum value of I co  in order to ensure frequency stability under varying input voltage conditions. The switching point is dependent on the current I co , i.e. the leakage collector current which flows in the absence of emitter current and I co  is dependent on the input voltage. However by injecting the aforesaid current this drawback is overcome and rapid and consistent switching is provided. As shown in Fig. 1 the means comprises an additional transformer winding 23 connected to the transistor bases through current limiting resistors 24, 26, and diodes 25, 27. The feedback circuit comprises the feedback winding and a capacitor 17 and resistor 18 which determine the frequency; the feedback current, when transistor 12 is conducting, flowing through 18, a diode 20 which biases off the transistor 13, and the emitter-base path of the transistor 12 to charge the capacitor 17. As this charges the base current decreases, transistor 12 comes out of saturation and switching commences whereafter transistor 13 is conducting, and transistor 12 is shut off. The aforesaid winding 23 injects currents to ensure rapid switching and hence stable operation. A further embodiment, Fig. 2, comprises a D.C.-A.C.-D.C. converter which comprises a main inverter controlled by a frequency stabilized control inverter and feeding a rectifier. As shown the control inverter comprises transistors 31, 32 and capacitors 33, 34 in a bridge. A D.C. input supply is smoothed by components 36, 37 (diode 38 being provided to take up surges on switching) and supplied to one diagonal. The output of the bridge across the other diagonal is applied to a winding 42 on a transformer 41, which has feedback winding 43, 44. Each of these supplies an RC timing circuit 45, 46 or 47, 48 in the base of one transistor and a current injection circuit 51, 52 or 53, 54 in the other. The transformer also has output windings 63, 63 for controlling the main inverter. This comprises a similar bridge 55, 56, 33, 34 feeding the primary winding 59 of a transformer having a saturable core 60 of material exhibiting a square hysteresis loop so that its output which is rectified and smoothed has a constant voltage. A pair of diodes 57, 58 is included in parallel with the capacitors 33, 34 their junction being connected to an end tapping on the primary winding 59, this arrangement ensuring equal voltage drops across the capacitors. The output winding 64 of the control inverter is connected in series with a capacitor 68, a resistor/capacitor combination 69, 70, and a winding 62 on the main transformer across a Zener diode 76 disposed across the emitter base path of the transistor 56. A similar circuit controls the transistor 55. Transistors 31 and 55 conduct together as do 32 and 56. The combined voltages of windings 62 and 64 forward bias the Zener diode 76 driving transistor 56 into full conduction and capacitors 68 and 70 are charged. Meanwhile windings 61, 63, provide a voltage to charge capacitors 65 and 67 and to break down Zener diode 75 rendering transistor 55 non-conductive. When the core 60 saturates all winding voltages fall to zero and both Zener diodes are broken down rendering both transistors 55, 56 non- conductive. When the transistors 31, 32 switch over the voltages across 63 and 64 are reversed, and the voltage across winding 63 and capacitor 65 sum to render the transistor 55 conductive. This cycle of operations is repeated alternately. The output waveform comprises alternate square pulses separated by zero intervals; however since the core is of square loop material and the switching frequency is constant the output is constant.