Patent GB-1177392-A (GB)
Title: Direct Coupled Static D.C. to A.C. Inverter
IPC: H02M7/483
Filed: 1968-03-28
Abstract: 1,177,392. D.C. to A.C. converters. TEXAS INSTRUMENTS Inc. 28 March, 1967 [31 March, 1966], No. 13969/67. Heading H2F. [Also in Divisions G3 and H3] A step approximated sine wave power wraveform (Fig. 7) is produced at each of terminals A, B, C (Fig. 4) of an inverter from a regulated D.C. supply. The inverter comprises transistor switches A0, A1, A3, A4; B0, B1 &c.,C0, C1 &c., which are controlled by logic circuits (Figs. 8, 9, not shown) including a clock source producing pulses at 12 times the desired inverter frequency so that each cycle is made up of 12 intervals as shown, Fig. 7, to selectively connect and in a predetermined sequence, each of the terminals A, B and C to D.C. bus-bars V0, V1, V3, V4 which are at different voltage levels. To by-pass inductive kicks from the load, diodes 116, 132 are provided, also transistors 120, 124 are bi-lateral. The inverter output is filtered by inductors 134-136 and capacitors 138-140 to improve the sine wave shape. Voltage regulation.-In the D.C. power supply unit the different D.C. voltage levels are produced by a ratio regulator 80 (Fig. 3) which receives at terminal 26 the output of a voltage regulator unit 22 which is fed by a battery 12 e.g. at 28 volts. The voltage regulator also steps up this voltage to 40 V. under normal conditions, and typical voltage levels on buses V4, V3, V1 are + 40 V. + 30 V. + 10 V. respectively. The voltage regulator maintains the voltage level V4 at terminal 26 substantially constant under varying load conditions of the inverter and the ratio regulator maintains each lower voltage level at a predetermined percentage of the level of V4. The voltage level of V0 represents the zero level of the sine wave. In the regulator 22 windings 30, 38 are secondaries of a transformer having a primary winding 34 in series with a transistor 36 across the filtered battery voltage. The output voltage at 26 is determined primarily by the duty cycle of T36 which is turned on by a network including secondary 38, R40, T42, R44, R46 and T48, transistor 36 being turned off when T48 is turned on by positive pulses from a control unit 50. The voltage induced in secondary 30, when primary 34 circuit is broken by turning off T36, is added to the battery voltage. If the period between the positive pulses (i.e. the duty cycle) is decreased, then the secondary 30 voltage is decreased and hence the total voltage at point 26. The inverter voltage output is compared with a reference potential in the unit 50, which is a one-shot multivibrator and produces the positive pulse train of uniform frequency the pulse width of which varies with the inverter output voltage, to vary the voltage at 26 and thus maintain it constant. Ratio regulator.-In the ratio regulator, if the output voltage at point 84 to bus VI 1 rises above the input at 78, transistor 98 tends to turn on since its base voltage is constant, and T96 also tends to turn on so that the voltage at 84 is maintained constant. If the voltage at 78 rises, T94, T92, T86 will become more conductive raising the potential at 82 to bus V3 proportionately. T96, T98 become less conductive and the potential at output 84 will also be raised proportionately. Protection.-If due to overload the voltage at 26 falls below that at 24, a normally on transistor 28 is turned off, and secondary 30 discharges through-the inverter load via a diode 74 and the total output voltage is supplied only by the inductive coupling between primary 34 and secondary 30, the secondary voltage being normally additive to the battery voltage to produce the output at 26. In an overload condition excess current through a sensing resistor 104 (Fig. 4) produces sufficient voltage drop which is fed back to the control unit 50 to cause it to reduce the duty cycle of T36 thus reducing the voltage at 26 preventing damage to the inverter.  Excess voltage protection is provided by double diodes 144 (Fig. 3), between V4 and V0.