Patent GB-907311-A (GB)
Title: Apparatus for converting d-c power to a-c power
IPC: H02M7/53846
Filed: 1957-08-12
Abstract: 907,311. Converting; automatic voltage control systems. CLIFTON, J. B. M. Sept. 18, 1958 [Aug. 12, 1957; Oct. 17, 1957 (4)], Nos. 25334/57, 32430/57, 32431/57, 32432/57 and 32433/57. Classes 38 (2) and 38 (4). [Also in Group XL (c)] An inverter comprises (Fig. 1) power amplifier 17 feeding an output circuit 13 to give a sinusoidal output, the control for the amplifier comprising in sequence an oscillator 10, a distorter 11, a pulse width modulator 12 and a limiter amplifier 16. The output power is controlled by a feedback from the output circuit to the pulse width modulator 12 through a rectifier 14 and amplifier 15, and a feedback to the limiter 16 through a rectifier 18. A three-phase system is disclosed in which the oscillator 10 feeds a phase splitter to give two additional outputs, the three outputs, which are at 120 degrees displacement, being fed to similar systems. In Fig. 2, one only of these is shown in detail. The alternating current derived from the output circuit 13 may be rectified to provide a D.C.-D.C. converter. In general, the system throughout comprises push-pull connected transistors. Either PNP or NPN types may be used. Oscillator 10.-A pair of transistors 22, 22<SP>1</SP> have their emitters connected to earth through biasing resistor 36 and their collectors connected to a transformer primary winding 24 which forms a tuned circuit with a capacitance 28. A negative D.C. supply 29 is connected to the primary winding centre tap. A feedback winding, whose centre tap is connected to potential divider across the supply 29, has its ends connected to the base electrodes and output is derived from a secondary winding 41. A Zener diode 39 limits the maximum collector feed potential. To the output from the winding 41 is added a negative feedback from the output circuit windings 43, 43<SP>1</SP> to minimize phase divergence and this combined signal is supplied to both the distorter 11 and pulse width modulator 12. Phase splitter.-The ends of the winding 24 are connected to the bases of transistors 51, 51<SP>1</SP> whose collectors are connected to the negative D.C. line and whose emitters are connected to a phase displacing network to give the two desired outputs. Distorter 11.-The input is fed to bases of transistors 57, 57<SP>1</SP> acting as a Class C amplifier, the emitters being connected through a biasing circuit 63, 64 to earth. The output comprising negative and positive pulses is derived from windings 69, 691. Pulse width modulator 12. The input to this circuit, which is applied to the bases of transistors 74, 74<SP>1</SP>, is the combined oscillator and distorter output and comprises steeply rising pulses. Alternative methods of deriving such pulses utilize saturable transformers or bi-stable circuits whose outputs are integrated to give triangular pulses. A temperature dependent biasing potential is maintained on the bases of transistors 69, 691, 74, 74<SP>1</SP> by means of transistor/resistor voltage dividing network 68, 67 connected to the winding 41. The emitters 76, 76<SP>1</SP> receive a biasing potential from the divider 79, 81 and a feedback control signal from the collector of a transistor 84 (amplifier 15). Limiter amplifier 16.-The output of the modulator 12 comprising square pulses of controlled width and these are fed to a limiter amplifier comprising transistors 101, 101<SP>1</SP> whose emitters are connected to earth through a biasing circuit comprising diode 104 and capacitor 106 (or a resistor/capacitor combination). The collectors are connected to outer ends of primary windings 111, 111<SP>1</SP> of a transformer 112 whose inner ends are connected to D.C. supply line 113. To prevent damage by high transient voltages the windings 111, 111<SP>1</SP> are shunted by diode resistor combinations 123, 125, 124, 125<SP>1</SP> (see Specification 907,315). The supply to the line 113 comprises the supply 29 supplemented by a voltage derived through a rectifier 18 from a current transformer 147 whose primary winding is fed from the output circuit of the inverter, and is thus dependent on output current (see Specification 907,313). Power amplifier 17.-Four secondary windings on the transformer 112 control two pairs of transistors 116, 117, 116<SP>1</SP>, 117<SP>1</SP> in parallel, the emitter-collector circuits being connected to primary winding 136, 137, 136<SP>1</SP>, 1371 on an output transformer 44. The emitters are connected together to the supply potential. An inductor 139 has a primary winding 138 connected in series in the emitter-collector circuits, and a secondary winding 141 in series with a diode 142 which shunts the D.C. supply. The choke 139 is shown constructionally in Fig. 3 and has a core with air gaps 201, 201<SP>1</SP> of different size which saturates at different current levels. The arrangement of choke 139 and diode prevent damage due to transient voltages on switching (see Specification 907,312). Output circuit 13.-The secondary winding of transformer 44 feeds a filter circuit 13 comprising series resonant circuits 152, 152<SP>1</SP>, 15211 tuned to the various harmonic frequencies, the complete shunt circuit being tuned, by a capacitor 144 if required, to the fundamental frequency so that a sinusoidal output is obtained (see Specification 907,314). Regulation and protection.-Bias on the modulator 12 to control pulse width is obtained from a transistor 15 so that a signal to the emitter-base circuit of this transistor sufficient to break down a Zener diode 167, controls output. Four such signals may arise: (a) To stabilize output power a signal indicative of output power is derived from a voltage winding 161 and a current winding on the transformer 147, rectified by a diode 168, filtered, and applied to the transistor 15. (b) A measure of output current only is obtained from a winding 169 and applied in parallel to the above signal. (c) Transient voltage protection is effected by a series combination of transformer 182 and capacitor 181 across any power amplifier transistor, transient voltage providing an output which is rectified by a diode 187 and applied to transistor 15. (d) Surges in the supply potential 29 produce outputs in a transformer 193 which are rectified by a diode 199 and applied to the transistor 15.