Patent GB-1436268-A (GB)
Title: Converter circuits
IPC: H02M3/337
Filed: 1973-07-27
Abstract: 1436268 Automatic control of voltage and current WESTERN ELECTRIC CO Inc 27 July 1973 [31 July 1972] 35900/73 Heading G3R [Also in Divisions H2 and H4] A converter circuit comprises an inverter 11  controlled by a constant-frequency oscillator 25 and having means for monitoring the output voltage and/or current, at least one modulated pulse generator 20 arranged to provide pulses the timing or phase of which in relation to those of the oscillator 25 is a function of the excess output over predetermined values, the pulses fed back from generator 20 being fed to a circuit 24 arranged to control the duration of conduction of the converting elements in the inverter 11 so as to tend to maintain constant output and a soft start circuit 26 arranged to produce a train of pulses after initial energization of the circuit, the time interval between successive pulses of said train increasing and the circuit 24 being arranged so as to give priority to the start circuit pulses so that the output of inverter 11 builds up gradually. The output of inverter 11 is preferably rectified at 28 to provide a balanced D.C. output to feed power to series-connected repeaters in a two-way four-wire communication link Fig. 2, (see Division H4). Inverting. Fig. 3 (not shown) relates to the inverter 11 which consists of a pair of main transistors alternately conductive so as to reverse the current in the centre-tapped primary of a transformer. Driver transistors are, in turn, driven at a constant frequency by means of Darlington pairs, the final transistors having a Zener diode capacitor arrangement for removing stored charge. The Darlington pairs are driven at a constant frequency from the oscillator 25 under control of the logic circuit 24 which determines the conductive period of the output transistors and hence the resulting voltage or current output. A shut-down circuit 27 removes the drive to the output transistors of the inverter 11 if no feedback pulses are provided by generators 20 or 21 for 10 cycles. This is achieved by a capacitor charge circuit which is discharged by each pulse from generator 20 but when charged sufficiently, triggers a SCR which inhibits the drive to inverter 11 and illuminates a fault indicator lamp. Automatic voltage and current control. The current and voltage in each side of the balanced D.C. output lines are monitored at 16...19 and compared with a reference Fig. 3 (not shown) such that the output current is kept constant until the voltage rises above a predetermined level when automatic voltage control takes over. Two pulse generators 20, 21 compare the error voltages from the voltage/current comparators 16...19 with ramps generated during each cycle of the oscillator 25 and determine the delay in output pulses from generators 20, 21 relative to that from the oscillator 25. Starting. When the circuit is initially energized, a capacitor Fig. 4 (not shown) is charged with a decreasing current and periodically discharged by the pulses from the oscillator 25 so that the time interval between successive pulses increases. This train of pulses is fed to the logic circuit 24 detailed in Fig. 4 (not shown) which gives it priority over the pulses from generators 20, 21 and so produces a gradual build-up of output from inverter 11, so as to charge the reservoir capacitors 14, 15 slowly.