Patent GB-1436106-A (GB)
Title: Power supply devices
IPC: H02M3/337
Filed: 1972-05-01
Abstract: 1436106 Automatic control of voltage and current ADVANCE ELECTRONICS Ltd 1 May 1973 [1 May 1972] 20220/72 Heading G3R [Also in Divisions H2 and H3] A power supply for providing a D.C. output from an A.C. input comprises means for rectifying A.C. to produce D.C., a pair of semi-conductor switching devices TR101, TR102, arranged to switch said D.C. alternately with opposite polarity to a transformer T105, means (D109, D110) for rectifying the output of said transformer T105 to provide a D.C. output and a control circuit (TR3...TR10) responsive to a pair of ramp voltage trains of which the respective ramps alternate in time and to a threshold voltage source the voltage of which varies in accordance with a condition of said D.C. output, the control circuit being arranged to generate a pair of pulse trains for the respective semi-conductor switching devices TR101, TR102 the duration of each pulse being a function of the voltage of said threshold voltage source. General. The A.C. input is passed via a fuse Fig. 2A (not shown) and a filter, without a transformer, to a diode bridge feeding D.C. to the inverter comprising TR101, TR102 and transformer T105 which operates at a frequency higher than that of the A.C. supply so that a small transformer can be used for T105, the output of which is rectified by diodes D109, D110 and fed to a L.C. smoothing circuit giving a D.C. supply balanced about earth. The A.C. supply is also transformer coupled to a further diode bridge to produce D.C. for the control circuit. Controlling inverters. A pair of spaced ramp voltage waves is derived from the collectors of transistors TR1, TR2, and fed to transistors TR5, TR6, which, with TR9 have a common emitter load formed by TR7 and TR8. At the beginning of each ramp, TR9 is conductive, but, when the ramp reaches a voltage determined primarily by the D.C. output voltage, either TR5 or TR6 becomes conductive, turning off TR9 and turning on TR3 or TR4 so as to provide a pulse in the secondaries of T102 to turn on one or other of the inverter transistors TR101, TR102. At the end of each ramp, TR5 or TR6 is turned off, turning on TR9 and TR10 which turns on TR8 so that one of diodes D4 or D8 provides a short circuit across T102 so as to facilitate dissipation of charge storage in the inverter transistors TR101, TR102. Alternatively, a pair of transistors (not shown) may be connected across the base/emitter paths of TR101 and TR102 so as to prevent simultaneous conduction of these transistors. The current switched by transistors TR3 or TR4 into the base circuit of inverter transistor TR101, TR102 is initially high to provide rapid turn-on but the inverter collector current is monitored by T101 and diodes D12...D15 and is coupled by D16 to TR15 to by-pass some of the current in R7 through TR3 or TR4 so as to facilitate turn-off. Over-current protection. The current monitored by T101 and rectified by diodes D12...D15 is passed to transistors TR14, TR13 and if excessive, cuts off TR13 so that D10 raises the threshold of TR9, reducing the duty cycle of the inverter and maintaining constant current into an external load. Over-voltage protection. The D.C. output voltage is monitored by TR19, TR20 and, if the voltage is excessive, these transistors will be turned on after slight delay due to C11, so that SCR1 is turned on, diverting the drive through R7 and preventing the inverter transistors TR101, TR102 from being turned on. Thermistor R44 provides temperature compensation for TR20. Starting. The D.C. voltage supplied to the control circuit is monitored by Zener diode D11 and transistor TR11 which is arranged to bias the base of TR9 to prevent fortuitous operation of the inverter transistors TR101, TR102 in the event of incorrect supply voltage. Automatic control of voltage. The D.C. output voltage is monitored by a potential divider R108...R114 a tapping of which is fed to the base of TR16 which compares it with a Zener reference D21 and provides an output to TR12 and thence to the base of TR9 so as to control the threshold voltage and hence the duty cycle of the inverting transistors TR101, TR102. A pulse width modulated voltage is derived from the collector of TR8 and fed across R38 so as to improve line and load regulation of output voltage which is adjustable by R114 or by an external control across terminals 5, 6.