Patent GB-1230441-A (GB)
IPC: H02M3/28
Filed: 1968-08-26
Abstract: 1,230,441. D.C. to D.C. converters. INTERNATIONAL BUSINESS MACHINES CORP. 26 Aug., 1968 [14 Sept., 1967 (2); 20 Dec., 1967], No. 40663/68. Heading H2F. [Also in Division G3] A D.C./A.C./D.C. converter supplying stepped down, regulated direct voltage comprises an invertor stage including transistor switches connected to the D.C. source, first rectifier output means coupled to the invertor by a transformer, an inductor at least part of which is in series with the transformer primary and which has a secondary winding which produces an output periodically (from stored energy) to a second rectifier connected to the same output terminal and with same polarity as the first rectifying means. Operation.-The converter operating a cycle comprises four timing periods T1 to T4. In the Fig. 1 embodiment for example, during period T1 (illustrated Fig. 2, not shown) a transistor switch S1 is made conductive by a control unit 11 (switch S2 being off) and passes current from a source 1 through inductor L1 transformer primary 7 and a capacitor C. The inductor and capacitor additionally serve to step-down the voltage across primary 7 to ¢E. A voltage is induced in the transformer secondary 8 of a polarity which causes a diode D2 to conduct current to a load 3, while reverse biasing a diode D1. A voltage is also induced in the inductor secondary L2 which reverse biases a diode D3 so that L1 acts simply as an inductor as L2 is virtually open circuited. During this period L1 and C are storing energy. During period T2 both transistor switches S1, S2 are OFF. Capacitor C is prevented from discharging by a diode D4. Energy stored in L1 however discharges via L2 and D3 to the load. This back biases diodes D1, D2. Energy stored in the transformer primary 7 also develops a potential tending to cause a free-wheel current through C, D4, L1 and 7, which current will be relatively small. During period T3 with S2 "ON" and S1 "OFF", capacitor C discharges via primary 7 and S2 and thus delivers energy to the load via secondary 8 and diode D1. During period T4 transistors S1 S2 are "OFF", and energy in L1 continues to supply load via L2, D3 while energy in C is stored. Voltage regulation.-The load voltage is sensed by the control unit 11 which accordingly controls the switches S1, S2 "ON" time and hence the duration of the D.C. feed from source E (i.e. period T1), which governs the output voltage. Uniform load current.-The turns ratio of the inductor primary to secondary and that of the transformer are made equal, to ensure that the same value currents are fed via L1, L2 and, via the transformer to the load. In an alternative embodiment Fig. 8 (not shown) there is a full bridge inverter switching arrangement and the capacitor is dispensed with, while in Figs. 4, 7, 8, 9 (not shown), the inductor primary is centre tapped, i.e. bi-phase. Fault protection may be provided in the form of devices which open the series current path of the primaries (Fig. 4, not shown). Fig. 10 embodiment.-The transformer and inductor windings may be combined on a common core (Fig. 10) also Fig. 12 (not shown). The core has a gap 12 to prevent saturation and carries close coupled primary and secondary windings N1-A, N1-B and N2-A, N2-B respectively and a loose coupled secondary winding N3 on the centre leg and having output terminals 35, 38. During period T1, switch 20 conducts and switch 22 is OFF. Current through N1-A induces voltages in N2-A, N2-B and N3 such that diode D2-B only conducts to supply the load. Energy is stored in the centre leg. During period T2 with 20 and 22 OFF no primary current flows and flux stored in centre leg C decays, inducing a voltage in N3 such that diode D3 conducts current I3 to the load. Decay of flux in the outer core produces current from secondary N2-A, which is added to current 13 to give substantially the same load current during periods T1, T2. During period T3 switch 22 only conducts and secondary N2-A supplies the load current via D2-A and energy is stored to produce load current during period T4, as in period T2.