Patent GB-1151112-A (GB)
Title: Improvements to Voltage Converter Circuits.
Inventor: KING KENNETH GORDON
IPC: H02M3/142
Filed: 1967-01-24
Abstract: 1,151,112. Converting; cyclical chopping circuits. WESTINGHOUSE BRAKE & SIGNAL CO. Ltd. 21 Dec., 1967 [24 Jan., 1967], No. 3528/67. Headings H2F and H2H. A D.C.-to-D.C. converting circuit comprises a plurality of inductances connected to a D.C. source via respective switching means each of which is turned on when the previous one turns off, whereby the rising ripple current through one inductance is compensated by the falling ripple current through the previously turned-off inductance; each inductance is provided with a free-wheeling path. Fig. 1 shows a D.C./D.C. converter using two inductances 3, 4 with freewheeling diodes 6, 7, the inductances being joined to the supply 1, 2 via thyristors Tr1 and Tr2, which are turned on alternately. The ripple currents through the load 13 due to the two inductances are equal and opposite. The thyristors are commutated by further thyristors Tr3-Tr6, capacitors 8/10 and inductors 9/11, thyristors 1, 4 and 5 being fired simultaneously and thyristors 2, 3 and 6 being fired when the first three turn off. The output voltage is approximately half the input voltage. In Fig. 4 the output voltage is boosted by connecting the free-wheeling diodes 6, 7 in series with the secondary windings of a transformer whose primary 12a is across the input to the inductances 3, 4. In addition, the inductances in the commutating circuit are replaced by a single inductance 14. If the load 13 draws only a light current the commutating capacitors 8, 10 may not be fully discharged before the next thyristor is turned on, and dangerous short-circuit paths may be established. To prevent this, each main thyristor Tr1 or Tr2 is shunted by a series RC circuit to limit voltage spikes and also by a diode in series with a winding of a saturable reactor. The other windings 21, 22 of the reactors are in series across an auxiliary D.C. supply 23, 24. At a certain instant after Tr3 or Tr4 is fired the saturable reactor becomes conducting and reverses the charge on the commutating capacitor. Resistors 17, 26 prevent excessive build up of charge on the commutating capacitors from one period to the next. This safety circuit for light load currents may be omitted (Fig. 3, not shown). A further boost to the output voltage may be obtained by tapping the free-wheeling diodes into the transformer secondaries and joining the ends of these to the inductances 3, 4 via further switches (Fig. 5, not shown). By making these switches conductive at variable instants the output voltage can be adjusted. The number of inductances and switches in the above circuits can be increased to provide output voltages which are a different submultiple of the input, e.g. one quarter. To obtain an output voltage greater than the input (Fig. 7), switches S1, S2 (corresponding to the thyristors Tr1 and Tr2 of Fig. 1) connect each inductance 3 or 4 in turn across the supply, and the free-wheeling diodes 6, 7 are in series with the load 13. The output may be boosted by a transformer (Fig. 8, not shown) in the same way as in Fig. 4 or 6. The number of inductances determines the ratio of output to input voltage. The boost transformer windings in the above circuits may be reversed to reduce the output voltage instead of increasing it. Varying degrees of boost may be obtained by altering the turns ratio. The commutating circuits shown may be replaced by others, e.g. as in Specification 968,512.