Patent GB-1277502-A (GB)
Title: Method for controlling a dc-to-dc converter
IPC: H02M3/10
Filed: 1969-10-07
Abstract: 1277502 Control systems for DC to DC converters BORG-WARNER CORP 7 Oct 1969 [15 Nov 1968 22 Nov 1968] 49309/69 Heading H2H [Also in Division H3] In a D.C. supply arrangement of the type comprising a conventional D.C. to D.C. chopper having two semi-conductor switches (such as transistors or silicon controlled rectifiers) respectively in series between a D.C. source and a load, and in parallel with the load, Fig. 1 (not shown), the switches are controlled to vary the operating frequency of the chopper and its on and off times so that for a given set of chopper operating characteristics (such as output voltage range, minimum switching time and maximum operating frequency) a minimum size can be employed for the inductor of the filter interposed between the chopper and the load. The method of operating the chopper consists in delivering an output voltage equal to one-half the input voltage of the source at a maximum operating frequency, delivering output voltages of less than one-half the input voltage by gradually decreasing the frequency while maintaining the on-time approximately constant or gradually decreasing it, and delivering output voltages of more than onehalf the input voltage by gradually decreasing the frequency while maintaining the off-time approximately constant or gradually decreasing it. The following three methods of operation are described:-(1) Progressive increase of the frequency up to its maximum value with the on-time maintained constant, followed by progressive decrease of the frequency with the offtime maintained constant, Fig. 7 (not shown). (2) Progressive increase of the frequency up to its maximum value with the on-time maintained constant, followed by progressive increase of the on-time with the frequency maintained constant at the maximum value, followed by progressive decrease of the frequency with the off-time maintained constant, Fig. 8 (not shown). In this method the maximum operating frequency is less than the theoretical value on which method (1) is based, (3) Simultaneous increase of the frequency up to its maximum value and increase of the on-time followed by simultaneous decrease of the frequency and decrease of the offtime, Fig. 11 (not shown). The following characteristics are derived and discussed:-(1) The voltage-time integral of the filter inductor (which is indicative of its size and is calculated for a desired output voltage range, a maximum operating frequency and a minimum on-time) plotted against the output voltage, Fig. 5 (not shown), for:-(a) method (1); (b) the known method of constant pulse width and variable frequency modulation; (c) the known method of constant frequency and variable pulse width modulation. (2) The voltage-time integral plotted against output voltage, Fig. 9 (not shown), for method (2) as resulting from the use of a number of different maximum frequencies. (3) The voltage-time integral plotted against output voltage, Fig. 10 (not shown), for method (3) based upon a predetermined value of the integral which must not be exceeded. (4) The voltage-time integral plotted against output voltage, Fig. 12 (not shown), for methods (1), (2) and (3) and the two known methods set out above. Figs. 13, 14 and 15 show a control circuit and details thereof for obtaining firing signals for the chopper switches, so that it may be operated in either one of the methods (2) and (3) described above. The unit 60 generates a controlled, variable signal for application to a compensating circuit 62 and thence to a difference amplifier 64. The latter produces a pair of output signals I 1 , I 2  whose sum is always constant but whose individual values vary according as the signal 61 from unit 60 varies above or below the value corresponding to equality of I 1  and I 2 . It is explained, Fig. 16 (not shown), how this relationship between I 1  and I 2  produces a constant value for the voltage-time integral, whose relationship with the on-time and the chopper period T (on plus off time) is discussed, Fig. 4 (not shown). Signals I 1 , I 2  are applied to sensing and clamping circuits 67, 68 whose outputs 70, 71 supply pulse circuits 72, 73 controlling a flip-flop 76 from which are derived signals 26, 27 for firing the chopper semiconductor switches. Details of members 62, 64 are shown in Fig. 14 wherein variation of the signal V a  produces a corresponding variation of signals I 1 , I 2  as discussed above. To this end, when V a  equals the reference voltage established at the base of transistor 81, the potentiometers 103, 108 are adjusted until the base voltages of transistors 82, 83 are equal, so equalizing I 1  and I 2 . Transistor 81 and its associated circuit compensate for changes in the base-emitter voltage drop of transistor 80 due to temperature variation. The currents I 1 , I 2  control unijunction transistor and capacitor timing circuits 117, 120 and 152, 156, so that each time a capacitor 120 or 156 is charged sufficiently to fire its associated transistor the state of the flipflop 76 is changed to produce a like change in its output signals 26, 27. A typical cycle of operations of the circuit shown in Fig. 15 is described, Fig. 17 (not shown).