Patent GB-1436770-A (GB)
Title: Pulse modulation switching circuit
IPC: G05F5/00
Filed: 1973-09-19
Abstract: 1436770 Commutating circuits BBC BROWN BOVERI & CO Ltd 19 Sept 1973 [21 Sept 1972] 44025/73 Heading H3T In a pulse modulation switching circuit comprising a control device 3, Fig. 1, a controllable load current semi-conductor switching device ST and a commutating circuit including a commutating capacitor Ck, a controllable commutating semi-conductor device SC, a reversing choke 4 and a reversing diode connected in parallel with the switching device ST as shown for operation on a D.C. supply voltage 1, 2 of varying amplitude, connected in parallel with the output load terminals 5, 6 of the switching circuits is an inductance L1 which after the load current switching device ST has been turned on, is magnetized by a magnetic flux which is proportional to the D.C. supply voltage 1, 2 and for a duration of flow of the load current which is determined by the control device 3 and when the load-current switching device ST is turned off, the inductance L1 can be demagnetized by decrease in magnetic flux which is proportional to the difference between the D.C. supply voltage and the voltage which the commutating capacitor is required to be charged to during charging of the commutating capacitor C k . When the load current thyristor ST is turned on by a turn on signal from 3a of the control device 3 current flows through the inductance L1 and at the same moment the reversal of the commutating capacitor Ck also begins through the load current thyristor ST, the reversing diode and the reversing choke 4. When the control 3 has measured that the energy stored in inductance L1 is sufficient to charge the commutating capacitor C c  to the rated voltage the control 3 turns on the commutating thyristor SC. The load current thyristor ST becomes blocked and the commutating capacitor Ck has its charge reversed by the load current and the inductance L1 until it is charged in the reverse polarity, to the value of the input voltage so that the output voltage has become zero. The load current is decoupled from the pulse modulation switching circuit by the diode D2 conducting and the diode D1 turning off. The current in the inductance L1 now further reverses the charge of the capacitor C k  until it has again reached the initial state previously determined by the control 3. As the current has fallen to zero so the commutating thyristor SC is again blocked. With a low input voltage Ue (Fig. 2a, not shown) the load current thyristor ST is turned on longer to store more energy in the inductance than for a high input voltage (Fig. 2b, not shown) where the load current thyristor ST is turned on for a shorter time so that the energy stored during the turn on period is sufficient to charge the commutating capacitor Ck to its rated voltage after the commutating thyristor SC has turned on. The load L 2  may be a field or armature coil. In a modification the inductance L 1  is replaced by a transformer TV, Fig. 3, feeding a battery load B through a diode and a smoothing choke L 3 . A regulating device may be connected between the terminals 6, 7 of the switching circuit so as to adjust the mean D.C. voltage between 6, 7 to a preset value by varying the period of time between two firings of the load current thyristor ST and hence the pulse repetition frequency of the switching circuit.