Patent GB-1214086-A (GB)
Title: Commutation system for static controlled rectifiers
IPC: H03K3/35
Filed: 1968-10-28
Abstract: 1,214,086. Automatic voltage and current control. GENERAL SYSTEMS Inc. Oct.28, 1968 [Nov.16, 1967], No.51012/68. Heading G3R. [Also in Divisions H2 and H3] In a D.C. chopper circuit comprising a D.C. source 40, 40 (Fig. 2) and a switch 42 e.g. a thyristor which is turned ON by a pulse from a circuit 62 to supply current via a diode 58 and smoothing filter 52 to a load e.g. a battery on charge, and is turned OFF by a charge on a capacitor 78 when a commutating thyristor 82 is turned ON, there is provided a Zener diode 94 which senses the charge on C78 and initiates production of a firing pulse to Th 42 only when C78 has been charged to a level necessary for subsequent reverse biassing and turning OFF Th 42. A free-wheel diode 56 accommodates reactive current due to the filter inductor. Alternatively the output may be A.C. LOAD RESPONSIVE PULSE CIRCUIT. The pulse circuit 62 includes a Shockley diode 64 and a capacitor 66 the charge on which fires the diode 64 and thereby fires Th 42. Transistors 68, 70 shunt the capacitor 66 and the amount of current shunted by the transistors, which are controlled in dependence upon the load voltage and current respectively determines the period between commutation and firing of Th 42 and hence the pulse frequency. The feedback voltage to the base of T68 is derived from a load voltage value taken from a voltage divider R100, R101, R102, and a voltage reference provided by the base-emitter barrier voltage of T68 and a Zener diode 104 connected via R109 to the output terminal 48. The feedback load current value to the base of T70 is derived by a current shunt 116. COMMUTATION. With D.C. applied to terminals 40, 41, a voltage developed across L86 turns ON Th 82 which allows C78 to charge from the source via Th 82, terminal 48, diode 58, L52, load circuit 46, and via terminal 49 back to the source terminal 41. When the charge in C78 reaches a predetermined level Th 82 turns OFF and Zener diode 94 'breaksover' so that C66 begins to charge up until sufficient to break down the Shockley diode 64. When this occurs a firing pulse is applied via a pulse transformer 60 to turn ON Th 42. Current thus flows in two paths, - firstly through the load circuit and secondly commutating current flows through C78, L86 and Th 42. This second loop path, which includes a series resonant circuit, allows the charge on C78 to be reversed while maintaining the energy stored in C78 within the loop irrespective of load or supply transients. Diode 88 will conduct commutating current from C78 for one half cycle of resonant frequency and block the other half cycle during which the charge on C78 is reversed and a turn ON signal is applied again to Th 82. Thereby C78 is connected across Th 42, reverse biasing and turning it OFF. Thus Th 42 will remain conducting for a fixed period after initiation of conduction, which period is determined by the natural frequency of the series resonance circuit. Also the firing pulse circuit is controlled in dependence upon the load, so that the firing pulse rate will increase as the load increases. In an embodiment Fig.3 (not shown) the Zener diode charge level sensing means controls a transistor switch connected between the firing pulse circuit and the main thyristor gate, to prevent firing pulses until the charge level is sufficient. In addition the load is coupled to the thyristor chopping circuit by a transformer having a biphase rectified output. In an embodiment Fig. 4 (not shown) firing pulses are prevented by connecting the Zener diode 94 to cause saturation of transistor 68 or 70.