Patent GB-1458985-A (GB)
Title: Generation of currents through television line deflection coils
IPC: H04N3/22
Filed: 1973-12-20
Abstract: 1458985 EHT supplies for television apparatus PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 20 Dec 1973 [2 Jan 1973] 59033/73 Heading H2F [Also in Divisions H3 and H4]  In an East-West raster correction circuit a field frequency parabolically varying voltage V M  (Fig. 1) generated by a driver stage Dr<SP>1</SP> under the control of field output stage 11 amplitude modulates via winding L2 on the line transformer A, the line frequency deflection current flowing in the circuit including line deflection coils Ly, trace capacitor C1, retrace capacitor C2, switching diodes D1 and D2 and a controllable (transistor) switch Tr. During the first half of the line trace period D1 conducts and the voltage across C1 produces a sawtooth deflection current in the coil Ly which, on reversing in the middle of the trace period, blocks D1 and flows through D2 and switch Tr which is made conductive before the middle of the trace by the output of a driver Dr. The latter is supplied with the output of the line oscillator 9 and with the rectified output of a winding L3 and includes a comparator and a modulator providing switching pulses for controlling the conduction period of switch Tr. When the latter conducts (at mid-line period), in addition to line deflection current, a linearly increasing current i 1  produced in winding L1 from the operating D.C. voltage supply B (in practice derived from the mains supply) also flows through switch Tr. Where the latter is subsequently "opened" the interruption of the current i 1  produces a current i 2  in winding L2 having an initial value equal to the final value of i 1  (assuming L1 and L2 to have the same number of turns). Current i 2  then allows diode D3 to conduct and the fly-back pulse at its cathode acts via winding L2 to induce a pulse in winding L 1 which appears at the cathode of D2 and blocks the latter. When the retrace period ends D3 remains conducting until Tr is rendered conducting, i 2  becomes zero and i 1  starts to flow. A mathematical analysis of the system is given with reference to Fig. 2 (not shown) and it is shown that the circuit operates to so control the instant at which Tr is made conductive that variations of the EHT voltage (derived from winding L4) and of the D.C. voltage derived from winding L3 (and of any other operating voltages derived in similar manner from windings not shown) due to the field frequency modulation of the line-deflection current are compensated and variations of such voltages and of the field modulated line current due to variations in the supply voltage V B  are likewise compensated. Variation of the circuit configuration are described with reference to Figs. 3 to 7 (none shown). Since the operation is such that the amplitude of the voltage across winding L3 is maintained constant only during fly-back the modulation voltage V M  need only occur during the line retrace period (Figs. 8, 9 and 10, none shown). The modulation source M may be a passive element such as a field frequency varying resistor (e.g. a field frequency controlled transistor) or it may be constituted by a winding of a saturable reactor (Fig. 9, not shown).