Patent GB-1503666-A (GB)
Title: Television display circuit arrangements which include a switched mode power supply
IPC: H03K4/62
Filed: 1974-04-01
Abstract: 1503666 Line deflection circuit MULLARD Ltd 25 Feb 1975 [1 April 1974] 14380/74 Heading H3T A circuit arrangement for television display apparatus includes a switched mode power supply and a line deflection circuit. The supply includes a first switch Tl and a first winding L11 on a transformer in series between terminals 11, 12 for connection to a first source of D.C. and also including a capacitor C13. The line deflection circuit includes a second series circuit of a second switch T12 and a second winding L12 on the transformer, which circuit is connected in parallel with the capacitor C13, and also including a line deflection coil L13. The first switch T1 is switched at line frequency or an integral multiple or submultiple of line frequency to produce a charge on the capacitor C13 by the coupling between the transformer windings L11, L12 and the capacitor when charged acts as a second source of direct voltage for the line deflection circuit. The first transformer winding L11 is tuned so that in each period when the first switch is off the voltage at the junction between switch and winding performs a half sinewave oscillation at a first frequency which occupies the non-conducting period. The second switch is controlled at line frequency to provide current to the deflection coil L13 in scan periods and to be off during the flyback periods with the line deflection circuit tuned so that in flyback, when the second switch is off, the voltage at the junction between switch and winding performs a half sinewave at a second frequency which occupies the flyback period. The transformer windings may be tightly coupled and the first may be tuned by reflection of the impedance of the tuned circuit associated with the second winding. The windings may be loosely coupled which permits the switches to be turned off asynchronously. Loose coupling may also be used with a transductor (TR11, Fig. 2, not shown) in parallel with the first winding and controlled by a signal dependent on a first voltage present in, or derived from, the arrangement and thus to stabilize a second voltage (not shown) where the first and second voltages may have the same magnitude. Alternatively such a voltage may be stabilized, when there is loose coupling, by varying the time at which the first switch is turned off relative to the turn off time of the second switch; this may be achieved by using alternate edges of a pulse train to turn the switches off, the mark-space ratio being varied for the stabilization action. The windings may be such that the voltage to which the capacitor C13 is charged is greater than the applied D.C. voltage. The second transformer winding is D.C. isolated from the first source and if this is obtained by rectifying the mains, mains isolation is obtained. The windings may be wound on opposite limbs of a line output transformer to provide the loose coupling, with further windings to derive additional voltages (not shown) preferably near the second winding L12 to provide the same isolation. Synch pulses applied at 22 control a pulse generator 33 which provides line frequency pulses which are applied via driver 34 to the second switch T12, they are also applied to ramp generator 35 and the ramp is applied to voltage sensor 41 to give variable width pulses, using a transformer TR4 to main D.C. isolation these pulses are applied to a pulse generator 37 whose output is of constant width and whose leading edge is coincident with the trailing edge of the variable-width pulse. A driver 38 passes these pulses to the first switch. The switches may be transistors, thyristors or gate-turn off switches; but in all cases current must pass in each direction and diodes may be needed in parallel with the device. The external D.C. may be connected to terminals 30, 31 and an output taken from terminals 11, 12. The capacitor C13 may be connected between the windings, as from 11 to 30.