Thomson Multimedia S.a.

Chun-Hsing Wu

Seng Huat Ng

Kian Meng Koh

Hsu Flame

Circuitry to control a power supply of a degaussing coil of a cathode ray tube, said degaussing coil is coupled to a main voltage supply through a switching device the circuitry has inputs coupled to a direct source of current of a switched mode power supply, the switched mode power supply comprises a switched mode transformer with primary windings coupled to the main source of voltage, and secondary windings and the switched mode power supply is switched in a first range of frequencies when the power supply is in a 'standby' mode and in a second range of frequencies when the power supply is in an 'on' mode wherein said circuitry comprising a detecting circuit which detectes a change in the range of switching frequencies when the power supply is switched from standby to on, the detecting circuit having an input coupled to a direct current source being downstream of a primary winding of the switched mode transformer and an output coupled to an input of a control circuit of the switching device the control circuit, having an output to carry an output signal having an 'on' state and an 'off' state, the 'on' state lasting for a predetermined duration after a change in a value present at the output of said detection circuit, the change occurring when the power supply is switched to the 'on' mode, in the 'on' state of said signal at the output of said control circuit, the switching device is on and current coming from the main is flows through the degaussing coil and in the 'off' state of said signal the switching device is off and no current is circulates in the degaussing coil.

Auto deguassing from stand-by to on

The most popular method for reducing stray emission effects in a cathode ray tube is to install a bleeder resistor at the flyback transformer of the tube. Such a bleeder resistor is, however, quite expensive. Therefore, the stray emission reduction circuit according to the invention differentiates the H pulse when the TV set is switched from 'on' to 'standby' which results in a higher duty cycle of the control transistor, implying a lower duty cycle of the horizontal output transistor HOT. This results in a lower voltage at the tuning capacitor and hence lowers the flyback peak voltage at the anode. This results in a drop of the EHT voltage.

Stray emission reduction circuit

The invention relates to a switch mode power supply (SMPS) comprising control means (IP1) including an oscillator (OS) for generating a pulse-width modulated signal (V2). The object is to provide a fast start-up time over a wide mains input-voltage range (AC). According to the invention, the switch mode power supply comprises a network (T2, R52, R51, R24) which provides in case of a high input voltage a soft start with a low oscillating frequency and in case of low input voltage a start-up with essentially the normal oscillating frequency. The invention provides a fast start-up time over a wide mains input voltage range of 90 Volt up to 265 Volt. The network includes a transistor (T2) used in inverse mode as a switching element which provides a threshold value of about 150 Volt. The power supply can be used advantageously as part of a TV receiver.

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Singapore, Singapore

Chun-Hsing Wu