Switched mode power supplies used in consumer electronics appliances usually have a low power mode, a so called standby mode, in which the essential circuits of the appliance are switched off, and only a few circuits, for example a microprocessor and a remote control receiver, are operating. To keep the power consumption in the standby mode as low as possible, it is known to use in the low power mode a so-called burst mode, in which the switching transistor is turned off regularly for defined time intervals.
For example, the switched mode power supply is switched off and on with a frequency of 100 Hz, in which during a short on phase the switching transistor operates at a switching frequency of 20 kHz, for transferring energy from the primary side to the secondary side of the switched mode power supply. Via the ratio between the on phase and the off phase the output power of the power supply can be controlled. During the on phase, the switching parameters for the switching transistor can be kept within a safe range. Switched mode power supplies with a burst mode are known for example from EP-A-0 386 989 and DE-A-195 18 863.
U.S. Pat. No. 6,434,030 discloses a circuit arrangement with a switched mode power supply, in which a low power burst mode is controlled by a microprocessor. An output of the microprocessor is coupled to the control loop of the switched mode power supply, and when the arrangement is switched to the standby mode, the microprocessor applies a square wave signal with a given duty factor to the control loop. With an off signal from the microprocessor, the power supply is disabled via the control loop, and with an on signal, the power supply is enabled for a short on phase, in which the switching transistor of the switched mode power supply is operating with a regular switching frequency.
Television sets and computer monitors having a cathode ray picture tube (CRT) as a display means comprise usually a demagnetization circuit for demagnetizing the picture tube in regular intervals, for example when the appliance is switched on. For the demagnetization, also known as degaussing, a time period of about 1,5 seconds is used, in which a magnetic field is applied to the picture tube in a known manner. Generally, the demagnetization circuit in a television set operates independently from the micro-processor, but microprocessor-controlled demagnetization circuits are known also.
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