Power-saving circuit for an AC/DC adaptor for a portable computer

An alternate current/direct current adaptor having a power-saving circuit, comprising:

• an input unit for inputting an alternating current voltage from an external alternating current voltage source and for converting the alternating current voltage into a direct current voltage;
• a power transistor selectively switched on and off based on a pulse width modulation control signal;
• a transformer for inputting the direct current voltage from the input unit to a primary coil of the transformer and for inducing an alternating current voltage at a secondary coil of the transformer based on a switch-on state and a switch-off state of the power transistor;
• a main voltage output unit for converting an alternating current voltage from a first secondary coil of the transformer into a direct current voltage to be output from the main voltage output unit;
• a feedback unit for producing a feedback voltage based on one of the direct current voltage output of the main voltage output unit and a direct current voltage output of an auxiliary voltage output unit;
• a driver for outputting to the power transistor the pulse width modulation control signal having a duty ratio controlled based on the feedback voltage from the feedback unit;
• an output load sensing unit for sensing a load of one of the main voltage output unit and the auxiliary voltage output unit and outputting a sensing signal when the sensed load is below a predetermined level;
• an output voltage dropping unit for receiving the sensing signal output by the output load sensing unit and for dropping one of the direct current voltage output from the main voltage output unit and the direct current voltage output from the auxiliary voltage output unit based on the sensing signal when the sensing signal is received by the output voltage dropping unit;
• a switching frequency reducer for receiving the sensing signal when output by the output load sensing unit and for reducing a switching frequency of the pulse width modulation control signal produced from the driver based on the sensing signal when the sensing signal is received by the switching frequency reducer;
• the auxiliary voltage output unit for converting an alternating current voltage input from a second secondary coil of the transformer into a direct current voltage and outputting the direct current voltage from the auxiliary voltage output unit;
• and
• a main switching unit selectively switched on and off based on the sensing signal when the sensing signal is received by the main switching unit to selectively cut off the direct current voltage output from the main voltage output unit and to supply the direct current voltage output from the auxiliary voltage output unit.

A power saving method in an alternating current/direct current adaptor, comprising the steps of:

• outputting a sensing signal by an output load sensor when a sensed load of an output unit of the alternating current/direct current adaptor for converting an alternating current voltage into a direct current voltage is below a predetermined level;
• dropping by an output voltage dropping unit a direct current voltage output from the output unit when the sensing signal is output by the output load sensor;
• and
• reducing a switching frequency of a pulse width modulation control signal produced by a driver for driving a power transistor to control inducing an alternating current voltage provided to the output unit when the sensing signal is output by the output load sensor.