Constant-power power supply apparatus and method of supplying constant-power power

A power supply apparatus for receiving power from a power source and supplying a loading with an electric power within a predetermined range of a default power, comprising:

• a driving unit for receiving the power from the power source and supplying the loading with a working voltage and a working current;
• a voltage sensing unit electrically connected to the driving unit to detect the working voltage;
• and
• a feedback control unit electrically connected to the voltage sensing unit and the driving unit, wherein the feedback control unit keeps at least two reference voltages, and each two neighboring reference voltages are defined to have a voltage section therebetween, and each voltage section has a slope parameter accordingly;
• wherein the feedback control unit receives the working voltage sensed by the voltage sensing unit, and finds the voltage section in which the working voltage falls to obtain the slope parameter of the voltage section, and then sends a current signal, which is according to the working voltage and the obtained slope parameter, to the driving unit to command the driving unit to change the working current accordingly, wherein a product of the working current and the working voltage is remained in the predetermined range of the default power.