The configurations of a switched-mode power supply and a controlling method thereof are provided. The proposed switched-mode power supply includes a first output converter receiving a DC input voltage and generating a first high power DC voltage output and at least one low power DC voltage output, and a second output converter receiving the DC input voltage and generating a second high power DC voltage output coupled to the first high power DC voltage output to generate a coupled output, wherein the first output converter works and the second output converter idles when a transient power of the coupled output is not larger than a rated output power of the first high power DC voltage output, and both the first and the second output converters work when the transient power is larger than the rated output power.