A method of manufacturing a power supply unit (PSU) is provided. The method includes providing at least one PSU supplying a dimming signal to at least one light source, performing a first test for electrical characteristics of the at least one PSU, detecting light emitted from the at least one light source, measuring a flicker of the at least one light source, and performing a second test for a state of the at least one PSU based on a flicker measurement result, and packing a PSU determined to be in a normal state among the at least one PSU, as a result of the first test and the second test.