Disclosed herein are embodiments of a power supply apparatus. In some embodiments, the power supply apparatus supplies power to a load including a programmable logic controller (PLC) and includes: a first power storing module comprising a first auxiliary power source; and a second power storing module including a second auxiliary power source. An output voltage from the second power storing module may be larger than an output voltage from the first power storing module. It is possible to prevent an accident such as memory data loss possibly occurring in the event of sudden power-off by way of reducing usage of a battery that has limited lifetime while utilizing the advantage of super capacitor that has a very high number of charge/discharge cycles.