A power supply is provided. The power supply comprises: a power input unit switching an input power; a transformer unit inducing an output power of the power input unit to a secondary side; a first power output unit connected to a first coil of the secondary side of the transformer unit; a second power output unit connected to a part of the first coil of the secondary side of the transformer unit; a synchronization signal detecting unit connected to a third coil of the secondary side of the transformer unit and detecting a switching period of the power input unit; and a synchronization signal output unit controlling an operation of the second power output unit according to the detected signal of the synchronization signal detecting unit.