Power source apparatus utilizing a synchronous rectification system including a main transformer having a primary coil, and two secondary coils connected to each other by a tap mutually electromagnetically coupled to the primary coil. The power source apparatus includes a gate driver generating a first drive signal and a second drive signal to a gate of a first field effect transistor and to a gate of a second field effect transistor, respectively. The first field effect transistor and the second field effect transistor are turned on and off alternative by the gate driver.