The invention provides a power supply changeover device as well as a controller and a method for controlling a transformer in the power supply changeover device. The power supply changeover device comprises a transformer which is provided with a primary winding connected with a power supply and a secondary winding connected with a load; a signal generator for generating a first signal; and a pulse signal generator connected with the signal generator and used for generating a pulse signal which controls the output power of the transformer according to the first signal. The transformer can be operated for a plurality of cycles, and at least one of the cycles comprises a charging phase, a discharging phase and an adjusting phase. The ratio of the duration time of the discharging phase to the total duration time of the charging phase, the discharging phase and the adjusting phase is a constant. The power supply changeover device has the advantages that an optical coupler, an error amplifier, and the like included in the traditional power supply changeover device can be omitted, and thus the size of the power supply changeover device is reduced, and the efficiency is enhanced.