Patent ID: 8260165

Claim:
An image forming apparatus comprising: a fixing unit; a heater provided in the fixing unit; a first temperature measuring unit that measures temperature of the fixing unit; an alternating-current power source that applies an alternating voltage to the heater; a storage unit that stores a plurality of patterns controlling on/off of the heater on a half-wavelength basis of a voltage cycle pertaining to the alternating-current power source; a half-wave control unit that determines a pattern from the patterns stored in the storage unit based on the temperature measured by the first temperature measuring unit and a target temperature of the fixing unit, and performs half-wave control to control the heater according to the determined pattern; a measuring unit that measures, when the heater is switched off, a heater-off period of time that elapses until the heater is next switched on; a determining unit that determines, based on the measured heater-off period of time, whether phase control is to be performed on the alternating voltage to control the heater, the phase control being performed by shifting phase of the alternating voltage; and a phase control unit that performs, when the determining unit determines that the phase control is to be performed, the phase control only for a period of time depending on the measured heater-off period of time after switch-on of the heater and before execution of the half-wave control.