Image forming apparatus that controls heater of fixing device, according to size of recording sheet

An image forming apparatus comprising:

• a fixing device including:
• a fixing belt having a cylindrical shape, and set to rotate;
• a presser opposed to the fixing belt, so as to define a nip region between the presser and the fixing belt, through which a recording medium to undergo a fixing operation passes;
• and
• a heater extending in a width direction, orthogonal to a rotation direction of the fixing belt, and configured to heat the fixing belt, the heater including a main heater and two sub heaters, divided in the width direction, the main heater being located at a central portion of the fixing belt in the width direction, and the two sub heaters being located on respective end portions of the fixing belt in the width direction;
• a first temperature sensor that detects a temperature of the main heater;
• a second temperature sensor that detects a temperature of at least one of the sub heaters;
• a cooling mechanism including a cooling fan, and configured to cool the end portions of the heater in the width direction;
• a controller that controls operation of the cooling fan, controls the main heater so as to adjust a first detected temperature, detected by the first temperature sensor while the cooling fan is operating, to a predetermined first target temperature which is an appropriate fixing temperature, and controls the sub heaters so as to adjust a second detected temperature, detected by the second temperature sensor while the cooling fan is operating, to a predetermined second target temperature;
• and
• a setting device that sets the second target temperature to a predetermined first set temperature, when the recording medium is a large size that reaches a temperature detection position of the second temperature sensor, in the width direction orthogonal to a transport direction of the recording medium, and sets the second target temperature to a predetermined second set temperature higher than the first set temperature, when a size of the recording medium in the width direction is a specific size that reaches a position corresponding to the sub heater in the width direction, but does not reach the temperature detection position of the second temperature sensor in the width direction,
• wherein the second set temperature corresponds to a predetermined temperature at which the temperature of a sheet passage region of the recording medium reaching the position corresponding to the sub heaters in the width direction is maintained at the appropriate fixing temperature.