In the field of image forming apparatuses, such as printers, the adoption of fixing units based on electromagnetic induction heating has been proposed. In a fixing unit based on electromagnetic induction heating, a fixing member, such as a fixing roller and a fixing belt, is provided with an electromagnetic induction heating layer. An excitation coil is caused to produce magnetic flux by switching the current flowing to the excitation coil via a switching element. The magnetic flux then causes the electromagnetic induction heating layer to heat up, so that the unfixed image on the transported sheet is fixed by the heat of the fixing member.
During such electromagnetic induction heating, the on (conduction) and off (cutoff) switching cycle of the switching element is controlled so as to maintain the temperature of the fixing member within a range of a few ° C. of the necessary temperature for during image formation operations, such as 180° C.
The switching cycle varies due to fluctuations in the temperature of the fixing member and the voltage input into the excitation coil. For example, the ratio of the on time to the off time (duty ratio) is raised when the temperature of the fixing member drops, or when the voltage input into the excitation coil drops. When the duty ratio is raised, the current flowing through the switching element increases, resulting in the switching element heating up. If the temperature of the switching element rises too high, the switching element may deteriorate or be damaged.
Japanese Patent Application Publication No. 2005-257898 discloses technology to prevent such deterioration of the switching element as follows. The temperature of both the fixing roller and the switching element is detected. While conduction of the switching element is controlled based on the detected temperature of the fixing roller, the current supplied to the switching element is reduced when the detected temperature of the switching element exceeds a predetermined value, so that the temperature of the switching element will not rise too high.
With the structure described in the above-cited publication, however, when the detected temperature of the switching element becomes higher than the predetermined value, the current supplied to the switching element is maintained at a reduced level until the detected temperature of the switching element falls to the predetermined value or less.
As a result, if the detected temperature of the switching element is higher than the predetermined value during a period from the start of image formation operations for a sheet until the tip of the sheet arrives at the fixing member, the current provided to the switching element will be reduced at the same time as the image formation operations for the sheet are performed.
If the temperature of the switching element does not fall to the predetermined temperature or lower by the time the tip of the sheet reaches the fixing member, the reduction in current provided to the switching element will continue, and the temperature of the fixing member will continue to drop. This easily leads to a situation in which, at the time when the tip of the sheet reaches the fixing member, the temperature of the fixing member is below the temperature necessary for fixing. If fixing is performed in this state, fixity will deteriorate, leading to defective fixing.