Duty ratio control is known in which AC power is supplied or stopped (hereinafter, referred to as “on” and “off”, respectively) when a voltage of AC power becomes 0 V, and a value of the AC power is controlled. An image forming apparatus is also known that uses such duty ratio control to control a value of AC power supplied to a heater of a fixing device to adjust a heating temperature for fixing a toner image being transferred to paper.
A frequency of the power supplied from a power company (hereinafter, referred to as a “commercial power source”) may vary depending on country or region.
A cycle of AC power and a cycle of turning on or off the AC power might not be synchronized if a duty ratio control cycle for the heater of a fixing device is not changed when the frequency of commercial power source is changed. In this case, since a waveform of the AC power supplied to the heater collapses, a harmonic component is increased. In this case, it may not be possible to control a duty ratio of power supply to the heater and a heating temperature of the heater with required resolution according to a configuration and working conditions of an image forming apparatus incorporating the fixing device.