FIELD OF THE INVENTION AND RELATED ART

The present invention relates to a fixing device and an image forming apparatus and particularly relates to control of the fixing device mounted in the image forming apparatus such as a copying machine or a laser beam printer.

There is a technique, as disclosed in Japanese Laid-Open Patent Application (JP-A) 2002-247758, such that in a circuit in which electric power is supplied from an AC voltage source (power source) to a load by controlling a switching element such as a bidirectional thyristor (hereinafter, referred to as a triac), a voltage source different from the AC voltage source is provided and the triac is controlled by passing a gate current through the triac. On the other hand, it has been known that due to distortion of an AC voltage of the AC voltage source and superposed noise, the triac turns off. As a method in which the triac is prevented from turning off and is controlled, as disclosed in JP-A 2001-32687, there is a technique such that in order to substantially continuously bring the triac into conduction, control is carried out so that a minus (−) potential is continuously supplied to a voltage source. Further, as disclosed in Japanese Patent 6152618, a technique such that an application time of a gate signal is increased has been proposed.

As in a conventional circuit in which the voltage source is provided separately from the AC voltage source and in which the gate current is passed and the triac is controlled, for continuously applying the potential to the voltage source in order to substantially continuously bring the triac into conduction, a circuit element such as a transformer or a bridge diode is needed. Further, in the circuit in which the voltage source is provided separately from the AC voltage source and in which the gate current is passed and the triac is controlled, when countermeasure against the turning-off of the triac due to the distortion of the voltage source and the noise is intended to be taken by prolonging a time in which the gate current is passed, a large voltage source capacity is needed. For this reason, in a circuit in which the switching element is controlled by the voltage source other than the AC voltage source, it has been required that the switching element is continuously controlled by a simple means while suppressing an increase in cost and while avoiding the influence due to the distortion of the AC voltage source and the noise.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, there is provided a fixing device comprising: a heater; a switching element configured to switch a state thereof between a conduction state in which electric power is supplied from an AC voltage source to the heater and a non-conduction state in which supply of the electric power is interrupted; a zero-cross detecting portion configured to detect a zero-cross point of an AC voltage supplied from the AC voltage source; a controller configured to control the switching element to the conduction state or the non-conduction state on the basis of a detection result of the zero-cross detecting portion; a power source chargeable by the electric power supplied from the AC voltage source and configured to supply, to the switching element, a current for changing the state of the switching element from the non-conduction state to the conduction state; and a driving portion configured to change the state of the switching element to the non-conduction state by supplying the current from the power source to the switching element depending on a signal outputted from the controller, wherein in a case that an amount of electric charge charged in the power source is predetermined value or more, the controller outputs a first signal to the driving portion so that a time in which a current is passed from the power source to the switching element is a first time, and in a case that the amount of the electric charge is less than the predetermined value, the controller outputs a second signal to the driving portion so that the time in which the current is passed from the power source to the switching element is a second time shorter than the first time.

Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.

