BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a heating apparatus and an image forming apparatus, and more particularly, to a control circuit configured to control a fixing heater of a heating apparatus.

Description of the Related Art

In the related art, there is known a configuration in which a fixing device includes a plurality of heating elements, and the plurality of heating elements are connected in parallel via a plurality of bidirectional thyristors so that a heating element of a length corresponding to a width of a recording sheet, for example, is selectively used to reduce temperature rise of a non-sheet passing portion (see Japanese Patent Application Laid-Open No. 2001-100558, for example). A bidirectional thyristor is hereinafter referred to as “triac.” Meanwhile, a triac is generally a semiconductor element intended to control an AC power source. A triac has three terminals: a gate (G) terminal, a T1 terminal, and a T2 terminal, and when a gate current flows through the G terminal and a voltage of the G terminal becomes a threshold voltage or more, an electric current flows bidirectionally between the T1 terminal and the T2 terminal. Further, a triac is known to have four types of trigger modes: trigger modes I, II, III, and IV. In this specification, the trigger mode I refers to a case in which the T2 terminal has a positive potential and the G terminal has the positive potential with respect to the T1 terminal, and the trigger mode II refers to a case in which the T2 terminal has the positive potential and the G terminal has a negative potential with respect to the T1 terminal. The trigger mode III refers to a case in which the T2 terminal has the negative potential and the G terminal has the negative potential with respect to the T1 terminal, and the trigger mode IV refers to a case in which the T2 terminal has the negative potential and the G terminal has the positive potential with respect to the T1 terminal. Here, it is known that the G terminal requires a larger gate current to turn on the triac in the trigger modes III and IV than in the trigger modes I and II (see Japanese Patent Application Laid-Open No. S60-022370, for example).

Meanwhile, when a surge having a high ratio of dv/dt is applied from the AC power source to the triac as in the electrical fast transient/burst immunity test (EFT/B immunity test), for example, the gate current may flow through the G terminal at an undesirable timing. As a result, the triac may be turned on (hereinafter referred to as “false turn-on”) to supply power to the heating element at an undesirable timing. To address the false turn-on, there is devised a controller configured to monitor a voltage across both terminals of the triac to detect excessive temperature rise of the heating element caused by the false turn-on of the triac (see Japanese Patent Application Laid-Open No. H06-297808, for example).

In a fixing device in which a plurality of heating elements are connected in parallel via a plurality of triacs, when a surge voltage having a high ratio of dv/dt is continuously applied from the AC power source as in the EFT/B immunity test, and the plurality of triacs are falsely turned on continuously, the following problem occurs. That is, there occurs a problem that power is supplied to the plurality of heating elements at undesirable timings to excessively increase the temperature of the fixing device, for example. In order to prevent such excessive temperature rise of the fixing device caused by the false turn-on of the triacs, there may be used a configuration in which a voltage across both terminals of each of the triacs is monitored as in the related art, but there is a problem that addition of circuit components increases cost and a circuit board area.

SUMMARY OF THE INVENTION

There is provided a heating apparatus, comprising: a first load; a first switch element which includes a T1 terminal, a T2 terminal, and a gate terminal, and is configured to switch between supply of power from an AC power source to the first load and shut-off of power supply; a second load; a second switch element which includes a T1 terminal a T2 terminal, and a gate terminal, and is configured to switch between supply of power from the AC power source to the second load and shut-off of power supply; and a control unit configured to control the first switch element and the second switch element to prevent power from being supplied to both of the first load and the second load, wherein the first switch element has the T1 terminal connected to one end of the first load, and the T2 terminal connected to the AC power source, and the second switch element has the T2 terminal connected to one end of the second load, and the T1 terminal connected to the AC power source, or wherein the first switch element has the T2 terminal connected to the one end of the first load, and the T1 terminal connected to the AC power source, and the second switch element has the T1 terminal connected to the one end of the second load, and the T2 terminal connected to the AC power source.

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

