Image forming device

Abstract

To prevent flicker that is generated by switching a heating element to be supplied with power.SOLUTION: In a heater control circuit 108, based on a temperature of a heater 201 detected by temperature detection means and a target temperature of the heater 201, in a period P in which power is supplied to a heating element 203, a composite current of a first current supplied from a commercial AC power source 101 to the heating element 203 and a third current supplied from the commercial AC power source 101 to a power unit 150 in at least one unit period is a current in which the timing for supplying the first current and the timing of the maximum value and the minimum value of the third current do not overlap each other, and in a period Q in which power is supplied to a heating element 205, a composite current of a second current supplied from the commercial AC power source 101 to the heating element 205 and the third current in at least one unit period is a current in which the timing for supplying the second current and the timing of the maximum value and the minimum value of the third current overlap each other.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus. Place Regarding. [Background technology]

[0002] An image forming apparatus that forms an image on a recording material includes a fixing device that heats and pressurizes a toner image transferred onto the recording material to fix it to the recording material. The fixing device includes a heating device that uses a ceramic heater as a heat source for heating the recording material. The heating device has a plurality of heating elements that have different lengths in the width direction (longitudinal direction of the ceramic heater), which is a direction that is substantially perpendicular to the conveying direction of the recording material, and a technology that maximizes performance by switching the heat generation timing of each heating element is proposed in, for example, Patent Documents 1 and 2. Patent Documents 1 and 2 propose a technology that suppresses the temperature rise of non-paper passing parts of the heating elements through which the recording material does not pass by exclusively switching heating elements that have different lengths in the width direction of the recording material at a high frequency. This increases the printing speed of narrow B5 size and A5 size recording materials and improves the productivity of small size recording materials.

[0003] Furthermore, in control for exclusively switching between a plurality of heating elements, it is known that the AC voltage of the commercial AC power source that supplies current fluctuates at the timing of switching between the heating elements to which power is supplied, causing flicker. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2020-115185 A [Patent Document 2] JP 2020-115186 A Summary of the Invention [Problem to be solved by the invention]

[0005] Conventionally, flicker occurs when the current flow changes due to switching the heating element to which power is supplied, causing the AC voltage of the power supply to fluctuate. In addition, the frequency at which the current changes due to switching the heating element to which power is supplied can reach 8.8 Hz, a frequency at which flicker is perceived as unpleasant. There is a demand for suppressing flicker that occurs in this way.

[0006] The present invention has been made under these circumstances, and has an object to suppress flicker that occurs when switching between heating elements to which power is supplied. [Means for solving the problem]

[0007] In order to solve the above-mentioned problems, the present invention has the following configuration.

[0008] (1) A heating device having a heating rotator, a first heating element, and a second heating element having a resistance value greater than that of the first heating element, the device being provided with a heater disposed in an internal space of the heating rotator and heating the heating rotator, a pressure rotator forming a nip portion with the heating rotator, a detection means for detecting a temperature of the heater, a switching means for switching a power supply path from an AC power source to the first heating element or the second heating element, a control means for controlling the switching means to supply power to the first heating element or the second heating element, and a power supply device for generating a DC voltage from an AC voltage supplied from the AC power source. the first heating element, the second heating element, and the power supply device are connected in parallel to the AC power supply, and the control means controls power supply by phase control to the first heating element in a first period including a plurality of unit cycles and to the second heating element in a second period including a plurality of unit cycles based on a temperature of the heater detected by the detection means and a target temperature of the heater, and switches from the first period to the second period or from the second period to the first period by switching a power supply path by the switching means; a first current is supplied from the AC power supply to the first heating element, a second current is supplied from the AC power supply to the second heating element, and a third current is supplied from the AC power supply to the power supply device; In the first period, In the unit period First Current and the above Combined current with a third current is a current in a case where, in a first unit period of the first period consecutive to the second period, a timing at which the first current is supplied does not overlap with a timing at which the third current has a maximum value and a minimum value, and where only the unit period in which the first current is supplied does not overlap with a timing at which the third current has a maximum value and a minimum value, or a current in a case where the unit period in which the first current is supplied does not overlap with a timing at which the third current has a maximum value and a minimum value, and the unit period in which the first current is supplied overlaps with a maximum value or a minimum value, and a plurality of the unit periods in which the first current is supplied does not overlap with a timing at which the third current has a maximum value and a minimum value, and the unit period in which the first current is supplied overlaps with a maximum value or a minimum value, are repeated; During the second period ,beforeIn the unit period The second current and the third current The image forming apparatus according to the present invention is characterized in that the composite current is a current in which the timing at which the second current is supplied overlaps with the timing at which the third current takes a maximum value and a minimum value. Effect of the Invention

[0011] According to the present invention, it is possible to suppress flicker that occurs when the heating element to which power is supplied is switched. [Brief description of the drawings]

[0012] [Figure 1] Schematic cross-sectional view showing the configuration of an image forming apparatus according to Examples 1 to 3. [Diagram 2] A circuit diagram showing the circuit configuration of a heating device and a power supply device according to the first embodiment. [Diagram 3] FIG. 1 is a waveform diagram of a capacitor input current of the power supply device according to the first embodiment, and a diagram for explaining heating element switching control. [Figure 4] FIG. 1 is a diagram for explaining a current waveform supplied from a commercial AC power supply to an image forming apparatus according to a first embodiment; [Diagram 5] A circuit diagram showing the circuit configuration of a heating device and a power supply device according to a second embodiment. [Figure 6] FIG. 11 is a waveform diagram of an AC current input to a power supply device according to a second embodiment of the present invention, and is a diagram for explaining a current waveform supplied from a commercial AC power supply to an image forming apparatus. [Figure 7] FIG. 11 is a block diagram showing the configuration of a control unit of an image forming apparatus according to a third embodiment of the present invention. [Figure 8] FIG. 11 is a schematic cross-sectional view illustrating a configuration of a fixing device according to a third embodiment of the present invention. [Figure 9] Schematic diagram showing the configuration of a heater according to Example 3. [Figure 10] Schematic diagram showing a cross section of a heater according to a third embodiment. [Figure 11] FIG. 11 is a schematic diagram showing a configuration of a fixing control circuit of a fixing device according to a third embodiment of the present invention; [Figure 12] FIG. 11 is a diagram for explaining the control of the power duty of the heating element in the third embodiment. [Figure 13] Flowchart for controlling the power duty of the heating element in the third embodiment [Figure 14] FIG. 11 is a diagram for explaining a method for controlling the power duty of a heating element in the third embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. EXAMPLES

[0014] [Configuration of image forming device] FIG. 1 is a schematic cross-sectional view showing the configuration of an image forming apparatus 400 including a fixing device 50 according to the first embodiment. In the image forming apparatus 400 shown in FIG. 1, an image forming section 401 (enclosed by a dashed line in the figure) that forms a toner image on a recording material P has four image forming stations Pa, Pb, Pc, and Pd that form toner images of yellow, magenta, cyan, and black. Furthermore, the image forming section 401 has laser scanners 3a, 3b, 3c, and 3d that are provided corresponding to the image forming stations Pa, Pb, Pc, and Pd and form electrostatic latent images on the photosensitive drums 1 (to be described later) of the image forming stations. Each of the image forming stations Pa, Pb, Pc, and Pd has cylindrical photosensitive drums 1a, 1b, 1c, and 1d that are image carriers, charging rollers 2a, 2b, 2c, and 2d, and developing units 4a, 4b, 4c, and 4d that include developing rollers 41a, 41b, 41c, and 41d. Each of the image forming stations Pa, Pb, Pc, and Pd has the same configuration, and the suffixes a, b, c, and d added to the end of each component indicate that it is a component of the image forming station Pa, Pb, Pc, or Pd. In the following, the suffixes a, b, c, and d will be omitted except when referring to a component of a specific image forming station.

[0015] The charging roller 2 charges the photosensitive drum 1 to a uniform potential. The photosensitive drum 1 is charged to a uniform potential by the charging roller 2, and a laser scanner 3 irradiates the photosensitive drum 1 with laser light according to image data, forming an electrostatic latent image on the surface of the photosensitive drum according to the image data. Then, a developing roller 41 of a developing device 4 attaches toner to the electrostatic latent image formed on the photosensitive drum 1, forming a toner image on the photosensitive drum 1. The toner images formed on the photosensitive drum 1 are transferred in sequence, superimposed on the intermediate transfer belt 7 rotating in the direction of the arrow in the figure (clockwise direction), by a primary transfer member 6 provided at a position facing the photosensitive drum 1. Note that the toner remaining on the photosensitive drum 1 without being transferred to the intermediate transfer belt 7 is removed by a cleaning blade 5C of a cleaner 5. The toner image transferred to the intermediate transfer belt 7 is conveyed to a secondary transfer nip portion formed by the contact between the intermediate transfer belt 7 and a secondary transfer roller 8 in order to be transferred to a recording material P.

[0016] Meanwhile, recording materials P are stored in a paper feed cassette 9, and when an image forming operation starts, the recording materials P are fed one by one to a conveying path by a feed roller 10. The recording materials P fed by the feed roller 10 are conveyed to a secondary transfer nip portion by a conveying roller 11, and in the secondary transfer nip portion, the toner image on the intermediate transfer belt 7 is transferred onto the recording material P. Toner that is not transferred to the recording material P and remains on the intermediate transfer belt 7 is removed by an intermediate transfer belt cleaning blade 45 of an intermediate transfer belt cleaner 40.

[0017] The recording material P to which the toner image has been transferred at the secondary transfer nip is conveyed to the fixing device 50 in order to fix the toner image to the recording material P. The fixing device 50 is disposed in the internal space of a cylindrical fixing film 51 (also referred to as a film 51), and has a heating device 100 (see FIG. 2) for heating the fixing film 51, and a pressure roller 53 for pressing the recording material P by contacting the fixing film 51 to form a nip. In the fixing device 50, the conveyed recording material P is heated and pressed when passing through the nip, and the toner image is fixed to the recording material P. Then, the recording material P that has passed through the fixing device 50 is discharged to the discharge section 13 by the discharge roller 12. In addition, the power supply device 150 generates a DC voltage required in the image forming apparatus 400 using an AC voltage input from a commercial AC power source 101 (see FIG. 2), and supplies the DC voltage to each device inside the image forming apparatus 400. The power supply device 300 is a power supply device used in the second embodiment described later. The fixing control device 56 is a control device for the fixing device 50 used in a third embodiment described later.

[0018] [Heating device configuration] Fig. 2 is a circuit diagram showing the circuit configuration of the heating device 100 and the power supply device 150 included in the image forming apparatus 400 in Fig. 1. As shown in Fig. 2, the heating device 100 is composed of a heater 201 having a plurality of heat generating elements, a heater control circuit 108 (hereinafter also referred to as a control unit 108) that controls the power supply to the heater 201, and the like.

[0019] 2, a first closed circuit through which a current flows from the commercial AC power supply 101 is constituted by a commercial AC power supply 101 that supplies an AC voltage, a current fuse 102, a heater 201, a bidirectional thyristor (hereinafter referred to as a triac) 104, and an electromagnetic relay 103. A second closed circuit through which a current flows from the commercial AC power supply 101 is constituted by the commercial AC power supply 101, the current fuse 102, the heater 201, an electromagnetic relay 106, a triac 105, and an electromagnetic relay 103. The power supply from the commercial AC power supply 101 to the first closed circuit is controlled by a control unit 108 switching between a conductive state and a non-conductive state of the triac 104. On the other hand, the power supply from the commercial AC power supply 101 to the second closed circuit is controlled by a control unit 108 switching between a conductive state and a non-conductive state of the triac 105.

[0020] [Heater configuration] The heater 201 is disposed inside the heating device 100 provided in the fixing device 50 of the image forming apparatus 400 of this embodiment, and supplies heat to be applied to the toner image when the fixing film 51 fixes the unfixed toner image onto the recording material P. The heater 201 is composed of a ceramic substrate 202, heating elements 203, 204, 205, and contacts 206a, 206b, 206c, 206d disposed on the ceramic substrate 202. The heating element 203, which is a component of the first closed circuit, is composed of two heating elements connected in parallel, and is supplied with power from the commercial AC power source 101 via the contacts 206a and 206d.

[0021] The heating elements 204 and 205 are components of the second closed circuit. The heating element 204 is supplied with power from the commercial AC power supply 101 via the contacts 206b and 206d. On the other hand, the heating element 205 is supplied with power from the commercial AC power supply 101 via the contacts 206b and 206c. The C-contact relay 106 selects whether the heating element 204 or the heating element 205 is connected to the second closed circuit, that is, whether power is supplied from the commercial AC power supply 101 to the heating element 204 or the heating element 205. When the contacts 106a and 106b of the C-contact relay 106 are connected, the heating element 205 is short-circuited by the C-contact relay 106, and the heating element 204 is selected as a component of the second closed circuit. As a result, power is supplied from the commercial AC power supply 101 to the heating element 204. On the other hand, when the contact 106a and the contact 106c are connected in the C-contact relay 106, the heating element 204 is short-circuited by the C-contact relay 106, and the heating element 205 is selected as a component of the second closed circuit. As a result, power is supplied to the heating element 205 from the commercial AC power supply 101. In this way, the heating element 204 or the heating element 205 is appropriately selected as a component of the second closed circuit by controlling the C-contact relay 106.

[0022] The relationship of the resistance values ​​of the heating elements 203, 204, and 205 is: resistance value of heating element 203<resistance value of heating element 204<resistance value of heating element 205. In this embodiment, the resistance values ​​of the heating elements 204 and 205 are 1.92 and 2.25 times the resistance value of the heating element 203, respectively.

[0023] [Heater control circuit] The heater control circuit 108 (also referred to as the control unit 108) has a CPU 109, SSRs (solid state relays) 113 and 118, transistors 111 and 116, and resistors 110, 112, 114, 115, 117, and 119. The triac 104 is controlled by the CPU 109 of the control unit 108. When the CPU 109 sets the T1 terminal and T2 terminal of the triac 104 to a conductive state, the CPU 109 supplies a current to the base terminal of the transistor 111 via a current limiting resistor 110. This turns on the transistor 111, and the collector terminal and emitter terminal of the transistor 111 are brought into a conductive state. When the collector terminal and emitter terminal of the transistor 111 are brought into a conductive state, a current flows from the power supply voltage Vcc2 through the current limiting resistor 112 to the light emitting diode of the SSR (solid state relay) 113, causing it to emit light. As a result, the light receiving section of SSR 113 is turned on (conductive state), a gate current flows from commercial AC power supply 101 to terminal G (gate terminal) of triac 104 via current limiting resistor 114, and conductive state is established between terminals T1 and T2 of triac 104. Here, power supply voltage Vcc2 is a DC voltage of a secondary side potential (potential insulated from commercial AC power supply 101) such as DC 3.3 V, and is generated by power supply device 150, which will be described later.

[0024] On the other hand, the triac 105 is also controlled by the CPU 109, like the triac 104. When the CPU 109 sets the T1 terminal and the T2 terminal of the triac 105 to a conductive state, the CPU 109 supplies a current to the base terminal of the transistor 116 via the current limiting resistor 115. This turns on the transistor 116, and the collector terminal and the emitter terminal of the transistor 116 are in a conductive state. When the collector terminal and the emitter terminal of the transistor 116 are in a conductive state, a current flows from the power supply voltage Vcc2 through the current limiting resistor 117 to the light-emitting diode of the SSR 118, which emits light, and the light-receiving portion of the SSR 118 is turned on (conductive state). When the light-receiving portion of the SSR 118 is turned on, a gate current flows from the commercial AC power supply 101 through the current limiting resistor 119 to the G terminal of the triac 105, and the T1 terminal and the T2 terminal of the triac 105 are in a conductive state. The CPU 109 controls the conductive and non-conductive states of the triacs 104 and 105 based on a power supply control table (Table 1) described later.

[0025] In the image forming apparatus 400 of this embodiment, the control unit 108 exclusively controls the conductive state of the triac 104 and the conductive state of the triac 105 so that power is not simultaneously supplied from the commercial AC power supply 101 to the first closed circuit and the second closed circuit. Specifically, when the T1 terminal and the T2 terminal of the triac 104 are in a conductive state, the control unit 108 sets the T1 terminal and the T2 terminal of the triac 105 to a non-conductive state. On the other hand, when the T1 terminal and the T2 terminal of the triac 105 are in a conductive state, the control unit 108 sets the T1 terminal and the T2 terminal of the triac 104 to a non-conductive state.

[0026] This is because the three types of heating elements of the heater 201 are exclusively selected according to the paper width of the recording material P used, in order to uniformly distribute heat in the longitudinal direction of the heater 201 (the width direction of the recording material P, which is a direction substantially perpendicular to the direction in which the recording material P is conveyed to the fixing device 50). For example, when a recording material P of B5 size passes through the nip portion of the fixing device 50, the control unit 108 alternately supplies power to the heating elements 203 and 204 of the heater 201 to generate heat. Also, when a recording material P of A5 size passes through the nip portion of the fixing device 50, the control unit 108 alternately supplies power to the heating elements 203 and 205 to generate heat. The reason for such power supply control is that the heater 201 is designed on the assumption that power is exclusively supplied to each heating element, and therefore if a state in which multiple heating elements generate heat simultaneously continues, the heater 201 may overheat and be damaged.

[0027] When there is no need to supply power to the heater 201 (for example, when the image forming apparatus 400 is in a power-saving sleep mode in which it is not printing), the electromagnetic relay 103 is set to a non-conductive state in order to save energy. The electromagnetic relay 103 is controlled by the CPU 109, similar to the triacs 104 and 105. The CPU 109 supplies a current to the base terminal of the transistor 121 via a current limiting resistor 120 to turn on the transistor 121, thereby bringing the collector terminal and the emitter terminal of the transistor 121 into a conductive state. As a result, a current flows from the power supply voltage Vcc1 to the coil of the electromagnetic relay 103, and the contacts of the electromagnetic relay 103 are set to a conductive state.

[0028] [Power supply configuration] Next, the power supply device 150 will be described. The power supply device 150 is an AC input / DC output type switching converter that generates a DC voltage (e.g., 24 V, 12 V, 5 V, 3.3 V, etc.) required in the image forming apparatus 400 from an AC voltage output from the commercial AC power supply 101. The power supply device 150 has a diode bridge 151, a primary smoothing capacitor 152, a transformer 153, a field effect transistor (FET) 154, and a control circuit 158 ​​that controls the switching operation of the FET 154. Furthermore, the power supply device 150 has a rectifier diode 155, a secondary smoothing capacitor 156, and a DC / DC converter 157.

[0029] In the power supply device 150, a diode bridge 151, which is a rectifier circuit, performs full-wave rectification on the AC voltage input from the commercial AC power supply 101, and a primary smoothing capacitor 152, which is a smoothing circuit, smoothes the full-wave rectified voltage. The voltage smoothed by the primary smoothing capacitor 152 is input to the primary side of a transformer 153, and a voltage is induced on the secondary side of the transformer 153 by switching operation of an FET 154 by a control circuit 158. The voltage induced on the secondary side of the transformer 153 is rectified and smoothed by a rectifier diode 155, a secondary smoothing capacitor 156, etc., to generate a DC voltage Vcc1 (24 V in this embodiment). The DC voltage Vcc1 is further converted into a DC voltage Vcc2 (3.3 V in this embodiment) via a DC / DC converter 157. Since the power supply device 150 is not a feature of the present invention, detailed description of the circuit operation will be omitted.

[0030] 3(a) is a diagram showing a voltage waveform (FIG. 3(a)(i)) of the voltage across primary smoothing capacitor 152, and a current waveform (FIG. 3(a)(ii)) of the current flowing through primary smoothing capacitor 152. The vertical axis of FIG. 3(a)(i) represents voltage, and the vertical axis of FIG. 3(a)(ii) represents current value. The horizontal axis of FIG. 3(a) represents time.

[0031] 3(a)(i), the thick solid line indicates the voltage waveform of the voltage across primary smoothing capacitor 152, and the dashed line indicates the voltage waveform of the AC voltage input from commercial AC power supply 101 and full-wave rectified by diode bridge 151. In FIG. 3(a)(ii), the thick solid line indicates the current waveform of the current supplied (output) from commercial AC power supply 101 to primary smoothing capacitor 152. Here, the current (capacitor input current) flowing through primary smoothing capacitor 152 shown in FIG. 3(a)(ii) flows only during the following period. That is, the capacitor input current flows only during a period (the charging period shown in FIG. 3(a)(ii)) when the voltage across primary smoothing capacitor 152 is lower than the voltage obtained by full-wave rectifying the voltage of commercial AC power supply 101 by diode bridge 151. On the other hand, in a period in which the voltage across primary smoothing capacitor 152 is higher than the voltage obtained by full-wave rectifying the voltage of commercial AC power supply 101 by diode bridge 151 (the discharge period shown in FIG. 3(a)(ii)), no capacitor input current flows. In this way, the capacitor input current has the characteristic of flowing only during a part of a half cycle (half wave) of the voltage waveform of the AC voltage output from commercial AC power supply 101.

[0032] [Power supply control to heating element] Next, the operation of the heating device 100 when forming an image on the recording material P will be described using an example of an A5 size recording material P. When forming an image on the A5 size recording material P, the contact 106 (FIG. 2) in the heating device 100 is in a state where the contact 106a and the contact 106c are connected, and the heating element 205 is in a state where it is selected as a component of the second closed circuit. Then, the heating device 100 operates to control the temperature of the heater 201 to a predetermined temperature. Specifically, in the heating device 100, the temperature control (temperature adjustment) of the heater 201 is performed by performing PID control for the target temperature of the heater 201 based on the temperature of the heater 201 detected by a temperature detection element (not shown). The temperature control (temperature adjustment) of the heater 201 is performed in common regardless of the size of the recording material P on which an image is formed. Based on the PID control, the current values ​​supplied to the first closed circuit, which is the power supply circuit to the heating element 203, and the second closed circuit, which is the power supply circuit to the heating element 205, follow the power supply control tables shown in Tables 1 and 2.

[0033] [Power supply control table] Table 1 is a power supply control table when power is supplied to the heating element 203, and Table 2 is a power supply control table when power is supplied to the heating elements 204 and 205.

[0034] The power supply control to the heating element 203 based on the power supply control table shown in Table 1 is performed with 8 half waves (4 periods) of the AC voltage waveform of the commercial AC power source 101 as one control period (one control cycle), as shown in the upper part of Table 1. Meanwhile, the numerical values ​​shown in the lower part of Table 1 indicate the power duty (duty) (unit: %) supplied to the heating element 203 in each half wave period with respect to the average target input power duty in one control period. In this embodiment, the power supply control is performed by dividing the power duty (written as power duty in Table 1) of the power that can be supplied from the commercial AC power source 101 to the heating element 203 into 40 stages in increments of 2.50%. That is, the gain update period of the above-mentioned PID control is in units of 8 half waves of the AC voltage waveform of the commercial AC power source 101, and Table 1 indicates what percentage of the power that can be supplied from the commercial AC power source 101 to the heating element 203 is supplied to the heating element 203 by the above-mentioned PID control. For example, when power with a power duty of 50.00% is supplied from commercial AC power supply 101 to heating element 203 over a total of eight half waves, the power supply is controlled as follows based on Table 1. That is, power is supplied with a power duty of 33.75% in the first half wave, a power duty of 33.75% in the second half wave, a power duty of 100.00% in the third half wave, and a power duty of 33.75% in the fourth half wave. Then, power supply is controlled so that power is supplied with a power duty of 33.75% in the fifth half wave, a power duty of 100.00% in the sixth half wave, a power duty of 33.75% in the seventh half wave, and a power duty of 33.75% in the eighth half wave. As shown in Table 1, the average input power duty when eight half-wave power is supplied is 50.31%, which is not 50.00% strictly speaking. However, a power duty of 50.00% is merely a target value, and such a small difference is within the margin of error.

[0035] Moreover, the power supply control to the heating element 205 is performed based on the power supply control table shown in Table 2. Table 2 is different from Table 1 described above only in the value of the power duty (unit: %) supplied to the heating element 205 in each half-wave period relative to the average target input power duty in one control period in the table, but the control period, control resolution, etc. are the same as Table 1.

[0036] [Table 1]

[0037] [Table 2]

[0038] When an image is formed on a recording material P of A5 size, a period during which power is supplied to the heating element 203 via a first closed circuit and a period during which power is supplied to the heating element 205 via a second closed circuit are switched exclusively based on a predetermined control sequence. FIG. 3B is a diagram for explaining the relationship between the AC voltage waveform of the commercial AC power source 101 and the period during which power is supplied to the heating element 203 and the heating element 205. In FIG. 3B, (i) is a diagram showing an AC voltage waveform supplied from the commercial AC power source 101 to the image forming apparatus 400. (ii) is a diagram showing a period during which power is supplied to the heating element 203 (shown as ON in the diagram) and a period during which power is not supplied to the heating element 203 (shown as OFF in the diagram). (iii) is a diagram showing a period during which power is supplied to the heating element 205 (shown as ON in the diagram) and a period during which power is not supplied to the heating element 205 (shown as OFF in the diagram). The horizontal axis of FIG. 3B indicates time. As shown in FIG. 3(b), power is supplied to the heating element 203 through the first closed circuit for one control cycle (=8 half waves), and then power is supplied to the heating element 205 through the second closed circuit for five control cycles (=40 half waves). Furthermore, power is supplied to the heating element 203 through the first closed circuit for one control cycle (=8 half waves), and then power is supplied to the heating element 205 through the second closed circuit for five control cycles (=40 half waves), and this cycle is repeated. In this way, in the heating device 100, the temperature of the heater 210 is adjusted while switching the power supply to the heating element. A detailed explanation of Tables 1 and 2 will be given later.

[0039] [Current waveform input to image forming device from commercial AC power supply] A period U shown in Fig. 3(b) indicates a period of one control cycle (=8 half waves) during which power is supplied to the heating element 203 via the first closed circuit, and a period of one control cycle (=8 half waves) during which power is supplied to the heating element 205 via the second closed circuit. Fig. 4 is a diagram for explaining the current waveform of the AC current supplied from the commercial AC power supply 101 to the image forming apparatus during the period U shown in Fig. 3(b).

[0040] 4[A] is a diagram showing the current waveform of the AC current when power is supplied to heating element 203 in period P, and the current waveform of the AC current when power is supplied to heating element 205 in period Q following period P. The horizontal axis of FIG. 4[A] indicates time. Period P is one control period (first period) of 8 half waves during which power is supplied to heating element 203 via the first closed circuit, and the average target input power duty is 42.50%. On the other hand, period Q is one control period (second period) of 8 half waves during which power is supplied to heating element 205 via the second closed circuit, and the average target input power duty is 100%.

[0041] 4[A](b) shows the current waveform of the AC current (first current) when power is supplied from the commercial AC power source 101 to the heating element 203 with an average target input power duty of 42.50%. When the average target input power duty is 42.50%, the current supplied from the commercial AC power source 101 to the heating element 203 has a waveform that is always phase-controlled with a duty of 42.50% throughout one control cycle of 8 half waves, based on the values ​​shown in the power supply control table of Table 1.

[0042] 4[A](c) shows the current waveform of the AC current (second current) when power is supplied with an average target input power duty of 100.00% from commercial AC power supply 101 to heating element 205. When the average target input power duty is 100.00%, the current supplied from commercial AC power supply 101 to heating element 205 always has a current waveform with a duty of 100.00% throughout one control cycle of 8 half waves, based on the values ​​shown in the power supply control table of Table 2.

[0043] As described above, the resistance value of heating element 205 is 2.25 times the resistance value of heating element 203. Therefore, the amount of power supplied from commercial AC power supply 101 to heating element 203 when the power duty is 42.50% is approximately the same as the amount of power supplied from commercial AC power supply 101 to heating element 205 when the power duty is 100.00%.

[0044] Fig. 4[A](d) shows the current waveform of the capacitor input current (Fig. 3(a)) (third current) that flows when charging the primary smoothing capacitor 152 in the power supply device 150 from the commercial AC power supply 101. Fig. 4[A](a) shows a composite waveform of the three AC currents shown in Fig. 4[A](b), Fig. 4[A](c), and Fig. 4[A](d).

[0045] During period P (period during which power is supplied from the commercial AC power supply 101 to the heating element 203), both the current supplied to the heating element 203 and the capacitor input current are supplied from the commercial AC power supply 101, and therefore a current waveform is formed by combining these two AC currents. On the other hand, during period Q (period during which power is supplied from the commercial AC power supply 101 to the heating element 205), both the current supplied to the heating element 205 and the capacitor input current are supplied from the commercial AC power supply 101, and therefore a current waveform is formed by combining these two currents. During period P, the current waveform when the AC current flows through the heating element 203 and the current waveform when the capacitor input current flows do not overlap from start to finish. Therefore, during period P, the value of the maximum point (maximum value) and the value of the minimum point (minimum value) (hereinafter also referred to as the current peak value or peak value) of the combined current of the AC current flowing through the heating element 203 and the capacitor input current are suppressed to be lower than when the two current waveforms overlap.

[0046] On the other hand, in the period Q, the current waveform when the AC current flows through the heating element 205 and the current waveform when the capacitor input current flows are overlapped from beginning to end. Therefore, in the period Q, the peak value of the composite current of the current flowing through the heating element 205 and the capacitor input current is larger than when the two current waveforms do not overlap. As a result, the difference ΔI1 (FIG. 4[A](a)) indicating the difference between the current peak value in the period P and the current peak value in the period Q in the composite current shown in FIG. 4[A](a) is as follows. That is, the difference ΔI1 can be made smaller than the difference between the peak value I203p (FIG. 4[A](b)) of the AC current flowing through the heating element 203 and the peak value I205p (FIG. 4[A](c)) of the AC current flowing through the heating element 205. That is, the difference ΔI1<(peak value ΔI203p-peak value I205p). The smaller the value of the difference ΔI1, the more the flicker can be suppressed when the image forming apparatus 400 is operated. Therefore, when the period (period P) during which power is supplied to the heating element 203 and the period (period Q) during which power is supplied to the heating element 205 are switched exclusively and frequently, the capacitor input current and the AC current flowing through each heating element (heating elements 203, 205) are controlled so as not to overlap. That is, in order to suppress flicker, it is important to reduce the difference ΔI1, which indicates the difference between the current peak value in period P and the current peak value in period Q. From another perspective, the resistance value of the heating element 205 is 2.25 times the resistance value of the heating element 203, and since the difference in resistance value is large, it is difficult to reduce the difference ΔI1 only by the current waveforms of the AC currents flowing through the heating elements 203 and 205. Therefore, in this embodiment, instead of focusing only on the AC currents flowing through the heating elements 203, 205 and reducing the difference in current peak values, we focused on the composite waveform of the capacitor input current and the AC currents flowing through the heating elements 203, 205. One of the features of the present invention is that the overlap of the composite currents is skillfully controlled to reduce the difference ΔI1 in the AC current peak values.

[0047] Fig. 4[B] shows the current waveform of the AC current when power is supplied to heating element 203 in period R, and the current waveform of the AC current when power is supplied to heating element 205 in period S following period R. The horizontal axis of Fig. 4[B] shows time. Period R is one control period of 8 half waves during which power is supplied via the first closed circuit, and the average target input power duty is 57.50%. On the other hand, period S is one control period of 8 half waves during which power is supplied via the second closed circuit, and the average target input power duty is 100%.

[0048] 4[B](g) shows the current waveform of the AC current when power is supplied from the commercial AC power source 101 to the heating element 203 with an average target input power duty of 57.50%. When the average target input power duty is 57.50%, the current supplied from the commercial AC power source 101 to the heating element 203 is controlled based on the values ​​shown in the power supply control table of Table 1. Specifically, as shown in Table 1 when the average target input power duty is 57.50%, in one control cycle, the power duty in the first half wave is 43.75%, the power duty in the second half wave is 43.75%, and the power duty in the third half wave is 100.00%. Next, the power supply is controlled so that the power duty of the fourth half wave is 43.75%, the power duty of the fifth half wave is 43.75%, the power duty of the sixth half wave is 100.00%, the power duty of the seventh half wave is 43.75%, and the power duty of the eighth half wave is 43.75%.

[0049] 4[B](h) shows the current waveform of the AC current when power is supplied from commercial AC power source 101 to heating element 205 with an average target input power duty of 100.00%. When the average target input power duty is 100.00%, the current supplied from commercial AC power source 101 to heating element 205 always has a current waveform for a duty of 100.00% throughout one control cycle of 8 half waves, based on the values ​​shown in the power supply control table of Table 2.

[0050] Fig. 4[B](i) shows the current waveform of the capacitor input current (Fig. 3(a)(ii)) that flows when charging the primary smoothing capacitor 152 in the power supply device 150 from the commercial AC power supply 101. Fig. 4[B](e) shows a composite waveform of the three AC currents shown in Fig. 4[B](g), Fig. 4[B](h), and Fig. 4[B](i).

[0051] During period R (the period when power is supplied from commercial AC power supply 101 to heating element 203), both the current to heating element 203 and the capacitor input current are supplied from commercial AC power supply 101, resulting in a current waveform that is a combination of these two AC currents. On the other hand, during period S (the period when power is supplied from commercial AC power supply 101 to heating element 205), both the current to heating element 205 and the capacitor input current are supplied from commercial AC power supply 101, resulting in a current waveform that is a combination of these two currents.

[0052] In the period R, there are both sections where the current peak value of the AC current flowing through the heating element 203 and the current peak value of the capacitor input current overlap, and sections where they do not overlap. Since the average input target power duty for the entire period is 57.50% in the period R, if phase control is performed at the same phase angle throughout the entire period, the current waveform of the AC current flowing through the heating element 203 and the current waveform of the capacitor input current will overlap in all half waves. In other words, if phase control is performed at the same phase angle throughout the entire period R, the difference ΔI2 (FIG. 4[B](e)) between the current peak value in the period R and the current peak value in the period S will be large, making it impossible to suppress flicker. Therefore, in the period R, the overlap between the current waveform of the AC current supplied to the heating element 203 and the current waveform of the capacitor input current is sharply accentuated to suppress flicker.

[0053] FIG. 4[B](f) shows a waveform that expresses the absolute value of the difference between the peak values ​​of the current waveform shown in FIG. 4[B](e) for each half wave. In the period R, the relationship between the AC current flowing through the heating element 203 and the capacitor input current is divided into two types of sections, a section where the two current peak values ​​overlap and a section where they do not overlap, and the two types of sections are not arranged unevenly (continuously) but are arranged sparsely. Therefore, the fluctuation frequency f1 of the current peaks in the period R (current fluctuation period t1) can be made higher than when the section where the two current peak values ​​overlap and the section where they do not overlap are arranged unevenly. Since flicker is felt most unpleasant when the frequency of the current fluctuation is 8.8 Hz (period 113.6 msec), when a current fluctuation occurs, it is desirable to set the frequency of the current fluctuation to a frequency away from 8.8 Hz. For example, when the power frequency of commercial AC power supply 101 is 50 Hz, one half wave is 10 msec (milliseconds), so the frequency f1 of the current fluctuation shown in Fig. 4[B](f) is 33.3 Hz (period 30 msec). In this case, it can be seen that the current fluctuation has a sufficiently high frequency, about 3.8 times higher than the 8.8 Hz frequency at which flicker is felt most unpleasant.

[0054] In the period R shown in FIG. 4[B](e), the average target input power duty (=57.50%) is larger than the average target input power duty (=42.50%) in the case of the period P shown in FIG. 4[A](a). Therefore, in the period R in FIG. 4[B](e), it is not possible to adopt a method of reducing the difference ΔI2 by preventing the peak of the AC current flowing through the heating element 203 from overlapping the peak of the capacitor input current throughout the entire period R, as in the period P in FIG. 4[A](a). Therefore, the relationship between the current flowing through the heating element 203 and the capacitor input current is divided into two types, a section in which the current peak values ​​of the two currents overlap and a section in which they do not overlap, and the two sections are arranged sparsely. As a result, one of the features of the present invention is that flicker is suppressed by setting the fluctuation frequency f1 of the current peak value high. Note that the table values ​​of the power supply control table in Table 1 may be configured so that the fluctuation frequency f1 of the current peak value is lower than 8.8 Hz, but this inevitably results in a longer control period (8 half waves in this embodiment). As a result, the gain update period of the PID control becomes longer, and the frequency of switching between the period in which power is supplied to the heating element 203 and the period in which power is supplied to the heating element 205 is reduced, resulting in problems such as an increase in the temperature ripple of the heater 201. For this reason, in practical terms, the configuration of this embodiment in which the fluctuation frequency f1 of the current peak value is increased to suppress flicker during the period R in FIG. 4[B](e) is desirable.

[0055] Here, the eighth half-wave section N, which is the last section of the period R, and the first half-wave section (N+1), which is the first section of the period S, will be described, which are the joints between the period R in which power is supplied to the heating element 203 and the period S in which power is supplied to the heating element 205. From the viewpoint of suppressing flicker, it is desirable that the difference ΔI3 (FIG. 4[B](f)) between the current peak values ​​of the two sections in the sections N and (N+1) is small. Therefore, it is desirable to suppress the peak value of the composite current of the two currents low by not overlapping the AC current flowing through the heating element 203 and the capacitor input current as much as possible in the section N. Also, it is desirable to increase the peak value of the composite current of the two AC currents by overlapping the AC current flowing through the heating element 205 and the capacitor input current as much as possible in the section (N+1). In this way, by optimizing the overlapping of the capacitor input current in the sections N and (N+1), the difference ΔI3 between the current peak values ​​of the two adjacent sections can be suppressed small, and flicker can be suppressed.

[0056] Here, a method has been described for minimizing the difference ΔI3 in current peak values ​​at the timing of transition from the period (period R) in which power is supplied to heating element 203 to the period (period S) in which power is supplied to heating element 205. Similarly, for the period from the period in which power is supplied to heating element 205 to the period in which power is supplied to heating element 203, it is desirable to optimize the overlapping of AC currents so as to minimize the difference in current peak values ​​when the heating elements are switched.

[0057] [Power supply control table applied to heating element 203] Next, the power supply control table of Table 1 will be described in detail. First, the power supply control table shown in Table 1 is a table used for controlling the power supply to the heating element 203, and is roughly composed of three sections. Section 1 (first section) is an area in which the average target input power duty is in the range of 0.00% to 42.50%. In section 1, phase control is always performed at a phase angle equal to the average target input power duty throughout one control cycle (8 half waves). In the range of section 1 in which the average target input power duty is 0.00% to 42.50%, even if control is performed at a phase angle equal to the average target input power duty, the timing of the current peak of the AC current flowing through the heating element 203 and the current peak of the capacitor input current do not overlap. As a result, the peak value of the composite current of the AC current flowing through the heating element 203 and the capacitor input current does not become large. Therefore, as shown in FIG. 4[A](a), flicker can be suppressed by reducing the difference ΔI1 between the current peak values.

[0058] Section 2 (second section) is an area where the average target input power duty is in the range of 45.00% to 72.50%. In section 2, the relationship between the current waveform of the AC current flowing through the heating element 203 and the current waveform of the capacitor input current is divided into two types of sections, where the peaks of the two current waveforms overlap and where they do not overlap, and the power supply control table has the two current peaks sparsely arranged. In the range of section 2, if control is performed with a phase angle equal to the average target input power duty throughout the entire control period, the current peak value of the AC current flowing through the heating element 203 will always overlap with the current peak value of the capacitor input current. Therefore, in the power supply control table for section 2, as shown in FIG. 4[B](e) and FIG. 4[B](f), the flicker is suppressed by increasing the fluctuation frequency f1 of the current peak.

[0059] Section 3 (third section) is an area where the average target input power duty is in the range of 75.00% to 100.00%. FIG. 4[C](j) shows the current waveform of the AC current supplied from the commercial AC power source 101 to the image forming apparatus 400 when power is supplied to the heating element 203 using the power supply control table shown in section 3. FIG. 4[C](j) shows a current waveform obtained by combining the current waveform of the current supplied from the commercial AC power source 101 to the heating element 203 and the current waveform of the capacitor input current. In FIG. 4[C](j), the first 8 half-wave period is a current waveform when power is supplied from the commercial AC power source 101 to the heating element 203 at a target input power duty of 100%. The latter 8 half-wave period is a current waveform when power is supplied from the commercial AC power source 101 to the heating element 203 at a target input power duty of 77.5%.

[0060] In section 3, since the average target input power duty is very high, the relationship between the current waveform of the AC current flowing through the heating element 203 and the current waveform of the capacitor input current cannot be divided into two types of sections, one in which the two current waveforms overlap and the other in which they do not overlap, as in section 2. That is, section 3 is a section in which it is unavoidable that the current waveform of the AC current flowing through the heating element 203 and the current waveform of the capacitor input current overlap throughout one control cycle (8 half waves). However, the average target input power duty of section 3 is only used for a few seconds when the heater 201 in a cold state is to be rapidly heated (for example, when the image forming apparatus 400 is started up). Furthermore, when the image forming apparatus 400 is started up, power is not supplied to the heating element 205, and power is supplied only to the heating element 203. This is because when the heater 201 is in a cold state, it is necessary to heat the entire heater 201 evenly and quickly, so it is required to supply power only to the heating element 203, which has a longer longitudinal region that is heated and generates a large amount of heat. In accordance with the average target input power duty of section 3, when power is supplied only to heating element 203 at the start of image forming apparatus 400, the peak value of the AC current is high, but the heating element to which power is supplied is not switched. Therefore, there is little fluctuation in the AC current, and almost no flicker occurs.

[0061] As described above, the power supply control table shown in Table 1 that is applied to the heating element 203 is composed of three categories: category 1 and category 2, which are implemented with flicker suppression measures during heating element switching control, and category 3, which is applied when heating element switching control is not performed. The power supply control table shown in Table 1 has power duty values ​​that can suppress flicker in all categories.

[0062] [Power supply control table applied to heating element 205] Next, a detailed explanation will be given of the power supply control table of Table 2. The power supply control table shown in Table 2 is a table used for controlling the power supply to the heating element 205.

[0063] As described above, the resistance value of the heating element 205 is about 2.25 times the resistance value of the heating element 203. Therefore, when the power duty is 100.00%, the peak value of the AC current flowing through the heating element 205 is about 1 / 2.25 of the peak value of the AC current flowing through the heating element 203. Therefore, in order to compensate for the peak value of the AC current flowing through the heating element 205, it is desirable to actively overlap the timing of the peak of the current waveform of the AC current flowing through the heating element 205 and the peak of the current waveform of the capacitor input current, and to increase the peak current value of the composite current of the two currents. Therefore, in the power supply control table of Table 2, in the entire range of the average target input power duty from 0.00% to 100.00%, phase control is always performed at a phase angle equal to the average target input power duty throughout one control cycle (8 half waves). When the image forming apparatus 400 is in a steady state (when the heater 201 is sufficiently warmed up and image formation is being performed continuously), the heating element 203 is supplied with power at an average target input power duty of about 40% to 50%. On the other hand, the heating element 205, which has a resistance value about 2.25 times larger, is supplied with power at an average target input power duty of about 90% to 100%. That is, since the conduction angle of the AC current flowing through the heating element 205 is large when the image forming apparatus 400 is in a steady state, the timing of the peak of the current waveform of the AC current flowing through the heating element 205 and the timing of the peak of the current waveform of the capacitor input current always overlap. That is, during the steady state, the timing of the peak of the current waveform of the AC current flowing through the heating element 205 always overlaps with the timing of the peak of the current waveform of the capacitor input current, so that the peak value of the composite current of the two currents can always be large. This makes it possible to reduce the difference between the current peak value during the period when power is supplied to the heating element 203 and the current peak value during the period when power is supplied to the heating element 205 (the difference ΔI1 shown in FIG. 4[A] and the difference ΔI2 shown in FIG. 4[B]). As a result, it is possible to effectively suppress flicker.

[0064] [Application of power supply control table according to recording material size] The above has described flicker suppression for AC current supplied from the commercial AC power source 101 to the image forming apparatus 400, taking as an example the case of forming an image on A5 size recording material P (also called A5 paper). Here, power supply control for heating elements for B5 size recording material P (also called B5 paper), A4 size recording material P (also called A4 paper), and LTR (letter) paper will be described.

[0065] Table 3 shows the correspondence between the type of recording material P on which an image is formed (paper on which an image is formed), the heating element to be used, and the power supply control table applied to the heating element to be used. The power supply control table to be used when forming an image on LTR paper, A4 paper, B5 paper, and A5 paper will be described using Table 3.

[0066] [Table 3]

[0067] When forming an image on A5 paper, as described above, power supply control is performed for heating element 203 based on the power supply control table of Table 1, and power supply control is performed for heating element 205 based on the power supply control table of Table 2. Note that when forming an image on A5 paper, heating element 204 is not used, so no power is supplied to it.

[0068] On the other hand, when forming an image on B5 paper, power supply control is performed for heating element 203 based on the power supply control table of Table 1, and power supply control is performed for heating element 204 based on the power supply control table of Table 2. Note that when forming an image on B5 paper, heating element 205 is not used, so no power is supplied to it.

[0069] Also, when forming an image on A4 paper and LTR paper, power supply control is performed for the heating element 203 based on the power supply control table of Table 2, and power is not supplied to the heating elements 204 and 205. When forming an image on A4 paper and LTR paper, power is supplied only to the heating element 203, so flicker does not occur due to the difference in current peak value (difference ΔI1 shown in FIG. 4[A] and difference ΔI2 shown in FIG. 4[B]) that occurs when switching the power supply path to the heating element. Therefore, when forming an image on A4 paper and LTR paper, flicker only needs to be taken care of for the AC current flowing through the heating element 203. In the power supply control table of category 2 of Table 1, there are cases where the timing of the peak of the current waveform of the AC current supplied to the heating element 203 overlaps with the timing of the peak of the current waveform of the capacitor input current, and there are cases where they do not overlap, causing unnecessary current changes. Therefore, from the viewpoint of taking care of flicker only for the current flowing through the heating element 203, unnecessary current changes occur, which is undesirable. Therefore, when forming images on A4 paper and LTR paper, it is optimal to control the power supply to the heating element 203 based on the power supply control table of Table 2 rather than Table 1.

[0070] In this manner, in this embodiment, different power supply control tables for the heating element 203 are used depending on whether the power supply path is switched between the heating element 203 and the other heating elements (the heating element 204 or the heating element 205) or whether power is supplied only to the heating element 203. This allows optimal control to be performed in either case from the viewpoint of suppressing flicker.

[0071] As described above, according to this embodiment, it is possible to suppress flicker that occurs when the heating elements to which power is supplied are switched. EXAMPLES

[0072] In the first embodiment, an embodiment using a power supply device to which a capacitor input current is supplied from a commercial AC power supply is described. In the second embodiment, an embodiment using a power supply device having a PFC (Power Factor Correction) circuit is described. Note that the configuration of the image forming apparatus is the same as in the first embodiment, and the same devices are designated by the same reference numerals and description thereof is omitted.

[0073] [Heating device configuration] FIG. 5 is a circuit diagram showing the circuit configuration of the heating device 100 and the power supply device 300 of this embodiment provided in the image forming apparatus 400 of FIG. 1. The circuit configuration of the heating device 100 shown in FIG. 5 is the same as the circuit configuration shown in FIG. 2 of the first embodiment, and the description will be omitted. On the other hand, the power supply device 300 is different from the power supply device 150 shown in FIG. 2 of the first embodiment in that it has a PFC circuit (power factor correction circuit) 301 between the diode bridge 151 and the primary smoothing capacitor 152, but the other circuit configurations are the same as those of the power supply device 150. The PFC circuit 301 has an inductor 302, a FET 303, a diode 304, and a control circuit 305 that controls the FET 303. Since the circuit operation of the PFC circuit 301 is not a feature of this embodiment, the detailed description of the operation of the PFC circuit 301 will be omitted.

[0074] FIG. 6[A] shows a voltage waveform (FIG. 6[A](i)) of the AC voltage input from the commercial AC power source 101 to the image forming apparatus 400, and a current waveform (FIG. 6[A](ii)) of the AC current supplied from the commercial AC power source 101 to the power supply device 300. The vertical axis of FIG. 6[A](i) indicates voltage, and the vertical axis of FIG. 6[A](ii) indicates current value. The horizontal axis of FIG. 6[A] indicates time. Since the power supply device 300 is equipped with a PFC circuit 301, the current waveform of the current supplied from the commercial AC power source 101 to the power supply device 300 becomes a sine wave-like waveform as shown in FIG. 6[A](ii). The circuit configuration of the power supply device 300 other than the PFC circuit 301 is the same as that of the power supply device 150 shown in FIG. 2 of the first embodiment, and therefore a description thereof will be omitted.

[0075] [Power supply control to heating element] Next, the operation of the heating device 100 of this embodiment when forming an image on the recording material P will be described using an A5 size recording material P as an example. As described in the first embodiment, when forming an image on the A5 size recording material P, power is supplied from the commercial AC power source 101 to the heating elements 203 and 205 in the heating device 100. The power supply to each heating element is controlled based on the power supply control table shown in Table 4.

[0076] Table 4 shows a power supply control table to be referred to when power is supplied to the heating element 203 through the first closed circuit and when power is supplied to the heating element 205 through the second closed circuit. As shown in the upper part of Table 4, the power supply control to the heating elements 203 and 205 is performed with 8 half waves (4 periods) of the AC voltage waveform of the commercial AC power source 101 as one control period (one control cycle), as in the first embodiment. Meanwhile, the numerical values ​​shown in the lower part of Table 4 indicate the power duty (written as power duty in Table 4) supplied to the heating elements 203 and 205 in each half wave period with respect to the average target input power duty in one control period. The conduction start phase angle will be described in detail later. In this embodiment, as in the first embodiment, the power supply control is performed by dividing the power duty of the power that can be supplied from the commercial AC power source 101 to the heating elements 203 and 205 into 40 stages at intervals of 2.50%. In the first embodiment, different power supply control tables are referenced in the power supply control for heating element 203 and heating element 205. In the present embodiment, the power supply control table of Table 4 is commonly used for the power supply control for heating element 203 and for the power supply control for heating elements 204 and 205.

[0077] [Table 4]

[0078] [Current waveform input to image forming device from commercial AC power supply] 6[B] is a diagram showing the current waveform of the AC current supplied from the commercial AC power source 101 to the image forming apparatus 400 when an image is formed on an A5 size recording material P. In detail, the diagram shows the current waveform of the AC current when power is supplied to the heating element 203 in a period V, and the current waveform of the AC current when power is supplied to the heating element 205 in a period W following the period V. The period V is one control period of 8 half waves during which power is supplied to the heating element 203 via a first closed circuit, and the period W is one control period of 8 half waves during which power is supplied to the heating element 205 via a second closed circuit.

[0079] 6[B](b) shows the current waveform of the AC current when power is supplied from the commercial AC power source 101 to the heating element 203 with an average target input power duty of 42.50%. When the average target input power duty is 42.50%, the current waveform of the AC current supplied from the commercial AC power source 101 to the heating element 203 is always a waveform phase-controlled with a power duty of 42.50% throughout one control cycle based on the power supply control table of Table 4.

[0080] 6[B](c) shows the current waveform of the AC current when power is supplied from the commercial AC power source 101 to the heating element 205 with an average target input power duty of 100.00%. When the average target input power duty is 100.00%, the current waveform of the AC current supplied from the commercial AC power source 101 to the heating element 205 is always a waveform phase-controlled with a power duty of 100.00% throughout one entire control cycle according to the power supply control table of Table 4.

[0081] 6[B](d) shows the current waveform of the AC current supplied from the commercial AC power source 101 to the power supply device 300. Since the power supply device 300 is equipped with a PFC circuit 301, the current waveform of the AC current supplied from the commercial AC power source 101 to the power supply device 300 becomes sinusoidal (FIG. 6[A]).

[0082] FIG. 6[B](a) shows a current waveform of a current supplied to the image forming apparatus 400, which is a combination of the three AC currents shown in FIG. 6[B](b), FIG. 6[B](c), and FIG. 6[B](d). In a period V (a period during which power is supplied from the commercial AC power source 101 to the heating element 203), both the AC current supplied to the heating element 203 and the AC current supplied to the power supply device 300 are supplied from the commercial AC power source 101. Therefore, the current waveform in the period V is a current waveform in which these two AC currents are combined. On the other hand, in a period W (a period during which power is supplied from the commercial AC power source 101 to the heating element 205), both the AC current supplied to the heating element 205 and the AC current supplied to the power supply device 300 are supplied from the commercial AC power source 101. Therefore, the current waveform in the period W is a current waveform in which these two AC currents are combined.

[0083] Here, the sinusoidal AC current supplied to the power supply device 300 shown in FIG. 6[B](d) is always flowing while power is being supplied from the commercial AC power source 101 to the power supply device 300, unlike the capacitor input current shown in FIG. 4[A](d) of the first embodiment. Therefore, it is not possible to suppress flicker by not overlapping the peak of the current waveform of the AC current flowing through each heating element with the peak of the sinusoidal current waveform of the AC current supplied to the power supply device 300, as in the first embodiment. Therefore, in this embodiment, the current peak value of the AC current itself flowing through each heating element (heating element 203 and heating element 205) is controlled to suppress flicker. In this embodiment, a synergistic effect of suppressing flicker is achieved by considering the relationship between the timing of the peak value of the sinusoidal AC current supplied to the power supply device 300 (phase angle timing of approximately 90 deg and 270 deg) and the current waveform of the AC current supplied to each heating element.

[0084] A method of suppressing flicker in this embodiment will be specifically described. In order to reduce the difference between the peak value of the AC current in period V and the peak value of the AC current in period W, it is desirable to do the following. That is, in period V in Fig. 6[B], it is desirable to make the AC current supplied from commercial AC power supply 101 to heating element 203 as small (low) as possible by avoiding 90 deg and 270 deg, which are phase angles at which the current peak value is largest (highest). By making the phase angle larger than 90 deg and larger than 270 deg, it is possible to make the peak value of the AC current in period V as small (low) as possible.

[0085] On the other hand, in the period W of Fig. 6[B], it is desirable to make the AC current supplied from the commercial AC power supply 101 to the heating element 205 as large (high) as possible by including 90 deg and 270 deg, which are phase angles at which the current peak value is largest (highest). That is, by setting the phase angle to 0 deg or more and 90 deg or less, and 180 deg or more and 270 deg or less, the peak value of the AC current in the period W can be made as large (high) as possible. Note that, at phase angles 90 deg and 270 deg, where the current value of the AC current flowing through each heating element 203, 205 is at its peak, the current value of the sinusoidal AC current supplied to the power supply device 300 is also at its peak. Therefore, supplying power to the heating element 203 while avoiding phase angles 90 deg and 270 deg, and supplying power to the heating element 205 while including phase angles 90 deg and 270 deg, can achieve the following effects. That is, an effect is achieved in that the difference in the peak value of the combined current in periods V and W can be made smaller than the difference in the peak values ​​of the currents of the heating elements 203, 205.

[0086] In FIG. 6[B], the peak value of the AC current flowing through the heating element 203 is defined as I203p (FIG. 6[B](b)), and the peak value of the AC current flowing through the heating element 203 is defined as I205p (FIG. 6[B](c)). In addition, the difference between the current peak value in period V and the current peak value in period W in the composite current shown in FIG. 6[B](a) is defined as ΔI4. Then, the magnitude relationship between the peak value I203p, the peak value I205p, and the difference ΔI4 is (peak value I203p-peak value I205p)>difference ΔI4. Incidentally, the timing at which the peak value I203p is observed does not include phase angles of 90 deg and 270 deg. Therefore, the instantaneous current value Is203 of the AC current (sine wave current) supplied to the power supply device 300 at the timing when the peak value I203p is observed is smaller than the peak current value Isp of the sine wave current (instantaneous current value Is203<peak current value Isp). Also, the instantaneous current value Is205 of the AC current (sine wave current) supplied to the power supply device 300 at the timing when the peak value I205p is observed is equal to the peak current value Isp of the sine wave current (instantaneous current value Is205=peak current value Isp). As a result, the magnitude relationship between the instantaneous current value Is203 and the instantaneous current value Is205 is the instantaneous current value Is203<instantaneous current value Is205.

[0087] [Power supply control to heating elements using a power supply control table] As described above, when power is supplied to the heating element 203, in order to perform power supply control that avoids phase angles 90 deg and 270 deg, the average target input power duty shown in category 4 of Table 4 is mainly used for power supply control to the heating element 203. In Table 4, the average target input power duty (Duty) (unit: %) of category 4 is in the range of 0.00% to 47.50%. When the average target input power duty is 0.00%, the conduction start phase angle (unit: deg) is 180 deg for a positive half wave and 360 deg for a negative half wave. Note that category 4 does not include an average target input power duty of 50.00%, so the phase angles 90 deg and 270 deg are not included, and the conduction start phase angle is greater than 90 deg or 270 deg. The conduction end phase angle (unit: deg) is approximately 180 deg (180 deg or less) in the case of a positive half wave, and approximately 360 deg (360 deg or less) in the case of a negative half wave.

[0088] On the other hand, when power is supplied to the heating element 205, power supply control including phase angles of 90 degrees and 270 degrees is performed, so that the power supply control to the heating element 205 mainly uses the average target input power duty shown in category 5 of Table 4. In Table 4, the average target input power duty (Duty) (unit: %) of category 5 is in the range of 50.00% to 100.00%. When the average target input power duty is 50.00%, the conduction start phase angle (unit: deg) is 90 degrees in the case of a positive half wave and 270 degrees in the case of a negative half wave. Therefore, the conduction start phase angle is 90 degrees or less or 270 degrees or less. Also, when the average target input power duty is 100.00%, the conduction start phase angle (unit: deg) is 0 degrees in the case of a positive half wave and 180 degrees in the case of a negative half wave. The conduction end phase angle (unit: deg) is approximately 180 deg for a positive half wave, and approximately 360 deg for a negative half wave.

[0089] The simplest and most reliable method for mainly using category 4 of Table 4 for controlling the power supply to heating element 203 and category 5 of Table 4 for controlling the power supply to heating element 205 is to set a limit on the average target input power duty value for each heating element to which power is supplied.

[0090] Specifically, it is determined whether or not the average target input power duty to heating element 203 calculated by PID control for the target temperature of heater 201 described above is within the range of category 4 in Table 4. If the calculated average target input power duty to heating element 203 is outside the range of category 4 in Table 4, the average target input power duty to be selected is set to 47.50%, which is the upper limit of category 4, so as not to include phase angles of 90 deg and 270 deg. Similarly, for power supply control to heating element 205, the average target input power duty to be selected is set as follows. That is, if the calculated average target input power duty to heating element 205 is outside the range of category 5 in Table 4, the average target input power duty to be selected is set to 50.00%, which is the lower limit of category 5 in Table 4, so as to include phase angles of 90 deg and 270 deg. However, when the heater 201, which is cold when the image forming apparatus 400 is started, needs to be rapidly heated, power is supplied only to the heating element 203, as in the first embodiment, and at that time, the power supply to the heating element 203 is controlled using the range of power duty included in category 5 of Table 4.

[0091] In this way, when the image forming apparatus 400 is started up (when the cold heater 201 is being rapidly heated), flickering can be suppressed by not switching the heating element. On the other hand, during steady operation, when the heater 201 is sufficiently warmed up and images are being continuously formed on the recording material P, if the heating element to which power is supplied is switched from the heating element 203 to another heating element (heating element 204 or heating element 205), the classification restrictions shown in Table 4 are set. This makes it possible to suppress flickering.

[0092] [Application of power supply control table according to recording material size] The above has described flicker suppression for AC current supplied to image forming apparatus 400 from commercial AC power source 101, taking the example of image formation on A5 size recording material P (A5 paper). Here, power supply control for heating elements for B5 size recording material P (also called B5 paper), A4 size recording material P (also called A4 paper), and LTR (letter) paper will be described.

[0093] Table 5 shows the correspondence between the type of recording material P on which an image is formed (paper on which an image is formed), the heating element to be used, and the power supply control table applied to the heating element to be used. Table 5 will be used to explain the power supply control table to be used when forming an image on LTR paper, A4 paper, B5 paper, and A5 paper.

[0094] [Table 5]

[0095] When forming an image on A5 paper, as described above, power supply control is performed for both heating element 203 and heating element 205 by referring to the power supply control table of Table 4. Note that when forming an image on A5 paper, heating element 204 is not used, so power is not supplied to it.

[0096] When forming an image on B5 paper, as described above, power supply control is performed for both heating element 203 and heating element 204 by referring to the power supply control table in Table 4. When forming an image on B5 paper, heating element 205 is not used and therefore no power is supplied. When forming an image on A5 paper and B5 paper, the heating element to which power is supplied is frequently switched, so flicker is suppressed by implementing the above-mentioned classification limit of the average target input power duty to each heating element.

[0097] Furthermore, when forming images on A4 paper and LTR paper, power is supplied only to the heating element 203 using the power supply control table of Table 4. When forming images on A4 paper and LTR paper, power is supplied only to the heating element 203, so flicker does not occur due to differences in current peak values ​​that occur when switching the power supply path to the heating element. Therefore, when forming images on A4 paper and LTR paper, the power supply control table of Table 4 can be used without classification restrictions.

[0098] In this way, by using the classification restrictions shown in Table 4 appropriately when switching between the heating element that supplies power to heating element 203 and the other heating elements (heating element 204 or heating element 205) and when supplying power only to heating element 203, optimal control for suppressing flicker can be performed.

[0099] A feature of the present invention in this embodiment is that when power is supplied to heating element 203, phase angles of 90 deg and 270 deg are avoided (not included), and when power is supplied to heating element 205, phase angles of 90 deg and 270 deg are included. Also, the method of achieving this is not limited to the method of classification restriction shown in Table 4 described in this embodiment. For example, methods other than classification restriction, such as changing the PID control gain when power is supplied to heating element 203 and the PID control gain when power is supplied to heating element 205, are also possible.

[0100] As described above, according to this embodiment, it is possible to suppress flicker that occurs when the heating elements to which power is supplied are switched. EXAMPLES

[0101] In the third embodiment, a flicker suppression is performed by controlling the power supply so that the average value of the maximum current value per control cycle during the period when power is supplied to a heating element with a small resistance does not exceed the maximum current value during the period when power is supplied to a heating element with a large resistance. Note that the configuration of the image forming apparatus is the same as that of the first embodiment, and the same devices are designated by the same reference numerals and the description thereof is omitted.

[0102] [Control block diagram of image forming device] Fig. 7 is a block diagram showing the configuration of a control unit of this embodiment related to the fixing operation of the image forming apparatus 400 shown in Fig. 1, and the fixing operation of the image forming apparatus 400 will be described with reference to Fig. 7. A PC 410, which is a host computer, transmits a print command including image data of a print image and print information to a video controller 81 inside the image forming apparatus 400.

[0103] When the video controller 81 receives a print command including image data from the PC 410, it transmits the print command to the engine controller 82. The engine controller 82 is equipped with a CPU 83, a memory 84, etc., and the CPU 83 controls the image forming operation including the fixing operation according to a program previously stored in the memory 84. The CPU 83 also has a timer that measures time.

[0104] The fixing control device 56 is composed of a triac 57 which is a power supply control unit, a heating element switch 58 (see FIG. 11) which is a switching unit for selecting a heating element to which power is supplied, and the like. The fixing control device 56 selects a heating element in the fixing device 50 to which power is supplied, and determines the amount of power to be supplied. The driving device 85 is composed of a main motor 86, a fixing motor 87, and the like. The sensor 88 is composed of a fixing temperature sensor 59 which is a temperature detection unit for detecting the temperature of the fixing device 50, a paper width sensor 89 which detects the width of the recording material P, and the like, and the detection result of the sensor 88 is transmitted to the CPU 83. The CPU 83 fixes the transferred toner image to the recording material P by controlling the fixing control device 56 and the driving device 85 based on the obtained detection result of the sensor 88. Note that the image forming device 400 to which the present invention is applied is not limited to the image forming device having the configuration described in FIG. 1, and may be any image forming device capable of printing recording materials P of different widths and equipped with a fixing device 50 having a heater 54 described later.

[0105] [Configuration of the fixing device] Next, the configuration of the fixing device 50 that controls the heating device (heater) that heats the toner image on the paper P with a heat generating element will be described with reference to Fig. 8. Here, the "longitudinal direction" refers to the direction of the rotation axis of the pressure roller 53 that is approximately perpendicular to the transport direction of the paper P described later. Also, the length of the recording material P in the direction (longitudinal direction) that is approximately perpendicular to the transport direction of the paper P is called the paper width.

[0106] 8 is a schematic cross-sectional view illustrating the configuration of the fixing device 50 of this embodiment. In the fixing device 50, the recording material P carrying the unfixed toner image T is transported from the left side of the figure toward the fixing nip N formed by the contact between the fixing film 51 (hereinafter referred to as the film 51) and the pressure roller 53 in the direction of the arrow in the figure. In the fixing nip N, the fixing film 51 is sandwiched between the pressure roller 53 and the heater 54. The recording material P is heated while being transported from the left side to the right side in the figure in the fixing nip N, so that the toner image T is fixed to the recording material P. The fixing device 50 is composed of a cylindrical film 51, a nip forming member 52 that holds the film 51, a pressure roller 53 that forms the fixing nip N together with the film 51, and a heater 54 (heater unit) that is a heating device that heats the recording material P.

[0107] The film 51 is a fixing film that serves as a heating rotary body. The film 51 uses, for example, polyimide as a base layer, and an elastic layer made of silicone rubber and a release layer made of PFA are formed on the base layer. In order to reduce the frictional force generated between the film 51 and the nip forming member 52 and heater 54 as the film 51 rotates, grease is applied to the inner surface of the film 51.

[0108] The nip forming member 52 guides the film 51 from the inside and forms a fixing nip N between the film 51 and the pressure roller 53. The nip forming member 52 is a member having rigidity, heat resistance, and heat insulation properties, and is formed of a liquid crystal polymer or the like. The film 51 is fitted onto the nip forming member 52. The pressure roller 53 is a roller as a pressure rotating body, and is composed of a core metal 53a, an elastic layer 53b, and a release layer 53c. The pressure roller 53 is rotatably held at both ends in the longitudinal direction and is driven to rotate by a fixing motor 87 (FIG. 7). When the pressure roller 53 rotates, the film 51 is rotated. The heater 54, which is a heating member, is disposed in the internal space of the fixing film 51, is held by the nip forming member 52, and is in contact with the inner surface of the film 51. Details of the heater 54 will be described later.

[0109] [Heater Overview] Next, the heater 54, which is a heating unit, will be described. FIG. 9 is a schematic diagram showing the configuration of the heater 54, on which the heating elements are arranged, when viewed from the pressure roller 53 side shown in FIG. 8. In FIG. 9, a reference line a is the center line in the longitudinal direction of the heating elements 54b1a, 54b1b, 54b2, and 54b3, and is also the center line in the longitudinal direction (paper width direction) of the paper P transported to the fixing nip portion N of the fixing device 50. As shown in FIG. 9, the heater 54 has a substrate 54a, heating elements 54b1a, 54b1b, 54b2, and 54b3, a conductor 54c, contacts 54d1 to 54d4, and a protective glass layer 54e. The conductor 54c is the part painted black in the figure. The substrate 54a in this embodiment is made of alumina (Al 2 O 3 ) is used as the ceramic substrate. 2 O 3 ), aluminum nitride (AlN), zirconia (ZrO 2 ), silicon carbide (SiC), etc. are widely known, and among them, alumina (Al 2 O 3) is inexpensive and easily available. Furthermore, metals having excellent strength may be used for the substrate 54a. When using a metal substrate, stainless steel (SUS) is preferable because it is inexpensive and strong. Furthermore, in either case of a ceramic substrate or a metal substrate, if it has electrical conductivity, an insulating layer may be provided before use. Heating elements 54b1a, 54b1b, 54b2, 54b3, a conductor 54c, and contacts 54d1 to 54d4 are arranged on the substrate 54a, and a protective glass layer 54e is coated thereon to ensure insulation between the heating elements and the film 51.

[0110] The heating elements have different lengths in the longitudinal direction (lengths in the left-right direction in FIG. 9). The heating elements 54b1a and 54b1b have a longitudinal length L1 of 222 mm, the heating element 54b2 has a longitudinal length L2 of 188 mm, and the heating element 54b3 has a longitudinal length L3 of 154 mm. The longitudinal lengths L1, L2, and L3 have a length L1>length L2>length L3 relationship. For example, when the paper P used is A4 size, the heating elements 54b1a and 54b1b are used, when the paper P used is B5 size, the heating element 54b2 is mainly used, and when the paper P used is A5 size, the heating element 54b3 is mainly used. The heating elements are arranged in the order of the heating elements 54b1a, 54b2, 54b3, and 54b1b in the short side direction (vertical direction in FIG. 9).

[0111] As shown in FIG. 9, the heating elements 54b1a and 54b1b are electrically connected at one end to a contact 54d2 (first contact) and at the other end to a contact 54d4 (fourth contact) via a conductor 54c. The heating element 54b2 is electrically connected at one end to a contact 54d2 and at the other end to a contact 54d3 (third contact) via a conductor 54c. Similarly, the heating element 54b3 is electrically connected at one end to a contact 54d1 (second contact) and at the other end to a contact 54d3 via a conductor 54c. As shown in FIG. 9, the heating elements 54b1a and 54b1b have substantially the same length L1 in the longitudinal direction, and the two heating elements 54b1a and 54b1b are always used simultaneously. Hereinafter, the pair of heating elements 54b1a and 54b1b will be collectively referred to as heating element 54b1 (first heating element). The resistance values ​​of the heating elements are as follows: heating element 54b1 is 10Ω (combined resistance value of heating elements 54b1a and 54b1b), heating element 54b2 (third heating element) is 24Ω, and heating element 54b3 (second heating element) is 24Ω.

[0112] 9, the fixing temperature sensor 59 is enclosed by a dashed line. The dashed line indicates that the fixing temperature sensor 59 is disposed on the rear surface of the substrate 54a (opposite the surface on which the heating elements 54b1, 54b2, and 54b3 are disposed) and indicates the position where the fixing temperature sensor 59 abuts against the substrate 54a. The thermistor 59a that detects the temperature of the fixing temperature sensor 59 is disposed on the center line of the heating elements 54b1, 54b2, and 54b3 in the longitudinal direction and on the reference line a that is the center line of the paper P transported to the fixing device 50.

[0113] [Heater configuration] FIG. 10 is a schematic diagram showing a cross section of the heater 54 shown in FIG. 9 when the heater 54 is cut along the center line (reference line a in FIG. 9) in the longitudinal direction of the recording material P conveyed to the fixing device 50. The fixing temperature sensor 59, which is a temperature detection means for detecting the temperature of the heater 54, is composed of the following members. That is, the fixing temperature sensor 59 is composed of a thermistor 59a, a holder 59b, a ceramic paper 59c that blocks heat conduction between the holder 59b and thermistor 59a, and an insulating resin sheet 59d that physically and electrically protects the thermistor 59a. The thermistor 59a is a temperature detection element whose resistance value changes according to the temperature of the heater 54 and whose output voltage changes, and is connected to the CPU 83 by a dumet wire (not shown) and wiring, and outputs a voltage according to the temperature of the heater 54 to the CPU 83. The CPU 83 controls the temperature of the heater 54 based on the temperature detection result of the fixing temperature sensor 59 (thermistor 59a). The fixing temperature sensor 59 is disposed on the surface of the substrate 54a opposite to the surface on which the heating elements 54b1, 54b2, and 54b3 covered with the protective glass layer 54e are disposed, and is in contact with the substrate 54a.

[0114] [Power control circuit] 11 is a schematic diagram showing the configuration of the fixing device 50 and the fixing control device 56. The fixing device 50 of this embodiment forms a desired temperature distribution in the longitudinal direction of the heater 54 by switching the heating element to which power is supplied depending on the size of the recording material P.

[0115] The fixing control device 56 of the fixing device 50 has triacs 57a and 57b, which are switching means for connecting or disconnecting the power supply path, a heating element switch 58, a triac state detection unit 55, and a relay 70 for cutting off the power supply to all the heating elements. The triacs 57a and 57b connect or disconnect the power supply path from the commercial AC power source 101 to each of the heating elements 54b1, 54b2, and 54b3. In this embodiment, the heating element switch 58 is configured by a C-contact relay (hereinafter referred to as relay 58). The triac state detection unit 55 monitors the on / off states of the triacs 57a and 57b.

[0116] The triac 57a (first switch) connects (ON state) or disconnects (OFF state) the power supply path between the commercial AC power supply 101 and the contact 54d4 of the heater 54. On the other hand, the triac 57b (second switch) connects (ON state) or disconnects (OFF state) the power supply path between the commercial AC power supply 101 and the contact 54d3 of the heater 54 via the relay 58, or between the commercial AC power supply 101 and the contact 54d1 of the heater 54. The relay 58 (first relay) is switchable so as to connect the contact 54d3 of the heater 54 to the commercial AC power supply 101 via the triac 57b or the relay 70.

[0117] For example, when power is supplied from the commercial AC power supply 101 to the heating element 54b1, the triac 57a is turned on to connect the commercial AC power supply 101 to the contact 54d4 of the heater 54, and the triac 57b is turned off. This connects the heating element 54b1 (54b1a, 54b1b) to the commercial AC power supply 101 via the contacts 54d2, 54d4 of the heater 54. When power is supplied from the commercial AC power supply 101 to the heating element 54b2, the triac 57b is turned on to connect the commercial AC power supply 101 to the relay 58, and the relay 58 is controlled to connect the contact 54d3 of the heater 54 to the triac 57b, and the triac 57a is turned off. As a result, one end of the heating element 54b2 is connected to the commercial AC power supply 101 via the contact 54d3 of the heater 54, the relay 58, and the triac 57b, and the other end of the heating element 54b2 is connected to the commercial AC power supply 101 via the contact 54d2 of the heater 54.

[0118] When power is supplied to heating element 54b3 from commercial AC power supply 101, triac 57b is turned on and relay 58 is controlled to connect contact 54d3 of heater 54 to commercial AC power supply 101 and triac 57a is turned off. As a result, one end of heating element 54b3 is connected to commercial AC power supply 101 via contact 54d3 of heater 54 and relay 58, and the other end of heating element 54b3 is connected to commercial AC power supply 101 via contact 54d1 of heater 54 and triac 57b. The on / off operations of triacs 57a and 57b are performed by commands (control signals) from CPU 83.

[0119] The triac state detection unit 55 detects the on / off state of the triacs 57a and 57b. For example, if the triacs 57a and 57b are simultaneously turned on due to an unexpected malfunction of the CPU 83, the triac state detection unit 55 sets the relay 70 to the off state and forcibly cuts off the power supply from the commercial AC power source 101 to the fixing device 50 (heater 54). This ensures that only one of the triacs 57a and 57b is turned on or both are turned off, preventing a malfunction of the fixing device 50.

[0120] In this way, triacs 57a, 57b, triac state detection unit 55, and relay 58 operate as a switching unit that switches the connection of the power supply path so that power is supplied from commercial AC power source 101 to one of three heating elements 54b1, 54b2, 54b3. In this embodiment, a switching unit with such a configuration is used, but it is only necessary that power can be supplied to only one of the heating elements, and the configuration for controlling the power supply path is not limited to the above-mentioned configuration.

[0121] Here, a recording material P having a width in the longitudinal direction shorter than that of the heating element 54b3 is called a small size paper, and a recording material P having a width in the longitudinal direction longer than that of the heating element 54b2 is called a large size paper. When printing on large size paper, only the heating element 54b1 is used for the fixing process. On the other hand, when printing on small size paper, the heating elements 54b1 and 54b3 are alternately used depending on the number of prints, from the viewpoint of deformation of the film 51. In this embodiment, the switching operation of the heating element 54b during continuous printing is performed, for example, in the case of continuous printing of small size paper.

[0122] [Method of controlling power supply to heating element] A method of controlling the power supply to the heating element in this embodiment will be described. In this embodiment, after power is supplied to the heating element 54b1 (first heating element) for a certain period of time, the state is switched to a state in which power can be supplied to the heating element 54b3 (second heating element) whose end in the longitudinal direction has a smaller amount of heat generation than the heating element 54b1. The power supply period to the heating element 54b1 at this time is defined as a first supply period. At the same time as the first supply period ends, power supply to the heating element 54b3 is started. Power is supplied to the heating element 54b3 until the amount of power supplied to the heating element 54b3 becomes a predetermined multiple of the amount of power supplied to the heating element 54b1. The power supply period to the heating element 54b3 at this time is defined as a second supply period. When the amount of power supplied to the heating element 54b3 reaches a predetermined multiple of the amount of power supplied to the heating element 54b1, the state is switched to a state in which power can be supplied to the heating element 54b1 again, and power is supplied to the heating element 54b1.

[0123] In this embodiment, in order to bring the temperature of the heating element 54b closer to the target temperature, power supply control is performed so that the detection result of the thermistor 59a, which is a temperature detection means, approaches the target temperature. As the power supply control to the heating element 54b, the power to be supplied to the heating element 54b is calculated at regular intervals according to the difference between the target temperature of the heating element 54b and the temperature detected by the thermistor 59a. Specifically, the CPU 83 calculates the amount of power required to make the temperature of the heater 54 reach the target temperature suitable for image formation on the recording material P based on the temperature information of the heater 54 detected by the thermistor 59a. The calculation of the amount of power is performed at regular intervals. In this embodiment, power is supplied to the heating element 54b by phase control of the AC power supply. In this embodiment, PI control is used to calculate the amount of power, and the unit period of the power calculation of the PI control is two half waves (one period of the power frequency of the commercial AC power supply 101). This means that when the power frequency of the commercial AC power supply 101 is 50 Hz, the power calculation control of the PI control is performed every 20 msec (milliseconds).

[0124] In PI control, the CPU 83 compares the temperature detected by the thermistor 59a with a target temperature value for each unit period, and controls the power supply based on the difference between the two temperatures. The CPU 83 determines a proportional term and an integral term according to the magnitude of the temperature difference between the two temperatures. The proportional term is an output value proportional to the magnitude of the temperature difference, and the integral term is an output value according to the integrated value of the temperature difference. The CPU 83 determines the power value to be supplied to the heating element based on the values ​​of the proportional term and the integral term. In this embodiment, a proportional term and an integral term are set for each heating element, and the amount of power for PI control is calculated using the values ​​of the proportional term and the integral term of the selected heating element.

[0125] Specifically, the PI control of this embodiment will be described. n is the timing (cycle) number for performing the PI control, and P n (unit:%), integral term is I n (Unit: %). Power duty D input to the heating element by PI control n (Unit: %) is expressed by the following (Equation 1) to (Equation 3). D n =P n +I n (100%≧P n +I n ≧0%) (Equation 1) D n =100 (P n +I n >100%) (Equation 2) D n =0 (0%>P n +I n (when ) (Equation 3)

[0126] Here, the power duty D n represents the input ratio of the amount of power that is determined by how much power is supplied to the AC voltage waveform of the commercial AC power source 101. n can take a value between 0% and 100% depending on the power supply pattern determined by phase control. The power supply pattern to the heating element by phase control is stored in advance in the memory 84, which is a storage unit. The CPU 83 determines the power duty D nIn response to the request, a corresponding power supply pattern is selected from the memory 84, and power is supplied to the heating element in accordance with the selected power supply pattern.

[0127] When printing on the recording material P is started, first, the initial value I of the output value of the integral term I operated by the integral control is 0 Table 6 shows the initial values ​​I of the integral terms I of the heating elements 54b1 and 54b3. 0 As shown in Table 6, the initial value I 0 is 32.5%, and the initial value I 0 is 50%.

[0128] [Table 6]

[0129] Next, the temperature of the heater 54 is detected by thermistor 59a every two half-wave periods of the voltage waveform, and the difference ΔT between the target temperature of the heater 54 and the temperature detected by thermistor 59a (=target temperature-temperature detected by thermistor 59a) is calculated. Table 7 shows the proportional terms P n In Table 7, the proportional terms P of the heating elements 54b1 and 54b3 for each 1°C change in the difference ΔT from -15°C to 15°C are shown. n For example, when the temperature difference ΔT between the two temperatures is −10° C., the proportional term P n is -27.5%, and the proportional term P n Similarly, when the temperature difference ΔT between the two temperatures is 5° C., the proportional term P n is 15%, and the proportional term P n is 22.5%.

[0130] [Table 7]

[0131] The CPU 83 also calculates an integrated value ΔT by integrating the temperature difference ΔT calculated over two half-wave periods. V is stored in the memory 84. The CPU 83 calculates the integral term I n is calculated using the following (Equation 4). I n =I n-1 +△I...(Formula 4)

[0132] For example, the integral term I 1 is the integral term I n The initial value I 0 Using I 1 =I 0 + △I. The value of △I is calculated by accumulating the difference △T. V Table 8 shows the integrated value △T V 8 is a table showing the value of ΔI of the heating elements 54b1 and 54b3 according to the temperature (unit: °C). As shown in Table 8, the integrated value ΔT V When the integrated value ΔT is greater than or equal to -400 and less than 400, the ΔI value of the heating elements 54b1 and 54b3 is 0%. V When the integrated value ΔT is 400 or more, the ΔI value of the heating element 54b1 is 5%, and the ΔI value of the heating element 54b3 is 10%. V If the value of ΔI is less than −400, the value of ΔI of the heating element 54b1 will be −5%, and the value of ΔI of the heating element 54b3 will be −10%.

[0133] [Table 8]

[0134] By the above process, the proportional term P n , integral term I n The value of the proportional term P is determined. n , integral term I n The power duty D n is determined, and the determined power duty D n In this embodiment, power according to the above-mentioned power duty D nThe determination of is performed over two half-wave periods, and the determined power duty D n The power based on the power duty D is supplied for the next two half-wave periods. n The power according to the power duty D is supplied to the heating element selected by the switching unit. In this embodiment, the timing for switching the heating element to which power is supplied is the timing for updating the PI control, that is, the power duty D n This is in line with the timing of updates.

[0135] [Power supply control to heating element] In this embodiment, the power supply to the heating element 54b1 is controlled so that the average value of the maximum current value per control cycle in the first supply period of the heating element 54b1 with a small resistance does not exceed the maximum current value in the second supply period of the heating element 54b3 with a large resistance. A specific control method will be described with reference to FIG.

[0136] In FIG. 12, a graph is shown on the upper side, and a table corresponding to the graph is shown on the lower side. The graph in FIG. 12 shows the amount of current flowing through the heating elements 54b1 and 54b3, with the horizontal axis showing time and the vertical axis showing current value. The graph shown with a dashed line shows the movement of the current value when the supply power rate (also called power duty) is 100%, and the graph shown with a solid line shows the movement of the current value when controlled at a desired amount of supply power. In addition, the table below the graph in FIG. 12 shows the items of timing n, supply power rate (power duty), maximum current value, name of period, selected heating element to be supplied with power, and current value to be compared, which correspond to the time on the horizontal axis of the graph. During continuous printing of small size paper, the heating elements 54b1 and 54b3 are heated by alternately switching between the heating elements 54b1 and 54b3 as shown in FIG. 12. The switching timing of the heating elements can be variably controlled depending on the warming state of the components of the fixing device 50.

[0137] At timing 1 (n=1) which is the first control cycle of the second supply period in which power is supplied to the heating element 54b3 in FIG. 12, power is supplied to the heating element 54b3 at a supply power rate (duty) of 70%, and the maximum current value at timing 1 is 7.1 A (amperes). This can be calculated from the fact that the resistance value of the heating element 54b3 is 24Ω and the effective voltage of the commercial AC power supply 101 is 120V. In this embodiment, the supply power rate (power duty) is controlled by the on (ON) state and off (OFF) state of the triacs 57a and 57b. Once the triacs 57a and 57b are set to the on state, the on state is maintained until the AC voltage supplied from the commercial AC power supply 101 becomes 0V (zero cross). Therefore, the supply power rate to the heating element is controlled by the waveform shown in FIG. 12. That is, the supply power rate is controlled by setting the latter half of one half wave until the AC voltage becomes zero cross to the on state.

[0138] At the next control period, timing 2 (n=2), the supply power rate (duty) is 40%, and the maximum current value at timing 2 is 5.7A. In this way, after power is supplied to the heating element 54b3 for four unit periods (n=1 to 4), the power supply destination is switched to the heating element 54b1. Here, the maximum current value in the second supply period of the heating element 54b3 is 7.1A. In this embodiment, when determining the supply power rate (duty) in the first supply period to the next heating element 54b1, a limit is set for the supply power rate derived by PI control so that a large current fluctuation does not occur between the first supply period and the second supply period. That is, at timing 5 (n=5), which is the first control period of the first supply period, power is supplied to the heating element 54b1 at a supply power rate (duty) of 40%, and the maximum current value at timing 5 is 7.2A. At timing 6 (n=6) in the next control cycle, power is supplied at a supply power rate (duty) of 30%, and the maximum current value at timing 6 is 6.8 A. When the average maximum current value within the first supply period (n=5, 6) is calculated, it is 7.0 A, which is lower than the maximum current value of 7.1 A in the immediately preceding second supply period (n=1 to 4).

[0139] [Example of power supply control to heating element] Next, we will explain the control of the power supply to the heating elements when actually printing on the recording material P. Table 9 corresponds to the above-mentioned FIG. 12 and explains how the amount of power supplied to the heating elements is determined over time when printing on the recording material P.

[0140] [Table 9]

[0141] Table 9 is composed of the following items from the top: Timing n (corresponding to timing n in FIG. 12), time (unit: sec (seconds)), target temperature (unit: °C), thermistor detection value (unit: °C), difference △T (unit: °C), integrated value △T V (unit: °C). Furthermore, Table 9 is composed of the selected heating element to which power is supplied, the power calculation results of heating elements 54b1 and 54b3 (proportional term P, integral term I, Duty (power duty)), the power duty under PI control, the actual power duty, and the maximum current value.

[0142] For example, when the timing n is 5 (when the time is 0.100 sec), the thermistor detection value indicating the temperature of the heater 54 detected by the thermistor 59a is 218° C., and the difference ΔT from the target temperature of 220° C. is 2° C. (=220° C.-218° C.). V is 399°C (=397°C + 2°C). For the power calculation of the heating element 54b1, the proportional term P is 7.5% when the difference △T is 2°C according to Table 7, and the integral term I is △I is 0% according to Table 8. Therefore, from (Equation 4), I 5 =I 0 + ΔI = 32.5%. As a result, the power duty is 40% (= 7.5% + 32.5%). Similarly, in the power calculation of the heating element 54b3, the proportional term P is 10.0% when the difference ΔT is 2°C according to Table 7, and the integral term I is ΔI = 0% according to Table 8, so I is calculated from (Equation 4). 5 =I 4 (=I 0) + ΔI = 50%. As a result, the power duty becomes 60.0% (= 10.0% + 50%). Since the selected heating element when timing n is 5 is 54b1, the power duty actually supplied to heater 54 becomes 40%, and the maximum current value at this time is 7.2A.

[0143] Next, when the timing n is 6 (when the time is 0.120 seconds), the thermistor detection value indicating the temperature of the heater 54 detected by the thermistor 59a is 218°C, and the difference ΔT from the target temperature of 220°C is 2°C (=220°C-218°C). V However, according to Table 8 above, the integrated value △T V When the temperature is 400° C. or higher, ΔI is not 0%, but is 5% in the case of the heating element 54b1 and 10% in the case of the heating element 54b3. V is reset once the integrated value exceeds 400°C. Therefore, the integrated value △T V is 1°C (=401°C - 400°C). For the power calculation of the heating element 54b1, the proportional term P is 7.5% when the difference △T is 2°C according to Table 7, and the integral term I is △I is 5% according to Table 8. Therefore, I is calculated from (Equation 4). 6 =I 5 +ΔI=32.5%+5%=37.5%. As a result, the power duty is 45% (=7.5%+37.5%). Similarly, for the power calculation of the heating element 54b3, the proportional term P is 10% when the difference ΔT is 2°C according to Table 7, and the integral term I is ΔI=10% according to Table 3, so I is calculated from equation 4. 6 =I 5+ΔI=50%+10%=60%. As a result, the power duty is 70% (=10%+60%). When the timing n is 6, the selected heating element is 54b1, so the power duty actually supplied to the heater 54 is 45%. When the timing n is 6, the switching unit selects the heating element 54b1 as the heating element to which power is supplied, and the average of the maximum current values ​​at timings n=5 and n=6 exceeds 7.1A, which is the maximum current value at timings n=1 to 4 (second supply period). Therefore, the duty actually supplied to the heating element 54b3 is limited to 30%. This makes it possible to suppress the average of the maximum current values ​​at timings n=5 and n=6 to 7.0A (=(7.2A+6.8A) / 2), which is smaller than 7.1A.

[0144] [Controlling the amount of power supplied to a heating element with a low resistance] In this embodiment, the power supply to the heating element 54b1 is controlled so that the average value of the maximum current value per control cycle in the first supply period during which power is supplied to the heating element 54b1 with a small resistance value does not exceed the maximum current value in the second supply period of the heating element 54b3 with a large resistance value. Fig. 13 is a flowchart showing a control sequence for controlling the amount of power supplied to the heating element 54b1 with a small resistance value. The process of Fig. 13 is started when a print job for printing on the recording material P is started, and is executed by the CPU 83 of the engine controller 82.

[0145] In step (hereinafter, S) 100, the CPU 83 performs a PI calculation (calculation of the amount of power by PI control) of the power supply rate (power duty) to the selected heating element. In S101, the CPU 83 determines whether it is time to switch the heating element to be supplied with power from heating element 54b1 to heating element 54b3 next time. If the CPU 83 determines that it is time to switch from heating element 54b1 with a low resistance value to heating element 54b3 with a high resistance value next time, the CPU 83 advances the process to S102. On the other hand, if the CPU 83 determines that it is not time to switch from heating element 54b1 with a low resistance value to heating element 54b3 with a high resistance value next time, the CPU 83 advances the process to S104.

[0146] In S102, the CPU 83 determines whether the average maximum current value of the heating element 54b1 when power is supplied according to the PI calculation calculated in S100 is equal to or less than the maximum current value of the heating element 54b3. If the CPU 83 determines that the average maximum current value of the heating element 54b1 is equal to or less than the maximum current value of the heating element 54b3, the process proceeds to S104. On the other hand, if the CPU 83 determines that the average maximum current value of the heating element 54b1 is not equal to or less than the maximum current value of the heating element 54b3 (the average maximum current value of the heating element 54b1 is greater than the maximum current value of the heating element 54b3), the process proceeds to S103.

[0147] In S103, the CPU 83 sets the power duty to the heating element 54b1 to a reduced power duty value in order to limit the power supplied to the heating element 54b1 so that the average maximum current value of the heating element 54b1 is equal to or less than the maximum current value of the heating element 54b3.

[0148] In S104, the CPU 83 determines the amount of power to be supplied to the selected heating element. In S105, the CPU 83 controls the fixing control device 56 according to the amount of power to be supplied determined in S104, and supplies power to the selected heating element. In S106, the CPU 83 determines whether the print job has ended. If the CPU 83 determines that the print job has ended, the process ends, and if the CPU 83 determines that the print job has not ended, the process returns to S100.

[0149] The limit value of the power supply to the heating element 54b1 may be obtained by calculation, or a table of possible limit values ​​may be stored in advance in the memory 84 and referenced when necessary to determine the limit value. Here, a method of obtaining the limit value by calculation will be described according to the present embodiment.

[0150] In the second supply period, the maximum current value flowing through the heating element 54b3 with the large resistance is I_max, and the maximum current value that can be supplied to the heating element 54b1 with the small resistance is I_max_small. In addition, at the timing n=5 in the first supply period shown in Fig. 12, the maximum current value flowing through the heating element 54b1 with the small resistance is I_1, and the duty at the timing n=5 is Duty_1. In addition, at the timing n=6 in the first supply period shown in Fig. 12, the maximum current value flowing through the heating element 54b1 with the small resistance is I_2, and the duty at the timing n=6 is Duty_2. In addition, the resistance value of the heating element 54b1 is R, and the effective voltage of the commercial AC power supply 101 is V.

[0151] The average value I_Avg_large of the maximum current value for each control cycle in the first supply period supplied to the heating element 54b1 having the small resistance value is expressed by the following (Equation 5). I_Avg_large=(I_1+I_2) / 2···(Formula 5)

[0152] In this embodiment, the power supply to the heating element is controlled so as to satisfy the following formula (6). I_max≧I_Avg_large=(I_1+I_2) / 2 (Formula 6)

[0153] If the current waveform of the commercial AC power supply 101 is a sine wave, Duty_1 supplied to the heating element 54b1 with a small resistance value is expressed as shown in Fig. 14. In Fig. 14, the area of ​​the region surrounded by the solid line and the x-axis is Duty_1, and the area surrounded by the dashed dotted line and the x-axis is drawn so as to be equal to the area of ​​the region surrounded by the solid line and the x-axis. Note that the vertical axis of Fig. 14 indicates the current value, and the horizontal axis indicates the time.

[0154] In FIG. 14, Duty_1 is equal to the integral of sin(x) from 0 (zero) to a desired time t, and the maximum current value I_1 at that time can be expressed as sin(t)×I_max_small. Then, cos(t)=1−Duty_1, and sin 2 (t)+cos 2Using the equation (t)=1, the maximum current value I_1 can be expressed by the following (Equation 7). I_1=sin(t)×I_max_small =(√(1-cos 2 (t)))×I_max_small =(√(1-(1-Duty_1) 2 )×I_max_small =(√(Duty_1×(2-Duty_1)))×I_max_small ...(Formula 7)

[0155] Similarly, the maximum current value I_2 can be expressed by the following (Equation 8). I_1=(√(Duty_2×(2-Duty_2)))×I_max_small ...(Formula 8)

[0156] As a result, the duties Duty_1 and Duty_2 can be determined so that the average of the maximum current value I_1 shown in (Equation 7) and the maximum current value I_2 shown in (Equation 8) does not exceed the maximum current value I_max flowing through the heating element 54b3. This makes it possible to control so that the average of the maximum current value per control cycle in the first supply period supplied to the heating element 54b1 with a small resistance does not exceed the maximum current value in the second supply period of the heating element 54b3 with a large resistance.

[0157] In this manner, in this embodiment, the average value of the maximum current flowing through the heating element 54b1 is limited while changing the magnitude of the power supply to the heating element by PI control. Specifically, the control and limitation of the power supplied to the heating element is performed by changing the power duty ratio that controls the power supply by phase control, thereby adjusting the temperature of the fixing device 50.

[0158] In addition, in this embodiment, PI control with two half-wave cycles is used, but the update cycle and control method of PI control are not limited to this. In addition, in this embodiment, a method of changing the on / off ratio of power supply by phase control is used, but the power limiting method is not limited to this. For example, the amount of power supply may be changed by providing a current limiting circuit and limiting the amplitude of the current supplied from the commercial AC power supply 101.

[0159] Table 10 shows the maximum current value of the heating element 54b3 and the maximum current value of the heating element 54b1 when the control to suppress the current value of the heating element 54b1 of the present embodiment described above is performed and when it is not performed. As shown in Table 10, when the control to suppress the current value of the heating element 54b1 is performed, the maximum current value of the heating element 54b3 is 7.1 A, and the maximum current value of the heating element 54b1 is 7.2 A. On the other hand, when the control to suppress the current value of the heating element 54b1 is not performed, the maximum current value of the heating element 54b3 is 7.1 A, and the maximum current value of the heating element 54b1 is 12 A. In this way, when the control of the present embodiment described above is used, the change in the current value when the heating element is switched is small, and when it is not used, the change in the current value when the heating element is switched is large, and it can be seen that the level of flicker is improved compared to when the control is not performed.

[0160] [Table 10]

[0161] As described above, according to this embodiment, it is possible to suppress flicker that occurs when the heating elements to which power is supplied are switched. [Explanation of symbols]

[0162] 101 Commercial AC power supply 108 Heater control circuit 150 Power supply 201 Heater 203 Heating element 205 Heating element

Claims

1. A heating rotor; a heater having a first heating element and a second heating element having a resistance value greater than that of the first heating element, the heater being disposed in an internal space of the heating rotator and configured to heat the heating rotator; a pressure rotating body that forms a nip portion with the heating rotating body; A detection means for detecting the temperature of the heater; A switching means for switching a power supply path from an AC power source to the first heating element or the second heating element; A control means for controlling the switching means to supply power to the first heating element or the second heating element; a power supply device that generates a DC voltage from the AC voltage supplied from the AC power supply; Equipped with the first heating element, the second heating element, and the power supply device are connected in parallel to the AC power supply; the control means controls power supply by phase control to the first heating element in a first period including a plurality of unit cycles and to the second heating element in a second period including a plurality of unit cycles based on the temperature of the heater detected by the detection means and a target temperature of the heater; the power supply path is switched by the switching means to switch from the first period to the second period or from the second period to the first period; A first current is supplied from the AC power source to the first heating element, A second current is supplied from the AC power source to the second heating element; a third current is supplied from the AC power source to the power supply device; In the first period, a combined current of the first current and the third current in the unit cycle is a current in a case where a timing at which the first current is supplied does not overlap a timing at which the third current has a maximum value and a minimum value in a first unit cycle of the first period that is continuous with the second period, a current having only the unit period in which the timing at which the first current is supplied does not overlap with the timing at which the third current has a maximum value and a minimum value, Or, a unit period in which the timing at which the first current is supplied does not overlap with the timing of the maximum and minimum values ​​of the third current, and a unit period in which the timing at which the first current is supplied overlaps with the timing of the maximum and minimum values ​​of the third current, and a plurality of unit periods in which the timing at which the first current is supplied does not overlap with the timing of the maximum and minimum values ​​of the third current, and a unit period in which the timing at which the first current is supplied overlaps with the timing of the maximum and minimum values ​​of the third current, an image forming apparatus characterized in that, during the second period, a composite current of the second current and the third current in the unit cycle is a current in which the timing at which the second current is being supplied overlaps with the timing at which the third current has a maximum value and a minimum value.

2. the power supply device includes a rectifier circuit that rectifies an AC voltage supplied from the AC power supply, and a smoothing circuit that smoothes the rectified AC voltage, 2. The image forming apparatus according to claim 1, wherein the third current is a current for charging a capacitor of the smoothing circuit.

3. The first heating element has a length in a longitudinal direction longer than that of the second heating element, the heater further includes a third heating element having a length in the longitudinal direction shorter than that of the first heating element and longer than that of the second heating element; The first heating element is a pair of heating elements having substantially the same length in a longitudinal direction, 3. The image forming apparatus according to claim 1, wherein the first heating element, the second heating element, the third heating element, and the first heating element are arranged in this order in the short side direction of a substrate having the heater.

4. The heater is a first contact point at which one end of the first heating element and one end of the third heating element are electrically connected; a second contact to which one end of the second heating element is electrically connected; a third contact at which the other end of the second heating element and the other end of the third heating element are electrically connected; a fourth contact to which the other end of the first heating element is electrically connected; and 4. The image forming apparatus according to claim 3, further comprising:

5. the switching means includes a first switch, a second switch, and a first relay; the first switch connects or disconnects the AC power source and the fourth contact; the second switch connects or disconnects the AC power supply to or from the first relay and the AC power supply to or from the second contact; 5. The image forming apparatus according to claim 4, wherein the first relay is capable of switching between a connection between the second switch and the third contact, or a connection between the AC power source and the third contact.

6. 6. The image forming apparatus according to claim 5, wherein the first switch and the second switch are bidirectional thyristors.

7. The heating rotatable body is a cylindrical film, the pressure rotating body is a pressure roller that forms the nip portion with the film, 7. An image forming apparatus according to claim 1, wherein the heater is disposed in an internal space of the film, the film is clamped between the heater and the pressure roller, and an image on a recording material is heated through the film at the nip portion.

Images