Image forming apparatus

Abstract

To provide an image forming apparatus that can quickly cut off energization to heating means at the occurrence of abnormality accompanying a sudden increase in temperature.SOLUTION: An image forming apparatus has: a fixing member; heating means; a power supply; temperature detection means that outputs output voltage according to a temperature of the fixing member or the heating means; control means that controls supply of power from the power supply to the heating means; reference voltage generation means that generates reference voltage; and cut-off means that, when a temperature corresponding to the output voltage from the temperature detection means becomes a higher temperature than a temperature corresponding to the reference voltage, cuts off the supply of power to the heating means. The reference voltage generation means outputs the reference voltage whose value changes according to the time elapsed from the start of heating so that, compared with a temperature corresponding to a value of the reference voltage at the time point when a first time is elapsed from the start of heating, a temperature corresponding to a value of the reference voltage at the time point when a second time longer than the first time is elapsed from the start of heating becomes high temperature.SELECTED DRAWING: Figure 9

Description

Technical Field

[0001] The present invention relates to an image forming apparatus including a fixing device for fixing a toner image on a recording material.

Background Art

[0002] Electrophotographic image forming apparatuses such as printers, copiers, and multifunction peripherals are provided with a heat fixing device that heats and fixes a toner image formed on a recording material such as paper. The heat fixing device of the heat fixing method includes a fixing member (heating member) that contacts the recording material and a heating means that heats the fixing member. The heat fixing method includes a film heating method in which a cylindrical film is heated by a ceramic heater that contacts the inner surface of the film, an induction heating method in which a cylindrical rotating body provided with a conductive layer is heated by induction heating, and a heat roller method in which a hollow roller is heated by radiant heat such as a halogen lamp. Patent Document 1 describes a film heating type fixing device, and Patent Document 2 describes an induction heating type fixing device.

[0003] In the heat fixing device of the heat fixing method, when continuously fixing a small-sized recording material, the fixing member may be heated at an end portion where the recording material does not pass, such as an end temperature rise, or the amount of electricity supplied to the heating means may become uncontrollable due to a failure of the drive circuit or the like, resulting in a overheated state. Therefore, in this type of fixing device, a safety device that detects an abnormal overheated state and cuts off the power supply to the heating means is provided to avoid problems caused by the overheated state. Patent Document 3 describes that by switching the threshold temperature for detecting abnormal heating by a thermistor according to the current amount of the heater, malfunction of the safety device due to the end temperature rise during normal operation is prevented, and the power supply to the heater is cut off at an early stage during a power runaway to prevent overheating.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

[0005] In the configuration of Patent Document 3, in a configuration for determining an abnormal heating state by comparing a detection signal (voltage) of a thermistor with a reference voltage representing a threshold temperature for detecting abnormal heating using a comparator, the circuit is configured such that the reference voltage changes according to the current amount of the heater. Therefore, a delay time occurs from when the current amount of the heater changes until the circuit state switches and the value of the reference voltage changes. However, depending on the cause of abnormal heating in the fixing device, it is conceivable that the temperature of the heater rises rapidly. In the configuration described in the above document, when an abnormality occurs in which the temperature of the heater rises rapidly, the temperature rise continues during the delay time required for switching the circuit state. Therefore, when an abnormality accompanied by a rapid temperature rise occurs, it has been desired to more quickly cut off the power supply to the heating means.

[0006] Therefore, the present invention provides an image forming apparatus capable of quickly cutting off the power supply to the heating means when an abnormality accompanied by a rapid temperature rise occurs. [Means for Solving the Problems]

[0007] One aspect of the present invention includes a fixing member that heats a toner image transferred onto a recording material to fix it onto the recording material, a heating means that heats the fixing member when energized, a power source that supplies power to the heating means, a temperature detection means that outputs an output voltage corresponding to the temperature of the fixing member or the heating means, a control means that controls the power supply from the power source to the heating means so that the temperature of the fixing member becomes a predetermined set temperature based on the output voltage of the temperature detection means, a reference voltage generation means that generates a reference voltage, and a cutoff means that cuts off the power supply to the heating means when the temperature corresponding to the output voltage of the temperature detection means becomes higher than the temperature corresponding to the reference voltage. The image forming apparatus is characterized in that the reference voltage generation means outputs a reference voltage whose value changes according to the passage of time from the start of heating, such that the temperature corresponding to the value of the reference voltage at a point in time when a second time longer than the first time has elapsed from the start of heating is higher than the temperature corresponding to the value of the reference voltage at a point in time when the first time has elapsed from the start of heating, at which time the control means starts supplying power to the heating means. Moreover, the reference voltage generating means is configured to linearly change the value of the reference voltage from the low temperature side to the high temperature side with respect to the elapsed time since the start of heating. An image forming apparatus characterized by the above. Another aspect of the present invention is an image forming apparatus having a fixing member that heats a toner image transferred onto a recording material to fix it onto the recording material, a heating means that heats the fixing member when energized, a power source that supplies power to the heating means, a temperature detection means that outputs an output voltage corresponding to the temperature of the fixing member or the heating means, a control means that controls the power supply from the power source to the heating means so that the temperature of the fixing member becomes a predetermined set temperature based on the output voltage of the temperature detection means, a reference voltage generating means that generates a reference voltage, and a cutoff means that cuts off the power supply to the heating means when the temperature corresponding to the output voltage of the temperature detection means becomes higher than the temperature corresponding to the reference voltage. The reference voltage generating means outputs the reference voltage whose value changes according to the passage of time from the start of heating such that the temperature corresponding to the value of the reference voltage at the time when a second time longer than the first time has elapsed from the start of heating is higher than the temperature corresponding to the value of the reference voltage at the time when the first time has elapsed from the start of heating, and the reference voltage generating means is configured to change the value of the reference voltage such that the reference voltage draws a curve convex to the high temperature side or a curve convex to the low temperature side with respect to the elapsed time from the start of heating. Another aspect of the present invention is a fixing member that heats a toner image transferred onto a recording material to fix it onto the recording material, a heating means that heats the fixing member when energized, a power source that supplies power to the heating means, a temperature detection means that outputs an output voltage corresponding to the temperature of the fixing member or the heating means, a control means that controls the power supply from the power source to the heating means so that the temperature of the fixing member becomes a predetermined set temperature based on the output voltage of the temperature detection means, a reference voltage generation means that generates a reference voltage, a cutoff means that cuts off the power supply to the heating means when the temperature corresponding to the output voltage of the temperature detection means becomes higher than the temperature corresponding to the reference voltage, and an initial voltage holding means that holds the output voltage of the temperature detection means at the start of heating when the control means starts supplying power to the heating means and delivers a voltage based on the output voltage to the reference voltage generation means. The image forming apparatus has a reference voltage generation means that outputs a reference voltage whose value changes according to the passage of time from the start of heating such that the temperature corresponding to the value of the reference voltage at the time when a second time longer than the first time has elapsed from the start of heating is higher than the temperature corresponding to the value of the reference voltage at the time when the first time has elapsed from the start of heating, and the value of the reference voltage generated by the reference voltage generation means changes according to the voltage delivered from the initial voltage holding means.

Effect of the Invention

[0008] According to the present invention, when an abnormality accompanied by a rapid temperature rise occurs, the energization of the heating means can be quickly cut off.

Brief Description of the Drawings

[0009]

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Embodiments for Carrying Out the Invention

[0010] Hereinafter, embodiments according to the present disclosure will be described with reference to the drawings.

[0011] In each of the following examples, a configuration example of a fixing device mounted on an electrophotographic image forming apparatus will be described. Therefore, first, an example of an image forming apparatus including a fixing device will be described using the schematic diagram shown in FIG. 26. The image forming apparatus 80 is an electrophotographic monochrome laser beam printer that directly transfers the toner image on the photosensitive drum 81 onto the recording material P. The photosensitive drum 81, which is an image carrier, is an electrophotographic photoreceptor formed in a drum shape (cylindrical shape). Around the photosensitive drum 81, in order along the rotation direction (clockwise direction in the figure), a charger 82, an exposure device 83, a developing device 85, a transfer roller 87, and a drum cleaner 88 are arranged. The image forming unit (process unit) including the photosensitive drum 81, the charger 82, the exposure device 83, the developing device 85, the transfer roller 87, and the drum cleaner 88 constitutes a toner image forming means for forming a toner image on the recording material.

[0012] The flow of the image forming operation (printing operation) by the image forming apparatus 80 will be described. When image information and an execution instruction for image formation (print instruction) are input to the image forming apparatus 80, the photosensitive drum 81 is rotationally driven in the direction of the arrow in the figure, and the surface of the photosensitive drum 81 is charged to a predetermined polarity by the charger 82. Next, the charged surface of the photosensitive drum 81 is irradiated with the laser beam L modulated based on the image information by the exposure device 83 to form an electrostatic latent image on the surface of the photosensitive drum 81. The developer containing the charged toner is carried on the developing roller of the developing device 85 and then adheres to the photosensitive drum 81 according to the distribution of the surface potential of the photosensitive drum 81. Thereby, the electrostatic latent image on the photosensitive drum 81 is developed and visualized as a toner image.

[0013] In parallel with the above process, the recording material P is fed one by one by the feeding roller 84 and conveyed by the conveying roller 86 toward the transfer nip Ntr. The transfer nip Ntr, which is a transfer section where the toner image is transferred, is a nip section formed between the photosensitive drum 81 and the transfer roller 87. By applying a voltage with a polarity opposite to the normal charging polarity of the toner to the transfer roller 87 from a power source (not shown), the toner image on the photosensitive drum 81 is transferred onto the recording material P at the transfer nip Ntr. The surface of the photosensitive drum 81 that has passed through the transfer nip Ntr is cleaned by a drum cleaner 88 having a cleaning member such as an elastic blade that contacts the surface of the photosensitive drum 81, and deposits such as residual transferred toner are removed.

[0014] The recording material P that has passed through the transfer nip Ntr and is carrying the unfixed toner image is conveyed to the fixing device 1, undergoes heat fixing processing by the fixing device 1, and is then discharged to the outside of the image forming apparatus 80 as a finished product by the discharge roller pair. As the fixing device 1, a known heat fixing type fixing device such as a film heating type or an induction heating type specifically described in the following embodiments can be used.

[0015] Here, an image forming unit of a direct transfer method in which the toner image is directly transferred from the image carrier to the recording material has been exemplified. However, an intermediate transfer method in which the toner image first transferred from the image carrier to an intermediate transfer body such as an intermediate transfer belt is secondarily transferred from the intermediate transfer body to the recording material may also be used. Further, an image forming unit that includes a plurality of image carriers and forms a full-color toner image by overlapping toner images formed using a plurality of colors of toner on the recording material may be used as the toner image forming means. The "image forming apparatus" is not limited to a printer that forms an image on a recording material based on image information input from the outside, but may also be a copying machine that forms an image on a recording material based on image information read from a document, or a multifunction machine having a plurality of functions.

Embodiment

[0016] FIG. 1 is a cross-sectional view of a fixing device 1 according to Embodiment 1. The fixing device 1 includes a fixing film 3 as a flexible cylindrical film member, a heater 4 that contacts the inner surface of the fixing film 3, a pressure roller 8 as an opposing member that opposes the heater 4 via the fixing film 3, and a metal stay 5. Hereinafter, the direction in which the recording material P is conveyed through the fixing nip N is defined as the recording material conveyance direction, and the longitudinal direction of the fixing nip N (the direction perpendicular to the recording material conveyance direction, the rotation axis direction of the pressure roller 8) is defined as the longitudinal direction of the fixing device 1.

[0017] The fixing device 1 is a film heating type thermal fixing device excellent in quick start performance (short warm-up time). In particular, here, as the heater 4, a ceramic heater having an integral structure in which a heating element (heating resistor) that generates heat by energization is embedded in an insulating substrate made of aluminum oxide (Al2O3) or aluminum nitride (AlN) is used. That is, a heater formed in a plate shape in such a manner that a heating resistor formed in a predetermined pattern is buried in a ceramic material having insulating properties is used. However, the heater 4 may be a heater having another configuration (for example, one in which an insulating layer such as glass is formed on a metal substrate and a heating resistor is formed thereon).

[0018] The fixing film 3 is a multilayer heat-resistant film formed in a cylindrical shape (endless endless belt), and has a base layer and a release layer formed on the base layer (surface side). The base layer is formed of a conductive resin obtained by adding conductive fine particles such as carbon black to a heat-resistant resin such as polyimide, polyamideimide, or PEEK (polyetheretherketone), or a conductive metal using a pure metal such as stainless steel, Al, Ni, Cu, Zn, etc. having heat resistance and high thermal conductivity, or an alloy. The release layer is formed by coating a heat-resistant resin excellent in releasability such as tetrafluoroethylene·perfluoroalkyl vinyl ether copolymer (PFA) on the base layer in order to prevent toner adhesion and ensure separability from the recording material P.

[0019] The pressure roller 8 has a core metal 9 made of a material such as iron or aluminum, and an elastic layer 10 formed on the outer periphery of the core metal 9 with a material such as silicone rubber. The heater 4 is held by a heater holding member 2 made of a heat-resistant resin and heats the fixing film 3. The heater holding member 2 also has a guide function for guiding the rotation of the fixing film 3. The metal stay 5 receives a pressing force from a pressing spring (not shown) and biases the heater holding member 2 toward the pressure roller 8. A fixing nip N is formed as a region where the pressure roller 8 sandwiches the fixing film 3 and is pressed against the heater 4. The heater 4 and the heater holding member 2 are disposed inside the fixing film 101 and form a fixing nip N as a nip portion together with the pressure roller 8. The pressure roller 8 receives power from a motor (not shown) and rotates in the direction of arrow R1. As the pressure roller 8 rotates, the fixing film 3 is driven to rotate in the direction of arrow R2.

[0020] The fixing device 1 rotates the pressure roller 8 while energizing the heater 4, sandwiches and conveys the recording material P between the fixing film 3 and the pressure roller 8 at the fixing nip N, and fixes the toner image on the recording material P by the fixing film 3 heated by the non-radiative heat of the heater 4. That is, the fixing film 3 is the fixing member of this embodiment, and the heater 4 is the heating means of this embodiment.

[0021] A temperature detection element 6, which is an example of temperature detection means, is in contact with the heater 4. The temperature detection element 6 is configured to output, as a detection signal, a voltage value corresponding to the temperature of the heater 4, for example. Regarding the energization control of the heater 4, the control means 11 (FIG. 3), which will be described later, controls the duty ratio and frequency of the AC voltage applied to the heater 4 according to the output of the temperature detection element 6 to maintain the temperature in the fixing nip N at a predetermined set temperature (temperature control temperature) suitable for image fixing. Further, a safety element 7 is also in contact with the heater 4. The safety element 7 refers to a thermoswitch, a temperature fuse, or the like that operates due to abnormal heating of the heater 4 and cuts off the power supply to the heater 4.

[0022] As shown in Fig. 2, the temperature detection element 6 has a characteristic that the output voltage decreases as the detected temperature (the surface temperature of the heater 4 with which the element is in contact) increases (monotonically decreases). That is, the output voltage when the detected temperature is the second temperature (e.g., 200°C) higher than the first temperature is lower than the output voltage when the detected temperature is the first temperature (e.g., 100°C). The control described below is generally applicable to a temperature detection member having a characteristic that the output voltage decreases as the detected temperature increases. In this embodiment, a thermistor is used as the temperature detection element 6.

[0023] (Simple Explanation of Overheating Prevention Means) Next, the operation of the overheating prevention means of this embodiment will be described with reference to the block diagram of Fig. 3 and the flowchart of the overheating prevention means of Fig. 4. As shown in Fig. 3, the image forming apparatus according to this embodiment includes an electric circuit including a power supply 12, a control means 11, an overheating prevention means, and a cutoff means 13 as a configuration related to the electrical control of the fixing device 1. The control means 11 includes a storage device such as a ROM that stores a program, and a CPU that reads and executes the program from the storage device. The overheating prevention means includes an energization signal generation means 15, an initial voltage holding means 16, a reference voltage generation means 17, and a comparison means 18.

[0024] When a print instruction is input to the image forming apparatus, a print start instruction is output by the control means 11. Then, the pressure roller 8 is rotationally driven by a motor (not shown), and the fixing film 3 rotates following the pressure roller 8. Also, energization to the heater 4 is started from the power supply 12, and the heater 4 generates heat. The temperature of the heater 4 is detected by the temperature detection element 6, and a detection signal corresponding to the temperature of the heater 4 is input from the temperature detection element 6 to the control means 11. The control means 11 controls the energization to the heater 4 by sending an energization control signal to the power supply 12 so that the heater 4 reaches a predetermined set temperature.

[0025] The energization control signal of the control means 11 is also input to the energization signal generation means 15. When the output (energization signal) of the energization signal generation means 15 is input to the initial voltage holding means 16, the output voltage of the temperature detection element 6 is held by the initial voltage holding means 16. Further, when the output of the energization signal generation means 15 is input to the reference voltage generation means 17, the voltage value held by the initial voltage holding means 16 is delivered to the reference voltage generation means 17. The reference voltage generation means 17 generates a reference voltage that changes with the passage of time, using the voltage value delivered from the initial voltage holding means 16 as an initial value. The output voltage of the temperature detection element 6 and the output voltage (reference voltage) of the reference voltage generation means 17 are compared by the comparison means 18. When the output of the temperature detection element 6 becomes higher than the output of the reference voltage generation means 17, the comparison means 18 outputs a control signal to the cutoff means 13. The cutoff means 13 that has received the control signal cuts off the energization of the heater 4 by the power supply 12 regardless of the energization control signal of the control means 11.

[0026] The operating conditions will be further described with reference to the flowchart of FIG. 4. During the period when the energization signal is OFF (S19: OFF), the initial voltage holding means 16 captures the output voltage Vth of the temperature detection element 6 before the start of printing at, for example, predetermined time intervals (S20). When the energization signal changes to ON (S19: ON), the initial voltage holding means 16 holds the output voltage Vth of the temperature detection element 6 immediately before the start of printing as the initial voltage Vpre, and the reference voltage generation means 17 captures the voltage corresponding to the initial voltage Vpre (S21).

[0027] Thereafter, the reference voltage generating means 17 generates (produces) a reference voltage that changes with the passage of time from the initial value corresponding to the initial voltage Vpre (S22). The reference voltage Vref generated by the reference voltage generating means 17 is a function of time in which the value changes with respect to time, and there are a first region (time region, time window) in which the value changes temporally and a second region (time region, time window) in which the value becomes a fixed value. The reference voltage generating means 17 outputs the reference voltage Vref to the comparing means 18. The comparing means 18 compares the reference voltage Vref output by the reference voltage generating means 17 with the output voltage Vth of the temperature detecting element 6 (S23). When the output voltage Vth of the temperature detecting element 6 is higher than the reference voltage Vref of the reference voltage generating means 17, the interrupting means 13 interrupts the power supply to the heater 4 by the power supply 12 (S24). In this embodiment, since the temperature detecting element 6 in which the output voltage decreases monotonically with respect to the detected temperature is used, "the output voltage Vth is higher than the reference voltage Vref" means the relationship of Vth ≦ Vref.

[0028] (Main circuit of the fixing device) The interruption of the power supply to the heater 4 will be described. Regarding the power control of the heater 4, in this embodiment, the main heater 25 and the sub-heater 26 of the heater 4 are independently controlled. FIG. 5 is a connection diagram of the heater 4 and the power control circuit.

[0029] As shown in FIG. 5, the heater 4 is incorporated in a circuit including a control means 11, a first triac 27, a second triac 28, an AC power supply 30 as the power supply 12 (FIG. 3), a relay 29 as the interrupting means 13 (FIG. 3), and an overheating prevention means 14. The first triac 27 and the main heater 25 are connected in series, and the second triac 28 and the sub-heater 26 are connected in series. Further, a partial circuit including the first triac 27 and the main heater 25 and a partial circuit including the second triac 28 and the sub-heater 26 are connected in parallel to the AC power supply 30.

[0030] The two triacs 27 and 28 are respectively turned on / off controlled by the on / off of the gate control signal from the control means 11. The relay 29 is inserted between the triacs 27 and 28 and the AC power supply 30, and is configured such that energization to the main heater 25 and the sub-heater 26 can be cut off by driving the relay 29. The relay 29 is connected to the over-temperature prevention means 14 described later and is driven by a control signal from the over-temperature prevention means 14. Further, the over-temperature prevention means 14 is controlled by a control signal from the control means 11.

[0031] (Description of the circuit of the over-temperature prevention means) The circuit of the over-temperature prevention means 14 in the first embodiment will be described using the circuit diagrams of FIGS. 6 and 7 and the timing diagrams of FIGS. 8 and 9. As shown in FIG. 6, the temperature detection element 6 is connected to the pull-up resistor 31, and outputs the output voltage Vth of the temperature detection element 6 to the initial voltage holding means 16 and the comparison means 18. The energization signal generation means 15 outputs an energization signal 77 to the initial voltage holding means 16 and the reference voltage generation means 17 in response to the gate signal 75 from the control means 11 to the triac 27.

[0032] As shown in FIG. 7, the energization signal generation means 15 outputs an H (ON) energization signal 77 while the gate signal 75 from the control means 11 is being output by a circuit (sample & hold circuit) including the capacitor 68, the diode 67, the capacitor 69, and the resistor 70. Further, the energization signal generation means 15 rapidly decreases the gentle decrease of the output of the sample & hold circuit when the output of the gate signal 75 stops by a circuit including the resistors 71, 72, 74, and the comparator 73. As a result, as shown in FIG. 8, while the control means 11 is outputting the gate signal 75, the energization signal generation means 15 outputs an H (ON) energization signal 77. When the gate signal 75 is input from the control means 11 to the triac 27, a drive voltage 76 shown in FIG. 8 is input to the main heater 25 while the gate signal 75 is being output.

[0033] As shown in FIG. 6, the initial voltage holding means 16 receives the output voltage Vth of the temperature detection element 6 by a buffer composed of a resistor 34 and an operational amplifier 32. Then, when the energization signal 77 is L (OFF), the FET switch 35 is short-circuited, and the output voltage Vth of the temperature detection element 6 is charged to the capacitor 36. The capacitor 36 is once received by a buffer composed of an operational amplifier 33, and outputs a voltage adjusted to a predetermined voltage value by resistors 37 and 38 to the reference voltage generation means 17.

[0034] The output of the energization signal generation means 15 is input to the FET switch 40 of the reference voltage generation means 17 via an inverter 39. When the output voltage of the energization signal generation means 15 is H (ON), the FET switch 40 is short-circuited, and power is supplied to the reference voltage generation means 17. The inverter 39 may be composed of a transistor or an FET, or may be composed of other elements.

[0035] The reference voltage generation means 17 is a circuit including a triangular wave voltage generation section (41, 42), a voltage division resistor generation section (43 to 47), a staircase waveform generation section including a plurality of comparators, and a voltage clamp section (63 to 65). The triangular wave voltage generation section is composed of a resistor 42 and a capacitor 41, and outputs a triangular wave voltage. The voltage division resistor generation section is composed of resistors 43 to 47, and outputs a voltage (voltage division output) obtained by dividing the voltage delivered from the initial voltage holding means 16 by the series-connected resistors 43 to 47. The staircase waveform generation section includes three comparators composed of comparators 51 to 53, resistors 54 to 56, 60, and diodes 57 to 59, 61, 62. The staircase waveform generation section generates (generates) a staircase waveform voltage that changes with time according to the elapsed time from the start of power supply to the energization signal generation means 15 by receiving the output voltages of the triangular wave voltage generation section and the voltage division resistor generation section. The generated staircase waveform voltage is limited by a voltage clamp composed of resistors 63, 64 and a diode 65 so as not to be less than a predetermined voltage Va, and is output as the reference voltage Vref.

[0036] Thus, until a predetermined time elapses from the start of power supply to the energization signal generating means 15, the reference voltage generating means 17 outputs a stepped waveform voltage that gradually drops as the reference voltage Vref, and after the predetermined time has elapsed, it outputs a constant voltage Va as the reference voltage Vref. In other words, the reference voltage generating means 17 outputs the reference voltage Vref having a waveform that changes to the high-temperature side (low-voltage side) according to the passage of time in a time zone (the first region on the time axis) where the elapsed time from the start of heating of the heater 4 (ON of the energization signal 77) is equal to or less than the predetermined time to the comparison means 18. Further, the reference voltage generating means 17 outputs a reference voltage Vref having a constant value (Va) to the comparison means 18 in a time zone (the second region on the time axis) where the elapsed time from the start of heating of the heater 4 (ON of the energization signal 77) is longer than the predetermined time, regardless of the passage of time. The operation of the over-temperature rise prevention means 14 using such a reference voltage Vref will be described later.

[0037] The above-mentioned predetermined time represents the length of the rising period of the heater temperature or the period during which it should be monitored whether a rapid temperature rise occurs after the start of heating of the heater 4. As an example, the length of the predetermined time is set to the time from the start of energization of a heater at room temperature (25°C) until the heater temperature reaches a temperature close to the set temperature (for example, 130°C). In this embodiment, the predetermined time is set to 3 seconds.

[0038] On the other hand, the period after the predetermined time has elapsed from the start of heating of the heater 4 is usually a period during which the heater temperature is maintained at a temperature close to the set temperature (temperature control temperature). The predetermined voltage Va is a threshold value for determining an abnormal heating state when the energization of the heater 4 continues even if the set temperature (for example, 150°C) is exceeded due to an abnormality in the heater temperature control by the control means 11 in the period after the predetermined time has elapsed from the start of heating of the heater 4. The voltage Va is set to 0.67V (Figure 2) corresponding to the detected temperature of 200°C in this embodiment. In other words, in this embodiment, a heater temperature of 200°C or higher is treated as an abnormal heating range.

[0039] The length of the specified time, the value of the voltage Va, the number of steps of the staircase waveform, the magnitude of the voltage drop, etc. can be changed according to the specific configuration of the fixing device. For example, when using a temperature detection element 6 having characteristics different from those shown in FIG. 2, the set value of the voltage Va can be changed from 0.67V.

[0040] The comparison means 18 is composed of a comparator 66, which compares the output voltage Vth of the temperature detection element 6 with the reference voltage Vref, and outputs a signal for shutting off (turning off) the shutting-off means 13 when the output voltage Vth is higher than the reference voltage Vref, that is, when Vth≦Vref. In this embodiment, a relay 29 (FIG. 5) is used as the shutting-off means 13, and the relay 29 can cut off the power supply to the heater 4. Here, the relay 29 employs a latching relay whose shut-off state is held (latched) once it is shut off (turned off).

[0041] (Explanation of the operation of the overheating prevention means) The operation of this embodiment will be described with reference to the timing charts of FIGS. 9(a - c) and FIGS. 10(a - c). FIGS. 9(a - c) and FIGS. 10(a - c) show the temperature of the heater 4 from the start of heating, the output voltage Vth of the temperature detection element 6, and the reference voltage Vref output by the reference voltage generation means 17. The reference voltage Vref output by the reference voltage generation means 17 has a staircase waveform in which the voltage value drops stepwise over time as described with reference to the circuit diagram of FIG. 6. The initial value of the reference voltage Vref is the voltage value delivered from the initial voltage holding means 16 to the reference voltage generation means 17 immediately before the start of printing. Also, the set temperature, which is the temperature target value (150°C) of the heater 4 in FIG. 9(a), is indicated by a thin dashed line. In FIGS. 9(a - c) and FIGS. 10(a - c), the temperature of the heater 4 before the power supply to the heater 4 is cut off is shown by a solid line, and for the case after the power supply to the heater 4 is cut off, the temperature of the heater 4 if the power supply had not been cut off is shown by a dashed line.

[0042] When the output voltage Vth of the temperature detection element 6 becomes higher than the reference voltage Vref having a staircase waveform, the overheating prevention means 14 cuts off the power supply to the heater 4 by the cutoff means 13. In this embodiment, "the output voltage Vth is higher than the reference voltage Vref" refers to the relationship of Vth ≦ Vref because the temperature detection element 6 whose output voltage decreases (monotonically decreases) as the detected temperature decreases is used (see FIG. 2). That is, the overheating prevention means 14 of this embodiment determines that the heater 4 has entered the abnormal heating region when Vth ≦ Vref, and cuts off the power supply to the heater 4. Here, taking the start time of heating of the heater 4 (when the energization signal 77 becomes ON) as a reference, the time region in which the reference voltage Vref drops stepwise is defined as the first region, and the time region in which the reference voltage Vref becomes a constant value (Vref = Va) is defined as the second region. The boundary between the first region and the second region is the time point when 3 seconds, which is a predetermined time, has elapsed since the start of heating of the heater 4. At this time point, the reference voltage Vref drops from a voltage value higher than Va to Va.

[0043] During normal printing (normal operation) in FIG. 9(a), the heater 4 heats up to a target of 150° C. and is maintained at approximately 150° C. from a certain time. Correspondingly, the output voltage Vth of the temperature detection element 6 drops during the heating of the heater 4, and when the temperature of the heater 4 becomes substantially constant at 150° C., the output voltage Vth of the temperature detection element 6 also becomes substantially constant at the voltage value corresponding to 150° C. In this case, when the output voltage Vth of the temperature detection element 6 is compared with the reference voltage Vref, Vth > Vref always holds, the overheating prevention means 14 does not operate, and the power supply to the heater 4 is not cut off by the overheating prevention means 14.

[0044] FIG. 9(b) shows a case where an abnormality (temperature control failure) occurs in the power supply control of the heater 4 by the control means 11, and an abnormal operation occurs in which the temperature of the heater 4 continues to rise without becoming constant at the target temperature of 150° C. In this case, corresponding to the continuous temperature rise of the heater 4, the output voltage Vth of the temperature detection element 6 also continuously drops and falls below the voltage value corresponding to 150° C. In this case, since the output voltage Vth of the temperature detection element 6 becomes equal to or lower than the reference voltage Vref (= Va) in the second region of the reference voltage Vref, the overheating prevention means 14 operates and the power supply to the heater 4 is cut off.

[0045] Specifically, the value (Va) of the reference voltage Vref in the second region is set to a value corresponding to the temperature of 200°C of the heater 4. Therefore, in the example shown in FIG. 9(b), when the temperature of the heater 4 rises to 200°C, the power supply to the heater 4 is cut off.

[0046] FIG. 9(c) shows a case where an abnormal operation occurs in which the heater 4 is energized with the rotation of the fixing film 3 and the pressure roller 8 stopped. In this case, since the heat generated by the heater 4 does not move outside the fixing nip N due to the rotation of the fixing film 3 or the pressure roller 8, the heater 4 rapidly heats up. In this case, since the output voltage Vth of the temperature detection element 6 rapidly decreases as shown by the broken line, Vth≤Vref in the first region, and the overheating prevention means 14 operates to cut off the power supply to the heater 4.

[0047] As described above, in the present embodiment, the reference voltage Vref generated by the reference voltage generation means 17 changes in value according to the elapsed time from the start of heating of the heater 4, and the value of the reference voltage Vref changes from the low temperature side to the high temperature side as time elapses. In other words, in the present embodiment, the reference voltage generation means is configured such that the temperature corresponding to the value of the reference voltage at the time when the second time longer than the first time has elapsed from the start of heating is higher than the temperature corresponding to the value of the reference voltage at the time when the first time has elapsed from the start of heating when the control means starts the power supply to the heating means. In the present embodiment, an example of the first time is the time point of 0 seconds from the start of heating (the state where the reference voltage Vref is the initial value), and an example of the second time is the time point of 1.5 seconds from the start of heating (the state where the reference voltage Vref has dropped by one step). In the present embodiment, as a specific example of waveforms in which the value of the reference voltage is different at the first time and the second time, a step waveform in which the reference voltage Vref changes stepwise to the high temperature side (low voltage side) as time elapses is adopted.

[0048] Here, if a rapid temperature rise of the heater 4 occurs after the start of heating of the heater 4, it is possible that the heater 4 is being heated with the rotation of the fixing film 3 and the pressure roller 8 stopped due to some abnormality. The rapid temperature rise referred to here is a temperature rise rate that is clearly greater than the temperature rise rate assumed from the calorific value of the heater 4 and the heat capacities of the fixing film 3, the heater holding member 2, the pressure roller 8, etc.

[0049] According to this embodiment, the value of the reference voltage Vref changes from the low temperature side to the high temperature side according to the elapsed time from the start of heating of the heater 4. Therefore, for example, when the temperature of the heater 4 rapidly rises immediately after the start of heating of the heater 4, even at a relatively low temperature, the output voltage Vth of the temperature detection element enters the cutoff region on the high temperature side of the reference voltage Vref (FIG. 9(c)), and the energization to the heater 4 is cut off. Thereby, when an abnormality accompanied by a rapid temperature rise occurs, the energization to the heating means can be quickly cut off. Here, since the value of the reference voltage Vref is not constant but changes from the low temperature side to the high temperature side, the rise of the heater temperature at a normal temperature rise rate in the first region is not hindered.

[0050] Also, in this embodiment, as the reference voltage Vref, a waveform is used in which the value changes to the high temperature side according to the elapsed time in the first region after the start of heating of the heater 4, and becomes a constant value (Va) regardless of the elapsed time in the second region. For this reason, whether it is an abnormality that causes a rapid temperature rise of the fixing film 3 or an abnormality that causes a gradual temperature rise, the energization to the heater 4 can be cut off at an appropriate timing before reaching the abnormal heating region.

[0051] In FIGS. 9(a - c) described above, the case where the temperature of the fixing device 1 is normal temperature at the start of printing is illustrated. However, there may be a case where the fixing device 1 is at a high temperature at the start of the printing operation, such as when the next printing operation is started immediately after the end of the previous printing operation. The operation in such a case will be described with reference to FIGS. 10(a - c).

[0052] Fig. 10(a) shows the same case as Fig. 9(c) for comparison. That is, Fig. 10(a) shows the case where an abnormality occurs in which power is supplied to the heater 4 while the rotation of the fixing film 3 and the pressure roller 8 has stopped, and the temperature of the heater 4 at the start of printing is normal temperature (25°C). On the other hand, Fig. 10(b) shows the case where an abnormality occurs in which power is supplied to the heater 4 while the rotation of the fixing film 3 and the pressure roller 8 has stopped, and the temperature of the heater 4 at the start of printing is high temperature (100°C).

[0053] As described above, the output voltage of the temperature detection element 6 corresponding to the temperature before the start of printing is held as the initial voltage Vpre by the initial voltage holding means 16, and the initial value of the reference voltage Vref based on the initial voltage Vpre is delivered to the reference voltage generation means 17. Therefore, when the heater 4 is at a high temperature at the start of printing, the voltage value delivered from the initial voltage holding means 16 to the reference voltage generation means 17 is lower than when the heater 4 is at normal temperature at the start of printing. As a result, the initial value of the reference voltage Vref (the value at the 0 sec time point) in Fig. 10(b) is lower than the initial value of the reference voltage Vref in Fig. 10(a).

[0054] Therefore, even if a rapid temperature rise of the heater 4 occurs when the fixing film 3 is at a high temperature at the start of printing, the power supply to the heater 4 can be cut off before entering the abnormal heating region. Note that the level of the reference voltage Vref in the first region becomes lower as the temperature of the heater 4 at the start of printing is higher, but the value (Va) of the reference voltage Vref in the second region is constant regardless of the temperature of the heater 4 at the start of printing. Therefore, in the case where the heater 4 is at a high temperature at the start of printing, the operation when the output voltage Vth of the temperature detection element 6 enters the cutoff region in the second region is the same as in the case of Fig. 9(b).

[0055] Here, FIG. 10(c) shows a case where the temperature of the heater 4 is high (100° C.) at the start of printing and the printing operation is performed normally. In this case, although the initial value of the reference voltage Vref is lower than that in the case of FIG. 10(a) where the temperature of the heater 4 is normal temperature (25° C.) at the start of printing, since the rotation of the fixing film 3 and the pressure roller 8 is performed normally, a rapid temperature rise of the heater 4 does not occur. Therefore, the output voltage Vth of the temperature detection element 6 gradually decreases in accordance with the temperature rise of the heater 4, and after the heater 4 reaches the target temperature of 150° C., Vth is also maintained substantially constant. Therefore, even when the temperature of the heater 4 is high at the start of printing, when the printing operation is performed normally, the output voltage Vth of the temperature detection element 6 does not become less than the reference voltage Vref, and the power supply to the heater 4 is not cut off.

[0056] If the waveform of the reference voltage Vref in the first region is fixed regardless of the temperature of the heater 4 at the start of printing, there is a possibility that the over-temperature prevention means 14 may not operate properly depending on the temperature of the heater 4 at the start of printing. That is, when the reference voltage Vref in the first region is fixed to the waveform of FIG. 10(a), when the temperature of the heater 4 is normal temperature at the start of printing, although the printing operation is performed normally, there is a possibility that the power supply to the heater 4 is cut off because Vth ≦ Vref. On the other hand, when the reference voltage Vref in the first region is fixed to the waveform of FIG. 10(b), when the temperature of the heater 4 is high at the start of printing, the timing at which Vth ≦ Vref occurs when a rapid temperature rise of the heater 4 occurs is delayed compared to this embodiment. Therefore, the power supply cut-off to the heater 4 is delayed compared to this embodiment, and there is a possibility that the temperature rise of the heater 4 continues during that time.

[0057] As described above, in this embodiment, the initial value of the reference voltage Vref is changed according to the temperature of the heater 4 at the start of printing. Thereby, according to the temperature of the heater 4 at the start of printing, while allowing a normal temperature rise of the heater 4, when a rapid temperature rise of the heater 4 is detected, the power supply to the heater 4 can be quickly cut off.

[0058] The circuit used in the above description is an example, and any other circuit with equivalent functions may be used instead.

Embodiment

[0059] Embodiment 2 is different from Embodiment 1 in that a fixing device using an induction heating method is used as the fixing device 1, and the circuit configuration of the reference voltage generating means 17 is different. Hereinafter, the differences from Embodiment 1 will be mainly described.

[0060] (Configuration of the fixing device) FIG. 11 is a cross-sectional view of a fixing device 100 according to this embodiment. The fixing device 100 includes a fixing film 101, an induction heating member 102, a heater holding member 2, a pressure roller 8 as an opposing member that opposes the heater holding member 2 with the fixing film 101 interposed therebetween, and a metal stay 5.

[0061] The fixing film 101 is a cylindrical rotating body provided with a conductive layer and is composed of a flexible film member (endless belt). The induction heating member 102 has a magnetic core 103 and an exciting coil 104, and when an alternating voltage is applied to the exciting coil 104 to energize it, the fixing film 101 is heated by induction heating. That is, by applying an alternating voltage, the exciting coil 104 generates an alternating magnetic field surrounding the conductive layer of the fixing film 101, and eddy currents flow in the conductive layer to cancel the change in the magnetic field, so that the fixing film 101 is heated by Joule heat. The induction heating member 102 and the heater holding member 2 are disposed inside the fixing film 101. The pressure roller 8 is pressed against the heater holding member 2 with the fixing film 101 interposed therebetween, and a fixing nip N is formed between the pressure roller 8 and the heater holding member 2. The heater holding member 2 is disposed inside the fixing film 101 and is a nip forming unit that forms a fixing nip N as a nip portion together with the pressure roller 8.

[0062] The metal stay 5 is made of a material such as stainless steel that is difficult to generate heat by induction heating. The fixing film 101 is a cylindrical rotating body with a composite structure including a heat-generating layer made of a conductive member using a pure metal or alloy such as stainless steel, Al, Ni, Cu, Zn, etc. with a diameter of 10 to 50 mm as a base layer, an elastic layer laminated on the outer surface thereof, and a release layer laminated on the outer surface thereof. The pressure roller 8 has a core metal 9 made of a material such as iron or aluminum and an elastic layer 10 made of a material such as silicone rubber. The magnetic core 103, which is the core material of the induction heating member 102, is a ferromagnetic body composed of an end-shaped fired ferrite, ferrite resin, amorphous alloy (amorphous alloy), or a high-permeability oxide or alloy material such as permalloy. The exciting coil 104 is composed of a wire spirally wound around the outer periphery of the magnetic core 103 along the longitudinal direction. The induction heating member 102 is held by a heater holding member 2 made of a heat-resistant resin. The heater holding member 2 also has a guide function for guiding the rotation of the fixing film 101. The metal stay 5 is urged toward the pressure roller 8 against the heater holding member 2 under a pressing force (not shown).

[0063] The pressure roller 8 receives power from a motor (not shown) and rotates in the direction of arrow R1. When the pressure roller 8 rotates, the fixing film 3 is driven to rotate in the direction of arrow R2. The fixing device 1 applies the heat of the fixing film 101 heated by induction heating while sandwiching and conveying the recording material P at the fixing nip N, thereby performing a fixing process on the unfixed toner image on the recording material P. That is, the fixing film 101 is the fixing member of this embodiment, and the induction heating member 102 is the heating means of this embodiment.

[0064] A temperature detection element 6, which is an example of temperature detection means, is in contact with the fixing film 101. As for the energization control of the induction heating member 102, a CPU (not shown) controls the duty ratio, frequency, output stop period, etc. of the AC voltage applied to the induction heating member 102 according to the output of the temperature detection element 6, thereby maintaining the temperature in the fixing nip at a predetermined fixing temperature. Also, safety elements such as a thermoswitch or a temperature fuse (not shown), which operate due to abnormal heating of the fixing film 101 and cut off the power supply to the induction heating member 102, are also close to the fixing film 101. As the temperature detection element 6, a thermistor can be used as in the first embodiment.

[0065] The operation of the over-temperature prevention means 14 shown in FIG. 12 is substantially the same as that in the first embodiment. The differences are that the power supply 12 is energized to the induction heating member 102, the temperature detection element 6 is internally connected to the fixing film 101, and the input of the energization signal generation means 15 is the power supply. Since the cutoff of the power supply 12 is the same as that in the first embodiment, the description is omitted.

[0066] The energization / cutoff circuit for the induction heating member 102 will be described. FIG. 13 is a connection diagram of the induction heating member 102 and the power control circuit. The power control circuit includes a control means 11, an AC power supply 105, a line filter 106, a rectifier 107, a coil 108, and a capacitor 109. The coil 108 and the capacitor 109 constitute a filter for smoothing the output of the rectifier 107. Also, the power control circuit has a full-bridge power supply composed of switching elements 110 to 113 such as IGBTs and FETs and capacitors 114 to 117. The output of the full-bridge power supply is connected to the terminals 104a and 104b of the exciting coil 104 of the induction heating member 102. The resistor 118 and the coil 119 are equivalent circuits when looking at the fixing device 100 including the induction heating member 102 from the terminals 104a and 104b of the exciting coil 104. Further, the power control circuit includes gate control means 122, cutoff means 121, insulation means 120, and a temperature detection element 6.

[0067] Upon receiving the control signal from the control means 11, the gate control means 122 outputs a gate control signal for driving the switching elements 110 to 113. Between the gate control means 122 and the switching elements 110 to 113, a cutoff means 121 for cutting off the gate control signal in case of an abnormality and an insulation means 120 for electrically insulating the full-bridge power supply and the control circuit are provided. When the gate signal is stopped by the cutoff means 121, the power supply to the induction heating member 102 can be cut off. The control signal of the cutoff means 121 is connected to the over-temperature rise prevention means 14 described later. Further, the over-temperature rise prevention means 14 is controlled by the presence or absence of the gate control signal of the gate control means 122.

[0068] (Description of the circuit of the over-temperature rise prevention means) The circuit of the over-temperature rise prevention means 14 in this embodiment will be described with reference to the circuit diagrams of FIGS. 13 and 14 and the timing diagrams of FIGS. 15(a, b) and 16(a to c). Note that the temperature detection element 6, the initial voltage holding means 16, the inverter 39, and the FET switch 40 are the same as those in the first embodiment, so the description thereof will be omitted.

[0069] The energization signal generation means 15 in FIG. 14 is the same sample hold circuit as that described with reference to FIG. 7. The energization signal generation means 15 outputs an energization signal 145 of H(ON) while at least one of the four gate control signals 140 to 143 output by the gate control means 122 in FIG. 15(a) is being output. A comparator composed of resistors 71, 72, 74 and a comparator 73 rapidly decreases the gentle decrease of the sample hold circuit output to generate the energization signal 145. As shown in FIG. 15(b), the combination of the gate control signals 140 to 143 generates the waveform of the AC voltage that is the input to the induction heating member 102.

[0070] The reference voltage generation means 17 in FIG. 14 includes a current source, a triangular wave voltage generation section, and a voltage clamp section. The current source is composed of resistors 123, 124, 126, 127 and transistors 125, 128, and supplies a constant current to the triangular wave voltage generation section via a diode 130. The triangular wave voltage generation section is composed of resistors 131, 133, 136, 136, capacitors 132, 134, and a transistor 135, and generates a triangular wave voltage. The triangular wave generated by the triangular wave voltage generation section is limited by a voltage clamp composed of resistors 137, 138 and a diode 139 so as not to be less than a predetermined voltage Va, and is output to the comparison means 18. As a result, as shown in FIGS. 16(a - c), the reference voltage generation means 17 outputs a reference voltage Vref that has a triangular wave shape that monotonically decreases until a predetermined time has elapsed from the start of heating, and becomes a constant value (Va) after the predetermined time has elapsed.

[0071] The comparison means 18 is composed of a comparator 66, compares the output voltage Vth of the temperature detection element 6 with the reference voltage Vref, and outputs a signal for shutting off the shut-off means 13 when Vth ≦ Vref.

[0072] (Operation of over-temperature rise prevention means) The operation of this embodiment will be described using the timing charts of FIGS. 16(a - c) and FIGS. 17(a - c). FIGS. 16(a - c) and FIGS. 17(a - c) show the temperature of the fixing film 101 from the start of heating, the output voltage Vth of the temperature detection element 6, and the reference voltage Vref of the reference voltage generation means 17. As described with reference to the circuit diagram, when the output voltage Vth of the temperature detection element 6 becomes higher than this reference voltage Vref (Vth ≦ Vref), the over-temperature rise prevention means 14 determines that the fixing film 101 has entered the abnormal heating region, and cuts off the power supply to the induction heating member 102. Here, the time region in which the reference voltage Vref linearly drops is defined as the first region, and the time region in which the reference voltage Vref is a constant value (Va) is defined as the second region.

[0073] During normal printing (normal operation) as shown in Fig. 16(a), the fixing film 101 is heated up to a target temperature of 150°C, and after a certain time, it reaches approximately 150°C. Correspondingly, the output voltage Vth of the temperature detection element 6 also becomes a voltage corresponding to 150°C. In this case, when comparing the output voltage Vth of the temperature detection element 6 with the reference voltage Vref, Vth > Vref always holds, and the overheating prevention means 14 does not operate, and the power supply to the induction heating member 102 is not cut off.

[0074] Fig. 16(b) shows a case (abnormal operation) where, due to a temperature control failure of the control means 11, the temperature of the fixing film 101 does not become constant at 150°C but continues to rise. In this case, the output voltage Vth of the corresponding temperature detection element 6 also does not become constant and continues to decrease. In this case, when comparing the output voltage Vth of the temperature detection element 6 with the reference voltage Vref, Vth = Vref at the time when the temperature of the fixing film 101 reaches 200°C in the second region, the overheating prevention means 14 operates, and the power supply to the induction heating member 102 is cut off.

[0075] Fig. 16(c) shows a case (abnormal operation) where the power supply to the induction heating member 102 is turned on while the rotational drive of the fixing film 101 and the pressure roller 8 is stopped. In this case, since the heat generated in the fixing film 101 is not taken away by the pressure roller 8, the fixing film 101 rapidly heats up. In this case, since the output voltage Vth of the temperature detection element 6 rapidly decreases as shown by the broken line, Vth ≤ Vref in the first region, and the overheating prevention means 14 operates to cut off the power supply to the induction heating member 102.

[0076] Thus, also in this embodiment, the reference voltage Vref generated by the reference voltage generation means 17 changes in value according to the elapsed time since the start of heating, and is configured such that the value of the reference voltage Vref changes from the low-temperature side to the high-temperature side as time elapses. In other words, in this embodiment, the reference voltage generation means is configured to change the value of the reference voltage according to the elapsed time since the start of heating such that the temperature corresponding to the value of the reference voltage at the time when the first time has elapsed since the start of heating when the control means starts the power supply to the heating means is lower than the temperature corresponding to the value of the reference voltage at the time when the second time longer than the first time has elapsed since the start of heating. In this embodiment, an example of the first time is the point in time of 0 seconds since the start of heating, and an example of the second time is the point in time of 1.5 seconds since the start of heating. In this embodiment, as a specific example of a waveform in which the value of the reference voltage is different at the first time and the second time, a waveform in which the reference voltage Vref changes linearly to the high-temperature side (low-voltage side) as time elapses is adopted.

[0077] According to this embodiment, the value of the reference voltage Vref changes from the low-temperature side to the high-temperature side according to the elapsed time since the start of heating of the fixing film 101. Therefore, for example, when the temperature of the fixing film 101 rises rapidly immediately after the start of heating, even at a relatively low temperature, the output voltage Vth of the temperature detection element 6 enters the cutoff region on the high-temperature side of the reference voltage Vref (FIG. 16(c)), and the energization of the induction heating member 102 is cut off. Thereby, the energization to the heating means can be quickly cut off when an abnormality accompanied by a rapid temperature rise occurs. Here, since the value of the reference voltage Vref is not constant but changes from the low-temperature side to the high-temperature side, the rise of the fixing film temperature at a normal temperature rise rate in the first region is not hindered.

[0078] Also, in this embodiment, as the reference voltage Vref, a waveform is used in which the value changes to the high-temperature side according to the elapsed time in the first region after the start of heating of the fixing film 101, and becomes a constant value (Va) regardless of the elapsed time in the second region. Therefore, whether it is an abnormality that causes a rapid temperature rise of the fixing film 101 or an abnormality that causes a gradual temperature rise, the energization of the induction heating member 102 can be cut off at an appropriate timing before reaching the abnormal heating region.

[0079] In FIGS. 16(a) to 16(c) described above, the case where the temperature of the fixing device 100 is normal temperature at the start of printing was illustrated. However, there may be a case where the fixing device 100 is at a high temperature at the start of the printing operation, such as when the next printing operation is started immediately after the end of the previous printing operation. The operation in such a case will be described with reference to FIGS. 17(a) to 17(c).

[0080] FIG. 17(a) shows the same case as FIG. 16(c) for comparison. That is, FIG. 17(a) shows the case where an abnormality occurs in which the induction heating member 102 is energized while the rotation of the fixing film 101 and the pressure roller 8 has stopped, and the temperature of the fixing film 101 at the start of printing is normal temperature (25° C.). On the other hand, FIG. 17(b) shows the case where an abnormality occurs in which the induction heating member 102 is energized while the rotation of the fixing film 101 and the pressure roller 8 has stopped, and the temperature of the fixing film 101 at the start of printing is high temperature (100° C.).

[0081] As described above, the output voltage of the temperature detection element 6 corresponding to the temperature before the start of printing is held as the initial voltage Vpre by the initial voltage holding means 16, and the initial value of the reference voltage Vref based on the initial voltage Vpre is delivered to the reference voltage generation means 17. Therefore, when the fixing film 101 is at a high temperature at the start of printing, the voltage value delivered from the initial voltage holding means 16 to the reference voltage generation means 17 is lower than when the fixing film 101 is at normal temperature at the start of printing. As a result, the initial value of the reference voltage Vref (the value at the 0 sec time point) in FIG. 17(b) is lower than the initial value of the reference voltage Vref in FIG. 17(a). Also, the slope of the triangular wave in the first region becomes gentle.

[0082] Therefore, even if a rapid temperature rise of the fixing film 101 occurs when the fixing film 101 is at a high temperature at the start of printing, the energization of the induction heating member 102 can be cut off before entering the abnormal heating region. Note that the level of the reference voltage Vref in the first region becomes lower as the temperature of the fixing film 101 is higher at the start of printing, but the value (Va) of the reference voltage Vref in the second region is constant regardless of the temperature of the fixing film 101 at the start of printing. Therefore, when the fixing film 101 is at a high temperature at the start of printing, the operation when the output voltage Vth of the temperature detection element 6 enters the cut-off region in the second region is the same as that in the case of FIG. 16(b).

[0083] Here, FIG. 17(c) shows the case where the temperature of the fixing film 101 is high (100 ° C.) at the start of printing and the printing operation is normally performed. In this case, the initial value of the reference voltage Vref is lower than that in the case of FIG. 17(a) where the temperature of the fixing film 101 is normal temperature (25 ° C.) at the start of printing. However, since the rotation of the fixing film 101 and the pressure roller 8 is normally performed, a rapid temperature rise of the fixing film 101 does not occur. Therefore, the output voltage Vth of the temperature detection element 6 gradually drops in response to the temperature rise of the fixing film 101, and Vth is maintained substantially constant after the fixing film 101 reaches the target temperature of 150 ° C. Therefore, even if the temperature of the fixing film 101 is high at the start of printing, when the printing operation is normally performed, the output voltage Vth of the temperature detection element 6 does not become lower than the reference voltage Vref, and the energization to the induction heating member 102 is not cut off.

[0084] Thus, also in this embodiment, the initial value of the reference voltage Vref is changed according to the temperature of the heater 4 at the start of printing. Thereby, according to the temperature of the heater 4 at the start of printing, while allowing a normal temperature rise of the heater 4, when a rapid temperature rise of the heater 4 is detected, the energization to the heater 4 can be quickly cut off.

[0085] The circuit used in the above description is an example, and any other circuit may be used as long as it has an equivalent function.

Example

[0086] Example 3 uses the same induction heating type fixing device 100 as in Example 2 as the fixing device 1, but is different from Example 2 in that the circuit configuration of the reference voltage generation means 17 is different. Hereinafter, the differences from Example 2 will be mainly described. Since the configuration of the fixing device 100 and the overall configuration of the power control circuit of the fixing device 100 are substantially the same as those in Example 2, the description will be omitted.

[0087] (Circuit of over-temperature prevention means) The circuit of the over-temperature prevention means 14 in Example 3 will be described with reference to the circuit diagram of FIG. 18 and the timing diagrams of FIGS. 19(a - c) and 20(a - c). Note that since the temperature detection element 6, the energization signal generation means 15, the initial voltage holding means 16, the inverter 39, and the FET switch 40 are the same as those in Example 1, the description thereof will be omitted.

[0088] The reference voltage generation means 17 consists of an upper convex curve voltage generation section composed of resistors 202, 203, 205, capacitors 201, 206, and transistor 20 4 . The upper convex curve voltage generation section is configured to draw a curve in which the voltage value with respect to time is convex upward (high voltage side, low temperature side) by utilizing the charging characteristics of the capacitor. The voltage of the upper convex curve waveform created by the upper convex curve voltage generation section is limited by a voltage clamp composed of resistors 207, 208, and diode 209 so as not to fall below a predetermined voltage and is output to the comparison means 18. As a result, in the first region shown in FIGS. 19(a - c) and 20(a - c), an upper convex curve is formed, and a reference voltage Vref having a waveform that becomes a constant value (Va) in the second region is output.

[0089] The comparison means 18 is composed of a comparator 66, compares the output voltage Vth of the temperature detection element with the reference voltage Vref, and outputs a signal for shutting off the shut-off means 13 when Vth ≦ Vref.

[0090] (Operation of over-temperature prevention means) The operation of this embodiment will be described with reference to the timing charts of FIGS. 19(a - c) and 20(a - c). FIGS. 19(a - c) show the temperature of the fixing film 101 from the start of heating, the output voltage Vth of the temperature detection element 6, and the reference voltage Vref of the reference voltage generation means 17. As described with reference to the circuit diagram, the reference voltage Vref forms an upwardly convex curve in the first region and has a constant value (Va) in the second region. When the output voltage Vth of the temperature detection element 6 becomes higher than this reference voltage Vref (Vth ≦ Vref), the overheating prevention means 14 determines that the fixing film 101 has entered the abnormal heating region and cuts off the power supply to the induction heating member 102. Here, the time region in which the reference voltage Vref descends while forming an upwardly convex curve is defined as the first region, and the time region in which the reference voltage Vref has a constant value (Va) is defined as the second region.

[0091] During normal printing (normal operation) in FIG. 19(a), the fixing film 101 is heated to a target temperature of 150°C, reaches approximately 150°C from a certain time, and correspondingly, the output voltage Vth of the temperature detection element 6 also becomes a voltage corresponding to 150°C. In this case, when comparing the output voltage Vth of the temperature detection element 6 with the reference voltage Vref, Vth > Vref always holds, the overheating prevention means 14 does not operate, and the power supply to the induction heating member 102 is not cut off.

[0092] FIG. 19(b) shows a case where the temperature of the fixing film 101 does not become constant at 150°C but continues to rise due to poor temperature control of the control means 11 (abnormal operation). In this case, the output voltage Vth of the corresponding temperature detection element 6 also does not become constant and continues to decrease. In this case, when comparing the output voltage Vth of the temperature detection element 6 with the reference voltage Vref, Vth = Vref at the time when the temperature of the fixing film 101 reaches 200°C in the second region, the overheating prevention means 14 operates, and the power supply to the induction heating member 102 is cut off.

[0093] FIG. 19(c) shows a case (during abnormal operation) where power is supplied to the induction heating member 102 with the rotational drive of the fixing film 101 and the pressure roller 8 stopped. In this case, since the heat generated in the fixing film 101 is not taken away by the pressure roller 8, the temperature of the fixing film 101 rises rapidly. In this case, since the output voltage Vth of the temperature detection element 6 drops rapidly as shown by the broken line, Vth≤Vref in the first region, and the overheating prevention means 14 operates to cut off the power supply to the induction heating member 102.

[0094] Thus, also in this embodiment, the reference voltage Vref generated by the reference voltage generation means 17 changes in value according to the elapsed time from the start of heating, and the value of the reference voltage Vref changes from the low temperature side to the high temperature side as time elapses. In other words, in this embodiment, the reference voltage generation means is configured such that the temperature corresponding to the value of the reference voltage at the time when the second time longer than the first time has elapsed from the start of heating is higher than the temperature corresponding to the value of the reference voltage at the time when the first time has elapsed from the start of heating, at which time the control means starts supplying power to the heating means. The reference voltage value changes according to the elapsed time from the start of heating. In this embodiment, as a specific example of a waveform in which the reference voltage values are different at the first time and the second time, a waveform is adopted in which the reference voltage Vref descends while drawing a curve convex upward (high pressure side) as time elapses. As an example of another waveform of the reference voltage, a waveform in which the reference voltage Vref descends while drawing a curve convex downward (low pressure side) as time elapses may be used.

[0095] According to this embodiment, the value of the reference voltage Vref changes from the low temperature side to the high temperature side according to the elapsed time since the start of heating of the fixing film 101. Therefore, for example, when the temperature of the fixing film 101 rapidly rises immediately after the start of heating, even at a relatively low temperature, the output voltage Vth of the temperature detection element 6 enters the cutoff region on the high temperature side of the reference voltage Vref (Fig. 19(c)), and the energization to the induction heating member 102 is cut off. As a result, when an abnormality accompanied by a rapid temperature rise occurs, the energization to the heating means can be quickly cut off. Here, since the value of the reference voltage Vref is not constant but changes from the low temperature side to the high temperature side, the rise of the fixing film temperature at a normal temperature rise rate in the first region is not hindered.

[0096] Also, in this embodiment, as the reference voltage Vref, a waveform is used in which the value changes to the high temperature side according to the elapsed time in the first region after the start of heating of the fixing film 101, and becomes a constant value (Va) regardless of the elapsed time in the second region. For this reason, whether it is an abnormality that causes a rapid temperature rise of the fixing film 101 or an abnormality that causes a slow temperature rise, the energization to the induction heating member 102 can be cut off at an appropriate timing before reaching the abnormal heating region.

[0097] In FIGS. 19(a to c) described above, the case where the temperature of the fixing device 100 is normal temperature at the start of printing is illustrated. However, there may be a case where the fixing device 100 is at a high temperature at the start of the printing operation, such as when the next printing operation is started immediately after the end of the previous printing operation. The operation in such a case will be described with reference to FIGS. 20(a to c).

[0098] Fig. 20(a) shows the same case as Fig. 19(c) for comparison. That is, Fig. 20(a) shows the case where an abnormality occurs in which the induction heating member 102 is energized while the rotation of the fixing film 101 and the pressure roller 8 has stopped, and the temperature of the fixing film 101 at the start of printing is normal temperature (25°C). On the other hand, Fig. 20(b) shows the case where an abnormality occurs in which the induction heating member 102 is energized while the rotation of the fixing film 101 and the pressure roller 8 has stopped, and the temperature of the fixing film 101 at the start of printing is high temperature (100°C).

[0099] As described above, the output voltage of the temperature detection element 6 corresponding to the temperature before the start of printing is held as the initial voltage Vpre by the initial voltage holding means 16, and the initial value of the reference voltage Vref based on the initial voltage Vpre is delivered to the reference voltage generation means 17. Therefore, when the fixing film 101 is at a high temperature at the start of printing, the voltage value delivered from the initial voltage holding means 16 to the reference voltage generation means 17 is lower than when the fixing film 101 is at normal temperature at the start of printing. As a result, the initial value of the reference voltage Vref (the value at the 0 sec time point) in Fig. 20(b) is lower than the initial value of the reference voltage Vref in Fig. 20(a).

[0100] Therefore, even if a rapid temperature rise of the fixing film 101 occurs when the fixing film 101 is at a high temperature at the start of printing, the energization of the induction heating member 102 can be cut off before entering the abnormal heating region. Note that the level of the reference voltage Vref in the first region becomes lower as the temperature of the fixing film 101 is higher at the start of printing, but the value (Va) of the reference voltage Vref in the second region is constant regardless of the temperature of the fixing film 101 at the start of printing. Therefore, when the fixing film 101 is at a high temperature at the start of printing, the operation when the output voltage Vth of the temperature detection element 6 enters the cutoff region in the second region is the same as in the case of Fig. 19(b).

[0101] Here, FIG. 20(c) shows the case where the temperature of the fixing film 101 is high (100° C.) at the start of printing and the printing operation is performed normally. In this case, the initial value of the reference voltage Vref is lower than that in the case of FIG. 20(a) where the temperature of the fixing film 101 is normal temperature (25° C.) at the start of printing. However, since the rotation of the fixing film 101 and the pressure roller 8 is performed normally, a rapid temperature rise of the fixing film 101 does not occur. Therefore, the output voltage Vth of the temperature detection element 6 gradually drops in response to the temperature rise of the fixing film 101, and Vth is maintained substantially constant after the fixing film 101 reaches the target temperature of 150° C. Therefore, even when the temperature of the fixing film 101 is high at the start of printing, when the printing operation is performed normally, the output voltage Vth of the temperature detection element 6 does not become lower than the reference voltage Vref, and the energization to the induction heating member 102 is not cut off.

[0102] Thus, also in this embodiment, the initial value of the reference voltage Vref is changed according to the temperature of the heater 4 at the start of printing. Thereby, according to the temperature of the heater 4 at the start of printing, while allowing a normal temperature rise of the heater 4, when a rapid temperature rise of the heater 4 is detected, the energization to the heater 4 can be quickly cut off.

[0103] The circuit used in the above description is an example, and other circuits may be used as long as they have the same function.

Embodiment

[0104] In the fourth embodiment, the same induction heating type fixing device 100 as in the second embodiment is used as the fixing device 1, but it is different from the second embodiment in that the circuit configurations of the temperature detection element 6 and the reference voltage generation means 17 are different. Hereinafter, the differences from the second embodiment will be mainly described. Since the configuration of the fixing device 100 and the overall configuration of the power control circuit of the fixing device 100 are substantially the same as those in the second embodiment, the description thereof will be omitted.

[0105] The temperature detection element 6 of this embodiment is configured such that, as shown in FIG. 21, the output voltage with respect to the detected temperature exhibits a characteristic of increasing towards the upper right (monotonically increasing). That is, the output voltage when the detected temperature is a second temperature (for example, 200°C) higher than the first temperature is higher than the output voltage when the detected temperature is the first temperature (for example, 100°C). The control described below is generally applicable to a temperature detection member having a characteristic that the output voltage increases towards the upper right with respect to the detected temperature. In this embodiment, a thermopile is used as the temperature detection element 6. The thermopile is installed outside the fixing film 101. Regarding points other than the temperature detection element 6, the configuration of the fixing device 100 and the overall configuration of the power control circuit are substantially the same as those in the second embodiment.

[0106] The operating conditions will be further described with reference to the flowchart of FIG. 22. During the period when the energization signal is OFF (S19: OFF), the initial voltage holding means 16 captures the output voltage Vth of the temperature detection element 6 before the start of printing at, for example, predetermined time intervals (S20). When the energization signal changes to ON (S19: ON), the initial voltage holding means 16 holds the output voltage Vth of the temperature detection element 6 immediately before the start of printing as the initial voltage Vpre, and the reference voltage generation means 17 captures the voltage corresponding to the initial voltage Vpre as the initial value (S21).

[0107] Thereafter, the reference voltage generation means 17 generates (produces) a reference voltage that changes with the passage of time from the initial value corresponding to the initial voltage Vpre (S22). The comparison means 18 compares the reference voltage Vref output by the reference voltage generation means 17 with the output voltage Vth of the temperature detection element 6 (S23). When the output voltage Vth of the temperature detection element 6 becomes higher than the reference voltage Vref of the reference voltage generation means 17, the cutoff means 13 cuts off the power supply to the heater 4 by the power supply 12 (S24). In this embodiment, since the temperature detection element 6 whose output voltage increases monotonically with respect to the detected temperature is used, "Vref is on the higher temperature side than Vth" means the relationship of Vth ≧ Vref.

[0108] (Circuit of over-temperature rise prevention means) Using the circuit diagram of FIG. 23 and the timing diagrams of FIGS. 24(a-c) and 25(a-c), the circuit of the over-temperature rise prevention means 14 in Example 4 will be described. Note that since the power supply signal generation means 15, the initial voltage holding means 16, the inverter 39, and the FET switch 40 are the same as those in Example 3, the description thereof will be omitted.

[0109] As shown in FIG. 23, in this embodiment, a thermopile is used as the temperature detection element 310, and the output voltage Vth of the temperature detection element 310 is output to the initial voltage holding means 16 and the comparison means 18.

[0110] The reference voltage generation means 17 consists of a lower convex curve voltage generation section composed of resistors 301, 303, 306, capacitors 302, 305, and a transistor 304. The lower convex curve voltage generation section is configured to draw a curve in which the voltage value with respect to time is convex downward (low voltage side, low temperature side) by utilizing the charging characteristics of the capacitor. The voltage of the lower convex curve waveform created by the lower convex curve voltage generation section is limited by a voltage limiter composed of resistors 307, 308, and a diode 309 so as not to exceed a predetermined voltage Vb and is output to the comparison means 18. As a result, in the first region shown in FIGS. 24(a-c) and 25(a-c), a lower convex curve is obtained, and a reference voltage Vref having a waveform that becomes a constant value (Vb) in the second region is output. Here, the constant voltage Vb = 2.4V in the second region will naturally change if temperature detection elements 6 having different characteristics from those shown in FIG. 21 are used.

[0111] The comparison means 18 is composed of a comparator 66, compares the output voltage Vth of the temperature detection element 310 with the reference voltage Vref, and outputs a signal for shutting off the shut-off means 13 when Vth ≧ Vref. That is, in this embodiment, since the temperature detection element 310 whose output voltage increases monotonically with respect to the detected temperature is used, "the output voltage Vth is on the high temperature side with respect to the reference voltage Vref" means the relationship Vth ≧ Vref.

[0112] (Operation of the over-temperature rise prevention means) The operation of this embodiment will be described with reference to the timing charts of FIGS. 24(a - c) and FIGS. 25(a - c). FIGS. 24(a - c) and FIGS. 25(a - c) show the temperature of the fixing film 101 from the start of heating, the output voltage Vth of the temperature detection element 310, and the reference voltage Vref of the reference voltage generation means 17. As described with reference to the circuit diagram, the reference voltage Vref is a downwardly convex curve in the first region and a constant value (Vb) in the second region. When the output voltage Vth of the temperature detection element 6 becomes higher than this reference voltage Vref (Vth ≧ Vref), the overheating prevention means 14 determines that the fixing film 101 has entered the abnormal heating region and cuts off the power supply to the induction heating member 102. Here, the time region in which the reference voltage Vref rises while drawing a downwardly convex curve is defined as the first region, and the time region in which the reference voltage Vref is a constant value (Vb) is defined as the second region.

[0113] During normal printing (normal operation) in FIG. 24(a), the fixing film 101 is heated to a target temperature of 150°C, and after a certain time, it becomes approximately 150°C. Correspondingly, the output voltage Vth of the temperature detection element 6 also becomes a voltage corresponding to 150°C. In this case, when comparing the output voltage Vth of the temperature detection element 6 with the reference voltage Vref, Vth < Vref always holds, and the overheating prevention means 14 does not operate, and the power supply to the induction heating member 102 is not cut off.

[0114] FIG. 24(b) shows a case where, due to a temperature control failure of the control means 11, the temperature of the fixing film 101 does not become constant at 150°C and continues to rise (abnormal operation). In this case, the output voltage Vth of the corresponding temperature detection element 6 also does not become constant and continues to rise. In this case, when comparing the output voltage Vth of the temperature detection element 6 with the reference voltage Vref, Vth = Vref at the time when the temperature of the fixing film 101 reaches 200°C in the second region, and the overheating prevention means 14 operates to cut off the power supply to the induction heating member 102.

[0115] FIG. 24(c) shows the case (during abnormal operation) where power is supplied to the induction heating member 102 while the rotational driving of the fixing film 101 and the pressure roller 8 has stopped. In this case, since the heat generated in the fixing film 101 is not taken away by the pressure roller 8, the temperature of the fixing film 101 rises rapidly. In this case, since the output voltage Vth of the temperature detection element 6 rises rapidly as shown by the broken line, Vth≧Vref in the first region, and the over-temperature prevention means 14 operates to cut off the power supply to the induction heating member 102.

[0116] Thus, also in this embodiment, the reference voltage Vref generated by the reference voltage generation means 17 changes in value according to the elapsed time from the start of heating, and the value of the reference voltage Vref changes from the low-temperature side to the high-temperature side as time elapses. In other words, in this embodiment, the reference voltage generation means is configured such that the temperature corresponding to the value of the reference voltage at the time when the second time longer than the first time has elapsed from the start of heating is higher than the temperature corresponding to the value of the reference voltage at the time when the first time has elapsed from the start of heating, which is the time when the control means starts the power supply to the heating means. In this embodiment, as a specific example of the waveform in which the value of the reference voltage is different at the first time and the second time, a waveform is adopted in which the reference voltage Vref rises while drawing a curve convex downward (low-pressure side) with the passage of time. Note that, as another example of the waveform of the reference voltage, a waveform in which the reference voltage Vref rises while drawing a curve convex upward (high-pressure side) with the passage of time may be used.

[0117] According to this embodiment, the value of the reference voltage Vref changes from the low temperature side to the high temperature side according to the elapsed time since the start of heating of the fixing film 101. Therefore, for example, when the temperature of the fixing film 101 rapidly rises immediately after the start of heating, even at a relatively low temperature, the output voltage Vth of the temperature detection element 6 enters the cutoff region on the high temperature side of the reference voltage Vref (Fig. 24(c)), and the energization to the induction heating member 102 is cut off. As a result, when an abnormality accompanied by a rapid temperature rise occurs, the energization to the heating means can be quickly cut off. Here, since the value of the reference voltage Vref is not constant but changes from the low temperature side to the high temperature side, the rise of the fixing film temperature at a normal temperature rise rate in the first region is not hindered.

[0118] Also, in this embodiment, as the reference voltage Vref, a waveform is used in which the value changes to the high temperature side according to the passage of time in the first region after the start of heating of the fixing film 101, and becomes a constant value (Vb) regardless of the passage of time in the second region. For this reason, whether it is an abnormality in which the temperature of the fixing film 101 rapidly rises or an abnormality in which the temperature rises gently, the energization to the induction heating member 102 can be cut off at an appropriate timing before reaching the abnormal heating region.

[0119] In FIGS. 24(a to c) described above, the case where the temperature of the fixing device 100 is normal temperature at the start of printing is illustrated. However, there may be a case where the fixing device 100 is at a high temperature at the start of the printing operation, such as when the next printing operation is started immediately after the end of the previous printing operation. The operation in such a case will be described with reference to FIGS. 24(a to c).

[0120] FIG. 25(a) shows the same case as FIG. 24(c) for comparison. That is, FIG. 25(a) shows a case where an abnormality occurs in which the induction heating member 102 is energized while the rotation of the fixing film 101 and the pressure roller 8 has stopped, and the temperature of the fixing film 101 at the start of printing is normal temperature (25° C.). On the other hand, FIG. 25(b) shows a case where an abnormality occurs in which the induction heating member 102 is energized while the rotation of the fixing film 101 and the pressure roller 8 has stopped, and the temperature of the fixing film 101 at the start of printing is high temperature (100° C.).

[0121] As described above, the output voltage of the temperature detection element 310 corresponding to the temperature before the start of printing is held as the initial voltage Vpre by the initial voltage holding means 16, and the initial value of the reference voltage Vref based on the initial voltage Vpre is delivered to the reference voltage generation means 17. Therefore, when the fixing film 101 is at a high temperature at the start of printing, the voltage value delivered from the initial voltage holding means 16 to the reference voltage generation means 17 is higher than when the fixing film 101 is at normal temperature at the start of printing. As a result, the initial value of the reference voltage Vref (the value at the 0 sec time point) in FIG. 25(b) is higher than the initial value of the reference voltage Vref in FIG. 25(a).

[0122] Therefore, even if a rapid temperature rise of the fixing film 101 occurs when the fixing film 101 is at a high temperature at the start of printing, the energization of the induction heating member 102 can be cut off before entering the abnormal heating region. Note that the level of the reference voltage Vref in the first region becomes higher as the temperature of the fixing film 101 is higher at the start of printing, but the value (Vb) of the reference voltage Vref in the second region is constant regardless of the temperature of the fixing film 101 at the start of printing. Therefore, when the fixing film 101 is at a high temperature at the start of printing, the operation when the output voltage Vth of the temperature detection element 310 enters the cutoff region in the second region is the same as in the case of FIG. 24(b).

[0123] Here, FIG. 25(c) shows the case where the temperature of the fixing film 101 is high (100° C.) at the start of printing and the printing operation is performed normally. In this case, the initial value of the reference voltage Vref is higher than that in the case of FIG. 25(a) where the temperature of the fixing film 101 is normal temperature (25° C.) at the start of printing. However, since the rotation of the fixing film 101 and the pressure roller 8 is performed normally, a rapid temperature rise of the fixing film 101 does not occur. Therefore, the output voltage Vth of the temperature detection element 310 gradually increases in accordance with the temperature rise of the fixing film 101, and after the fixing film 101 reaches the target temperature of 150° C., Vth is also maintained substantially constant. Therefore, even when the temperature of the fixing film 101 is high at the start of printing, when the printing operation is performed normally, the output voltage Vth of the temperature detection element 310 does not become equal to or higher than the reference voltage Vref, and the energization to the induction heating member 102 is not cut off.

[0124] Thus, also in this embodiment, the initial value of the reference voltage Vref is changed according to the temperature of the heater 4 at the start of printing. Thereby, according to the temperature of the fixing film 101 at the start of printing, while allowing a normal temperature rise of the fixing film 101, when a rapid temperature rise of the fixing film 101 is detected, the energization to the induction heating member 102 can be quickly cut off.

[0125] The circuit used in the above description is an example, and any other circuit having the same function may be used.

Explanation of Reference Numerals

[0126] 3,101... Fixing member (fixing film) / 4,102... Heating means (heater, induction heating member) / 6... Temperature detection means (temperature detection element, thermistor, thermopile) / 11... Control means / 12... Power supply / 13... Cut-off means / 17... Reference voltage generation means

Claims

1. A fixing member that heats a toner image transferred onto a recording material to fix it onto the recording material, heating means for heating the fixing member by being energized, a power source for supplying power to the heating means, temperature detection means for outputting an output voltage corresponding to the temperature of the fixing member or the heating means, control means for controlling the power supply from the power source to the heating means so that the temperature of the fixing member becomes a predetermined set temperature based on the output voltage of the temperature detection means, reference voltage generation means for generating a reference voltage, interruption means for interrupting the power supply to the heating means when the temperature corresponding to the output voltage of the temperature detection means becomes higher than the temperature corresponding to the reference voltage, An image forming apparatus having: The reference voltage generation means outputs a reference voltage whose value changes according to the passage of time from the start of heating such that the temperature corresponding to the value of the reference voltage at the time when a second time longer than the first time has elapsed from the start of heating is higher than the temperature corresponding to the value of the reference voltage at the time when the first time has elapsed from the start of heating when the control means starts supplying power to the heating means, The image forming apparatus is characterized in that the reference voltage generation means is configured to linearly change the value of the reference voltage from a lower temperature side to a higher temperature side with respect to the elapsed time from the start of heating.

2. A fixing member that heats a toner image transferred onto a recording material to fix it onto the recording material, heating means for heating the fixing member by being energized, a power source for supplying power to the heating means, temperature detection means for outputting an output voltage corresponding to the temperature of the fixing member or the heating means, control means for controlling the power supply from the power source to the heating means so that the temperature of the fixing member becomes a predetermined set temperature based on the output voltage of the temperature detection means, reference voltage generation means for generating a reference voltage, interruption means for interrupting the power supply to the heating means when the temperature corresponding to the output voltage of the temperature detection means becomes higher than the temperature corresponding to the reference voltage, An image forming apparatus having: The reference voltage generating means outputs a reference voltage whose value changes according to the passage of time from the start of heating, such that the temperature corresponding to the value of the reference voltage at the time when a second time longer than the first time has elapsed since the start of heating is higher than the temperature corresponding to the value of the reference voltage at the time when the first time has elapsed since the start of heating when the control means starts power supply to the heating means. The image forming apparatus is characterized in that the reference voltage generating means is configured to change the value of the reference voltage so that the reference voltage draws a curve convex on the high-temperature side or a curve convex on the low-temperature side with respect to the elapsed time from the start of heating.

3. The reference voltage generating means outputs the reference voltage having a waveform whose value changes according to the passage of time from the start of heating until a predetermined time has elapsed since the start of heating, and outputs a reference voltage having a constant value regardless of the passage of time from the start of heating after the predetermined time has elapsed since the start of heating. The image forming apparatus according to claim 1 or 2, characterized in that it is configured as described above.

4. The image forming apparatus according to claim 3, characterized in that the temperature corresponding to the value of the reference voltage after the predetermined time has elapsed since the start of heating is on the higher temperature side than the value of the reference voltage during the period until the predetermined time has elapsed since the start of heating.

5. A fixing member that heats the toner image transferred to the recording material and fixes it to the recording material, Heating means for heating the fixing member by being energized, A power supply for supplying power to the heating means, Temperature detection means for outputting an output voltage corresponding to the temperature of the fixing member or the heating means, Control means for controlling the power supply from the power supply to the heating means so that the temperature of the fixing member becomes a predetermined set temperature based on the output voltage of the temperature detection means, Reference voltage generating means for generating a reference voltage, Cut-off means for cutting off the power supply to the heating means when the temperature corresponding to the output voltage of the temperature detection means becomes higher than the temperature corresponding to the reference voltage, Initial voltage holding means for holding the output voltage of the temperature detection means at the start of heating when the control means starts power supply to the heating means, and delivering a voltage based on the output voltage to the reference voltage generating means, An image forming apparatus having The reference voltage generating means outputs a reference voltage whose value changes in accordance with the passage of time from the start of heating, such that the temperature corresponding to the value of the reference voltage at the time when a second time longer than the first time has elapsed from the start of heating is higher than the temperature corresponding to the value of the reference voltage at the time when the first time has elapsed from the start of heating. An image forming apparatus, wherein the value of the reference voltage generated by the reference voltage generating means changes in accordance with the voltage delivered from the initial voltage holding means. **Claim 6** The fixing member is a flexible cylindrical film member. The heating means includes a heater having a heating resistor that generates heat upon energization and is disposed inside the film member. The apparatus further includes a pressure roller that faces the heater with the film member interposed therebetween and forms a nip portion between the heater and the pressure roller. The image forming apparatus according to any one of claims 1 to 5, wherein the toner image is fixed to the recording material by the film member heated by non-radiative heat of the heater while the recording material is sandwiched and conveyed between the film member and the pressure roller in the nip portion. **Claim 7** The heater has an integrated structure in which the heating resistor is embedded inside an insulating substrate made of Al ２ O ３ or AlN. The image forming apparatus according to claim 6, characterized in that it has such a structure. **Claim 8** The fixing member is a rotating body having a conductive layer. The heating means includes a coil that generates an alternating magnetic field when an alternating voltage is applied, and is configured to inductively heat the conductive layer by the alternating magnetic field. The apparatus further includes an opposing member that forms a nip portion between the rotating body and the opposing member. The image forming apparatus according to any one of claims 1 to 5, wherein the toner image is fixed to the recording material by the rotating body heated by inductive heating while the recording material is sandwiched and conveyed between the rotating body and the opposing member in the nip portion.

Images