Image forming apparatus

Abstract

To achieve both protection of a fixing device against an overvoltage and reduction in size of the fixing device and an image forming apparatus.SOLUTION: An image forming apparatus has a metal frame that provides a ground potential, image forming means that forms a toner image on a sheet, and a fixing device that fixes the toner image to the sheet. The fixing device has a heating element, an insulating layer, a conductive layer, a flexible cylindrical member heated by the heating element, and pressure means. The image forming apparatus is further provided with a capacitive element that is connected with the metal frame at one end, and connected with the conductive layer of the fixing device at the other end.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus having a fixing device. [Background technology]

[0002] A fixing device generally includes an endless belt (also called a fixing film), a flat heater that contacts the inner surface of the endless belt, and a pressure roller that forms a nip portion in cooperation with the heater via the endless belt. Because AC power supplied from a commercial AC power source is applied to the heater, excessive voltages such as lightning surges may be applied to the heater. Such excessive voltages may destroy the insulating member (insulating layer) between the heater and the core of the pressure roller. Patent Document 1 proposes protecting the fixing device from excessive voltages by connecting a capacitor between the core of the pressure roller and a metal frame that constitutes an image forming apparatus. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 5305931 Summary of the Invention [Problem to be solved by the invention]

[0004] In Patent Document 1, capacitance also occurs between the heater and the core of the pressure roller. Because the distance between the heater and the core and between the heater and the core of the pressure roller is long, this capacitance is insufficient to withstand excessive voltage. Furthermore, because the pressure roller has an elastic layer, the capacitance between the heater and the core of the pressure roller is prone to variation. As a result, the potential of the fixing film also varies. Considering this variation, the creepage distance and spatial distance between the fixing film and the metal frame must be sufficiently long, making it difficult to miniaturize the image forming apparatus. Therefore, an object of the present invention is to simultaneously protect the fixing device from excessive voltage and miniaturize the fixing device and image forming apparatus. [Means for solving the problem]

[0005] The present invention is, for example, a metal frame providing a ground potential; an image forming means for forming a toner image on a sheet; a fixing device that fixes the toner image to the sheet, a heating element that generates heat when power is supplied from an AC power source; an insulating layer covering the heating element; a conductive layer in contact with the insulating layer; a flexible tubular member that is heated by the heating element; a fixing device having a pressure means provided opposite the cylindrical member and cooperating with the cylindrical member to form a nip portion; a capacitance element having one end connected to the metal frame and the other end connected to the conductive layer of the fixing device; The present invention provides an image forming apparatus comprising: [Effects of the Invention]

[0006] According to the present invention, it is possible to protect the fixing device from overvoltage while also reducing the size of the fixing device and the image forming apparatus. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 1 is a diagram illustrating an image forming apparatus. [Figure 2] FIG. 1 is a diagram illustrating a fixing device according to a first embodiment. [Figure 3] FIG. 1 is a diagram illustrating an equivalent circuit of a fixing device according to a first embodiment. [Figure 4] FIG. 1 is a diagram illustrating the structure of a heater according to a first embodiment. [Figure 5] FIG. 1 is a diagram illustrating a power supply circuit (control circuit) according to a first embodiment. [Figure 6] FIG. 10 is a diagram illustrating the structure of a heater according to a second embodiment. [Figure 7] FIG. 10 is a diagram illustrating the configuration of a fixing device according to a third embodiment. [Figure 8]FIG. 10 is a diagram illustrating the structure of a heater according to a third embodiment. [Figure 9] FIG. 10 is a diagram illustrating the structure of a heater according to a third embodiment. [Figure 10] FIG. 10 is a diagram illustrating a fixing device according to a fourth embodiment. [Figure 11] FIG. 10 is a diagram illustrating the structure of a heater according to a fourth embodiment. [Figure 12] FIG. 10 is a diagram illustrating the structure of a heater according to a fourth embodiment. [Figure 13] FIG. 10 is a diagram illustrating a power supply circuit (control circuit) according to a third embodiment. [Figure 14] FIG. 10 is a diagram illustrating a power supply circuit (control circuit) according to a third embodiment. [Figure 15] FIG. 10 is a diagram illustrating a drive signal according to a fourth embodiment. [Figure 16] FIG. 10 is a diagram illustrating insulation in Example 4. [Figure 17] FIG. 10 is a diagram illustrating an equivalent circuit of a fixing device according to a fourth embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0008] Hereinafter, embodiments will be described in detail with reference to the accompanying drawings. Note that the following embodiments do not limit the scope of the invention claimed. Although multiple features are described in the embodiments, not all of these multiple features are necessarily essential to the invention, and multiple features may be combined arbitrarily. Furthermore, in the accompanying drawings, the same reference numerals are used to designate the same or similar components, and redundant explanations will be omitted.

[0009] Example 1 [Structure of image forming device] 1, the image forming apparatus 100 is a printer that uses electrophotographic recording technology to form an image on a sheet P. The image forming apparatus 100 may also be realized as a copying machine, a multifunction machine, or a facsimile machine.

[0010] The photoconductor 19 is an image carrier that rotates and carries an electrostatic latent image and a toner image. The charging roller 16 uniformly charges the surface of the photoconductor 19. The scanner unit 21 has a laser light source 22, a rotating polygon mirror 23, and a reflecting mirror 24. The laser light source 22 outputs a laser beam modulated according to image information. The rotating polygon mirror 23 deflects the laser beam as it rotates. The reflecting mirror 24 deflects the laser beam toward the photoconductor 19. As a result, an electrostatic latent image is formed on the surface of the photoconductor 19.

[0011] The process cartridge 15 is a replaceable consumable part that includes a photosensitive member 19, a charging roller 16, a developing roller 17, and a cleaning member 18. The developing roller 17 develops the electrostatic latent image with toner to form a toner image.

[0012] The feed cassette 11 is a storage container that stores a plurality of sheets P. The pickup roller 12 feeds the sheets P one by one from the feed cassette 11. The transport roller 13 is provided downstream of the pickup roller 12 in the transport direction of the sheet P, and transports the sheet P toward the registration roller 14. The registration roller 14 corrects skew of the sheet P and transports the sheet P so that the timing at which the toner image arrives at the transfer position and the timing at which the sheet P arrives at the transfer position coincide with each other.

[0013] The transfer position is a transfer nip formed by the photoreceptor 19 and the transfer roller 20. The toner image is transferred from the photoreceptor 19 to the sheet P by conveying the sheet P between the transfer roller 20 and the photoreceptor 19. The sheet P is then conveyed to the fixing device 50. The cleaning member 18 cleans the photoreceptor 19 by removing any toner remaining on the photoreceptor 19.

[0014] The fixing device 50 applies heat and pressure to the sheet P and the toner image, thereby fixing the toner image to the sheet P. Conveying rollers 26 and 27 are provided downstream of the fixing device 50. The conveying rollers 26 and 27 eject the sheet P that has passed through the fixing device 50 to the outside of the image forming apparatus 100.

[0015] The motor 30 is a drive source that drives the fixing device 50 and the like. The conveying speed of the sheet P is proportional to the rotation speed of the motor 30. The power supply circuit 40 is connected to a commercial AC power source 41, and supplies power supplied from the commercial AC power source 41 to the fixing device 50. The photosensitive member 19, the charging roller 16, the scanner unit 21, the developing roller 17, and the transfer roller 20 form an image forming unit that forms an image on the sheet P.

[0016] The image forming apparatus 100 can form images on sheets P of a plurality of different sizes. The feed cassette 11 can accommodate, for example, Letter paper (approximately 216 mm x 279 mm) and Legal paper (approximately 216 mm x 356 mm). Furthermore, the feed cassette 11 can accommodate A4 paper (210 mm x 297 mm), Executive paper (approximately 184 mm x 267 mm), JIS B5 paper (182 mm x 257 mm), and A5 paper (148 mm x 210 mm). JIS is an abbreviation for Japanese Industrial Standards. The image forming apparatus 100 basically feeds sheets P vertically (conveys sheets P so that the long sides of the sheets P are parallel to the conveyance direction). However, the embodiment can also be applied to a printer that feeds sheets P horizontally (conveys sheets P so that the short sides of the sheets P are parallel to the conveyance direction). Among the standard sheet P widths (nominal sheet P widths) that can be loaded into the image forming apparatus 100, the sheet P with the largest width is letter paper and legal paper. These widths are approximately 216 mm. A sheet P that is smaller than the maximum size sheet P that can be loaded into the image forming apparatus 100 may be called a small size sheet.

[0017] [Fixing device structure] FIG. 2 is a cross-sectional view of the fixing device 50. Arrow F indicates the conveyance direction of the sheet P. The film 202 is a flexible cylindrical film (endless belt). The heater 230 is disposed so as to contact the inner circumferential surface of the film 202 and heats the film 202. The pressure roller 208 is a nip forming member that forms the fixing nip N in cooperation with the film 202 and heater 230. The base layer of the film 202 is made of a heat-resistant resin such as polyimide or a metal such as stainless steel. The surface layer of the film 202 may have an elastic layer made of heat-resistant rubber, for example. The heater 230 is held by a holding member 201 made of heat-resistant resin. The holding member 201 also has a guide function for guiding the rotation of the film 202. The metal stay 204 is a metal stay that applies pressure from a spring (not shown) to the holding member 201. The safety element 212 is a thermoswitch or a thermal fuse that is activated by abnormal heat generation in the heater 230 to cut off the power supplied to the heater 230. The safety element 212 is in direct contact with the heater 230 or indirectly via the holding member 201.

[0018] The pressure roller 208 has a core 209 made of a metal material such as iron or aluminum, and an elastic layer 210 made of a material such as silicone rubber. The pressure roller 208 receives power from a motor 30 and rotates in the direction of the arrow. As the pressure roller 208 rotates, the film 202 rotates in conjunction with the pressure roller 208. The sheet P carrying an unfixed toner image is heated while being sandwiched and conveyed by the fixing nip N.

[0019] 2, heater 230 has a back surface (non-sliding surface) and a front surface (sliding surface). The back surface is the surface that comes into contact with holding member 201. The front surface is the surface that faces pressure roller 208. Heater 230 has a ceramic substrate 235 that is an insulating member. Resistance heating elements 232 and 233 are provided on the back surface side of substrate 235. Resistance heating element 233 is provided on the upstream side in the conveying direction of sheet P, and resistance heating element 232 is provided on the downstream side. Surface protection layer 237 is glass that covers and insulates resistance heating elements 232 and 233.

[0020] A conductor 234 is provided on the sliding surface side of the substrate 235. The conductor 234 is a conductive layer formed over almost the entire sliding surface of the substrate 235. A terminal (not shown) connected to the conductor 234 is provided at one end of the substrate 235 in the longitudinal direction. The capacitor 236 is disposed inside the fixing device 50. One end of the capacitor 236 is connected to the conductor 234. The other end of the capacitor 236 is connected to a frame ground 239. The frame ground 239 is a metal frame connected to the electrical ground (earth) of the image forming apparatus 100. The surface protection layer 238 is glass that protects the conductor 234 and improves the sliding properties of the fixing nip portion N.

[0021] In the image forming apparatus described in Patent Document 1, a capacitor is connected between the pressure roller and the frame ground (FG) to prevent breakdown of the insulating layer and protect the fixing device from excessive voltages such as lightning surges without increasing the size of the fixing device. However, because the shaft (core) of the pressure roller is connected to the frame ground via a capacitor, the distance from the resistance heating element to the core of the pressure roller is likely to be long. As a result, the electrostatic capacitance formed between the resistance heating element and the pressure roller (hereinafter referred to as the electrostatic capacitance of the pressure roller) is small. The pressure roller has an elastic layer. Therefore, the electrostatic capacitance of the pressure roller is likely to fluctuate significantly due to variations in the thickness and deformation of the elastic layer. In other words, when excessive voltage occurs, the potential of the fixing film varies. To account for this potential variation, the creepage distance and spatial distance between the fixing film and the frame ground must be increased. As a result, it has been difficult to miniaturize the fixing device and image forming apparatus. In other words, the fixing device described in Patent Document 1 has room for further miniaturization.

[0022] Therefore, in this embodiment, the connection destination of the capacitor 236 connected to the frame ground 239 is changed from the core metal 209 to the conductor 234 provided on the sliding surface side of the substrate 235. This will make it easier to reduce the size of the fixing device 50 and the image forming apparatus 100.

[0023] 3 shows the electrical equivalent circuit of the fixing device 50. An electrostatic capacitance Chf exists between the resistance heating elements 232, 233 and the film 202. An electrostatic capacitance Cfg exists between the film 202 and the conductor 234. An electrostatic capacitance Chg exists between the resistance heating elements 232, 233 and the conductor 234. The electrostatic capacitance of a capacitor 236 connected between the conductor 234 and the frame ground 239 is Cx.

[0024] When an excessive voltage Vsurge is applied to point A, the excessive voltage Vsurge is divided by the capacitances Chf, Cfg, Chg, and Cx. As a result, the voltages Vhg, Vhf, Vfg, and V become lower than the excessive voltage Vsurge. Here, voltage Vhg is the voltage between the resistance heating elements 232 and 233 and the conductor 234. Voltage Vhf is the voltage between the resistance heating elements 232 and 233 and the film 202. Voltage Vfg is the voltage between the film 202 and the conductor 234. Voltage V is the voltage between the film 202 and the frame ground 239. In this way, voltage V between the frame ground 239 and the film 202 becomes lower than the excessive voltage Vsurge, making it possible to shorten the distance x.

[0025] Furthermore, the conductor 234 is disposed near the resistance heating elements 232 and 233 and the film 202. Therefore, the capacitance Cfg between the film 202 and the conductor 234 and the capacitance Chg between the resistance heating elements 232 and 233 and the conductor 234 are large, reducing the effect of variations in the thickness of the elastic layer 210. On the other hand, the thickness of the substrate 235 and the thickness of the surface protection layer 238 are unlikely to change. Therefore, variations in the capacitance Cfg between the film 202 and the conductor 234 and variations in the capacitance Chg between the resistance heating elements 232 and 233 and the conductor 234 are small.

[0026] Therefore, the creepage distance and spatial distance required to prevent discharge from the film 202 to the frame ground 239 when an excessive voltage Vsurge occurs can be made shorter than before, which makes it possible to reduce the size of the fixing device 50 and the image forming apparatus 100 while maintaining the protective performance of the fixing device 50.

[0027] In this embodiment, the capacitor 236 is disposed inside the fixing device 50, but this is merely an example. The capacitor 236 may be disposed on a substrate (not shown) provided outside the fixing device 50. In this case, the capacitor 236 is electrically connected to the conductor 234 via a wire bundle. This reduces the number of components inside the fixing device 50, allowing the fixing device 50 to be further miniaturized.

[0028] In this embodiment, the resistance heating elements 232 and 233 are arranged on the rear surface of the substrate 235, and the conductor 234 is arranged on the front surface, but this is merely an example. The conductor 234 may be arranged on the rear surface, and the resistance heating elements 232 and 233 may be arranged on the front surface. Details of this structure will be described later.

[0029] [Heater structure] 4(A) and 4(B) show the structure of the heater 230. FIG. 4(A) is a cross-sectional view of the heater 230 taken at the transport reference position Y shown in FIG. 4(B). FIG. 4(B) is a plan view illustrating the structure of the heater 230 in the longitudinal direction. In FIG. 4(B), the upper surface of the first back surface layer and the upper surface of the second back surface layer indicate the upper surfaces of the respective layers when looking down on the heater 230 from above. The lower surface of the first front surface layer and the lower surface of the second front surface layer indicate the lower surfaces of the respective layers when looking up on the heater 230 from below. The transport reference position Y coincides with the center of the sheet P in the width direction (the direction perpendicular to the transport direction). Regardless of size differences, the sheet P is centered and transported so that the center of the sheet P coincides with the transport reference position Y.

[0030] As shown in FIG. 4(A), the first back surface layer of the heater 230 has a resistance heating element 232 and a resistance heating element 233 provided on a substrate 235. The second back surface layer of the heater 230 has an insulating surface protective layer 237 formed to cover the resistance heating element 232 and the resistance heating element 233. The first surface layer of the heater 230 has a conductor 234 formed on the substrate 235. Furthermore, the second surface layer of the heater 230 has an insulating surface protective layer 238 formed to cover the conductor 234. The surface protective layer 237 fills the gaps and both ends of the resistance heating element 232 and the resistance heating element 233 on the first back surface layer. Similarly, the surface protective layer 238 fills both ends of the conductor 234 on the first surface layer. The surface protective layers 237 and 238 are, for example, glass.

[0031] 4(B), the first back surface layer of the heater 230 is provided with the resistance heating element 232, the resistance heating element 233, and the electrodes E1 and E2. The conductor 401a electrically connects the electrode E1 to one end of the resistance heating element 232. The conductor 401b electrically connects the electrode E2 to one end of the resistance heating element 233. The conductor 401c electrically connects the other end of the resistance heating element 232 to the other end of the resistance heating element 233. The electrodes E1 and E2 are provided on one end side of the substrate 235 in the longitudinal direction.

[0032] In the second back surface layer of the heater 230, an insulating surface protective layer 237 covers the resistance heating elements 232, 233 and the conductors 401a to 401c, except for the electrodes E1 and E2. The electrodes E1 and E2 are not covered by the surface protective layer 237 and are exposed.

[0033] The first surface layer of the heater 230 has a conductor 234 and an electrode E3 formed on the sliding surface side of the substrate 235. The conductor 234 and the power supply electrode E3 are directly connected. Furthermore, the second surface layer of the heater 230 has an insulating surface protection layer 238 that exposes the electrode E3 and covers the conductor 234.

[0034] [Power supply circuit] FIG. 5 shows the power supply circuit 40 of the first embodiment. The resistance heating elements 232 and 233 constituting the heater 230 are electrically connected to the power supply circuit 40 via electrodes E1 and E2 provided on the heater 230. The power supply voltages Vcc1 and Vcc2 are DC voltages generated by an AC / DC converter (not shown) connected to a commercial AC power supply 41. AC is an abbreviation for alternating current. DC is an abbreviation for direct current. The commercial AC power supply 41 is connected to the heater 230 via a relay 530 and a triac 540. The triac 540 is turned on and off by a control signal FUSER1 from the CPU 510. The driver circuit for the triac 540 is not shown. The zero-cross circuit 520 generates a ZEROX signal corresponding to the zero-cross timing of the commercial AC power supply 41 and inputs it to the CPU 510. The zero-cross circuit 520 is internally insulated. For example, in zero-cross circuit 520, reinforced insulation may be provided between a primary circuit connected to commercial AC power supply 41 and a secondary circuit that outputs a ZEROX signal.

[0035] The thermistors T1 and T2 form a temperature detection circuit. The detection voltage VTh1 of the thermistor T1 is generated by dividing the power supply voltage Vcc2 using the resistance of the thermistor T1 and the combined resistance of the pull-up resistor 561 and the variable resistor 562. The detection voltage VTh2 of the thermistor T2 is generated by dividing the power supply voltage Vcc2 using the resistance of the thermistor T2 and the resistance of the pull-up resistor 563. The detection voltages VTh1 and VTh2 are input to the CPU 510. The CPU 510 has a memory 511. The CPU 510 converts the detection voltages VTh1 and VTh2 into temperatures according to the information stored in the memory 511.

[0036] The thermistor T1 is used as a temperature sensor for controlling the temperature of the heater 230. The CPU 510 calculates the power to be supplied to the heater 230 through control (e.g., PI control) based on a set target temperature and the temperature detected by the thermistor T1. The CPU 510 converts the calculated power into a control level for a phase angle (phase control) and a wave number (wave number control) corresponding to the calculated power. PI stands for proportional-integral. The CPU 510 controls the triac 540 according to the zero-cross timing of the commercial AC power supply 41 detected by the zero-cross circuit 520 and the control level. This maintains the temperature detected by thermistor T1 at the target temperature. The thermistor T2 plays an auxiliary role. For example, if the temperature detected by thermistor T2 is equal to or higher than a preset threshold temperature, the CPU 510 reduces the conveying speed of the sheet P. In other words, the CPU 510 reduces the rotation speed of the motor 30. This prevents the heater 230 from excessively increasing in temperature at its longitudinal end (non-sheet passing area).

[0037] Next, the operation of relay 530 will be described. When CPU 510 sets the RLON signal to a high state, the RLON signal turns on transistor 565 via resistor 564. This causes current to flow from power supply voltage Vcc1 to the secondary coil L of relay 530, and the primary contact of relay 530 turns on. When the RLON signal turns to a low state, transistor 565 turns off. This interrupts the current flowing from power supply voltage Vcc1 to the secondary coil L of relay 530, and the primary contact of relay 530 turns off.

[0038] Next, the operation of the safety circuit using the relay 530 will be described. The comparison circuit 551 has resistors 566 and 567 that generate a reference voltage (threshold voltage). Resistors 566 and 567 generate the threshold voltage by dividing the power supply voltage Vcc2. The threshold voltage is a voltage corresponding to a temperature (threshold temperature) that is not reached during normal printing and that can safely shut down the heater 230 in the event of an excessive temperature rise. The threshold voltage is determined by the voltage division ratio between resistors 566 and 567. A comparator 568 compares the threshold voltage with the detection voltage VTh1 of the thermistor T1. If the detection voltage VTh1 is lower than the threshold voltage, the RLOFF signal output from the comparator 568 goes high. If the detection voltage VTh1 is equal to or higher than the threshold voltage, some abnormality has occurred in the power supply to the heater 230. Therefore, the comparator 568 sets the RLOFF signal to low. The latch circuit 552 latches the RLOFF signal to a low state. When the RLOFF signal is latched to a low state, the transistor 565 is maintained in an off state even if the CPU 510 sets the RLON signal to a high state. As a result, the relay 530 is forcibly maintained in an off state (a safe state). In this way, when an abnormal state occurs due to a malfunction of the CPU 510 or a failure of the triac 540, the relay 530 also functions as a power cutoff circuit that prevents the heater 230 from overheating.

[0039] The thermistor T1 has manufacturing variations (individual differences). Therefore, the resistance value of the thermistor T1 at a certain temperature differs for each individual thermistor T1. The greater this manufacturing variation, the greater the deviation of the detection voltage of the thermistor T1 from the ideal value. For this reason, it has been difficult to use a thermistor T1 with large manufacturing variations in the safety circuit (comparison circuit 551).

[0040] In this embodiment, the thermistors T1 and T2 have a mechanism for correcting individual differences. In particular, hardware correction is applied to thermistor T1, and software correction is applied to thermistor T2. This reduces the influence of manufacturing variations in thermistors T1 and T2.

[0041] ●Hardware correction The detected voltage VTh1 of the thermistor T1 is expressed by the following equation (1): Here, it is assumed that the variable resistor 562 is not provided.

[0042] VTh1=Vcc2×RT1 / (RT1+R561)...(1) Here, RT1 is the resistance of thermistor T1, and R561 is the resistance of resistor 561.

[0043] The thermistor T1 has manufacturing variations. When a thermistor T1 with large manufacturing variations is installed, the discrepancy between the actual heater 230 temperature and the temperature recognized by the CPU 510 becomes larger. As a result, the power supplied to the heater 230 during printing by the image forming apparatus 100 may be too high or too low compared to the power required by the heater 230. Furthermore, even in the safety circuit using the relay 530 described above, the discrepancy between the actual heater 230 temperature and the detected temperature becomes large. As a result, there may be cases where the relay 530 cannot be turned off when the actual heater 230 temperature reaches the expected temperature. Regardless of the manufacturing variations of the thermistor T1 installed in the heater 230, the detected voltage VTh1 at a given temperature should be constant. Therefore, in this embodiment, hardware correction is applied to the thermistor T1.

[0044] By connecting the variable resistor 562 in parallel with the pull-up resistor 561, the detection voltage VTh1 is expressed by equation (2).

[0045] VTh1=Vcc2 / (1+R561×R562 / (RT1(R561+R562))) ···(2) Here, R562 is the resistance value of the variable resistor 562.

[0046] For example, during the manufacturing process of fixing device 50, the resistance value RT1 of thermistor T1 at a certain predetermined temperature is measured in advance and stored in memory 511 of CPU 510. CPU 510 uses resistance value RT1 to adjust variable resistor 562 so as to obtain an ideal voltage VTh1 at the certain predetermined temperature. By adjusting variable resistor 562 in this manner, the detected voltage VTh1 at the certain predetermined temperature is always constant, regardless of the manufacturing variations of the thermistor T1. Here, it is assumed that variations in power supply voltage Vcc2 and resistor 561 are much smaller than the manufacturing variations of the thermistor T1.

[0047] In this embodiment, the thermistor T1 can accurately detect the actual temperature of the heater 230, so that appropriate power is supplied to the heater 230. Furthermore, even if some abnormality occurs and the temperature of the heater 230 rises excessively, the comparison circuit 551 operates appropriately. This is because there is almost no discrepancy between the actual temperature of the heater 230 and the detected temperature input to the comparison circuit 551. This allows the relay 530 to cut off the power supply to the heater 230 when an excessive temperature rise of the heater 230 is detected. Note that the adjustment of the variable resistor 562 may be performed manually when the fixing device 50 is shipped from the factory.

[0048] ● Software correction Software correction is applied to the thermistor T2. During the manufacturing process of the fixing device 50, the resistance value RT2 of the thermistor T2 at a certain predetermined temperature is measured in advance, and the deviation amount D is stored in the memory 511 of the CPU 510. The deviation amount D is the difference between the resistance value RT2 and the resistance value RT2ref of the reference thermistor T2. The CPU 510 performs analog-to-digital conversion on the detected voltage VTh2 of the thermistor T2 to obtain the detected value VTh2AD. The CPU 510 corrects the detected value VTh2AD using the deviation amount D read from the memory 511. The detected voltage VTh2 of the thermistor T2 is expressed by equation (3).

[0049] VTh2=Vcc2×RT2 / (RT2+R563) ···(3) Here, RT2 is the resistance value of thermistor T2. R563 is the resistance value of resistor 563. Like thermistor T1, thermistor T2 also has manufacturing variations. Therefore, if a thermistor T2 with large manufacturing variations is installed, the difference between the actual temperature of heater 230 and the temperature recognized by CPU 510 will be large.

[0050] As an example, the CPU 510 processes the detected voltage VTh2 in 10 bits. In this case, the detected value VTh2AD is expressed by equation (4).

[0051] VTh2AD=VTh2×1023 / Vcc2 (4) On the other hand, the detection voltage VTh2 is affected by manufacturing variations in the thermistor T2. When the detected voltage VTh2 is corrected by the deviation amount D, the formula (5) is obtained.

[0052] VTh2=Vcc2×D×RT2ref / (D×RT2ref+R563) ···(5) Furthermore, substituting equation (5) into equation (4) gives equation (6).

[0053] VTh2AD=(D×RT2ref / (D×RT2ref+R563))×1023 ···(6) Here, RT2ref is expressed by equation (7).

[0054] RT2ref=VTh2AD×R563 / (D×(1023-VTh2AD)) ···(7) Therefore, the corrected detection value is expressed by equation (8).

[0055] VTh2AD'=(VTh2AD / ((1-D)VTh2AD+1023×D))×1023 ···(8) The CPU 510 can calculate the corrected detection value VTh2AD' based on the detection value VTh2AD and the deviation amount D. In this way, the temperature correction is realized by using the deviation amount D obtained during the manufacturing process of the fixing device 50 and the detection value VTh2AD obtained by the CPU 510. No matter what manufacturing variations the thermistor T2 has, the influence of the manufacturing variations can be reduced. In other words, the difference between the actual temperature of the heater 230 and the detected temperature recognized by the CPU 510 becomes sufficiently small.

[0056] Software correction is a method of correcting the detection value VTh2AD of the detection voltage VTh2 through internal calculations by the CPU 510. Therefore, the detection voltage VTh2 is not directly corrected. Meanwhile, the detection voltage VTh2 is input to the comparator 568 of the comparison circuit 551 without correction. Therefore, software correction is inapplicable to the comparison circuit 551, which is a safety circuit using the relay 530. For example, a comparison circuit 551 using a thermistor T2 with a certain manufacturing variation may operate correctly when the actual temperature of the heater 230 is equal to or higher than the threshold temperature. However, a thermistor T2 with a different manufacturing variation may cause the comparison circuit 551 to operate when the actual temperature is lower than the threshold temperature. Therefore, the thermistor T2 to which software correction is applied is not connected to a safety circuit using the relay 530.

[0057] The advantage of software correction is that it reduces manufacturing costs because it is a correction method via the CPU 510. Hardware correction requires the cost of the variable resistor 562 itself, the cost of connecting the variable resistor 562, and the cost of adjusting the variable resistor 562 during the manufacturing process of the fixing device 50. For this reason, hardware correction is applied to the thermistor T1 connected to a safety circuit using the relay 530. This protects the fixing device 50 from abnormal states caused by malfunctions of the CPU 510 or failures of the triac 540.

[0058] According to this embodiment, the conductor 234 is formed on one of the rear and front surfaces of the substrate 235 of the heater 230, which is different from the rear surface on which the resistance heating elements 232 and 233 are mounted. Furthermore, the conductor 234 and the frame ground 239 are connected via a capacitor 236. Compared to the comparative example in which the capacitor 236 is connected to the core metal 209, this embodiment can reduce the cause of the potential variation that occurs in the film 202 when an excessive voltage occurs. This makes it possible to both reduce the size of the fixing device 50 and protect the fixing device 50 from excessive voltage.

[0059] This embodiment absorbs the effects of manufacturing variations (individual differences) in the thermistors T1 and T2 that detect the temperature of the heater 230. As a result, the accuracy of temperature detection is improved, and the safety of the heater 230 is improved.

[0060] <Example 2> In the first embodiment, the resistance heating elements 232 and 233 are disposed on the rear surface of the substrate 235, and the conductor 234 is disposed on the front surface of the substrate 235. However, as shown in Figures 6(A) and 6(B), the resistance heating elements 232 and 233 may be disposed on the front surface (sliding surface) of the substrate 235, and the conductor 234 may be disposed on the rear surface (non-sliding surface) of the substrate 235. In the second embodiment, the heater 230 of the first embodiment is turned upside down. Therefore, the remaining explanations of the second embodiment are the same as those of the first embodiment.

[0061] Example 3 In Examples 1 and 2, the resistance heating elements 232 and 233 and the conductor 234 are arranged on different surfaces of the substrate 235. However, the resistance heating elements 232 and 233 and the conductor 234 may be arranged on the same surface of the substrate 235.

[0062] The configuration of the image forming apparatus 100 of the third embodiment is the same as that of the first embodiment, except for the fixing device 50. Therefore, the differences between the third embodiment and the first embodiment will be described in detail below, and the description of the first embodiment will be used to explain the commonalities between the third embodiment and the first embodiment.

[0063] As shown in Figures 7, 8(A), and 8(B), heater 730 is used in place of heater 230. Conductor 734 has the same function as conductor 234 (a plate of a capacitor). As shown in Figure 8(A), conductor 734 and resistance heating elements 232 and 233 are arranged on the same side of substrate 235. Resistance heating elements 232 and 233 are mounted on the back surface of substrate 235. Furthermore, surface protective layer 237 is laminated so as to cover resistance heating elements 232 and 233. Furthermore, conductor 734 is laminated on surface protective layer 237. Therefore, no conductive layer is formed on the sliding surface side.

[0064] As shown in Fig. 8(B), conductor 734 is formed over the entire back surface side of substrate 235, and terminal E3 is formed at an end in the longitudinal direction of substrate 235. Terminal E3 is electrically connected to one terminal of capacitor 236. The other terminal of capacitor 236 is electrically connected to frame ground 239. As shown in Fig. 8(B), surface protection layer 238 is formed on the sliding surface of substrate 235.

[0065] In this embodiment, a conductor 734 formed on the same surface as the resistance heating elements 232 and 233 is connected to a frame ground 239 via a capacitor 236. However, the equivalent circuit of the third embodiment is the same as the equivalent circuit of the first embodiment shown in FIG.

[0066] The conductor 734 is disposed near the resistance heating elements 232 and 233 and near the film 202. This increases the capacitance Cfg between the film 202 and the conductor 734 and the capacitance Chg between the resistance heating elements 232 and 233 and the conductor 734. As a result, the effects of the above-mentioned variations are reduced. Because the thickness of the substrate 235 and the thickness of the surface protection layers 237 and 238 are unlikely to change, variations in the capacitance Cfg between the film 202 and the conductor 734 and variations in the capacitance Chg between the resistance heating elements 232 and 233 and the conductor 734 are reduced.

[0067] This reduces the influence of potential variations and further shortens the creepage distance and spatial distance required to prevent discharge from the film 202 to the frame ground 239. This allows the fixing device 50 to be both miniaturized and protected.

[0068] In the third embodiment, it is assumed that the capacitor 236 is disposed inside the fixing device 50, but this is merely an example. The capacitor 236 may also be disposed outside the fixing device 50. In this case, the conductor 734 and the capacitor 236 are connected via a bundled wire. This reduces the number of parts inside the fixing device 50, and further miniaturization can be achieved.

[0069] In the third embodiment, the resistance heating elements 232 and 233 and the conductor 734 are disposed on the back surface side of the substrate 235. However, as shown in Fig. 9, the resistance heating elements 232 and 233 and the conductor 734 may be disposed on the sliding surface side of the substrate 235. In this case, an additional surface protection layer may be further employed to cover the conductor 734. Providing the additional surface protection layer will improve the sliding performance of the heater 730.

[0070] A conductor 734 is formed on the same surface of the substrate 235 of the heater 730 as the resistance heating elements 232 and 233, and the conductor 734 is connected to the frame ground 239 by a capacitor 236, thereby suppressing potential variations in the fixing film when an excessive voltage occurs. This makes it possible to both miniaturize the fixing device and protect the fixing device from excessive voltage.

[0071] Example 4 The technical idea of ​​the fourth embodiment is basically the same as the technical idea of ​​the first embodiment. Therefore, the following will describe in detail the differences between the fourth embodiment and the first embodiment. The explanation of the first embodiment will be cited for the explanation of the commonalities between the fourth embodiment and the first embodiment.

[0072] FIG. 10 shows a fixing device 50 of a fourth embodiment. One difference is that a heater 1000 is used instead of the heater 230. FIG. 11(A) is a cross-sectional view of the heater 1000 taken at position Y1 near the transport reference position Y shown in FIG. 11(B). FIG. 11(B) is a plan view illustrating the structure of the heater 1000 in the longitudinal direction. A capacitor C3 is disposed between the frame ground 239 and the heater 1000.

[0073] As shown in FIG. 11(A), the first back surface layer of the heater 1000 has conductors 1a, 1b, and a conductor 3 provided on a substrate 235. The conductor 1a is disposed upstream of the conductor 1b in the conveying direction of the sheet P. The first back surface layer also has resistance heating elements 2a and 2b. The resistance heating element 2a is provided on the substrate 235 between the conductors 1a and 3. The resistance heating element 2b is provided on the substrate 235 between the conductors 1b and 3. The conductors 1a, 1b, and 3 function as wiring that supplies power to the resistance heating elements 2a and 2b. The first back surface layer also has a power supply electrode E4. In this example, the electrode E4 is connected to the conductor 3.

[0074] The second back surface layer has an insulating surface protection layer 237 made of glass or the like. The surface protection layer 237 covers the back surface of the substrate 235, the conductors 1a, 1b, and 3, and the resistance heating elements 2a and 2b, except for the electrode E4.

[0075] The first surface layer of the heater 1000 is a layer in contact with the surface of the substrate 235. The first surface layer has a conductor ET for supplying power to a plurality of printed thermistors T1 and T2, and conductors EGa and EGb. The conductor EGa is disposed upstream of the conductor EGb in the conveyance direction of the sheet P. The second surface layer of the heater 1000 is laminated on the first surface layer. The second surface layer has a surface protection layer 238 that covers the conductor ET and the conductors EGa and EGb. The surface protection layer 238 is formed so as to fill the gaps and both ends of the conductors EGa, EGb, and ET provided on the first surface layer.

[0076] As shown in FIG. 11(B), on the first back surface layer of the heater 1000, the conductors 1a, 1b, and 3, the resistance heating elements 2a and 2b, and the electrode E4 form a heat generating block. In this example, seven heat generating blocks HB1 to HB7 exist along the longitudinal direction of the heater 1000. The heat generating block HBi is formed by the conductors 1a, 1b, and 3-i, the resistance heating elements 2a-i and 2b-i, and the electrode E4-i (i is an index and an integer from 1 to 7). Specifically, the resistance heating element 2a-i is provided between the conductor 1a and the conductor 3-i and is electrically connected to the conductor 1a and the conductor 3-i. The resistance heating element 2b-i is provided between the conductor 1b and the conductor 3-i and is electrically connected to the conductor 1b and the conductor 3-i. The conductor 3-i is electrically connected to the electrode E4-i. Electrical conductors 1a and 1b are electrically connected to electrodes E5 and E6, respectively, at both ends of the substrate 235 in the longitudinal direction. Resistance heating element 2a-i is insulated from resistance heating element 2a-i+1. Resistance heating element 2b-i is also insulated from resistance heating element 2b-i+1. Electrical conductor 3-i is also insulated from conductor 3-i+1. This allows heat generation blocks HB1 to HB7 to generate heat selectively.

[0077] Surface protective layer 237 provided on the second back surface layer exposes electrodes E4-1 to E4-7, electrode E5, and electrode E6, and covers conductors 1a, 1b, and 3-i and resistance heating elements 2a-i and 2b-i. As shown in FIG. 11(B), heat generating blocks HB1 to HB7 form heat generating areas AREA1 to AREA4. For example, heat generating area AREA1 is heated by heat generating block HB4. Heat generating area AREA2 is heated by heat generating blocks HB3, HB4, and HB5. Heat generating area AREA3 is heated by heat generating blocks HB2 to HB6. Heat generating area AREA4 is heated by heat generating blocks HB1 to HB7. Heating area AREA1 corresponds to A5-size sheets P. Heating area AREA2 corresponds to B5-size sheets P. Heating area AREA3 corresponds to A4-size sheets. Heating area AREA4 corresponds to Letter-size sheets.

[0078] The CPU 510 can control seven heat generating blocks HB1 to HB7. Therefore, the CPU 510 selects the heat generating block HBi to be supplied with power according to the size of the sheet P. The number of heat generating regions HBi and the number of heat generating blocks HB are merely examples, and may be more or less than the numbers shown in FIG. 11(B). The shapes and areas of the resistance heating elements 2a-i and 2b-i provided in the heat generating block HBi are also merely examples. As shown in FIG. 12, the resistance heating elements 2a-i and 2b-i may each be configured with a plurality of heat generating patterns (striped patterns) separated by gaps.

[0079] As shown in Figure 11 (B), the first surface layer (sliding surface layer) of heater 1000 has thermistors T1 and T2 that detect the temperature of each heat generating block HBi. Thermistor T1-1 detects the temperature of heat generating block HB1. Thermistor T2-2 detects the temperature of heat generating block HB2. Thermistor T1-3 detects the temperature of heat generating block HB3. Thermistor T1-4 and thermistor T2-4 detect the temperature of heat generating block HB4. Thermistor T2-5 detects the temperature of heat generating block HB5. Thermistor T1-6 detects the temperature of heat generating block HB6. Thermistor T1-7 detects the temperature of heat generating block HB7. Thermistors T1 and T2 are printed on the surface of substrate 235. Thermistors T1-1, T1-3, T1-4, T1-6, and T1-7 are used to adjust the temperatures of the corresponding heat generating blocks HB1, HB3, HB4, and HB7. Therefore, thermistors T1-1, T1-3, T1-4, T1-6, and T1-7 are arranged at the centers (longitudinal centers) of the corresponding heat generating blocks HB1, HB3, HB4, and HB7. Furthermore, thermistors T1-1, T1-3, T1-4, T1-6, and T1-7 are arranged downstream in the conveyance direction of sheet P. This is because the temperature downstream is higher than the temperature upstream. This reduces the bias in the temperature distribution of heat generating block HBi.

[0080] Thermistors T2-2, T2-4, and T2-5 are installed to detect the temperature of the non-paper passing areas. The non-paper passing areas are areas of the heat generating area where heat is not absorbed by the sheet P. For example, when a narrow sheet P is transported, heat generating areas away from the transport reference position Y cannot supply heat to the sheet P. Therefore, if the size of the sheet P is narrower than the width of the selected heat generating area AREAi, the temperature of the edge areas of the selected heat generating area AREAi is likely to rise. For this reason, thermistors T2-2, T2-4, and T2-5 are arranged in the corresponding heat generating areas HB2, HB4, and HB5 at positions away from the transport reference position Y (toward the outside) and upstream in the transport direction of the sheet P.

[0081] One end of each of the thermistors T1-1, T1-3, T1-4, T1-6, and T1-7 is connected to the conductors ET1-1, ET1-3, ET1-4, ET1-6, and ET1-7, respectively. The other end of each of the thermistors T1-1, T1-3, T1-4, T1-6, and T1-7 is connected in common to the conductor EGb. One end of each of the thermistors T2-2, T2-4, and T2-5 is connected in common to the conductors ET2-2, ET2-4, and ET2-5, respectively. The other end of each of the thermistors T2-2, T2-4, and T2-5 is connected in common to the conductor EGa. In this way, the width of the heater 1000 in the short direction increases as the number of thermistors T1 and T2 and the number of conductors ET1 and ET2 increase. Alternatively, if the number of thermistors T1, T2 and the number of conductors ET1, ET2 increase, the distance between the conductors ET1, ET2 decreases.

[0082] The conductors ET1-1, ET1-3, ET1-4, and ET2-2 have electrical contacts at the end on the same side (the left end in FIG. 11(B)) in the longitudinal direction of the substrate 235. The conductors ET1-6, ET1-7, ET2-4, and ET2-5 have electrical contacts at the end on the opposite side (the right end in FIG. 11(B)).

[0083] The second surface layer exposes the electrical contacts provided on the first surface layer and has a surface protection layer 238 that covers almost the entire sliding surface of the heater 1000. The material of the surface protection layer 238 is glass, which has excellent properties such as sliding properties, abrasion resistance, and sealing properties.

[0084] Fig. 13 shows a circuit diagram of a power supply circuit 40 that supplies power to the heater 1000 of Example 4. Fig. 14 shows details of the circuit near the heater 1000. Unlike the power supply circuit 40 of Example 1, the power supply circuit 40 of Example 4 has a primary side circuit 1301, a secondary side circuit 1302, and a temperature detection circuit 1303, which are electrically insulated from each other.

[0085] The primary circuit 1301 is a circuit that supplies power from a commercial AC power supply 41 connected to the image forming apparatus 100 to the resistance heating elements 2a and 2b of the heater 1000. The resistance heating elements 2a and 2b are provided in the primary circuit 1301, which is electrically connected to the commercial AC power supply 41.

[0086] A commercial AC power supply 41 is connected to the heater 1000 via a relay RL1, a relay RL2, and triacs TA1, TA3, TA4, TA6, and TA7.

[0087] Triac TA1 controls the on / off of resistive heating elements 2a-1 and 2b-1 in response to a control signal FUSER1 supplied from CPU 510 via isolation circuit 1312. Resistive heating elements 2a-1 and 2b-1 are connected to triac TA1 via terminal E4-1. Triac TA3 controls the on / off of resistive heating elements 2a-3, 2b-3, 2a-5, and 2b-5 in response to a control signal FUSER3 supplied from CPU 510 via isolation circuit 1312. Resistive heating elements 2a-3 and 2b-3 are connected to triac TA3 via terminal E4-3. Resistive heating elements 2a-5 and 2b-5 are connected to triac TA3 via terminal E4-5. Triac TA4 controls the on / off of resistive heating elements 2a-4 and 2b-4 in response to a control signal FUSER4 supplied from CPU 510 via isolation circuit 1312. Resistive heating elements 2a-4 and 2b-4 are connected to triac TA4 via terminal E4-4. Triac TA6 controls the on / off of resistive heating elements 2a-2, 2b-2, 2a-6, and 2b-6 in response to a control signal FUSER6 supplied from CPU 510 via isolation circuit 1312. Resistive heating elements 2a-2 and 2b-2 are connected to triac TA3 via terminal E4-2. Resistive heating elements 2a-6 and 2b-6 are connected to triac TA6 via terminal E4-6. Triac TA7 controls the on / off of resistive heating elements 2a-7 and 2b-7 in response to a control signal FUSER7 supplied from CPU 510 via isolation circuit 1312. Resistive heating elements 2a-7 and 2b-7 are connected to triac TA7 via terminal E4-7. Triacs TA1 and TA7 are connected to commercial AC power supply 41 via triac TA6. The insulating circuit 1312 is a circuit that electrically insulates the primary circuit 1301 and the secondary circuit 1302 from each other.

[0088] Triac TA4 controls heat generating block HB4. Triac TA3 controls heat generating block HB3 and heat generating block HB5. Here, one triac TA3 is electrically connected to the two heat generating blocks HB3 and HB5. Triac TA6 controls heat generating block HB2 and heat generating block HB6. One triac TA6 is electrically connected to the two heat generating blocks HB2 and HB6. Here, triac TA1 that drives heat generating block HB1 is connected in series with triac TA6 that drives the adjacent heat generating block HB2. Triac TA7 that drives heat generating block HB7 is connected in series with triac TA6 that drives the adjacent heat generating block HB6.

[0089] When only triac TA6 is driven, heat generating blocks HB2 and HB6 generate heat. When both triac TA1 and triac TA6 are driven, heat generating blocks HB1, HB2, and HB6 generate heat. When both triac TA6 and triac TA7 are driven, heat generating blocks HB2, HB6, and HB7 generate heat. Furthermore, when triac TA1, triac TA6, and triac TA7 are driven, heat generating blocks HB1, HB2, HB6, and HB7 generate heat. Therefore, in the example shown in Figure 14, heat generating blocks HB1 and HB7 do not generate heat alone. Furthermore, with this circuit configuration, it is possible to select control to cause heat generating blocks HB2 and HB6 to generate heat, or control to cause heat generating blocks HB1 and HB7 to generate heat in addition. Therefore, the CPU 510 can select heat generating block HBi depending on the size of the sheet P.

[0090] 13, an isolated AC / DC converter 1311 is a switching power supply circuit that supplies power from a primary circuit 1301 to a secondary circuit 1302. The AC / DC converter 1311 generates a power supply voltage Vcc-21 for the secondary circuit 1302. The AC / DC converter 1311 has a built-in transformer that ensures reinforced insulation between the primary circuit 1301 and the secondary circuit 1302.

[0091] The temperature detection circuit 1303 includes thermistors T1 and T2, a CPU 1350, and detects the temperature of the heater 1000. As shown in FIG. 14, the conductors EGa and EGb are connected to a ground potential GND-3. A pull-up resistor R11 is connected between the thermistor T1-1 and a power supply voltage Vcc-3. A variable resistor R31 is connected in parallel to the pull-up resistor R11. The power supply voltage Vcc-3 is divided by the pull-up resistor R11, the variable resistor R31, and the resistance of the thermistor T1-1 to generate a detection voltage Th1-1 for thermistor T1-1. A pull-up resistor R13 is connected between the thermistor T1-3 and the power supply voltage Vcc-3. A variable resistor R33 is connected in parallel to the pull-up resistor R13. The power supply voltage Vcc-3 is divided by pull-up resistor R13, variable resistor R33, and the resistance of thermistor T1-3, generating the detection voltage Th1-1 for thermistor T1-3. Pull-up resistor R14 is connected between thermistor T1-4 and the power supply voltage Vcc-3. Variable resistor R34 is connected in parallel to pull-up resistor R14. The power supply voltage Vcc-4 is divided by pull-up resistor R14, variable resistor R34, and the resistance of thermistor T1-4, generating the detection voltage Th1-4 for thermistor T1-4. Pull-up resistor R16 is connected between thermistor T1-6 and the power supply voltage Vcc-3. Variable resistor R36 is connected in parallel to pull-up resistor R16. The power supply voltage Vcc-3 is divided by pull-up resistor R16, variable resistor R36, and the resistance of thermistor T1-6, generating the detection voltage Th1-6 for thermistor T1-6. A pull-up resistor R17 is connected between the thermistor T1-7 and the power supply voltage Vcc-3. A variable resistor R37 is connected in parallel to the pull-up resistor R17. The detection voltage Th1-7 of the thermistor T1-7 is generated by dividing the power supply voltage Vcc-3 with the pull-up resistor R17, variable resistor R37, and the resistance of the thermistor T1-7.

[0092] A pull-up resistor R22 is connected between thermistor T2-2 and power supply voltage Vcc-3. The power supply voltage Vcc-3 is divided by pull-up resistor R22 and the resistance of thermistor T2-2, generating a detection voltage Th2-2 for thermistor T2-2. A pull-up resistor R24 ​​is connected between thermistor T2-4 and power supply voltage Vcc-3. The power supply voltage Vcc-3 is divided by pull-up resistor R24 ​​and the resistance of thermistor T2-4, generating a detection voltage Th2-4 for thermistor T2-4. A pull-up resistor R25 is connected between thermistor T2-5 and power supply voltage Vcc-3. The power supply voltage Vcc-3 is divided by pull-up resistor R25 and the resistance of thermistor T2-5, generating a detection voltage Th2-5 for thermistor T2-5.

[0093] The detection voltages Th1-1, Th-3, Th1-4, Th1-6, and Th1-7 and the detection voltages Th2-2, Th2-4, and Th2-5 are input to A / D ports of the CPU 1350. The CPU 1350 converts each detection voltage into a temperature according to a conversion table stored in the memory 1351. The CPU 1350 outputs a CLK_OUT signal and a DATA_OUT signal to transmit temperature information to the CPU 510. The CPU 510 receives a CLK_IN signal corresponding to the CLK_OUT signal and a DATA_IN signal corresponding to the DATA_OUT signal, thereby acquiring temperature information detected by the temperature detection circuit 1303.

[0094] The CLK_OUT signal and the CLK_IN signal are isolated by photocoupler PC1. Resistor R80 limits the current flowing through the light-emitting diode of photocoupler PC1. Resistor R82 limits the current flowing through the phototransistor of photocoupler PC1. The DATA_OUT signal and the DATA_IN signal are isolated by photocoupler PC2. Resistor R81 limits the current flowing through the light-emitting diode of photocoupler PC2. Resistor R83 limits the current flowing through the phototransistor of photocoupler PC2.

[0095] The CPU 510 performs PI control and calculates the power supply based on the detected temperature of the thermistor T1-3 provided in the heat generating block HB3. The thermistor T2-5 provided in the heat generating block HB5 plays an auxiliary role. For example, if the detected temperature is equal to or higher than a preset temperature, the CPU 510 reduces the conveying speed of the sheet P (the rotation speed of the motor 30). This prevents excessive temperature rise in the non-sheet passing area. The heat generating blocks HB2 and HB6 are controlled in a similar manner. The CPU 510 controls the power supplied to the heat generating block HB6 based on the detected temperature of the thermistor T1-6 provided in the heat generating block HB6. The thermistor T2-2 provided in the heat generating block HB2 plays an auxiliary role. The heat generating blocks HB3 and HB5 are arranged line-symmetrically with respect to the conveying reference position Y of the sheet P. The heat generating blocks HB2 and HB6 are also arranged line-symmetrically with respect to the conveying reference position Y of the sheet P. Here, the temperature distribution of the heat generating block HB3 and the temperature distribution of the heat generating block HB5 are approximately the same. The temperature distribution of heat generating block HB2 and the temperature distribution of heat generating block HB6 are almost the same. Therefore, a thermistor T is placed on only one of the pair of heat generating blocks. Therefore, compared to when the heat generating blocks HB are controlled individually, it is possible to reduce the number of triacs TA and their drive circuits, the number of thermistors T and their detection circuits, and the number of connectors and wires that connect them. As a result, it is possible to reduce costs and make the fixing device 50 more compact.

[0096] Heat generating block HB4 is controlled by triac TA4. Heat generating block HB4 is located in the center of heater 1000 and does not have a paired heat generating block HB. Heat generating block HB4 has a relatively large area. Therefore, in this embodiment, two thermistors, thermistor T1-4 and thermistor T2-4, are located therein. Thermistor T1-4 is a temperature sensor for temperature control. Thermistor T2-4 is a temperature sensor that plays an auxiliary role.

[0097] The heat generating areas of the heat generating blocks HB1 and HB7 are relatively narrow, so the thermistors T1-1 and T1-7 function as temperature sensors for detecting the temperature of the non-sheet passing areas (edges) and also as temperature sensors for temperature control.

[0098] 11(B), one end of the heater 1000 on the sliding surface side is provided with electrodes of conductors ET1-1, ET1-3, ET1-4, ET2-2, conductor EGa, and conductor EGb connected to thermistor T. The other end of the heater 1000 on the sliding surface side is provided with electrodes of conductors ET1-6, ET1-7, ET2-4, and ET2-5 connected to thermistor T. The electrode of conductor ET1-i (i is 1, 4, 6, or 7) is connected to one end of resistor R1i and one end of variable resistor R3i on a substrate having a power supply circuit 40 mounted on the image forming apparatus 100 or the fixing device 50. The electrode of conductor ET2-j (j is 2, 4, or 5) is connected to one end of resistor R2j on a substrate having the power supply circuit 40.

[0099] A flexible flat wiring board (FPC) or the like may be used as a connection interface. The FPC has multiple conductor foil patterns inside a film-like insulating inner layer. The conductors of the FPC can be connected to the electrodes of the heater 1000, namely, conductors ET1-1, ET1-3, ET1-4, ET1-6, ET1-7, conductors ET2-2, ET2-4, ET2-5, conductors EGa, and conductors EGb, by soldering. The other end of the FPC can be connected to a board having the power supply circuit 40 by a connector or the like. To use a connector for a general-purpose flexible flat cable (FFC), the pitch between the conductors may be 0.5 mm or 1.0 mm.

[0100] The temperature detection circuit 1303 includes a DC / DC converter 1313 that generates a power supply voltage Vcc-3. The power supply voltage Vcc-3 is supplied to the CPU 1350 and other devices. The transformer TR1 is a switching transformer for generating the power supply voltage Vcc-3. The transformer TR1 has a primary winding N1 and a secondary winding N2, with reinforced insulation between the primary winding N1 and the secondary winding N2. The field-effect transistor FET1 switches the power supply voltage Vcc-21 supplied to the primary winding N1 in response to a TR1_DRIVE signal input from the CPU 510. This transfers energy stored in the primary winding N1 of the transformer TR1 to the secondary winding N2. The pulsating current generated in the secondary winding N2 is rectified by the diode D1 and smoothed by the capacitor C1 to become a direct current. The DC / DC converter 1313 and the capacitor C2 step down the DC voltage generated by the capacitor C1 to the specified power supply voltage Vcc-3. Capacitor C3 is a capacitor connected between secondary side circuit 1302 and temperature detection circuit 1303. The effect of capacitor C3 will be described later.

[0101] The voltage across the capacitor C1 fluctuates depending on the load current of the temperature detection circuit 1303. Therefore, by employing the DC / DC converter 1313, the power supply voltage Vcc-3 is stabilized. The power supply voltage Vcc-3 is, for example, 3.3 V. Instead of the DC / DC converter 1313, a linear regulator such as an LDO (low dropout) may be employed.

[0102] For example, the duty cycle of the TR1_DRIVE signal is set to 50%, and the switching frequency is set from 80 kHz to 120 kHz. In other words, the CPU 510 changes the frequency of the TR1_DRIVE signal. This distributes the switching frequency and reduces EMC noise. EMC stands for electromagnetic compatibility.

[0103] Figure 15 shows an example of the waveform of the TR1_DRIVE signal. The vertical axis represents voltage, and the horizontal axis represents time. The switching frequency gradually increases from 100 kHz to 120 kHz. Then, the switching frequency gradually decreases from 120 kHz to 80 kHz. Then, the switching frequency gradually increases from 80 kHz to 100 kHz. This cycle is repeated. The high level of the TR1_DRIVE signal is 3.2 V, and the low level is 0.1 V. The method of changing the switching frequency shown in Figure 15 is just one example. The method of changing the switching frequency can be changed as appropriate depending on the characteristics of the power supply voltage Vcc-3 and EMC noise.

[0104] Incidentally, basic insulation or reinforced insulation is provided between the primary circuit 1301 and the temperature detection circuit 1303. Furthermore, the temperature detection circuit 1303 is a circuit that cannot be touched by the user. Supplementary insulation or reinforced insulation may be provided between the temperature detection circuit 1303 and the secondary circuit 1302. In this way, the secondary circuit 1302 is a circuit that has electrical components and wiring that can be touched by the user. On the other hand, the temperature detection circuit 1303 does not have electrical components or wiring that can be touched by the user. The effect of insulating the temperature detection circuit 1303 from both the primary circuit 1301 and the secondary circuit 1302 will be described later.

[0105] 13, the secondary circuit 1302 has a DC / DC converter 1314. The DC / DC converter 1314 is a step-down circuit that steps down a power supply voltage Vcc-21 (e.g., 5 V) to generate a power supply voltage Vcc-22 (e.g., 3.3 V). The input and output terminals of the DC / DC converter 1314 are connected to a ground potential GND-2 via a capacitor (not shown). The ground potential GND-2 is a ground potential provided by the frame ground 239.

[0106] CPU 510 calculates the supply power using PI control based on the set temperature and the temperatures detected by thermistors T1-1, T1-3, T1-4, T1-6, and T1-7 input by the CLK_IN signal and the DATA_IN signal. CPU 510 recognizes the zero-cross timing of commercial AC power supply 41 using zero-cross circuit 520. CPU 510 converts the supply power into control conditions (control levels of phase angle (phase control) and wave number (wave number control)). CPU 510 controls triacs TA1, TA3, TA4, TA6, and TA7 according to the control conditions.

[0107] As shown in Figure 13, two relays RL1 and RL2 are provided. The CPU 510 is connected to the base terminal of the transistor Q1 via a resistor R44. The collector of the transistor Q1 is connected to the power supply voltage Vcc-21 via the secondary coil L1 of the relay RL1. The emitter of the transistor Q1 is connected to the ground potential GND-2. The CPU 510 is connected to the base terminal of the transistor Q2 via a resistor R46. The collector of the transistor Q2 is connected to the power supply voltage Vcc-21 via the secondary coil L2 of the relay RL2. The emitter of the transistor Q2 is connected to the ground potential GND-2.

[0108] When the CPU 510 sets the RLON signal to a high state, the RLON signal turns on transistor Q1 via resistor R44. Current flows from the power supply voltage Vcc-21 to the secondary coil L1 of relay RL1, turning the primary contact of relay RL1 on. When the CPU 510 sets the RLON signal to a low state, transistor Q1 turns off, cutting off the current flowing from the power supply voltage Vcc-21 to the secondary coil L1 of relay RL1. This turns off the primary contact of relay RL1. The same operation applies to relay RL2. When the CPU 510 sets the RLON signal to a high state, the RLON signal turns on transistor Q2 via resistor R46. Current flows from the power supply voltage Vcc-21 to the secondary coil L2 of relay RL2, turning on the primary contact of relay RL2. When the CPU 510 sets the RLON signal to a low state, transistor Q2 turns off, cutting off the current flowing from the power supply voltage Vcc-21 to the secondary coil L2 of relay RL2. As a result, the primary contact of relay RL2 is turned OFF.

[0109] The safety circuit using relays RL1 and RL2 operates as follows. When the detected temperature of any of the thermistors T1-1, T1-3, T1-4, T1-6, and T1-7 exceeds a threshold value set by comparison circuit 551, comparison circuit 551 activates latch circuit 552. Latch circuit 552 latches the RLOFF signal to a low state. When the RLOFF signal is low, even if CPU 510 sets the RLON signal to a high state, transistors Q1 and Q2 are maintained in an off state. Therefore, relays RL1 and RL2 are maintained in an off state (safe state). In this way, when heater 1000 overheats, relays RL1 and RL2 can cut off the power supplied to heater 1000. The operation of comparison circuit 551 and latch circuit 552 is similar to that described in the first embodiment, and therefore a description thereof will be omitted. If at least one of the thermistors T1-1, T1-3, T1-4, T1-6, and T1-7 detects an abnormality, the RLOFF signal is maintained in a low state, and the relays RL1 and RL2 are turned off, thereby cutting off the power supply to the heater 1000.

[0110] A user or the like may need to remove a jammed sheet P. In this case, the user opens a door (maintenance door) of the image forming apparatus 100. The image forming apparatus 100 may have electrical components and wiring that the user can touch when the door is open. As shown in FIG. 16 , an interface cable 1601 connecting an external device 1600, such as a PC, to the image forming apparatus 100 is also an electrical component that the user can touch. The interface cable 1601 is, for example, a cable that complies with USB (Universal Serial Bus) or LAN (Local Area Network). As shown in FIG. 16 , the electrical component (interface cable 1601) provided in a location that the user can touch is connected to a secondary-side circuit 1302. Reinforced insulation is provided between the primary-side circuit 1301, to which the commercial AC power supply 41 is connected, and the secondary-side circuit 1302. This circuit configuration enhances the safety of the user with respect to electrical components and wiring provided in a location that the user can touch.

[0111] 17 shows an electrical equivalent circuit of fixing device 50 in Example 4. Example 4 differs from Example 1 in that resistance heating elements 232 and 233 are replaced by resistance heating elements 2a and 2b, conductor 234 is replaced by conductors EGa and EGb, and capacitor 236 (capacitance=Cx) is replaced by capacitor C3.

[0112] In conventional fixing devices, the pressure roller's core metal is connected to the frame ground via a capacitor. This increases the distance between the fixing nip and the pressure roller's core metal, resulting in a small capacitance of the pressure roller. The capacitance of the pressure roller can fluctuate significantly due to variations in the pressure roller's thickness, resulting in variations in the potential of the fixing film when excessive voltage occurs. Considering this variation, the creepage distance and clearance distance of the fixing device must be long, which hinders the downsizing of the fixing device.

[0113] As shown in Figure 17, in Example 4, conductors EGa and EGb provided on the sliding surface of substrate 235 are connected to frame ground 239 via capacitor C3. Furthermore, temperature detection circuit 1303 is insulated from both primary circuit 1301 and secondary circuit 1302. Therefore, capacitance Chf occurs between resistance heating elements 2a, 3b and film 202. Capacitance Cfg occurs between film 202 and temperature detection circuit 1303. Capacitance Chg occurs between resistance heating elements 2a, 2b and temperature detection circuit 1303. Capacitor C3 is connected between temperature detection circuit 1303 and frame ground 239. The capacitance of capacitor C3 is also denoted as C3.

[0114] The equivalent circuit of the fixing device 50 of Example 4 corresponds to the equivalent circuit of the fixing device 50 of Example 1, in which the conductor 234 is replaced with a temperature detection circuit 1303, and the resistance heating elements 232 and 233 are replaced with resistance heating elements 2a and 2b. Therefore, in Example 4, by arranging the temperature detection circuit 1303 near the resistance heating elements 2a and 2b and the film 202, the electrostatic capacitances Cfg and Chg are increased, and the influence of variations in the potential of the film 202 is reduced. Furthermore, the thicknesses of the substrate 235 and the surface protection layer 238 are unlikely to change. Therefore, variations in the electrostatic capacitance Cfg and the electrostatic capacitance Chg are reduced.

[0115] In this way, the influence of variations in the potential of the film 202 is suppressed, and it is possible to shorten the creepage distance and the spatial distance required to prevent discharge from the film 202 to the frame ground 239. As a result, it is possible to achieve both further miniaturization of the fixing device 50 and protection of the fixing device 50.

[0116] The temperature detection circuit 1303 insulated from the primary circuit 1301 and the secondary circuit 1302 of the fourth embodiment is employed as the conductor 234 of the first embodiment. This reduces the creepage distance and the spatial distance that suppress discharge from the film 202 to the frame ground 239 without adding the conductor 234. As a result, it is possible to achieve both further miniaturization of the fixing device 50 and protection of the fixing device 50.

[0117] In the fourth embodiment, the capacitor C3 is disposed in a circuit provided outside the fixing device 50, but this is merely an example. The capacitor C3 may be disposed inside the fixing device 50.

[0118] In the fourth embodiment, the resistance heating elements 2a and 2b are disposed on the back surface side of the substrate 235, and the conductive layers EGa and Egb are disposed on the sliding surface side of the substrate 235, but this is merely an example. The resistance heating elements 2a and 2b may be disposed on the sliding surface side of the substrate 235, and the conductive layers EGa and Egb may be disposed on the front surface side of the substrate 235.

[0119] In the fourth embodiment, as in the first embodiment, the comparison circuit 551 of the safety circuit using the relays RL1 and RL2 compares the threshold voltage with the detection voltage Th1-i of the thermistor T1-i (where i is 1, 3, 4, 6, or 7). However, the thermistor T1-i has manufacturing variations. That is, the resistance value of the thermistor T1-i at a certain temperature varies from one to another. The greater this manufacturing variation, the greater the deviation of the detection voltage Th1-i of the thermistor T1-i from the ideal value. For this reason, it has been difficult to incorporate thermistor T1-i, which has large manufacturing variations, into a safety circuit.

[0120] In Example 4, hardware correction is applied to thermistor T1-i, and software correction is applied to thermistor T2-j (j is 2, 4, or 5). This absorbs manufacturing variations in thermistors T1 and T2, improving temperature detection accuracy. The hardware correction and software correction are as described in Example 1. As in Example 1, thermistors T1-1, T1-3, T1-4, T1-6, and T1-7, to which hardware correction is applied, are connected to a safety circuit using relays RL1 and RL2. Thermistors T2-2, T2-4, and T2-5, to which software correction is applied, are not connected to the safety circuit. The reason for this is the same as in Example 1.

[0121] According to the fourth embodiment, the thermistors T1 and T2 arranged on the substrate 235 of the heater 1000 are connected to a temperature detection circuit 1303 that is insulated from a primary circuit 1301 and a secondary circuit 1302. A capacitor C3 is connected between the temperature detection circuit 1303 and the secondary circuit 1302. This reduces the potential variation of the film 202 when an excessive voltage occurs. As a result, the fixing device 50 can be made smaller and protected from excessive voltage at the same time.

[0122] Furthermore, a configuration is adopted that can absorb manufacturing variations in the thermistors T1-1, T1-3, T1-4, T1-6, and T1-7 and thermistors T2-2, T2-4, and T2-5 that are printed on the heater 1000. This achieves high temperature detection accuracy, improving the safety of the fixing device 50.

[0123] <Technical ideas derived from examples> [Item 1] a metal frame providing a ground potential; an image forming means for forming a toner image on a sheet; a fixing device that fixes the toner image to the sheet, a heating element that generates heat when power is supplied from an AC power source; an insulating layer covering the heating element; a conductive layer in contact with the insulating layer and insulated from the metal frame; a cylindrical member heated by the heating element; a fixing device having a pressure means provided opposite the cylindrical member and cooperating with the cylindrical member to form a nip portion; a capacitance element having one end connected to the metal frame and the other end connected to the conductive layer of the fixing device; An image forming apparatus comprising:

[0124] The frame ground 239 is an example of a metal frame. The photoconductor 19 is an example of an image forming means. The resistance heating elements 232, 233, 2a, and 2b are examples of heating elements. The surface protection layer 237 is an example of an insulating layer that covers the heating elements. The conductors 234, 734, EGa, and EGb are examples of a conductive layer in contact with an insulating layer. The film 202 is an example of a (flexible) cylindrical member that is heated by the heating element. The pressure roller 208 is an example of a pressure means. The capacitors 236 and C3 are examples of capacitive elements. This makes it possible to protect the fixing device 50 from overvoltage while also miniaturizing the fixing device 50 and the image forming apparatus 100.

[0125] Item 1. The image forming apparatus according to item 1, wherein a surge voltage (e.g., Vsurge) applied between the heating element and the metal frame is divided by the capacitance of the capacitive element (e.g., Cx, C3), the capacitance between the heating element and the conductive layer (e.g., Chg), the capacitance between the conductive layer and the cylindrical member (e.g., Cfg), and the capacitance between the cylindrical member and the heating element (e.g., Chf).

[0126] This reduces the creepage distance and the clearance between the frame (for example, frame ground 239) and film 202, and the fixing device 50 and image forming apparatus 100 can be made smaller.

[0127] [Item 2] The heating element is provided on the first surface side of the substrate, a first insulating layer of the insulating layers is provided to cover the heating element provided on the first surface side of the substrate, the conductive layer is provided on a second surface of the substrate that faces an inner circumferential surface of the cylindrical member, The image forming apparatus described in item 1, characterized in that a second insulating layer of the insulating layers is provided on the second surface side of the substrate so as to cover the conductive layer and slides against the inner surface of the cylindrical member.

[0128] This corresponds to the structure shown in Figures 2, 4(A), and 4(B). When this structure is adopted, the distance between the cylindrical member and the conductive layer is shortened, and therefore the capacitance between the cylindrical member and the conductive layer is increased.

[0129] [Item 3] The heating element is provided on the first surface side of the substrate, a first insulating layer of the insulating layers is provided to cover the heating element provided on the first surface side of the substrate, 3. The image forming apparatus according to item 1 or 2, wherein the conductive layer is provided on the first surface side.

[0130] This corresponds to the structure shown in Figures 6(A) and 6(B). By adopting this structure, it is easy to shorten the distance between the cylindrical member and the conductive layer, which may make it easier to increase the capacitance between the cylindrical member and the conductive layer.

[0131] [Item 4] 3. The image forming apparatus according to item 2, wherein the conductive layer is disposed outside the first insulating layer and in contact with the first insulating layer.

[0132] This corresponds to the structure shown in Figures 7, 8(A), and 8(B). By adopting this structure, it is easy to shorten the distance between the cylindrical member and the conductive layer, which will make it easy to increase the capacitance between the cylindrical member and the conductive layer.

[0133] the heating element includes a first heating element and a second heating element mounted on the substrate; the first heating element is disposed downstream of the second heating element in a conveying direction of the sheet, An image forming apparatus as described in any one of the above items, characterized in that a first terminal for supplying power to the first heating element and a second terminal for supplying power to the second heating element are arranged on one of the two longitudinal ends of the substrate.

[0134] This structure is illustrated in Figures 4(B), 6(B), and 8(B), which makes it easier to ensure a power supply path to the first heating element (e.g., resistance heating element 232) and the second heating element (e.g., resistance heating element 233).

[0135] An image forming apparatus described in any of the above items, characterized in that a terminal provided on the conductive layer on the second surface of the substrate, which is for connecting to the capacitive element, is located on the other end side of the two ends in the longitudinal direction of the substrate, which is opposite to the one end.

[0136] This structure is illustrated in Figures 4(B), 6(B) and 8(B), which allows both ends of the heaters 230 and 730 in the longitudinal direction to be used effectively.

[0137] [Item 5] a first detection element (e.g., thermistor T1) and a second detection element (e.g., thermistor T2) for detecting the temperature of the heating element; a temperature control means (e.g., CPU 510) for controlling the temperature of the heating element based on the detection result of the first detection element; a cutoff means (e.g., a relay 530) for forcibly cutting off power supplied to the heating element when the detection result of the first detection element indicates that a predetermined event has occurred; a correction circuit (e.g., a variable resistor 562) that corrects the detection result of the first detection element according to individual differences of the first detection element; 2. The image forming apparatus according to item 1, further comprising: an arithmetic circuit (e.g., CPU 510) that corrects the detection result of the second detection element by calculation according to individual differences of the second detection element.

[0138] In addition, a speed control means (e.g., CPU 510) may be further provided that reduces the throughput or conveying speed of the sheet when the detection result of the second detection element indicates that a second event has occurred.

[0139] The detection result of the second detection element corrected by the arithmetic circuit is not used for forcibly shutting off (safe operation) the power supplied to the heating element. In other words, the detection result of the first detection element corrected by the correction circuit is used for forcibly shutting off (safe operation) the power supplied to the heating element. This makes it possible to appropriately protect the fixing device 50 while reducing manufacturing costs. The detection result of the second detection element corrected by the arithmetic circuit may also be used for throughput control (speed control). The throughput is, for example, the number of sheets on which images are formed per unit time. The throughput is controlled, for example, by changing the sheet transport speed, or by changing the gap between the preceding and succeeding sheets while maintaining the transport speed.

[0140] [Item 6] The image forming apparatus described in item 5 is characterized in that the correction circuit has a fixed resistance element (e.g., resistor 561) connected in series to the first detection element and a variable resistance element (e.g., variable resistor 562) connected in parallel to the fixed resistance element, and the detection result of the first detection element is corrected by changing the resistance value of the variable resistance element.

[0141] The correction circuit may be realized by such a simple circuit.

[0142] [Item 7] The image forming apparatus described in item 5 or 6 is characterized in that the arithmetic circuit has a memory circuit (e.g., memory 511) that stores a correction coefficient (e.g., deviation amount D), and corrects the detection result of the second detection element using the correction coefficient.

[0143] In this way, the detection result of the second detection element is corrected by calculation, which makes it possible to reduce manufacturing costs.

[0144] [Item 8] 8. The image forming apparatus according to any one of items 5 to 7, wherein the first detection element is disposed downstream of the second detection element in the sheet transport direction.

[0145] This structure is illustrated in Fig. 11(B). In the heaters 230, 730, and 1000, the temperature on the downstream side in the conveying direction of the sheet P is higher than the temperature on the upstream side. Therefore, by arranging the first detection element on the downstream side, it is possible to quickly detect an excessive temperature rise.

[0146] An image forming apparatus according to any of the preceding items, characterized in that the temperature detection circuit including the first detection element and the second detection element is arranged so as to be in contact with the insulating layer, and the temperature detection circuit including the first detection element and the second detection element is electrically insulated from both a primary side circuit including the heating element and a secondary side circuit electrically insulated from the primary side circuit, and the conductive layer (e.g., conductors EGa, Egb) is electrically connected to the first detection element and the second detection element.

[0147] [Item 9] 9. The image forming apparatus according to any one of items 1 to 8, wherein the capacitive element is provided inside the fixing device.

[0148] [Item 10] 9. The image forming apparatus according to any one of items 1 to 8, wherein the capacitive element is provided outside the fixing device.

[0149] [Item 11] an image forming means for forming a toner image on a sheet; a fixing device that fixes the toner image to the sheet, a heating element that generates heat when power is supplied from an AC power source; a cylindrical member heated by the heating element; a fixing device having a pressure means provided opposite the cylindrical member and cooperating with the cylindrical member to form a nip portion; a first detection element and a second detection element that detect the temperature of the heating element; a correction circuit that corrects the detection result of the first detection element according to individual differences of the first detection element; an arithmetic circuit that corrects the detection result of the second detection element according to individual differences of the second detection element; a temperature control means for controlling the temperature of the heating element based on the detection result of the first detection element corrected by the correction circuit without using the detection result of the second detection element; a cutoff means for forcibly cutting off the power supplied to the heating element using the detection result of the first detection element corrected by the correction circuit, without using the detection result of the second detection element; An image forming apparatus comprising:

[0150] In this way, the detection result of the second detection element corrected by the calculation circuit is used for speed control, but is not used for forcibly cutting off (safe operation) the power supplied to the heating element. In other words, the detection result of the first detection element corrected by the correction circuit is used for forcibly cutting off (safe operation) the power supplied to the heating element. This makes it possible to appropriately protect the fixing device 50 while reducing manufacturing costs.

[0151] Furthermore, a throughput control means may be provided that reduces the throughput of the sheet based on the detection result of the second detection element corrected by the arithmetic circuit.

[0152] [Item 12] Item 12. The image forming apparatus according to item 11, wherein the correction circuit has a fixed resistance element connected in series to the first detection element and a variable resistance element connected in parallel to the fixed resistance element, and the detection result of the first detection element is corrected by changing the resistance value of the variable resistance element.

[0153] The correction circuit may be realized by such a simple circuit.

[0154] [Item 13] 13. The image forming apparatus according to item 11 or 12, wherein the arithmetic circuit has a memory circuit that stores a correction coefficient, and the correction coefficient is used to correct the detection result of the second detection element.

[0155] In this way, the detection result of the second detection element is corrected by calculation, which makes it possible to reduce manufacturing costs.

[0156] [Item 14] 14. The image forming apparatus according to any one of items 11 to 13, wherein the first detection element is disposed downstream of the second detection element in the sheet transport direction.

[0157] This structure is illustrated in Fig. 11(B). In the heaters 230, 730, and 1000, the temperature on the downstream side in the conveying direction of the sheet P is higher than the temperature on the upstream side. Therefore, by arranging the first detection element on the downstream side, it is possible to quickly detect an excessive temperature rise.

[0158] [Item 15] The first sensing element and the second sensing element are 15. The image forming apparatus according to any one of items 11 to 14, characterized in that the fixing device is configured to detect the temperature of a substrate that includes the heating element, the cylindrical member, or the pressure means in a contact or non-contact manner.

[0159] [Item 16] The fixing device further includes a substrate that includes the heat generating element, the heating element is formed on a first surface of the substrate, 15. The image forming apparatus according to any one of items 11 to 14, wherein the first detection element and the second detection element are printed on a second surface of the substrate, the second surface being different from the first surface.

[0160] [Item 17] a primary side circuit including the heating element; a secondary circuit insulated from the primary circuit and controlling the power supplied from the AC power supply to the heating element based on the temperature of the fixing device detected by a temperature detection circuit, The temperature sensing circuit including the first sensing element and the second sensing element is Insulation is provided between the primary side circuit and the Item 17. The image forming apparatus according to item 16, wherein insulation is also provided between the secondary side circuit.

[0161] [Item 18] a metal frame providing a ground potential; an image forming means for forming a toner image on a sheet; a fixing device that fixes the toner image to the sheet, a heating element that generates heat when power is supplied from an AC power source; an insulating layer covering the heating element; a detection means for detecting the temperature of the heating element; a cylindrical member heated by the heating element; a fixing device having a pressure means provided opposite the cylindrical member and cooperating with the cylindrical member to form a nip portion; a temperature detection circuit including the detection means for detecting the temperature of the fixing device and insulated from the metal frame; a capacitance element (e.g., capacitor C3) having one end connected to the metal frame and the other end connected to the temperature detection circuit; An image forming apparatus comprising:

[0162] This structure was described in Example 4. By connecting a capacitive element (e.g., capacitor C3) between the metal frame and the temperature detection circuit in this manner, it is possible to protect the fixing device 50 from overvoltage and reduce the size of the fixing device 50 at the same time.

[0163] [Item 19] a primary side circuit including the heating element; a secondary side circuit that controls the power supplied from the AC power supply to the heating element based on the temperature of the fixing device detected by the temperature detection circuit, Item 19. The image forming apparatus according to item 18, wherein insulation is provided between the temperature detection circuit and the primary side circuit, and insulation is also provided between the temperature detection circuit and the secondary side circuit.

[0164] This structure is shown in Fig. 16. By providing insulation (for example, reinforced insulation) in this manner, the safety of the image forming apparatus 100 is further improved.

[0165] [Item 20] 20. The image forming apparatus according to item 19, wherein the capacitive element is connected to the metal frame via the secondary side circuit.

[0166] 13, the capacitive element (e.g., capacitor C3) may be connected to the frame ground 239 via the ground potential GND-2 of the secondary side circuit 1302. The conductors Ega and Egb of the temperature detection circuit 1303 are connected to the ground potential GND-3 of the temperature detection circuit 1303. The capacitive element (e.g., capacitor C3) connects the ground potential GND-2 of the secondary side circuit 1302 and the ground potential GND-3 of the temperature detection circuit 1303. Therefore, in other words, the capacitive element (e.g., capacitor C3) connects the frame ground 239 and the conductors Ega and Egb.

[0167] the fixing device has conductors (e.g., Ega, Egb) that are provided on a substrate on which the heating element and the detection means are provided and that are connected to the detection means; An image forming apparatus described in any of the above items, characterized in that the capacitive element connects the conductor provided on the substrate to the metal frame via the secondary side circuit and the temperature detection circuit.

[0168] This is illustrated in Fig. 13. The occurrence of capacitance cfg between the conductors Ega and Egb of the temperature detection circuit 1303 and the film 202 makes it easier to prevent overvoltage. As a result, the creepage distance and clearance distance between the film 202 and the frame ground 239 can be shortened, which may realize a reduction in the size of the fixing device 50 (image forming apparatus 100).

[0169] The invention is not limited to the above-described embodiments, and various changes and modifications can be made without departing from the spirit and scope of the invention. Accordingly, the following claims are appended to apprise the public of the scope of the invention. [Explanation of symbols]

[0170] 100: Image forming apparatus, 239: Frame ground, 19: Photosensitive member, 50: Fixing device

Claims

1. a metal frame providing a ground potential; an image forming means for forming a toner image on a sheet; a fixing device that fixes the toner image to the sheet, a heating element that generates heat when power is supplied from an AC power source; an insulating layer covering the heating element; a conductive layer in contact with the insulating layer and insulated from the metal frame; a cylindrical member heated by the heating element; a fixing device having a pressure means provided opposite the cylindrical member and cooperating with the cylindrical member to form a nip portion; a capacitance element having one end connected to the metal frame and the other end connected to the conductive layer of the fixing device; An image forming apparatus comprising:

2. The heating element is provided on the first surface side of the substrate, a first insulating layer of the insulating layers is provided to cover the heating element provided on the first surface side of the substrate, the conductive layer is provided on a second surface of the substrate that faces an inner circumferential surface of the cylindrical member, 2. The image forming apparatus according to claim 1, wherein a second insulating layer of the insulating layers is provided on the second surface side of the substrate so as to cover the conductive layer and slide against the inner circumferential surface of the cylindrical member.

3. The heating element is provided on the first surface side of the substrate, a first insulating layer of the insulating layers is provided to cover the heating element provided on the first surface side of the substrate, 2. The image forming apparatus according to claim 1, wherein the conductive layer is provided on the first surface side.

4. 3. The image forming apparatus according to claim 2, wherein the conductive layer is disposed outside the first insulating layer and in contact with the first insulating layer.

5. a first detection element and a second detection element that detect the temperature of the heating element; a temperature control means for controlling the temperature of the heating element based on the detection result of the first detection element; a cutoff means for forcibly cutting off power supplied to the heating element when a detection result of the first detection element indicates that a predetermined event has occurred; a correction circuit that corrects the detection result of the first detection element according to individual differences of the first detection element; 2. The image forming apparatus according to claim 1, further comprising: a calculation circuit that corrects the detection result of the second detection element by calculation in accordance with individual differences of the second detection element.

6. 6. The image forming apparatus according to claim 5, wherein the correction circuit has a fixed resistance element connected in series to the first detection element and a variable resistance element connected in parallel to the fixed resistance element, and the detection result of the first detection element is corrected by changing the resistance value of the variable resistance element.

7. 7. The image forming apparatus according to claim 5, wherein the arithmetic circuit has a memory circuit that stores a correction coefficient, and corrects the detection result of the second detection element using the correction coefficient.

8. 7. The image forming apparatus according to claim 5, wherein the first detecting element is disposed downstream of the second detecting element in the sheet transport direction.

9. 2. The image forming apparatus according to claim 1, wherein the capacitance element is provided inside the fixing device.

10. 2. The image forming apparatus according to claim 1, wherein the capacitance element is provided outside the fixing device.

11. an image forming means for forming a toner image on a sheet; a fixing device that fixes the toner image to the sheet, a heating element that generates heat when power is supplied from an AC power source; a cylindrical member heated by the heating element; a fixing device having a pressure means provided opposite the cylindrical member and cooperating with the cylindrical member to form a nip portion; a first detection element and a second detection element that detect the temperature of the heating element; a correction circuit that corrects the detection result of the first detection element according to individual differences of the first detection element; an arithmetic circuit that corrects the detection result of the second detection element according to individual differences of the second detection element; a temperature control means for controlling the temperature of the heating element based on the detection result of the first detection element corrected by the correction circuit without using the detection result of the second detection element; a cutoff means for forcibly cutting off the power supplied to the heating element using the detection result of the first detection element corrected by the correction circuit, without using the detection result of the second detection element; An image forming apparatus comprising:

12. 12. The image forming apparatus according to claim 11, wherein the correction circuit has a fixed resistance element connected in series to the first detection element and a variable resistance element connected in parallel to the fixed resistance element, and the detection result of the first detection element is corrected by changing the resistance value of the variable resistance element.

13. 13. The image forming apparatus according to claim 11, wherein the arithmetic circuit has a memory circuit that stores a correction coefficient, and corrects the detection result of the second detection element using the correction coefficient.

14. 13. The image forming apparatus according to claim 11, wherein the first detecting element is disposed downstream of the second detecting element in the sheet transport direction.

15. The first sensing element and the second sensing element are 12. The image forming apparatus according to claim 11, wherein the fixing device is configured to detect the temperature of a substrate enclosing the heating element, the cylindrical member, or the pressure means in a contact or non-contact manner.

16. The fixing device further includes a substrate that includes the heat generating element, the heating element is formed on a first surface of the substrate, 12. The image forming apparatus according to claim 11, wherein the first sensing element and the second sensing element are printed on a second surface of the substrate, the second surface being different from the first surface.

17. a primary side circuit including the heating element; a secondary circuit insulated from the primary circuit and controlling the power supplied from the AC power supply to the heating element based on the temperature of the fixing device detected by a temperature detection circuit, The temperature sensing circuit including the first sensing element and the second sensing element is Insulation is provided between the primary side circuit and the 17. The image forming apparatus according to claim 16, wherein insulation is also provided between the image forming apparatus and the secondary circuit.

18. a metal frame providing a ground potential; an image forming means for forming a toner image on a sheet; a fixing device that fixes the toner image to the sheet, a heating element that generates heat when power is supplied from an AC power source; an insulating layer covering the heating element; a detection means for detecting the temperature of the heating element; a cylindrical member heated by the heating element; a fixing device having a pressure means provided opposite the cylindrical member and cooperating with the cylindrical member to form a nip portion; a temperature detection circuit including the detection means for detecting the temperature of the fixing device and insulated from the metal frame; a capacitance element having one end connected to the metal frame and the other end connected to the temperature detection circuit; An image forming apparatus comprising:

19. a primary side circuit including the heating element; a secondary side circuit that controls the power supplied from the AC power supply to the heating element based on the temperature of the fixing device detected by the temperature detection circuit, 19. The image forming apparatus according to claim 18, wherein insulation is provided between the temperature detection circuit and the primary side circuit, and insulation is also provided between the temperature detection circuit and the secondary side circuit.

20. 20. The image forming apparatus according to claim 19, wherein the capacitive element is connected to the metal frame via the secondary circuit.

Images