JP4817862B2 - Heat fixing device - Google Patents

Description

The present invention relates to a copying machine, a printer, a facsimile or thermal fixing apparatus and a control method thereof definitive in image forming equipment for the complex machine or the like using an electrophotographic method.

  As a fixing device in an image forming apparatus, a heating type fixing apparatus is known in which a recording material on which an unfixed image is formed and supported by an image forming unit of the image forming apparatus is heated to fix the image. The heating-type fixing device includes an energization heating element (hereinafter simply referred to as a heating element) that is a heat source, a power source that supplies current to the heating element, a temperature detection unit that detects a temperature in the vicinity of the heating, and the temperature. Control means for controlling the supply current of the power source based on a signal from the detection means. With this configuration, the image fixing temperature is controlled to a predetermined temperature for image fixing.

  Such a heating-type fixing device does not function as a fixing device when any one of the heating element, power source, temperature detecting means, and control means does not function normally. Furthermore, when energization runaway occurs, there is a risk that the fixing device may malfunction due to overheating.

  Therefore, in such a fixing device, as disclosed in Patent Document 1, the following abnormal overheat safety device is provided to avoid various problems that may occur during energization runaway.

  (1) A safety device (thermo protector) such as a thermal fuse or a thermo switch intervenes in the energization circuit of the heating element, and when overheating occurs due to energization runaway, the energization to the heating element is cut off.

  (2) Temperature detection means such as a thermistor is placed near the heating element, and when the heating element is in an abnormally overheated state, current supply to the heating element is interrupted by a current interruption means such as a relay interposed in the energizing circuit. Let The temperature at which the abnormal overheat safety device operates is set to be higher than the temperature reached during normal operation to prevent malfunction during normal operation and to operate only during abnormal overheating.

  After the abnormal overheat safety device as described above is activated, the fixing component or unit (heating element, power supply, temperature detection means, control means, etc.) that originally caused the trouble, It is only necessary for a service person to replace a safety device such as a thermo switch.

  However, in actuality, there may be an overtemperature increase in the ambient temperature between the occurrence of an overheating trouble in the fixing device and the operation of a safety device such as a thermoswitch. As a result, not only members such as a pressure roller and a ceramic heater inside the fixing device but also peripheral devices thereof may be damaged due to overheating. In some cases, the entire fixing device and its peripheral devices may be repaired or replaced. In order to suppress the damage that occurs during such energization runaway, the operating temperature of the abnormal overheat safety device may be set as low as possible. However, if the operating temperature is set low, the abnormal overheat safety device is activated during normal operation and the image forming apparatus malfunctions.

  For this problem, an abnormal overheat safety device (hereinafter simply referred to as a safety device) as disclosed in Patent Document 2 has been proposed.

  FIG. 16 is a block diagram illustrating a safety device in the above-described conventional example. 2012 is a relay inserted between the power source and the heating element, and is operated by the transistor 2010. Here, 2009 is a bias resistor. When the relay 2012 is off, the power supply to the heating element is cut off. The transistor 2010 is connected to a temperature comparison circuit composed of an operational amplifier 2007. A detection signal of a thermistor provided around the heating element is input to the negative input of the operational amplifier 2007. On the other hand, the reference voltage Vref is input to the positive input. The reference voltage Vref is generated by resistors 2004, 2005, 2006, and a transistor 2003. Here, 2002 is a bias resistor. The transistor 2003 is connected to a current detection circuit 2001 whose output state changes according to the amount of current flowing through the heating element, and the reference voltage Vref is switched according to the amount of current of the heating element. In this way, a circuit for detecting abnormal overheating and cutting off relay 2012 is referred to as an abnormal overheat detection circuit. Reference numeral 2013 denotes a latch circuit, which holds the state when the relay 2012 is continuously turned off for a predetermined time. Note that 2008 and 2011 are resistors.

  With such a configuration, the abnormal overheat detection temperature of the abnormal overheat detection circuit is switched according to the amount of current flowing through the heating element of the image forming apparatus. When the current amount is large, the safety circuit operates at a low temperature to cut off the current, and when the current amount is small, the safety circuit operates at a high temperature to cut off the current.

That is, during energization runaway, the energization of the heating element is interrupted at a low temperature to minimize damage to the fixing device, and during normal operation, it is possible to avoid malfunction of the safety circuit.
Japanese Patent Application Laid-Open No. 08-248813 JP 2005-321573 A

  However, in the safety device having the configuration in which the abnormal overheat detection temperature of the abnormal overheat detection circuit is switched according to the amount of current flowing through the heating element as described above, a phenomenon occurs in which the safety device is repeatedly activated / deactivated during energization runaway. Therefore, there were the following problems.

  (1) The time until the latch circuit operates and the energization is completely cut off is delayed. If the overheating state continues for a long time, damage to the fixing device may increase.

  (2) When the relay 2012 is switched on / off, discharge occurs between the secondary contacts of the relay 2012. The electric circuit in the image forming apparatus malfunctions due to noise generated by the discharge.

  The operation of the safety device during energization runaway will be described below. When a large current flows through the heating element due to energization runaway, the circuit operates in the following steps.

  Step S1: The output of the current detection circuit 2001 is switched to a LOW output, and the level of the reference voltage Vref input to the positive electrode of the operational amplifier 2007 increases. That is, the abnormal overheat detection temperature is switched to Tp (LOW) on the low temperature side.

  Step S2: When the detection temperature of the thermistor rises and becomes higher than the abnormal overheat detection temperature, the output of the operational amplifier 2007 is switched to the LOW output, and the transistor 2010 is turned off.

  Step S3: The relay 2012 is turned off and the power supply to the heating element is cut off.

  Step S4: The output of the current detection circuit 2001 is switched to a HIGH output, and the level of the reference voltage Vref input to the positive electrode of the operational amplifier 2007 decreases. That is, the abnormal overheat detection temperature is switched to Tp (HIGH) on the high temperature side.

  Step S5: The output of the operational amplifier 2007 is switched to a HIGH output, and the transistor 2010 is turned on.

  Step S6: The relay 2012 is turned on and energization of the heating element is started.

  After step S6, the process proceeds to step S1 again, and the operation / non-operation of the safety device is repeated. That is, energization / non-energization of the heating element is repeated. This phenomenon is repeated until the detection temperature of the thermistor rises and the abnormal overheat detection temperature becomes higher than Tp (HIGH) on the high temperature side.

The present invention has been made paying attention to the above points, and prevents the malfunction of the safety device due to the temperature rise at the end during normal operation. It is an object of the present invention to provide a heat fixing device that can be shut off in time.

In order to solve the above problems, the present onset Ming comprises the following configuration.

(1) a heater, a temperature detection means for detecting a temperature of said heater, said temperature sensing hand stage of control to that control means the power supplied to based power or al the heater to the detected temperature, from the power supply a relay provided in the electricity supply path to the heater, and current detecting means for detect the flowing current to the heater, the detected temperature and the reference temperature comparing the detected temperature is the reference temperature of the temperature detecting means A comparator that sends a signal for shutting off the power supply path to the relay when reaching the relay, and a latch circuit that latches the relay in a power supply path shut-off state when the signal sent from the comparator to the relay continues. the current detection when the detected current value of the unit is lower than the reference current value is set to the reference temperature to a first reference temperature, the detected current value is the reference current wherein the case higher is the reference temperature value first Reference temperature And a reference temperature switch unit for setting the lower second reference temperature, anda safety circuit for driving the relay independently of the control means, a recording material an unfixed image formed on a recording material In the heat fixing device for fixing to the safety circuit, the safety circuit has the detected temperature exceeding the second reference temperature in a state where the reference temperature is set to the second reference temperature, and the relay is connected to the power supply path. When the power supply path is cut off, a holding unit is provided to hold the reference temperature at the second reference temperature regardless of the detected current value. The heating and fixing device , wherein the latch circuit latches the relay in a state where the power supply path is cut off before the temperature falls to the second reference temperature .

  According to the present invention, it is possible to cut off the power supply to the heating element in a short time at a low temperature during energization runaway without causing the safety device to malfunction even when the end temperature rise occurs during normal operation. It becomes possible. Thereby, it is possible to minimize damage to the image forming apparatus and to minimize the cost of replacement parts and service cost. This is realized by switching the abnormal overheat detection temperature of the abnormal overheat detection circuit according to the amount of current flowing through the heating element of the image forming apparatus, and latching the level of the abnormal overheat detection temperature according to the output of the temperature comparison means. Is done.

  Further, the same effect can be achieved by latching the abnormal overheat detection temperature level according to the outputs of the plurality of temperature detection means.

  Hereinafter, the best mode for carrying out the present invention will be described in detail with reference to examples.

  Embodiment 1 of the present invention will be described below with reference to the drawings.

(1) Image Forming Apparatus FIG. 1 is a schematic sectional view showing an embodiment of an image forming apparatus according to the present invention, and 100 is an image forming apparatus such as a laser beam printer. The image forming apparatus 100 includes a deck 101 that stores recording paper (corresponding to a recording material ) P. Also, a deck paper presence / absence detection sensor 102 that detects the presence or absence of the recording paper P in the deck 101, a paper size detection sensor 103 that detects the size of the recording paper P in the deck 101, and a pickup roller 104 that feeds the recording paper P from the deck 101. Is provided. Further, a deck sheet feeding roller 105 that conveys the recording sheet P fed by the pickup roller 104 and a retard roller 106 that is paired with the deck sheet feeding roller 105 and prevents double feeding of the recording sheet P are provided. ing.

  Downstream of the deck paper feed roller 105, there are a paper feed sensor 107 for detecting a paper feed conveyance state from the deck 101 and a double-side reversing unit described later, and a paper feed conveyance roller 108 for conveying the recording paper P further downstream. It is arranged. A registration roller pair 109 that conveys the recording paper P in synchronization with the printing timing and a pre-registration sensor 110 that detects the conveyance state of the recording paper P to the registration roller pair 109 are provided. Further, a process cartridge 112 that forms a toner image on a photosensitive drum (corresponding to an image carrier) 1 based on a laser beam from the laser scanner unit 111 is disposed downstream of the registration roller pair 109. Then, a roller member 113 (hereinafter referred to as a transfer roller) for transferring the toner image formed on the photosensitive drum 1 onto the recording paper P, and removing the charge on the recording paper P to promote separation from the photosensitive drum 1. A discharge member 114 (hereinafter referred to as a static elimination needle) is provided.

  Further, downstream of the static elimination needle 114, a conveyance guide 115, a fixing device 116 for thermally fixing the toner image transferred onto the recording paper P, and a fixing paper discharge sensor 119 for detecting the conveyance state from the fixing device 116 are arranged. ing. In addition, a double-sided flapper 120 is provided for switching the destination of the recording paper P conveyed from the fixing device 116 to a paper discharge unit or a double-side reversing unit. A paper discharge sensor 121 that detects the paper conveyance state of the paper discharge unit and a paper discharge roller pair 122 that discharges the recording paper P are disposed downstream of the paper discharge unit. On the other hand, to print on both sides of the recording paper P, the recording paper P after the single-sided printing is turned upside down, and the recording paper P is switched by forward / reverse on the double-sided reversing part side for feeding again to the image forming part. A reverse roller pair 123 for backing is disposed. In addition, a reversing sensor 124 for detecting the state of paper conveyance to the reversing roller 123, and a D-cut roller 125 for conveying the recording paper P from a lateral registration portion (not shown) for adjusting the lateral position of the recording paper P are provided. It is arranged. Further, a double-sided sensor 126 for detecting the conveyance state of the recording paper P in the double-side reversing unit and a double-sided conveyance roller pair 127 for conveying the recording paper P from the double-side reversing unit to the paper feeding unit are provided.

(2) Fixing Device FIG. 2 is a schematic sectional view of the fixing device 116. The fixing device 116 of the present embodiment is a film heating type fixing device disclosed in JP-A-4-44075 to JP-A-4-44083, JP-A-4-204980 to JP-A-4-204984, and the like. is there.

Reference numeral 204 denotes a heat-resistant, heat-insulating, and rigid body stay for fixing the ceramic heater and guiding the inner surface of the film, and is a horizontally long member having a longitudinal direction in the direction crossing the conveyance path of the recording paper 210 (perpendicular to the drawing). 205 is a ceramic heater data to be described later, and fitted into the groove formed along the length on the lower surface of the stay 204 was fixedly supported by the heat-resistant adhesive, Horizontal to the direction transverse to the transfer material conveying path and longitudinal It is a member. Reference numeral 201 denotes a cylindrical heat-resistant film material (hereinafter referred to as a fixing film), which is loosely fitted on a stay 204 to which a ceramic heater 205 is attached. For example, it is a cylindrical single layer film such as PTFE, PFA, FEP having a thickness of about 40 to 100 μm and having heat resistance, releasability, strength, durability and the like. Moreover, the composite layer film which coated PTFE, PFA, FEP etc. on the outer peripheral surface of cylindrical films, such as a polyimide, polyamide, PEEK, PES, and PPS, may be sufficient. A pressure roller 202 is an elastic roller in which a heat-resistant elastic layer 207 such as silicone rubber is provided concentrically and integrally on the outer periphery of the core metal 203. The pressure roller 202 and the side of the stay 204 to which the ceramic heater 205 is attached are pressed against the elasticity of the pressure roller 202 with the fixing film 201 interposed therebetween. A range indicated by an arrow N is a fixing nip portion formed by the pressure contact.

  The pressure roller 202 is rotationally driven in the direction of arrow B at a predetermined peripheral speed by a fixing drive motor M2 (118 in FIG. 1). The rotational force generated by the rotational driving of the pressure roller 202 directly acts on the fixing film 201 by the frictional force between the pressure roller 202 and the outer surface of the fixing film 201 in the fixing nip portion N. As shown in FIG. 2, when the recording paper 210 is introduced into the fixing nip N in the direction of arrow A, a rotational force indirectly acts on the fixing film 201 via the recording paper 210. As a result, the fixing film 201 is rotationally driven in the clockwise direction C as indicated by an arrow while being slidably pressed against the lower surface of the ceramic heater 205. The stay 204 also functions as a film inner surface guide member and facilitates the rotation of the film 201. In order to reduce the sliding resistance between the inner surface of the film 201 and the lower surface of the ceramic heater 205, a small amount of lubricant such as heat-resistant grease can be interposed between them.

  Eventually, the rotation of the film 201 due to the rotation of the pressure roller 202 becomes steady, and the temperature of the ceramic heater 205 rises to a predetermined level. Here, the recording paper 210 to be image-fixed is introduced between the film 201 and the pressure roller 202 in the fixing nip N formed by the ceramic heater 205 and the pressure roller 202 with the film 201 interposed therebetween. When the recording paper 210 is nipped and conveyed together with the film 201 through the fixing nip portion N, the heat of the ceramic heater 205 is applied to the unfixed image on the recording paper 210 via the film 201, and the unfixed image on the recording paper 210 is unfixed. An image is heat-fixed on the surface of the recording paper 210. The recording paper 210 that has passed through the fixing nip N is separated from the surface of the film 201 and conveyed. 2 indicates the conveyance direction of the recording paper 210.

(3) Ceramic heater FIG. 3 is a schematic sectional view of the ceramic heater 205. The ceramic heater 205 is long disposed in a direction perpendicular to the conveyance direction of the recording paper 210. Alumina (Al ₂ O ₃ ) is used as the substrate 301, and two heat generation patterns 302a and 302b are formed on one side by printing. The heat generation patterns 302a and 302b are covered with a glass protective film as an electrical insulating layer. In this embodiment, the heater portion formed by the heat generation pattern 302a is referred to as a main heater, and the heater portion formed from the heat generation pattern 302b is referred to as a sub heater. Reference numerals 303a, 303b, and 303c denote power supply electrodes, which are formed so that a voltage can be applied to both ends of the heat generation pattern.

  The main heater 302a and the sub heater 302b have greatly different heat generation distributions. FIG. 4 is a diagram showing a correspondence between the longitudinal direction of the ceramic heater 205 and a graph representing the heat distribution of the main heater 302a and the sub heater 302b. The main heater 302a is formed so that the amount of heat generated at the central portion Cn of the ceramic heater 205 is large. On the other hand, the sub-heater 302b is formed so as to increase the amount of heat generated at both ends Ed.

(4) Thermistor The fixing device 116 of this embodiment has three thermistors (corresponding to temperature detecting means) 206 (see FIG. 2) for measuring the temperature of the ceramic heater 205. Each thermistor 206 is pressed onto the ceramic heater 205 with a predetermined pressure. FIG. 4 shows an arrangement relationship between the thermo switch on the ceramic heater 205 and the three thermistors 206 (hereinafter referred to as the thermistor 1, the thermistor 2, and the thermistor 3). Further, the arrangement of the thermo switch and the three thermistors 1, 2, and 3 in the longitudinal direction of the ceramic heater 205 is indicated by arrows D, E, F, and G. The thermo switch (arrow D) will be described later. The thermistor 1 is disposed at the center of the ceramic heater 205 (arrow E). On the other hand, the thermistors 2 and 3 are arranged at the ends of the ceramic heater 205 (arrows F and G). Each thermistor 1, 2, 3 is connected to a temperature detection circuit (not shown). FIG. 6 is an internal configuration circuit diagram of the temperature detection circuit. Thermistor 1 601, thermistor 2 602, and thermistor 3 603 are connected in series with resistors 604, 606, and 607, respectively. S6, S7, and S8 are detection signals, which vary according to the resistance values of the thermistors 1, 2, and 3 that change with temperature. Since the resistance values of the thermistors 1, 2, and 3 are characteristics that decrease as the temperature increases, the detection signals S6, S7, and S8 have characteristics that the voltage level decreases as the detection temperature increases. The detection signal S7 is connected only to the CPU 501 (see FIG. 5). On the other hand, the detection signals S6 and S8 are connected to the CPU 501 and a safety circuit described later.

(5) Thermo switch The fixing device 116 of this embodiment has one thermo switch (not shown) as a current interrupting means in the event of abnormal overheating.

  The thermo switch is pressed against the ceramic heater 205 with a predetermined pressure. FIG. 4 shows the position of the thermoswitch in the longitudinal direction of the ceramic heater 205 (arrow D). The operating temperature of the thermoswitch is 250 ° C.

  Here, the operating temperature of the thermoswitch will be described. The operating temperature of the thermoswitch is largely related to the rate of temperature rise until the operating temperature is reached. FIG. 15 is a graph showing the relationship between the heater temperature of the ceramic heater 205 and the temperature at which the thermoswitch actually operates. LINE-B is a case where the ceramic heater 205 is heated at a low temperature rise rate, and the thermoswitch operates at a point B where the ceramic heater 205 is heated by ΔTb from 250 ° C. On the other hand, LINE-A indicates a case where the ceramic heater 205 is heated at a high temperature rise rate. In this case, the thermo switch does not operate at 250 ° C., and operates at the point A where ΔTa is heated more than 250 ° C. ΔTa is larger than ΔTb. That is, the thermoswitch has a characteristic of operating at a temperature closer to the operating temperature of 250 ° C. as the rate of temperature rise until reaching the operating temperature is lower. Such characteristics are caused by the heat capacity of the thermoswitch itself.

(6) (corresponding to control means) power control circuit supplies power to the power control circuit then the ceramic heater 205 will be described. The power control is controlled independently by the main heater 302a and the sub heater 302b. FIG. 5 is a connection diagram of the ceramic heater 205 and the power control circuit. 501 CPU, 502, 503 are first and second triac, 504 AC power source, 505 relays, 507 denotes a current detection circuit (corresponding to a current detecting means). The first triac 502 and the main heater 302a are connected in series, the second triac 503 and the sub-heater 302b are connected in series, and they are connected to the AC power source 504 in parallel. The first and second triacs 502 and 503 are on / off controlled by turning on and off the first and second heater drive signals S1 and S2 from the CPU 501, respectively. The relay 505 is inserted between the first and second triacs 502 and 503 and the AC power source 504, and is configured to cut off the energization to the main heater 302a and the sub heater 302b by driving the relay 505. The control signal S4 of the relay 505 is connected to a safety circuit 509 described later. The current detection circuit 507 is inserted between the relay 505 and the AC power source 504. The safety circuit 509 is controlled by a control signal S3 from the CPU 501. The operation of the current detection circuit 507 will be described later.

(7) Power Control Sequence A power control method in the image forming apparatus of this embodiment will be described. In the present embodiment, the main heater 302a and the sub heater 302b are turned on / off at a phase angle within one half wave to perform phase control for controlling the power applied to each heater.

[1] Control at Startup When the CPU 501 receives a print start signal from a controller (not shown), it executes an image forming sequence. At the same time, the first and second heater drive signals S1 and S2 are controlled to turn on the first and second triacs 503 and 504, and the ceramic heater 205 is heated. As the ceramic heater 205 rises in temperature, the resistance value of the thermistor 1 decreases, and the CPU 501 monitors and recognizes the temperature state at the center of the ceramic heater 205 by monitoring the output signal S6 from the thermistor 1. At this time, the electric power Pup applied to the ceramic heater 205 is set from the difference between the temperature detected by the thermistor 1 and a predetermined fixing target temperature. By setting the level of the applied power Pup large, the temperature of the ceramic heater 205 can be raised in a short time. The applied power Pup is set at a power of 80% or more, assuming that the power is 100% when all phases are on.

[2] Steady Temperature Control When it is detected that the temperature detected by the thermistor 1 has risen to a predetermined fixing target temperature, the power applied to the ceramic heater 205 is reduced to suppress the temperature of the ceramic heater 205. Thereafter, the applied power is increased or decreased according to the difference between the detected temperature of the thermistor 1 and a predetermined fixing target temperature, so that the temperature at the center of the ceramic heater 205 is controlled to be the fixing target temperature.

  The applied power Psat at this time is smaller than the applied power Pup at the start-up, and is controlled at 0% to 60%.

  In the above processing, the energization method of the sub heater 302b is changed according to the length of the recording paper 210 in the direction perpendicular to the recording paper conveyance direction, that is, the width size. FIG. 7A is a diagram showing the positional relationship between the type of the width size of the recording paper 210 (referred to as the paper width in the drawing) and the ceramic heater 205. FIG. 7B is a table showing the relationship between the paper width and the power ratio related to the energization setting of the sub-heater 302b (hereinafter referred to as the sub-heater power ratio). The control method is classified into four according to the paper width size. Switch. Here, the sub-heater power ratio indicates the ratio of the power applied to the sub-heater 302b with respect to the power applied to the main heater 302a. The second heater drive signal S2 is controlled according to this setting. By setting the power ratio of the sub-heater 302b to be smaller as the paper width is smaller, a phenomenon in which the temperature at the end of the fixing device 116 increases during printing (hereinafter referred to as end temperature rise) is suppressed. Edge temperature rise is greatly related to the width size of the recording paper 210. When the width of the recording paper 210 is smaller than the width of the heating area of the fixing device 116, the end of the fixing device 116 is a non-sheet passing area. The portion of the recording paper 210 that passes through and the portion that does not pass through the paper have a large difference in the amount of heat taken away, so that the temperature at the end of the ceramic heater 205 increases. This edge temperature increase may cause various problems such as wrinkles and offsets. If the recording paper 210 to be passed is a recording paper having a narrow width, the non-uniformity of heat of the ceramic heater 205 tends to become larger.

(8) Edge temperature rise countermeasure control In the image forming apparatus of the present embodiment, the temperature of the end of the ceramic heater 205 is detected by the thermistors 2 and 3, and the temperature of the end of the ceramic heater 205 is predetermined by the temperature rise of the edge. When the temperature is higher than the temperature, control is performed to increase the sheet passing interval of the recording paper 210. By increasing the paper passing interval, the temperature difference between the portion where the recording paper 210 is passed and the portion where the recording paper 210 is not passed is reduced, and the temperature rise at the edge can be suppressed.

  FIG. 8 is a graph showing the transition of the temperature rise at the edge when the recording paper 210 whose paper width is classified as “A” in FIG. 7 is passed. The horizontal axis represents the time from the start of printing, and the vertical axis represents the detected temperature of the thermistor 1 disposed at the center of the ceramic heater 205 and the thermistor 3 disposed at the end. After printing is started, electric power is supplied to the ceramic heater 205 so that the detected temperature of the thermistor 1 becomes a predetermined value, and the detected temperature of the thermistor 1 rises to 200 ° C. (point <1>). Thereafter, the detected temperature of the thermistor 1 is controlled to be constant. On the other hand, the temperature detected by the thermistor 3 rises gradually after printing is started. This is because the sub-heater 302b is not energized when a recording paper whose paper width is classified as “A” is passed. Even if the detection temperature of the thermistor 3 exceeds the detection temperature 200 ° C. of the thermistor 1, it further increases (point <2>) and reaches 240 ° C. (point <3>). When the temperature of the thermistor 3 reaches 240 ° C., control for increasing the sheet passing interval is started. This stops the temperature rise at the end.

(9) Current detection circuit The current detection circuit 507 is inserted between the relay 505 and the AC power source 504. The current detection circuit 507 detects the total current value of the current flowing through the main heater 302a and the sub heater 302b (hereinafter referred to as a heater current detection value), and outputs a current detection signal S5. The current detection signal S5 is connected to a safety circuit 509 described later. The current detection signal S5 is in a LOW state when the heater current detection value is larger than the reference current Ipr. The reference current Ipr is 12 A, which corresponds to the heater current that flows when the applied power is 80%. As described above, the applied power becomes 80% or more only in the case of control at the time of start-up, so that the current detection signal S5 is in a LOW state only at the time of start-up during normal operation.

(10) Safety circuit In the image forming apparatus of this embodiment, a safety device is provided to avoid overheating of the ceramic heater 205 during energization runaway. In addition to the above-described thermoswitch as a safety device, a thermistor 1 and 3 are used to detect an abnormal overheating of the ceramic heater 205, and a safety circuit 509 that cuts off the power supply when abnormal heating occurs is provided.

  FIG. 10 is a setting table of operating temperatures of the safety circuit 509 for the thermistors 1 and 3. The operating temperature of the safety circuit 509 is different for each thermistor.

  The abnormal overheat detection temperature of the thermistor 1 is 220 ° C. regardless of the state of the heater current detection value. As described above, in the image forming apparatus of this embodiment, the ceramic heater 205 is controlled so that the temperature detected by the thermistor 1 is 200 ° C. Therefore, the safety circuit 509 does not operate during normal operation. On the other hand, during energization runaway, the safety circuit 509 operates at a detected temperature of 220 ° C., and the energization to the ceramic heater 205 can be cut off.

  The abnormal overheat detection temperature of the thermistor 3 is switched depending on the state of the heater current detection value. When the heater current detection value is 12 A (reference current Ipr) or more, that is, when the heater current detection value is at a high level, the abnormal overheat detection temperature is set to 220 ° C. On the other hand, when the heater current detection value is 12 A or less, that is, when the heater current detection value is at a low level, the abnormal overheat detection temperature is set to 260 ° C.

  By setting in this way, the safety circuit 509 operates when the detected temperature of the thermistors 1 and 3 is 220 ° C. during energization runaway, and the energization to the ceramic heater 205 is cut off.

  The heater current detection value becomes 12 A or more in the state where the power control is normally performed only when the ceramic heater 205 is started up. As shown in FIG. 8, the edge temperature rise is a phenomenon that occurs when the paper is continuously passed, and the temperature of the edge does not exceed 220 ° C. when the ceramic heater 205 is started up. Therefore, the safety circuit 509 does not operate during normal operation. In addition, the heater current is controlled to be 12 A or less except when the ceramic heater 205 is started up. The detected temperature of the thermistor 3 becomes higher than that of the thermistor 1 due to the temperature rise at the end, but does not exceed 260 ° C., so the safety circuit 509 does not operate.

  Next, details of the configuration of the safety circuit 509 will be described with reference to FIG. FIG. 9 is a block diagram illustrating the safety circuit 509.

Detection signal S6, S8 of the thermistor 1 and 3 are input to the negative input of the comparator (ratio corresponding to compare unit) 909,915, abnormal by performing the comparison with the reference voltage V ref1, Vref3 being inputted to the positive electrode Determine overheating condition.

  The detection signal S6 of the thermistor 1 is compared with a reference voltage Vref1, which is a value obtained by dividing the voltage Vcc by the resistors 910 and 911. When the output of the comparator 909 is in a HIGH state as a result of the voltage comparison, a base current flows from the power source Vcc through the resistor 908 to the transistor 906 and is turned on. Here, 907 is a bias resistor. As a result, the potential at the point C is decreased, the current applied to the base of the transistor 903 is decreased, and the transistor 903 is turned off. Here, 904 is a bias resistor. When the transistor 903 is turned off, the energization to the relay 901 is stopped and the relay 901 is cut off. Note that 939 is a diode, and 902, 905, 938, and 941 are resistors.

On the other hand, the detection signal S8 of the thermistor 3 is compared with a reference voltage Vref3 determined by the resistors 916, 917 and 919. A transistor 920 (corresponding to a reference temperature switching unit ) is connected to the resistor 917 in series. The transistor 920 is driven by the current detection signal S5 of the current detection circuit 507, and the reference voltage Vref3 is switched according to the detection result of the current detection circuit 507. Since the current detection signal S5 is in the LOW state when the heater current detection value is larger than the reference current Ipr, the reference voltage Vref3 is the combined resistance value of the parallel connection of the resistor 917 and the resistor 919 when the heater current detection value is large. And the divided voltage value of the resistor 916. On the other hand, when the heater current detection value is smaller than the reference current Ipr, the reference voltage Vref3 is determined by the divided voltage of the resistor 916 and the resistor 919. When the output of the comparator 915 is in a HIGH state as a result of the voltage comparison, a base current flows from the power source Vcc through the resistor 913 to the transistor 912 and is turned on. Here, 914 is a bias resistor. As a result, the potential at the point C is lowered, the transistor 903 is turned off, and the relay 901 is cut off. Reference numerals 921 and 925 denote resistors.

  Here, the latch circuit 940 connected to the point C will be described. The latch circuit 940 is provided for the purpose of fixing the potential at the point C to a low level and maintaining the cutoff state of the relay 901 when the transistor 906 or the transistor 912 remains on for a predetermined time. When the transistor 906 or the transistor 912 is turned on, a current flows through the resistors 934 and 933, and a bias is applied between the emitter and the base of the transistor 930. A capacitor 932 connected between the emitter and the base of the transistor 930 suppresses a rapid increase in the voltage between the emitter and the base. The voltage between the emitter and the base is increased by a time constant determined by the resistor 933 and the capacitor 932, and the transistor 930 is turned on after a predetermined time. Accordingly, a current flows through the resistor 936 and the transistor 935 is turned on. When the transistor 935 is turned on, the point C is latched at a low voltage regardless of the states of the transistors 906 and 912. The latch circuit 940 holds the latched state until the power supply Vcc is turned off. The latch circuit 940 operates when the transistor 906 or the transistor 912 is continuously turned on for 500 msec. Reference numeral 931 is a resistor.

It will now be described transistor 918 (corresponding to the hold portion). The transistor 918 is provided to latch the reference voltage Vref3 of the comparator 915 when the detection temperature of the thermistor 3 is in a high temperature state and the comparator 915 switches to the HIGH state. The transistor 918 is connected between the output terminal of the comparator 915 and the base of the transistor 920. When the detection temperature of the thermistor 3 becomes a high temperature state and the output terminal of the comparator 915 switches to the HIGH state, the base current is applied to the transistor 918 through the resistors 913 and 926, and the transistor 918 is turned on. When the transistor 918 is on, the transistor 920 is off regardless of the state of the current detection signal S5 output from the current detection circuit 507. That is, the abnormal overheat detection temperature of the thermistor 3 is latched at 220 ° C. The latched state is released when the detected temperature of the thermistor 3 falls below 220 ° C.

  Note that 922 is a transistor that operates in response to a control signal S3 from the CPU 501, and 923 is a bias resistor.

  By providing the transistor 918, it is possible to avoid a phenomenon in which the heater current flows again after the current detection signal S5 is reversed and the safety circuit 509 is released after the heater current is interrupted by the operation of the safety circuit 509. .

  Next, the operation timing of the safety circuit 509 configured using the thermistor 3 will be described.

  FIG. 11 is a partial waveform diagram of each part in the safety circuit 509 when the energization runaway of the ceramic heater 205 occurs. Time T0 to T1 is a section in which the power control of the ceramic heater 205 is normally executed. The heater current is controlled to 12 A or less, and the current detection signal S5 is in a HIGH state. Therefore, the abnormal overheat detection temperature of the thermistor 3 is set to 260 ° C., which is the heater current detection value at a low level (see FIG. 10).

  Here, it is assumed that energization runaway occurs at time T1. The heater current increases due to energization runaway, and the temperature detected by the thermistor 3 rises.

  The heater current becomes 12 A or more, and the current detection signal S5 that is the output of the current detection circuit 507 is switched from HIGH to LOW. Along with this, the point A (see FIG. 9) is also in the LOW state, and the abnormal overheat detection temperature is switched from 260 ° C. to 220 ° C. At this time, since the detected temperature of the thermistor 3 is 220 ° C. or lower, the output of the comparator 915 is not inverted and the relay is not cut off.

  However, the detected temperature of the thermistor 3 continues to rise, and eventually the detected temperature of the thermistor 3 reaches the abnormal overheat detection temperature 220 ° C. at time T2. The comparator 915 is inverted from LOW to HIGH, the transistor 912 is turned on, and the potential at the point C is lowered. Due to the potential drop at point C, the relay 901 is turned off and the energization to the ceramic heater 205 is interrupted. Along with this, the current detection signal S5 is switched from LOW to HIGH. On the other hand, at the point A, the LOW state is latched by the transistor 918 turned on by the inversion of the comparator 915, and the abnormal overheat detection temperature remains set at 220 ° C. That is, the output of the comparator 915 is maintained at HIGH.

  At time T3, the latch circuit 940 is activated. As described above, the latch circuit 940 operates 500 msec after the potential at the point C decreases. As a result, the potential at the point B rises and the transistor 935 is turned on.

  At time T4, the temperature detected by the thermistor 3 is lowered to 220 ° C. again. Accordingly, the transistor 918 is turned off, the potential at the point A is increased, and the transistor 920 is turned on. When the transistor 920 is turned on again, the abnormal overheat detection temperature is switched from 220 ° C. to 260 ° C., and the comparator 915 changes from HIGH to LOW. However, since the latch circuit 940 is activated at time T3, the potential at the point C is held at a low level, the relay 901 is turned on again, and energization to the ceramic heater 205 is not resumed.

  As described above, in the image forming apparatus of the present embodiment, the level of the abnormal overheat detection temperature of the abnormal overheat detection circuit by the plurality of thermistors is switched according to the level of the current flowing through the ceramic heater 205.

  Further, when an abnormal overheat is detected, the level of the abnormal overheat detection temperature is latched.

  This makes it possible to instantaneously cut off the energization of the ceramic heater 205 at a low temperature during energization runaway without causing the safety device to malfunction even when the end temperature rises during normal operation. Become.

  Embodiment 2 of the present invention will be described below with reference to the drawings.

  The basic configuration of the image forming apparatus according to the present embodiment is the same as that of the first embodiment, and the temperature of a plurality of thermistors is compared with a reference temperature that is switched according to the heater current state. In the case of abnormal overheating, the relay 901 is turned off. Turn off the heater current. In the first embodiment, the detection signal S7 of the thermistor 2 is connected only to the CPU 501 (see FIG. 5). However, in this embodiment, the detection signal S7 is also connected to the safety circuit 509.

  FIG. 12 is a setting table of operating temperatures of the safety circuit 509 for the thermistors 1, 2, and 3. The levels of the thermistors 1 and 3 are the same as in the first embodiment. The thermistor 2 varies depending on the level of the heater current, and is set to 220 ° C. when the current is 12 A or higher and 260 ° C. when the current is 12 A or lower.

  FIG. 13 is a block diagram illustrating the safety circuit 509.

Detection signal S7 is the output signal of the thermistor 2 is compared with a reference voltage Vref2 by the comparator 1404 (corresponding to comparison unit). The reference voltage Vref2 is connected to the positive input of the comparator 915 that sets the abnormal overheat detection temperature of the thermistor 3. When the detected temperature of the thermistor 2 becomes equal to or higher than the abnormal overheat detection temperature, the comparator 1404 is inverted from LOW to HIGH, the base current is applied to the transistor 1401 through the resistor 1402, and the transistor 1401 is turned on. Here, 1403 is a bias resistor. As a result, the potential at the point C is lowered and the relay 901 is turned off. Further, through the inverted by resistors 1402 and 1406 of the comparator 1404 base current is applied to the transistor 1405 (corresponding to hold portion), the transistor 1405 is turned on.

  When the transistor 918 or the transistor 1405 is on, the transistor 920 is off regardless of the state of the current detection signal S5 output from the current detection circuit 507. That is, the abnormal overheat detection temperature of the thermistor 2 and the thermistor 3 is latched at 220 ° C. The latch state is released when the detected temperature of the thermistor 2 or thermistor 3 is lowered to 220 ° C. or lower.

  FIG. 14 is a partial waveform diagram of each part of the safety circuit 509 when the energization runaway of the ceramic heater 205 occurs.

  Time T0 to T1 is a section in which the power control of the ceramic heater 205 is normally executed.

  Assume that energization runaway occurs at time T1. The heater current increases, and the temperature detected by the thermistor 2 and the thermistor 3 rises.

  The heater current becomes 12 A or more, and the current detection signal S5 that is the output of the current detection circuit 507 is switched from HIGH to LOW. Along with this, the point A is also in the LOW state, and the abnormal overheat detection temperature is switched from 260 ° C. to 220 ° C. At this time, since the detected temperature of the thermistors 2 and 3 is 220 ° C. or lower, the outputs of the comparators 915 and 1404 are not inverted, and the relay 901 is not cut off.

  At time T2, the detected temperature of the thermistor 2 reaches the abnormal overheat detection temperature of 220 ° C.

  The comparator 1404 is inverted from LOW to HIGH, the transistor 1401 is turned on, and the potential at the point C decreases. Due to the potential drop at point C, the relay 901 is turned off and the power supply to the heater is cut off. Along with this, the current detection signal S5 is switched from LOW to HIGH. On the other hand, at the point A, the LOW state is latched by the transistor 1405 that is turned on by the inversion of the comparator 1404, and the abnormal overheat detection temperature remains 220 ° C. and does not change. That is, the output of the comparator 1404 is maintained HIGH.

  At this time, the detected temperature of the thermistor 3 has not reached 220 ° C.

  At time T3, the latch circuit 940 is activated. As described above, the latch circuit 940 operates 500 msec after the potential at the point C decreases. The potential at the point B rises and the transistor 935 is turned on.

  At time T4, the temperature detected by the thermistor 2 is lowered to 220 ° C. again. Accordingly, the transistor 1405 is turned off, and the comparator 1404 changes from HIGH to LOW. However, since the latch circuit 940 is activated at time T3, the potential at the point C is held at a low level, the relay 901 is turned on again, and energization to the ceramic heater 205 is not resumed.

  In this way, the detected temperature of the thermistors 2 and 3 is compared with the abnormal overheat detection temperature that switches according to the current level, and when the abnormal overheat is detected, the setting of the abnormal overheat detection temperature is latched. It has become.

  During energization runaway, the relay 901 is turned off and the energization to the ceramic heater 205 is cut off at the time of the thermistor 2 or 3 that first reaches the abnormal overheat detection temperature.

  Here, the temperature rise speed of the thermistor 2 and the thermistor 3 during energization runaway will be described.

  The temperature rise rate of the thermistors 1, 2, and 3 during energization runaway varies depending on the operating conditions of the image forming apparatus. FIG. 14 shows an example in which the thermistor 2 reaches the abnormal overheat detection temperature in a shorter time than the thermistor 3. This is an example in the case where energization runaway occurs in a state where there is no recording paper 210 in the fixing device 116. This is because the position of the thermistor 3 is close to the electrode 303c (see FIGS. 3 and 4), and the temperature of the ceramic heater 205 becomes lower than the position of the thermistor 2 due to heat radiation from the electrode 303c. On the other hand, when a runaway energization occurs while the recording paper having the paper width B of FIG. 7 passes through the fixing device 116, the thermistor 3 reaches the abnormal overheat detection temperature in a shorter time than the thermistor 2. This is because, at the position of the thermistor 2 on the ceramic heater 205, the temperature rise slows due to heat radiation to the recording paper 210, whereas at the position of the thermistor 3, there is no heat radiation of the recording paper 210 and the temperature rise speed increases. .

  As described above, in the image forming apparatus according to the present embodiment, the detected temperature of the plurality of thermistors is compared with the abnormal overheat temperature that switches according to the current level, and when the abnormal overheat temperature is detected, the abnormal overheat temperature is detected. The configuration is latched. As a result, regardless of the conditions of the image forming apparatus during energization runaway, it is possible to operate the safety circuit 509 in the shortest time and instantaneously cut off the energization to the ceramic heater 205 at a low temperature.

Schematic configuration sectional view showing an embodiment of an image forming apparatus according to this embodiment 1 is a schematic cross-sectional view of a fixing device according to a first embodiment. Schematic configuration cross-sectional view of a ceramic heater in Example 1 The figure which shows a response | compatibility with the longitudinal direction of the ceramic heater in Example 1, and the graph showing the heat_generation | fever distribution of a main heater and a subheater. Connection diagram of ceramic heater and power control circuit in embodiment 1 Circuit diagram of internal configuration of temperature detection circuit in embodiment 1 (A) The figure which shows the positional relationship of the kind of paper width in Example 1, and a ceramic heater, (b) The table | surface which shows the relationship between the paper width and the power ratio of a sub heater. The graph which shows transition of edge part temperature rising when the recording paper of the paper width A in Example 1 is passed. Block diagram for explaining a safety circuit in the first embodiment Setting table of operating temperature of safety circuit for each thermistor in embodiment 1 Partial waveform diagram of each part of the safety circuit when energization runaway occurs in the ceramic heater in Example 1 Setting table of operating temperature of safety circuit for each thermistor in Example 2 Block diagram for explaining a safety circuit in the second embodiment Partial waveform diagram of each part of the safety circuit when energization runaway occurs in the ceramic heater in Example 2 Graph showing the relationship between the heater temperature of the ceramic heater and the temperature at which the thermoswitch actually operates Block diagram for explaining a safety device in a conventional example

Explanation of symbols

100 image forming apparatus 116 fixing device 201 fixing film 202 pressure roller 203 core metal 204 stays 205 ceramic heater motor 206 thermistor (corresponding to the temperature detecting means)
207 Heat-resistant elastic layer 210 Recording paper (corresponding to recording material )
301 Substrate 302a Main heater 302b Sub heaters 303a, 303b, 303c Feed electrode 501 CPU
502, 503 triac 504 AC power 505,901 relays <br/> 507 current detection circuit (corresponding to a current detecting means)
509 safety circuit 903,906,912,1401 transistor 909 comparator 915 comparator (corresponding to the comparison unit)
918 transistor (corresponding to the hold portion)
920 transistor (equivalent to reference temperature switching part )
1404 Comparator (corresponding to comparison unit)
1405 transistor (corresponding to the hold portion)

Claims (2)

A heater ,
Temperature detecting means for detecting the temperature of the heater ;
And said temperature detecting hands stage system that controls the power to be supplied to on the basis of power or al the heater to the detected temperature control means,
A relay provided in a power supply path from the power source to the heater;
A current detecting means for detect the current flowing through the heater,
A comparator that compares the detected temperature of the temperature detecting means with a reference temperature and sends a signal to the relay to shut off the power supply path when the detected temperature reaches the reference temperature;
A latch circuit that latches the relay in a power supply path cutoff state when the signal sent from the comparison unit to the relay continues;
The current If detected current value of the detection means is lower than a reference current value by setting the reference temperature to a first reference temperature, when the detected current value is higher than the reference current value is the reference temperature the first reference A reference temperature switching unit for setting a second reference temperature lower than the temperature ;
Equipped with a,
A safety circuit that drives the relay independently of the control means;
In a heat fixing apparatus for fixing an unfixed image formed on a recording material to the recording material,
In the safety circuit, when the detected temperature exceeds the second reference temperature and the relay cuts off the power supply path in a state where the reference temperature is set to the second reference temperature, the detected current value Regardless of the above, there is provided a holding unit for holding the reference temperature at the second reference temperature until the temperature of the heater is lowered by the interruption of the power supply path and the detected temperature is lowered to the second reference temperature. heat fixing device wherein the latch circuit is characterized in that latches the relay in the power supply path blocking state. The apparatus further includes a cylindrical film in contact with the heater on the inner surface,
A pressure roller that forms a fixing nip portion for conveying a recording material together with the heater through the cylindrical film;
Heat fixing device according to claim 1, characterized in that it comprises a.

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