JP2010197585A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of interrupting heater current by a thermoswitch or a temperature fuse before a ceramic heater is damaged, by delaying increase in temperature of the ceramic heater of a fixing device. <P>SOLUTION: The image forming apparatus includes: a power supply means 201 for supplying power to a heater 109c of the fixing device 109 from a power source; a current detection means 225 for detecting the current supplied to the heater and outputting an output voltage according to the detected value; a circuit control means 126 that operates with a plurality of threshold values set for the output voltage of the current detection means and interrupts the power supply of the power supply means; a threshold value alteration means for altering the threshold values at which the circuit control means operates; and a detection means for detecting that the circuit control means has operated. When the detection means detects that the circuit control means has operated, the threshold value alteration means alters the threshold value at which the circuit control means operates. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an electrophotographic image forming apparatus.

  Conventionally, an image forming apparatus using an electrophotographic process is known. In this image forming apparatus, an unfixed image (toner image) formed on transfer paper by image forming means such as an electrophotographic process is thermally fixed. It is fixed on the transfer paper by an apparatus. As the heat fixing device, for example, a heat roller type heat fixing device using a halogen heater as a heat source and a film heating type heat fixing device using a ceramic heater as a heat source described in Patent Document 1 or 2 are known. Yes.

  In general, such a heater is supplied with electric power from an AC power supply via a switching element such as a triac.

  In a fixing device using a halogen heater as a heat source, the temperature of the fixing device is detected by a temperature detection element such as a thermistor temperature sensing element, and the switching element is controlled on / off by a sequence controller based on the detected temperature. The That is, power supply to the halogen heater is turned on / off, and the temperature is controlled so that the temperature of the fixing device becomes the target temperature.

  On the other hand, in a fixing device using a ceramic heater as a heat source, the power ratio supplied to the ceramic heater is calculated based on the temperature difference between the temperature detected by the temperature detection element and a preset target temperature by the sequence controller. The corresponding phase angle or wave number is determined. Then, the switching element is ON / OFF controlled at the determined phase or wave number, and the temperature of the fixing device is controlled.

  In addition, in order to prevent damage to the heater in the event of an abnormality, when the current detection means detects that an overcurrent is flowing through the heater, some control is performed to cut off the energization of the heater by a control circuit.

Japanese Unexamined Patent Publication No. 64-77887 JP 2007-212868 A

  However, when the ceramic heater becomes hot, the resistance value of the ceramic heater increases and the heater current decreases. In this case, even when there is an abnormality, it cannot be detected that an overcurrent flows through the ceramic heater, and the current cannot be cut off. Further, even when the power supply voltage fluctuates and becomes small, the heater overcurrent at the time of abnormality cannot be detected, so that the protection by the thermoswitch or the thermal fuse is not in time, and the ceramic heater may be damaged.

  The present invention has been made to solve the above-described problems, and by slowing the temperature rise of the ceramic heater of the fixing device, the heater current can be interrupted by a thermo switch or a thermal fuse earlier than the ceramic heater is damaged. An object is to provide an image forming apparatus.

  In order to achieve the above-mentioned object, the present invention comprises the following configurations (1) to (8).

(1) power supply means for supplying power from the power source to the heater of the fixing device;
Current detecting means for detecting a current supplied to the heater and outputting an output voltage corresponding to the detected value;
Circuit control means for operating at a plurality of thresholds set for the output voltage of the current detection means, and for cutting off the power supply of the power supply means;
Threshold changing means for changing the threshold at which the circuit control means operates;
Detecting means for detecting the operation of the circuit control means,
The image forming apparatus according to claim 1, wherein when the detection unit detects that the circuit control unit is operated, the threshold value changing unit changes a threshold value at which the circuit control unit operates.

  (2) At least one of the plurality of thresholds is set to a voltage higher than the output voltage set from a current value calculated based on a power value required for normal operation of the image forming apparatus. The image forming apparatus according to (1), wherein the threshold value is 1.

  (3) The image forming apparatus according to (2), wherein the plurality of threshold values are equal to or lower than an upper limit based on the power value at which the heater may be damaged.

  (4) At least one threshold among the plurality of thresholds is a second threshold smaller than the output voltage set from a current value calculated based on an electric power value at which the heater is not likely to be damaged. The image forming apparatus as described in (2) or (3) above.

  (5) The second threshold value is set to a voltage smaller than the output voltage set from a current value calculated based on a power value necessary for normal operation of the image forming apparatus. The image forming apparatus according to 4).

  (6) The threshold value of the circuit control unit is not changed to the first threshold value after the first threshold value is changed to the second threshold value by the threshold value changing unit. The image forming apparatus according to 4) or (5).

(7) further comprising a power shut-off means that is disposed in the vicinity of the heater and shuts off the power supply to the heater when the heater exceeds a predetermined temperature;
The image forming apparatus according to any one of (1) to (6), wherein the power cut-off unit operates in a state in which cut-off of power supply to the heater by the circuit control unit is intermittently performed. .

  (8) The image forming apparatus according to any one of (1) to (7), wherein the detection unit is an ASIC.

  The image forming apparatus according to the present invention detects an overcurrent even when an abnormality occurs in the apparatus and an abnormal current flows through the ceramic heater, even if the ceramic heater becomes hot and the heater current decreases. Can cut off the current. As a result, the temperature rise of the ceramic heater can be delayed, and the heater current can be interrupted by the thermo switch or the thermal fuse before the ceramic heater is damaged.

1 is a diagram illustrating an image forming apparatus according to a first embodiment. The figure which showed the control and drive circuit of the fixing device in Example 1. The figure which showed the outline of the ceramic heater which is a heating means in Example 1. FIG. 3 is a diagram illustrating a schematic configuration of a fixing device according to the first exemplary embodiment. The figure which showed the electric current detection circuit in Example 1. The figure which showed the heater temperature curve in Example 1 The figure explaining the flowchart in Example 1 The figure which showed the control and drive circuit of the fixing device in Example 2. The figure explaining ASIC in Example 2 The figure which showed the heater temperature curve in Example 2 The figure explaining the flowchart in Example 2.

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

  FIG. 1 is a schematic configuration diagram of an image forming apparatus (laser printer) using an electrophotographic process according to the present embodiment.

  A laser printer main body 101 (hereinafter referred to as “main body 101”) can be loaded with a cassette 102 for storing a recording sheet S, and forms an image on the recording sheet S supplied from the cassette 102. Reference numeral 103 denotes a cassette presence sensor that detects the presence or absence of the recording sheet S in the cassette 102.

  Reference numeral 104 denotes a cassette size sensor that detects the size of the recording sheet S accommodated in the cassette 102, and includes, for example, a plurality of micro switches. Reference numeral 105 denotes a paper feed roller that picks up and conveys the recording sheet S from the cassette 102. A registration roller pair 106 that synchronously conveys the recording sheet S is provided downstream of the paper feed roller 105.

  Further, an image forming unit 108 that forms a toner image on the recording sheet S based on the laser beam from the laser scanner unit 107 is provided downstream of the registration roller pair 106. Further, a fixing device 109 that thermally fixes the toner image formed on the recording sheet S is provided downstream of the image forming unit 108.

  Downstream of the fixing device 109, a paper discharge sensor 110 that detects the conveyance state of the paper discharge unit, a paper discharge roller pair 111 that discharges the recording sheet S, and a recording sheet S on which an image is formed and fixed are displayed. A loading tray 112 for loading and storing is provided. Here, the conveyance reference of the recording sheet S is set so as to be approximately in the center with respect to the length in the direction orthogonal to the conveyance direction of the recording sheet S, that is, the width of the recording sheet S.

  The laser scanner unit 107 includes a laser unit 113 that emits a laser beam modulated based on an image signal (image signal VDO) transmitted from the external device 131. The laser beam from the laser unit 113 is reflected by a polygon mirror that is rotationally driven by a polygon motor 114, reflected by an imaging lens 115, a folding mirror 116, and the like, and scans on the photosensitive drum 117.

  The image forming unit 108 includes a photosensitive drum 117, a primary charging roller 119, a developing device 120, a transfer charging roller 121, a cleaner 122, and the like necessary for an electrophotographic process. The fixing device 109 includes a fixing film 401a, a pressure roller 401b, a ceramic heater 109c provided inside the fixing film 401a, and a thermistor 109d that detects the surface temperature of the ceramic heater 109c.

  The main motor 123 applies a rotational force to the paper feed roller 105 via the paper feed roller clutch 124. A rotational force is applied to the registration roller pair 106 via a registration roller clutch 125. Further, a driving force is applied to each unit of the image forming unit 108 including the photosensitive drum 117, the fixing device 109, and the paper discharge roller pair 111.

  An engine controller 126 controls the electrophotographic process by the laser scanner unit 107, the image forming unit 108, and the fixing unit 109, and controls the conveyance of the recording sheet S in the main body 101. Reference numeral 127 denotes a video controller, which is connected to an external device 131 such as a personal computer via a general-purpose interface (Centronics, RS232C, etc.) 130. The video controller 127 expands image information sent via the general-purpose interface 130 into bit data, and sends the bit data to the engine controller 126 as a VDO signal.

  FIG. 2 is a block diagram showing a configuration of a heater control circuit for controlling energization driving to the ceramic heater 109c in the embodiment of the present invention.

  Reference numeral 201 denotes an AC power source to which the image forming apparatus 101 is connected. In this image forming apparatus, an AC power source 201 is supplied to heating elements 303 and 320 of the ceramic heater 109c via an AC filter 202, a current transformer 220, and a relay 203. Thereby, the heating elements 303 and 320 constituting the ceramic heater 109c generate heat. The power supply to the heating element 303 is controlled by energization and interruption of the triac 219.

  The resistors 217 and 218 are bias resistors of the triac 219, and the phototriac coupler 215 is a device for ensuring a creepage distance between the primary and secondary. When the light emitting diode of the phototriac coupler 215 is energized, the triac 219 is turned on. The resistor 216 is a resistor for limiting the current flowing through the phototriac coupler 215, and energization of the phototriac coupler 215 is turned on / off by the transistor 214. The transistor 214 operates in accordance with a signal (ON) supplied from the engine controller 126 via the resistor 213.

  The AC power supply 201 is input to the zero cross detection circuit 204 via the AC filter 202. The zero cross detection circuit detects the zero cross point at which the positive / negative of the power supply voltage is switched, or detects that the voltage falls below a certain threshold voltage including the zero cross point and notifies the engine controller 126 as a pulse signal.

  Hereinafter, a signal sent to the engine controller 126 is referred to as a ZEROX signal. The engine controller 126 detects the edge of the pulse of the ZEROX signal and controls on / off of the triac 219 by phase control or wave number control.

  The heater current controlled by the triac 219 and energized to the heating elements 303 and 320 is converted into a voltage by the current transformer 220 and input to the current detection circuit 225. This current detection circuit 225 converts the voltage-converted heater current waveform into an effective value or a square value thereof, and inputs it to the engine controller 126 as an HCRRT signal. The HCRRT signal input in this way is A / D converted by the engine controller 126 and managed as a digital value.

  The temperature detection element 109d is, for example, a thermistor temperature sensing element for detecting the temperature of the ceramic heater 109c in which the heating elements 303 and 320 are formed. The temperature detection element 109d is disposed on the ceramic heater 109c via an insulator having a withstand voltage so as to ensure an insulation distance from the heating elements 303 and 320.

  The temperature (detected value) detected by the temperature detection element 109d is detected as a partial pressure between the resistor 212 and the temperature detection element 109d, and is input to the engine controller 126 as a TH signal. The TH signal input in this way is A / D converted by the engine controller 126 and managed as a digital value.

  The temperature of the ceramic heater 109c is monitored by the engine controller 126 as a TH signal. Then, by comparing with the set temperature of the ceramic heater 109c set by the engine controller 126, the power ratio to be supplied to the heating elements 303 and 320 constituting the ceramic heater 109c is calculated.

  Then, it is converted into a phase angle (phase control) or a wave number (wave number control) corresponding to the supplied power ratio, and the engine controller 126 sends an ON signal to the transistor 214 according to the control condition. Thus, the temperature of the ceramic heater 109c is controlled. Here, when calculating the power ratio supplied to the heat generating elements 303 and 320, the upper limit power ratio is accurately calculated based on the HCRRT signal notified from the current detection circuit 225, and the power below the upper limit of the power ratio is calculated. Is controlled to be energized. For example, in the case of phase control, the following control table is provided in the engine controller 126, and control is performed based on this control table.

  Further, when a circuit for supplying electric power to the heating elements 303 and 320 to control it fails and the heating elements 303 and 320 are brought into a thermal runaway, an overtemperature prevention unit is provided as a means for preventing the excessive temperature rise. 223 is provided in the vicinity of the ceramic heater 109c. This excessive temperature rise prevention part 223 is a power interruption means, for example, a temperature fuse or a thermo switch. When the heating elements 303 and 320 become thermal runaway and the excessive temperature rise prevention unit 223 reaches a predetermined temperature or more, the excessive temperature rise prevention unit 223 is opened and the heating elements 303 and 320 are de-energized. .

  Further, in order to control the temperature of the ceramic heater 109c monitored as the TH signal, an abnormal temperature value for detecting an abnormally high temperature is set by the engine controller 126 separately from the set temperature for temperature control. Accordingly, when the temperature information indicated by the TH signal becomes equal to or higher than the abnormal temperature value, the engine controller 126 sets the / RLD signal to the high level. Accordingly, the transistor 209 is turned on, whereby the transistor 206 is turned off and the relay 203 is opened. That is, the engine controller 126 functions as a circuit control unit.

  Therefore, after the abnormally high temperature is detected by the TH signal, the energization to the heat generating elements 303 and 320 remains cut off. During temperature control during normal operation, the engine controller 126 always outputs the / RLD signal at a low level to turn off the transistor 209 and turn on the transistor 206 to turn on the relay 203 (conducting state). The resistor 244 is a current limiting resistor, and the resistor 243 is a base-emitter bias resistor of the transistor 209. The diode 205 is an element for absorbing the back electromotive force when the relay 203 is off, and the transistor 206 forms a ground circuit for the relay 203.

  FIGS. 3A and 3B are diagrams for explaining the outline of the ceramic heater 109c according to the present embodiment. 3A is a sectional view of the ceramic heater, and FIG. 3B is a plan view and a bottom view. 3B shows a surface on which the heating elements 303 and 320 are formed, and 302 shows a surface opposite to the surface indicated by 301 (see FIG. 3A).

The ceramic heater 109c includes a ceramic insulating substrate 331 such as SiC, AlN, Al ₂ O _3, heating elements 303 and 320 formed on the surface of the insulating substrate by paste printing or the like, and two heating elements. It is comprised from protective layers 334, such as glass which is protecting. On the protective layer 334, a temperature detecting element 109d for detecting the temperature of the ceramic heater 109c and an excessive temperature rise preventing unit 223 are arranged. These are disposed at a position symmetrical to the conveyance sheet conveyance reference, that is, the center in the length direction of the heating elements 303a and 320a, and at a position inside the minimum recording sheet width capable of passing paper. .

  The heating element 303 includes a heating element 303a that generates heat when power is supplied thereto, an electrode part 303c that is supplied with power via a connector, and a conductive part 303b that connects the electrode part 303c and the heating element 303a. Yes. The heating element 320 includes a heating element 320a that generates heat when electric power is supplied, an electrode part 320c to which electric power is supplied via a connector, and a conductive part 320b connected to the electrode part 320c. The conductive portion 310 is connected to the two heating elements 303 and 320. In addition, a glass layer may be formed on the side facing the insulating substrate 331 on which the heating elements 303 and 320 are printed in order to improve the slidability.

  The electrode part 303 c is connected from the HOT side terminal of the AC power supply 201 via the overheat prevention part 223. The electrode part 320 c is connected to a triac 219 that controls the heating element 303, and is connected to the neutral terminal of the AC power supply 201. The ceramic heater 109c is supported by a film guide 402 as shown in FIGS. 4 (a) and 4 (b).

  FIGS. 4A and 4B are diagrams showing a schematic configuration of the fixing device 109 according to the present embodiment. FIG. 4A shows the nip portion of the heating elements 303 and 320 with respect to the insulating substrate 331. The case is on the opposite side. FIG. 4B shows a case where the heating elements 303 and 320 are located on the nip portion side with respect to the insulating substrate 331.

  The fixing film 401a is a fixing film made of a cylindrical heat-resistant material, and is externally fitted to a film guide 402 that supports a ceramic heater 109c on the lower surface side. Then, the ceramic heater 109c on the lower surface of the film guide 402 and a pressure roller 401b as a pressure member are pressed against each other with a predetermined pressure against the elasticity of the pressure roller 401b with the fixing film 401a interposed therebetween. ing.

  Thus, a fixing nip portion having a predetermined width as a heating portion is formed. Further, the excessive temperature rise prevention unit 223, for example, a thermo switch is brought into contact with the surface of the insulating substrate 331 or the protective layer 334 of the ceramic heater 109c. The excessive temperature rise prevention unit 223 is corrected in position by the film guide 402, and the heat sensitive surface of the over temperature rise prevention unit 223 is in contact with the surface of the ceramic heater 109c. Although not shown, the temperature detecting element 109d is also in contact with the surface of the ceramic heater 109c.

  Here, as shown in FIG. 4A, the ceramic heater 109c may have the heating elements 303 and 320 on the side opposite to the nip portion. Alternatively, as shown in FIG. It may be on the nip side. In order to improve the slidability of the fixing film 401a, slidable grease may be applied to the interface between the fixing film 401a and the ceramic heater 109c.

  When any abnormality occurs in the apparatus, it is necessary to satisfy the standards such as withstand voltage and leakage current and stop the operation of the image forming apparatus. For this reason, the thermoswitch and the thermal fuse must be operated before the ceramic heater reaches an abnormally high temperature and is damaged.

  However, the power required for fixing is increasing due to the recent demand for higher printing speed. Therefore, the resistance value of the ceramic heater is reduced, and the temperature rise rate of the ceramic heater at the time of abnormality is increased. On the other hand, since the thermoswitch and the thermal fuse require a certain period of time to operate, the protection by the thermoswitch and the thermal fuse may not be in time.

  For this reason, in order to slow down the temperature rise rate of the ceramic heater, some control is performed to detect the overcurrent of the ceramic heater and repeatedly cut off the heater energization. By doing so, the ceramic heater is energized intermittently, the heater temperature gradually rises, and the thermo switch and the thermal fuse can be operated before the heater temperature reaches the temperature at which the heater is damaged. It is a feature of the present invention that the threshold value for detecting this overcurrent is variable.

  FIG. 5 is a diagram showing details and peripheral circuits of the current detection circuit 225 shown in FIG. FIG. 6 is a diagram showing a heater temperature curve for explaining the present embodiment.

  2 and 5, the comparator 501 compares the reference voltage (CPREF) by the voltage dividing resistors 226, 227 and 228 with the output voltage value (Vout) corresponding to the heater current value from the current detection circuit 225.

  When Vout is larger than the reference voltage value CPREF (first threshold value) of the voltage dividing resistors 226 and 227, the comparator 501 outputs High, and the transistor 505 is turned on via the resistor 503 and the capacitor 504. Then, since CURLIM becomes Low, the transistor 206 for driving the relay 203 is turned off, and the energization to the ceramic heater is cut off. Further, when CURLIM becomes Low, the transistor 237 is turned on, and the FET 230 and the FET 241 are turned on.

  At this time, the resistance ratio of the resistors 235 and 238 is set so that the FET 230 and the FET 241 are turned on. When the FET 241 is turned on, the voltage value of CPREF is set from the first threshold value set by the ratio of the resistor 226 and the resistor 227 to the second threshold value set by the ratio of the combined resistance of the resistor 226 and the resistor 227 and the resistor 228. Changed to

  Once the FET 230 is turned on, since the FET 230 and the FET 241 continue to be turned on until the power is turned off thereafter, the voltage value of CPREF is fixed to the second threshold value. Here, the second threshold value is set to a value smaller than the first threshold value, and if a current that causes Vout to be smaller than the second threshold value flows through the ceramic heater, the thermoswitch or the The value at which the thermal fuse operates.

  When the heater current is cut off, Vout becomes small, and when it becomes smaller than the second threshold value, the output of the comparator 501 becomes Low and CURLIM becomes High. Then, the relay 203 is turned on, and energization to the ceramic heater is resumed. When energization to the ceramic heater is resumed and Vout becomes larger than the second threshold, the heater current is cut off as described above.

  When CPREF becomes the second threshold value, the energization is cut off with a heater current lower than that at the first threshold value, so that the temperature rise of the ceramic heater can be delayed. By doing so, it becomes possible to slow down the temperature rise rate of the ceramic heater regardless of the value of the current supplied to the ceramic heater at the time of abnormality. As a result, it becomes possible to operate the thermoswitch and the thermal fuse faster than the ceramic heater breaks, satisfy the standards such as withstand voltage and leakage current, and stop the operation of the image forming apparatus.

  In addition, since the thermoswitch and the thermal fuse are operating, the ceramic heater cannot be energized by turning the power off / on. Therefore, the ceramic heater of the image forming apparatus in which an abnormality has occurred is not energized again, and a state in which an abnormal current flows through the ceramic heater is not repeated.

  FIG. 7 is a flowchart for explaining the present embodiment. In particular, the present invention assumes that the fixing device cannot be controlled due to an abnormality in the image forming apparatus. When the ceramic heater falls into an uncontrollable state and energization is started, a protection element such as a thermo switch or a thermal fuse is operated according to the temperature of the ceramic heater (S701).

  When the ceramic heater is at or above the operating temperature of the protective element, energization of the ceramic heater is terminated. However, in the above-described state, the temperature rise rate of the ceramic heater is fast, and there is a higher possibility that the ceramic heater will be damaged before the protective element operates. Therefore, the present invention aims to suppress the temperature rise rate of the ceramic heater, and is configured to cut off the energization of the ceramic heater before reaching the electric power at which the ceramic heater is damaged. Here, the first threshold value is calculated in advance by examination, and is set to be equal to or smaller than the current value at which the ceramic heater is damaged and equal to or larger than the current value calculated from the power value necessary for steady printing. .

  Next, it is determined whether CPREF is the first threshold value (S702). If CPREF is the first threshold value, it is determined whether Vout exceeds the first threshold value (S703). If the first threshold value is not exceeded in S702, the ceramic heater is heated at a temperature rising rate at which the ceramic heater is not damaged, and the heater energization is terminated by operating the thermo switch. When Vout exceeds the first threshold value in S702, the output of the comparator 501 becomes High and CURLIM becomes Low (S704).

  When CURLIM becomes Low, CPREF is set to the second threshold (S705). When CURLIM becomes Low, the relay 203 is cut off and the heater energization is cut off (S706). The heater energization is interrupted until Vout becomes smaller than the second threshold after the heater energization is interrupted. When Vout becomes smaller than the second threshold (S707), CURLIM becomes High (S708), and the heater energization is resumed. (S709). Thereafter, the process returns to S701, and if the protection element is not operating, it is determined in S702 whether CPREF is the first threshold value.

  Since CPREF is set to the second threshold value in S705, Vout is compared with the second threshold value (S710). When Vout does not exceed the second threshold value, the ceramic heater is heated at a temperature rising rate at which the ceramic heater is not damaged, and the heater energization is terminated by operating the thermo switch. When Vout exceeds the second threshold, CURLIM becomes Low (S711), and the heater energization is cut off.

  The heater energization is interrupted until Vout becomes smaller than the second threshold after the heater energization is interrupted. When Vout becomes smaller than the second threshold (S713), CURLIM becomes High (S714), and the heater energization is resumed. (S715). Thereafter, the process returns to S701, and the above control is performed until the thermo switch operates.

  It has been proved by examination that the temperature curve of the ceramic heater is larger in the energized state than in the cut-off state. Therefore, when the energization and interruption state of the ceramic heater are repeated, the temperature of the ceramic heater gradually rises and eventually exceeds the apparatus operating temperature, and the energization of the ceramic heater is interrupted. It is a feature of the present invention that the first and second threshold voltages are set to values that draw a temperature rise curve so that the apparatus works reliably and the ceramic heater is not damaged.

  FIG. 8 is a diagram showing a circuit for driving and controlling the ceramic heater in this embodiment. In FIG. 8, the same points as those in the first embodiment are omitted. When an abnormality occurs in the apparatus and an overcurrent flows through the ceramic heater, Vout becomes higher than the first threshold voltage value, and the comparator 501 outputs High. Then, CURLIM becomes Low, the triac driving transistor 214 is turned off, and the light emitting diode of the phototriac coupler 215 is not energized.

  Therefore, the triac 219 is turned off, and the energization to the ceramic heater is interrupted. When energization to the ceramic heater is interrupted, Vout becomes smaller than CPREF, the output of the comparator 501 becomes Low, and energization to the ceramic heater is resumed. In this way, energization and de-energization of the ceramic heater are repeated.

  FIG. 9 is a diagram for explaining an ASIC 901 that is a threshold changing method according to the present embodiment. FIG. 10 is a diagram illustrating a heater temperature curve for explaining the present embodiment. CURLIM is input to the ASIC 901. CURLIM is filtered by filter 906 to remove chattering due to external noise. Thereafter, the falling edge detection unit 902 detects the falling edge of CURLIM (a rising edge may be used instead of the falling edge).

  When the falling edge detection unit 902 detects the falling edge, the timer value comparison unit 908 measures the interval from the previous falling edge. If it is within a predetermined time from the previous falling detection to the next falling detection, the counter 903 counts up (may be counted down). In addition, the timer 907 is reset. The comparison unit 904 compares the count value output from the counter 903 with a predetermined counter threshold value.

  When the counter value is equal to or less than a predetermined value by the comparison unit 904, the comparison unit 904 outputs the CHANGE signal as a low level. When the falling detection unit detects the falling again within a predetermined time, the counter 903 counts up (may be counted down). If a falling edge is not detected within a predetermined time, the counter 903 is reset. When the counter value is greater than or equal to a predetermined value (below), the comparison unit 904 outputs CHANGE as High level.

  At this time, the counter value of the counter 903 is held by the counter holding unit 905. For this reason, the CHANGE signal remains at a low level until the falling edge (rising edge) of CURLIM is detected a predetermined number of times. By comparing the counter values, it is possible to prevent malfunction of threshold change during normal operation. The predetermined counter value is a value at which the temperature of the ceramic heater at the time of abnormality does not reach the heater breakage temperature. When the ASIC 901 outputs the CHANGE signal as a low level, the FET 241 is turned on. When the FET 241 is turned on, the voltage value of CPREF is set from the first threshold value set by the ratio of the resistor 226 and the resistor 227 to the second threshold value set by the ratio of the combined resistance of the resistor 226 and the resistor 227 and the resistor 228. Changed to

  Once the FET 230 is turned on, since the FET 230 and the FET 241 continue to be turned on until the power is turned off thereafter, the voltage value of CPREF is fixed to the second threshold value. Here, the second threshold value is set to a value smaller than the first threshold value, and if a current that causes Vout to be smaller than the second threshold value flows to the ceramic heater, the thermoswitch or temperature is earlier than the ceramic heater is damaged. The value at which the fuse operates. By doing so, it becomes possible to slow down the temperature rise rate of the ceramic heater regardless of the value of the current supplied to the ceramic heater at the time of abnormality.

  FIG. 11 is a flowchart for explaining the present embodiment. In FIG. 11, the same points as those in the first embodiment are omitted. First, the counter 903 is reset (S1101). When the ceramic heater falls into an uncontrollable state and energization is started, a protection element such as a thermo switch or a thermal fuse is operated according to the temperature of the ceramic heater (S1102). Next, it is determined whether CPREF is the first threshold value (S1103). If CPREF is the first threshold value, it is determined whether Vout exceeds the first threshold value (S1104).

  If Vout does not exceed the first threshold value in S1104, the ceramic heater is heated at a temperature rising rate at which the ceramic heater is not damaged, and the heater energization is terminated by operating the thermo switch. When Vout exceeds the first threshold value in S1104, the output of the comparator 501 becomes High and CURLIM becomes Low (S1105). When the ASIC 901 detects a falling edge, it is determined whether a predetermined time has elapsed since the previous count-up (S1106). Here, the predetermined time is a time sufficiently longer than the interval of falling of CURLIM when an overcurrent is intermittently supplied to the ceramic heater.

  By doing so, it is possible to prevent malfunction of changing CPREF. If a predetermined time has elapsed since the previous count-up in S1106, the counter 903 is reset (S1107), and the count-up is performed (S1108). If the predetermined time has not elapsed since the previous count-up in S1106, the count-up is performed (S1108). When CURLIM becomes Low, energization to the ceramic heater is cut off (S1109).

  Next, the counter value is compared. If the counter value is not a predetermined value, Vout is compared with the first threshold value (S1115). When Vout is larger than the first threshold, the process returns to S1109 and the heater energization is continued to be cut off. Since the heater energization is cut off, Vout eventually becomes smaller than the first threshold value. Then, CURLIM is set to High (S1116), and heater energization is resumed. Thereafter, the process returns to S1102 and the above control is repeated. When the counter value reaches a predetermined value in S1110, CPREF is set to the second threshold value (S1111). Thereafter, Vout is compared with the second threshold value (S1112). If Vout is larger than the second threshold value, the process returns to S1109 and the above control is repeated. When Vout becomes smaller than the second threshold, CURLIM is set to High (S1113), and heater energization is resumed (S1114).

  Thereafter, the process returns to S1102, and if the protection element is not operating, it is determined whether or not CPREF is the first threshold value in S1103. Since CPREF is set to the second threshold value in S1111, Vout is compared with the second threshold value (S1118). When Vout does not exceed the second threshold value, the ceramic heater is heated at a temperature rising rate at which the ceramic heater is not damaged, and the heater energization is terminated by operating the thermo switch.

  When Vout exceeds the second threshold, CURLIM becomes Low (S1119), and the heater energization is cut off (S1120). The heater energization is interrupted until Vout becomes smaller than the second threshold after the heater energization is interrupted. When Vout becomes smaller than the second threshold (S1121), CURLIM becomes High (S1122), and the heater energization is resumed. (S1123). Thereafter, the process returns to S1102, and the above control is performed until the thermo switch operates.

  By performing the above control, the thermo switch and thermal fuse can be operated faster than the ceramic heater breaks, and the operation of the image forming apparatus is stopped while satisfying the standards such as withstand voltage and leakage current. Can be made.

101 Laser printer main body 109 Fixing device 109c Ceramic heater 109d Thermistor 126 Engine controller (corresponding to circuit control means)
201 AC power supply
204 Zero cross detection circuit 223 Over temperature rise prevention unit 225 Current detection circuit (corresponding to current detection means)
303 Heating element 320 Heating element

Claims (8)

Power supply means for supplying power from the power source to the heater of the fixing device;
Current detection means for detecting a current supplied to the heater and outputting an output voltage corresponding to the detected value;
Circuit control means for operating at a plurality of thresholds set for the output voltage of the current detection means, and for cutting off the power supply of the power supply means;
Threshold changing means for changing the threshold at which the circuit control means operates;
Detecting means for detecting the operation of the circuit control means,
The image forming apparatus according to claim 1, wherein when the detection unit detects that the circuit control unit is operated, the threshold value changing unit changes a threshold value at which the circuit control unit operates.   At least one threshold among the plurality of thresholds is a first threshold set to a voltage greater than the output voltage set from a current value calculated based on a power value required for normal operation of the image forming apparatus. The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus.   3. The image forming apparatus according to claim 2, wherein the plurality of threshold values are equal to or lower than an upper limit based on the power value at which the heater may be damaged.   At least one threshold among the plurality of thresholds is a second threshold smaller than the output voltage set from a current value calculated based on an electric power value at which the heater is not likely to be damaged. The image forming apparatus according to claim 2 or 3.   5. The second threshold value is set at a voltage smaller than the output voltage set from a current value calculated based on a power value necessary for a normal operation of the image forming apparatus. Image forming apparatus.   The threshold value of the circuit control means is not changed to the first threshold value after the first threshold value is changed to the second threshold value by the threshold value changing means. The image forming apparatus described in 1. A power shut-off means arranged near the heater, and shuts off the power supply to the heater when the heater exceeds a predetermined temperature;
The image forming apparatus according to claim 1, wherein the power cut-off unit operates in a state in which cut-off of power supply to the heater by the circuit control unit is intermittently performed.   The image forming apparatus according to claim 1, wherein the detection unit is an ASIC.

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