Patent Publication Number: US-9411277-B1

Title: Fixing device and image forming apparatus having the same

Description:
FIELD 
     Embodiments described herein relate generally to a fixing device and an image forming apparatus having the same. 
     BACKGROUND 
     A fixing device fixes an image (toner image) formed on a sheet with heat when the sheet passes through the fixing device. One type of the fixing device detects a temperature of the fixing device (e.g., fixing belt or heating roller) and, based on the detected temperature, controls a temperature of a heating region to be within a target temperature range that is preferable to fix the image. One way of controlling the temperature would be turning on and off a heating unit of the fixing device. However, controlling the temperature only by turning on and off the heating unit would be difficult. It would be desirable to control the temperature to be within the target temperature range in an easier manner. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an MFP having a fixing device according to one embodiment. 
         FIG. 2  illustrate the fixing device. 
         FIG. 3  illustrates a temperature detection portion of a fixing belt in the fixing device. 
         FIG. 4  illustrate switching a maximum value of power that can be allocated to the fixing device. 
         FIG. 5  is a flowchart of a power supply control carried out by an IH control unit. 
         FIGS. 6 and 7  each illustrate a relationship between a detected temperature of the fixing belt and a value of supplied power in time sequence. 
         FIG. 8  illustrates a specific example of a power supply control carried out by the IH control unit according to a first modification example of the embodiment. 
         FIG. 9  illustrates a relationship between the detected temperature of the fixing belt and a value of supplied power in time sequence according to a second modification example of the embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, a fixing device includes a fixing unit configured to fix an image on a sheet with heat when the sheet passes through a nip formed in the fixing unit, a temperature detection unit disposed adjacent to the fixing unit, and a control unit configured to change a maximum amount of power that is suppliable to the fixing unit from a first value to a second value that is smaller than the first value, based on a temperature detected by the temperature detection unit. 
     Hereinafter, a fixing device according to the embodiment will be described.  FIG. 1  illustrates a Multi Functional Peripheral (hereinafter, “MFP”)  1  which is an image forming apparatus having the fixing device. The MFP  1  includes a scanner unit  13  which reads an image, a printer unit  14  which is an image forming unit, a sheet feeding unit  21  which feeds a sheet P which is a recording medium, and a sheet discharging unit  52  which includes first and second trays  52   a  and  52   b  which store the sheet P discharged from the printer unit  14 . The MFP  1  includes a manual sheet feeding unit  23  at a side portion of a housing  11 . The MFP  1  includes a transport mechanism  40  of the sheet P along a sheet conveyance path along which the sheet is conveyed from the sheet feeding unit  21  or the manual sheet feeding unit  23  to the sheet discharging unit  52  through the printer unit  14 . 
     The scanner unit  13  generates image information after scanning the original document which is supplied from an automatic document feeder (AFD)  35 . When the image information is generated using the scanner unit  13 , the ADF  35  discharges the original document to an original document discharging unit  31 . 
     The printer unit  14  forms on the sheet P an image corresponding to image information input from an external device or the image information generated by the scanner unit  13 . The printer unit  14  includes an exposure device  42  and a transfer unit  44 , in addition to four sets of image forming stations  50  of yellow (Y), magenta (M), cyan (C), and black (K). The transfer unit  44  transfers a toner image which is formed on the sheet P, which has an arbitrary size, in the image forming station  50  on a sheet P. The printer unit  14  includes a fixing device  45  which fixes the toner image on the sheet P. 
     The four sets of image forming stations  50  have the same structure, and include a photosensitive drum  41 , a charging device  48 , and a developing device  43 . The charging device  48  uniformly charges the surface of the photosensitive drum  41  which is an image carrier. The developing device  43  supplies toner such that an electrostatic latent image which is formed on the surface of the photosensitive drum  41  is developed as a toner image after the surface is irradiated with exposure light using the exposure device  42  while the photosensitive drum  41  is charged. 
     The transfer unit  44  includes an intermediate transfer belt  44   a , a primary transfer roller  44   c , and a secondary transfer roller  44   b . The sheet feeding unit  21  includes an upper sheet feeding cassette  21   a , a lower sheet feeding cassette  21   b , and a large cassette  21   c.    
     The transport mechanism  40  includes a transport roller  24  and a resist roller  16 . The transport roller  24  supplies a sheet P which is taken out from the sheet feeding unit  21  or the manual sheet feeding unit  23  using a pickup roller  22  to the transfer unit  44 . The transport mechanism  40  transports the sheet P including a fixed toner image which is formed while passing through the transfer unit  44  and the fixing device  45 , to the sheet discharging unit  52  or a circulation path  51 . 
     The sheet discharging unit  52  discharges the sheet P to the first tray  52   a  or the second tray  52   b , or reverses the sheet P in a direction towards the circulation path  51 . The circulation path  51  guides the sheet P to the transfer unit  44  again. In addition, the transport mechanism  40  includes a sheet sensor  40   a  which detects the sheet P while the sheet P reaches the fixing device  45  from the transfer unit  44 . 
     The sheet P which is supplied from any one of the sheet feeding unit  21  and the manual sheet feeding unit  23  passes through the transport mechanism  40 , and reaches a nip between the intermediate transfer belt  44   a  and the secondary transfer roller  44   b  in synchronization with conveyance of the toner image which is primarily transferred onto the intermediate transfer belt  44   a . The secondary transfer roller  44   b  performs secondary transfer of the toner image on the intermediate transfer belt  44   a  to the sheet P passing through the nip between the intermediate transfer belt  44   a  and the secondary transfer roller  44   b . The fixing device  45  fixes the toner image on the sheet P. 
     The sheet discharging unit  52  discharges the sheet P with the fixed toner image to the first tray  52   a  or the second tray  52   b . The circulation path  51  guides the sheet P after the toner image is fixed thereon, in a direction of the secondary transfer roller  44   b  of the transfer unit  44  again. 
     Subsequently, the fixing device  45  will be described in detail.  FIG. 2  illustrates the fixing device  45 . In addition,  FIG. 3  illustrates a temperature detection portion of a fixing belt  60  in the fixing device  45 . As illustrated in  FIGS. 2 and 3 , the fixing device  45  includes the fixing belt  60 , a press roller  61 , an induced current generation coil (hereinafter, referred to as “IH coil”)  70 , a fixing pad  72  which is a nip forming member, an auxiliary heat generating member  74 , and a contact-type thermistor  67 . The fixing device  45  includes a separation blade  64 , which is a separation member, on the outer periphery of the fixing belt  60  on the discharging side of the sheet P with respect to a nip  63 . 
     The fixing belt  60  is in an endless shape and has a multilayered structure. The fixing belt  60  includes, for example, a metallic heat generating layer formed of nickel (Ni) with a thickness of 40 μm, an adhesive layer with a thickness of 20 μm, a silicone rubber layer with a thickness of 200 μm, and a mold release layer formed of fluororesin with a thickness of 30 μm at a periphery of a support layer. As a material of the metallic heat generating layer, stainless steel, copper (CU), silver (Ag), a compound material of iron and nickel (Ni), or the like, may be used. A flange  62  supports both sides of the fixing belt  60 . The fixing belt  60  rotates following the press roller  61  integrally with the flange  62 , or independently rotates. 
     The fixing pad  72  is formed of silicon sponge or silicon rubber having a heat-resisting property, for example. The fixing pad  72  includes a mold release layer which is formed of fluororesin, for example, on the surface. A stay  73  supports the fixing pad  72 , and fixes the fixing pad  72  into the fixing belt  60 . 
     The press roller  61  is a pressurizing member which includes a heat-resistant silicon sponge or a silicon rubber layer at the periphery of a core metal, for example, and includes a mold separation layer of PFA on the surface thereof. The press roller  61  is connected to a pressurizing changing mechanism  87  which adjusts a pressurizing force of the press roller  61  with respect to the fixing pad  72 . The pressurizing changing mechanism  87  includes a cam  81 , a bearing  82 , and a pressurizing spring  85 . The pressurizing spring  85  pressurizes the press roller  61  in a direction of the arrow r. 
     When the fixing device  45  is used, a cam face  83   b  of the elliptic cam  81  which is close to a rotation center  81   a  comes into contact with the bearing  82 , and pressurizes the press roller  61  toward the fixing pad  72  with a pressure in a direction of the arrow r using the pressurizing spring  85 . While the fixing device  45  is not used, a cam face  83   a  of the cam  81  which is far from the rotation center  81   a  comes into contact with the bearing  82 . A press roller frame  80  rotates in a direction of the arrow t, a pressure to the fixing pad  72  of the press roller  61  decreases, and an occurrence of a permanent strain in the press roller  61  is prevented. 
     The press roller frame  80  fixes and supports the separation blade  64 . While the fixing device  45  is used, the separation blade  64  faces the fixing belt  60  along the fixing pad  72 . While the fixing device  45  is not used, if a pressure with respect to the fixing pad  72  of the press roller  61  decreases, the shape of the fixing pad  72  that is shrunk with the pressure is restored. When the shape of the fixing pad  72  is restored, the press roller  61  is separated from the fixing pad  72 . When performing separation of the sheet P, it is possible for the separation blade  64  to cause a tip end of the separation blade  64  to be closer to the fixing belt  60  in order to reliably separate the sheet P. When performing separation, it is possible to maintain a gap between the tip end of the separation blade  64  and the fixing belt  60  in a range of 0.1 mm to 0.4 mm, for example. 
     The IH coil  70  includes a magnetic core  70   a  and a coil  71 . The magnetic core  70   a  includes an upstream core  70   b  at an end portion on the upstream side, and a downstream core  70   c  at an end portion on the downstream side along a rotational direction of the fixing belt  60  in a direction of the arrow u. The magnetic core  70   a  intensifies a magnetic field using the coil  71 . A magnetic flux generation region (heating region) of the IH coil  70  generated due to excitation in the rotational direction of the fixing belt  60  is determined in accordance with positions of the upstream core  70   b  and the downstream core  70   c . Specifically, in the magnetic flux generation region of the IH coil  70 , a magnetic flux generation upstream end portion is determined based on the position of the upstream core  70   b , and a magnetic flux generation downstream end portion is determined based on the position of the downstream core  70   c.    
     The coil  71  includes a first coil  71   a  and a second coil  71   b . The first coil  71   a  generates a magnetic flux in whole length of the fixing belt  60  in the longitudinal direction. A current direction of the second coil  71   b  is opposite to a current direction of the first coil  71   a  in both sides of the fixing belt  60  in the longitudinal direction, and the magnetic flux of the second coil  71   b  cancels the magnetic flux of the first coil  71   a . As a material of the coil  71 , a litz wire which is formed by bundling one hundred of copper wire rods with a line diameter of 0.2 mm covered with heat resistant polyamide imide as an insulation material can be used. 
     An eddy current is generated in the metallic heat generating layer of the fixing belt  60  by applying a high frequency current to the first coil  71   a , and by generating a magnetic flux. Joule heat is generated due to the eddy current and a resistance value of the metallic heat generating layer, and the surface of the fixing belt  60  in the whole length in the longitudinal direction is heated. Using heat generated by exciting the first coil  71   a , a toner image is fixed onto the sheet P with a width of an A4 vertical size (297 mm) of the JIS standard, for example. 
     When the first coil  71   a  and the second coil  71   b  are excited, the second coil  71   b  cancels excitation of the first coil  71   a . When the first coil  71   a  and the second coil  71   b  are excited, the sheet P with a width of an A4 horizontal size (210 mm) of the JIS standard is fixed, for example. 
     The auxiliary heat generating member  74  is provided so as to face a magnetic flux generation region of the IH coil  70  in the fixing belt  60 , and generates heat due to a magnetic flux which penetrates the fixing belt  60 . The auxiliary heat generating member  74  is fixed with a gap of approximately 1 mm, for example, from the inner periphery of the fixing belt  60 . The auxiliary heat generating member  74  includes, for example, a mold release layer which is formed of fluororesin with a thickness of 15 μm, a metallic heat generating layer with a thickness of 0.2 mm, a soaking layer which is formed of aluminum with a thickness of 0.5 mm, and a protecting layer which is formed of a white PFA resin with a thickness of 10 μm from the inner peripheral face side of the fixing belt  60  in order. The auxiliary heat generating member  74  prevents a temperature of the fixing belt  60  from falling by warming the fixing belt  60  from an inner peripheral side thereof. 
     In addition, as the metallic heat generating layer of the auxiliary heat generating member  74 , for example, magnetic shunt metal of which Curie point is 230° C. may be used in order to prevent an abnormal heat generation. The thermistor  67  is provided inside and in contact with the fixing belt  60 , detects a temperature of the fixing belt  60  as a voltage value, and inputs a detection result thereof to a main body control unit  10 . 
     The main body control unit  10  controls a thermostat  92 , an IH control unit  10   a , and a driving control unit  10   b . The IH control unit  10   a  controls supply of a high frequency current to the IH coil  70 . The driving control unit  10   b  controls a pressure adjustment and rotation driving of the press roller  61 . The thermostat  92  interrupts a power supply to the IH coil  70  from a power supply circuit  93 , and prevents abnormal heat generation of the fixing device  45  when abnormal heat generation of the fixing device  45  is detected thorough the main body control unit  10 . 
     The IH control unit  10   a  excites the coil  71  according to a size of the sheet P. The IH control unit  10   a  performs a feedback control of the IH coil  70  based on a detection result of the thermistor  67 , and maintains a temperature of the fixing belt  60  in a target temperature range. The magnetic flux of the coil  71  generates an eddy current in the metallic heat generating layer of the fixing belt  60 , and heats the fixing belt  60 . 
     When a change in belt temperature in time sequence which is detected by the thermistor  67  satisfies a predetermined temperature change pattern, which serves as an index for determining a state in the device, the IH control unit  10   a  switches a setting upper limit of power supplied to the IH coil  70  from a first upper limit corresponding to a maximum power value which is allocated to the fixing device  45  in advance to a second upper limit which is lower than the first upper limit. The temperature change pattern may be arbitrarily defined. According to the embodiment, it is determined that a change in the detected temperature satisfies the temperature change pattern when one cycle which is formed of an ascending curve and a descending curve within the target temperature range is completed, after the supplied power is changed between the first upper limit and the lower limit. 
       FIG. 4  illustrates switching of the maximum value (first upper limit) of power that can be allocated to the fixing device  45  of the MFP  1 . As illustrated in  FIG. 1 , the fixing device  45  is a part of the MFP  1 ; however, a maximum amount of power which may be supplied to the entire MFP  1  is fixed. When a commercial power supply is used, it is 100 V/15 A in Japan. Accordingly, a maximum value of power which may be used in each of the fixing device  45 , an image forming portion, and an optional device (supplement) is predetermined such that the total value of power used by the entire MFP  1  is 15 A or less. According to the embodiment, the main body control unit  10  performs distribution of power to each unit, and the IH control unit  10   a  controls an amount of power supplied to the IH coil  70 . 
     The main body control unit  10  instantly determines whether or not an optional device such as a scanner is connected to the MFP  1  when the MFP  1  is turned on. When an optional device is not connected, the main body control unit  10  may cause the fixing device  45  to use power for the optional device. 
     Further, it is possible to switch a maximum power value which the fixing device  45  may use according to an operation state (ON or OFF) of the optional device. As illustrated in  FIG. 4 , at a time of non-operation of a scanner (T 0  to T 1  and T 2  to T 3 ), since power Δ W which is allocated to the scanner may be used by the fixing device  45 , the maximum power value of the fixing device  45  is 960 W. At an operation time of the scanner (T 1  to T 2 ), the maximum power value of the fixing device  45  is 900 W which is lower than 960 W by Δ W. 
     In addition, when the value of power supplied to the fixing device  45  is instantly lowered to 0 W from the maximum value, since the thickness of a base material of the fixing belt  60  is small and a heat capacity is small, a temperature of the fixing device  45  rapidly falls and a fixing failure may occur. Therefore, in  FIG. 4 , in order to prevent a rapid temperature fall of the fixing device  45 , a lower limit of 510 W is set, in addition to two types of maximum values (first and second upper limits). It is preferable to set the lower limit to be a half or more of the first upper limit. 
     Subsequently, operations of the MFP  1  will be described based on drawings.  FIG. 5  is a flowchart of a power control process of the IH control unit  10   a . The process is started when a printing job is received. Here, for ease of descriptions, there is no change in the first upper limit in accordance with connection or disconnection of the optional device, as illustrated in  FIG. 4 . 
     When the MFP  1  is turned on or a stand-by time is terminated, a temperature of the fixing device  45  is low. The IH control unit  10   a  controls the supply power W having the first upper limit to the IH coil  70 , and starts heating the fixing device  45  (Act  101 ). 
     Subsequently, the IH control unit  10   a  obtains a temperature of the fixing belt  60  (hereinafter, “belt temperature”) detected by the thermistor  67 , and determines whether or not the belt temperature is equal to or higher than a lower limit temperature of the target temperature range (Act  102 ). Here, when it is determined that the belt temperature is equal to or higher than the lower limit temperature (Yes in Act  102 ), the process proceeds to Act  103 . When it is determined that the belt temperature is lower than the lower limit temperature (No in Act  102 ), the process returns to Act  101 . That is, the IH control unit  10   a  continues heating with the power of the first upper limit (maximum power) until the belt temperature reaches the lower limit temperature. 
     In Act  103 , the IH control unit  10   a  determines whether or not the belt temperature exceeds the upper limit temperature of the target temperature range. Here, when it is determined that the belt temperature exceeds the upper limit temperature (Yes in Act  103 ), the process proceeds to Act  104 . When it is determined that the belt temperature is equal to or lower than the upper limit temperature (No in Act  103 ), the process proceeds to Act  108 . 
     In Act  104 , the IH control unit  10   a  determines whether or not the belt temperature is lower than a predetermined abnormal temperature. Here, when it is determined that the belt temperature is lower than the abnormal temperature (Yes in Act  104 ), the process proceeds to Act  105 . When it is determined that the belt temperature is equal to or higher than the abnormal temperature (No in Act  104 ), the value of the supplied power W is set to 0 W in order to lower the belt temperature (Act  106 ), and the process returns to Act  104 . 
     In Act  105 , the IH control unit  10   a  controls the value of the power W supplied to the IH coil  70  so as to be the lower limit in order to lower the belt temperature down to the target temperature range, and the process proceeds to Act  107 . 
     In Act  107 , the IH control unit  10   a  determines whether or not the belt temperature is equal to or lower than the upper limit temperature of the target temperature range. Here, when it is determined that the belt temperature is equal to or lower than the upper limit temperature, that is, the belt temperature is in the target temperature range (Yes in Act  107 ), the process proceeds to Act  108 . When it is determined that the belt temperature exceeds the upper limit temperature (No in Act  107 ), the process returns to Act  105 . 
     In Act  108 , the IH control unit  10   a  determines whether or not the belt temperature is equal to or higher than the lower limit temperature. Here, when it is determined that the belt temperature is equal to or higher than the lower limit temperature (Yes in Act  108 ), the process proceeds to Act  109 . When it is determined that the belt temperature is lower than the lower limit temperature (No in Act  108 ), the process proceeds to Act  111 . 
     In Act  109 , the IH control unit  10   a  determines whether or not the belt temperature is rising. Here, when it is determined that the belt temperature is rising (Yes in Act  109 ), since the currently supplied power W is excessive, the power W is lowered by a predetermined variable width Δ W (Act  110 ), and the process proceeds to Act  112 . In contrast, when it is determined that the belt temperature is falling (No in Act  109 ), the process proceeds to Act  111 . 
     In Act  111 , in the IH control unit  10   a , since the currently supplied power W is insufficient, the power W is increased by the variable width Δ W, and the process proceeds to Act  112 . 
     In Act  112 , the IH control unit  10   a  determines whether or not predetermined one cycle of temperature change in the target temperature range is completed, and when it is determined that one cycle is completed (Yes in Act  112 ), the process proceeds to Act  113 . When it is determined that the one cycle is not completed (No in Act  112 ), the process returns to Act  103 . 
     In Act  113 , the IH control unit  10   a  switches the setting upper limit value of the power W supplied to the IH coil  70  to the second upper limit which is lower than the first upper limit (initial value), and ends the process. Switching of the setting upper limit value means that the fixing device  45  is sufficiently warmed. 
       FIG. 6  is a diagram which describes a relationship between a detected temperature of the fixing belt  60  and a power control in time sequence. The top column (A) illustrates a relationship between a detected temperature of the fixing belt  60  and a time. The middle column (B) illustrates a relationship between the power supplied to the IH coil  70  and the time. The lower column (C) illustrates a relationship between an increasing trend (UP) or a decreasing trend (DOWN) of the supplied power illustrated in the middle column (B) and the time. Time axes in (A) to (C) are common. 
     In a time zone of T 0  to T 1 , power of 960 W, which is the first upper limit, is supplied to the IH coil  70 . The belt temperature in time T 1  reaches the lower limit temperature of the target temperature range. 
     In a time zone of T 1  to T 2 , the belt temperature rises within the target temperature range. At this time, the power supplied to the IH coil  70  is controlled so as to decrease by a predetermined step width Δ Ws from 960 W periodically. In T 2 , when the belt temperature reaches the upper limit temperature of the target temperature range, the supplied power is controlled so as to be lowered to a predetermined lower limit (for example, 510 W). 
     In a time zone of T 2  to T 3 , the supplied power is maintained to the lower limit. Here, the belt temperature changes between the upper limit temperature and the predetermined abnormal temperature, and in T 3 , the belt temperature is lowered to the upper limit temperature. 
     In a time zone of T 3  to T 4 , the belt temperature falls within the target temperature range. At this time, the power supplied to the IH coil  70  is controlled so as to periodically increase from the lower limit by the step width Δ Ws. However, the belt temperature keeps falling, and the belt temperature becomes lower than the lower limit temperature in T 4 . 
     In a time zone of T 4  to T 5 , the supplied power is controlled so as to increase by the step width Δ Ws&#39; (&gt;Δ Ws) in a predetermined cycle (for example, 200 ms). As a result, the belt temperature turns to an increasing trend, and in T 5 , the belt temperature reaches the lower limit temperature. 
     In time zones of T 5  to T 6  and T 6  to T 7 , the same control as that in the time zones of T 1  to T 2  and T 2  to T 3  is performed. 
     In a time zone of T 7  to T 8 , the belt temperature becomes the abnormal temperature or more. At this time, the supplied power is maintained at 0 W until T 8 , and the fixing device  45  is allowed to be cooled. In a time zone of T 8  to T 9 , the same control as that in the time zone of T 2  to T 3  is performed. 
     However, between T 0  to T 9 , the fixing device  45  has not stably changed within the target temperature range yet, even though the supplied power is increased or decreased. For this reason, in T 9 , it is considered that a predetermined temperature changing pattern has not been satisfied yet. 
       FIG. 7  describes a relationship between a detected temperature of the fixing belt  60  and a power control according to the embodiment. The top column (A) illustrates a relationship between a detected temperature of the fixing belt  60  and a time. The middle column (B) illustrates a relationship between the power supplied to the IH coil  70  and the time. The lower column (C) illustrates a relationship between an increasing trend (UP) or a decreasing trend (DOWN) of the supplied power illustrated in the middle column (B) and the time. Time axes in (A) to (C) are common.  FIG. 7  illustrates a state at which it is determined that the fixing device  45  is sufficiently warmed after the temperature control illustrated in  FIG. 6  is executed. That is, the first point of time when one cycle of a temperature change is completed within the target temperature range is T 12 , and a predetermined temperature changing pattern according to the embodiment is satisfied in T 12 . For this reason, in T 12 , the setting upper limit W max  of the supply power is switched to the second upper limit from the first upper limit. As illustrated in  FIG. 7 , the second upper limit is 900 W, and is set to a value which is subtracted from the first upper limit 960 W by 60 W. Here, the difference of the setting upper limit is referred to as a first variable width. The first variable width may be arbitrarily defined. 
     In this manner, according to the embodiment, when the fixing device  45  is sufficiently warmed, and it is possible to maintain a temperature of the fixing device  45  within the target temperature range without supplying the maximum power to the IH coil, the upper limit of the supplied power is switched to the second upper limit which is lower than the maximum power value (first upper limit). Since it is possible to reduce a time period during which a maximum amount of power is supplied, power consumption can also be reduced. That is, it is possible to reduce power consumption using heat which is accumulated in the fixing device  45  or the MFP  1  in which the fixing device  45  is installed. 
     Modification Example 
     Hereinafter, some of modification examples of the above described embodiment will be described. 
     In the above described embodiment, when the temperature change of the fixing belt  60  satisfies the predetermined temperature changing pattern, the setting upper limit of the supplied power is switched from the first upper limit to the second upper limit. However, it is also possible to switch the setting upper limit when a certain period of time has passes since the startup of the MFP, without referring to the detected temperature. Here, it is assumed that the device is sufficiently warmed after the certain period of time.  FIG. 8  illustrates a specific example of a power supply control of the IH control unit in a modification example (1) of the embodiment. Here, the power of a maximum power value (first upper limit) is supplied only for thirty seconds from the startup of the fixing device  45 , and then the value of the power is switched to the second upper limit. The switching time may be arbitrarily changed. 
     In addition, in the above described embodiment, the second upper limit is a constant value. However, when it is determined that the temperature of the device is continuously stable, it is possible to further lower the second upper limit.  FIG. 9  illustrates a relationship between a detection temperature of the fixing belt  60  and a power control in a modification example (2) of the embodiment. Here, switching of the setting upper limit from the first upper limit to the second upper limit (900 W) is completed in T 20 . In addition, an ascending curve and a descending curve of the detection temperature of the fixing belt  60  are continued five times within the target temperature range in of T 22 . At this time, it is possible to determine that the detected temperature is stably within the target temperature range for a sufficiently long time. At T 22 , the second upper limit is updated from 900 W to 850 W according to this determination. Here, the difference of the second upper limit is referred to as a second variable width. The second variable width may be arbitrarily defined. 
     In addition, when the ascending curve and the descending curve of five cycles are continued within the target temperature range after lowering the second upper limit, it is possible to further lower the second upper limit by the second variable width. In contrast, when the detected temperature goes out of the target temperature range, the second upper limit may be raised by the second variable width. When the detected temperature does not reach the target temperature range even when the second upper limit is raised, the second upper limit may be further raised by the second variable width. The second upper limit may be increased or decreased between the initial upper limit (first upper limit) and the lower limit. It is possible to further reduce power consumption by appropriately adjusting the second upper limit to an optimal value according to a state of the device. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the invention.