Patent Publication Number: US-2012045240-A1

Title: Fixing device and image forming apparatus

Description:
This application is based on an application No. 2010-183733 filed in Japan, the contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates to a fixing device and an image forming apparatus, and in particular to a technology for preventing overheating in regions where recording sheets do not pass through, together with aiming to reduce size, weight, and cost of the device and the apparatus. 
     (2) Description of the Related Art 
     In recent years, for electrophotographic image forming apparatuses, an electromagnetic induction-heating method has been actively studied to realize low heat capacity and improve temperature rise performance of a fixing member that fuses toner on recording sheets. The electromagnetic induction-heating method is used to heat the fixing member by applying induced current to a metal heat generating layer included in the fixing member with use of an excitation coil. 
     Recording sheets on which toner is fixed are various in size, and the fixing member has an enough size for fixing toner on recording sheets having the maximum size (hereinafter, referred to as “maximum-size recording sheets”) as a specification. Also, an effective heating length of the excitation coil that heats the fixing member corresponds to the maximum-size recording sheet. Therefore, when toner is fixed on recording sheets having a smaller size (hereinafter, small-size recording sheets), a region where the small-size recording sheets pass through on a surface of the fixing member is deprived of heat by the small-size recording sheets and needs to be heated. However, there is a problem in that if heating continues, the regions where small-size recording sheets do not pass through are overheated, which results in failure of the fixing device. 
     In view of the above problem, the following three conventional technologies have been proposed, for example. 
     (1) A fixing member  1505  is a rotator like a fixing roller or a fixing belt, and demagnetization coils  1502  that cancel a magnetic flux generated by an excitation coil  1501  are provided at both ends of the fixing member  1505  in a rotational axis direction thereof. A connection of each of the demagnetization coils  1502  is switched ON and OFF in accordance with a size of recording sheets or a temperature of the regions where recording sheets do not pass through (see Japanese Unexamined Patent Application Publication No. 2007-226126 and  FIG. 1 ). 
     (2) A plurality of excitation coils  1601  to  1603  each having a short effective heating length are aligned on a fixing member  1504  in the rotational axis direction, and power supply to each of the excitation coils  1601  to  1603  is switched ON and OFF in accordance with a size of recording sheets or a temperature of the regions where recording sheets do not pass through (see Japanese Unexamined Patent Application Publication No. 2001-235962 and  FIG. 2 ). 
     (3) A main excitation coil  1701  and an auxiliary excitation coil  1702  are aligned in a circumferential direction of the fixing member  1504 , and power is supplied to one of the main excitation coil  1701  and the auxiliary excitation coil  1702  in accordance with a size of recording sheets and a temperature of the regions where recording sheets do not pass through. The main excitation coil  1701  has an effective heating length that corresponds to a width of the maximum-size recording sheets, and the auxiliary excitation coil  1702  has an effective heating length that is smaller than an effective heating length of the main excitation coil (see Japanese Unexamined Patent Application Publication No. 2001-332377 and  FIG. 3 ). 
     However, according to the conventional art (1), the demagnetization coils  1502  cannot completely cancel the magnetic flux generated by the excitation coil  1501 , and as shown in  FIG. 4 , a temperature in regions where small-size recording sheets do not pass through becomes high (solid line  1802 ). Therefore, for example, when small-size recording sheets pass through at a high speed, output of the excitation coil has to be large, and as a result, the demagnetization coils  1502  cannot effectively prevent overheating in the regions where the small-size recording sheets do not pass through. 
     Also, according to the conventional art (2), magnetic fluxes generated by the plurality of excitation coils  1601  to  1603  interfere with one another. As a result, as shown in  FIG. 5 , an uneven temperature distribution occurs in the rotational axis direction of the fixing member, and in particular at joints of the excitation coils  1601  to  1603  (solid line  1901 ). Accordingly uneven fixation might occur. 
     According to the conventional art (3), in order to align the excitation coils  1701  and  1702  in the circumferential direction of a fixing rotational body, each of the excitation coils has to be small. As a result, heat generation efficiency decreases. That is, by increasing a distance through which the magnetic flux generated by the excitation coil  1501  passes in the circumferential direction of the fixing rotational body, the heat generation efficiency of each coil can be increased ( FIG. 6B ). 
     However, in the case of aligning the excitation coils  1701  and  1702  in the circumferential direction of the fixing rotational body, it is impossible to increase a distance through which the magnetic fluxes generated by the excitation coils  1701  and  1702  pass in the circumferential direction of the fixing rotational body. As a result, it is impossible to prevent reduction of heat generation efficiency of each of coils that correspond to different sizes of recording sheets ( FIG. 6A ). In addition, if the excitation coils are made large, the fixing member also has to be large, and accordingly the fixing device becomes large. 
     Thus, each of the conventional arts has a different problem. 
     SUMMARY OF THE INVENTION 
     The present invention has been achieved in view of the above problems, and aims to provide a fixing device and an image forming apparatus that can realize reduction in size, weight, and cost thereof without having a harmful effect such as uneven temperature distribution and reduction of heat generation efficiency, while preventing overheating in the regions where recording sheets do not pass through. 
     In order to achieve the above aim, a fixing device pertaining to the present invention includes a fixing rotational body and fixes toner images on recording sheets of various sizes by using the fixing rotational body, the fixing device comprising: a main excitation coil that heats the fixing rotational body by electromagnetic induction and has an effective heating length L 1  corresponding to a recording sheet of a maximum size; an auxiliary excitation coil that heats the fixing rotational body by electromagnetic induction and has an effective heating length L 2  that is shorter than the effective heating length L 1  of the main excitation coil; a high-frequency power source that supplies power to the main excitation coil and the auxiliary excitation coil; and a switch that selectively connects the main excitation coil and the auxiliary excitation coil to the high-frequency power source, wherein the main excitation coil is positioned along a part of an outer circumferential surface of the fixing rotational body, the auxiliary excitation coil is positioned farther from the fixing rotational body than the main excitation coil is and layered on a substantially central portion of the main excitation coil in a longitudinal direction of the main excitation coil, and the effective heating length L 2  of the auxiliary excitation coil satisfies the following relationship: L 2 ≦L 1 ·η 2 /η 1 , where η 1  is a thermal conversion efficiency of the main excitation coil and η 2  is a thermal conversion efficiency of the auxiliary excitation coil. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings which illustrate a specific embodiment of the invention. 
       In the drawings: 
         FIG. 1  shows a structure of a fixing device pertaining to a conventional art using demagnetization coils; 
         FIG. 2  shows a structure of a fixing device pertaining to a conventional art aligning a plurality of excitation coils in a rotational axis direction of a fixing member; 
         FIG. 3  shows a structure of a fixing device pertaining to a conventional art aligning a plurality of excitation coils in a circumferential direction of the fixing member; 
         FIG. 4  is a graph showing a temperature distribution of a surface of the fixing member pertaining to the conventional art using the demagnetization coils; 
         FIG. 5  is a graph showing a temperature distribution of a surface of the fixing member pertaining to the conventional art aligning the plurality of excitation coils in the rotational axis direction of the fixing member; 
         FIGS. 6A and 6B  explain heat generation efficiency of the fixing device pertaining to the conventional art aligning the plurality of excitation coils in the circumferential direction of the fixing member; 
         FIG. 7  shows a main structure of an image forming apparatus pertaining to an embodiment of the present invention; 
         FIG. 8  is a cross-sectional view showing a main structure of a fixing device  115 ; 
         FIG. 9  is a cross-sectional view showing a structure of a fixing belt  206 ; 
         FIG. 10  shows a circuit structure for controlling power supply to a main excitation coil  207  and an auxiliary excitation coil  215 ; 
         FIG. 11  is a lateral view showing a positional relationship between the main excitation coil  207  and the auxiliary excitation coil  215  in a rotational axis direction of a fixing roller  202 ; 
         FIG. 12  is a graph showing a relationship between a ratio of an effective heating length of the auxiliary excitation coil to the main excitation coil and heat generation amount per unit length generated by the auxiliary excitation coil having the above ratio within an effective heating area of the fixing belt; 
         FIG. 13  is a plan view showing a shape of the auxiliary excitation coil  215 ; 
         FIG. 14  is a graph showing a temperature distribution during electromagnetic induction heating by the main excitation coil  207  and a temperature distribution during electromagnetic induction heating by the auxiliary excitation coil  215  in the rotational axis direction of the fixing belt  206 ; 
         FIG. 15  is an external view of a main structure of a fixing device pertaining to a modification of the present invention; 
         FIG. 16  is a flowchart showing control of power supply to the main excitation coil  207  and the auxiliary excitation coil  215 , which is performed by a controller pertaining to the modification of the present invention; 
         FIG. 17  is a flowchart showing processing for maximum-size recording sheets pertaining to the modification of the present invention; 
         FIG. 18  is a flowchart showing processing for recording sheets having a middle size pertaining to the modification of the present invention; 
         FIG. 19  is a flowchart showing processing for small-size recording sheets pertaining to the modification of the present invention; and 
         FIG. 20  shows a main structure of the fixing device pertaining to the modification of the present invention. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     The following describes an embodiment of a fixing device and an image forming apparatus pertaining to the present invention, with reference to the drawings. 
     1. Structure of Image Forming Apparatus 
     Firstly, the following describes a structure of the image forming apparatus pertaining to the embodiment. 
       FIG. 7  shows a main structure of the image forming apparatus pertaining to the embodiment. As shown in  FIG. 7 , an image forming apparatus  1  includes a document reader  100 , an image forming section  110 , and a paper feeder  120 . The document reader  100  generates image data by optically reading a document. 
     The image forming section  110  includes image forming units  111 Y to  111 K, a controller  112 , an intermediate transfer belt  113 , a secondary transfer roller pair  114 , a fixing device  115 , a sheet ejecting roller  116 , an ejected-sheet tray  117 , and a cleaner  118 . 
     The image forming units  111 Y to  111 K respectively form toner images of yellow (Y), magenta (M), cyan (C) and black (K) under control of the controller  112 , and electrostatically transfer (i.e., primarily transfer) the toner images onto the intermediate transfer belt  113  such that the toner images are superimposed. The intermediate transfer belt  113  is an endless belt that rotates in the direction of an arrow A so as to convey the toner images to the secondary transfer roller pair  114 . 
     The paper feeder  120  includes feeding cassettes  121  each containing recording sheets P of a different size, and supplies the recording sheets P to the image forming section  110 . The supplied recording sheets P are conveyed to the secondary transfer roller pair  114  in parallel with the transportation of the toner images formed on the intermediate transfer belt  113 . 
     The secondary transfer roller pair  114  is composed of a pair of rollers having a potential difference and being pressed against each other to form a transfer nip portion. At the transfer nip portion, the toner images on the intermediate transfer belt  113  are electrostatically transferred onto the recording sheets P (i.e., secondary transfer). The recording sheets P, onto which the toner images have been transferred, are conveyed to the fixing device  115 . 
     The fixing device  115  employs an electromagnetic induction-heating method. The fixing device  115  heats and fuses the toner images, and then presses the toner images against the recording sheets P. The recording sheets P, on which the toner images have been fused, are ejected onto the ejected-sheet tray  117  by the sheet ejecting roller  116 . 
     2. Structure of Fixing Device  115   
     Next, the following describes a structure of the fixing device  115 . 
       FIG. 8  is a cross-sectional view showing a main structure of the fixing device  115 . As shown in  FIG. 8 , the fixing device  115  includes within a housing  201  a fixing roller  202  and a pressurizing roller  203 . Rotational axes of the fixing roller  202  and the pressurizing roller  203  are in parallel with each other. The fixing device  115  presses the fixing roller  202  against the pressurizing roller  203  to sandwich the fixing belt  206  between the fixing roller  202  and the pressurizing roller  203 , and rotates the pressurizing roller  203  by a drive motor (not illustrated). 
     The fixing roller  202  includes an insulating elastic layer  205  that is made of materials such as silicone sponge around a circumferential surface of an elongated metal core  204 . The metal core is, for example, made of metal such as aluminum and stainless and has a diameter of 18 mm. The insulating elastic layer  205  is made of heat-resistant rubber, such as silicone rubber or fluoro rubber, or a foamed material obtained by foaming such rubber. Alternatively, the insulating elastic layer  205  may be formed by layering the heat-resistant rubber and the foamed material. The insulating elastic layer  205  has a thickness of, for example, 5 mm. 
     An endless fixing belt  206  is freely fit around a circumferential surface of the fixing roller  202 . That is, an outer diameter of the fixing roller  202  is smaller (e.g., 28 mm) than an inner diameter of the fixing belt  206 . The fixing roller  202  is in contact with the fixing belt  206  at a fixing nip N. There is a gap (space) between the fixing roller  202  and the fixing belt  206  except for the fixing nip N. 
     With the above structure, an area through which heat from the fixing belt  206  transfers to the fixing roller  202  becomes small compared with a case in which the fixing belt  206  closely attaches to the fixing roller  202 , and it is possible to reduce heat transfer loss caused when a part of heat generated by the fixing belt  206  transfers via the metal core of the fixing roller  202  to the housing of the fixing device  115  that rotatably supports the metal core. Accordingly, high heat efficiency can be realized. 
     As shown in  FIG. 9 , the fixing belt  206  is formed by layering three layers including a metal heat generating layer  301 , an elastic layer  302  and a release layer  303  in this order with the metal heat generating layer  301  being closest to the circumferential surface of the fixing roller  202 . The metal heat generating layer  301  is formed of a Ni electroformed sleeve, and generates heat by electromagnetic induction by an alternating magnetic flux generated by a main excitation coil  207  or an auxiliary excitation coil  215 . In order to improve strength of the fixing belt  206 , a heat resistant reinforced layer may be added under the metal heat generating layer  301 . 
     The pressurizing roller  203  is formed by layering an elastic layer and a release layer in the stated order on a circumferential surface of an elongated metal core. The pressurizing roller  203  is provided outside a belt rotation path of the fixing belt  206  and pressed (not illustrated) against the fixing roller  202  via the fixing belt  206  from outside of the fixing belt  206  by a pressing mechanism. In this way, the fixing nip N is formed between a surface of the fixing roller  202  and a surface of the fixing belt  206 . An outer diameter of the pressurizing roller  203  is preferably in a range of 20 mm to 100 mm inclusive. In the present embodiment, the outer diameter of the pressurizing roller  203  is 35 mm. 
     The metal core has a hollow pipe-shape, and is made of metal such as aluminum or iron. An outer diameter of the metal core is, for example, 27 mm. A thickness of the metal core is preferably in a range of 0.1 mm to 10 mm inclusive. In the present embodiment, the thickness of the metal core is 2.5 mm. Note that the metal core may have a solid cylindrical shape or a Y-shaped cross-section. 
     The elastic layer is made of heat-resistant rubber, such as silicone rubber or fluoro rubber, or a foamed material obtained by foaming such rubber. A thickness of the elastic layer is preferably in a range of 1 mm to 20 mm inclusive. In the present embodiment, the thickness of the elastic layer is 4 mm. 
     The release layer is made of a fluororesin tube or a fluororesin coating that uses PFA (perfluoroalkoxy). The release layer may be conductive so as to prevent offset phenomenon of toner which is caused by electrostatic charge. A thickness of the release layer is preferably in a range of 5 μm to 100 μm inclusive. In the present embodiment, the thickness of the release layer is 30 μm. 
     The pressurizing roller  203  is rotated by a driving mechanism (not illustrated). In correspondence with rotation of the pressurizing roller  203 , the fixing belt  206  and the fixing roller  202  are rotated. Note that instead of rotatably driving the pressurizing roller  203  by a drive motor, the fixing belt  206  and the pressurizing roller  203  may be rotated by rotating the fixing roller  202 . 
     Moreover, in vicinity to the circumferential surface of the fixing belt  206 , a temperature detecting element (sensor)  208  is disposed. The temperature detecting element  208  that is out of contact with the fixing belt  206  detects a signal indicating a surface temperature of substantially a central portion of the circumferential surface in a rotational axis direction thereof, and then transmits the detected signal. The controller  120  receives the detected signal and controls power supply to the main excitation coil  207  and the auxiliary excitation coil  215  so that the temperature of fixing belt  206  is controlled to be a predetermined value. 
     The main excitation coil  207 , the auxiliary excitation coil  215 , a center core  209  and hem cores  210  and  211  are held by a coil bobbin  212 , and a plurality of main cores  213  are held by a core holding member  214 . The main excitation coil  207  and the auxiliary excitation coil  215  can generate a magnetic flux with necessary density such that a part of the fixing belt  206  whose width corresponds to a width of a region where either of the maximum recording sheets and the small-size recording sheets pass through is heated up to a temperature that is necessary for the fixing belt  206  to fix toner images on the recording sheets (hereinafter, fixing temperature). 
     The center core  209 , the hem cores  210  and  211 , and the main cores  213  are made of a magnetic material with high permeability and low loss characteristics, such as a ferrite alloy and a permalloy alloy, and form a magnetic circuit with the fixing belt  206  and the main excitation coil  207 . Thus, it is possible to prevent leaks of a magnetic flux to outside of the magnetic circuit, and accordingly heat generation efficiency improves. Note that in the present embodiment, the main cores  213  are rib-like, and provided along the fixing roller  202  in the rotational axis direction thereof. 
     The main cores  213  are bent like ribs so as to cover an outer surface of the main excitation coil  207 . The main cores  213  that are some to dozen in number are held by the core holding member  214  at a predetermined interval therebetween in a direction parallel to an axis direction of the fixing roller  202 . Two of the main cores  213  that are positioned at both ends in the axis direction have high magnetic coupling in order to compensate heat dissipation from both ends of the fixing belt. 
     Each of the center core  209  and the hem cores  210  and  211  has an elongated shape and is parallel to the axis direction of the fixing roller  202 , and is bonded to the coil bobbin  212  with use of a heat resistant adhesive agent such as a silicone adhesive agent. Each of the hem cores  210  and  211  may be divided into two in the axis direction, but it is preferable that each of the hem cores  210  and  211  be arranged without space therebetween. 
     The center core  209  uniformly leads a magnetic flux generated by the main excitation coil  207  to the fixing belt  206 . A magnetic flux penetrating through the fixing belt  206  induces eddy current, and then the fixing belt  206  generates Joule heat. 
     The coil bobbin  212  and the core holding member  214  are fixed by bolts and nuts at hem portions thereof. Alternatively, components other than the bolts and nuts, such as rivets may be used. 
     The main excitation coil  207  is held by the coil bobbin  212 . The auxiliary excitation coil  215  is positioned on a central portion of the main excitation coil  207  in the rotational axis direction of the fixing belt  206  so as to correspond to the region where the small-size recording sheets pass through. The auxiliary excitation coil  215  is attached firmly to an outer surface of the main excitation coil  207  and an insulating sheet is sandwiched between the auxiliary excitation coil  215  and the main excitation coil  207 . Note that the central portion represents an area of the main excitation coil  207  except for the both ends thereof, and the center of the main excitation coil  207  and the center of the auxiliary excitation coil  215  may not necessarily match. 
     Each of the main excitation coil  207  and the auxiliary excitation coil  215  is connected to an unillustrated high-frequency inverter (high-frequency power source), and high-frequency power of 10-100 kHz and 100-2000 W is supplied to each of the main excitation coil  207  and the auxiliary excitation coil  215 . Accordingly, each of the main excitation coil  207  and the auxiliary excitation coil  215  is preferably made by winding litz wire consisting of thin wires that are covered with heat resistant resin and bundled together. The present embodiment employs the main excitation coil  207  and the auxiliary excitation coil  215  that are each made by winding the litz wire 10 turns. The litz wire consists of  114  wires bundled and twisted together and a diameter of each of the wires is Ø0.17. 
       FIG. 10  shows a circuit structure for controlling power supply to the main excitation coil  207  and the auxiliary excitation coil  215 . As shown in  FIG. 10 , the main excitation coil  207  is electrically connected to a high-frequency inverter  403  through a switching relay  401 . The auxiliary excitation coil  215  is electrically connected to the high-frequency inverter  403  through a switching relay  402 . The switching relays  401  and  402  are each under control of the controller  112 . 
     The controller  112  causes one of the switching relays  401  and  402  to be ON in accordance with a size of fed recording sheets, and supplies high-frequency power to one of the main excitation coil  207  and the auxiliary excitation coil  215  so as to heat the fixing belt  206  by electromagnetic induction. The controller  112  monitors a temperature of the region where the recording sheets pass through with use of the temperature detecting element  208 , and when the temperature reaches a predetermined value, the controller  112  disconnects the switching relays  401  and  402  so as to stop temperature rise. Thereby, the region where the recording sheets pass through of the fixing belt  206  remains at the fixing temperature. 
       FIG. 11  is a partially cutaway lateral view showing a positional relationship between the main excitation coil  207  and the auxiliary excitation coil  215  in the rotational axis direction of the fixing roller  202 . As shown in  FIG. 11 , the auxiliary excitation coil  215  has an effective heating length that corresponds to the small-size recording sheets and is shorter than an effective heating length of the main excitation coil  207 . 
     Also, the auxiliary excitation coil  215  is layered substantially on a central portion of the main excitation coil  207  in the rotational axis direction of the fixing roller  202 . In addition, although not shown in  FIG. 11 , the main excitation coil  207  and the auxiliary excitation coil  215  firmly attach to each other, sandwiching an insulating sheet therebetween. 
     Note that, as a distance between an excitation coil and the fixing belt  206  becomes larger, density of the magnetic flux penetrating through the fixing belt  206  decreases, and then heat generation efficiency by electromagnetic induction decreases. Generally, as a size of the recording sheets becomes larger, a more amount of heat is required for fixing. However, there is a limit to power to be supplied to the excitation coils due to conditions such as power source capacity. 
     Accordingly, it is preferable that the main excitation coil  207  that requires higher power be positioned closest to the fixing belt  206 . In addition, in the case where a plurality of auxiliary excitation coils  215  are provided, the plurality of auxiliary excitation coils  215  should be positioned closer to the fixing belt  206  in descending order of effective heating length and required power. Thereby, even when toner images are fixed on recording sheets having a larger size, power shortage can be prevented. 
     As described above, as a distance between the excitation coils and the fixing belt  206  becomes larger, the heat generation efficiency decreases. However, if an effective heating length L 2  of the auxiliary excitation coil  215  satisfies the following inequality with reference to an effective heating length L 1  of the main excitation coil  207 , it is possible to guarantee a required amount of heat by supplying the same amount of power as power supplied to the main excitation coil  207 . 
     
       
         
           
             
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     Note that η 1  is a thermal conversion efficiency of the main excitation coil  207 , and η 2  is a thermal conversion efficiency of the auxiliary excitation coil  215 . 
       FIG. 12  is a graph showing a relationship between a ratio of the effective heating length of the auxiliary excitation coil to the main excitation coil and a heat generation amount per unit length generated by the auxiliary excitation coil having the above ratio within an effective heating area of the fixing belt. Note that in  FIG. 12 , a solid line  601  indicates a heat generation amount of a first auxiliary excitation coil, which is second closest to the fixing belt after the main excitation coil, and a solid line  602  indicates a heat generation amount of a second auxiliary excitation coil that is positioned on the first auxiliary excitation coil. A dashed line  603  indicates a heat generation amount required by the main excitation coil. 
     As shown in  FIG. 12 , in order to cause the first auxiliary excitation coil to reliably generate the same amount of heat as the main excitation coil, it is necessary that the effective heating length of the first auxiliary excitation coil is equal to or less than 93% of the effective heating length of the main excitation coil. Similarly, if the effective heating length of the second auxiliary excitation coil is equal to or less than 86% of the effective heating length of the main excitation coil, the second auxiliary excitation coil reliably generates the same amount of heat as the main excitation coil. 
       FIG. 13  is a plan view showing a shape of the auxiliary excitation coil  215 . As shown in  FIG. 13 , the auxiliary excitation coil  215  has a center hole in a plan view, and a width W 2  of the center hole is smaller than a width W 1  of the center hole. The width W 1  is a width of a central position of the center hole in a longitudinal direction thereof, and the width W 2  is a Width of an end portion of the center hole in the longitudinal direction. The heat generation efficiency of the auxiliary excitation coil  215  is higher at the center hole with a larger width, and lower at the center hole with a smaller width. Therefore, when the auxiliary excitation coil  215  heats the fixing belt  206  by electromagnetic induction, temperature gradient at a boundary between an effective heating area of the auxiliary excitation coil  215  and outside thereof is mild. 
       FIG. 14  is a graph showing a temperature distribution during electromagnetic induction heating by the main excitation coil  207  and a temperature distribution during electromagnetic induction heating by the auxiliary excitation coil  215  in the rotational axis direction of the fixing belt  206 . Note that a dashed line  801  indicates a temperature distribution in the case of the main excitation coil  207 , and a solid line  802  indicates that a temperature distribution in the case of the auxiliary excitation coil  215 . 
     As shown in  FIG. 14 , when the main excitation coil  207  generates heat by electromagnetic induction, the region where the maximum-size recording sheets pass through is at substantially the fixing temperature. In addition, when the auxiliary excitation coil  215  generates heat by electromagnetic induction, the region where the small-size recording sheets pass through is at substantially the fixing temperature. On the other hand, a temperature outside of the region is low and accordingly overheating can be prevented. Also, the auxiliary excitation coil  215  has milder temperature gradient outside the region where recording sheets pass through than the main excitation coil  207 . Thereby, it is possible to prevent undesired variations in fixing that is caused by a difference in luster level when the maximum-size recording sheets pass through after the small-size recording sheets pass through. 
     Thus, the fixing device pertaining to the present embodiment can effectively prevent overheating in the regions where recording sheets do not pass through. In addition, reduction in size, weight, and cost of the fixing device can be realized without problems such as undesired variations in fixing and reduction of heat generation efficiency. 
     [3] Modifications 
     The present invention has been described based on the above embodiment. However, it is natural that the contents of the present invention are not limited to the above embodiment. For example, the following modifications are possible. 
     (1) The above embodiment has explained the case where one of the main excitation coil  207  and the auxiliary excitation coil  215  is used in accordance with a size of recording sheets to be passed. The present invention is of course not limited to this. For example, the following structure is acceptable. 
       FIG. 15  is an external view of a main structure of the fixing device pertaining to the present modification. In the following description, a member that has been described in the above embodiment is referred to by the same reference sign. As shown in  FIG. 15 , a fixing device  9  pertaining to the present modification includes, in addition to substantially the same structure as the fixing device  115  pertaining to the above embodiment, a temperature detecting element  901  for monitoring a surface temperature of the fixing belt  206  in the regions where recording sheets do not pass through. 
     When images are being fixed on small-size recording sheets, the controller (not illustrated) refers to the surface temperature of the fixing belt  206  in the region where the small-size recording sheets do not pass through, which is monitored by the temperature detecting element  901  and connects one of the main excitation coil  207  and the auxiliary excitation coil  215  to the high-frequency inverter  403 . Note that instead of the temperature detecting element  901 , another temperature sensor may be used. 
       FIG. 16  is a flowchart showing control of power supply to the main excitation coil  207  and the auxiliary excitation coil  215 , which is performed by a controller pertaining to the present modification. As shown in  FIG. 16 , the controller checks a size of recording sheets prior to fixing. In the case of the maximum-size recording sheets whose width corresponds to the effective heating length of the main excitation coil  207  (S 1000 : Maximum), processing for the maximum-size recording sheets is performed (S 1001 ). In the case of recording sheets having a middle size (hereinafter, middle-size recording sheets) whose width is smaller than the effective heating length of the main excitation coil  207  and larger than the effective heating length of the auxiliary excitation coil  215  (S 1000 : Middle), processing for the middle-size recording sheets is performed (S 1002 ). In the case of small-size recording sheets whose width corresponds to the effective heating length of the auxiliary excitation coil  215  (S 1000 : Small), processing for the small-size recording sheets is performed (S 1003 ). 
       FIG. 17  is a flowchart showing processing for the maximum-size recording sheets. As shown in  FIG. 17 , in the processing for the maximum-size recording sheets, firstly, the high-frequency inverter  403  is connected to the main excitation coil  207  to supply power (S 1100 ). After fixing images on the maximum-size recording sheets (S 1101 : YES), the processing ends. 
     When fixing continues (S 1101 : NO), the temperature detecting element  208  monitors a temperature t of the region where recording sheets pass through on the fixing belt  206  (S 1102 ). If the temperature t is lower than a reference temperature T 1  (S 1103 : NO), power supply to the main excitation coil  207  continues. Here, the reference temperature T 1  is higher than but close to the fixing temperature in the range where abnormal fixing does not occur. When the temperature t of the region where the recording sheets pass through is higher than the reference temperature T 1  (S 1103 : YES), power supply to the main excitation coil  207  stops (S 1104 ). After fixing images on the maximum-size recording sheets (S 1105 : YES), the processing ends. 
     When fixing continues (S 1105 : NO), the temperature t of the region where the recording sheets pass through is monitored (S 1106 ). When the temperature t of the region where the recording sheets pass through is higher than a reference temperature T 2  (S 1107 : NO), power supply to the main excitation coil  207  remains stopped. When the temperature t of the region where the recording sheets pass through is lower than the reference temperature T 2  (S 1107 : YES), power is supplied to the main excitation coil  207  (S 1100 ). Here, the reference temperature T 2  is lower than and close to the fixing temperature in the range where abnormal fixing does not occur. When images are being fixed on the maximum-size recording sheets, the temperature of the fixing belt  206  is kept substantially at the fixing temperature. 
       FIG. 18  is a flowchart showing processing for the middle-size recording sheets. As shown in  FIG. 18 , also in the processing for the middle-size recording sheets, firstly, the high-frequency inverter  403  is connected to the main excitation coil  207  to supply power (S 1200 ). After fixing images on the middle-size recording sheets (S 1201 : YES), the processing ends. 
     When fixing continues (S 1201 : NO), the temperature detecting element  901  monitors a temperature t of the region where recording sheets do not pass through on the fixing belt  206  (S 1202 ). If the temperature t is lower than a reference temperature T 3  (S 1203 : NO), power supply to the main excitation coil  207  continues. Here, the reference temperature T 3  is lower than a temperature of the regions where the middle-size recording sheets do not pass through in an overheated state. On the other hand, a temperature t of the region where the recording sheets pass through is higher than the reference temperature T 3  (S 1203 : YES), power supply to the main excitation coil  207  stops and power is supplied to the auxiliary excitation coil  215  (S 1204 ). 
     Thereby, overheating in the region where the middle-size recording sheets do not pass through can be prevented. After fixing images on the middle-size recording sheets (S 1205 : YES), the processing ends. When fixing continues (S 1205 : NO), the temperature t of the region where the recording sheets pass through is monitored (S 1206 ). When the temperature t of the region where the recording sheets pass through is lower than the reference temperature T 1  (S 1207 : NO), power supply to the main excitation coil  215  continues. When a temperature t of the regions where the recording sheets do not pass through is higher than the reference temperature T 1  (S 1207 : YES), power supply to the auxiliary excitation coil  215  stops (S 1208 ). 
     In this case, power is also not supplied to the main excitation coil  207 . Thereby, it is possible to prevent the region where the recoding sheets pass through from departing from the fixing temperature and then being overheated. After fixing images on the middle-size recording sheets (S 1209 : YES), the processing ends. When fixing continues (S 1209 : NO), a temperature t of the region where the recording sheets pass through is monitored (S 1210 ). When the temperature t of the region where the recording sheets pass through is higher than the reference temperature T 2  (S 1211 : NO), power supply continues to be stopped. When the temperature t of the regions where the recording sheets do not pass through is lower than the reference temperature T 2  (S 1211 : YES), power supply to the main excitation coil  207  resumes (S 1200 ). 
     Thereby, in the case where a width of recording sheets is smaller than the effective heating length of the main excitation coil and larger than the effective heating length of the auxiliary excitation coil, like the middle-size recording sheets, overheating in the regions where recording sheets do not pass through can be prevented while keeping the temperature of the region where the middle-size recording sheets pass through at the fixing temperature. 
       FIG. 19  is a flowchart showing processing for the small-size recording sheets. As shown in  FIG. 19 , in the processing for the small-size recording sheets, processing that is similar to the processing for the maximum-size recording sheets is performed. A difference is that the auxiliary excitation coil  215  is used instead of the main excitation coil  207 . Firstly, the high-frequency inverter  403  is connected to the auxiliary excitation coil  215  to supply power (S 1300 ). After fixing images on the small-size recording sheets (S 1301 : YES), the processing ends. 
     When fixing continues (S 1301 : NO), the temperature detecting element  208  monitors a temperature t of the region where recording sheets pass through on the fixing belt  206  (S 1302 ). If the temperature t is lower than the reference temperature T 1  (S 1303 : NO), power supply to the auxiliary excitation coil  215  continues. When the temperature t of the region where the recording sheets pass through is higher than the reference temperature T 1  (S 1303 : YES), power supply to the auxiliary excitation coil  215  stops (S 1304 ). After fixing images on the small-size recording sheets (S 1305 : YES), the processing ends. 
     When fixing continues (S 1305 : NO), the temperature t of the region where the recording sheets pass through is monitored (S 1306 ). When the temperature t of the region where the recording sheets pass through is higher than the reference temperature T 2  (S 1307 : YES), power supply to the auxiliary excitation coil  215  remains stopped. When the temperature t of the region where the recording sheets pass through is lower than the reference value T 2  (S 1307 : NO), power is supplied to the auxiliary excitation coil  215  (S 1300 ). When images are being fixed on the small-size recording sheets, a temperature of the fixing belt  206  in the region where small-size recording sheets pass through remains substantially at the fixing temperature, as described above. Note that, the reference temperature T 2  is a predetermined temperature lower than the reference temperature T 1 . 
     (2) The above embodiment has described the case where overheating in the region where recording sheets do not pass through is prevented by combining the main excitation coil and the auxiliary excitation coil. The present invention is of course not limited to this. In addition to the above, a demagnetization coil may be combined. 
       FIG. 20  shows a main structure of a fixing device according to the present modification. As shown in  FIG. 20 , a fixing device  14  includes demagnetization coils  1401  layered on the both end portions of the main excitation coil  207  in a rotational axis direction of the fixing belt  206 . The demagnetization coils  1401  are provided at positions corresponding to the regions where the middle-size recording sheets do not pass through. The demagnetization coils  1401  are each connected to a switch under control of the controller. The switch is ON when images are fixed on the middle-size recording sheets so that demagnetization effect of the demagnetization coils  1401  works, and the switch is OFF when images are fixed on the maximum-size recording sheets or the small-size recording sheets so that the demagnetization effect of the demagnetization coils  1401  does not work. 
     Thereby, even when the above modification (1) cannot control overheating in the regions where the recording sheets do not pass through, it is possible to prevent overheating in the regions where the recording sheets do not pass through with use of the demagnetization coils. 
     (3) The above embodiment has described the case of using a single auxiliary excitation coil. The present invention is of course not limited to this. A plurality of auxiliary excitation coils may be used in accordance with the number of sizes of fed recording sheets. 
     [4] Features and Effects of the Present Invention 
     A fixing device of the present invention includes a fixing rotational body and fixes toner images on recording sheets of various sizes by using the fixing rotational body, the fixing device comprising: a main excitation coil that heats the fixing rotational body by electromagnetic induction and has an effective heating length L 1  corresponding to a recording sheet of a maximum size; an auxiliary excitation coil that heats the fixing rotational body by electromagnetic induction and has an effective heating length L 2  that is shorter than the effective heating length L 1  of the main excitation coil; a high-frequency power source that supplies power to the main excitation coil and the auxiliary excitation coil; and a switch that selectively connects the main excitation coil and the auxiliary excitation coil to the high-frequency power source, wherein the main excitation coil is positioned along a part of an outer circumferential surface of the fixing rotational body, the auxiliary excitation coil is positioned farther from the fixing rotational body than the main excitation coil is and layered on a substantially central portion of the main excitation coil in a longitudinal direction of the main excitation coil, and the effective heating length L 2  of the auxiliary excitation coil satisfies the following relationship: L 2 ≦L 1 ·η 2 /η 1 , where η 1  is a thermal conversion efficiency of the main excitation coil and η 2  is a thermal conversion efficiency of the auxiliary excitation coil. 
     Thereby, since the auxiliary excitation coil is provided outside the main excitation coil as viewed from the fixing rotational body, and layered substantially on the central portion of the main excitation coil in the longitudinal direction thereof, overheating in the regions where recording sheets do not pass through can be effectively prevented, and reduction in size, weight, and cost of the fixing device can be realized without problems such as uneven temperature distribution and reduction of heat generation efficiency. 
     In this case, the auxiliary excitation coil is provided in plurality, the plurality of the auxiliary excitation coils may have effective heating lengths that are different from each other, and the plurality of the auxiliary excitation coils may be layered on the main excitation coil so that the effective heating lengths decrease with distance from the main excitation coil. Thereby, when the maximum-size recording sheets that consume the most energy (power) are fed, higher heat generation efficiency can be retained and heat energy supplied to the recording sheets is guaranteed. 
     Also, the auxiliary excitation coil has a center hole, and a width of the center hole in a circumferential direction of the fixing rotational body at each end portion of the center hole in a rotational axis direction of the fixing rotational body may be smaller than a width of the center hole in the circumferential direction at a central portion of the center hole in the rotational axis direction. Thereby, when the small-size recording sheets are fed, rapid temperature change occurring at the both ends of the region where small-size recording sheets pass through can be suppressed, and accordingly, when recording sheets having a large size are fed after that, uneven fixation (glossiness) can be prevented. 
     Also, the switch may connect, to the high-frequency power source, one of the main excitation coil and the auxiliary excitation coil whose effective heating length is closer to a width of a fed recording sheet on which toner images are to be fixed than an effective heating length of the other. Thereby, overheating in the regions where the recording sheets do not pass through can be prevented. 
     Also, in order to raise a temperature of the fixing rotational body, the switch may connect, to the high-frequency power source, one of the main excitation coil and the auxiliary excitation coil whose effective heating length is longer than a width of a fed recording sheet on which toner images are to be fixed, and in order to reduce a temperature of the fixing rotational body, the switch may connect, to the high-frequency power source, one of the main excitation coil and the auxiliary excitation coil whose effective heating length is shorter than the width of the fed recording sheet on which toner images are to be fixed. Thereby, even in the case where an auxiliary excitation coil having an effective heating length that matches a width of fed recording sheets is not provided, overheating can be prevented by monitoring a temperature of the regions where the recording sheets do not pass through and switching excitation coils. 
     Also, a fixing device that includes a fixing rotational body and fixes toner images on recording sheets of various sizes by using the fixing rotational body, the fixing device comprising: a main excitation coil that heats the fixing rotational body by electromagnetic induction and has an effective heating length L 1  corresponding to a recording sheet of a maximum size; an auxiliary excitation coil that heats the fixing rotational body by electromagnetic induction and has an effective heating length L 2  that is shorter than the effective heating length L 1  of the main excitation coil; a high-frequency power source that supplies power to the main excitation coil and the auxiliary excitation coil; and a switch that selectively connects the main excitation coil and the auxiliary excitation coil to the high-frequency power source, wherein the main excitation coil is positioned along a part of an outer circumferential surface of the fixing rotational body, the auxiliary excitation coil is positioned farther from the fixing rotational body than the main excitation coil is and layered on a substantially central portion of the main excitation coil in a longitudinal direction of the main excitation coil, and the effective heating length L 2  of the auxiliary excitation coil satisfies the following relationship: L 2 ≦L 1 ·η 2 /η 1 , where η 1  is a thermal conversion efficiency of the main excitation coil and η 2  is a thermal conversion efficiency of the auxiliary excitation coil. If the effective heating length L 2  of the auxiliary excitation coil satisfies the above range, it is possible to cause the auxiliary excitation coil to reliably generate the same amount of heat as the main excitation coil, even if an amount of power supply to the auxiliary excitation coil is not larger than an amount of power supply to the main excitation coil. 
     An image forming apparatus pertaining to the present invention includes the fixing device pertaining to the present invention. Thereby, an effect of the fixing device pertaining to the present invention can be obtained. 
     Although the present invention has been fully described by way of examples with reference to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art. 
     Therefore, unless otherwise such changes and modifications depart from the scope of the present invention, they should be construed as being included therein.