Abstract:
An image heating apparatus includes a rotatable member; a first heater and a second heater, for heating the rotatable member, wherein a ratio of a heat generation amount of the second heater at a end portion thereof to that at a central portion thereof is larger than that of the second heater at a end portion thereof to that at a central portion thereof with respect to the generatrix direction; a back-up member; a sensor, and an electric power supply controller capable of controlling the first heater and the second heater, independently. The controller effects switching to decrease a ratio of the power to the second heater to the power to the first heater, when heating a small width sheet. Immediately after the switching, the power to the second heater is smaller and the power to the first heater is larger than those immediately before the switching.

Description:
FIELD OF THE INVENTION AND RELATED ART 
       [0001]    The present invention relates to an image heating apparatus (device) with which an image forming apparatus such as an electrophotographic copying machine, an electrophotographic printer, and the like, is equipped. 
         [0002]    There have been known various image heating apparatuses (devices) which are mountable as a fixing device in an image forming apparatus such as a copying machine, a laser beam printer, and the like. One of them is an image heating apparatus (device) which employs a heating film, and is known to virtually instantly startup. Generally speaking, it is made up of a ceramic heater (as heat source), a fixation film, and a pressure roller. The pressure roller is pressed against the ceramic heater, with the presence of the fixation film between the pressure roller and ceramic heater, forming thereby a nip between itself and heater. In operation, the film is circularly driven in contact with the heater and pressure roller. While the fixation film is circularly driven, a sheet of recording medium on which an unfixed toner image is present is conveyed through this nip, while being subjected to heat and pressure. Consequently, the unfixed toner image on the sheet of recording medium becomes thermally fixed to the sheet. 
         [0003]    A fixing device such as the above described one which employs a heating film (fixation film) has been know to suffer from the problem that in a case where a substantial number of small sheets of recording medium, that is, sheets of recording medium which are narrower than the heat generation range of the heater of the fixing device, are continuously conveyed for fixation through its fixation nip, the portions of the ceramic heater, fixation film pressure roller, etc., which are outside the path of the small sheets of recording medium, are likely to unnecessarily increase in temperature. 
         [0004]    Thus, there have been proposed various fixing apparatuses (devices) capable of dealing with the above-described problem. One of them is disclosed in Japanese Laid-open Patent Application H10-177319. It is structured so that when a substantial number of prints of a small size are continuously outputted, the portions of the heater, which are outside the sheet path, are reduced in the amount of heat generation, compared to the portion of the heater, which are within the sheet path. More specifically, the fixing device is provided with a heater and a heater driving circuit. The heater has multiple heat generating resistors which are different in heat generation pattern in terms of the direction (lengthwise direction) perpendicular to the recording medium conveyance direction. The multiple heat generating resistors are disposed in parallel in the recording medium conveyance direction. Further, the heater driving circuit is enabled to control in duty the power supply to each heat generating resistor, in order to control each heat generation resistor in such a manner that, they are changed in the gradient of their heat generation pattern, in terms of the lengthwise direction, according to the size of a sheet of recording medium used for image formation. 
         [0005]    More concretely, when a large sheet of recording medium is used for image formation, both the duty of the power supply to the first heat generating resistor  610 , and that of the power supply to the second heat generating resistor  620  are set to 40% (200 W in heat generation amount). On the other hand, when a small sheet of recording medium is used for image formation, the duty of the power supply to the first heat generating resistor  610  is set to 50% (250 W in power generation amount), and the duty of the power supply to the second heat generating resistor  620  is set to 20% (100 W in heat generation amount). 
       SUMMARY OF THE INVENTION 
       [0006]    According to an aspect of the present invention, there is provided an image heating apparatus for heating a recording material having a toner image formed thereon while feeding the recording material, in a nip, said apparatus comprising: 
         [0000]    a rotatable member;
 
a first heat generating element and a second heat generating element, for heating said rotatable member, said heat generating elements being elongated in a generatrix direction of said rotatable member, wherein a ratio of a heat generation amount of said second heat generating element at a end portion thereof to that at a central portion thereof with respect to the generatrix direction is larger than a ratio of a heat generation amount of said second heat generating element at a end portion thereof to that at a central portion thereof with respect to the generatrix direction;
 
a back-up member contacting said rotatable member to form the nip;
 
a temperature detecting portion, and
 
a controller for controlling an electric power supplied to said first heat generating element and said second heat generating element in accordance with a detected temperature acquired by said temperature detecting portion, said controller being capable of controlling the electric power to said first heat generating element and the electric power to said second heat generating element, independently from each other;
 
wherein said controller effects switching to decrease a ratio of the electric power to said second heat generating element to the electric power to said first heat generating element, when said apparatus heats a recording material having a width smaller than a maximum width usable with said apparatus, and wherein immediately after the switching by said controller, the electric power to said second heat generating element is smaller and the electric power to said first heat generating element is larger than immediately before the switching.
 
         [0007]    According to another aspect of the present invention, there is provided an image heating apparatus for heating a recording material having a toner image formed thereon while feeding the recording material, in a nip, said apparatus comprising: 
         [0000]    a rotatable member;
 
a first heat generating element and a second heat generating element, for heating said rotatable member, said heat generating elements being elongated in a generatrix direction of said rotatable member, wherein a ratio of a heat generation amount of said second heat generating element at a end portion thereof to that at a central portion thereof with respect to the generatrix direction is larger than a ratio of a heat generation amount of said second heat generating element at a end portion thereof to that at a central portion thereof with respect to the generatrix direction;
 
a back-up member contacting said rotatable member to form the nip;
 
a temperature detecting portion for detecting a temperature of said rotatable member or said heat generating element; and
 
a controller for controlling electric power supplied to said heat generating element so that a detected temperature of said temperature detecting portion is at a target temperature, said controller being capable of controlling the electric power to said first heat generating element and the electric power to said second heat generating element, independently from each other;
 
wherein said controller effects switching to decrease a ratio of the electric power to said second heat generating element to the electric power to said first heat generating element, when said apparatus heats a recording material having a width smaller than a maximum width usable with said apparatus, and wherein said controller effects the control so that a sum of the electric power supplied to said first heat generating element and the electric power supplied to said second heat generating element immediately before the switching is the same as the sum immediately after the switching.
 
         [0008]    According to a further aspect of the present invention, there is provided an image heating apparatus for heating a recording material having a toner image formed thereon while feeding the recording material, in a nip, said apparatus comprising: 
         [0000]    a rotatable member;
 
a first heat generating element and a second heat generating element, for heating said rotatable member, said heat generating elements being elongated in a generatrix direction of said rotatable member, wherein a ratio of a heat generation amount of said second heat generating element at a end portion thereof to that at a central portion thereof with respect to the generatrix direction is larger than a ratio of a heat generation amount of said second heat generating element at a end portion thereof to that at a central portion thereof with respect to the generatrix direction;
 
a back-up member contacting said rotatable member to form the nip;
 
a temperature detecting portion, and
 
a controller for controlling an electric power supply duty cycle which is electric power supply time per unit time to said heat generating element, on the basis of a detected temperature of said temperature detecting portion, said controller being capable of controlling the duty cycle for said first heat generating element and the duty cycle for said second heat generating element, independently from each other, wherein said controller effects switching to decrease a ratio of the electric power supply duty cycle to said second heat generating element to the electric power supply duty cycle to said first heat generating element, when said apparatus heats a recording material having a width smaller than a maximum width usable with said apparatus, and
 
         [0009]    Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings). 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a drawing which shows the temperature of the fixation heater before and after the power supply to the fixation heater was switched in duty to control in temperature the fixation heater of the image heating device in the first embodiment of the present invention. 
           [0011]      FIG. 2  is a schematic sectional view of the image forming apparatus equipped with the image heating apparatus in the first embodiment of the present invention, and shows the structure of the image forming apparatus. 
           [0012]      FIG. 3  is a schematic sectional view of the fixing device (as image heating apparatus) in the first embodiment, and shows the structure of the fixing device. 
           [0013]      FIGS. 4(   a ) and  4 ( b ) are schematic sectional and plan view of the fixation heater in the first embodiment. 
           [0014]      FIG. 5  is a drawing for showing the heat generation pattern of the fixation heater in the first embodiment, in terms of the lengthwise direction of the heater. 
           [0015]      FIG. 6  is a schematic drawing of the fixation heater driving circuit in the first embodiment. 
           [0016]      FIG. 7  is a drawing which shows the heat generation pattern of the fixation heater in the first embodiment, in terms of the lengthwise direction of the heater. 
           [0017]      FIG. 8  is a drawing which shows the proportional control component of the PI control in the first embodiment. 
           [0018]      FIG. 9  is a flowchart of the heater drive control in the first embodiment. 
           [0019]      FIG. 10  is a drawing for schematically showing the temperature dependency of the viscosity of melted toner. 
           [0020]      FIG. 11  is a drawing which shows a temperature range in which the target temperature for the fixing device can be set. 
           [0021]      FIG. 12  is a drawing which shows the temperature and power supply duty before and after the switching of the power supply duty ratio, in the conventional fixation heater drive control. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiment 1 
       [0022]    Hereinafter, some of the preferred embodiments of the present invention are described in detail with reference to appended drawings. However, the measurements, materials, and shapes of the structural components of the apparatuses and devices in the following embodiments of the present invention, and the positional relationship among the structural components, are not intended to limit the present invention in scope. That is, they are to be modified as necessary according to the structure of the apparatuses and devices to which the present invention is applied, and also, according to various conditions under which the apparatus and devices are used. 
       (Image Forming Apparatus) 
       [0023]    First, referring to  FIG. 2 , the image forming apparatus equipped with a fixing device (as image heating device) which is in accordance with the present invention is described about its overall structure.  FIG. 2  is a schematic sectional view of a full-color printer (which hereafter will be referred to simply as printer  10 ), which is an example of an image forming apparatus to which the present invention is applicable. It shows the overall structure of the apparatus. Incidentally, the image forming apparatus in this embodiment is a full-color printer having multiple photosensitive drums. However, this embodiment is not intended to limit the present invention in terms of the number of photosensitive drums. That is, the present invention is also applicable to a monochromatic copying machine, a monochromatic printer, and the like, which is equipped with only one photosensitive drum. 
         [0024]    The printer  10  has a cassette  11 , which is in the bottom portion of the printer. The cassette  11  can be pulled out of the printer. It can store in layers a substantial number of sheets of recording medium. The sheets of recording medium in the cassette  11  are pulled out of the cassette  11  by a pickup roller  13  while being separated by a pair of rollers  14  (feed roller and retard rollers). Then, each sheet is conveyed to a pair of registration rollers  15 . The printer  10  is also provided with an image forming portion  7 , as an image forming means, having image formation stations  7 Y,  7 M,  7 C and  7 K which are for yellow, magenta, cyan and black colors, respectively, and which are aligned parallel in tandem in the recording medium conveyance direction. 
         [0025]    The image forming portion  7  has: photosensitive drums  1 Y,  1 M,  1 C and  1 K (which hereafter will be referred to together simply as photosensitive drum  1 ); charging device  2 Y,  2 M,  2 C and  2 K for uniformly charging the peripheral surface of the photosensitive drum  1 ; and developing devices  4 Y,  4 M,  4 C and  4 K. There are development rollers  5 Y,  5 M,  5 C and  5 K in the developing devices  4 Y,  4 M,  4 C and  4 K, respectively, The development rollers  5  are for adhering toner to the electrostatic latent image on the photosensitive drum  1  to develop the toner image. 
         [0026]    The image forming portion  7  has also primary transfer portion  8 Y,  8 M,  8 C and  8 K (which hereafter will be referred to together as primary transfer portion  8 ), which transfer the toner image on the photosensitive drum  1  onto an electrostatic transfer belt  29 . Further, the image forming portion  7  has cleaning blades  6 Y,  6 M,  6 C and  6 K which remove the toner which has failed to be transferred in the primary transfer portion  8 , being therefore remaining on the photosensitive drum  1  after the primary transfer. The printer  10  is provided with scanner units  3 YM and  3 CK, which is on the bottom side of the image forming portion  7 . The scanner units  3 YM and  3 CK project a beam of laser light while modulating the beam according to the information of the image to be formed, in order to form an electrostatic latent image on the peripheral surface of the photosensitive drum  1 . 
         [0027]    After the transfer of the toner image onto the transfer belt  29 , in the primary transfer portion  8 , the toner image on the transfer belt  29  is transferred by a secondary transfer roller  31  onto a sheet P of recording medium, in the secondary transfer portion M formed by a belt-backing roller  30  and the secondary transfer roller  31 . The secondary transfer residual toner, that is, the toner having failed to be transferred onto the sheet P of recording medium, in the secondary transfer portion M, and therefore remaining on the transfer belt  29 , is removed and recovered by a belt cleaning device  34 . After being conveyed through the secondary transfer portion M, the sheet P is conveyed through a fixing device  12 , in which the toner image is fixed to the surface of the sheet P. 
         [0028]    After the fixation of the toner image to the sheet P of recording medium, the sheet P is conveyed to a pair of discharge rollers  32 . Then, the sheet P is discharged by the pair of discharge roller  33 , through the interface between the pair of discharge roller, into a delivery tray, in a manner to be accumulated therein. Incidentally, in terms of the direction (which hereafter may be referred to simply as lengthwise direction) perpendicular to the recording medium conveyance direction, the widths of the widest and narrowest sheets of recording medium conveyable through the printer  10  in this embodiment are 297 mm and 76 mm, respectively. Also in terms of the lengthwise direction, the point of reference for positioning a sheet of recording medium, relative to the printer, is roughly the center of the printer (central reference). 
       (Image Heating Apparatus (Device)) 
       [0029]    Next, referring to  FIG. 3 , the fixing device  12 , as an image heating apparatus in accordance with the present invention, is described. The fixing device  12  in this embodiment is of the so-called heating film type, and also, of the so-called tensionless type (heating film is driven by pressure roller which presses heating film). 
       1) Overall Structure 
     1-a) Heat Source (Heater) 
       [0030]    A referential numeral  16  stands for a fixation heater as a heat source (heat generating member), which is made up of a substrate and heat generating resistors. The substrate is formed of ceramic or the like substance. The heat generating resistors are formed on the substrate. The fixation heater  16  generates heat as it is supplied with electric power by a heater driving circuit  21 , which will be described later. The details of the heater will be described later. 
       1-b) Heater Holder 
       [0031]    A referential code  17  stands for a heater holder as a supporting member. It is roughly semicircular in cross-section. The heater holder  17  is made of highly heat resistant liquid polymer. It supports the heater  16 , and guides the fixation sleeve  20  by the inward surface of the fixation sleeve  20 . The liquid polymer used as the material for the heater holder  17  is Sumikasuper E5204L (commercial name) produced by Sumitomo Chemical Co., Ltd. The heater  16  is supported by the heater seating portion of the heater holder  17 , in such a manner that its long edges become parallel to the long edges of the heater holder  17 . The fixation sleeve  20  is loosely fitted around the combination of the heater  16  and heater holder  17 , making up a fixation sleeve unit. 
       1-c) Temperature Detecting Portion (Thermistor) and Heater Driving Circuit 
       [0032]    A referential code  18  stands for a thermistor for detecting the temperature of the fixation sleeve  20 , which will be described later. In this embodiment, the thermistor  18  is disposed so that its temperature detecting portion contacts the inward surface of the fixation sleeve  20 . The temperature information of the fixation sleeve  20  is transmitted to the heater driving circuit  21 . The thermistor  19  is for detecting the temperature of the heater  16 . In this embodiment, the image heating device  12  is provided with two thermistors  19 , which are disposed on the opposite surface of the heater  16  from the fixation sleeve  20 . In terms of the lengthwise direction, one of the thermistors  19  is at the center of the heater  16 , and the other is at one of the end portions of the heater  16 . 
         [0033]    In terms of the lengthwise direction, the thermistor  19   a  ( FIG. 6 ) is at the center of the heater  16 , whereas the thermistor  19   b  ( FIG. 6 ) is at one of the lengthwise ends of the heater  16 . It is disposed so that it is within the path of the largest sheet (size SRA3, which is 320 mm in width) of recording medium conveyable through the fixing device  12 , and on the outward side of the path of the smallest sheet (size LTR, which is 279 mm in landscape attitude) of recording medium conveyable through the fixing device  12 . The temperature information of the heater  16  is transmitted to the heater driving circuit  21 . 
         [0034]    A referential code  21  stands for the heater driving circuit, which controls the amount by which electric power is supplied to the heater  16  to control the amount by which the heater  16  generates heat. Basically, it keeps the temperature of the heater  16  or fixation sleeve  20  at a preset level, by controlling the amount by which electric power is supplied to the heater  16 , based on the temperature information transmitted thereto from the thermistors  18  and  19 . The heater driving circuit  21  is described later in detail. 
       1-d) Rotational Heating Member 
       [0035]    A referential code  20  stands for the fixation sleeve (fixation film) as a rotational heating member. It is a cylindrical component (endless belt) made up of a cylindrical substrate, an elastic layer, and a surface layer. More specifically, the substrate is a cylindrical metallic endless belt, which is 24 mm in internal diameter, and 30 μm in thickness. It is made of stainless steel or the like. The elastic layer is made of silicone rubber, and covers the outward surface of the substrate. It is roughly 300 μm in thickness. The surface layer covers the elastic layer. It is a piece of PFA resin tube. It is 30 μm in thickness. Regarding the external shape of the fixation sleeve  20 , the fixation sleeve  20  in this embodiment is uniform in external diameter. However, the shape of the fixation sleeve  20  may be such that its cross-section, at a plane which coincides with its axial line, is in the form of an inverted crown, that is, its center portion is different in external diameter from its lengthwise end portions. 
       1-e) Pressure Bearing Member (Belt-backing Member) and Pressure Application Mechanism 
       [0036]    A referential code  22  stands for a pressure roller as a pressure bearing member (belt-backing member). The pressure roller  22  is made up of a metallic core, an elastic layer, and a piece of PFA resin tube. The metallic core is made of stainless steel. The elastic layer is formed of silicone rubber, by injection molding, in a manner to cover the peripheral surface of the metallic core. It is roughly 3 mm in thickness. The PFA resin tube is roughly 40 μm in thickness. It covers the silicone rubber layer. The pressure roller  22  in this embodiment is 25 mm in external diameter. It is rotatably supported by the unshown rear and front lateral plates of the frame of the fixing device  12 . More specifically, the lengthwise ends of the metallic core are rotatably supported by a pair of bearings attached to the rear and front lateral plates, one for one. 
         [0037]    The above-described fixation sleeve unit is on the bottom side of the pressure roller  22 , being positioned in parallel to the pressure roller  22 , with its heater side facing upward. It is kept pressured toward the axial line of the pressure roller  22  by the pressure (147 N (15 kgf) per lengthwise end; 294 N (30 kgf) in total) applied to the lengthwise ends of the heater holder  17  by an unshown pressure application mechanism. Thus, the upwardly facing surface of the heater  16  is pressed by the application of a preset amount of pressure, against the pressure roller  22 , with the presence of the fixation sleeve  20  between the heater  16  and pressure roller  22 , while being subjected to the resiliency of the elastic layer of the pressure roller  22 . Consequently, a fixation nip N with a preset width, which is necessary for thermal fixation, is formed. 
         [0038]    The pressure roller  22  is rotationally driven by an unshown driving means at a preset peripheral velocity in the clockwise direction indicated by an arrow mark. Thus, the cylindrical fixation sleeve  20  is subjected to the rotational friction generated between the peripheral surface of the pressure roller  22  and fixation sleeve  20 , in the fixation nip N, by the rotation of the pressure roller  22 . Thus, the fixation sleeve  20  is rotated by the rotation of the pressure roller  22  in the counterclockwise direction indicated by another arrow mark, around the heater holder  17 , with its inward surface sliding on (while remaining in contact with) the upwardly facing surface of the heater  16 . The inward surface of the fixation sleeve  20  is coated with grease to minimize the friction between the heater holder  17  and the inward surface of the fixation sleeve  20 . 
         [0000]    1-f) Recording Medium Conveyance through Nip 
         [0039]    It is after the fixation sleeve  20  begins to be rotationally driven by the rotation of the pressure roller, and the temperature of the heater  16  has been increased to a preset level, and kept at the preset level, by the electric power supplied to the heater  16 , that a sheet P of recording medium begins to be conveyed through the nip N while remaining pinched between the pressure roller  22  and fixation sleeve  20 . That is, a sheet P of recording medium, by which an unfixed toner image is borne, is introduced between the fixation sleeve  20  and pressure roller  22 , in the nip N, while being guided by an entrance guide  23 . Then, the sheet P is moved with the fixation sleeve  20  through the nip N, with the toner image bearing surface of the sheet P being kept in contact with the outward surface of the fixation sleeve  20 . 
         [0040]    While the sheet P of recording medium is conveyed through the nip N, the heat from the heater  16  is given to the sheet P through the fixation sleeve  20 . Consequently, the unfixed toner image on the sheet P is welded (fixed) to the sheet P by the heat and pressure applied thereto in the nip N. After being conveyed through the nip N, the sheet P is separated from the fixation sleeve  20  by the curvature of the fixation sleeve  20 . Then, it is discharged from the fixing device  12  by a discharge roller  26  of the fixing device  12 . 
         [0041]    Incidentally, referential codes  23  and  26  stand for the entrance guide and discharge roller, respectively, attached to the frame  24  of the fixing device  12 . The entrance guide  23  plays the role of accurately guiding the sheet P after the sheet P is conveyed through the secondary transfer nip. The material for the entrance guide  23  in this embodiment is polyphenylenesulfide (PPS) resin. In this embodiment, the distance from the secondary transfer nip to the fixation nip N is 100 mm. 
       2) Parallelly Disposed Two Heat Generating Resistors 
       [0042]      FIGS. 4(   a ) and  4 ( b ) are schematic sectional and plan views, respectively, of the heater  16 . The heater  16  is provided with multiple (two) heat generating resistors. More concretely, it is provided with first and second heat generating resistors, which are disposed in parallel so that when they are in the hollow of the fixation sleeve  20 , they are perpendicular to the recording medium conveyance direction, and also, so that they can be independently supplied with electric power from each other. The first heat generating resistor is structured so that its lengthwise end portions are smaller in the amount of heat generation than its center portion, whereas the second heat generating resistor is structured so that its lengthwise end portions are greater in the amount of heat generation than its center portion. 
         [0043]    The heater  16  is made up of a substrate  41  and a pair of heat generating resistor layers  42  and  43  (first and second heat generating resistors). The substrate  41  is in the form of a long and narrow rectangular plate. It is formed of ceramic. In this embodiment, the substrate  41  is 370 mm in length, 10 mm in width, and 0.6 mm in thickness. The heat generating resistor layers  42  and  43  are on one of the primary surfaces of the substrate  41 . They extend in the lengthwise direction of the substrate  41 . They are roughly 10 μm in thickness, roughly 1 mm in width (dimension in terms of recording medium conveyance direction), and roughly 303 mm in length). Their material is silver/palladium (Ag/Pd). They are formed by drawing fine lines or narrow stripes on one of the primary surface of the substrate  41 , with electrically conductive paste which contains silver and paradigm, by screen printing. They generate heat as electric current is flowed through them. 
         [0044]    The heater  16  is also provided with electrodes  44   a ,  44   b , and  44   c  formed of silver paste or the like, in a preset pattern, on the substrate  41 , by screen printing, to supply the heat generating resistors  42  and  43  with electric power. Further, the heater  16  is provided with a thin glass coat  45  for protecting and insulating the heat generating resistors  42  and  43 . The glass coat  45  is roughly 30 μm in thickness. Moreover, the heater  16  is provided with a low-friction layer  46 , which is on the opposite surface of the substrate  41  from where the heat generating resistors are located, so that the low-friction layer  46  contacts the inward surface of the fixation sleeve  20 . 
         [0045]    Referring to  FIG. 4 , the electrodes  44  ( 44   a ,  44   b  and  44   c ) of the heater  16  are fitted with power supply connector, through which electric power is supplied to the electrodes  44  from the heater driving circuit  21 , which will be described later. As electric power is supplied to the electrodes  44 , the heat generating resistors  42  and  43  generate heat, and the heater  16  quickly increases in temperature. In this embodiment, the electrode  44   c  is a common electrode. Thus, it is through the electrodes  44   a  and  44   b  that electric power is supplied to the heater  16  to cause the heat generating resistors  42  and  43 , respectively, to generate heat. Thus, the heat generating resistors  42  and  43  can be independently driven from each other. 
         [0046]    In this embodiment, the two heat generating resistors  42  and  43  are different from each other, in heat generation pattern in terms of the lengthwise direction. More specifically, the heat generating resistor  42  is such that its lengthwise end portions are smaller in the amount of heat generation than its lengthwise center portion, whereas the heat generating resistor  43  is such that its lengthwise end portions are greater in the amount of heat generation than its lengthwise center portion. 
         [0047]      FIG. 5  shows the heat generation pattern of each of the heat generating resistors  42  and  43  in terms of the lengthwise direction, and heat generation pattern of the heater  16  (combination of the heat generating resistors  42  and  43 ) in terms of the lengthwise direction. In this embodiment, the design of the heater  16  is such that when the electric power supplied to the heat generating resistor  42  is the same in voltage and duty cycle as the electric power supplied to the heat generating resistor  43 , the heater  16  (combination of heat generating resistors  42  and  43 ) is uniform in heat generation (flat in heat generation pattern in  FIG. 5 ). The heat generating resistors  42  and  43  are 15Ω and 23Ω, respectively, in electrical resistance, and 960 W and 630 W, respectively, in maximum amount of heat generation, when the electric power supplied thereto is 120 V in voltage. 
       3) Heater Driving Circuit and Method for Controlling Heater in Amount of Heat Generation 
       [0048]    Next, referring to  FIGS. 6 and 9 , the details of the heater driving circuit  21  and the method for controlling the heater  21  in the amount of heat generation are described. By the way, in the following description of the embodiments of the present invention, “power supply duty (power supply duty cycle)” means the length of time electric power is actually supplied per unit length of time. “Power supply duty ratio” means the ratio of the duty cycle of the electric power supplied to one of the multiple heat generating resistors of a heater, relative to that of another heat generating resistor. For example, in a case where a heater has two heat generating resistors A and B, which are Da and Db in power supply duty, the power supply duty ratio of the heat generating resistor B, relative to that of the heat generating resistor A is Db/Da. 
         [0049]    Referring to  FIG. 6 , the heater driving circuit  21  is an example of circuit for controlling the power supply to the heater  16 . As the temperature information obtained by the thermistors  18  and  19  is inputted into the CPU  211 , the CPU  211  controls the triacs  212   a  and  212   b  in power delivery timing, in order to keep the temperature of the heater  16  (fixation sleeve  20 ) at a preset level, based on the results of temperature detection by the thermistors  18  and  19 . The triac  212   a  is in connection to the heat generating resistor  42  through the electrode  44   a , whereas the triac  212   b  is in connection to the heat generating resistor  43  through the electrode  44   b.    
         [0050]    Therefore, the CPU  211  can individually control the triacs  212   a  and  212   b  in power supply duty ratios Da and Db, respectively, and therefore, can control the heater  16  in heat generation pattern in terms of the lengthwise direction. The method for controlling the triacs  212   a  and  212   b  in terms of the power supply duties Da and Db, respectively, is described later in detail. 
         [0051]    By connecting the heater  16  to the heater driving circuit  21 , and controlling the triacs  212   a  and  212   b  in power duty ratio Db/Da, it is possible to make the heat generation pattern of the heater  16 , in terms of the lengthwise direction, have a desired gradient.  FIG. 7  shows examples of the heat generation pattern of the heater  16 , in terms of the lengthwise direction, which reflect power supply duty ratio. 
         [0052]    In this embodiment, in a case where the power supply duty ratio Db/Da is one (Db/Da=1/1), the heat generation pattern in the lengthwise direction is flat. In comparison, in a case where Db/Da&lt;1, for example, Db/Da=0.7/1, the heat generation pattern in terms of the lengthwise direction, becomes higher across the center portion (shaped like mountain), where as in a case where Db/Da&gt;1, for example, Db/Da=1/0.7, the heat generation pattern becomes higher across the lengthwise end portions (shaped like valley). 
         [0053]    In this embodiment, the power supply duty ratio of the heat generating resistor  43 , relative to the heat generating resistor  42 , that is, Db/Da, can be switched as shown in Table 1 according to the size of a sheet P of recording medium. That is, in a case where a sheet P of recording medium of size A3, or the widest sheet P of recording medium conveyable through the fixing device  12 , is conveyed through the fixing device  12 , it is possible to set the power supply duty ratio Db/Da to one (Db/Da=1/1) to make the heater  16  flat in heat generation pattern in the lengthwise direction, in order to make the entirety of the path of the sheet P of size A3 uniform in fixation. 
         [0054]    On the other hand, in a case where a small sheet P of recording medium, for example, a sheet P of recording medium of the letter size, the width of which is less than that of the widest sheet P of recording medium conveyable through the fixing device  12 , is conveyed in the portrait attitude through the fixing device  12 , it is possible to set the power supply duty ratio Db/Da to 0.5 (Db/Da=0.5/1 to reduce the heat generating resistor  43  in the amount of heat generation, in order to reduce in the amount of heat generation, the portions of the recording medium passage, through which a sheet P of recording medium of letter size, does not pass. By doing so, it is possible to minimize the unnecessary temperature increase across the out-of-sheet-path portions. 
         [0000]    
       
         
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Ratio Db/Da 
                 Sheet size 
               
               
                   
                   
               
             
             
               
                   
                 1/1 
                 290-300 mm (A3 Portrait, 
               
               
                   
                   
                 A4 Landscape) 
               
               
                   
                 0.5/1   
                 268-289 mm (Ledger Portrait, 
               
               
                   
                   
                 Letter Landscape 
               
               
                   
                   
               
             
          
         
       
     
         [0055]    Next, referring to  FIG. 9  (flowchart), the method for controlling the heater driving circuit  21  in this embodiment is described.  FIG. 9  (flowchart) is related to a case where multiple sheets of recording medium of the letter size are continuously conveyed in the portrait attitude through the fixing device  12 . 
         [0056]    As a print command for a given printing job is inputted into the image forming apparatus, electric power begins to be supplied to the heater  16  in synchronism with the starting of the driving of the fixing device  12  (S 11 ). During the warm-up period, the triacs  212   a  and  212   b  are controlled so that the power supply duty ratio Db/Da remains to be 1 (Db/Da=1/1, Da=100%, Db=100%). 
         [0000]    “Target temperature” means a temperature level to be set as the desired peripheral surface temperature level for the fixation sleeve  20  when the fixing device  12  is used for thermal fixation. “Warm-up period” means the period from when electric power begins to be supplied to the heat generating resistor  42  and/or  43  to when the first sheet P of recording medium, on which an unfixed toner image is present, begins to be conveyed to the fixation nip N. 
         [0057]    By setting the power supply duties Da and Db to 100% during the warm-up period, it is possible to make the heat generating resistors  42  and  43  generate heat at their highest capacity. Therefore, it is possible to increase the heater  16  (film  36 ) in temperature as fast as possible. As the temperature detected by the thermistor  18  reaches 170° C. (S 12 ), an initial offset value, which will be described later, is set to 40%, and PI control is started (S 13 ). 
         [0058]    Then, sheets P of recording medium begin to be fed in S 14 . The timing with which the sheets P begin to be fed may be before the starting of the PI control (S 13 ), as long as it is ensured that the unfixed toner image on the sheet P is satisfactorily fixed. As the trailing edge of the first sheet P comes out of the nip N (S 15 ), it is checked whether or not the power supply duty ratio Db/Da has been switched to 0.5/1 (S 16 ). If the power supply duty ratio Db/Da has not been switched in S 16 , the CPU compares the temperature detected by the thermistor  19   a  with the temperature detected by the heater end thermistor  19   b  (S 17 ). 
         [0059]    If the temperature detected by the thermistor  19   b  is higher, the CPU  211  switches the power supply duty ratio Db/Da to 0.5/1 while sheets P of recording medium which are the same in size in terms of the lengthwise direction are conveyed for thermal image fixation (S 18 ). By switching the power supply duty ratio Db/Da to 0.5/1 in this way, it is possible to minimize the amount by which the out-of-sheet-path portions of the heater  16  (fixation sleeve  20 ) increase in temperature while the sheets P of recording medium which are the same in size in terms of the lengthwise direction are conveyed for thermal fixation through the fixing device  12 . On the other hand, if it is determined in S 16  that the power supply duty ratio Db/Da has already been switched to 0.5/1, Steps S 16  and S 17  are skipped. 
         [0060]    If it is determined in S 19  that the printing job has not been completed, the CPU  211  repeats Steps S 14 -S 18 . If it is determined in S 19  that the print job has been completed, the CPU  211  stops controlling. 
         [0061]    Next, the method in this embodiment for determining the proper value for power supply duty Da for the triac  212   a , and the proper value for the power supply duty Db for the triac  212   b , is described in comparison to a comparative method. 
       1) Comparative Method (Conventional Method) 
       [0062]    First, an interim power supply duty D for the heater  16  is determined with the use of the so-called PI control, which is a combination of proportional control (P) and integral control (I). D is defined by the following equation: 
         [0000]        D =Proportional Control Component+Integral Control Component+Initial Offset Value  (Formula 1)
 
         [0063]    The proportional control component is set every 20 ms based on the value of ΔT (=detected temperature level·target temperature level) with reference to  FIG. 8  (graph). As for the integral control component, it is set every 20 ms based on ΣΔT, which is the sum of ΔT. That is, the integral control component is increased by 1.0% as ΣΔT becomes greater than 36 (ΣΔT&gt;36), and decreased by 1.0% as ΣΔT becomes smaller than −60 (ΣΔT&lt;−60). 
         [0064]    The initial offset value is provided for ensuring that the temperature of the heater  16  (fixation sleeve  20 ) is reliably controlled during the initial stage of the PI control. It is to be optimally set to prevent the temperature detected by the thermistor  28  from overshooting or undershooting the target temperature. In this embodiment, the initial offset value is 40%. 
         [0065]    The power supply duty Da for the triac  212   a  and the power supply duty Db for the triac  212   b , are set with the use of the following formulas, and the interim power supply duty D is set as described above. That is, the relationship between the temperature detected by the thermistor  18  and the power supply duty for the heat generating resistors  42  and  43  are preset. Therefore, the values therefor are selected according to the detected temperature. 
         [0000]        Da=D×α   (Formula 2)
 
         [0000]        Db=D×β   (Formula 3)
 
         [0000]    α and β stand for the values of the Db and Da, respectively. They are no less than 0, and no more than 1. If α&gt;β, α=1. If α&lt;β, β=1. Further, if α=β, α=β=1. 
         [0066]    Table 2 shows the results of comparison between the amount of heat generation by the heater  16  before the power supply duty ratio was switched, and the amount of heat generation by the heater  16  after the power supply duty ratio was switched, while a substantial number of sheets P of recording medium which were the same in size were continuously conveyed, more specifically, a substantial number of sheets P of recording medium of the letter size, were continuously conveyed in portrait attitude (120 V). Immediately after the switching of the power supply duty ratio Db/Da from 1/1 to 1/0.5, the power supply duty for the heat generating resistor  43  reduces from 55% to 28%. Since the maximum amount of heat generation of the heat generating resistor  43  is 630 W, the amount of heat generation of the heat generating resistor  43  reduces from 344 W (630 W×55%) to 172 W (=630 W×28%). 
         [0067]    In comparison, in the case of the comparative method, the power supply duty of the heat generating resistor  42  is not changed. Thus, the amount of heat generation of the heat generating resistor  42  remains to be 528 W (=960 W (maximum amount of heat generation of heat generating resistor  42 )×55%). Therefore, the sum of the amount of heat generation of the heat generating resistor  42  and the amount of heat generation of the heat generating resistor  43  reduced from 872 W to 700 W. Thus, the amount of heat generation by the heater  16  is insufficient to keep the temperature detected by the thermistor  18  at the target level. That is, the temperature of the fixation sleeve  20  fell below its target level ( FIG. 12 ), making it possible for an unfixed toner image to be unsatisfactorily fixed to a sheet P of recording medium. 
         [0068]    A phenomenon such as the above described one is likely to occur in a case where the distance between a temperature detection element used for temperature control and a heat generating resistor is relatively large, or the heater substrate is relatively large in thermal resistance. For example, the phenomenon is likely to occur in a case where a fixing device is of the heating film type, like the one in this embodiment, and the temperature detection element is placed on the outward or inward surface of the fixation sleeve, instead of one of the surfaces of the heater, to control the fixation film in temperature. This kind of structural arrangement has merit that the fixation film can be more precisely controlled in temperature. On the other hand, it has demerit that it takes longer to detect the heater temperature reduction attributable to the reduction in the amount of heat generation of the heater  16 , and therefore, it is likely for the heater temperature to fall below the target level. 
         [0000]                                                  TABLE 2                       Before   After           switching   switching                                        Duty ratio Db/Da   1/1   0.5/1           Temporary Duty ratio (%)   55   55           Power supply duty Da (%)   55   55           Power supply duty Db (%)   55   28           Heat Generation amount by   528   528           element 42 (W)           Heat Generation amount by   344   172           element 43 (W)                   Sum of heat generation   872   700           amount (W)                        
2) Controlling Method in this Embodiment
 
         [0069]    Next, the method, in this embodiment, for preventing the heater  16  from reducing in the amount of heat generation after the switching of power supply duty ratio, in order to prevent the temperature detected by the thermistor  18  from reducing, is described. In terms of the procedure to calculate the value for the interim power supply duty D, the method in this embodiment is the same as the comparative method. However, in this embodiment, the power supply duty Da for the triac  212   a  and the power supply duty Db for the triac  212   b  are set with the use of the following formulas: 
         [0000]        Da=D ×α×( R 42+ R 43)/( R 43×α+ R 42×β)  (Formula 4)
 
         [0000]        Db=D ×β×( R 42+ R 43)/( R 43×α+ R 42×β)  (Formula 5)
 
         [0070]    R42 and R43 are resistance values of the heat generating resistors  42  and  43 , respectively. Table 3 shows the comparison between the amount of heat generation by the heater  16  immediately before the power supply duty ratio was switched and the amount of heat generation by the heater  16  immediately after the power supply duty ratio was switched, while a substantial number of sheets P of recording medium which are the same in size were continuously conveyed, more specifically, a substantial number of sheets P of recording medium of the letter size, were continuously conveyed in portrait attitude. In this embodiment, the values for the power supply duties Da and Db are set with the use of Formulas 4 and 5, which are for setting the values for the power supply duties Da and Db to ensure that the overall amount of heat generation of the heater  16  after the switching of the power supply duty ratio Db/Da remains the same as that prior to the switching. 
         [0071]    Since (R42+R43)/(R43×α+R42×β) is greater than 1, the relationship between the detected temperature and the power supply duty set according to the detected temperature prior to the switching of the power supply duty ratio is different from that after the switching. That is, in the case of the heat generating resistor  42 , the power supply duty (Formula 4) set according to the detected temperature after the switching of the power supply duty ratio is greater than the power supply duty (Formula 2) set according to the detected temperature prior to the switching. Similarly, in the case of the heat generating resistor  43 , the power supply duty (Formula 5) set according to the detected temperature after the switching of the power supply duty ratio is greater than the power supply duty (Formula 3) set according to the detected temperature prior to the switching. 
         [0072]    That is, in this embodiment, when the power supply duty ratio Db/Da of the heat generating resistor  43  relative to the heat generating resistor  42  is switched, the power supply duty of the heat generating resistor  42  is increased or decreased in the opposite direction from the direction in which the power supply duty of the heat generating resistor  43  is increased or decreased. Therefore, it is possible to keep the temperature of the fixation sleeve  20  at the target level, and therefore, to prevent an unfixed toner image from being unsatisfactorily fixed to a sheet P of recording medium. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Before 
                 Conventional 
                 After 
               
               
                   
                 switching 
                 Example 
                 switching 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Duty ratio Db/Da 
                 1/1 
                 0.5/1 
                 0.5/1 
               
               
                 Temprary Duty ratio (%) 
                 55 
                 55 
                 55 
               
               
                 Power supply duty Da (%) 
                 55 
                 55 
                 69 
               
               
                 Power supply duty Db (%) 
                 55 
                 28 
                 34 
               
               
                 Heat Generation 
                 528 
                 528 
                 658 
               
               
                 amount by element 42 (W) 
               
               
                 Heat Generation 
                 344 
                 172 
                 214 
               
               
                 amount by element 43 (W) 
                   
                   
                   
               
               
                 Sum of heat 
                 872 
                 700 
                 872 
               
               
                 generation amount (W) 
               
               
                   
               
             
          
         
       
     
       Effects of this Embodiment 
       [0073]    According to this embodiment, the multiple (two) heat generating resistors are switched in power supply duty while sheets of recording medium which are the same in size are continuously conveyed through the fixing device  12 . Therefore, not only is it possible to minimize the length of time it takes for the fixing device  12  to become ready for image formation after the fixing device  12  is turned on, but also, to minimize the amount of temperature increase of the out-of-sheet-path portions of the heater  16  (fixation sleeve  20 ). Further, it is possible to prevent (preferably, to keep) the amount by which heat is generated by the heater  16  after the switching of the power supply duty ratio is made while a substantial number of sheets P of recording medium are continuously conveyed through the fixing device  12 , from becoming different from (the same as) that prior to the switching. Therefore, it is possible to prevent the temperature of the fixation sleeve  20  from substantially deviating from the target level (and therefore, to prevent occurrence of unsatisfactory fixation). 
         [0074]    In particular, this embodiment is effective in a case where unsatisfactory fixation is likely to occur, for example, in a case where an image forming apparatus is high in process speed (Case 1), and in a case where the portion of a fixing device, which is between the heater and heater temperature detecting portion of the fixing device, is high in thermal resistance (Case 2). 
         [0075]    Hereafter, referring to  FIGS. 10 and 11 , first, the reason why unsatisfactory fixation is likely to occur in Case 1 is described.  FIG. 10  is a schematic drawing for showing the dependency of the viscosity of melted toner upon temperature. The higher the melted toner in temperature, the lower the melted toner in viscosity, being therefore more likely to unsatisfactorily solidify after its permeation into recording medium. Therefore, the higher the melted toner in temperature, the more likely for the melted toner to offset back onto a fixing member (this phenomenon will be referred to as “hot offset”, hereafter). On the other hand, the lower in temperature the melted toner, the higher in viscosity the melted toner, and therefore, the less satisfactory the melted toner in its permeation into recording medium. That is, if the melted toner is excessively low in temperature, it fails to be satisfactorily fixed to recording medium. 
         [0076]      FIG. 11  is a drawing for showing the relationship between the desirable temperature range in which the target temperature for image fixation can be set, and the process speed (peripheral velocity of fixation sleeve). The slower the process speed, the greater the amount by which the toner layer can be supplied with heat by the fixation sleeve  20  per unit length of time, and therefore, the gentler the temperature gradient within the toner layer. That is, referring to the left side of  FIG. 12 , the difference between the temperature of the bottom surface (interface between sheet of recording paper and toner layer) of the toner layer, and the temperature of the top surface (interface between toner layer and fixing member) of the toner layer, is relatively small. Thus, the moment when the temperature of the bottom surface of the toner layer reaches the lowest point in the temperature range in which unsatisfactory fixation does not occur, the temperature of the top surface of the toner layer will have not risen to the level above which hot-offset occurs. That is, there is sufficient margin for preventing hot-offset. 
         [0077]    That is, the slower the process speed, the wider the temperature range in which the target temperature can be set, that is, the range which is lower than the temperature level at or above which hot-offset occurs, and higher than the temperature level at or above which the top surface of the toner layer fixes itself to recording medium. 
         [0078]    On the other hand, the higher the process speed, the steeper in temperature gradient the toner layer. Thus, the moment when the temperature of the top surface of the toner layer reaches the lowest temperature level at which the toner layer is satisfactorily fixable to the recording medium, is close to the temperature level at or above which hot-offset occurs. That is, there is not enough margin for preventing hot-offset. 
         [0079]    In other words, the temperature range in which the target temperature can be set is rather narrow. In this case, it is sometimes impossible to set the target temperature to a level which is lower than the level at or above which hot-offset occur, and is higher than the lowest temperature level for the top surface of the toner layer to be fixed to the recording medium. That is, if the target temperature for the heater  16  (fixation sleeve  20 ) is set to a level which is lower than the lowest level at which the top surface of the toner layer is fixable to the recording medium, unsatisfactory fixation will occur. 
         [0080]    Next, the reason why unsatisfactory fixation is likely to occur in Case 2 is described, with reference to fixing devices which employ a fixation film. Some fixing devices (type A) which employ a fixation film are structured so that their temperature detecting portions are positioned so that they can directly detect the temperature of their heaters, whereas the others (type B) are structured so that their temperature detecting portions are placed in contact with the inward or outward surface of the fixation film. The fixing devices of the type A correspond to the case where the portion of a fixing device, which is between the heater of the device, and the heater temperature detecting portion of the device, is relatively small in thermal resistance, whereas the fixing devices of the type B correspond to the case where the portion is relatively large in thermal resistance. 
         [0081]    In the case of the fixing devices of the type A, the reduction in the heater temperature, which occurs as the heater is changed in power supply duty ratio, can be quickly detected. Therefore, the amount by which electric power is supplied to the heater can be instantly increased to minimize the temperature reduction. In comparison, the fixing devices of the type B have merit that they can be more precisely controlled in the temperature of their fixation film than the fixing devices of the type A. However, they require a longer length of time for them to detect the reduction in the temperature of their heaters, which occurs as the electric power to be supplied to the heater is switched in power supply duty ratio, than the fixing devices of the type A. Therefore, it takes longer for them to be increased in the amount by which electric power is supplied to the heater, which in turn makes it likely for the temperature of the heater (fixation film) to become lower than the target temperature. This is why the fixing devices of the type B are more likely to suffer from unsatisfactory fixation than the fixing devices of the type A, after the power supply to the heater is switched in duty ratio. 
         [0082]    According to this embodiment, it is possible to prevent the occurrence of unsatisfactory fixation, even if the process speed is relatively high and/or the portion of a fixing device, which is between the heater and temperature detecting portion of the fixing device is relatively large in thermal resistance. 
       (Modification 1) 
       [0083]    In the above-described embodiment, the heat generating resistor  42  was such that in terms of the lengthwise direction, its center portion is greater in the amount of heat generation than its center portion, whereas the heat generating resistor  43  was such that in terms of the lengthwise direction, its lengthwise end portions are greater in the amount of heat generation than its center portion. That is, the first and second heat generating resistors  42  and  43  may be similar in characteristic in terms of the heat generation pattern in terms of the lengthwise direction, which is perpendicular to the recording medium conveyance. Further, the first heat generating resistor  42  may be flat (uniform) in the heat generation pattern in terms of the lengthwise direction, which is perpendicular to the recording medium conveyance direction. 
         [0084]    Incidentally, in terms of electrical connection, the heat generating resistors  42  and  43 , which are disposed parallel in tandem in the recording medium conveyance direction may be connected in parallel or in series. 
       (Modification 2) 
       [0085]    In the above-described embodiment, the number of choices to which the power supply duty of each heat generating resistor can be switched during a printing operation was limited to only two. However, the embodiment is not intended to limit the present invention in terms of the number of power supply duty choices. That is, the present invention is also applicable to fixing devices provided with three or more choices of power supply duty for their heat generating resistors, which can be selected in steps. Also in this embodiment, the heater  16  was structured so that the heat generation pattern of its heat generating resistors  42  and  43  has a smooth curvature in terms of the lengthwise direction. However, the present invention is also compatible with a heater, the heat generation pattern of the heat generation resistors of which is stair-stepped in terms of the lengthwise direction. 
       (Modification 3) 
       [0086]    Also in the above-described first embodiment, the timing with which the power supply duty ratio Db/Da was switched was the same as the timing with which the power supply duties Da and Db were switched. However, the timing with which the power supply duty Da is switched dos not need to be the same as the timing with which the power supply duty Db is switched. That is, the power supply duty Db may be switched after the switching of the power supply duty Da. In other words, the fixing device may be structured so that the timing with which the first heat generating resistor is increased or decreased in power supply duty comes before the timing with which the second heat generating resistor is switched in power supply duty. 
         [0087]    Instead, it may be after the switching of the power supply duty Db that the power supply duty Db is switched. In other words, the fixing device may be structured so that the timing with which the second heat generating resistor is increased or decreased in power supply duty comes before the timing with which the first heat generating resistor is increased or decreased in power supply duty. 
       (Modification 4) 
       [0088]    Further in the above-described first embodiment, the power supply duty ratio Db/Da was switched so that it became smaller. However, the present invention is also applicable to a case where the power supply duty ratio is switched so that it becomes larger. Conventionally, switching the power supply duty ratio Db/Da so that it becomes larger resulted in increase in power consumption, which in turn caused the following problems. That is, fixation sleeve temperature exceeded the target temperature level. Therefore, it was possible that “hot-offset” would occur, that is, a part of the melted toner would adhere to the surface of rotational heating member, which would result in the contamination of the following sheets of recording medium. 
         [0089]    Even in such a case as those described above, application of the present invention can prevent the occurrence of high temperature offset (hot-offset), which will possibly occur during the switching of power supply duty ratio. 
       (Modification 5) 
       [0090]    Also in the first embodiment described above, the heat generation pattern of the combination of the heat generating resistors  42  and  43  was roughly flat. However, the embodiment is not intended to limit the present invention in terms of the heat generation pattern of the combination. That is, the present invention is also compatible with a heater ( 16 ) which generates more heat across its lengthwise end portions than its center portion. Incidentally, also in the case of such a heater, it is desired that the heat generation pattern of the heater (combined heat generation amount of heat generating resistors) after the switching of power supply duty ratio remains the same as that before the switching. 
       (Modification 6) 
       [0091]    Also in the above described first embodiment, the heater (fixation sleeve) temperature was controlled based on the results of the indirect detection of the heater temperature. However, the first embodiment is not intended to limit the present invention in terms of the method for detecting heater temperature. That is, the present invention is also applicable to a fixing device which controls heater temperature based on the results of direct detection of heater temperature with the use of a thermistor. 
         [0092]    By the way, in the first embodiment, power supply duty was used as the parameter for controlling the electric power to be supplied to the heater. However, the first embodiment is not intended to limit the present invention in terms of the parameter for controlling the power supply to the heater. That is, any parameter may be used as long as it is correlated to the amount of power supply to the heater per unit length of time. Further, in this embodiment, power supply duty ratio was used as the parameter for switching the heater in heat generation pattern. However, any parameter may be used in place of power supply duty ratio, as long as it is correlated to the ratio of the amount of electric power supplied to one of the heat generating resistor, relative to that of the other. 
         [0093]    While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
         [0094]    This application claims priority from Japanese Patent Application No. 171154/2013 filed Aug. 21, 2013, which is hereby incorporated by reference.