Patent Publication Number: US-10321518-B2

Title: Heating device for heating recording material, and image forming apparatus having the same

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a heating device that heats a recording material formed with an unfixed image to fix the image to the recording material, and an image forming apparatus having the heating device. 
     Description of the Related Art 
     In an electrophotographic copying machine or printer, there is used a heating device that thermally fixes an unfixed toner image formed on a recording material (such as a transfer sheet or a photosensitive sheet) to the recording material. For example, an on-demand film-heating-type device has been known (see, for example, Japanese Laid-open Patent Publication No. 2002-296955). 
     In the film-heating-type heating device, a ceramic heater or other heating member is used. The ceramic heater includes a ceramic base plate (e.g. of alumina or aluminum nitride) which is electrically resistant, heat-resistant, and excellent in thermal conductivity. The ceramic heater further includes heating resistors (e.g. of silver palladium) which are pattern-shaped on the base plate e.g. by printing or baking and which generate heat when supplied with electric power, power supply electrodes which are pattern-shaped e.g. by printing or baking, and a conductor pattern which is low in resistance and which connects the power supply electrodes and the heating resistors to one another. The ceramic heater, which is configured as described above to generate heat when the heating resistors are supplied with electric power through the power supply electrodes and the conductor pattern, is low in heat capacity as a whole and therefore high in temperature rise speed. 
     When recording materials that are small in size in a longitudinal direction of the heating member (i.e., small in widthwise size of recording materials) are continuously thermally fixed in the heating device whose heating member is formed by the ceramic heater which is small in heat capacity, heating member temperature tends to more easily rise at longitudinally opposite end portions of the heating member where recording materials do not pass than at a longitudinal central portion thereof where recording materials pass. In that case, due to the presence of a temperature difference in the longitudinal direction of the heating member, gloss unevenness tends to occur in fixed images. To avoid this, print speed must be decreased or temperature difference in the longitudinal direction of the heating member must be reduced. 
     Thus, there have been proposed heating devices each having a heating member provided with a plurality of heating resistors and a plurality of temperature detecting elements for detecting temperatures of the heating member at plural longitudinal positions and each configured to control power supply to the heating resistors based on the detected temperatures, thereby preventing gloss unevenness from occurring in fixed images due to a temperature difference in the longitudinal direction of the heating member. 
     As such a heating device, there is for example a fixing device that has a heating member formed by heating resistors disposed and configured to generate a large amount of heat at parts of the heating member where recording materials of any size pass and by heating resistors disposed and configured to generate a large amount of heat at parts of the heating member where only recording materials which are large in size pass (see, Japanese Laid-open Patent Publication No. 10-177319). 
     This fixing device controls power supply to the heating resistors based on a temperature detection result, thereby reducing a temperature rise at parts of the heating member where recording materials do not pass and thereby controlling temperatures at parts of the heating member where recording materials pass to predetermined temperatures. However, a conductor pattern (hereinafter, referred to as the electrically conductive parts or the conductive parts) that connects the heating resistors and the power supply electrodes sometimes generates heat, which can cause a problem. 
     To improve the reliability of connection between component parts formed on the base plate of the ceramic heater and the power supply electrodes, a relative large area is provided for installation of the power supply electrodes. Even in a small-sized heating device having a base plate which is compact in size, a large area is ensured to install the power supply electrodes. Thus, a space occupied by the conductive parts that connect the heating resistors and the power supply electrodes is naturally constrained in the heating device where a large area is provided for installation of the power supply electrodes. 
     It should be noted that the electrically conductive parts each have a resistance value although it is small. More specifically, the resistance value of each conductive part is inversely proportional to a cross-sectional area of the conductive part, and hence increases with decrease of the cross-sectional area of the conductive part. When electric power is supplied to the heating resistors, there occur power losses each represented by the product of current value and resistance value of the conductive part, and heat is generated. 
     In a heating device configured to control power supply to a plurality of heating resistors, power supply electrodes are provided for respective ones of the heating resistors, and therefore electrically conductive parts are installed in a more restrained space. In particular, a space for conductive parts, which are connected to power supply electrodes disposed remoter from the heating resistors, is more restrained. This requires the electrically conductive parts to be thinned, and much heat tends to generate. 
     When heat is generated by the electrically conductive parts that connect the heating resistors and the power supply electrodes, temperatures of the power supply electrodes rise and a temperature of a power supply connector connected to the power supply electrodes also rises. As a result, metallic parts such as copper alloy parts of the connector cannot ensure contact pressures that depend on thermal stress characteristics thereof, and the reliability of the connector can be impaired. On the other hand, in a case where metallic parts of the connector are formed by e.g. copper-titanium alloys to ensure the contact pressures, the reliability of the connector can be ensured, but a problem of increased costs is caused. 
     SUMMARY OF THE INVENTION 
     The present invention provides a heating device capable of suppressing electrically conductive parts that electrically connect power supply electrodes and heating resistors from generating heat, and provides an image forming apparatus having the heating device. 
     According to one aspect of this invention, there is provided a heating device comprising a base plate, a plurality of heating resistor members disposed in a longitudinal direction of the base plate, a plurality of power supply electrodes disposed on one longitudinal end portion of the base plate and including power supply electrodes configured to independently supply electric power to heating resistor members of said plurality of heating resistor members together with another power supply electrode connected to said plurality of heating resistor members, and a plurality of electrically conductive parts configured to connect the plurality of heating resistor members and the plurality of power supply electrodes to one another, wherein the plurality of electrically conductive parts are formed such that electrically conductive parts of said plurality of electrically conductive parts that are capable of having larger amounts of current flow therethrough have larger cross-sectional areas thereof with respect to a perpendicular direction to the longitudinal direction of said base plate. 
     With this invention, electrically conductive parts are formed such that electrically conductive parts capable of providing larger amounts of power supply have larger areas of cross section perpendicular to a power supply direction, and therefore power losses in the conductive parts are made uniform to one another, whereby the conductive parts are suppressed from generating heat. As a result, temperature ratings of peripheral members disposed near the conductive parts can be lowered, thereby contributing to improve the reliability of the heating device and to reduce costs. 
     Further features of the present invention will become apparent from the following description of an exemplary embodiment with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a section view schematically showing the construction of an image forming apparatus having a thermal fixing device that serves as a heating device according to one embodiment of this invention; 
         FIG. 2  is a section view schematically showing the construction of the thermal fixing device; 
         FIG. 3A  is a schematic view of a heating member of the thermal fixing device as seen from a side of a pressure roller; 
         FIG. 3B  is a section view taken along line A-A shown in  FIG. 3A ; 
         FIG. 4  is a view showing a resistance distribution in heating resistors of the heating member along a longitudinal direction of a base plate of the heating member; 
         FIG. 5  is a wiring diagram schematically showing an electrical connection relationship between the heating member and its peripheral elements; and 
         FIG. 6  is a flowchart showing procedures of a power supply control process executed by a CPU of the thermal fixing device. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     The present invention will now be described in detail below with reference to the drawings showing a preferred embodiment thereof. 
       FIG. 1  schematically shows in section the construction of an image forming apparatus that has a heating device according to one embodiment of this invention. The image forming apparatus is e.g. a digital multifunction peripheral, and includes a heating device such as an on-demand film-heating-type thermal fixing device. It should be noted that the image forming apparatus can be implemented by an electrophotographic apparatus, an electrostatic recording apparatus, or the like, other than the digital multifunction peripheral. 
     In  FIG. 1 , reference numeral  10  denotes the digital multifunction peripheral that mainly includes an image forming unit  11  and a document reading unit  12  disposed above the image forming unit  11 . The document reading unit  12  has an operation unit  13  operable by a user, and reads an image of a document and transmits an image signal to the image forming unit  11  according to a user&#39;s instruction given via the operation unit  13 . 
     The image forming unit  11  is provided with process cartridges  31  to  34  for respective colors that include photosensitive drums  21  to  24 , respectively. An optical unit  15  having a polygon mirror, a scanner motor, a laser light source, lens groups, etc. (none of which are shown) is disposed facing the process cartridges  31  to  34 . An endless transfer belt  16  is disposed facing and in contact with the photosensitive drums  21  to  24 . A conveyance unit  14  having conveyance rollers and a registration roller is disposed in contact with the transfer belt  16 . On a downstream side of the conveyance unit  14  in a sheet conveyance direction, a thermal fixing device  60  having a heating member  61 , a fixing film  62 , and a pressure roller  63  is disposed. On a downstream side of the thermal fixing device  60  in the sheet conveyance direction, an upper part of the image forming unit  11  constitutes a sheet discharge tray  17 . 
     Sheet feed units  41  to  44  are disposed below the optical unit  15 . The sheet feed units  41  to  44  have sheet feed cassettes  41   a  to  44   a  in which transfer sheet bundles  51  to  54  (recording materials) are stored. The sheet feed cassettes  41   a  to  44   a  are provided with pickup rollers  41   b  to  44   b , which are disposed in press contact with separation pads (not shown) and which cooperate with the separation pads to separate and feed transfer sheets one by one from the sheet feed cassettes, and also provided with sheet size sensors  41   c  to  44   c  that detect sheet sizes of the transfer sheet bundles  51  to  54 , respectively. 
     When receiving an image signal from the document reading unit  12 , the image forming unit  11  irradiates laser light on the photosensitive drums  21  to  24  for respective colors to thereby form electrostatic latent images on surfaces of the photosensitive drums, and develops the latent images to form toner images. 
     In synchronism with the formation of the toner images, a transfer sheet is fed from one of the sheet feed units  41  to  44  (e.g., the sheet feed unit  41 ) through the conveyance unit  14 . The toner images formed on the photosensitive drums  21  to  24  are transferred to the transfer belt  16  and further transferred from the transfer belt  16  to the transfer sheet. The transfer sheet (denoted by reference numeral  110  in  FIG. 2 ) formed with the toner images is conveyed to the thermal fixing device  60  in which the transfer sheet  110  is applied with heat and pressure, whereby the toner images are fixed thereto. The transfer sheet  110  fixed with the toner images is discharged to the sheet discharge tray  17 . 
       FIG. 2  schematically shows in section the construction of the thermal fixing device  60 . In  FIG. 2 , arrow A indicates a direction to which the transfer sheet  110  is conveyed. 
     As described above, the thermal fixing device  60  includes the heating member  61 , fixing film  62 , and pressure roller  63 . The heating member  61  is an elongated member that extends longitudinally in a direction transverse to a conveyance path for the transfer sheet  110  (i.e. in a direction perpendicular to the drawing paper of  FIG. 2 ), and that is surrounded by the fixing film  62 . The pressure roller  63  is disposed facing the fixing film  62 . 
     The thermal fixing device  60  further includes a rigid stay  65  and thermistors  66 ,  67 . The rigid stay  65  is an elongated heat-resistant and heat-insulating member that extends longitudinally in the direction transverse to the conveyance path for the transfer sheet  110 . The rigid stay  65  has a surface facing the transfer sheet  110  and formed with a groove in the direction transverse to the conveyance path for the transfer sheet  110 . The heating member  61  is fitted into the groove, and fixed and held therein by a heat-resistant adhesive. 
     The fixing film  62  is obtained by forming a heat-resistant flexible member, e.g., a heat-resistant film member, into a hollow cylindrical shape, and loosely fitted on the rigid stay  65  attached with the heating member  61 . The fixing film  62  is comprised of a hollow cylindrical single-layer film having a thickness of about 40 to 100 μm and formed e.g. of PTFE, PFA, or FEP having heat resistance, releasability, high strength, durability, etc. Alternatively, the fixing film  62  may be a multi-layer film having a cylindrical film of polyimide, polyamide, PEEK, PES, PPS, or the like whose outer peripheral surface is coated with PTFE, PFA, FEP, or the like. 
     The pressure roller  63  is an elastic roller that has a core shaft  63   a  and a heat-resistant elastic cylinder  63   b  made of e.g. silicone rubber and fixedly fitted on the core shaft  63   a  and that is disposed in press contact with the heating member  61 . The heating member  61  cooperates with the pressure roller  63  to form a fixing nip N where the fixing film  62  is held between the heating member and the pressure roller. 
     The pressure roller  63  is driven to rotate at a predetermined circumferential speed in a direction indicated by arrow B. With the rotation of the pressure roller  63 , a friction force is generated at the fixing nip N between the pressure roller  63  and the fixing film  62 . A rotation force of the pressure roller  63  acts on the fixing film  62 . When the transfer sheet  110  is introduced into the fixing nip N, the rotation force of the pressure roller  63  acts on the fixing film  62  via the transfer sheet  110 , whereby the fixing film  62  is rotated about the rigid stay  65  in a direction indicated by arrow C, while being in pressure sliding contact with the heating member  61 . 
     The rigid stay  65  has a function of guiding the fixing film  62  that rotates about the rigid stay  65 , whereby the fixing film  62  can easily rotate around the rigid stay  65 . It should be noted that a small amount of lubricant such as heat-resistant grease can be applied between the heating member  61  and the fixing film  62  to reduce sliding resistance therebetween. 
     In a state where the rotation of the fixing film  62  caused by the rotation of the pressure roller  63  becomes steady, it is waited that the temperature of the heating member  61  reaches a predetermined temperature, while monitoring the temperature of the heating member  61  by the thermistors  66 ,  67  (see,  FIG. 5  described later) disposed on the heating member  61  to be apart from each other in the longitudinal direction of the heating member. 
     After the temperature of the heating member  61  is raised to the predetermined temperature, the transfer sheet  110  to which a toner image is to be fixed is introduced to the fixing nip N where the transfer sheet  110  is heated by the heating member  61 , while being sandwiched and conveyed between the fixing film  62  and the pressure roller  63 . As a result, heat generated by the heating member  61  is efficiently transferred and applied to the transfer sheet  110  via the fixing film  62 , and an unfixed toner image formed on the transfer sheet  110  is thermally fixed thereto. Thereafter, the transfer sheet  110  passes through the fixing nip N, and is separated from the fixing film  62  and conveyed in the direction of arrow A. 
       FIG. 3A  schematically shows the heating member  61  of the thermal fixing device  60  as seen from the side of the pressure roller, and  FIG. 3B  is a section view taken along line A-A in  FIG. 3A . 
     As shown in  FIGS. 3A and 3B , the heating member  61  has a base plate  64  disposed parallel to a surface of the transfer sheet  110  which is introduced to the thermal fixing device  60 . The base plate  64  is formed into an elongated shape that extends laterally in the direction of conveyance of the transfer sheet  110  subjected to thermal fixing. The base plate  64  is disposed such that one longitudinal end portion (hereinafter, referred to as the E-side end portion) thereof is positioned on the front side of the drawing paper of  FIG. 2  and another longitudinal end portion (hereinafter, referred to as the F-side end portion) thereof is positioned on the rear side of the drawing paper of  FIG. 2 . 
     The base plate  64  is made from a ceramic material such as alumina or aluminum nitride, and has first and second faces  64   a ,  64   b . On the first face  64   a  of the base plate  64 , there are formed by printing and baking a plurality of (e.g., four) heating resistors  71  to  74  and a plurality of (e.g., three) power supply electrodes  81  to  83 . The heating resistors  71  to  74  are each formed of silver palladium and each generate heat when supplied with electric power. The power supply electrodes  81  to  83  are provided on the E-side end portion of the base plate  64 , and serve as electrical contacts which are disposed for contact with contacts of a connector  100 . 
     Hereinafter, the heating resistors  71  to  74  will be referred to as the heating resistor group, the heating resistors  72 ,  73  (first heating resistor group) will be referred to as the inside heating resistors Rin, and the heating resistors  71 ,  74  (second heating resistor group) will be referred to as the outside heating resistors Rout. A symbol C 0  represents a virtual axis that passes through a predetermined reference position, e.g., an intermediate point, in the lateral direction of the base plate  64 . 
     The inside heating resistors Rin (heating resistors  72 ,  73 ) are disposed substantially symmetrically with each other with respect to the virtual axis C 0  in the lateral direction of the base plate  64 . The outside heating resistors Rout (heating resistors  71 ,  74 ) are disposed remoter from the virtual axis C 0  than the inside heating resistors Rin and substantially symmetrically with each other with respect to the virtual axis C 0  in the lateral direction of the base plate  64 . 
     The heating resistors  72 ,  73  have the same resistance value and the same resistance distribution as each other. The heating resistors  71 ,  74  have the same resistance value and the same resistance distribution as each other. The resistance distribution and resistance value of the heating resistors  72 ,  73  differ from those of the heating resistors  71 ,  74 . 
     The heating resistors  72 ,  73  each have a longitudinal intermediate portion which is narrower in width than each of longitudinal opposite end portions. The heating resistors  71 ,  74  each have a longitudinal intermediate portion which is wider in width than each of longitudinal opposite end portions ( FIG. 3A ). As a result, in each of the heating resistors  72 ,  73 , the amount of heat generated at the intermediate portion is greater than that generated at each of the opposite end portions. On the other hand, in each of the heating resistors  71 ,  74 , the amount of heat generated at each of the opposite end portions is greater than that generated at the intermediate portion. 
     Since the inside heating resistors Rin (heating resistors  72 ,  73 ) are disposed symmetrically with each other with respect to the virtual axis C 0 , the heating resistors  72 ,  73  generate the same amount of heat at symmetrical positions with respect to the virtual axis C 0 . Similarly, the heating resistors  71 ,  74  (outside heating resistors Rout) generate the same amount of heat at symmetrical positions with respect to the virtual axis C 0 . 
     The F-side end portions of the heating resistors  72 ,  73  are connected with each other by an electrically conductive part (hereinafter simply referred to as the conductive part)  92  and are in an electrically conductive state with each other. The E-side end portions of the heating resistors  72 ,  73  are connected to conductive parts  94 ,  93  whose tip end portions constitute electrical end portions Eb, Ea, respectively. 
     The F-side end portions of the heating resistors  71 ,  74  are connected with each other by an electrically conductive part  91 , and are in an electrically conductive state with each other. The E-side end portion of the heating resistor  71  is connected to an electrically conductive part  95  whose tip end portion constitutes an electrical end portion Ec. The E-side end portion of the heating resistor  74  is connected to the conductive part  93 . 
     It should be noted that the F-side end portions of the inside heating resistors Rin and the outside heating resistors Rout are not electrically conductive to each other. 
     The power supply electrodes  81  to  83  are concentratedly disposed on the E-side end portion of the base plate  64 . The electrical end portions Ea to Ec of the conductive parts  93  to  95  are respectively connected to the power supply electrodes  81  to  83 . In other words, the power supply electrode  83  is an electrode for power supply to the outside heating resistors Rout, and the power supply electrode  82  is an electrode for power supply to the inside heating resistors Rin. The power supply electrode  81  is a common electrode for power supply to the inside heating resistors Rin and to the outside heating resistors Rout. The connector  100  is inserted in a direction of arrow G in  FIG. 3A  and removed in a direction opposite to the direction of arrow G. 
       FIG. 4  shows a resistance distribution in the heating resistors of the heating member  61  along the longitudinal direction of the base plate  64 . 
     The heating resistor group is configured to have a total resistance value that is slightly smaller at a longitudinal central portion of the base plate  64  than at each of opposite end portions as shown by a solid line in  FIG. 4 , whereby a heat distribution becomes substantially uniform in the longitudinal direction of the base plate  64 . 
     The inside heating resistors Rin are configured to have a resistance value that is greater at the central portion of the base plate  64  than at each of the opposite end portions as shown by a one-dotted chain line in  FIG. 4 , and generate much heat at the central portion of the base plate  64  when supplied with electric power across the electrical end portions Ea, Eb. 
     The outside heating resistors Rout are configured to have a resistance value that is smaller at the central portion of the base plate  64  and greater at each of the opposite end portions of the base plate  64  as shown by a broken line in  FIG. 4 , and generate much heat at each end portion of the base plate  64  when supplied with electric power across the electrical end portions Ea, Ec. 
     Transfer sheets  110  conveyed to the thermal fixing device  60  pass through the base plate  64  of the heating member  61 . At that time, each transfer sheet  110  necessarily passes through the longitudinal central portion of the base plate  64 . 
     For this reason, the total resistance value of the inside heating resistors Rin is made smaller than that of the outside heating resistors Rout to make the amount of heat of the heating member  61  greater at the longitudinal central portion than at each of the opposite end portions of the base plate  64 , thereby making an electric current flowing through the inside heating resistors Rin greater than an electric current flowing through the outside heating resistors Rout. Furthermore, a ratio between power supply to the outside heating resistors Rout and power supply to the inside heating resistors Rin is controlled according to the width of transfer sheet  110 , thereby controlling the heat distribution of the heating member  61  in the longitudinal direction of the base plate  64  according to the width of transfer sheet  110 . 
       FIG. 5  schematically shows in wiring diagram an electrical connection relationship between the heating member  61  and its peripheral elements. 
     As shown in  FIG. 5 , the thermistors  66 ,  67  of the thermal fixing device  60  are respectively disposed on a central portion and one end portion of a longitudinal side face of the heating member  61 , and connected to the CPU  70 . The thermistors  66 ,  67  function as temperature sensors. Based on output signals from the thermistors  66 ,  67 , the CPU  70  monitors a temperature of the longitudinal central portion of the heating member  61  and a temperature of the one longitudinal end portion of the heating member  61 . 
     The power supply electrode  81  of the thermal fixing device  60  is connected to a commercial power source  120 , and the power supply electrodes  82 ,  83  are connected via breaker devices  69 ,  68  to the commercial power source  120 . The breaker device  68  controls power supply from the power source  120  to the electrically conductive parts  95 ,  93  and to the outside heating resistors Rout (heating resistors  71 ,  74 ). The breaker device  69  controls power supply from the power source  120  to the electrically conductive parts  94 ,  93  and to the inside heating resistors Rin (heating resistors  72 ,  73 ). These breaker devices  68 ,  69  are connected to the CPU  70 . 
     The CPU  70  controls the breaker devices  68 ,  69  based on temperatures of the longitudinal central portion and the one longitudinal end portion of the heating member  61 , which are represented by output signals of the thermistors  66  and  67 , respectively, thereby controlling the power supply to the conductive parts  95 ,  93  and to the outside heating resistors Rout (heating resistors  71 ,  74 ) and controlling the power supply to the conductive parts  94 ,  93  and to the inside heating resistors Rin (heating resistor  72 ,  73 ). 
     As described above, the conductive part  93  having the electrical end portion Ea is connected to the heating resistors  73 ,  74 , and the conductive part  94  having the electrical end portion Eb and the conductive part  95  having the electrical end portion Ec are respectively connected to the heating resistors  72 ,  71 . 
     In  FIG. 5 , symbols Ia, Ib, Ic respectively denote electric currents flowing through the conductive parts  93 ,  94 , and  95 . The electric currents Ia to Ic are mainly decided by resistance values of the heating resistors  71  to  74  since each of the heating resistors  71  to  74  (heating resistor group) has a resistance value greater than that of a corresponding one of the conductive part  93  to  95 . 
     As described above, the electric current flowing through the inside heating resistors Rin (heating resistors  72 ,  73 ) are greater than the current flowing through the outside heating resistors Rout (heating resistors  71 ,  74 ). More specifically, the electric current Ib flowing through the heating resistor  72  and through the conductive part  94  is larger than the current Ic flowing through the heating resistor  71  and through the conductive part  95 . The current Ia flowing through the heating resistors  73 ,  74  and through the conductive part  93  is represented by the sum of the currents Ib, Ic. In other words, the current Ia is the largest, the current Ib is the next largest, and the current Ic is the smallest. Roughly speaking, the ratio between the currents Ia, Ib, and Ic is 5:3:2. 
     Power loss in a resistor is represented by the product of square of an electric current flowing through the resistor and a resistance value of the resistor. To minimize power losses in resistors that are disposed within a limited space (base plate width), it is preferable to make power losses in these resistors substantially uniform to one another. In other words, it is preferable to configure the resistors to have resistance values whose ratio is proportional to reciprocals of squares of currents flowing through the resistors. 
     In this embodiment, the conductive parts  93  to  95  are configured to have resistance values whose ratio becomes e.g. 1:2.78:6.25 in consideration of the ratio of 5:3:2 among the currents Ia, Ib, and Ic flowing through the conductive parts  93  to  95 . The conductive parts  93  to  95  and the power supply electrodes  81  to  83  connected thereto are configured such that the resistance values of the conductive parts  93  to  95  become smaller in this order. 
     More specifically, the power supply electrode  81  is disposed closest to the heating resistor group, and the conductive part  93  is configured to have a largest cross-sectional area in a power supply direction (i.e., a largest conductive width) and to have a shortest length. The power supply electrode  82  is disposed next closest to the heating resistor group, and the conductive part  94  is configured to have a next largest conductive width and to have a next shortest length. The power supply electrode  83  is disposed remotest from the heating resistor group, and the conductive part  95  is configured to have a narrowest conductive width and to have a longest length. In other words, conductive parts that can provide larger amounts of power supply have wider conductive widths. 
     The electrically conductive parts  93  to  95  are formed to have conductive widths that can substantially achieve the above-described ratio of resistance values. It should be noted that in a case where the conductive parts  93  to  95  are configured to achieve a predetermined ratio of resistance values under constraints about the size of the base plate  64 , etc., the conductive widths of the conductive parts  93  to  95  sometimes become excessively narrow. In that case, the conductive parts  93  to  95  can be formed to have conductive widths that are as wide as possible, irrespective of the conductive widths that are determined based on the predetermined ratio of resistance values. 
     In the thermal fixing device  60 , a power supply control process is executed by the CPU  70  when a printing operation is requested. 
       FIG. 6  shows in flowchart the procedures of the power supply control process executed by the CPU  70  of the thermal fixing device  60 . 
     The power supply control process is started when a request for a printing operation is input to the CPU  70  shown in  FIG. 5 . The CPU  70  determines whether transfer sheets  110  to be fed to the thermal fixing device  60  are large or small in size based on an output from a corresponding one of the sheet size sensors  41   c  to  44   c , which are shown in  FIG. 1  (step S 11 ). For example, transfer sheets  110  each having a width equal to or larger than 270 mm are determined as being large in size, whereas transfer sheets  110  each having a width less than 270 mm are determined as being small in size. 
     Next, the CPU  70  starts temperature control to control the breaker devices  68 ,  69  according to the size of transfer sheets  110 . More specifically, if determined in step S 11  that transfer sheets  110  are small in size, the CPU  70  controls the breaker devices  68 ,  69  such that power supply from the commercial power source  120  to the outside heating resistors Rout is performed at a ratio of 100% and power supply to the inside heating resistors Rin is performed at a ratio of 50% (step S 12 ), thereby making it possible to prevent temperatures of the heating member  61  at its opposite end portions where transfer sheets  110  do not pass from excessively increasing. 
     On the other hand, if determined in step S 11  that transfer sheets  110  are large in size and pass through substantially the entire face of the heating member  61 , the CPU  70  controls the breaker devices  68 ,  69  such that power supply to the outside heating resistors Rout and power supply to the inside heating resistors Rin are performed both at a ratio of 75% (step S 13 ), whereby temperature unevenness on the surface of the heating member  61  can be eliminated. 
     Next, the CPU  70  performs control to cause each transfer sheet  110  to pass through the fixing nip N of the thermal fixing device  60  and to cause a toner image to be heated and fixed to the sheet (step S 14 ). Each time each transfer sheet  110  passes through the heating member  61 , the CPU  70  compares outputs of the thermistors  66 ,  67  to each other, thereby comparing a temperature of a longitudinal central portion of the heating member  61  and a temperature of one longitudinal end portion thereof to each other (step S 15 ). 
     If determined in step S 15  that the temperatures of the central portion and one end portion of the heating member are nearly equal to each other, the CPU  70  holds current ratios of power supply to the outside heating resistors Rout and to the inside heating resistors Rin unchanged, and proceeds to step S 18 . 
     If determined in step S 15  that the temperature of the central portion of the heating member is higher than the temperature of one end portion thereof, the CPU  70  controls the breaker device  69  such that the ratio of power supply to the inside heating resistors Rin is decreased by a predetermined amount, e.g., about 5% (step S 16 ). As a result, the temperature of the central portion of the heating member decreases relative to the temperature of one end portion of the heating member. 
     On the other hand, if determined in step S 15  that the temperature of one end portion of the heating member is higher than the temperature of the central portion thereof, the CPU  70  controls the breaker device  69  such that the ratio of power supply to the inside heating resistors Rin is increased by a predetermined amount, e.g., about 5% (step S 17 ), whereby the temperature of the central portion of the heating member increases. As a result, the temperature of one end portion of the heating member decreases relative to the temperature of the central portion of the heating member. 
     Next, the CPU  70  determines whether or not the printing operation is completed (step S 18 ). If determined in step S 18  that the printing operation is completed, the CPU  70  controls the breaker devices  68 ,  69  such that the power supply to the heating resistor group is shut off (step S 19 ), whereupon the power supply control process is completed. 
     According to the control process of  FIG. 6 , the power supply to the heating resistors is controlled by utilizing characteristic of resistance distribution, i.e., characteristic of heat distribution in the inside heating resistors Rin and that in the outside heating resistors Rout of the heating member  61  along the longitudinal direction of the base plate. More specifically, the power supply is controlled so as to decrease the ratio of power supply to the inside heating resistors Rin when the temperature of the longitudinal central portion of the heating member  61  is higher than the temperature of one end portion thereof, but to increase the ratio of power supply to the inside heating resistors Rin when the temperature of one end portion of the heating member  61  is higher than that of the longitudinal central portion thereof. 
     As a result, a temperature difference between the central portion and one end portion of the heating member  61  becomes small, whereby the electrically conductive parts  93  to  95  that electrically connect the power supply electrodes  81  to  83  to the heating resistors  71  to  74  can be suppressed from generating heat. As a result, thermal affections to peripheral members can be prevented. In particular, a temperature rise of the power supply connector  100  can be suppressed to ensure the reliability of the connector  100 . Since it becomes unnecessary to use a high-priced material to form metallic parts of the connector  100 , increase in cost is not caused. 
     While the present invention has been described with reference to an exemplary embodiment, it is to be understood that the invention is not limited to the disclosed exemplary embodiment. 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. 
     This application claims the benefit of Japanese Patent Application No. 2012-165866, filed Jul. 26, 2012, which is hereby incorporated by reference herein in its entirety.