Patent Publication Number: US-11640130-B2

Title: Heating unit, fixing unit, and image forming apparatus for heat generation performance and miniaturization

Description:
BACKGROUND 
     Field 
     This disclosure relates to a heating unit for use in heat fixing of an image, a fixing unit including the heating unit, and an image forming apparatus including the fixing unit. 
     Description of the Related Art 
     In an image forming apparatus such as an electrophotographic printer, a copier, and a multifunction printer (MFP), a heat fixing type fixing unit is mounted. The fixing unit heats a toner image, which is transferred on a recording material, to fix the toner image to the recording material. As the fixing unit, a unit which includes a heater (heating unit) having a pattern of a resistance heating element formed on a board of a ceramic material, a fixing film rotating while sliding on the heater, and a pressing roller forming a nip portion with the heater therebetween across the fixing film is known. Japanese Patent Laid-Open No. H10-275671 describes a heater for use in the fixing unit which adopts a metal board having a higher strength against thermal stress than common ceramic materials. 
     Incidentally, to achieve an increased printing speed and an energy saving of the image forming apparatus, improvement in heat generation performance of the fixing heater is required. However, necessity to provide a countermeasure, such as thickening pattern widths of the resistance heating element and a conductor pattern, which supplies electricity to the resistance heating element, to prevent the resistance heating element and the conductor pattern from breakage due to overheating causes difficulties in miniaturizing the heater. 
     SUMMARY 
     The present disclosure provides a heating unit, a fixing unit and an image forming apparatus that can achieve both ensuring heat generation performance and miniaturization. 
     According to an aspect of the present disclosure, a heating unit includes a board including metal, an insulating layer including insulating material and formed on a surface of the board, a heating element disposed on the insulating layer and configured to generate heat by passing an electric current through the heating element, a conductive portion electrically connecting the heating element and the board to each other, a first power supplying electrode electrically connected to the heating element, and a second power supplying electrode electrically connected to the board, wherein the heating element, the conductive portion and the board constitute an electric circuit between the first power supplying electrode and the second power supplying electrode, and wherein the heating element is configured to generate the heat in a case where the first power supplying electrode and the second power supplying electrode are electrically connected to a power source and the electric current is passed through the electric circuit. 
     Further features of the present disclosure will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS.  1 A,  1 B, and  1 C  are respectively a cross-sectional view in a short direction, a plan view, and a cross-sectional view in a longitudinal direction of a heater according to a first embodiment. 
         FIG.  2    is diagram showing a drive circuit of the heater according to the first embodiment. 
         FIGS.  3 A,  3 B, and  3 C  are respectively a cross-sectional view in a short direction, a plan view, and a cross-sectional view in a longitudinal direction of a heater according to a comparative example. 
         FIGS.  4 A,  4 B, and  4 C  are respectively a cross-sectional view in a short direction, a plan view, and a cross-sectional view in a longitudinal direction of a heater according to a second embodiment. 
         FIGS.  5 A,  5 B, and  5 C  are respectively a cross-sectional view in a short direction, a plan view, and a cross-sectional view in a longitudinal direction of a heater according to a third embodiment. 
         FIG.  6    is a cross-sectional view of a fixing unit according to a fourth embodiment. 
         FIG.  7    is a schematic view of an image forming apparatus according to the fourth embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, embodiments of this disclosure will be described with reference to attached drawings. 
     First Embodiment 
       FIGS.  1 A to  1 C  are schematic views showing a configuration of a heater  100  serving as a heating unit for a fixing unit according to a first embodiment of this disclosure. 
     In the following descriptions, a direction along the longest side of a board constituting the heater  100  is referred to as a longitudinal direction X of the heater  100 . The longitudinal direction X is also a direction perpendicular to a conveyance direction of a recording material in the fixing unit, a longitudinal direction of a nip portion of the fixing unit, and a main scanning direction in an image forming operation. Among directions perpendicular to the longitudinal direction X of the heater  100 , a representative direction along a principal surface of the board is referred to as a short direction Y of the heater  100 . The principal surface is a surface on which a heating element is disposed. Further, a direction perpendicular to the longitudinal direction and the short direction (i.e., a normal direction of the principal surface of the board) is referred to as a thickness direction Z of the heater  100 . 
     Layer Structure of Heater 
       FIG.  1 A  is a cross-sectional view of the heater  100  taken along a virtual plane spreading in the short direction Y and thickness direction Z, and viewed in the longitudinal direction X.  FIG.  1 B  is a plan view of the heater  100 , when viewed from a side, in the thickness direction Z, on which a heating element  102  is disposed.  FIG.  1 C  is a cross-sectional view of the heater  100  taken along a virtual plane spreading in the longitudinal direction X and thickness direction Z, and viewed in the short direction Y. 
     As shown in  FIGS.  1 A to  1 C , the heater  100  includes a board  101  having an elongated plate shape made of a metal or an alloy at least as a chief material and the heating element  102 , serving as a heating layer generating heat by passing an electric current therethrough. The board  101  is a metal substrate. The heater  100  further includes an insulating layer  103  insulating the heating element  102  and the board  101 , and a protective layer  104  protecting the heating element  102 . Further, so as to prevent a warpage of a base material for the board  101  at manufacturing, the heater  100  includes an insulating layer  105  also on a surface of the board  101  opposite to the surface on which the heating element  102  is disposed. 
     As a material for the board  101 , stainless steel, nickel, copper, aluminum, or alloy using these metals as the chief material are suitably used. Among these, the stainless steel is preferred in view of strength, a heat resisting property, and corrosion. A type of stainless steel is not limited, and it is acceptable to appropriately choose the type considering such as required mechanical strength, a linear expansion coefficient tailored to formation of the insulating layers  103  and  105  and the heating element  102  described below, and easiness of procurement of a plate in a market. To cite an example, martensitic or ferritic chromium-based stainless steel (400 series stainless) have a relatively low linear expansion coefficient even in stainless steel, and are suitably used because of easiness in the formation of the insulating layers  103  and  105  and the heating element  102 . 
     A thickness of the board  101  is determined considering the strength, a heat capacity, and a heat radiation performance. In a case where the thickness of the board  101  is small (that is, thin), since the heat capacity is small, it is favorable to a quick start performance, but issues such as a distortion at calcination of the heating element  102  easily occurs if the thickness is too thin. On the other hand, in a case where the thickness of the board  101  is large (that is, thick), it is favorable in respect of the distortion at the calcination of the heating element  102 , but unfavorable to the quick start since the heat capacity is large if the thickness is too thick. In considering of a balance of mass productivity, a cost, and a performance, the preferred thickness of the board  101  is 0.2 to 2.0 mm. To be noted, the quick start performance indicates a shortness of a time required for increasing a temperature, when the heating of the heater  100  is started in a state where the image forming apparatus is in a stand-by or power OFF state not performing the image forming operation, to a proper value for a heat fixing so that it becomes possible to perform an image forming operation. 
     While a material for the insulating layers  103  and  105  and the protective layer  104  is not particularly limited, it is necessary to choose an insulating material having a heat resistance in view of an actual use temperature. As the material, glass and PI (polyimide) are preferred in consideration of the heat resistance, and, in a case of the glass, it is acceptable to particularly choose a powder material suitably within a range which does not hamper characteristics of this embodiment. When necessary, it is also acceptable to mix a thermally conductive filler and the like having an insulation property. 
     Either the same or different material(s) is/are used for the insulating layer  103 , the protective layer  104 , and the insulating layer  105 . Regarding thicknesses of the insulating layers  103  and  105  and the protective layer  104 , similarly, it is acceptable to adopt either the same thickness or the thicknesses different to each other as necessary. When an insulating layer of the glass and PI (polyimide) is formed on a surface of the board  101 , it is preferred to properly adjust the linear expansion coefficients of the board and the insulating material so that neither a crack nor a peeling occurs on the insulating layer due to differences in the linear expansion coefficients between the materials. 
     Composition of Heating Element 
     The heating element  102  is calcinated after printing a heating resistor paste mixed with (A) a conductive component, (B) a glass component, and (C) an organic binder component on the insulating layer  103 . Since, when the heating resistor paste is calcinated, the organic binder component (C) is burned off and the components (A) and (B) remained, so that the heating element  102  containing the conductive component and the glass component is formed. 
     As the conductive component (A), a silver and palladium alloy (Ag—Pd), ruthenium oxide (RuO 2 ), and the like are used alone or in combination, and a suitable sheet resistance is 0.1 Ω/sq (ohms per square) to 100 kΩ/sq. Further, it is acceptable to include a very small quantity of a material other than (A) to (C) above to an extent that does not hamper the characteristics of this embodiment. 
     Configuration of Power Supplying Electrode and Conductor Pattern 
     Next, a circuit configuration so as to passing an electric current to (i.e., to energize) the heating element  102  in the heater  100  will be described. As shown in  FIGS.  1 B and  1 C , the heater  100  includes power supplying electrodes  105   a  and  106   a  and conductor patterns  105   b  and  106   b . Further, as described below, in this embodiment, also the board  101  made of metal constitutes a part of an electric circuit in which the electric current flows so as to cause the heating element  102  to generate heat. 
     In  FIGS.  1 B and  1 C , the power supplying electrodes  105   a  and  106   a  and the conductor patterns  105   b  and  106   b  include silver (Ag), platinum (Pt), gold (Au), silver and platinum alloy (Ag—Pt), silver and palladium alloy (Ag—Pd), and the like as the conductive component. Similar to the heating resistor paste for the heating element  102 , the power supplying electrodes  105   a  and  106   a  and the conductor patterns  105   b  and  106   b  are each formed by printing and thereafter calcinating a paste mixed with (A) a conductive component, (B) a glass component, and (C) an organic binder component. 
     The power supplying electrode  105   a  and the conductor pattern  105   b  are formed on the insulating layer  103 . The power supplying electrode  105   a  serves as a first power supplying electrode electrically connected to the heating element  102 . Extending in the longitudinal direction X on the insulating layer  103 , the conductor pattern  105   b  electrically connects the power supplying electrode  105   a  and a first end of the heating element  102  to each other, and is covered at least partially by the protective layer  104 . On the other hand, the power supplying electrode  105   a  is exposed at least partially from the protective layer  104  so that the power supplying electrode  105   a  can be connected to a power circuit (drive circuit), described later. The power supplying electrode  105   a  and the conductor pattern  105   b  serve as a first conductive part to energize the heating element  102 . 
     The power supplying electrode  106   a , which serves as a second power supplying electrode electrically connected to the board  101 , is directly formed on the board  101 . The power supplying electrode  106   a  is exposed at least partially from the protective layer  104  so that the power supplying electrode  106   a  can be connected to the power circuit, described later. In this embodiment, two power supplying electrodes  105   a  and  106   a  are disposed on the same side in the longitudinal direction X of the heating element  102  (i.e., right side of the heating element  102  in  FIG.  1 B ), and on the same side as the heating element  102  in the thickness direction Z (i.e., upper side of the board  101  in  FIG.  1 C ). Further, in the longitudinal direction X, two power supplying electrodes  105   a  and  106   a  and the conductor pattern  105   b  are positioned outside an area in which the heating element  102  is disposed. The power supplying electrode  106   a  serves as a connecting portion connected to the power circuit with the first conductive part so as to energize the heating element  102 . 
     The conductor pattern  106   b  extends in the longitudinal direction X along a surface of the insulating layer  103  from a second end opposite to the first end of the heating element  102  in the longitudinal direction X, and, bending along an end of the insulating layer  103  in the longitudinal direction X, is connected to the board  101  (refer to  FIG.  1 C ). That is, the conductor pattern  106   b  serves as a conductive portion (or, second conductive part) electrically connecting the heating element  102  and the electrically conductive board  101  to each other. Further, in the longitudinal direction X, the conductor pattern  106   b  is positioned outside the area in which the heating element  102  is disposed. 
     Since the power supplying electrodes  105   a  and  106   a  and the conductor patterns  105   b  and  106   b  are members through which the electric current flows to supply an electricity to the heating element  102 , volume resistances are all set at sufficiently low in comparison with the heating element  102 . 
     For the heating resistor paste, the paste for forming the power supplying electrode  105   a  and  106   a , and the paste for forming the conductor pattern  105   b  and  106   b , described above, it is necessary to choose a material which softens and melts at a temperature below a melting point of the board  101  and has the heat resistance in view of the actual use temperature. Further, it is acceptable to mix a glass filler and the like in the power supplying electrode  106   a  and the conductor pattern  106   b  depending on required adhesion strength to the board  101 . 
     While a forming method of the insulating layers  103  and  105 , the protective layer  104 , the power supplying electrodes  105   a  and  106   a , and the conductor patterns  105   b  and  106   b  is not particularly limited, as an example, it is possible to smoothly perform formation by a screen printing method and the like. In addition, it is acceptable to perform the formation using a vapor deposition method and the like. 
     Heater Drive Circuit 
       FIG.  2    shows a configuration example of a drive circuit of the heater  100  of this embodiment. As shown in the figure, by connecting the heater  100  to a commercial alternating current power source  200 , serving as a power source, it is possible to supply a source voltage to the heating element  102 , and generate the heat at the heating element  102 . At this time, power supply to the heating element  102  is performed via the power supplying electrodes  105   a  and  106   a , the conductor patterns  105   b  and  106   b , and the board  101  of the heater  100 . 
     Further, it is possible to control an amount of heat generated by the heater  100  by energizing and shutting off the electricity to the heating element  102  by energizing/shutting off of a triac  202  disposed between the source voltage and the power supplying electrode  106   a . Both of resistors  203  and  204  are bias resistors for the triac  202 , and a phototriac coupler  205  is a device to control the triac  202  while securing an insulation between the primary side and the secondary side of the circuit. 
     A CPU (central processing unit)  209  controls the triac  202  based on a temperature detected by a thermistor  210 , serving as a temperature detection element, so as to, for example, bring a temperature close to a preset target temperature. In particular, a change in a resistance value of the thermistor  210  in response to a temperature change is detected as a change in a partial voltage between the thermistor  210  and a resistor  211 , and is input to the CPU  209  as temperature information (i.e., detected temperature signal) converted into a digital value by A/D (analog to digital) conversion. The CPU  209  outputs a heater drive instruction signal based on the input detected temperature signal. The heater drive instruction signal is input to a transistor  207  via a resistor  208 , and the phototriac coupler  205  is turned ON and OFF by the transistor  207 . Then, by energizing/shutting off of the triac  202  in accordance with lighting/extinction of a light emitting diode  205   a , the energizing/shutting off of the heater  100  is performed. To be noted, a resistor  206  is a resistor to regulate an electric current of the light emitting diode  205   a.    
     To be noted, the drive circuit shown here is an example, and it is acceptable to function the heater  100  by connecting a drive circuit with a different circuit configuration to the power supplying electrodes  105   a  and  106   a.    
     Comparison of First Embodiment and Comparative Example 
     So as to describe an advantage of this embodiment, this embodiment will be described while comparing with a heater  300  of a comparative example shown in  FIGS.  3 A to  3 C . 
     As shown in  FIG.  3 A , the heater  300  of the comparative example includes, similar to this embodiment, a board  301  made of metal, a heating element  302  generating the heat by passing an electric current therethrough, an insulating layer  303  insulating the board  301  and the heating element  302  from each other, and a protective layer  304  protecting the heating element  302 . Further, so as to prevent a warpage of a base material for the board  301  at manufacturing, an insulating layer  305  is included also on a surface of the board  301  opposite to the surface on which the heating element  302  is disposed. 
     A difference from this embodiment is that, as shown in  FIGS.  3 B and  3 C , in the comparative example, all of the power supplying electrode  306   a  and the conductor pattern  306   b  are printed and calcinated on the insulating layer  303 . That is, in the comparative example, a heater circuit (i.e., an electric circuit consisting of the heating element  302 , the power supplying electrodes  305   a  and  306   a , and the conductor patterns  305   b  and  306   b ) to supply the electricity to the heating element  302  is all disposed on the insulating layer  303 . Since the board  301  is insulated from the heater circuit by the insulating layer  303 , even if the power supplying electrodes  305   a  and  306   a  are connected to the source voltage, the electric current does not flow to the board  301 . 
     At this point, as shown in  FIG.  1 A , a short width W of a circuit layout area on the board  101  of this embodiment is equal to a short width W 1  which is the maximum width of the heating element  102  in the short direction Y, and expressed by an equation (1) below.
 
 W=W 1  (1)
 
     Note that a circuit layout area means a necessary area on the board  101 , when viewed in the thickness direction Z, so as to mount the heater circuit, and the short width W is the maximum width of the circuit layout area in the short direction Y. 
     On the other hand, a short width W′ of a circuit layout area on the board  301  of the comparative example is expressed by an equation (2) below. Note that W′ 1  indicates the maximum width of the heating element  302  in the short direction Y, W 2  indicates the maximum width of the conductor pattern  306   b  in the short direction Y, and W 3  indicates a necessary distance between the heating element  302  and the conductor pattern  306   b  for manufacturing.
 
 W′=W′ 1+ W 2+ W 3  (2)
 
     In a case where the short widths W 1  and W′ 1  in this embodiment and the comparative example are equal, the short width of the circuit layout area of this embodiment will be smaller than the short width of the circuit layout area of the comparative example by (W 2 +W 3 ). This is because, although the conductor pattern  306   b  is disposed alongside the heating element  302  in the short direction Y in the comparative example, in this embodiment, the metal board  101  is utilized as a circuit element substituting a function of the conductor pattern  306   b . To be noted, in the configuration of the comparative example, miniaturization in the short direction Y by disposing the power supplying electrode  306   a  and the conductor pattern  306   b  on an opposite side of the power supplying electrode  305   a  across the heating element  302  is also considered. However, in a case where the power supplying electrodes  305   a  and  306   a  are far apart from each other, contacts of the power circuit supplying the power to the heater  300  are also brought into far apart positions, and, therefore, it is necessary to provide a wiring space for the contacts so that the miniaturization of a fixing unit in whole is not attained. That is, since, in this embodiment, the power supplying electrodes  105   a  and  106   a  are disposed on the same side as the heating element  102  in the longitudinal direction X (on a right-hand side in  FIG.  1 B ), it is possible to miniaturize a layout of connectors and wiring connected to the power supplying electrodes  105   a  and  106   a.    
     Incidentally, if a reduction in the short width W′ in the comparative example is intended, it is necessary to reduce W′ 1  or W 3 . However, if W′ 1  or W 3  is reduced (narrowing a width of the heating element  302 ), there is a possibility of breakage due to overheating, or it is necessary to accept a decrease in heat generation performance to prevent the breakage. On the other hand, in this embodiment, since it becomes possible to keep the short width W of the circuit layout area small while securing the short width W 1  of the heating element  102 , it is possible to compatibly ensure the heat generation performance of the heater  100  and miniaturize the heater  100 . Especially, in this embodiment, the power supplying electrodes  105   a  and  106   a , the heating element  102 , and the conductor pattern  106   b  are arranged in a line in the longitudinal direction X, and positions, in the short direction Y, of the power supplying electrodes  105   a  and  106   a , the heating element  102 , and conductor pattern  106   b  overlap each other. The layout as described above is especially effective in compatibly ensuring the heat generation performance of the heater  100  and miniaturizing the heater  100 . It is acceptable if the positions of the power supplying electrodes  105   a  and  106   a , the heating element  102 , and the conductor pattern  106   b  in the short direction Y overlap each other at least partially. 
     To be noted, in the equation (1), it was described that the short width W 1  of the heating element  102  is larger than the maximum widths of the power supplying electrode  105   a  and the conductor pattern  105   b  in the short direction Y. Generally, this condition is met so as to prevent the overheating of the heating element  102  generating the heat by the energization. However, even in a case where the width of the power supplying electrode  105   a  or the conductor pattern  105   b  in the short direction Y is larger than the short width W 1  of the heating element  102 , it is similarly not necessary to dispose such circuit element and the conductor pattern  106   b  alongside in the short direction Y as shown in  FIG.  3 B . Accordingly, regardless of a width relation between the short width W 1  of the heating element  102  and the short widths of the power supplying electrode  105   a  and the conductor pattern  105   b , it is possible to compatibly ensure the heat generation performance of the heater  100  and miniaturize the heater  100 . 
     Second Embodiment 
     As a second embodiment, an embodiment in which the heating element and the board are electrically connected to each other through an opening portion disposed in the insulating layer will be described using  FIGS.  4 A to  4 C . Hereinafter, the elements put with the same reference characters as the first embodiment have substantially the same configurations and functions as the first embodiment, and differences from the first embodiment will be mainly described. 
       FIG.  4 A  is a cross-sectional view of a heater  100 A of this embodiment taken along a virtual plane spreading in the short direction Y and the thickness direction Z, and viewed in the longitudinal direction X.  FIG.  4 B  is a plan view of the heater  100 A, when viewed from a side, in the thickness direction Z, on which the heating element  102  is disposed.  FIG.  4 C  is a cross-sectional view of the heater  100 A taken along a virtual plane spreading in the longitudinal direction X and the thickness direction Z, and viewed in the short direction Y. 
     As shown in  FIGS.  4 B and  4 C , different from the first embodiment, the opening portion  401  piercing through from the surface of the insulating layer  103  to the board  101  is disposed inside a periphery of the insulating layer  103  insulating the heating element  102  and the board  101  when viewed in the thickness direction Z. Further, the conductor pattern  106   b , serving as the second conductive portion, is formed from an end of the heating element  102  in the longitudinal direction X to the board  101  via the opening portion  401 . Herewith, the heating element  102  and the board  101 , which is electrically conductive, are electrically connected to each other. 
     At this point, a case where, similar to the first embodiment, the conductor pattern  106   b  ( FIG.  4 C ) bending along the insulating layer  103  is formed by the screen printing method is considered. In this case, since there is a level difference of as much as a thickness of the insulating layer  103  at an end of the insulating layer  103 , it is sometimes difficult to secure a sufficient film thickness in the conductor pattern  106   b . In a case where the film thickness of the conductor pattern  106   b  is insufficient, an occurrence of a conduction failure between the heating element  102  and the board  101  is concerned. 
     On the other hand, as shown in  FIGS.  4 A to  4 C , by disposing the opening portion  401  in the insulating layer  103  and coating an inside of the opening portion  401  with the paste of the conductor pattern  106   b , printing formation of the conductor pattern  106   b  becomes easier. Accordingly, without depending on conditions such as the thickness of the insulating layer  103 , it is possible to secure the thickness of the conductor pattern  106   b , and further reduce a possibility of the occurrence of the conduction failure between the heating element  102  and the board  101 . 
     Third Embodiment 
     As a third embodiment, an embodiment in which a layout of the power supplying electrodes is changed will be described using  FIGS.  5 A to  5 C . Hereinafter, the elements put with the same reference characters as the first and second embodiments have substantially the same configurations and functions as the first and second embodiments, and differences from the first embodiment will be mainly described. 
       FIG.  5 A  is a cross-sectional view of a heater  100 B of this embodiment taken along a virtual plane spreading in the short direction Y and the thickness direction Z, and viewed in the longitudinal direction X.  FIG.  5 B  is a plan view of the heater  100 B, when viewed from a side, in the thickness direction Z, on which the heating element  102  is disposed.  FIG.  5 C  is a cross-sectional view of the heater  100 B taken along a virtual plane spreading in the longitudinal direction X and the thickness direction Z, and viewed in the short direction Y. 
     As shown in  FIGS.  5 B and  5 C , in this embodiment, different from the first and second embodiments, a power supplying electrode  506   a  (connecting portion) that is connected to the board  101  is disposed on a surface (i.e., second surface) different from the surface (i.e., first surface, upper surface in  FIGS.  5 A and  5 C ) on which the heating element  102  of the heater  100 B is disposed. In a configuration example shown in  FIGS.  5 A to  5 C , the power supplying electrode  506   a  is disposed on an opposite side, in the thickness direction Z, of the surface on which the heating element  102 , the power supplying electrode  105   a , and the conductor patterns  105   b  and  106   b  are disposed. 
     At this point, in the configurations of the first and second embodiments shown in  FIGS.  1 B and  1 C  and  FIGS.  4 B and  4 C , the power supplying electrode  106   a  is disposed on the same surface as the surface on which the heating element  102 , the power supplying electrode  105   a , and the conductor patterns  105   b  and  106   b  are disposed. Therefore, the power supplying electrode  106   a  is disposed in a line in the longitudinal direction X with these circuit elements, accepting that the width of the circuit layout area in the longitudinal direction X is enlarged by the width of the power supplying electrode  106   a  in the longitudinal direction X. 
     On the other hand, in this embodiment, the power supplying electrode  506   a  is disposed on the different surface from the surface on which the heating element  102 , the power supplying electrode  105   a , and the conductor patterns  105   b  and  106   b  are disposed. Therefore, it is possible to overlap a position of the power supplying electrode  506   a  in the longitudinal direction X ( FIG.  5 C ) with, for example, the position of the power supplying electrode  105   a  in the longitudinal direction X. Accordingly, by the configuration of this embodiment, a required length of the board  101  in the longitudinal direction X can be reduced at least by the maximum width L of the power supplying electrode  506   a  in the longitudinal direction X, and it is possible to further miniaturize the heater  100 B. 
     To be noted, while, in this embodiment, the power supplying electrode  506   a  is disposed on the surface of the board  101  opposite to the heating element  102  and the power supplying electrode  105   a  in the thickness direction Z, it is acceptable to dispose the power supplying electrode  506   a  on a further different surface (for example, on a side surface in the short direction Y). 
     Fourth Embodiment 
     As a fourth embodiment, a fixing unit  600  including the heater  100  described in the first embodiment will be described using  FIGS.  6  and  7   . Hereinafter, the elements put with the same reference characters as the first embodiment have substantially similar configurations and functions to the first embodiment. 
     The fixing unit  600  shown in  FIG.  6    is an image heating unit of the heat fixing type which fixes a toner image transferred onto a recording material P on the recording material P by heating at a nip portion. The fixing unit  600  includes a tubular film  601 , which is a fixing member, the heater  100  disposed in an internal space of the film  601 , a holding member  602  holding the heater  100 , and a pressing roller  604 , which is a pressing member. The heater  100  held by the holding member  602  and the pressing roller  604  facing the heater  100  come into pressure contact with each other across the film  601 , and herewith the nip portion N is formed. That is, the heater  100  and the holding member  602  function as a nip portion forming unit in this embodiment. 
     The film  601  is a heat resistance film formed into a tubular shape, which is also called an endless belt or an endless film, and at least includes a base layer. A material for the base layer is a heat resistance resin such as polyimide or metal such as stainless steel. Further, it is acceptable to dispose an elastic layer such as a heat resistance rubber on a surface of the film  601 . The pressing roller  604  includes a core metal  605  made of iron, aluminum, and the like and an elastic layer  606  made of a silicone rubber and the like. 
     The heater  100  is held by the holding member  602  made of a heat resistance resin. In the illustrated configuration example, the heater  100  is disposed so that the longitudinal direction X of the heater  100  is substantially parallel to rotational axis directions of the film  601  and the pressing roller  604  and the short direction Y is approximately parallel to the conveyance direction of the recording material P at the nip portion N. Further, with respect to the thickness direction Z, the heater  100  is disposed so that a surface (i.e., surface of the protective layer  104 ) of the heater  100  on a side on which the heating element  102  is disposed, comes into contact with an inner surface of the film  601 . 
     The holding member  602  also includes a guide function guiding rotation of the film  601 . The holding member  602  is applied a downward urging force in the figure from a stay  603  fixed to a frame member of the fixing unit  600  by a spring, not shown. Pressure to press the toner image at the nip portion N is generated by this urging force of the spring. 
     The pressing roller  604  receives a power from a drive source, not shown, and rotates counter-clockwise in the figure. By the rotation of the pressing roller  604 , the film  601  is rotatably driven clockwise in the figure. Further, before the recording material P with the toner image formed has reached the nip portion N, the energization of the heater  100  is started, and a temperature at the nip portion N is maintained at a target temperature suitable for the heat fixing during a passage of the recording material P through the nip portion N. 
       FIG.  7    shows a laser beam printer (hereinafter simply referred to as a printer  700 ) adopting an electrophotographic system as an example of the image forming apparatus. When the printer  700  has received an execution instruction of the image forming operation, a scanner unit  3  irradiates a photosensitive member  1 , serving as an image bearing member, with a laser beam in accordance with image information. By scanning a surface of the photosensitive member  1 , which has been charged in a predetermined polarity by a charge roller  2  beforehand, with the laser beam, an electrostatic latent image is formed on the surface of the photosensitive member  1  in accordance with the image information. Thereafter, a developing unit  4  supplies a toner to the photosensitive member  1 , and the electrostatic latent image is developed and visualized as a toner image. 
     By rotation of the photosensitive member  1  in an arrow R 1  direction, the toner image carried on the photosensitive member  1  reaches a transfer nip, serving as a transfer portion. The transfer nip is a nip portion formed between the photosensitive member  1  and a transfer roller  5 , serving as a transfer unit. By applying a voltage to the transfer roller  5 , the toner image is transferred to the recording material P sent from a cassette  6  by a pickup roller  7 . The surface, which has passed through the transfer nip, of the photosensitive member  1  is cleaned by a cleaner  8 . The recording material P with the toner image transferred is conveyed to the fixing unit  600 . 
     Then, the fixing unit  600  shown in  FIG.  6    performs a fixing process in which the toner image on the recording material P is provided with the heat and pressure at the nip portion N, while nipping and conveying the recording material P. Herewith, the toner is melted and thereafter cooled and solidified so that a fixed image fixed on the recording material P is obtained. 
     The recording material P passed through the fixing unit  600  is discharged to a tray  11  by a sheet discharge roller  10  ( FIG.  7   ). To be noted, for the recording material P, it is possible to use various kinds of sheets different in sizes and materials including, but not limited to, a paper such as a standard paper and a cardboard, a plastic film, a cloth, various kinds of sheet materials applied with a surface treatment such as a coated paper, and a specially shaped sheet such as an envelope and an index paper. Further, while a direct transfer system directly transferring the toner image from the photosensitive member  1  to the recording material P is described in this description, it is acceptable to apply a technique described below to an image forming apparatus which transfers the toner image formed on the image bearing member to the recording material via an intermediate transfer member such as an intermediate transfer belt. In that case, a transfer mechanism including a primary transfer member primarily transferring the toner image from the image bearing member to the intermediate transfer member and a secondary transfer member secondarily transfer the toner image from the intermediate transfer member to the recording material serves as the transfer unit. 
     As described above, by using the heater  100  of this embodiment for the fixing unit  600 , it is possible to miniaturize the fixing unit  600  and, furthermore, the printer  700 . 
     To be noted, it is acceptable to use the heaters  100 A and  100 B of the second and third embodiments for the fixing unit  600  in place of the heater  100  of the first embodiment. Further, it is not limited to the configuration example shown in  FIG.  6   , and acceptable to dispose in a configuration in which an opposite side (the side of the insulating layer  105 ), in the thickness direction Z, of the surface on which the heating element  102  of the heater  100  is disposed comes into contact with the inner surface of the film  601 . 
     Further, while the heater  100  directly comes into contact with the inner surface of the film  601  in the fixing unit  600  of  FIG.  6   , it is acceptable to dispose a plate shaped or sheet shaped member having a high heat conductivity (for example, sheet shaped member made of ferroalloy and aluminum) between the heater  100  and the inner surface of the film  601 . That is, it is acceptable to use a nip portion forming unit in which the heater  100  is configured to heat the film via a sliding member sliding along the inner surface of the film  601 . 
     OTHER EMBODIMENTS 
     While the present disclosure has been described with reference to exemplary embodiments, it is to be understood that the disclosure 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. 
     This application claims the benefit of Japanese Patent Application No. 2020-112790, filed on Jun. 30, 2020, which is hereby incorporated by reference herein in its entirety.