Patent Description:
Conventionally, as a fixing apparatus provided in an image forming apparatus, there is an apparatus which has an endless belt (also referred to as an endless film), a flat heater which is in contact with the inner surface of the endless belt, and a roller which forms a nip portion with the heater via the endless belt. PTL <NUM> proposes a method for sensing the temperature of the nip portion with high accuracy by forming a thermistor on the surface of a heater substrate on the side of the endless belt.

PTL <NUM> discloses a heater for a fixing unit in an image forming apparatus.

However, in the case where the thermistor is formed on the surface of the heater on the side of the nip portion, in order to secure an adequate withstand voltage of the fixing apparatus, it is necessary to form the thermistor such that the thermistor has a thick surface protective layer, or increase the width of the substrate of the heater. When the thickness of the surface protective layer of the thermistor is increased, a problem arises in that the heat transfer efficiency of the heater and accuracy in sensing the nip temperature are reduced. When the width of the substrate of the heater is increased, a problem arises in that the size of the apparatus is increased.

An object of the present invention is to provide a technique which allows a temperature sensing element to be disposed on a sliding surface of a heater which slides on a film while preventing a reduction in each of the thermal responsiveness and the heat transfer efficiency of the heater and preventing an increase in the size of the heater.

In order to achieve the above object, an image forming apparatus of the present invention is an image forming apparatus comprising:.

According to the present invention, it is possible to dispose the temperature sensing element on the sliding surface of the heater which slides on the film while preventing the reduction in each of the thermal responsiveness and the heat transfer efficiency of the heater and preventing the increase in the size of the heater.

Hereinafter, a description will be given, with reference to the drawings, of embodiments (examples) of the present invention. However, the sizes, materials, shapes, their relative arrangements, or the like of constituents described in the embodiments may be appropriately changed according to the configurations, various conditions, or the like of apparatuses to which the invention is applied. Therefore, the sizes, materials, shapes, their relative arrangements, or the like of the constituents described in the embodiments do not intend to limit the scope of the invention to the following embodiments.

<FIG> is a schematic cross-sectional view of an image forming apparatus of an embodiment of the present invention. An image forming apparatus <NUM> of the present embodiment is a laser printer which forms an image on a recording material by using the electrophotographic system.

When a print signal is generated, a scanner unit <NUM> emits laser light modulated according to image information, and scans the surface of a photosensitive drum (electrophotographic photosensitive member) <NUM> which is charged to a predetermined polarity by a charging roller <NUM>. With this, an electrostatic latent image is formed on the photosensitive drum <NUM> serving as an image bearing member. Toner charged to a predetermined polarity is supplied to the electrostatic latent image from a developing roller <NUM>, and the electrostatic latent image on the photosensitive drum <NUM> is thereby developed as a toner image (developer image). On the other hand, a recording material (recording sheet) P stacked on a sheet feeding cassette <NUM> is fed one by one by a pickup roller <NUM>, and is transported toward a resist roller pair <NUM> by a transport roller pair <NUM>. Further, the recording material P is transported to a transfer position from the resist roller pair <NUM> in synchronization with timing at which the toner image on the photosensitive drum <NUM> reaches the transfer position formed by the photosensitive drum <NUM> and a transfer roller <NUM> serving as a transfer member. The toner image on the photosensitive drum <NUM> is transferred to the recording material P when the recording material P passes through the transfer position. Thereafter, the recording material P is heated in a fixing apparatus <NUM> serving as a fixing portion, and the toner image is heated and fixed to the recording material P. The recording material P which bears the fixed toner image is discharged to a sheet discharge tray in the upper portion of the image forming apparatus <NUM> by transport roller pairs <NUM> and <NUM>. Note that the photosensitive member <NUM> is cleaned by a cleaner <NUM>. A motor <NUM> drives the fixing apparatus <NUM> and the like. The reference numeral <NUM> denotes a control circuit connected to a commercial AC power supply (commercial power supply) <NUM>, and power is supplied to the fixing apparatus <NUM> by the control circuit <NUM>.

The photosensitive drum <NUM>, the charging roller <NUM>, the scanner unit <NUM>, the developing roller <NUM>, and the transfer roller <NUM> which are described above constitute an image forming portion which forms an unfixed image on the recording material P. In addition, in the present embodiment, a developing unit including the photosensitive drum <NUM>, the charging roller <NUM>, and the developing roller <NUM> and a cleaning unit including the cleaner <NUM> are configured to be attachable to and detachable from the apparatus main body of the image forming apparatus <NUM> as a process cartridge <NUM>.

The image forming apparatus <NUM> of the present embodiment supports a plurality of recording material sizes. In the sheet feeding cassette <NUM>, it is possible to set, e.g., Letter paper (about <NUM> × <NUM>), Legal paper (about <NUM> × <NUM>), A4 paper (<NUM> × <NUM>), and Executive paper (about <NUM> × <NUM>). Further, it is also possible to set JIS B5 paper (<NUM> × <NUM>), and A5 paper (<NUM> × <NUM>). The image forming apparatus of the present embodiment is basically a laser printer which longitudinally sends a sheet (transports a sheet such that the long side of the sheet is horizontal to a transport direction). Note that, similarly to the present embodiment, the present invention can also be applied to a printer which laterally sends a sheet. The recording materials having, among the widths of a standard-sized recording material supported by the apparatus (the widths of the recording material in a catalogue), the largest width are Letter paper and Legal paper, and the width of each paper is about <NUM>. The recording material P with a paper width smaller than the maximum size supported by the apparatus is defined as small-sized paper in the present embodiment.

<FIG> is a cross-sectional view of the fixing apparatus <NUM> of the present embodiment. The fixing apparatus <NUM> has a fixing film (hereinafter referred to as a film) <NUM>, a heater <NUM> which is in contact with the inner surface of the film <NUM>, a pressure roller <NUM> which forms a fixing nip portion N with the heater <NUM> via the film <NUM>, and a metal stay <NUM>.

The film <NUM> is a heat-resistant film formed into a tubular shape which is also referred to as an endless belt or an endless film, and the material of its base layer is a heat-resistant resin such as a polyimide, or metal such as stainless steel. An elastic layer made of heat-resistant rubber or the like may be provided on the surface of the film <NUM>. The pressure roller <NUM> has a core metal <NUM> made of a material such as iron or aluminum, and an elastic layer <NUM> made of a material such as silicone rubber. The heater <NUM> is held by a holding member <NUM> made of a heat-resistant resin. The holding member <NUM> also has a guide function of guiding the rotation of the film <NUM>. The stay <NUM> applies pressure of a spring which is not shown to the holding member <NUM>. The pressure roller <NUM> receives power from the motor <NUM> and rotates in an arrow direction. The film <NUM> is caused to rotate by the rotation of the pressure roller <NUM>. The recording material P bearing an unfixed toner image is heated and subjected to a fixing process while being held and transported by the fixing nip portion N.

The heater <NUM> has resistance heating elements (hereinafter referred to as heating elements) <NUM> and <NUM> provided on a surface of a ceramic substrate <NUM> on a side where the heater <NUM> is in contact with the holding member <NUM> (hereinafter, this surface is defined as a back surface). On a surface on the side of the fixing nip portion N where the heater <NUM> is in contact with the film <NUM> (hereinafter, this surface is defined as a sliding surface), a thermistor T2 (T1 to T3) serving as a temperature sensing element is provided. A surface protective layer <NUM> is a layer for protecting the thermistor T2 (T1 to T3) and securing slidability of the fixing nip portion N, and the material of the surface protective layer <NUM> is insulating glass. The surface protective layer <NUM> is formed so as to cover the thermistor T2 (T1 to T3) on an opposing surface which opposes the fixing nip portion N in the ceramic substrate <NUM>. A surface protective layer <NUM> serving as an insulating layer provided on a side opposite to the fixing nip portion N is used for insulating the heating elements, and the material of the surface protective layer <NUM> is insulating glass.

In addition, a safety element <NUM> such as a thermo switch or a thermal fuse, which operates in response to abnormal heat generation of the heater <NUM> to interrupt power supplied to the heater <NUM>, abuts directly or indirectly on the heater <NUM> via the holding member <NUM>.

The configuration of the heater <NUM> according to the present embodiment will be described by using <FIG> is a cross-sectional view of the heater <NUM>, and <FIG> is a plan view of each layer of the heater <NUM>. <FIG> shows a transport reference position X of the recording material P in the image forming apparatus <NUM> of the present embodiment. The transport reference in the present embodiment is a center reference, and the recording material P is transported such that the center line in a direction orthogonal to the transport direction of the recording material P (i.e., a width direction) moves along the transport reference position X. The sheet feeding cassette <NUM> has a position control plate which controls the position of the recording material P in the width direction. The recording material P stacked on the sheet feeding cassette <NUM> is fed and then transported such that the central portion of the recording material P passes through the transport reference position X. <FIG> is a cross-sectional view of the heater <NUM> at the transport reference position X.

The heater <NUM> has the heating elements <NUM> and <NUM> on a back surface layer <NUM>. In addition, on a back surface layer <NUM> of the heater <NUM>, the insulating surface protective layer <NUM> (made of glass in the present embodiment) which covers the heating elements <NUM> and <NUM> is provided. On a sliding surface layer <NUM> of the heater <NUM>, the thermistor T2 (T1 to T3) and electrical conductors (EG1, ET1-<NUM> to ET1-<NUM>) for connection with the thermistors are provided. Further, on a sliding surface layer <NUM> of the heater <NUM>, the insulating surface protective layer <NUM> (made of glass in the present embodiment) which covers the thermistor T2 (T1 to T3) and the electrical conductors (EG1, ET1-<NUM> to ET1-<NUM>) is provided. The surface protective layer (second insulating layer) <NUM> of the present embodiment is thinner than the surface protective layer (first insulating layer) <NUM> that requires basic insulation. Although details will be described later, the surface protective layer (second insulating layer) <NUM> of the present embodiment does not need to be subjected to the basic insulation. It is only required that the surface protective layer <NUM> is subjected to functional insulation such that the thermistors T1 to T3 are not damaged. Consequently, the surface protective layer <NUM> can be made thinner than the surface protective layer <NUM>, and thermal conductivity from the heater <NUM> to the film <NUM> can be increased by making the surface protective layer <NUM> thinner than the surface protective layer <NUM>.

As shown in <FIG>, on the back surface layer <NUM> of the heater <NUM>, the heating element <NUM> and the heating element <NUM> are connected in series via an electrical conductor <NUM>, and power can be supplied from electrodes E1 and E2. On the back surface layer <NUM> of the heater <NUM>, the surface protective layer <NUM> is provided so as to cover the back surface layer <NUM> except the portions of the electrodes E1 and E2. By covering the electrical conductor <NUM> and the heating elements <NUM> and <NUM> with the surface protective layer <NUM> and the substrate <NUM>, the basic insulation is provided between the electrical conductor <NUM> and the heating elements <NUM> and <NUM> on the primary side of the commercial power supply <NUM>, and the film <NUM> and the thermistor T2. Herein, the basic insulation denotes insulation which is provided for performing basic protection against an electric shock. In addition, double insulation which will appear in the following description denotes insulation in which additional insulation for protection in the case where the basic insulation fails is further performed in addition to the basic insulation. Reinforced insulation is single insulation which provides protection against an electric shock at a level similar to the level of protection by the double insulation. Note that, in the present embodiment, the reinforced insulation and the double insulation are collectively referred to as reinforced insulation.

On the sliding surface layer <NUM> of the heater <NUM>, the thermistors T1, T2, and T3 formed of a material having a positive TCR (temperature coefficient of resistance) (PTC: positive temperature coefficient) or a negative TCR (NTC: negative temperature coefficient) are installed for sensing the temperature of the heater <NUM>. The property of each of the thermistors T1, T2, and T3 of the present embodiment displays the NTC. The thermistor T2 disposed at a central portion is a thermistor for temperature control of the heater <NUM>, and each of the thermistors T1 and T3 is a thermistor which is used sensing an increase in the temperature of a non-sheet passing portion caused when the small-sized paper is fed. The thermistor T1 is connected to an electrical conductor ET1, the thermistor T2 is connected to an electrical conductor ET2, and the thermistor T3 is connected to an electrical conductor ET3. An electrical conductor EG is a common electrical conductor which is shared by the thermistors T1, T2, and T3. On the sliding surface layer <NUM> of the heater <NUM>, the surface protective layer <NUM> is provided so as to cover the sliding surface layer <NUM> except the electrode portions of the electrical conductors ET <NUM> to ET3 and EG.

<FIG> shows a circuit diagram of a power supply circuit <NUM> of the heater <NUM> of Embodiment <NUM>. The power supply circuit <NUM> is constituted by three electrically insulated circuit blocks: a primary side circuit <NUM>, a secondary side circuit <NUM>, and a temperature sensing circuit <NUM>.

The primary side circuit <NUM> is a circuit which supplies power supplied from the commercial power supply <NUM> connected to the image forming apparatus <NUM> to the heating elements <NUM> and <NUM> of the heater <NUM>. The heating elements <NUM> and <NUM> are provided in the primary side circuit <NUM> which is electrically connected to the commercial power supply <NUM>. Power control of the heater <NUM> is performed by using energization/interruption of a triac Q1. The triac Q1 is controlled with a Q1_DRIVE signal outputted from a CPU <NUM> serving as the control portion (secondary control portion) of the secondary side circuit <NUM>. The control portion <NUM> is provided in the secondary side circuit <NUM> which is electrically insulated from the primary side circuit <NUM>. The reinforced insulation (hereinafter, the reinforced insulation includes the double insulation though the description thereof will be omitted) is provided between the primary side circuit and the secondary side circuit (secondary control portion) by a phototriac coupler SSR1. When the Q1_DRIVE signal is brought into a LoW state, a current flows to a secondary photodiode of SSR1, and a primary triac of SSR1 operates. Subsequently, when the current flows to resistors <NUM> and <NUM>, the triac Q1 is brought into an ON state. An isolated AC/DC converter <NUM> is a switched-mode power supply circuit which supplies power to the secondary side circuit <NUM> from the primary side circuit <NUM>, and secures the reinforced insulation between the primary side circuit <NUM> and the secondary side circuit <NUM> with a transformer which is not shown.

Incidentally, when a process for removing a jammed sheet is performed, a user opens the door of the image forming apparatus <NUM>. The image forming apparatus <NUM> has electric components and wiring which can be touched by the user in a state in which the door is opened. As shown in <FIG>, an interface cable <NUM> (USB, LAN) used for connection to external equipment <NUM> such as a PC is also one of the electric components which can be touched by the user. In the present embodiment, as shown in <FIG>, the electric component at a position which allows the user to touch the electric component is connected to the secondary side circuit <NUM>, and the reinforced insulation is provided between the primary side circuit <NUM> to which the commercial power supply <NUM> is connected and the secondary side circuit <NUM>. With this configuration, even when the user touches the electric component or the wire at the position which allows the user to touch the electric component or the wiring, an electric shock can be prevented.

Next, the temperature sensing circuit <NUM> will be described. The resistance values of the thermistors T1 to T3 change according to the temperature of the heater <NUM>. The resistance values of the thermistors T1 to T3 and the divided voltages of resistors <NUM> to <NUM> are inputted to a CPU <NUM> as Th1 to Th3 signals. The CPU <NUM> senses the heater temperature based on the Th1 to Th3 signals. Temperature information sensed by the CPU <NUM> of the temperature sensing circuit <NUM> is outputted as a CLK_OUT signal and a DATA_OUT signal, and the signals are transmitted to the CPU <NUM> of the secondary side circuit <NUM> by data transmission. The reinforced insulation is provided between CLK_OUT and CLK_IN, and between DATA_OUT and DATA_IN by photocouplers PC2 and PC3.

Incidentally, the basic insulation or the reinforced insulation is provided between the temperature sensing circuit <NUM> and the primary side circuit <NUM>. In addition, the temperature sensing circuit <NUM> is a circuit which cannot be touched by the user. Further, the basic insulation or the reinforced insulation is provided between the temperature sensing circuit <NUM> and the secondary side circuit <NUM>. Thus, the secondary side circuit <NUM> is different from the temperature sensing circuit <NUM> in that, while the secondary side circuit <NUM> has the electric component or the wiring which can be touched by the user, the temperature sensing circuit <NUM> does not have the electric component or the wiring which can be touched by the user. An effect obtained by insulating the temperature sensing circuit <NUM> from both of the primary side circuit <NUM> and the secondary side circuit <NUM> will be described later.

A transformer TR1 is an insulated transformer which is used for performing power supply to the temperature sensing circuit <NUM> from the secondary side circuit <NUM>, and is subjected to the reinforced insulation. A power supply voltage is supplied to the side of the temperature sensing circuit <NUM> of the transformer TR1 by switching an FET <NUM> with a TR1_DRIVE signal of the CPU <NUM>. A diode <NUM> and a capacitor <NUM> serve as a rectifying-smoothing circuit of the output of the transformer TR1.

Thus, the temperature information of the heater <NUM> sensed by the temperature sensing circuit <NUM> is transmitted to the secondary side circuit <NUM> by information transmission. Subsequently, the secondary side circuit <NUM> performs control of power supplied to the heater <NUM> from the primary side circuit <NUM> based on the temperature information of the heater <NUM>. In internal processing of the CPU <NUM>, power to be supplied is calculated by using, e.g., PI control based on the set temperature of the heater <NUM> and the sensed temperature by the thermistor. Further, a phase angle (phase control) or a wave number (wave number control) corresponding to the calculated power to be supplied is determined, and the triac Q1 is controlled at timing of the determined phase angle or wave number.

Herein, a description will be given of an advantage obtained by insulating the thermistors T1 to T3 and the temperature sensing circuit <NUM> of the heater <NUM> from both of the primary side circuit <NUM> and the secondary side circuit <NUM>.

First, the thermistors T1 to T3 are insulated from the primary side circuit <NUM>, and hence the potentials of the thermistors T1 to T3 are safe potentials, and it is not necessary to insulate the thermistors T1 to T3 from the film <NUM>. Accordingly, as described above, it is possible to reduce the thickness of the surface protective layer <NUM>.

In addition, the thermistors T1 to T3 are insulated from the secondary side circuit <NUM>, and hence it is not necessary to provide the reinforced insulation between the thermistors T1 to T3 and the heating elements <NUM> and <NUM>. The basic insulation between the heating elements <NUM> and <NUM> and the thermistors T1 to T3 is achieved by the substrate <NUM> and the surface protective layer <NUM>. Consequently, it is possible to dispose the thermistors T1 to T3 and the electrical conductors ET1 to ET3 and EG to which the thermistors are connected at any positions on the sliding surface layer (the end potion of the substrate <NUM> in a lateral direction and the like).

A description will be given of a disadvantage in the case where the thermistors T1 to T3 and the temperature sensing circuit <NUM> are not insulated from the primary side circuit <NUM>. In order to insulate the film <NUM> from the primary side circuit, it is necessary to increase the thickness of the surface protective layer <NUM> of the thermistors T1 to T3. In general, the thermal conductivity of glass used in the surface protective layer <NUM> is several tens of times to several hundred times lower than that of ceramic used in the substrate <NUM>, and hence, when the thickness of the surface protective layer <NUM> is increased, heat resistances between the heating elements <NUM> and <NUM> and the nip portion N are increased. Therefore, when the thickness of the surface protective layer <NUM> is increased, the heat transfer efficiency from the heater <NUM> to the nip portion N is reduced, and accuracy in sensing the temperature of the nip portion N by the thermistors T1 to T3 is also reduced.

A description will be given of a disadvantage in the case where the thermistors T1 to T3 and the temperature sensing circuit <NUM> are not insulated from the secondary side circuit <NUM>. It is necessary to provide the reinforced insulation between the primary side circuit <NUM> and the secondary side circuit <NUM>, and hence it is necessary to secure a sufficient creepage distance between the heating elements <NUM> and <NUM> of the heater <NUM> and the thermistors T1 to T3 in addition to the insulation by the surface protective layer <NUM>. Accordingly, it is necessary to dispose the thermistors T1 to T3 and the electrical conductors ET1 to ET3 and EG such that the thermistors T1 to T3 and the electrical conductors ET1 to ET3 and EG are spaced a predetermined creepage distance from the end portion of the substrate <NUM> in the lateral direction. When the width of the substrate <NUM> in the lateral direction is increased for securing the sufficient creepage distance, the size of the heater is increased. As a result, the material cost of the substrate <NUM> is increased and the heat capacity of the heater <NUM> is also increased, and hence a problem arises in that the start-up time of the heater <NUM> is increased.

As described thus far, the heater <NUM> and the power supply circuit <NUM> of the present embodiment have the following features. · Insulation is provided between the heating elements <NUM> and <NUM> serving as the primary side circuit, and the film <NUM> and the thermistors T1 to T3 by covering the heating elements <NUM> and <NUM> with the surface protective layer <NUM> and the substrate <NUM> of the heater <NUM>. · The temperature sensing circuit <NUM> is insulated from both of the primary side circuit <NUM> and the secondary side circuit <NUM>. · The thermistors T1 to T3 are insulated from both of the primary side circuit <NUM> and the secondary side circuit <NUM>, and hence it is possible to reduce the thickness of the surface protective layer <NUM>. · It is possible to dispose the thermistors T1 to T3 and the electrical conductors ET1 to ET3 and EG to which the thermistors are connected at any positions on the sliding surface layer of the substrate <NUM>. Therefore, it is possible to reduce the width of the substrate of the heater <NUM> in the lateral direction (a direction orthogonal to a longitudinal direction), and increase the thermal responsiveness of the heater <NUM>. Thus, the image forming apparatus of Embodiment <NUM> can dispose the temperature sensing element on the sliding surface of the heater which slides on the film while preventing a reduction in each of the thermal responsiveness and the heat transfer efficiency of the heater and preventing an increase in the size of the heater.

Embodiment <NUM> of the present invention will be described. Components in Embodiment <NUM> which are the same as those in Embodiment <NUM> are designated by the same reference numerals, and the description thereof will be omitted. Matters which are not described specifically in Embodiment <NUM> are the same as those in Embodiment <NUM>. A heater <NUM> of Embodiment <NUM> has heating blocks HB1 to HB7 which can be controlled individually. An increase in the temperature of the non-sheet passing portion in the case where the small-sized paper is fed can be prevented by individually controlling the temperatures of the heating blocks HB1 to HB7 based on the recording material size and image information, and power consumption of a fixing apparatus <NUM> can be reduced by reducing heat generation at a place where heating is not necessary.

<FIG> is a cross-sectional view of the fixing apparatus <NUM>. The fixing apparatus <NUM> has an electrode (herein, an electrode E4 is shown as a representative) on a surface of the heater <NUM> opposite to a surface thereof opposing the fixing nip portion N. In addition, in the fixing apparatus <NUM>, a plurality of electrical contacts (herein, an electrical contact C4 is shown as a representative) connected to the electrodes of the heater <NUM> are provided, and power is supplied from each electrical contact to each electrode. The detailed description of the heater <NUM> will be made in <FIG>.

The heater <NUM> has a heating element <NUM> provided on the side of a back surface of a substrate <NUM> opposite to the side of a surface thereof (the side of a sliding surface) opposing the fixing nip portion N (a sliding portion which slides on the film <NUM>). A surface protective layer <NUM> is glass used for insulating the heating element <NUM>. A thermistor T4 (T1 to T7) is provided on the side of the sliding surface of the substrate <NUM>. A surface protective layer <NUM> is glass used for protecting the thermistor T4 (T1 to T7) and obtaining slidability of the fixing nip portion N. In addition, in a holding member <NUM> which holds the heater <NUM>, holes for connecting the electrodes and the electrical contacts are provided. The detailed description thereof will be made in <FIG>.

The configuration of the heater <NUM> according to Embodiment <NUM> will be described by using <FIG> is a cross-sectional view of the heater <NUM> (a cross-sectional view in the vicinity of the transport reference position X in <FIG> is a plan view of each layer of the heater <NUM>, and <FIG> is a plan view of the holding member <NUM> of the heater <NUM>. The heater <NUM> is provided with two first electrical conductors <NUM> (601a, 601b) which are provided along the longitudinal direction of the heater <NUM> on the substrate <NUM>. Further, the heater <NUM> is provided with a second electrical conductor <NUM> (<NUM>-<NUM>) at a position different from that of the first electrical conductor <NUM> in the lateral direction of the heater <NUM> on the substrate <NUM>.

The first electrical conductor <NUM> is separated into an electrical conductor 601a which is disposed on the upstream side in the transport direction of the recording material P, and an electrical conductor 601b which is disposed on the downstream side therein. Further, the heater <NUM> has the heating element <NUM> (602a, 602b) which is provided between the first electrical conductor <NUM> and the second electrical conductor <NUM>, and generates heat with power supplied via the first electrical conductor <NUM> and the second electrical conductor <NUM>.

The heating element <NUM> is separated into a heating element 602a which is disposed on the upstream side in the transport direction of the recording material P, and a heating element 602b which is disposed on the downstream side therein. When a heat generation distribution in the lateral direction of the heater <NUM> (the transport direction of the recording material) becomes asymmetrical, a stress which occurs in the substrate <NUM> when the heater <NUM> generates heat is increased. When the stress occurring in the substrate <NUM> is increased, there are cases where the substrate <NUM> is cracked. To cope with this, the heat generation distribution in the lateral direction of the heater <NUM> is made symmetrical by separating the heating element <NUM> into the heating element 602a disposed on the upstream side in the transport direction and the heating element 602b disposed on the downstream side therein.

On the back surface layer <NUM> of the heater <NUM>, the insulating surface protective layer <NUM> (made of glass in the present embodiment) which covers the heating element <NUM>, the first electrical conductor <NUM> (601a, 601b), and the second electrical conductor <NUM> (<NUM>-<NUM>) is provided so as not to cover the electrode portion (E4).

As shown in <FIG>, on the back surface layer <NUM> of the heater <NUM>, a plurality of heating blocks each including a combination of the first electrical conductor <NUM>, the second electrical conductor <NUM>, and the heating element <NUM> are provided in the longitudinal direction of the heater <NUM>. The heater <NUM> of the present embodiment has seven heating blocks HB1 to HB7 at the central portion and both end portions of the heater <NUM> in the longitudinal direction. The heating blocks HB1 to HB7 are constituted by heating elements 602a-<NUM> to 602a-<NUM> and heating elements 602b-<NUM> to 602b-<NUM> which are formed symmetrically in the lateral direction of the heater <NUM>. The first electrical conductor <NUM> is constituted by the electrical conductor 601a connected to the heating elements 602a-<NUM> to 602a-<NUM> and the electrical conductor 601b connected to the heating elements 602b-<NUM> to 602b-<NUM>. Similarly, in order to correspond to the seven heating blocks HB1 to HB7, the second electrical conductor <NUM> is divided into seven electrical conductors <NUM>-<NUM> to <NUM>-<NUM>.

Electrical contacts C1 to C7, C8-<NUM>, and C8-<NUM> for supplying power from a power supply circuit <NUM> of the heater <NUM> described later are connected to electrodes E1 to E7, E8-<NUM>, and E8-<NUM>. Each of the electrodes E1 to E7 is an electrode for supplying power to each of the heating blocks HB <NUM> to HB7 via each of the electrical conductors <NUM>-<NUM> to <NUM>-<NUM>. Each of the electrodes E8-<NUM> and E8-<NUM> is an electrode to which a common electrical contact for supplying power to the seven heating blocks HB1 to HB7 via the electrical conductor 601a and the electrical conductor 601b is connected.

The surface protective layer <NUM> on the back surface layer <NUM> of the heater <NUM> is formed so as to cover the back surface layer <NUM> except the portions of the electrodes E1 to E7, E8-<NUM>, and E8-<NUM>. That is, the electrical contacts C1 to C7, C8-<NUM>, and C8-<NUM> can be connected to the respective electrodes from the side of the back surface of the heater <NUM>, and power can be supplied from the side of the back surface of the heater <NUM>.

Thus, a necessity to provide wiring based on a conductive pattern on the substrate <NUM> is eliminated by providing the electrodes on the back surface of the heater <NUM>, and hence it is possible to reduce the width of the substrate <NUM> in the lateral direction. Accordingly, it is possible to obtain effects of reducing the material cost of the substrate <NUM> and reducing the start-up time required to increase the temperature of the heater <NUM> by reducing the heat capacity of the substrate <NUM>.

Incidentally, the electrodes E2 to E6 are provided in an area in which the heating element is provided in the longitudinal direction of the substrate, and the surface protective layer <NUM> is formed in the area except the portions of the electrodes E2 to E6. As a result, in the configuration of Embodiment <NUM>, unlike the description in Embodiment <NUM>, it is not possible to insulate the heating element <NUM> by covering the heating element <NUM> with the surface protective layer <NUM> and the substrate <NUM>. To cope with this, in the present embodiment, as indicated by a dotted-line arrow in <FIG>, the basic insulation is provided by increasing the creepage distance from the heating element <NUM> to the film <NUM> and the sliding surface layer by using the surface protective layer <NUM>.

On the sliding surface layer <NUM> of the heater <NUM>, the thermistors T1 to T7 are installed to sense the temperatures of the respective heating blocks HB1 to HB7 of the heater <NUM>. One or more thermistors are provided for each of the heating blocks HB1 to HB7, and hence it is possible to sense the temperature of each of the heating blocks. In order to energize the seven thermistors T1 to T7, electrical conductors ET1 to ET7 for sensing the resistance value of the thermistor and a common electrical conductor EG of the thermistors are formed.

On the sliding surface (a surface in contact with the film <NUM>) layer <NUM> of the heater <NUM>, the surface protective layer <NUM> constituted by a coating of glass having slidability is provided. For connecting the electrical conductors ET1 to ET7 for sensing the resistance value of the thermistor and the electrical conductor EG, and electrical contacts, the surface protective layer <NUM> is provided at least in an area which slides on the film <NUM> except the end portion of the heater <NUM> in the longitudinal direction.

As shown in <FIG>, the holding member <NUM> of the heater <NUM> is provided with holes for connecting the electrodes E1, E2, E3, E4, E5, E6, E7, E8-<NUM>, and E8-<NUM> and the electrical contacts C1 to C7, C8-<NUM>, and C8-<NUM>. The above-described safety element <NUM> and the electrical contacts C1-C7, C8-<NUM>, and C8-<NUM> are provided between the stay <NUM> and the holding member <NUM>. The electrical contacts C1 to C7, C8-<NUM>, and C8-<NUM> which come into contact with the electrodes E1-E7, E8-<NUM>, and E8-<NUM> are electrically connected to the electrode portions of the heater by a method such as biasing with a spring or welding. Each electrical contact is connected to the power supply circuit <NUM> of the heater <NUM> described later via a conductive material such as a cable or a thin metal plate provided between the stay <NUM> and the holding member <NUM>.

<FIG> is a circuit diagram of the power supply circuit <NUM> of the heater <NUM> of Embodiment <NUM>. The details of the driving circuit and the insulated circuit are the same as those in <FIG>, and hence the depiction thereof is omitted in <FIG>. In a primary side circuit <NUM>, control of power to the heater <NUM> is performed by using energization/interruption of triacs Q1 to Q7. Each of the triacs Q1 to Q7 operates according to a control signal of the CPU <NUM> of an insulated secondary side circuit <NUM>.

To the CPU <NUM>, the resistance values of the thermistors T1 to T7 and the divided voltages of resistors <NUM> to <NUM> are inputted as Th1 to Th7 signals. The CPU <NUM> senses the heater temperature based on the Th1 to Th7 signals. The temperature information of the heater <NUM> sensed by the CPU <NUM> is transmitted to the CPU <NUM> of the secondary side circuit <NUM> which is insulated from the temperature sensing circuit by information transmission. The CPU <NUM> controls the power of each of the heating blocks HB1 to HB7 based on the temperature information of the heater <NUM>.

Incidentally, as described above, the electrodes E2 to E6 of the heater <NUM> are positioned in the area in which the heating element is provided in the longitudinal direction of the substrate. Accordingly, the surface protective layer <NUM> is formed in the area except the portions of the electrodes E2 to E6. According to the configuration of the heater <NUM>, a method in which the thermistors T1 to T7 and a temperature sensing circuit <NUM> are insulated from both of the primary side circuit <NUM> and the secondary side circuit <NUM> is more effective.

A disadvantage in the case where the thermistors T1 to T7 and the temperature sensing circuit <NUM> are not insulated from the primary side circuit <NUM> is the same as that in the description in Embodiment <NUM>, and hence the description thereof will be omitted.

A description will be given of a disadvantage in the case where the thermistors T1 to T7 and the temperature sensing circuit <NUM> are not insulated from the secondary side circuit <NUM>. It is necessary to provide the reinforced insulation between the primary side circuit <NUM> and the secondary side circuit <NUM>, and a required creepage distance is increased. Therefore, it is necessary to increase the creepage distance shown in <FIG> to a distance corresponding to the reinforced insulation, and it is necessary to increase the width of the heater substrate <NUM> in the lateral direction. Alternatively, it is necessary to increase the thickness of the surface protective layer <NUM> to insulate the thermistors T1 to T7. In either case, a disadvantage that the thermal responsiveness of the heater <NUM> or the heat transfer efficiency to the nip portion N is reduced is caused. Accordingly, in the configuration in which the electrode is provided on the side of the back surface such as the configuration of the heater <NUM>, a method in which the thermistors T1 to T7 and the temperature sensing circuit <NUM> are insulated from both of the primary side circuit <NUM> and the secondary side circuit <NUM> is more effective. Therefore, even in the configuration in which the seven heating blocks HB1 to HB7 can be controlled individually such as the configuration of the heater <NUM>, it is possible to dispose the temperature sensing element on the sliding surface of the heater which slides on the film while preventing a reduction in each of the thermal responsiveness and the heat transfer efficiency of the heater and preventing an increase in the size of the heater.

As described thus far, the heater <NUM> and the power supply circuit <NUM> of the present embodiment have the following features. · The surface protective layer <NUM> and the substrate <NUM> of the heater <NUM> cover the heating elements 602a and 602b while not covering the electrode portions (E1 to E7, E8-<NUM>, E8-<NUM>) of the heating elements 602a and 602b. With this, the sufficient creepage distance is secured, and insulation is provided between the heating elements 602a and 602b serving as the primary side circuit, and the film <NUM> and the thermistors T1 to T7. · The seven heating blocks HB1 to HB7 can be controlled individually, and at least part of the electrodes (the electrodes E2 to E6) of the heating blocks HB1 to HB7 is provided in the area in which the heating element is provided in the longitudinal direction of the substrate. · The temperature sensing circuit <NUM> is insulated from both of the primary side circuit <NUM> and the secondary side circuit <NUM>. · The thermistors T1 to T7 are insulated from both of the primary side circuit <NUM> and the secondary side circuit <NUM>, and hence it is possible to reduce the thickness of the surface protective layer <NUM>. · It is possible to dispose the thermistors T1 to T7 and the electrical conductors ET1 to ET7 and EG to which the thermistors are connected at any positions on the sliding surface layer of the substrate <NUM> (It is possible to reduce the width of the substrate of the heater <NUM> in the lateral direction, and increase the thermal responsiveness of the heater <NUM>). Thus, the image forming apparatus of Embodiment <NUM> can also dispose the temperature sensing element on the sliding surface of the heater which slides on the film while preventing a reduction in each of the thermal responsiveness and the heat transfer efficiency of the heater and preventing an increase in the size of the heater.

Embodiment <NUM> of the present invention will be described. Components in Embodiment <NUM> which are the same as those in Embodiment <NUM> are designated by the same reference numerals, and the description thereof will be omitted. Matters which are not described specifically in Embodiment <NUM> are the same as those in Embodiment <NUM>. A power supply circuit <NUM> of Embodiment <NUM> shown in <FIG> is different from the power supply circuit <NUM> of Embodiment <NUM> in that the CPU <NUM> also performs control of the triac Q1.

Claim 1:
An image forming apparatus (<NUM>) comprising:
an image forming portion (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>) for forming an image on a recording material (P); and
a fixing portion (<NUM>) including
a tubular film (<NUM>) and
a heater (<NUM>, <NUM>) including a substrate (<NUM>, <NUM>), a heating element (<NUM>, <NUM>, 602a-<NUM>-602a-<NUM>, 602b-<NUM>-602b-<NUM>) provided on the substrate, and a temperature sensing element (T1-T7) provided on a surface of the substrate opposite to a surface on which the heating element is provided, wherein
the fixing portion is configured to fix
the image formed on the recording material to the recording material with heat from the heater which is controlled according to a sensed temperature by the temperature sensing element, wherein
the image forming apparatus includes a temperature sensing circuit (<NUM>, <NUM>) to which the temperature sensing element is electrically connected,
a surface of the heater on a side where the temperature sensing element is provided faces an inner surface of the film,
the heating element is provided in a primary side circuit (<NUM>, <NUM>) which is electrically connected to a commercial power supply (<NUM>),
characterized in that
between the primary side circuit and the temperature sensing circuit, basic insulation or reinforced insulation or double insulation in which additional insulation is added to the basic insulation is provided, and between a secondary side circuit (<NUM>, <NUM>) which is reinforced insulated or double insulated with respect to the primary side circuit and the temperature sensing circuit, the basic insulation or the reinforced insulation or the double insulation is provided, and
the temperature sensing circuit is a circuit different from the primary side circuit and the secondary side circuit.