45b2 can be controlled independently of each other. Similarly, heat generation of the first end heating element 45b1 and the second end heating element 45b2 can be controlled.

The wiring set 55 (also referred to as a wiring group) is formed of a metal material such as silver.

The wiring set 55 includes a central junction 52a, a central wiring 53a, an end junction 52b, a first end wiring 53b1, a second end wiring 53b2, a common junction 58, and a common wiring 57.

The central junction 52a is arranged in the −y direction of the heating element group 45. The central routing 53a is arranged in the +x direction of the heating element group 45. The central routing 53a connects the end side in the +x direction of the central heating element 45a and the central junction 52a.

The end junction 52b is arranged in the −y direction of the central junction 52a. The first end holding 53b1 is arranged in the +x direction of the heating element group 45 and in the +x direction of the central routing 53a.

The first end holding 53b1 connects an end side of the first end heating element 45b1 in the +x direction and an end of the end junction 52b in the +x direction. The second end holding 53b2 is arranged in the +x direction of the heating element group 45 and in the −x direction of the central routing 53a. The second end holding 53b2 connects the end side in the +x direction of the second end heating element 45b2 and the end in the −x direction of the end junction 52b.

The common junction 58 is arranged in the +y direction of the heating element group 45. The common wiring 57 is arranged in the −x direction of the heating element group 45. The common wiring 57 connects the end sides in the −x direction of the central heating element 45a, the first end heating element 45b1, and the second end heating element 45b2 to the common junction 58.

In this way, in the +x direction of the heating element group 45, the second end holding 53b2, the central routing 53a, and the first end holding 53b1 are arranged. On the other hand, only the common wiring 57 is disposed in the −x direction of the heating element group 45. Therefore, the center 45c of the heating element group 45 in the x direction is offset in the −x direction from the center 41c in the x direction of the substrate 41.

As shown in FIG. 3, a straight line CL connecting the center pc of the pressing roller 30p and the center hc of the film unit 30h is defined. The center 41c in the x direction of the substrate 41 is arranged in the +x direction from the straight line CL. Accordingly, the substrate 41 extends in the +x direction of the nip N, and the sheet S that has passed through the nip N is easily peeled off from the film unit 30h.

The center 45c of the heating element set 45 in the x direction is disposed on the straight line CL. The heating element group 45 is entirely contained in the region of the nip N, and is disposed in the center of the nip N. Accordingly, heat distribution of the nip N becomes uniform, and the sheet S passing through the nip N is uniformly heated.

As shown in FIG. 4, the heating element group 45 and the wiring group 55 are formed on the +z direction surface of the insulating layer 43. The protective layer 46 is formed of a glass material or the like so as to cover the heating element group 45 and the wiring set 55. The protective layer 46 improves sliding (reduces friction) between the heater unit 40 and the cylindrical film 35.

As shown in FIG. 3, the heater unit 40 is disposed inside the cylindrical film 35. A lubricant is applied to an inner peripheral surface of the cylindrical film 35. The heater unit 40 contacts the inner circumferential surface of the cylindrical film 35 via a lubricant. When the heater unit 40 generates heat, the viscosity of the lubricant decreases. Accordingly, sliding friction between the heater unit 40 and the cylindrical film 35 is low.

As described above, the cylindrical film 35 is a belt-shaped thin film that slides along a surface of the heater unit 40 while being in contact with the heater unit 40 on one side.

The heat conductor 49 is formed of a metal material having a high thermal conductivity such as copper. An outer shape of the heat conductor 49 is equal to an outer shape of the substrate 41 of the heater unit 40. The heat conductor 49 is disposed in contact with the surface of the heater unit 40 in the −z direction.

The support member 36 is formed of a resin material such as a liquid crystal polymer. The support member 36 is disposed so as to cover the −z direction and both sides in the x direction of the heater unit 40. The support member 36 supports the heater unit 40 via the heat conductor 49. Rounded chamfers are formed at both ends of the support member 36 in the x direction. The support member 36 supports the inner peripheral surface of the cylindrical film 35 at both ends in the x direction of the heater unit 40.

When the sheet S passing through the fixing unit 30 is heated, a temperature distribution is generated in the heater unit 40 in accordance with the size of the sheet S. When the heater unit 40 locally reaches a high temperature, the temperature may exceed heat resistant temperature of the support member 36 formed of a resin material. The heat conductor 49 averages the temperature distribution of the heater unit 40. Thereby, the heat resistance of the support member 36 is ensured.

FIG. 6 is a front cross-sectional view of a heat conductor, a heater unit, and a cylindrical belt. The heat conductor 49 is disposed on a surface of the heater unit 40 that does not come into contact with the cylindrical film 35. Further, the heat conductor 49 is configured so as not to come into contact with the heater unit 40 at a position where heat generation distribution in the heater unit 40 becomes a peak. More specifically, as shown in FIG. 6, the heater unit 40 and the heat conductor 49 are in contact with each other in regions a1 and a2. Then, a non-contact portion forms a groove portion of the heat conductor 49. a width of the groove portion is set to be wider than a width of the heating element group 45 of the heater unit 40 by length d1 and length d2, respectively. For example, the heating element group 45 of the heater unit 40 has a width of 4.5 to 4.9 mm, and the groove portion has a width of about 5 mm.

The stay 38 shown in FIG. 3 is formed of a steel plate material or the like. A cross section perpendicular to the y direction of the stay 38 is formed in a U-shape. The stay 38 is mounted in the −z direction of the support member 36 so as to close an opening portion of the U shape with the support member 36. The stay 38 extends in the y direction. Both ends of the stay 38 in the y direction are fixed to the housing of the image forming apparatus 100. Thereby, the film unit 30h is supported by the image forming apparatus 100. The stay 38 improves rigidity of the film unit 30h. Flanges that restrict movement of the cylindrical film 35 in the y direction can be attached near both ends of the stay 38 in the y direction.

The heater temperature sensor 62 is disposed in the −z direction of the heater unit 40 with the heat conductor 49 interposed therebetween. For example, the heater temperature sensor 62 is a thermistor. The heater temperature sensor 62 is mounted on and supported by a surface of the support member 36 in the −z direction. A temperature sensitive element of the heater temperature sensor 62 contacts the heat conductor 49 through a hole that passes through the support member 36 in the z direction. The heater temperature sensor 62 measures the temperature of the heater unit 40 through the heat conductor 49.

The thermostat 68 is disposed in the same manner as the heater temperature sensor 62. The thermostat 68 is incorporated in an electric circuit, which will be described later. When the temperature of the heater unit 40 detected through the heat conductor 49 exceeds a predetermined temperature, the thermostat 68 cuts off the energization of the heating element group 45.

FIG. 7 is a plan view (a view from the −z direction) of a heater temperature sensor and a thermostat. In FIG. 7, description of the support member 36 is omitted. Note that the following description of arrangement of the heater temperature sensor, the thermostat, and the film temperature sensor describes arrangement of the respective temperature sensitive elements.

A plurality of heater temperature sensors 62 (central heater temperature sensor 62a and end heater temperature sensor 62b) are arranged side by side in the y direction. The plurality of heater temperature sensors 62 are disposed near the heating element group 45 in the y direction. The plurality of heater temperature sensors 62 are disposed in the center of the heating element group 45 in the x direction. That is, when viewed in the z direction, the plurality of heater temperature sensors 62 and the heating element group 45 overlap at least partially.

The plurality of thermostats 68 (central thermostat 68a and end thermostat 68b) are also arranged in the same manner as the plurality of heater temperature sensors 62 described above.

The plurality of heater temperature sensors 62 include the central heater temperature sensor 62a and the end heater temperature sensor 62b.

The central heater temperature sensor 62a measures temperature of the central heating element 45a. The central heater temperature sensor 62a is disposed within the range of the central heating element 45a. That is, when viewed from the z direction, the central heater temperature sensor 62a and the central heating element 45a overlap each other.

The end heater temperature sensor 62b measures the temperature of the second end heating element 45b2. As described above, the heat generation of the first end heating element 45b1 and the second end heating element 45b2 is similarly controlled. Therefore, the temperature of the first end heating element 45b1 and the temperature of the second end heating element 45b2 are equal to each other. The end heater temperature sensor 62b is disposed in the range of the second end heating element 45b2. That is, when viewed in the z direction, the end heater temperature sensor 62b and the second end heating element 45b2 overlap each other.

The plurality of thermostats 68 comprise the central thermostat 68a and the end thermostat 68b.

The central thermostat 68a interrupts the energization of the heating element group 45 when the temperature of the central heating element 45a exceeds the predetermined temperature. The central thermostat 68a is located within the range of the central heating element 45a. That is, when viewed from the z direction, the central thermostat 68a and the central heating element 45a overlap each other.

When the temperature of the first end heating element 45b1 exceeds the predetermined temperature, the end thermostat 68b cuts off the energization of the heating element group 45. As described above, the heat generation of the first end heating element 45b1 and the second end heating element 45b2 is similarly controlled. Therefore, the temperature of the first end heating element 45b1 and the temperature of the second end heating element 45b2 are equal to each other. The end thermostat 68b is located within the range of the first end heating element 45b1. That is, when viewed from the z direction, the end thermostat 68b and the first end heating element 45b1 overlap each other.

As described above, the central heater temperature sensor 62a and the central thermostat 68a are disposed within the range of the central heating element 45a. As a result, the temperature of the central heating element 45a is measured. In addition, when the temperature of the central heating element 45a exceeds the predetermined temperature, the energization of the heating element group 45 is cut off. On the other hand, an end heater temperature sensor 62b and an end thermostat 68b are disposed within the range of the first end heating element 45b1 and the second end heating element 45b2. Accordingly, the temperatures of the first end heating element 45b1 and the second end heating element 45b2 are measured. Further, when the temperature of the first end heating element 45b1 and the second end heating element 45b2 exceeds the predetermined temperature, the energization of the heating element group 45 is cut off.

The plurality of heater temperature sensors 62 and the plurality of thermostats 68 are arranged alternately along the y direction. As described above, the first end heating element 45b1 is disposed in the +y direction of the central heating element 45a. The end thermostat 68b is disposed within the range of the first end heating element 45b1. The central heater temperature sensor 62a is disposed in the +y direction from the center of the central heating element 45a in the y direction. The central thermostat 68a is disposed in the −y direction from the center of the central heating element 45a in the y direction. As described above, the second end heating element 45b2 is disposed in the −y direction of the central heating element 45a. An end heater temperature sensor 62b is disposed within the range of the second end heating element 45b2. Accordingly, from the +y direction, the end thermostat 68b, the central heater temperature sensor 62a, the central thermostat 68a, and the end heater temperature sensor 62b are arranged in this order from the +y direction to the −y direction.

Generally, the thermostat 68 connects and disconnects an electrical circuit by utilizing bending deformation of a bimetal with temperature change. The thermostat is formed to be elongated to match the shape of the bimetal. Further, terminals extend outward from both ends in the longitudinal direction of the thermostat 68. The connector of the external sling is connected to the terminal by caulking. Therefore, it is necessary to secure a space on an outer side in the longitudinal direction of the thermostat 68. Since there is no spatial margin in the fixing unit 30 in the x-direction, the longitudinal direction of the thermostat 68 is arranged along the y-direction.

With this arrangement, when the plurality of thermostats 68 are arranged side by side in the y direction, it becomes difficult to secure a connection space for an external routing.

As described above, the plurality of heater temperature sensors 62 and the plurality of thermostats 68 are alternately arranged along the y direction. Thereby, the heater temperature sensor 62 is disposed adjacent to the thermostat 68 in the y direction. Therefore, it is possible to secure a connection space for the external routing to the thermostat 68. Further, a degree of freedom in a layout of the thermostat 68 and the heater temperature sensor 62 in the y direction is increased. Accordingly, the thermostat 68 and the heater temperature sensor 62 may be disposed at an optimal position, and the temperature of the fixing unit 30 may be controlled. Further, an isolation of an AC wiring connected to the plurality of thermostats 68 and an DC wiring connected to the plurality of heater temperature sensors 62 is facilitated. Accordingly, generation of noise in the electric circuit is suppressed.

The film temperature sensor 64 is disposed inside the cylindrical film 35 and in the +x direction of the heater unit 40, as shown in FIG. 3. The film temperature sensor 64 contacts the inner circumferential surface of the cylindrical film 35, and measures temperature of the cylindrical film 35.

Note that the image forming apparatus 100 may further include an environment temperature sensor 65 in addition to the heater temperature sensor 62 and the film temperature sensor 64. The environment temperature sensor 65 measures temperature around its mounted position. The environment temperature sensor 65 may be attached to any position near the fixing unit 30. The vicinity of the fixing unit 30 is a position at which the environment temperature sensor 65 can measure temperature of the space in which the fixing unit 30 is located (ambient temperature). For example, as shown in FIG. 3, the environment temperature sensor 65 may be attached to the housing 10 located outside of the film unit 30h.

If the image forming apparatus 100 comprises the environment temperature sensor 65, the controller 6 may control the energization of the heating element group 45 based on the temperatures measured by the heater temperature sensor 62, the film temperature sensor 64, and the environment temperature sensor 65. For example, when the temperature measured by the environment temperature sensor 65 is higher than a predetermined value or when the temperature is lower than the predetermined value, the controller 6 may stop the energization of the heating element group 45.

FIG. 8 is an electric circuit diagram of the heating device according to the first embodiment. In FIG. 8, a bottom view of FIG. 5 is arranged above, and a plan view of FIG. 8 is arranged below, respectively. FIG. 8 also illustrates the plurality of film temperature sensor meters 64, along with a cross section of the cylindrical film 35, above the plan view below.

The plurality of film temperature sensors 64 comprise a central film temperature sensor 64a and an end film temperature sensor 64b.

The central film temperature sensor 64a contacts the center of the cylindrical film 35 in the y direction. The central film temperature sensor 64a contacts the cylindrical film 35 within the range of the central heating element 45a in the y direction. The central film temperature sensor 64a measures the temperature of the center in the y direction of the cylindrical film 35.

The end film temperature sensor 64b contacts the end of the cylindrical film 35 in the −y direction. The end film temperature sensor 64b contacts the cylindrical film 35 within the range of the second end heating element 45b2 in the y direction. The end film temperature sensor 64b measures temperature of the end in the −y direction of the cylindrical film 35. As described above, the heat generation of the first end heating element 45b1 and the second end heating element 45b2 is similarly controlled. Therefore, the temperature at the end in the −y direction of the cylindrical film 35 and the temperature at the end in the +y direction are equal to each other.

A power source 95 is connected to the central junction 52a via a central triac 96a. The power source 95 is connected to the end junction 52b via an end triac 96b. The controller 6 controls ON/OFF of the central triac 96a and the end triac 96b independently of each other.

When the controller 6 turns on the central triac 96a, electric power is supplied from the power source 95 to the central heating element 45a. As a result, the central heating element 45a generates heat. When the controller 6 turns on the end triac 96b, electric power is supplied from the power source 95 to the first end heating element 45b1 and the second end heating element 45b2. Accordingly, the first end heating element 45b1 and the second end heating element 45b2 generate heat.

As described above, the central heating element 45a and the first end heating element 45b1 and the second end heating element 45b2 are controlled independently of each other. The central heating element 45a, the first end heating element 45b1, and the second end heating element 45b2 are connected in parallel with respect to the power source 95.

The power source 95 is connected to the common junction 58 via the central thermostat 68a and the end thermostat 68b. The central thermostat 68a and the end thermostat 68b are connected in series.

When the temperature of the central heating element 45a abnormally rises, detection temperature of the central thermostat 68a exceeds the predetermined temperature. At this time, the central thermostat 68a cuts off the power supply from the power source 95 to the entirety of the heating element group 45.

When the temperature of the first end heating element 45b1 abnormally increases, detection temperature of the end thermostat 68b exceeds the predetermined temperature. At this time, the end thermostat 68b cuts off the power supply from the power source 95 to the entirety of the heating element group 45. As described above, the heat generation of the first end heating element 45b1 and the second end heating element 45b2 is similarly controlled. Therefore, when the temperature of the second end heating element 45b2 rises abnormally, the temperature of the first end heating element 45b1 increases as well. Therefore, similarly, when the temperature of the second end heating element 45b2 abnormally rises, the end thermostat 68b cuts off the power supply from the power source 95 to the entire heating element group 45.

The controller 6 measures the temperature of the central heating element 45a by the central heater temperature sensor 62a. The controller 6 measures the temperature of the second end heating element 45b2 by the end heater temperature sensor 62b. The temperature of the second end heating element 45b2 is equal to the temperature of the first end heating element 45b1. The controller 6 measures the temperature of the heating element group 45 by the heater temperature sensor 62 at the time of starting of the fixing unit 30 (warming-up time) and return from a pause state (sleep state).

When the temperature of at least one of the central heating element 45a and the second end heating element 45b2 is lower than the predetermined temperature during the start of the fixing unit 30 and the return from the pause state, the controller 6 causes the heating element group 45 to generate heat for a short time. Thereafter, the controller 6 starts the rotation of the pressing roller 30p. The heating of the heating element group 45 causes viscosity of the lubricant applied to the inner surface of the cylindrical film 35 to decrease. This improves slidability (reduces sliding friction) between the heater unit 40 and the cylindrical film 35 at the start of the rotation of the pressing roller 30p.

The controller 6 measures the temperature of the central portion of the cylindrical film 35 with the central film temperature sensor 64a. The controller 6 measures the temperature of the end (in the −y direction) of the cylindrical film 35 with the end film temperature sensor 64b. The temperature at the end in the y direction of cylindrical film 35 is equal to the temperature at end in the +y direction of cylindrical film 35. The controller 6 measures the temperature of the central and end of the cylindrical film 35 in the y direction during the operation of the fixing unit 30.

The controller 6 performs phase control or wave number control of the power supplied to the heating element group 45 with the central triac 96a and the end triac 96b. The controller 6 controls energization of the central heating element 45a based on the temperature measurement result of the central portion in the y direction of the cylindrical film 35. The controller 6 controls energization of the first end heating element 45b1 and the second end heating element 45b2 based on the temperature measurement result of the end in the y direction of the cylindrical film 35.

In the present embodiment, the heating element group 45 (the central heating element 45a, the first end heating element 45b1, and the second end heating element 45b2) uses a TCR material that has a higher resistance value as the temperature increases. In this case, due to the characteristics of the TCR material, the power in the heating element group 45 decreases with the temperature rise. More specifically, as the heating element group 45 generates heat, a change in power output as shown in the following equation (1) occurs:

P=P0/{1+(αTCR/10⁶)×(T−T0)} (1)

Here, P represents an output [unit: Watts (W)] at an arbitrary temperature, P0 represents an output [unit: W] at a reference temperature, and T represents the arbitrary temperature [unit: ° C.], T0 represents the reference temperature [unit: ° C.], and TCR represents a resistance temperature coefficient [unit: ppm]. In the heating element group 45 of the present embodiment, for example, a TCR material having a resistance temperature coefficient of 1700 ppm is used. When using the heating element group 45 in which the TCR material is used, as shown in FIG. 9, the power becomes lower as the temperature increases.

In general, during starting of the fixing unit 30 and returning from the sleep state (hereinafter, collectively referred to as “start-up time”), heating of the heating element group 45 is performed until the cylindrical film reaches a predetermined temperature. That is, at the time of start-up, the heating element group 45 is continuously energized. This causes the heating element group 45 to be heated continuously. Therefore, the heating element group 45 continuously increases in temperature at the time of start-up, and thus the above-described reduction in power becomes significant.

When a start-up processing start condition is satisfied, the controller 6 according to the present embodiment energizes the heating element group 45 by a start-up time energization method. The energization of the heating element group 45 means, in this context, that the central heating element 45a, the first end heating element 45b1, and the second end heating element 45b2 are energized, respectively.

The start-up processing start condition refers to the start-up of the fixing unit 30 from an idle or unheated state to a target operation temperature. Note that at least one of a heater temperature range condition, a film temperature sensor range condition, or an ambient temperature range condition may be further added to the start-up processing start condition. The heater temperature range condition is that at least one of the temperatures measured by the heater temperature sensors 62 is within a predetermined range. The film temperature sensor range condition is that at least one of the temperatures measured by the film temperature sensors 64 is within a predetermined range. The environmental temperature range condition is that the temperature measured by the environment temperature sensor 65 is within a predetermined range.

The varying energization method used during the start-up processing may be any energization method as long as the energization method satisfies the following: the heating element group 45 (the central heating element 45a, the first end heating element 45b1, and the second end heating element 45b2) is energized at a duty ratio of X % at the start of energization, and then is energized at a duty ratio that has been increased by x % at intervals of to seconds.

FIG. 10 is a diagram illustrating a change in the duty ratio according to the energization system during the start-up process. As shown in FIG. 10, at the start of energization (t=0), the heating element group 45 starts to be energized at a duty ratio of X %. After that, the duty ratio is changed to (X+x) % when to seconds have elapsed. After that, when 2t₀seconds, 3t₀seconds, and 4t₀seconds respectively elapse, the duty ratio is changed to (X+2x) %, (X+3x) %, and (X+4x) %, respectively. Note that, when the duty ratio has reached 100%, the duty ratio is not further changed.

When the start-up processing start condition is satisfied, the controller 6 controls the central triac 96a and the end triac 96b so that the heating element group 45 is energized by the start-up time energization method. Note that the controller 6 includes a timing unit capable of measuring times for changing the duty ratio by measuring the elapsed time increments of to seconds (for example, issuing a signal).

In addition, when a start-up processing termination condition is satisfied, the controller 6 stops the energization of the heating element group 45. The start-up processing termination condition means that at least one of the temperatures measured by heater temperature sensors 62 reaches a predetermined temperature (target temperature). The start-up processing termination condition may be that one (or all) of the temperatures measured by the film temperature sensors 64 reach a predetermined temperature.

Note that a temperature range deviation condition may be further added to the start-up processing termination condition. The temperature range deviation condition means that at least one of the heater temperature range condition, the film temperature sensor range condition, or the environment temperature range condition is not satisfied.

FIG. 11 is a flowchart illustrating an example of a process at the time of start-up by the controller 6 according to the first embodiment.

The controller 6 determines whether or not the start-up processing start condition is satisfied (ACT 001). As described above, the start-up processing start condition refers to the start-up time of the fixing unit 30 (for example, the start-up time, the return time from the sleep state, or the like). Note that, the controller 6 may determine that the start-up processing start condition is not satisfied if at least one of the temperatures measured by a heater temperature sensor 62, the film temperature sensor 64, or an environment temperature sensor 65 is not within a predetermined range.

When it is determined that the start-up processing start condition is satisfied (Yes in ACT 001), the controller 6 starts to energize the heating element group 45 by the start-up time energization method (ACT 002). As described above, the energization method during start-up processing is an energization method in which the heating element group 45 is energized at the duty ratio of X % at the start of energization, and is energized at the duty ratio that has been changed by x % at every time increment to.

The controller 6 acquires the temperature measured by the film temperature sensor 64. The controller 6 checks whether the acquired temperature has reached a predetermined target temperature. When the controller determines that the acquired temperature has reached the target temperature (Yes in ACT 003), the controller stops the energization of the heating element group 45 (ACT 008).

On the other hand, when the controller 6 determines that the acquired temperature has not reached the target temperature (No in ACT 003), the controller 6 waits for a notification (signal) to be output from a timing unit or the like. Note that the timing unit notifies (transmits a signal) every time an increment of to seconds has elapsed since the start of the energization to the heating element group 45 in ACT 002. As a result, the controller 6 can recognize the times at which time increment to elapses from the start of the energization.

When it is determined that time increment to has elapsed since the reception of the signal (Yes in ACT 004), the controller 6 changes the duty ratio of the power in the current supply to the heating element group 45 to a value that is higher by x % (ACT 005). Note that, when the duty ratio has already reached 100%, the controller 6 does not further change the duty ratio.

Thereafter, the controller 6 acquires the temperature measured by the film temperature sensor 64 again. The controller 6 determines whether the acquired temperature has reached a predetermined target temperature (that is, whether or not the temperature is equal to or higher than the target temperature) (ACT 003).

On the other hand, when it is determined that the start-up processing start condition is not satisfied (ACT 001, No), the controller 6 starts to energize the heating element group 45 with a normal energization method (ACT 006). The normal energization method is an energization method in which the heating element group 45 is energized with a constant duty ratio (that is, without changing the duty ratio until the set temperature is reached). Note that the controller 6 may prevent the heating element group 45 from being energized if at least one of the temperatures measured by the heater temperature sensor 62, the film temperature sensor 64, or the environment temperature sensor 65 is not within a predetermined range.

The controller 6 acquires the temperature measured by the film temperature sensor 64. The controller 6 checks whether the acquired temperature has reached a predetermined target temperature (that is, whether or not the temperature is equal to or higher than the target temperature). When the controller determines that the acquired temperature has reached the target temperature (Yes in ACT 007), the controller stops the energization of the heating element group 45 (ACT 008).

As described above, the processing at the time of start-up by the controller 6 shown in the flowchart in FIG. 11 ends.

Hereinafter, an example of the first embodiment described above will be described. Experiments were carried out under the following conditions:

- The image forming apparatus 100 having the above-described configuration was used.
- When starting, the energization of the heating element group 45 was performed by the varying energization method during the start-up processing and the normal energization method, respectively.
- In the varying energization system during the start-up process, the duty ratio was changed so that the duty ratio of the power at the start of the energization was 80%, and the duty ratio was increased by 5% every 1.5 seconds after that. (that is, X=0.8, x=0.05, t₀=1.5 seconds).
- In the normal energization method, the duty ratio of the power is always set to 100%.
- In both of the varying start-up processing and the normal energization method, a power of 1485 W was applied.

FIG. 12 is a diagram illustrating an example of an experimental result indicating a relationship between elapsed time from start of energization to the heating element group 45 and the temperature of the cylindrical film 35. The horizontal axis in FIG. 12 represents the elapsed time [unit: seconds] from the start of the energization of the heating element group 45. The vertical axis of FIG. 12 represents the temperature [unit: ° C.], and power [unit: W] of the cylindrical film 35.

As shown in FIG. 12, when energization is performed on the heating element group 45 with the normal energization method (that is, an energization method with a fixed duty ratio), power output decreases as a temperature of the TCR material increases. For example, as shown in FIG. 12, the power, which is approximately 1200 W immediately after the start of energization, is reduced to approximately 1000 W approximately after 9 seconds from the start of energization. This power drop is due to characteristics of the TCR material used in heating element group 45. As a result, when energization is performed by the normal energization method for the heating element group 45, as shown in FIG. 11, the rate of rise in the temperature of the cylindrical film 35 decreases as time is elapsed from the start of the energization.

On the other hand, as shown in FIG. 12, when energization is performed with the heating element group 45 in the varying energization method (that is, the energization method with a variable duty ratio) in the start-up processing, the duty ratio of the power is stepped up in increments after a certain period of time (1.5 seconds in the present experiment). This increases the power being used again for a certain period of time. In the present experiment, the energization to the heating element group 45 is started at a duty ratio of 80%, and thereafter, the duty ratio is changed by a total of four times, once after every increment of 1.5 [seconds] at a particular duty ration level, from initially 80%, to 85%, to 90%, to 95%, and then to 100%, respectively. Accordingly, as shown in FIG. 12, the power is raised four times. Accordingly, the decrease in power resulting from any increased resistance of TCR-based heating element group 45 is suppressed.

As shown in FIG. 12, immediately after the start of the energization, approximately 1200 W of power is being used, and this power level is maintained at approximately 1200 W even after approximately 9 seconds from the start of the energization. Due to this, the decrease in the rate of temperature increase of the temperature of the cylindrical film 35 is reduced as compared with the normal energization method.

FIG. 13 is a diagram illustrating an example of experimental results. FIG. 13 shows a comparison result between the start-up time and average power at start-up completion when the energization to the heating element group 45 is performed by the normal energization method and the start-up time energization method, respectively.

Here, the “start-up time” is a time required for starting the fixing unit 30 from an idle or reference state. That is, the start-up time is the time required for the cylindrical film 35 to reach the target operating temperature from the start of the energization of the heating element group 45. The “average power at start-up completion” is the average power level used by the fixing unit 30 during the starting (start-up) process of the fixing unit 30 until completed. That is, the average power level used from the initial start time to start-up completion (i.e., when the cylindrical film 35 reaches the target operating temperature).

As shown in FIG. 13, the start-up time in the case where the normal energization method (that is, the energization method with the duty ratio fixed) is used was 8.6 [seconds]. On the other hand, the start-up time in the case where the start-up time (varying) energization method (that is, the energization method with the variable duty ratio) was used in the startup processing was 7.5 [seconds]. In this way, when the start-up time varying energization method during the start-up processing is used, the start-up time is shortened by about 12.8% as compared with the case where the normal energization method is used.

As shown in FIG. 13, the average power at start-up completion when the normal energization method was used was 1067 W. On the other hand, the average power at start-up completion when the start-up time energization method (that is, the duty ratio varying energization method) was 1183 W. As described above, when the varying energization method during the start-up processing is used, the average power at start-up completion is improved (increased) by about 10.9% as compared with the case where the normal energization method is used.

As described above, the image forming apparatus 100 according to the first embodiment includes the heating element group 45, as a heat generating portion, and the controller 6. The heating element group 45 uses a TCR material (that is, a material having a resistance value that increases with an increase in temperature), and generates heat when subjected to energization. The controller 6 changes the duty ratio of the supplied electric power during the heating of the heating element group 45 as the fixing unit 300 is starting up.

With the above-described configuration, the image forming apparatus 100 can change the duty ratio of the power supplied to the heating element group 45 over time. Generally, for a heating element in which the TCR material is used consumed power decreases as the temperature increases (resistance goes up, current goes down). According to this, there is a problem that the time required for starting (heating) of the fixing unit becomes longer when a TCR material is used. On the other hand, the image forming apparatus 100 according to the first embodiment causes the duty ratio of the power to be changed to a higher value, for example, after every increment of a fixed period of time. This allows the image forming apparatus 100 to compensate for the reduced power resulting from the temperature increase after every fixed period of time. That is, the image forming apparatus 100 can avoid (or limit) a decrease in power. Accordingly, the image forming apparatus 100 according to the first embodiment can shorten the time required for the start-up of the fixing unit 300 as compared to the related art.

In general, when the image forming apparatus is starting up, the power usable by a fixing unit may be set in advance. In this case, the heating start-up must be carried out with available set power. On the other hand, with the image forming apparatus 100 according to the first embodiment, it is possible to perform heating while suppressing power consumption after the start-up time.

Note that, in the above-described embodiment, the controller 6 changes the duty ratio of the power to be supplied to the heating element group 45 at a constant time increment (at regular intervals), but the present disclosure is not limited to this. For example, the controller 6 may change or more specifically lengthen the time interval for changing the duty ratio as the time elapses from the start of the energization as depicted in FIG. 16. That is, the frequency at which the duty ratio is varied (increased) may be higher closer to the point in time at which the energization is started. In this case, the decrease in utilized power is suppressed at times close to the time when the energization is started.

Further, for example, the controller 6 may further reduce or alter the change amount of the duty ratio as the time elapses. That is, the duty ratio may be changed by a greater amount at points in time closer to the initial startup time as compared to later in time.

Second Embodiment

Generally, during start-up of the fixing unit, the temperature at the ends, in the width direction, of the cylindrical film 35 may be lower than the temperature at the center of the cylindrical film 35. This is because the center is sandwiched between both ends that are heated similarly to the center, whereas the end is at a position that is heated on only one side.

In the image forming apparatus 100 according to the second embodiment, when the start-up processing start condition is satisfied, the controller 6 energizes the central heating element 45a, the first end heating element 45b1, and the second end heating element 45b2 by an energization method different from each other. When the start-up processing start condition is satisfied, the controller 6 energizes the central heating element 45a by a specific, central energization method. The controller 6 energizes the first end heating element 45b1 and the second end heating element 45b2 by a specific, end energization method.

The central energization method may be any energization method as long as the energization method satisfies the following: the central heating element 45a is energized at a duty ratio of X % at the start of energization, and then is energized at a duty ratio that is changed by x % after every time increment to.

The end energization method may be any energization method as long as the energization method satisfies the following: the first end heating element 45b1 and the second end heating element 45b2 are energized at the duty ratio of X % at the start of energization, and then are energized at a duty ratio that has been changed by y % after every time increment to. Here, it should be assumed that x<y.

FIG. 14 is a diagram illustrating a change in the duty ratio according to the central energization method. As shown in FIG. 14, at the start of energization (t=0), the energization of the central heating element 45a is started at a duty ratio of X %. After that, the duty ratio is changed to (X+x) % when to seconds have elapsed. After that, the duty ratio is changed to (X+2x) % and (X+3x) % after the lapse of 2t₀seconds and the lapse of 3t₀seconds, respectively. Note that, when the duty ratio has reached 100%, the duty ratio is not further changed.

FIG. 15 is a diagram illustrating a change in the duty ratio according to the end energization method. As shown in FIG. 15, at the start of energization (t=0), the first end heating element 45b1 and the second end heating element 45b2 both start energizing at the duty ratio of X %. After that, the duty ratio is changed to (X+y) % when to seconds have elapsed. After that, the duty ratio is changed to (X+2y) % after the elapse of 2t₀seconds. Note that, when the duty ratio has reached 100%, the duty ratio is not further changed. FIG. 15 illustrates an example in which the duty ratio reaches 100% once 2 to seconds have elapsed. Therefore, the duty ratio is not changed when the elapse of 3t₀seconds has elapsed. Note that, as described above, x<y.

As described above, in the image forming apparatus 100 of the second embodiment, when the start-up processing start condition is satisfied, the central heating element 45a is energized at a duty ratio that changes by x % after every time increment t₀. On the other hand, the first end heating element 45b1 and the second end heating element 45b2 are energized with a duty ratio that changes by y %, which is greater than x %, after every time increment t₀. Accordingly, the first end heating element 45b1 and the second end heating element 45b2 are relatively more heated/powered than the central heating element 45a, but the power is still increased at regular intervals of to seconds.

With the above configuration, the image forming apparatus 100 in the second embodiment can suppress possible differences in temperature at the ends of the cylindrical film 35 and the temperature at the central portion of the cylindrical film 35 when the fixing unit 30 is starting up.

The image forming apparatus 100 in the second embodiment has a configuration in which the increase amount (x) in the duty ratio for the energization to the central heating element 45a and the increase amount (y) of the duty ratio for the first end heating element 45b1 and the second end heating element 45b2 are different from each other. However, the disclosure is not limited thereto, and for example, the image forming apparatus 100 may have a configuration in which the frequency (timer intervals) for changing the duty ratio for energization of the central heating element 45a and the first end heating element 45b1 and the second end heating element 45b2 are different from each other.

Specifically, for example, the duty ratio may be changed every t1 seconds for the central heating element 45a, and the duty ratio may be changed for every t2 seconds for the first end heating element 45b1 and the second end heating element 45b2. Here, it can be assumed that t1>t2 is satisfied. In such a case as this, the increase amount (change increment) for the duty ratio for the central heating element