JP5282378B2 - Fixing apparatus and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fixing device capable of keeping image quality constant by shortening a warmup time and further stabilizing a fixing temperature, and to provide an image forming apparatus. <P>SOLUTION: The fixing device has a control circuit 40 that controls a fixing heat source so that a first temperature detected by a temperature detecting means 29 used for detecting the temperature of a fixing roller 27 has a constant temperature near a first target temperature. The control circuit 40 includes: a constant power controller that continuously supplies constant power; and a PID controller 41 that makes supply power variable according to a difference between the first target temperature and the first temperature detected by the means 29. The control circuit 40 is capable of selectively using them. After a warmup operation is started, a controller switching temperature is determined according to heat accumulated in the fixing device for a specific length of time. The controller switching temperature is the temperature at which a controller for the control circuit 40 is switched from the constant power controller for continuously supplying constant power to the PID controller 41. <P>COPYRIGHT: (C)2009,JPO&amp;INPIT

Description

  The present invention relates to a fixing device and an image forming apparatus, and more particularly, to an electrophotographic system and a fixing device and an image forming apparatus that perform fixing using heat.

  2. Description of the Related Art Conventionally, image forming apparatuses such as printers, copiers, and facsimile machines using a fixing device that fixes a toner image formed on a sheet of recording medium by heating and melting are known. Generally, a fixing device that fixes an electrophotographic image formed as a toner image on a sheet supplies power to a heater to generate heat in a roller or the like in the fixing unit, and this heat heats and melts the toner image on a sheet. Perform fixing process.

  Such a fixing device supplies electric power to the heater in order to make the fixing temperature constant while fixing the electrophotographic image onto the paper, and raises the temperature to a predetermined temperature to enable fixing. The sheet is passed through the fixing device while maintaining a predetermined fixing temperature.

  Here, the power driving (heating control) of the heater is performed by controlling the power supplied to the heater. Generally, as a method for controlling the power supplied to the heater, the temperature is detected by a temperature sensor such as a thermopile or thermistor provided in the fixing device. When the detected temperature is lower than the target control temperature, the heater is turned on. There is an ON / OFF control method of turning OFF when the temperature is higher than the target control temperature.

  However, in the temperature control by the ON / OFF control method, a large temperature ripple occurs with respect to the target control temperature, and the image quality of the fixed image changes according to the change in the fixing roller surface temperature, so that good image fixing cannot be performed. There is a problem.

  In order to reduce this problem, for example, it is conceivable to increase the heat capacity of the fixing roller pair by increasing the core metal thickness or surface layer thickness of the fixing roller pair heated in the fixing device. However, when the heat capacity of the fixing roller pair is set to be large as described above, although the temperature ripple can be reduced, a new problem arises that the warm-up time for raising the temperature to the target control temperature becomes longer.

  In order to solve the increase in the warm-up time, it is conceivable to keep the fixing device at a high temperature when the image forming apparatus is at rest. However, this causes a disadvantage that the power consumption of the image forming apparatus increases. . Therefore, in recent years, in order to comprehensively solve these problems, a roller having a relatively small heat capacity or a seamless belt is used as a heat medium, a heater having a relatively large heat generation amount is used as a fixing heat source, and a large temperature overshoot, A method of controlling without causing temperature ripple has become necessary.

  There is PID control as a control algorithm replacing the above-described ON / OFF control. PID control is controlled by three combinations of P: Proportional, I: Integral, and D: Differential. Different parameters are optimized according to the deviation between the target value and the current value. By controlling, This PID control can reduce the ripple as compared with the ON / OFF control.

  However, since the PID control is a control method in which the supplied power is changed in accordance with the deviation between the target value and the current value, the supplied power is weakened when the temperature of the controlled object approaches the target control temperature, and the rate of temperature increase accordingly. Has a problem that the warm-up time described above becomes long.

  FIG. 8 shows a state in which the temperature of the fixing roller is controlled to the target control temperature by the control methods of PID control and ON / OFF control. In this way, PID control and There are advantages and disadvantages in ON / OFF control.

  In order to solve this problem, in the prior art, as shown in FIG. 9, when the power is turned on, the heater is fully energized continuously or heated at a constant power, and after reaching a predetermined temperature, the PID control system is switched to energize the heater. A control method for performing control is adopted.

JP 58-38972 A JP 60-163102 A Japanese Patent Laid-Open No. 3-116208 JP-A-9-258601 JP 2005-257945 A JP 2006-10943 A

  For example, in Patent Document 1, the heater is fully energized and continuously heated until the heating temperature of the heater reaches a switching temperature lower than the control target temperature, and after reaching the switching temperature, the heater is energized by I (integral) control. Is configured to control.

  In Patent Document 2, the heater is continuously energized continuously until the switching temperature is reached, and the heater energization is controlled by PID control after reaching the switching temperature.

  Further, in Patent Document 3, the heater is fully energized continuously until the switching temperature is reached, and the heater energization is controlled by P (proportional) control after reaching the switching temperature.

  As described above, in Patent Documents 1 to 3, overshoot is prevented and the target control temperature is quickly followed by performing control by switching the control algorithm based on the temperature information.

However, since the control algorithm has a single switching temperature value, there is a limit to satisfy both the overshoot and the target control temperature follow-up performance in a short time.
Therefore, in Patent Document 4, a plurality of switching temperature values are prepared, and the control accuracy is improved by changing the PID control parameter a plurality of times at each switching temperature.

As described above, it is very important to change the control algorithm or the control parameter according to the temperature and perform the control in order to make the fixing temperature a desired behavior.
However, in Patent Documents 1 to 4, the switching temperature is a fixed value set in advance, and no consideration is given to the internal heat storage state of the fixing device.

  Since the internal heat storage state of the fixing device varies depending on the operation state, the temperature increase rate of the fixing member when the heater generates heat also varies. Therefore, for example, as shown in FIG. 10, if the controller switching temperature is set to be optimal when the heat storage amount inside the fixing device is small, and the set controller switching temperature is applied when the internal heat storage amount is large, excessive overheating will occur. May cause a shoot. Therefore, it is not sufficient to set the controller switching temperature to a single value without considering the heat storage state inside the fixing device.

In response to the above problem of the heat storage state, Patent Documents 5 and 6 propose a method of switching the PID control coefficient during warm-up depending on the heat storage state inside the fixing device.
However, as described above, there is a limit in achieving both shortening of the warm-up time and overshoot only by switching the PID control in multiple stages.

  SUMMARY OF THE INVENTION In view of the above-described conventional problems, an object of the present invention is to provide a fixing device and an image forming apparatus that can shorten the warm-up time, further stabilize the fixing temperature, and maintain a constant image quality. .

To achieve this object, the present invention includes a fixing member, a pressure member pressed against the fixing member, and a fixing heat source that heats at least the fixing member among the fixing member and the pressure member. In the fixing device in which the surface carrying the toner image to be fixed in the fixing nip formed by the fixing member and the pressure member passes the recording medium in a direction in contact with the surface of the fixing member, the fixing member First temperature detecting means for detecting temperature and second temperature detecting means for detecting the temperature of the pressurizing member are provided so that the first detected temperature becomes a constant temperature in the vicinity of the first target temperature. A heat source control means for controlling the fixing heat source, and the heat source control means is a constant power controller that keeps supplying a constant power, and a variable supply power according to a deviation between the first target temperature and the first detected temperature. PID con And a roller, and it is possible to use by switching them to reach after starting the warm-up operation, the time when the first detection temperature reaches 60% of the warm-up target temperature 90% At a certain time before, the heat storage state inside the fixing device is determined based on the second detected temperature by the second temperature detection unit, and the controller of the heat source control unit of the heat source control unit is determined according to the heat storage state. Determining a controller switching temperature for switching from the constant power controller to the PID controller, and switching the controller from the constant power controller to the PID controller when the first temperature detecting means detects the controller switching temperature. A fixing device is proposed.

  Further, according to the present invention, a fixing belt that is wound around a plurality of support rollers is used as the fixing member, and a pressure member that forms a fixing nip with the fixing belt is provided, and the recording medium is sandwiched between the fixing nips. It is effective to fuse the toner image on the recording medium to the recording medium while being conveyed.

  Furthermore, according to the present invention, the determination of the controller switching temperature of the heat source control means is performed during a predetermined time after starting the warm-up operation when returning from the low power mode, and then the determination during the warm-up operation. It is effective to switch the controller from the constant power controller to the PID controller at the controller switching temperature.

In order to achieve this object, the present invention provides an image forming apparatus for forming a toner image on a recording medium and fixing the toner image on the recording medium to obtain a recorded image. An image forming apparatus using the fixing device according to claim 1 as a fixing device for fixing is proposed.

  According to the present invention, the warm-up time can be shortened, the fixing temperature can be controlled with high accuracy, and fluctuations in the fixing temperature can be reduced. As a result, the set temperature can be lowered and the power consumption can be reduced.

The best mode for carrying out the present invention will be described below with reference to the accompanying drawings.
FIG. 1 is a schematic view showing an example of an image forming apparatus provided with a fixing device according to the present invention.

  In FIG. 1, the image forming apparatus employs an electrophotographic system, and includes a drum-shaped photoconductor 10 that rotates clockwise in the center of an image forming apparatus main body 100. The photosensitive member is not limited to a drum shape, and a belt shape can be used. A charging device 11, a developing device 12, a transfer device 13, and a cleaning device 14 are arranged around the photoconductor 10 in a clockwise direction, and an exposure device 15 is provided obliquely above the charging device 11.

  A sheet feeding cassette 16 is provided below the photosensitive member 10, and a sheet 20 of recording material stacked and accommodated in the sheet feeding cassette 16 is transferred from the sheet feeding cassette 16 to the photosensitive member 10 and the transfer device 13. A recording material conveyance path 18 that passes through to the sheet discharge stack unit 17 on the image forming apparatus main body 100 is conveyed.

  When the photosensitive member 10 is rotated in the clockwise direction by a driving device (not shown), the surface of the photosensitive member 10 is uniformly charged by the charging device 11. The charging device 11 is a charging charger, but a roll-shaped one can be used instead of the charger. The charged portion of the photoreceptor is irradiated with writing light L (light modulated laser light in the illustrated example) L emitted from the exposure device 15, thereby forming an electrostatic latent image on the photoreceptor 10. Instead of the laser type exposure apparatus, an exposure apparatus having an LED array and an imaging means can be used. The electrostatic latent image on the photoconductor 10 is visualized by the developing device 12 to form a toner image.

  On the other hand, in the paper feed unit, the paper feed roller 21 is rotated to feed the paper 20 from the paper feed cassette 16 and is fed between the photoconductor 10 and the transfer device 13 at a predetermined timing via the transport roller 22 and the registration roller 37. . Then, the toner image formed on the photoconductor 10 is transferred to the paper 20 by the transfer device 13, and the transferred paper 20 is separated from the photoconductor 10 and sent to the fixing device 24 via the recording material transport device 23. The paper 20 on which the toner image is fixed by the fixing device 24 is stacked on the paper discharge stack unit 17 via the paper discharge roller 25.

  The photoreceptor 10 after the toner image is transferred is prepared for the next image formation after dirt such as residual toner is removed by the cleaning device 14 and residual charges are removed by a charge eliminating means (not shown). Note that the transfer device 13 may be a roll instead of a charger as in the charging device. The cleaning device 14 includes a cleaning blade, but a cleaning brush or a combination of a cleaning blade and a cleaning brush can be used instead of the cleaning blade.

FIG. 2 is an explanatory diagram showing a configuration of the fixing device 24.
In FIG. 2, the fixing device 24 includes a fixing roller 27 that is a roller-shaped fixing member, and a pressure roller 28 that is a roller-shaped pressure member, and one of these rollers 27, 28. The rotating shaft is fixed, the rotating shaft of the other roller is movable, and the other roller is supported so as to be able to contact and separate from one roller. A fixing nip n is formed between the fixing roller 27 and the pressure roller 28 by urging the other roller toward the one roller by a pressing means such as a spring. The fixing roller 27 includes a fixing heat source 30 therein, and the fixing roller 27 is heated by the fixing heat source 30. In this example, a heater such as a halogen heater or a carbon heater is used as the fixing heat source 30, but a fixing heat source that generates heat using, for example, electromagnetic induction can be used instead of the heater.

  A temperature detecting means 29 for detecting the temperature of the fixing roller 27 is disposed in the vicinity of the fixing roller 27, and similarly, the temperature of the pressure roller 28 is detected around the pressure roller 28. The temperature detection means 102 for this is arrange | positioned closely. The temperature detection means 29 and 102 include a thermopile thermistor and the like, but at least the temperature detection means 29 is preferably provided with a thermopile that is not in contact with the fixing roller 27 and is suitable for the present invention.

  In the fixing device 24, the fixing roller 27 is heated by the fixing heat source 30, and the fixing roller 27 comes into contact with the image surface of the paper 20 passing through the fixing nip n between the fixing roller 27 and the pressure roller 28, The toner image on the image surface is melted and fixed on the paper 20.

FIG. 3 is a block diagram showing a control circuit 40 as heat source control means of the fixing device 24.
The control circuit 40 includes a constant power controller 101 that continues to supply constant power to the fixing heat source 30, a PID controller 41 that varies the power according to a deviation between the target control temperature and the temperature of the fixing roller 27, and a PWM drive circuit. 42, and controls the power applied to the fixing heat source 30 of the fixing roller 27.

  When the temperature of the fixing roller 27 detected by the temperature detecting unit 29 is equal to or higher than the controller switching temperature Tsw, the control circuit 40 fixes the target control temperature of the fixing roller 27 specified in advance and the fixing detected by the temperature detecting unit 29. Based on the information on the temperature deviation between the temperature of the roller 27 and the PID controller 41, the calculation is performed. This calculation result indicates a rate at which the fixing heat source 30 is turned on per certain time (control cycle), and the fixing heat source 30 is turned on through the PWM drive circuit 42 based on the calculated DUTY. In this example, the rate at which the rated AC voltage per certain time is applied to both ends of the fixing heat source 30 is controlled. The control cycle is often about 100 [ms] to 2 [s], but an optimum value is determined in accordance with the characteristics of the fixing device.

For example, when the control cycle is T [s] and the DUTY of the calculation result by the PID calculation is a [%], as shown in FIG. 4, the heater is only aT / 100 [s] in the control cycle T [s]. Lights up. Thus, the temperature of the fixing roller 27 is detected by the temperature detecting means 29, and the fixing heat source 30 is controlled by the PID controller 41 based on the detection result, thereby maintaining the temperature of the fixing roller 27 at the target control temperature.
On the other hand, when the temperature of the fixing roller 27 detected by the temperature detecting unit 29 is less than the controller switching temperature Tsw, the fixing heat source 30 is passed through the PWM drive circuit 42 based on a constant DUTY value designated by the constant power controller. Lights up. (DUTY = 100 [%] if all energized)

Here, the details of the procedure for calculating DUTY by the PID arithmetic expression will be described. For the calculation of DUTY, Formula (1), which is a PID basic formula, is converted into a digital system and used.

However,
Kp: proportional gain T _I : integration time T _D : differentiation time e (t): error between target control temperature and fixing roller 27 temperature (= r (t) -y (t))
r (t): target control temperature y (t): temperature of the fixing roller 27 T: control cycle

が得られる。数式（２）を用いて、制御周期ごとに目標制御温度と定着ローラ２７の温度の誤差情報からＤＵＴＹを計算する。 When Equation (1) is converted by bilinear transformation, a digital PID control equation

Is obtained. Using equation (2), DUTY is calculated from error information between the target control temperature and the temperature of the fixing roller 27 for each control cycle.

In addition, although it is the method of calculating DUTY shown in Formula (2), the example of the PID controller which performed digital conversion by the step-like approximation was shown here, However, Digital conversion by bilinear conversion shown in Formula (3) The PID algorithm performed may be used.

In addition, although the position type algorithm is shown in Formula (3), a speed type algorithm that calculates the amount of change in DUTY for each control period may be used as shown in Formula (4).

Moreover, although the example which calculates with a PID controller was shown here, according to the objective, feedback controllers, such as an I-PD controller shown to Numerical formula (5) and a PID controller shown to Numerical formula (6), may be used, Any feedback controller may be used as long as the temperature of the fixing roller follows the target control temperature.

  FIG. 5 shows the most basic mode in which the image forming apparatus operates. In general, when the fixing roller temperature reaches the target control temperature through a warm-up operation after turning on the power, the printer is ready for printing. Thereafter, when there is no print request, a standby state is entered, and when a print request is received, a printing operation is started. In the present invention, when the controller switching temperature Tsw is reached during the warm-up operation, the controller is switched from the constant power controller to the PID controller.

  Next, a method for determining the controller switching temperature Tsw will be described. In the prior art, the controller switching temperature Tsw is a fixed value regardless of the internal heat storage state of fixing.

  Here, a procedure for determining the controller switching temperature Tsw according to the fixing internal heat storage state will be described. Several means for grasping the heat storage state inside the fixing are conceivable. Generally, the temperature detecting means 102 is attached to the pressure roller 28. In a recent color fixing device, the heat capacity of the pressure roller 28 is increased. Therefore, the internal heat storage state can be estimated by the temperature of the pressure roller. Further, by using the temperature detecting means 102 attached in advance, the present invention can be applied with the current configuration without newly attaching a temperature detecting means or the like, which is advantageous in terms of cost.

  Therefore, in this embodiment, the heat storage state is grasped from the temperature of the pressure roller 28. However, in order to grasp the amount of heat storage more accurately, a separate temperature detection means is attached to a member with a larger heat capacity, the fixing heat source of the fixing member, the cumulative heating time of the heat source of the pressure member, or printing so far The determination may be made based on the history of past information such as the number of printed sheets.

  First, when the warm-up operation is started, the temperature of the pressure roller 28 is measured by the temperature detecting means 102. The temperature Tp of the pressure roller 28 and the temperature of the fixing roller 27 (controller switching temperature Tsw) for switching the controller from the constant power controller to the PID controller are preliminarily related, and the controller switching temperature Tsw is referred to them. Is set. The relationship between the two may be related by a table as shown in Table 1 below, or may be described by a mathematical formula Tsw = f (Tp). In general, the lower the temperature of the pressure roller 28, the smaller the heat storage amount of the fixing device, and the temperature increase rate of the fixing roller 27 becomes slower. Therefore, the controller switching temperature Tsw is designed to be higher. The controller switching temperature Tsw is better associated with the difference value from the target control temperature during warm-up.

  Once the controller switching temperature Tsw is set, power is supplied with constant power up to the controller switching temperature Tsw as described above, and then the PID control is switched to control the temperature of the fixing roller 27 to the target control temperature. The temperature of the pressure roller 28 may be measured immediately after the start of the warm-up operation, and the controller switching temperature Tsw may be set. The temperature of the fixing roller 27 is 60% of the warm-up target control temperature of the fixing roller 27. If the temperature of the pressure roller 28 is measured at a certain point from when the temperature reaches 90% until reaching the temperature of 90%, and the controller switching temperature Tsw is set at that time, the controller switching temperature Tsw is immediately before switching the controller. The latest heat storage information can be set and temperature control can be performed more accurately.

  In such a fixing device, the controller switching temperature Tsw is set based on the temperature Tp of the pressure roller 28 after the start of the warm-up operation by turning on the power source such as the power switch ON, and the control during the subsequent warm-up is performed. The temperature of the fixing roller 27 can be controlled quickly and stably from the charging to the fixing possible temperature. Therefore, as shown in FIG. 11, it is possible to shorten the warm-up time while suppressing overshoot.

  In addition, when a state in which printing is not performed continues for a predetermined time, the mode normally shifts to a low power mode or an energy saving mode. In this state, when a command for starting an image forming operation (for example, document insertion, a job from a printer) is issued, the fixing heater is turned on in order to return to the fixable temperature from the low power mode or the energy saving mode. The warm-up operation is started.

  In the fixing device configured as described above, the controller switching temperature Tsw is set based on the temperature Tp of the pressure roller 28 after starting the warm-up operation when returning from the low power mode or the energy saving mode, and then the warm-up is performed. By performing the internal control, the temperature of the fixing roller 27 can be quickly and stably controlled from the start of the image forming operation to the fixable temperature.

  By the way, in the image forming apparatus described above, a fixing roller 27 is used as a fixing member for convenience, and a pressure roller 28 for forming a fixing nip n is provided between the fixing roller 27 and a recording material is provided in the fixing nip n. A roller fixing device for fixing the toner image on the image surface to a sheet 20 through a certain sheet 20 was used.

  However, as shown in FIG. 6, for example, the fixing device according to the present invention uses a fixing belt 52 wound around a plurality of support rollers (two support rollers 50 and 51 in the illustrated example) as a fixing member. While the fixing heat source 30 is built in the support roller 50, another support roller 51 is pressed against the pressure roller 53 with the fixing belt 52 interposed therebetween, and a fixing nip n is formed between the pressure roller 53 and the fixing belt 52. It is also possible to use a belt fixing device that forms and fixes the toner image on the image surface on the paper 20 through the paper 20 in the fixing nip n.

  In this belt fixing device, the fixing belt 52 is heated by the fixing heat source 30 via the support roller 50. The temperature of the pressure roller 28 after the start of warm-up is determined based on the temperature of the fixing belt 52 as a fixing member. Controller switching temperature Tsw.

  FIG. 7 shows a comparison of the fixing temperatures of the belt fixing device and the roller fixing device. As shown in the figure, in the case of the belt fixing device, since the heat capacity is smaller than that of the roller fixing device described above, the warm-up time can be shortened, and the temperature of the fixing roller 27 is reduced because of the low heat capacity. There is a feature that the ripple tends to increase. Therefore, the effect of stabilizing the fixing temperature by switching the controller as in the present invention can be further increased.

  In the illustrated example, the fixing heat source 30 is built in the support roller 50. However, a fixing heat source may be disposed around the fixing belt 52 to heat the fixing belt 52. As in the case of the roller fixing device, not only the heater but also a fixing heat source that generates heat using electromagnetic induction may be used.

  In the belt fixing device and the roller fixing device described above, the pressure roller 28 that rotates together with the fixing member such as the fixing roller 28 and the fixing belt 52 is used as the pressure member. For example, a pressure pad that does not rotate with the fixing member can be used.

1 is a schematic diagram illustrating an example of an image forming apparatus including a fixing device according to the present invention. 1 is a configuration diagram of a roller fixing device showing an embodiment of a fixing device according to the present invention. FIG. It is a block diagram of a control circuit showing a heat source control means of the fixing device. It is a relationship diagram of a heater ON time with respect to a control period and DUTY. It is a figure which shows the temperature of the fixing roller at the time of PID control, and the state of DUTY of a fixing heat source. It is a block diagram of a belt fixing device showing another embodiment of the fixing device according to the present invention. It is a comparison figure of the fixing temperature of a belt fixing device and a roller fixing device. It is a comparison figure of PID control and ON / OFF control. It is an example of control when conventional constant power control and PID control are used in combination. It is a graph which shows the example of control when the conventional controller switching temperature is made into a fixed value irrespective of the fixing internal heat storage state. It is a graph which shows the example of control at the time of making controller switching temperature variable with the internal heat storage amount of this invention.

Explanation of symbols

24 Fixing Device 27 Fixing Roller 28 Pressure Roller 29 Fixing Member Temperature Detection Unit 30 Fixing Heat Source 40 Control Circuit 41 PID Controller 50, 51 Support Roller 52 Fixing Belt 102 Pressure Member Temperature Detection Unit

Claims (4)

A fixing member; a pressing member pressed against the fixing member; and a fixing heat source for heating at least the fixing member of the fixing member and the pressing member. The fixing member and the pressing member are formed. A fixing device that passes a recording medium in a direction in which a surface carrying a toner image to be fixed to the fixing nip is in contact with the surface of the fixing member;
A first temperature detecting means for detecting the temperature of the fixing member; and a second temperature detecting means for detecting the temperature of the pressure member, wherein the first detected temperature is constant near the first target temperature. A heat source control means for controlling the fixing heat source so as to reach a temperature;
The heat source control means has a constant power controller that continues to supply constant power, and a PID controller that makes the supply power variable according to the deviation between the first target temperature and the first detected temperature, and switches between them. Can be used,
After starting the warm-up operation, at a certain time between the time when the first detected temperature reaches 60% of the warm-up target temperature and the time when the first detected temperature reaches 90%, A heat storage state inside the fixing device is determined based on the detected temperature, and a controller switching temperature for switching the controller of the heat source control unit of the heat source control unit from the constant power controller to the PID controller is determined according to the heat storage state,
A fixing device, wherein the controller is switched from a constant power controller to a PID controller when the first temperature detecting means detects a controller switching temperature .   As the fixing member, a fixing belt that is wound around a plurality of support rollers is used, and a pressure member that forms a fixing nip with the fixing belt is provided, and the recording medium is sandwiched and conveyed at the fixing nip. The fixing device according to claim 1, wherein the upper toner image is fused to the recording medium.   The controller switching temperature of the heat source control means is determined for a predetermined time after starting the warm-up operation when returning from the low power mode, and then at the determined controller switching temperature at the time of the warm-up operation. The fixing device according to claim 1, wherein the controller is switched from a constant power controller to a PID controller. Forming a toner image on a recording medium, an image forming apparatus for obtaining a recording image by fixing the toner image to a recording medium, a fixing device that fixes the toner image on the recording medium, any one of claims 1 to 3 An image forming apparatus using the fixing device described in 1 above.

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