JP6562599B2 - Fixing device - Google Patents

Description

  The present invention relates to a fixing device mounted on an image forming apparatus such as an electrophotographic copying machine or a printer.

  A fixing device mounted on an image forming apparatus such as an electrophotographic copying machine or a printer is a recording material that carries an unfixed toner image at a nip formed by a heating rotator and a pressure roller that contacts the heating rotator. In general, a toner image is fixed on a recording material by heating while conveying the toner.

  In recent years, an electromagnetic induction heating type fixing device capable of generating heat in a conductive layer of a heating rotator has been developed and put into practical use. The electromagnetic induction heating type fixing device has an advantage that the warm-up time is short.

  Patent Document 1 discloses a fixing device in which the restrictions on the thickness of the conductive layer and the material of the conductive layer are small.

JP 2014-026267 A

  Also in the fixing device disclosed in Japanese Patent Application Laid-Open No. 2004-133620, the temperature rise of the non-sheet passing portion when fixing a small size recording material becomes a problem.

  An object of the present invention is to provide a fixing device that can easily control the temperature of a region through which a recording material of a rotating body passes while forming a heat generation distribution according to the size of the recording material.

The present invention for solving the above-described problems includes a cylindrical rotating body having a conductive layer, a coil disposed inside the rotating body and having a helical axis substantially parallel to the generatrix direction of the rotating body, and a resonant capacitor And a first converter that drives the resonance circuit, and the conductive layer is heated by electromagnetic induction by the magnetic flux generated by the coil, and is formed on the recording material by the heat of the rotating body. In the fixing device for fixing the recorded image to the recording material, there is one coil disposed inside the rotating body, and the conductive layer has the same material and thickness from one end to the other end in the busbar direction. A second converter configured to control power supplied to the first converter, and the first converter according to at least one of a size of a recording material and a temperature of a non-sheet passing portion of the rotating body. Converter A frequency setting unit that changes a dynamic frequency, and a power control that controls the second converter according to the temperature of the sheet passing portion of the rotating body and controls the power supplied from the second converter to the first converter And when the frequency setting unit changes the driving frequency, the heat generation distribution of the rotating body in the bus direction changes.

  It is possible to provide a fixing device that can easily control the temperature of a region through which a recording material of a rotating body passes while forming a heat generation distribution according to the size of the recording material.

Schematic sectional view of the image forming apparatus Cross section of fixing unit Front view of fixing unit Perspective view of coil unit provided in fixing unit and block diagram of drive circuit Drive circuit diagram Diagram showing the relationship between the drive frequency and the temperature distribution of the fixing sleeve The figure showing operation of the 1st converter The figure showing operation | movement of the 1st converter at the time of switching a drive frequency The figure showing operation of the 2nd converter The figure showing operation | movement of the 2nd converter at the time of switching ON duty ratio of the switch element in a 2nd converter The figure showing the difference in the emitted-heat amount of a rotary body at the time of switching ON duty ratio of the switch element in a 2nd converter Explanatory drawing of Example 2.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, embodiments for carrying out the invention will be exemplarily described in detail with reference to the drawings. However, the dimensions, materials, shapes, and relative arrangements of the components described in this embodiment should be appropriately changed according to the configuration of the apparatus to which the invention is applied and various conditions. That is, it is not intended to limit the scope of the present invention to the following embodiments.

Example 1
FIG. 1 is a schematic configuration diagram of an image forming apparatus 100 according to the present embodiment. The image forming apparatus 100 of this embodiment is a laser beam printer using an electrophotographic process.

  A controller 31 is a control unit of the image forming apparatus, and includes a CPU (Central Processing Unit) 32 having a ROM 32a, a RAM 32b, a timer 32c and the like, various input / output control circuits (not shown), and the like. Reference numeral 101 denotes a rotating drum type electrophotographic photosensitive member (hereinafter referred to as a photosensitive drum) serving as an image carrier, which is rotationally driven in a clockwise direction indicated by an arrow at a predetermined peripheral speed. The photosensitive drum 101 is uniformly charged to a predetermined polarity and potential by the contact charging roller 102 during the rotation process. Reference numeral 103 denotes a laser beam scanner, which outputs laser light L that is on / off modulated in accordance with image information input from an external device such as an image scanner or a computer (not shown). The charged surface of the photosensitive drum 101 is exposed by the laser light L, and an electrostatic latent image corresponding to image information is formed on the surface of the photosensitive drum 101. A developing device 104 supplies developer (toner) to the surface of the photosensitive drum 101 from the developing roller 104a, and develops the electrostatic latent image on the surface of the photosensitive drum 101 as a toner image. Reference numeral 105 denotes a paper feed cassette in which the recording material P is stored. A registration roller 107 conveys the recording material P so that the leading end of the toner image formed on the photosensitive drum and a predetermined position of the recording material are aligned. When a paper feed start signal is input, the paper feed roller 106 is driven to feed the recording material P in the paper feed cassette 105 one by one. The fed recording material is adjusted in conveyance timing by a registration roller 107 and then introduced into a transfer portion 108T where the photosensitive drum 101 and the transfer roller 108 abut. While the recording material P is nipped and conveyed at the transfer site 108T, a transfer bias is applied to the transfer roller 8 from a power source (not shown). The transfer roller 108 is applied with a transfer bias having a polarity opposite to the charging polarity of the toner, whereby the toner image on the photosensitive drum 101 is transferred to the recording material P. Thereafter, the recording material P onto which the toner image has been transferred is separated from the surface of the photosensitive drum 101 and is introduced into the fixing unit A through the conveyance guide 109. The toner image on the recording material is heated by the fixing unit and fixed on the recording material. The recording material P that has passed through the fixing unit is discharged onto the paper discharge tray 112 through the paper discharge port 111. On the other hand, the surface of the photosensitive drum 101 after the recording material P is separated is cleaned by the cleaning unit 110.

  The fixing unit A is an electromagnetic induction heating type fixing device. Specifically, the fixing device fixes the image formed on the recording material to the recording material by the electromagnetic induction heat generation of the conductive layer of the rotating material by the magnetic flux generated by the coil. 2 is a cross-sectional view of the fixing unit, FIG. 3 is a front view of the fixing unit, and FIG. 4 is a perspective view of a coil unit provided in the fixing unit. The fixing unit A includes a heating unit having a fixing sleeve 1 and a coil unit, which will be described later, and a pressure member 8, and sandwiches and conveys a recording material P carrying an unfixed toner image between the heating unit and the pressure member. A fixing nip N is formed.

  The pressure roller 8 as a pressure member includes a cored bar 8a, an elastic layer 8b formed of silicone rubber or the like, and a release layer 8c formed of fluorine resin or the like. Both ends of the cored bar 8a are rotatably held via bearings between device chassis (not shown) of the fixing unit. Also, pressure springs (compression springs in this example) 17a and 17b are provided between both ends of the pressure stay (metal reinforcing member) 5 shown in FIG. 3 and the spring receiving members 18a and 18b on the apparatus chassis side, respectively. By providing, a pressing force is applied to the pressurizing stay 5. In the fixing unit A of this embodiment, a total pressure of about 100 N to 250 N (about 10 kgf to about 25 kgf) is applied. As a result, the lower surface of the sleeve guide member 6 made of a heat resistant resin (PPS or the like) and the pressure roller 8 are pressed against each other with the fixing sleeve 1 interposed therebetween to form the fixing nip portion N. The pressure roller 8 is driven in a direction indicated by an arrow by a driving unit (not shown), and the fixing sleeve 1 rotates following the rotation of the pressure roller. Reference numerals 12a and 12b denote flange members that rotate following the rotation of the fixing sleeve. The flange member is rotatably disposed at the longitudinal end portion of the sleeve guide 6. When the fixing sleeve moves toward the generatrix while rotating, it abuts against the flange member, and the flange member pushed by the fixing sleeve abuts against the regulating member 13a (13b). Thereby, the shift movement of the fixing sleeve is regulated by the regulating member. The flange member is formed of a material having good heat resistance such as LCP (Liquid Crystal Polymer).

  The fixing sleeve 1 as a rotatable cylindrical rotating body preferably has a diameter of 10 to 50 mm, and includes a heat generating layer (conductive layer) 1a as a base layer, an elastic layer 1b laminated on the outer surface, and a release layer 1c on the sleeve surface. Have. The heat generating layer 1a is a metal film (the material of the sleeve in this example is stainless steel), and the film thickness is preferably 10 to 50 μm. The elastic layer 1b is made of silicone rubber, and preferably has a hardness of about 20 degrees (JIS-A, 1 kg load) and a thickness of 0.1 mm to 0.3 mm. The release layer is a fluororesin tube, and the thickness is preferably 10 to 50 μm. An induced current is generated in the heat generating layer 1a by the action of an alternating magnetic flux described later. The heat generation layer generates heat by this induced current, and this heat is transmitted to the elastic layer 1b and the release layer 1c, and the entire circumferential direction of the fixing sleeve 1 is heated. The temperature detection elements 9, 10, and 11 for detecting the temperature of the fixing sleeve will be described later.

  Next, a mechanism for generating an induced current in the heat generating layer 1a will be described in detail. FIG. 4 is a perspective view of a coil unit provided in the heating unit. The coil unit has a spiral-shaped portion that is disposed inside the rotating body (fixing sleeve) and has a spiral axis that is substantially parallel to the generatrix direction of the rotating body, and generates an alternating magnetic field for causing the conductive layer of the rotating body to generate electromagnetic induction heat. It has a coil 3 to be formed. Furthermore, the core 2 for arrange | positioning in a spiral shape part and guide | inducing a magnetic flux is provided. The magnetic core 2 as a magnetic core material is disposed through the hollow portion of the fixing sleeve 1 by fixing means (not shown). NP and SP indicate the magnetic poles of the core 2. The core 2 has a shape that does not form a loop outside the rotating body (that is, an end shape), and the magnetic flux generated by the coil forms an open magnetic path. The material of the core is composed of a material having a small hysteresis loss and a high relative magnetic permeability, for example, a sintered ferrite, a ferrite resin, an amorphous alloy (amorphous alloy), a high permeability oxide such as permalloy, an alloy, or the like. Ferromagnetic materials are preferred. In this example, sintered ferrite having a relative magnetic permeability of 1800 is used. The core of this example has a cylindrical shape, and the diameter is preferably 5 to 30 mm. In the case of a fixing device mounted on an A4 printer, the length of the core is preferably about 240 mm. The core 2 around which the coil 3 is wound is covered with a resin cover 4.

  The exciting coil 3 is formed by spirally winding a single conducting wire around the magnetic core 2 in the hollow portion of the fixing sleeve 1. In that case, it winds so that a space | interval may become denser in an edge part rather than a core center part. The exciting coil 3 is wound 18 times around the magnetic core 2 having a longitudinal dimension of 240 mm. The winding interval is 10 mm at the end, 20 mm at the center, and 15 mm in the middle. Thus, the coil is wound in a direction intersecting the axis X of the core.

  When a high frequency current is passed through the exciting coil 3 by the high frequency converter through the power supply contact portions 3a and 3b, a magnetic flux is generated. In the apparatus of this example, most of the magnetic flux that emerges from the end of the core 2 (70% or more, preferably 90% or more, more preferably 94% or more) passes outside of the heat generation layer of the fixing sleeve and other than the core. Designed to return to the edge. For this reason, an induced current flowing in the circumferential direction is generated in the heat generating layer of the fixing sleeve so that a magnetic flux that cancels the magnetic flux passing outside the sleeve is generated. Thereby, the whole circumferential direction of the heat generating layer generates heat. As described above, the configuration in which the induced current flows in the circumferential direction of the fixing sleeve generates heat in the entire circumferential direction of the fixing sleeve, so that there is an advantage that the time for warming up the fixing device to a temperature at which fixing can be performed can be shortened. Further, the core 2 is formed in an end shape, so that most of the magnetic flux passes through the outside of the heat generating layer by an open magnetic path. For this reason, there is also an advantage that the size can be reduced as compared with an apparatus having a configuration in which the core is formed in a loop shape to form a closed magnetic circuit.

  As shown in FIG. 2, the temperature detection elements 9, 10, and 11 of the fixing unit A are disposed upstream of the fixing nip portion N in the fixing sleeve rotation direction and detect the surface temperature of the fixing sleeve. Further, in the longitudinal direction of the fixing unit, as shown in FIG. 3, the temperatures of the center and both ends of the fixing sleeve are detected. The temperature detection elements 9, 10, and 11 are configured by a thermistor or the like. The power supply to the coil is controlled so that the temperature detected by the temperature detecting element 9 at the center maintains a control target temperature suitable for fixing. Further, the temperature detection elements 10 and 11 disposed near the end of the fixing sleeve 1 can detect the temperature rise in the non-sheet passing area of the fixing sleeve when the small size recording material P is continuously printed. . The temperature detecting elements 10 and 11 are arranged at the end of the pressure roller 8 in the axial direction to detect the temperature rise in the non-sheet passing area of the pressure roller when the small size recording material P is continuously printed. Also good.

  FIG. 4 is a block diagram showing the relationship among the CPU 32, the printer controller 41, and the host computer 42, which are control units that perform printer control. The printer controller 41 communicates with the host computer 42, receives image data, and develops the received image data into information that can be printed by the image forming apparatus 100. Furthermore, signal exchange and serial communication are performed with the engine control unit 121. The engine control unit 121 exchanges signals with the printer controller 41 and further controls each unit of the image forming apparatus 100 through serial communication. The engine control unit 121 controls the temperature of the fixing unit A based on the temperatures detected by the temperature detection elements 9, 10, and 11, and detects abnormality of the fixing unit A and the like.

  By the way, in order to generate an induced current flowing in the sleeve circumferential direction in the conductive layer of the sleeve, most of the magnetic flux emitted from the end of the core passes outside the heat generation layer of the fixing sleeve and returns to the other end of the core. The designed fixing device has been found to have the following problems.

  A general electromagnetic induction type fixing device is provided with a high-frequency converter that drives a resonance circuit including a coil. Then, in the fixing device that couples the magnetic flux generated by the coil to the conductive layer of the rotating body and generates heat by generating an eddy current in the conductive layer, the drive frequency of the high-frequency converter is adjusted to keep the sleeve temperature constant. Adjust the heat generation.

  However, in a fixing device that generates an induced current that flows in the circumferential direction of the sleeve as in this example, it has been found that when the drive frequency of the high-frequency converter is changed, the heat generation distribution in the direction of the bus of the sleeve changes. FIG. 6 shows the temperature distribution of the sleeve when the drive frequency of the high-frequency converter is changed in the range of 20 kHz to 50 kHz so that the center of the rotor body (sleeve) in the busbar direction (longitudinal direction) is maintained at 200 ° C. . It can be seen that the lower the drive frequency, the lower the amount of heat generated at both ends of the sleeve. Therefore, for example, in a case where the drive frequency must be set to 20 kHz in order to keep the temperature of the central portion at 200 ° C., the amount of heat generated at both ends of the sleeve is insufficient. As a result, the image on the recording material corresponding to both ends of the sleeve is insufficiently fixed.

  Therefore, in this embodiment, as shown in FIGS. 4 and 5, in addition to the high-frequency converter 16 (first converter) that drives the resonance circuit 191, a second for controlling the power supplied to the first converter. A converter 15 is provided. The resonant circuit 191 includes a cylindrical rotating body having a conductive layer, a coil that is disposed inside the rotating body and whose spiral axis is substantially parallel to the generatrix direction of the rotating body, and a resonant capacitor 1113. In FIG. 5, R is an equivalent resistance of the fixing unit A, and L is an equivalent inductance of the fixing unit A. The resonance circuit 191 of this example is a current resonance circuit. Further, a frequency setting unit 120 that sets the drive frequency of the first converter 16 according to at least one of the size of the recording material and the temperature of the non-sheet passing portion of the rotating body is provided. Furthermore, a power control unit 119 is provided that controls the second converter according to the temperature of the sheet passing portion of the rotating body and controls the power supplied from the second converter to the first converter.

  More specifically, the frequency setting unit 120 sets the drive frequency of the converter 16 according to the detected temperature of the temperature detecting elements 10 and 11 so that the temperature of the rotating body in the region that is a non-sheet passing portion of the small size paper does not become too high. Set. The power control unit 119 controls the output voltage of the second converter 15 so that the temperature of the sheet passing portion of the rotating body (detected temperature of the temperature detecting element 9) maintains a control target temperature that is a fixable temperature. Note that the drive frequency of the converter 16 may be set according to the size information of the recording material.

  Next, the drive circuit shown in FIG. 5 will be described in detail. A commercial power source (AC power supply) 50 connects the image forming apparatus 100 and supplies AC power to the image forming apparatus 100. The waveform of the commercial power supply 50 is a waveform like waveform 1 when the horizontal axis is time and the vertical axis is voltage. The electric power input from the commercial power supply 50 is input to the diode bridge 1102 via the AC filter 1101 and full-wave rectified. After the capacitor 1103 is charged, the rectified voltage becomes a voltage waveform like waveform 2 when the horizontal axis is time and the vertical axis is voltage.

  Reference numeral 71 denotes a power supply unit that generates a DC voltage, and outputs a predetermined voltage to a load (motor, CPU, etc.) (not shown) on the secondary side.

  First, the first converter 16 will be described. As will be described later, the first converter 16 is connected to the output of the second converter 15. The switch elements 1108 and 1109 constitute a half bridge circuit of the first converter 16. Reference numeral 1110 denotes a voltage resonance capacitor, which is connected between the drain (D) and source (S) of the switch element 1109 (between the collector and emitter when the switch element is an IGBT) in this embodiment. Reference numeral 1118 denotes a drive circuit for the switch elements 1108 and 1109. Reference numeral 191 denotes a series resonance (current resonance) circuit including an equivalent inductance L, an equivalent resistance R, and a current resonance capacitor 1113. The equivalent resistance R is a resistance that expresses the resistance of the rotating body 1 and the exciting coil 3 as a series equivalent resistance viewed from the exciting coil 3.

  FIG. 7 shows the voltage between the gate (G) and the source (S) of the switch element 1108, the voltage between the gate (G) and the source (S) of the switch element 1109, the drain (D) voltage of the switch element 1109, the current of the coil 3, And the voltage of the capacitor 1113. Since the current resonance circuit is used, the switch elements 1108 and 1109 are alternately driven with a duty ratio of about 50% in the period 1 + period 2 + period 3 + period 4. Specifically, the ON period of the switch element 1108 is period 1, and (period 1 / (period 1 + period 2 + period 3 + period 4)) ≈50%. The ON period of the switch element 1109 is period 3, and (period 3 / (period 1 + period 2 + period 3 + period 4)) ≈50%. The reason for driving at a duty ratio of 50% is that the current resonance capacitor 1113 needs to be charged with a voltage that is ½ of the voltage input to the first converter 16. When not driven at a duty ratio of 50%, the voltage amplitude allowable for the current resonance capacitor 1113 decreases, and the power that can be output to the coil 3 decreases. Further, in order to avoid conduction of both of the switch elements 1108 and 1109, a dead time is always provided as a period during which the switch elements 1108 and 1109 are turned off simultaneously (period 2 and period 4 in FIG. 7).

  A voltage resonant capacitor 1110 is connected between the drain (D) and source (S) terminals of the switch element 1109. When the switch element 1108 is turned on and a current flows from the capacitor 1107, the voltage of the capacitor 1110 becomes substantially the same as that of the capacitor 1107, and then a current starts to flow through the exciting coil 3 and the capacitor 1113 in the fixing unit A (FIG. 7). Period 1). The current flowing through the coil 3 and the capacitor 1113 is sinusoidal. The switch element 1109 is turned off during the period when the current of the coil 3 charges the capacitor 1113. Since current continues to flow through the exciting coil 3, the current flows through a reverse conducting diode (not shown) provided in the capacitor 1113 and the switch element 1109 (period 2 in FIG. 7).

  The drain (D) voltage of the switch element 1109 is lower than the source (S) voltage by the forward voltage of the reverse conducting diode. In the period 2 of FIG. 7, the frequency setting unit 120 turns on the switch element 1109 via the switch element driving circuit 1118 during the period in which the reverse conducting diode of the switch element 1109 is conducting. The current flowing through the exciting coil 3 decreases with time. The voltage stored in the capacitor 1113 becomes the highest voltage, and thereafter, current starts to flow in the reverse direction (period 3 in FIG. 7).

  Next, the switch element 1109 is turned off before the current flowing in the reverse direction becomes 0 A. Then, the flowing current starts to charge the capacitor 1110, and the drain (D) voltage of the switch element 1109 increases (period 4 in FIG. 7). When the drain (D) voltage of the switch element 1109 becomes higher than the voltage of the capacitor 1107, a current starts to flow through a reverse conducting diode (not shown) provided in the switch element 1108.

  The voltage of the capacitor 1110 is the sum of the voltage of the capacitor 1107 and the forward voltage of a reverse conducting diode (not shown) provided in the switch element 1109. The frequency setting circuit 120 turns on the switch element 1108 through the switch element drive circuit 1118 during the period when the reverse conducting diode of the switch element 1108 is energized (period 1 in FIG. 7). Thereafter, the above-described switch control from period 1 to period 4 is repeated.

  As described above, by appropriately setting the capacitance of the voltage resonance capacitor 1110, the current at turn-off, and the dead time (period 2 and period 4), the switch elements 1108 and 1109 become a soft switch operation, and high efficiency is achieved. It is possible to keep.

  By the way, the switch frequency (drive frequency) of the current resonance circuit 191 is controlled by the frequency setting unit 120. The frequency setting unit 120 controls the drive frequency of the resonance circuit 191 based on the detected temperature of the temperature detecting element 10 or 11 provided in the area (non-sheet passing area) where the recording material P of the rotating body 1 does not pass. The non-sheet-passing area is an area through which a recording material having a maximum size that can be used by the apparatus passes but does not pass a recording material having a size smaller than the maximum size. For example, when the detected temperature of the temperature detecting element 10 or 11 reaches a predetermined upper limit temperature, the drive frequency of the resonance circuit 191 is lowered to suppress the heat generation of the non-sheet passing portion of the rotating body and the temperature of the non-sheet passing portion increases. Is limiting. In this way, a heat generation distribution that matches the size of the recording material is formed. FIG. 8 shows voltage waveforms between the gate (G) and the source (S) of the switch element 1108 and the switch element 1109 when the drive frequency is set to 36 kHz and 50 kHz. At any frequency, the ON duty ratio of the switch element 1108 and the ON duty ratio of the switch element 1109 are about 50%. Thus, by switching the drive frequency of the first converter, a heat generation distribution suitable for the size of the recording material as shown in FIG. 6 can be formed. In this embodiment, since the drive frequency is controlled so that the temperature detected by the temperature detecting elements 10 and 11 does not exceed the upper limit temperature, the drive frequency fluctuates during the period for fixing one recording material. Sometimes. On the other hand, when the driving frequency of the resonance circuit is set according to the size information of the recording material without providing the temperature detecting elements 10 and 11, a predetermined driving frequency may be set for each recording material size. This will be described in Example 2.

  Next, the operation of the second converter 15 will be described. The second converter 15 is provided to control the electric power supplied to the first converter 16, and the temperature (the temperature detecting element 9 of the temperature detecting element 9) of the rotating body through which the recording material passes regardless of the recording material size. The power supplied to the first converter is controlled according to the detected temperature. Specifically, the power control unit 119 sends a signal to the drive circuit 1117 according to the temperature detected by the temperature detection element 9 to control the ON duty ratio of the switch element 1104. As a result, the power supplied to the first converter 16 (the output voltage of the second converter 15) is controlled.

  The second converter 15 includes a switch element 1104, a diode 1105, a coil 1106, a capacitor 1107, and the like, and is a step-down converter. When a voltage is applied between the gate (G) and the source (S) of the switch element 1104 and the switch element 1104 is turned on, a voltage is applied to the coil 1106. A voltage difference between the capacitor 1103 and the capacitor 1107 is applied to both ends of the coil 1106. The slope of the current flowing through the coil 1106 is determined by the inductance of the coil 1106 and the voltage applied to the coil 1106.

  The current flowing through the coil 1106 charges the capacitor 1107. When the voltage of the capacitor 1107 increases, the voltage applied to the coil 1106 decreases even when the switch element 1104 is turned on. As described above, although the current flowing through the coil 1106 varies depending on the voltage applied to the coil 1106, the current flowing through the coil 1106 increases almost linearly when the voltage rise of the capacitor 1107 is slow. This period is period 1 in FIG.

  When the switch element 1104 is turned off, a current continuously flows to the coil 1106 through the diode 1105. The capacitor 1107 is charged by the electric power stored in the coil 1106 in the form of a magnetic field. If the capacity of the capacitor 1107 is sufficiently large, the current of the coil 1106 decreases due to the substantially linear characteristic. If a current flows through the coil 1106 when the switch element 1104 is turned on, the current value becomes an initial value when the switch element 1104 is turned on. The second converter 15 operates by repeating the series of operations as described above.

  PWM control is used as a driving method of the switch element 1104. When the output power of the second converter 15 is to be increased in order to maintain the temperature of the sheet passing portion of the rotating body at the control target temperature, the PWM ON time ratio, that is, the ON duty ratio (period 1 / (period in FIG. 9) 1 + period 2)) is increased. Conversely, when it is desired to reduce the output power of the second converter 15, the ON duty ratio is reduced.

  In the PWM control, the current flowing through the coil 1106 does not become zero. FIG. 9 shows the current of the coil 1106 and the voltage of the K terminal of the diode 1105 when PWM control is performed. As described above, the switch element 1104 performs a hard switch operation in which a turn-on and turn-off operation is performed in a state where a current flows. Note that the voltage of the capacitor 1107 in FIG. 9 is the output voltage of the second converter.

  FIG. 10 is a comparison diagram when the ON duty ratio is set to 80% and 50%. As shown in FIG. 10, when the ON duty ratio changes, the voltage of the capacitor 1107 changes and the output voltage of the second converter changes. Thereby, the power supplied to the first converter 16 changes.

  Note that noise may occur depending on the ON / OFF timing of the switch element 1104. In such a case, a critical mode in which the switch element 1104 is kept off until the current flowing through the coil 1106 becomes 0 A during the OFF period of the switch element 1104 may be used.

  Since the source (S) terminal of the switch element 1104 serves as a contact point between the coil 1106 and the diode 1105, the voltage becomes equal to the negative terminal voltage of the capacitor 1103 when turned off. At turn-on, the voltage is equal to the positive terminal voltage of the capacitor 1103. Thus, since the source (S) voltage of the switch element 1104 fluctuates greatly, in order to continue to apply a voltage between the gate (G) and the source (S) of the switch element 1104, it is driven by transformer coupling or not shown. It is necessary to use a bootstrap circuit.

  The switch element 1104 is connected to the commercial AC power supply 50 without being insulated. In this embodiment, in order to apply to an apparatus that requires insulation according to safety standards, as an example, the drive circuits 1117 and 1118 are configured to ensure insulation.

  As described above, the power control unit 119 controls the ON duty ratio of the PWM control described above based on the temperature detected by the temperature detection element 9. As a control method, PI control or PID control is used. Then, the power control unit 119 drives the drive circuit 1117 to control the ON duty ratio of the switch element 1104 so that the detected temperature of the temperature detection element 9 maintains the control target temperature that is the fixable temperature. When the ON duty ratio of the switch element 1104 is changed, the voltage of the capacitor 1107 is changed, and the power supplied to the first converter 16 is changed.

  Thus, in order to keep the temperature of the sheet passing area (the detected temperature of the temperature detecting element 9) at the control target temperature, the output voltage of the second converter is controlled rather than controlling the drive frequency of the first converter 16. To do.

  FIG. 11 is a diagram comparing the amount of heat generated by the rotating body when the ON duty ratio of the switch element 1104 of the second converter 15 is set to 80% and 50%. As described above, the heat generation distribution in the generatrix direction of the rotating body is determined by the frequency setting unit 120 according to the detected temperature of the temperature detecting element 10 or 11 that detects the temperature of the non-sheet passing region of the rotating body. It is adjusted by setting the driving frequency. Then, the power control unit 119 controls the output voltage of the second converter 15 in accordance with the detected temperature of the temperature detecting element 9 that detects the temperature of the sheet passing unit, thereby controlling the temperature of the sheet passing unit to be constant. Executed. When the ON duty ratio of the second converter 15 is set to 80% and 50%, the voltage of the capacitor 1113 is different as shown in FIG. Using this voltage difference, the amount of heat generated by the rotating body is adjusted.

  As described above, in this example, the driving frequency of the first converter 16 is set according to the temperature of the non-sheet passing portion of the rotating body, and the second converter 15 is set according to the temperature of the sheet passing portion of the rotating body. Control the output voltage. As a result, the temperature of the sheet passing portion can be maintained at the control target temperature while maintaining the heat generation distribution of the rotator at a heat generation distribution corresponding to the size of the recording material.

(Example 2)
Whereas in the first embodiment, the driving frequency of the first converter is set according to the temperature of the non-sheet passing portion, the present embodiment sets the driving frequency of the first converter 16 according to the size of the recording material P. It is to set.

  As described above, as the driving frequency of the first converter 16 is lowered, the amount of heat generated at the longitudinal end of the rotating body becomes smaller than that at the center. In the present embodiment, by utilizing this property, the driving frequency of the first converter 16 is set lower as the width of the recording material P (width in the direction orthogonal to the transport direction) becomes narrower. Table 1 shows the driving frequency for each recording material size.

  In this embodiment, the frequency setting means 120 sets the drive frequency according to the size information of the recording material P designated by the user via the host computer 42.

  Note that the driving frequency used when fixing the recording material of one size may be switched alternately between the first driving frequency and the second driving frequency lower than the second frequency. FIG. 12 shows the temperature distribution in the rotation axis direction of the rotating body 1 when the ratio per unit time between the driving period at a driving frequency of 20 kHz and the driving period at 50 kHz is changed. For example, in the case of A5 size, the drive time ratio of 20 kHz: 50 kHz is set to 10: 0. In the case of B5 size, the drive time ratio of 20 kHz: 50 kHz is set to 5: 5. In the case of A4 size, the driving time ratio of 20 kHz: 50 kHz is 1: 9, and in the case of letter size, the driving time ratio of 20 kHz: 50 kHz is 0:10.

  In Examples 1 and 2 described above, a rotating body (fixing sleeve) was used as a film. However, the present invention can also be applied to a fixing device using a hard rotating body having little flexibility as a rotating body in which the core and the coil are arranged.

DESCRIPTION OF SYMBOLS 1 Rotating body 2 Magnetic core 3 Excitation coil 16 1st converter 15 2nd converter

Claims (6)

A resonant circuit comprising: a cylindrical rotating body having a conductive layer; a coil disposed inside the rotating body and having a helical axis substantially parallel to a generatrix direction of the rotating body; and a resonant capacitor;
A first converter for driving the resonant circuit;
In the fixing device for causing the conductive layer to generate electromagnetic induction heat by the magnetic flux generated by the coil and fixing the image formed on the recording material by the heat of the rotating body on the recording material,
The coil disposed inside the rotating body is one,
The conductive layer is formed in the same thickness with the same material from one end to the other end in the busbar direction,
A second converter for controlling the power supplied to the first converter;
A frequency setting unit that changes the drive frequency of the first converter according to at least one of the size of the recording material and the temperature of the non-sheet passing portion of the rotating body;
A power control unit that controls the second converter according to the temperature of the sheet passing portion of the rotating body, and that controls the power supplied from the second converter to the first converter;
The fixing device is characterized in that when the frequency setting unit changes the driving frequency, a heat generation distribution of the rotating body in the bus direction changes.   The fixing device according to claim 1, wherein the power control unit controls the second converter so that a temperature of a sheet passing portion of the rotating body maintains a target temperature.   The fixing device according to claim 1, wherein the resonance circuit is a current resonance circuit.   The fixing device according to claim 1, wherein the second converter is a step-down converter.   5. The spiral-shaped portion of the coil has a core for inducing magnetic flux, and an induced current that flows in a circumferential direction of the rotating body is generated in the conductive layer. The fixing device according to claim 1.   The fixing device according to claim 5, wherein the core has an end shape.

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