Abstract:
A fixing device includes a heating body; first and second coils for induction heating that heat the heating body by an induction heating method; a first switching element connected to the first coil for induction heating; a second switching element connected to the second coil for induction heating; a driving signal transmission unit that sends a first drive signal to the first switching element and a second drive signal to the second switching element so that the first switching element and the second switching element operate at the same timing to apply a first input voltage to the first coil for induction heating and a second input voltage to the second coil for induction heating; and a voltage drop unit that steps down the first input voltage applied to the first coil for induction heating so that the first input voltage is lower than the second input voltage.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The disclosures herein generally relate to a fixing device and an image forming apparatus, and in particular relate to a fixing device that heats a heating body by an induction heating method using plural coils for induction heating. 
     2. Description of the Related Art 
     An electrophotographic image forming apparatus in the related art has a fixing device for fixing a toner on a paper. The heating methods of the fixing device include an IH (Induction Heating) method. In a power unit for induction heating (inverter), an alternate current value applied to a coil for induction heating is controlled according to an operation frequency of a switching element mounted on the inverter (conduction time width in the switching element). 
     Moreover, in the image forming apparatus in the related art, papers having various sizes (widths) are used according to a specification. For example, Japanese Published Patent Applications No. 2006-206813 and 2001-312178 and Japanese Patent No. 4021707 describes an image forming apparatus in which plural coils for induction heating are arranged along an axis direction of a heating roller so as to fit a width of passing paper, and an electric power supplied to the coils for induction heating is controlled. 
     Moreover, in the image forming apparatus in the related art, when an electric power is supplied to each of the plural coils for induction heating arranged along a direction of a roller axis, an electric energy generated by each of the coils for induction heating is changed according to a temperature difference in a longitudinal direction of the roller. In the above operation, the plural coils for induction heating are driven in frequencies different from each other, and an interference sound may be generated. 
     In order to inhibit the interference sound as above, it is known to perform control by switching the timing of supplying the electric power to the plural coils for induction heating. However, when the timing of supplying the electric power is switched while the distribution ratio is maintained in order to inhibit the interference sound, the plural coils for induction heating heat with time division, and a time to reach the fixing temperature becomes long. 
     SUMMARY OF THE INVENTION 
     It is a general object of at least one embodiment of the present invention to provide a fixing device and an image forming apparatus that substantially obviate one or more problems caused by the limitations and disadvantages of the related art. 
     In one embodiment, a fixing device includes a heating body; a first coil for induction heating that heats the heating body by an induction heating method; a second coil for induction heating that heats the heating body by the induction heating method; a first switching element connected to the first coil for induction heating; a second switching element connected to the second coil for induction heating; a driving signal transmission unit that sends a first drive signal to the first switching element and a second drive signal to the second switching element so that the first switching element and the second switching element operate at the same timing to apply a first input voltage to the first coil for induction heating and a second input voltage to the second coil for induction heating; and a voltage drop unit that steps down the first input voltage applied to the first coil for induction heating so that the first input voltage is lower than the second input voltage. 
     In another embodiment, an image forming apparatus includes a fixing device which fixes a toner image generated based on image data on a recording medium. The fixing device includes a heating body; a first coil for induction heating that heats the heating body by an induction heating method; a second coil for induction heating that heats the heating body by the induction heating method; a first switching element connected to the first coil for induction heating; a second switching element connected to the second coil for induction heating; a driving signal transmission unit that sends a first drive signal to the first switching element and a second drive signal to the second switching element so that the first switching element and the second switching element operate at the same timing to apply a first input voltage to the first coil for induction heating and a second input voltage to the second coil for induction heating; and a voltage drop unit that steps down the first input voltage applied to the first coil for induction heating so that the first input voltage is lower than the second input voltage. 
     According to the present invention, a fixing device and an image forming apparatus that continuously supply an electric power to a heating coil without generating an interference sound are provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects and further features of embodiments will be apparent from the following detailed description when read in conjunction with the accompanying drawings, in which: 
         FIG. 1  is an explanatory diagram illustrating a fixing device according to a first embodiment; 
         FIG. 2  is an explanatory diagram illustrating a heating roller according to the first embodiment; 
         FIGS. 3A to 3C  are explanatory diagrams illustrating a control of a voltage by a voltage control circuit according to the first embodiment; 
         FIG. 4  is a diagram illustrating an example of the voltage control circuit according to the first embodiment; 
         FIG. 5  is a diagram illustrating an example of an operational waveform in the voltage control circuit according to the first embodiment; 
         FIG. 6  is an explanatory diagram illustrating a comparative example for comparison with the fixing device according to the first embodiment; 
         FIG. 7  is a diagram illustrating a result of implementation of heating experiments in the first embodiment and in the comparative example; 
         FIG. 8  is a diagram illustrating an example of a voltage control circuit according to a second embodiment; 
         FIG. 9  is a diagram illustrating an example of an operational waveform in the voltage control circuit according to the second embodiment; 
         FIG. 10  is a first explanatory diagram illustrating a fixing device according to a third embodiment; 
         FIG. 11  is a second explanatory diagram illustrating the fixing device according to the third embodiment; 
         FIG. 12  is an explanatory diagram illustrating a fixing device according to a fourth embodiment; 
         FIG. 13  is an explanatory diagram illustrating a fixing device according to a fifth embodiment; 
         FIG. 14  is a first explanatory diagram illustrating a fixing device according to a sixth embodiment; 
         FIG. 15  is a second explanatory diagram illustrating the fixing device according to the sixth embodiment; and 
         FIG. 16  is an explanatory diagram illustrating a fixing device according to a seventh embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     (First Embodiment) 
     In the following, a first embodiment of the present invention will be described with reference to the accompanying drawings.  FIG. 1  is an explanatory diagram illustrating a fixing device according to the first embodiment. 
     The fixing device  10  according to the present embodiment includes a power source  100 , an input AC (Alternating Current) power detection circuit  102 , a rectifier circuit  103  and a CPU (Central Processing unit)  104 . Moreover, the fixing device  10  according to the present embodiment further includes voltage control circuits  110 ,  120 , coil drive units  210 ,  220  and  230  and heating coils  213 ,  223  and  233 . 
     The CPU  104  of the fixing device  10  according to the present embodiment, upon receiving an instruction for fixing from an external control CPU  150  via an external communication IF (interface)  140 , makes the coils for induction heating  213 ,  223  and  233  heat by the coil drive unit  210 ,  220  and  230 , to heat the heating roller  300 . In the present embodiment, the external communication IF (Interface)  140  may be provided in the fixing device  10  or may be provided outside the fixing device  10 . Generally, the external communication IF is insulated by a photo coupler or the like in order to prevent a breakage to an internal electronic circuit. Moreover, the external control CPU  150  according to the present embodiment is a main control unit in the image forming apparatus which mounts the fixing device  10 , for example. Moreover, the heating roller  300  according to the present embodiment may be included in the fixing device  10  or may be arranged outside the fixing device  10 . 
     The voltage control circuits  110 ,  120  according to the present embodiment supplies voltage to the coil drive units  210 ,  230 , to drive the heating coils  213 ,  233 . Moreover, in the coil drive unit according to the present embodiment a drive signal to drive the heating coil  223  is supplied from the CPU  104 . 
     The coil drive unit  210  according to the present embodiment is a resonance circuit including a resonance capacitor  211  and a switching element  212 . The coil drive unit  220  according to the present embodiment is a resonance circuit including a resonance capacitor  221  and a switching element  222 . The coil drive unit  230  according to the present embodiment is a resonance circuit including a resonance capacitor  231  and a switching element  232 . 
     In the coil drive unit  210 ,  220  and  230  according to the present embodiment, the resonance capacitors are connected to the heating coils  213 ,  223  and  233  in parallel to form resonance circuits, respectively. Moreover, the switching elements  212 ,  222  and  232  are connected to the heating coils  213 ,  223  and  223  in series, to control the drive of the resonance circuits, respectively. 
     The switching elements  212 ,  222  and  232  according to the present embodiment are for example, power MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors), IGBTs (Insulated Gate Bipolar Transistors), or the like. A signal from the CPU  104  is applied to a gate of each of the switching elements. The ON/OFF of each of the switching elements  212 ,  222  and  232  according to the present embodiment is controlled by an instruction from the CPU  104 . 
     In the fixing device  10  according to the Present embodiment, a voltage V rect  which is obtained by rectifying a voltage output from the power source voltage  100  at a rectifier circuit  103  is supplied to the voltage control circuits  110 ,  120  and the coil drive unit  220 . Moreover, in the fixing device  10  according to the present embodiment, a control signal V cont  output from the CPU  104  is supplied to the voltage control circuits  110  and  120 . 
     Next, with reference to  FIG. 2 , the heating roller  300  according to the present invention will be described.  FIG. 2  is an explanatory diagram illustrating the heating roller according to the first embodiment. 
       FIG. 2  schematically shows the heating coils  213 ,  223  and  233  in the longitudinal direction of the fixing device  10  and the heating roller  300 . 
     In the present embodiment, the heating coils  213 ,  223  and  233  are divided in the longitudinal direction of the heating roller  300  according to the paper width are provided. 
     In the present embodiment, when the width of the heating coil  213  in the longitudinal direction is W1, the width of the heating coil  223  in the longitudinal direction is W2 and the width of the heating coil  233  in the longitudinal direction is W3, the respective heating coils are provided so that W1 is equal to W3 and less than W2. 
     That is, in the present invention, the heating coil  223  that heats a central part of the heating roller  300  is provided, and the heating coils  213  and  233  having smaller width than the heating coil  223  are provided on both ends of the heating coil  223 . 
     In the fixing device  10  according to the present embodiment, by driving the coil drive units  210 ,  220  and  230  that drive these three heating coils  213 ,  223  and  233  with the same frequency, an electric power is supplied continuously to the heating coils  213 ,  223  and  233  without generating interference sound. 
     Moreover, in the fixing device  10  according to the present embodiment, the voltage supplied to the heating coils  213  and  233  and the voltage supplied to the heating coil  223  are adjusted. Specifically, the region where the heating coil  223  heats on the heating roller  300  is larger than the regions where the heating coils  213  and  233  heat. Accordingly, the electric power supplied to the heating coil  223  is required to be greater than the electric powers supplied to the heating rollers  213 ,  233 . 
     The coil drive units  210 ,  220  and  230  according to the present embodiment operate by the drive signals with the same frequency. That is, this shows that time when the voltages are supplied to the respective coil drive units  210 ,  220  and  230  are the same. Accordingly, in the present embodiment, the voltage control circuits  110 ,  120  control so that the voltage V out  applied to the heating coils  213 ,  233  is less than the voltage V rect  applied to the heating coil  223 . 
     That is, in the present embodiment, the plural heating coils  213 ,  223  and  233  with the same frequency and voltages applied to the plural heating coils  213 ,  223  and  233  are controlled so that the electric powers provided to each of the plural heating coils are the target values set in advance. 
     Specifically, the coil drive unit  220  according to the present embodiment is driven by a PWM (pulse-width modulation) signal corresponding to the electric power (target value) set in the CPU  104 . Moreover, in the present embodiment, when the control signal V cont  output from the CPU  104  is provided to the voltage control circuits  110 ,  120 , the voltage control circuits  110 ,  120  output a voltage corresponding to the electric power which is set according to the control signal V cont . The voltage output from the voltage control circuits  110 ,  120  is supplied to the coil drive units  210 ,  230  and applied to the heating coils  213 ,  233 . 
     The operation of the CPU  104  included in the fixing device  10  according to the present embodiment will be explained in the following. 
     The CPU  104  according to the present embodiment detects the electric power supplied from the power source voltage  100  by the AC input power detection circuit  102 . Next, the CPU  104  refers to a target value of the electric power, which is set from a CPU  150  outside the fixing device  10 , and outputs a PWM signal that makes the switching elements  212 ,  222  and  232  of the respective coil drive units turn on/off at the same timing. 
     In the present embodiment, according to the control as described above, the drive frequencies of the switching elements  212 ,  222  and  232  are matched, and the interference sound between the heating coils  213 ,  223  and  233  can be suppressed. 
     In the following, with reference to  FIGS. 3A to 3C , a control of a voltage of the voltage control circuit  110  according to the present embodiment will be explained.  FIGS. 3A to 3C  are explanatory diagrams illustrating the control of the voltage by the voltage control circuit according to the first embodiment.  FIGS. 3A to 3C  show, for example, the case where the voltage control circuit  110  controls a voltage applied to the heating coil  213 . 
     In the coil drive unit  210 , when the PWM signal supplied to the switching element  212  is in an ON state (H level), a coil current I coil  flows in the heating coil  213 . In this stage, a path between the collector and the emitter of the switching element  212  is in a conducting state, and a voltage between the collector and the emitter V ce  is 0 V as shown in  FIG. 3A . Next, when the PWN signal becomes OFF state (L level), the coil current I coil  does not flow into GND, the resonance capacitor  211  is charged and the voltage between the collector and the emitter V ce  of the switching element  212  increases. 
     Moreover, since the electric charges charged in the resonance capacitor  211  are discharged, the coil current I coil  which is an opposite direction to the heating coil  213  flows, and the coil current I coil  goes from zero to negative. Then, a diode embedded in the switching element  212  conducts and the voltage between the collector and the emitter becomes nearly 0 V. In the coil drive unit  210 , during the period when the embedded diode conducts, by changing the PWM signal to the ON state again, it becomes possible to make the switching element  212  work with low loss. By repeating the switching operation using the resonance operation, it becomes possible to make a current with high frequency flow in the heating coil  213 . 
       FIG. 3B  is a diagram showing that by making the voltage V 2  applied to the heating coil  213  less than the voltage V 1  in  FIG. 3A , the coil current in the heating coil  213  I coil  decreases while the switching operation is repeated in a state where the ON width of the PWM signal is the same. In the present embodiment, by lowering the voltage V 1 , the coil current becomes smaller. 
       FIG. 3C  is a diagram showing that by making the voltage V 3  applied to the heating coil  213  greater than the voltage V 1  in  FIG. 3A , the coil current in the heating coil  213  I coil  increases while the switching operation is repeated in a state where the ON width of the PWM signal is the same. 
     In this way, in the present embodiment, by controlling the voltage applied to the heating coil  213 , the set electric power can be supplied without changing the ON width of the PWM signal supplied to the switching element  212  of the coil drive unit  210 . 
     That is, in the present embodiment, while controlling the respective coil drive units with the PWM signal having the same frequency, the voltage applied to the heating coils  213 ,  233  can be controlled. Accordingly, in the present embodiment in the case of inputting a large electric power, for example, while making the ON width of the PWM signal the same, the voltage applied to the heating coils  213 ,  233  can be made lower than the voltage applied to the heating coil  222 . 
     Next, with reference to  FIGS. 4 and 5 , the voltage control circuits  110 ,  120  will be explained. Meanwhile, in the present embodiment, since the configurations of the voltage control circuits  110 ,  120  are almost the same, in the following explanation, the configuration of the voltage control circuit  110  will be explained as an example.  FIG. 4  is a diagram illustrating an example of the voltage control circuit according to the first embodiment. 
     In the voltage control circuit  110  as shown in  FIG. 4 , a flyback type AC (alternate current)/DC (Direct Current) conversion circuit is used. 
     The voltage control circuit  110  according to the present embodiment includes a power transformer  111 , the switching element  112 , the CPU  113 , the diode D 1 , the capacitor C 1 , and resistors R 1  and R 2 . 
     To the voltage control circuit  110 , the voltage V rect  obtained by rectifying an AC voltage at the rectifier circuit  103  is supplied. 
     An input terminal Tin of the voltage control circuit  110  is connected with the switching element  112  via a primary winding of the power transformer  111 . The ON/OFF of the switching element  112  is controlled by the drive signal, which will be denoted V ctrl  signal in the following, from the CPU  113 . 
     A secondary winding of the power transformer  111  is connected with an output terminal Tout of the voltage control unit  110  via the diode D 1 . An output voltage V out  output from the output terminal Tout is supplied to the coil drive unit  210 . One end of the capacitor C 1  is connected between the diode D 1  and the output terminal Tout. The other end of the capacitor C 1  is connected to ground. Moreover, to the output terminal Tout, a voltage-dividing circuit  114  is connected which divides the output voltage V out . The voltage-dividing circuit  114  is a circuit in which the resister R 1  and the resister R 2  are connected in series. The CPU  113  is connected to the connection point at which the resister R 1  and the resister R 2  are connected. 
     To the CPU  113  according to the present embodiment, the control signal V cont  is supplied from the CPU  104 . In the CPU  113 , a target value of electric power supplied to the coil drive unit  210  is set by the control signal V cont . The CPU outputs the V ctrl  signal that controls the ON/OFF of the switching element  112 , so that the output voltage V out  from the voltage control circuit  110  is a voltage corresponding to the target value, based on the target value of the electric power and a voltage V fb  at the connection point of the resister R 1  and the resister R 2 . 
       FIG. 5  is a diagram illustrating an example of an operational waveform in the voltage control circuit according to the first embodiment. 
     In the voltage control circuit  110  according to the present embodiment, when the V ctrl  signal is changed to the ON state (high level), an electric current I ds  having a triangular waveform flows in the primary winding. When the V ctrl  signal is changed to the OFF state (low level) from the ON state, the current Ids in the primary winding becomes zero, a current I out  flows in the diode D 1  on a secondary side of the power transformer  111 . A step-up/step-down of the output voltage V out  becomes possible according the ON width of the V ctrl  signal to the switching element  112 . 
     That is, the CPU  113  according to the present embodiment steps up the output voltage V out  by broadening the ON width of the V ctrl  signal, and steps down the output voltage V out  by narrowing the ON width. 
     As described above, in the voltage control circuit  110  according to the present embodiment, based on the divided voltage V fb  of the output voltage V out  and the target value for the electric power supplied to the heating coil  213 , the output value V out  is assumed to be a voltage corresponding to the target value. Accordingly, the coil drive unit  210  according to the present embodiment, based on the output voltage V out , can make the electric power supplied to the heating coil  213  as the target value. That is, in the present embodiment, the electric power supplied to the heating coil can be controlled in accordance with the size of the heating coil. 
     Moreover, in the present embodiment, the controls of the ON/OFF for the switching elements  212 ,  222  and  232  of the coil drive units  210 ,  220  and  230  are performed at the same timing. 
     Accordingly, in the present embodiment, the electric power can be continuously supplied to heating coils without generating an interference sound. 
     Meanwhile, in the present embodiment, the explanation is provided for the case where all the heating coils  213 ,  223  and  233  are driven, but the present invention is not limited to this. In the present embodiment, for example, according to the width of the recording medium which is a fixing object, the drive of the heating coils  213  and  233  may be halted. The CPU  104  according to the present embodiment may drive only the heating coil  223 , for example, in case where the width of the recording medium is less than a coil width of the heating coil  223  or the like. In the present embodiment, for example, the external control CPU  150  may detect the width of the recording medium based on image data and send the width to the CPU  104 . 
     In the following, with reference to  FIGS. 6 and 7 , effects in the present embodiment will be explained. 
       FIG. 6  is an explanatory diagram illustrating a comparative example for comparison with the fixing device according to the first embodiment. In a fixing device  1  shown in  FIG. 6 , in the preceding stages of all the coil drive unit  210 ,  220  and  230 , the voltage control circuits  110 ,  130  and  120  are inserted. 
       FIG. 7  is a diagram illustrating a result of implementation of heating experiments in the first embodiment and in the comparative example.  FIG. 7  shows a result of comparison of a temperature rise performance of the heating coil  223  in the fixing device  10  according to the first embodiment and a temperature rise performance of a heating coil  223  in the fixing device of the comparative example. 
     In the fixing device  10  according to the first embodiment, it is found that the heating coil  223  can be heated earlier. 
     Meanwhile, in the heating experiments, the model of the image forming apparatus is “Imagio MP C5000”. The incident power is 1000 W. The size of the heating coil  223  corresponds to the length of an A4-size paper in the longitudinal direction. The position of measurement is in a central portion of the coil in front of a nip. 
     Usually, in the case of using a voltage control circuit in a fixing device, since a loss of electric power occurs in the AC/DC voltage conversion, the heating roller  300  is heated with an electric power less than the target value of the set electric Power, and the fixing performance degrades. 
     Moreover, the larger the size of the heating coil, the more prominently the loss of electric power appears. For this reason, the voltage control circuit is not adequate to a case of, for example, providing a large amount of power to the coil drive unit. In the present embodiment, a voltage control circuit is not included in the preceding stage of the coil drive unit  220  that drives the heating coil  223  requiring a large amount of power. 
     Accordingly, in the present embodiment, the loss of electric power generated by the voltage control circuit  130  provided in the preceding stage of the heating coil  223  is removed, and the electric power supplied to the coil drive unit  220  can be brought closer to the target value. Consequently, as shown in  FIG. 7 , it can be heated in a shorter time than in the fixing device  1  having the voltage control circuit  130 . 
     (Second Embodiment) 
     A second embodiment of the present invention will be explained with reference to the drawings in the following. In the second embodiment of the present invention, a forward type voltage control circuit is used, which is different from the first embodiment. Accordingly, in the following explanation in the second embodiment, only a difference from the first embodiment will be explained. To the member having the same functional configuration as in the first embodiment, the same reference numeral is assigned as the reference numeral used in the explanation in the first embodiment, and the explanation thereof will be omitted. 
       FIG. 8  is a diagram illustrating an example of a voltage control circuit according to the second embodiment. The voltage control circuit  110 A according to the present embodiment includes a power transformer  111 A, a switching element  112 , a CPU  113 , diodes D 1  and D 2 , a capacitor C 1 , a coil L 1 , and resistors R 1  and R 2 . 
     A secondary winding of the power transformer  111 A is connected with an output terminal Tout via the diode D 1  and the coil L 1 . The diode D 2  is connected between a connection point of the diode D 1  and the coil L 1  and the ground. 
       FIG. 9  is a diagram illustrating an example of an operational waveform in the voltage control circuit according to the second embodiment. 
     In the voltage control circuit  110 A according to the present embodiment, when the Vctrl signal is changed to the ON state (high level), an electric current ids having a triangular waveform flows in the primary winding. Moreover, an electric current I out  flows in the diodes D 1  and D 2  on a secondary side of the power transformer  111 , and charged in the coil L 1 . When the V ctrl  signal is changed to the OFF state (low level) from the ON state in the voltage control circuit  110 A, the current Ids in the primary winding becomes zero, the electric current I out  charged in the coil L 1  flows. A step-up/step-down of the output voltage V out  becomes possible according to the ON width of the V ctrl  signal to the switching element  112 . 
     (Third Embodiment) 
     A third embodiment of the present invention will be explained with reference to the drawings in the following. In the third embodiment of the present invention, plural heating coils arranged at both ends of the heating roller  300  are connected to the same voltage control circuit, which is different from the first embodiment. Accordingly, in the following explanation in the third embodiment, only a difference from the first embodiment will be explained. To the member having the same functional configuration as in the first embodiment, the same reference numeral is assigned as the reference numeral used in the explanation in the first embodiment, and the explanation thereof will be omitted. 
       FIG. 10  is a first explanatory diagram illustrating a fixing device according to the third embodiment. 
     In the fixing device  10 A according to the present embodiment, the heating coils  213  and  233 , which are connected in series, are connected to the coil drive unit  210 . 
       FIG. 11  is a second explanatory diagram illustrating the fixing device according to the third embodiment. 
     In the fixing device  10 B according to the present embodiment, the heating coils  213  and  233 , which are connected in parallel, are connected to the coil drive unit  210 . 
     The coil drive unit  210  shown in  FIG. 10  and  FIG. 11 , to which the output voltage V out  is supplied from the voltage control circuit  110 , drives the heating coils  213  and  233 . 
     As described above, by using the same voltage control circuit  110  and the coil drive unit  210  for driving the two heating coils  213  and  233 , the electric power supplied to the heating coil  213  can be the same as the electric power supplied to the heating coil  233 . That is, in the present embodiment, positions on the heating roller  300  which correspond to the heating coils  213  and  233  can be heated to the same temperature. 
     Moreover, in the present embodiment, the voltage control circuit  120  and the coil drive unit  230  corresponding to the heating coil  233  are not necessary. Accordingly, in the fixing device  10 A according to the present embodiment, the circuit size can be reduced. 
     (Fourth Embodiment) 
     A fourth embodiment of the present invention will be explained with reference to the drawings in the following. In the fourth embodiment of the present invention, plural heating coils are connected to the same voltage control circuit, which is different from the first embodiment. Accordingly, in the following explanation in the fourth embodiment, only a difference from the first embodiment will be explained. To the member having the same functional configuration as in the first embodiment, the same reference numeral is assigned as the reference numeral used in the explanation in the first embodiment, and the explanation thereof will be omitted. 
       FIG. 12  is an explanatory diagram illustrating a fixing device according to the fourth embodiment. 
     In the fixing device  10 C according to the present embodiment, the heating coil  213  and the heating coil  214  are connected in series to the coil drive unit  210 . Moreover, in the fixing device  100 , the heating coil  233  and the heating coil  234  are connected in series to the coil drive unit  230 . 
     In the fixing device  10 C according to the present embodiment, in the case where, for example, the paper width is comparable with the width of the heating roller  300  in the longitudinal direction, all the heating coils are driven. Moreover, when the paper width is shorter than the width of the heating roller  300  in the longitudinal direction, for example, the fixing device  100  according to the present embodiment may drive only the heating coils  223 ,  233  and  234 . 
     Moreover, regarding the heating coil according to the present embodiment, for example, a heating coil may be provided corresponding to the heating region of the heating roller  300 . 
     (Fifth Embodiment) 
     A fifth embodiment of the present invention will be explained with reference to the drawings in the following. In the fifth embodiment of the present invention, the coil drive unit is an electric current resonance circuit, which is different from the first embodiment. Accordingly, in the following explanation in the fifth embodiment, only a difference from the first embodiment will be explained. To the member having the same functional configuration as in the first embodiment, the same reference numeral is assigned as the reference numeral used in the explanation in the first embodiment, and the explanation thereof will be omitted. 
       FIG. 13  is an explanatory diagram illustrating a fixing device according to the fifth embodiment. The fixing device  10 D according to the Present embodiment includes coil drive units  210 A,  220 A and  230 A. 
     Since configurations of the coil drive units  210 A,  220 A and  230 A are the same, in the following explanation, the coil drive unit  210 A will be explained as an example. 
     The coil drive unit  210 A according to the present embodiment includes switching elements  211 A and  212 A. To gates of the switching elements  211 A and  212 A, a PWM signal is supplied from the CPU  104 . 
     In the coil drive unit  210 A, when the PWM signal supplied to the switching element  211 A is changed to the ON state (H level), an electric current Icoil flows in the heating coil  213 , and electric charges are charged in the resonance capacitor C 10 . Next, when the PWM signal supplied to the gate of the switching element  211 A changes to the OFF state (L level) and a PWM signal supplied to the gate of the switching element  212 A changes to the ON state (H level), an electric current of an opposite direction I coil  flows in the heating coil  213 . 
     In the present embodiment, by switching the PWM signals supplied to the switching element  211 A and to the gate of the switching element  212 A between ON and OFF alternately, an electric current with high frequency flows in the heating coil  213 . The switching elements  221 A and  222 A included in the coil drive unit  220 A and the switching elements  231 A included in the coil drive unit  230 A are also driven by the same operation as the coil drive unit  210 A. In the present embodiment, the same effect as in the first embodiment can be obtained. 
     (Sixth Embodiment) 
     A sixth embodiment of the present invention will be explained with reference to the drawings in the following. In the sixth embodiment of the present invention, the coil drive unit in the third embodiment is an electric current resonance circuit. Accordingly, in the following explanation in the sixth embodiment, only a difference from the third embodiment will be explained. To the member having the same functional configuration as in the third embodiment, the same reference numeral is assigned as the reference numeral used in the explanation in the third embodiment, and the explanation thereof will be omitted. 
       FIG. 14  is a first explanatory diagram illustrating a fixing device according to the sixth embodiment. 
     In the fixing device  10 E according to the present embodiment, the heating coil  213  and the heating coil  233  which are connected in series are connected to the coil drive unit  210 A, and a resonance capacitor C 10  is connected between the heating coil  233  and the ground. Moreover, the heating coil  223  is connected to the coil drive unit  220 A according to the present embodiment, and a resonance capacitor C 20  is connected between the heating coil  223  and the ground. 
       FIG. 15  is a second explanatory diagram illustrating the fixing device according to the sixth embodiment. 
     In the fixing device  10 F according to the present embodiment, the heating coil  213  and the heating coil  233  which are connected in parallel are connected to the coil drive unit  210 A, and the resonance capacitor C 10  is connected between the connection point at which the heating coil  213  and the heating coil  233  are connected and the ground. Moreover, the heating coil  223  is connected to the coil drive unit  220 A according to the present embodiment, and the resonance capacitor C 20  is connected between the heating coil  223 , and the resonance capacitor C 20  is connected between the heating coil  223  and the ground. 
     In the present embodiment, the same effect as in the third embodiment can be obtained. 
     (Seventh Embodiment) 
     A seventh embodiment of the present invention will be explained with reference to the drawings in the following. In the seventh embodiment of the present invention, the coil drive unit in the fourth embodiment is an electric current resonance circuit. Accordingly, in the following explanation in the seventh embodiment, only a difference from the fourth embodiment will be explained. To the member having the same functional configuration as in the fourth embodiment, the same reference numeral is assigned as the reference numeral used in the explanation in the fourth embodiment, and the explanation thereof will be omitted. 
       FIG. 16  is an explanatory diagram illustrating a fixing device according to the seventh embodiment. 
     In the fixing device  10 G according to the present embodiment, the heating coil  213  and the heating coil  214  are connected in series to the coil drive unit  210 A, and a resonance capacitor C 10  is connected between the heating coil  214  and the ground. Moreover, in the fixing device  10 G, the heating coil  233  and the heating coil  234  are connected in series to the coil drive unit  230 A, and a resonance capacitor C 30  is connected between the heating coil  234  and the ground. 
     In the present embodiment, the same effect as in the fourth embodiment can be obtained. 
     Further, the present invention is not limited to these embodiments, but various variations and modifications may be made without departing from the scope of the present invention. 
     The present application is based on and claims the benefit of priority of Japanese Priority Application No. 2013-140726 filed on Jul. 4, 2013, the entire contents of which are hereby incorporated by reference.