Patent Publication Number: US-9411279-B2

Title: Image forming and fixing apparatuses having fixing and pressing rotating member and rectification element

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a technique to suppress an offset of toner in a fixing unit that is mounted to an image forming apparatus. 
     2. Description of the Related Art 
     A fixing unit in an image forming apparatus applies heat to a toner image and a sheet in order to fix the toner image that is transferred to the sheet. Although ceramic heaters and halogen heaters were mainly used as a heat source, an electromagnetic induction heating method has come to be utilized in recent years. The electromagnetic induction heating method is a method to let the fixing roller generate heat by generating an eddy current in a fixing roller with an electromagnetic induction coil. 
     Incidentally, when a sheet passes the nip of the fixing unit, the surface of the fixing roller is charged by friction of the sheet with the fixing roller and a pressing roller. Meanwhile, the toner on the sheet that arrives at the fixing unit is charged by an image forming process. When the polarity of the surface of the fixing roller and the polarity of the toner are opposite from each other, or the polarity of the surface of the pressing roller and the polarity of the toner are the same, a so-called offset occurs, which is a phenomenon that the toner on the sheet adheres to the fixing roller. 
     In Japanese Patent No. 4040348, a bias circuit is proposed, which is configured to prevent the offset in a fixing unit in the electromagnetic induction heating method. This bias circuit collects electric charge by a collector member contacting a magnetic core, stores the collected electric charge in an external rectification circuit and a charging capacitor, and applies the stored electric charge to an electrically conductive layer of the fixing roller. 
     The bias circuit described in the Japanese Patent No. 4040348 has the advantage that it is capable to prevent the offset without providing a high voltage power supply for applying bias. However, in this bias circuit, since a collector member and a charging capacitor are required, a new problem has arisen, namely that it tends to increase the size and cost of the fixing unit. 
     SUMMARY OF THE INVENTION 
     Thus, the feature of the present invention is to suppress a fixing offset without adding a collector member or a charging capacitor. 
     The present invention provides a fixing apparatus comprising the following elements. A fixing roller, which includes an electrically conductive layer and a magnetic field generation unit that generates an eddy current in the electrically conductive layer by generating a magnetic field, generates heat by the eddy current flowing in the electrically conductive layer. An electric power supply unit causes the magnetic field generation unit of the fixing roller to generate the magnetic field by supplying electric power to the magnetic field generation unit. A pressing roller, which is arranged opposite to the fixing roller, and, together with the fixing roller, forms a pressing part that presses an un-fixed toner image to a sheet. A first rectification element, which is connected between the electrically conductive layer of the fixing roller and ground, and causes the surface of the fixing roller that contacts the un-fixed toner image to hold electric charge with the same polarity as the charged polarity of the un-fixed toner image. A second rectification element, which is connected between the surface of the pressing roller that does not contact the un-fixed toner image and ground, causes the surface of the pressing roller to hold electric charge with the reverse polarity of the charged polarity of the un-fixed toner image. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating a general configuration of an image forming apparatus according to an embodiment of the present invention. 
         FIG. 2  is a diagram illustrating a fixing unit according to a first embodiment of the present invention. 
         FIG. 3A  is a diagram illustrating capacitance elements of a first embodiment of the present invention. 
         FIG. 3B  is a diagram illustrating an equivalent circuit according to a first embodiment of the present invention. 
         FIG. 4  is a block diagram of a main circuit according to a first embodiment of the present invention. 
         FIG. 5  is a flowchart illustrating an image forming operation according to an embodiment of the present invention. 
         FIG. 6  is a flowchart of temperature control according to an embodiment of the present invention. 
         FIG. 7  is a diagram illustrating potential waveforms of an electrically conductive layer of a fixing roller and the surface of a pressing roller according to the present invention. 
         FIG. 8  is a diagram illustrating a fixing unit according to a second embodiment of the present invention. 
         FIG. 9A  is a diagram illustrating capacitance elements of a second embodiment of the present invention. 
         FIG. 9B  is a diagram illustrating an equivalent circuit according to a second embodiment of the present invention. 
         FIG. 10  is a block diagram of a main circuit according to a second embodiment of the present invention. 
         FIG. 11  is a flowchart illustrating fixing potential control according to the present invention. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Embodiment 1 
     An image forming apparatus including a fixing unit will be described, with reference to  FIG. 1 . The image forming apparatus  900  is an image forming apparatus that uses an electrophotographic method. A photoreceptor drum  901  is an image carrier that carries an electrostatic latent image and a toner image. A primary charging roller  902  charges the surface of the photoreceptor drum  901  uniformly. A laser unit  903  irradiates the surface of the photoreceptor drum  901  with a laser beam, which is modulated by density information of the image,  1  to form the electrostatic latent image thereon. The laser unit  903  may be referred as an exposure apparatus or a light scanning apparatus. A developing sleeve  904  is a main unit of the developing unit, which forms a toner image by developing the electrostatic latent image formed on the surface of the photoreceptor drum  901  with toner. The toner is, for example, negatively charged. An intermediate transfer belt  906  is sandwiched between the photoreceptor drum  901  and a primary transfer roller  905 . The toner that is carried on the surface of the photoreceptor drum  901  is primary-transferred to the surface of the intermediate transfer belt  906  by applying a primary transfer bias to the primary transfer roller  905 . Accordingly, a toner image that is formed by negatively charged toner is formed on the intermediate transfer belt  906 . The intermediate transfer belt  906  passes a nip unit that is formed by a secondary transfer internal roller  907  and a secondary transfer external roller  908 . A cassette  910  accommodates a plurality of sheets  913 . When a sheet  913  fed from the cassette  910  passes the nip unit, the toner image on the intermediate transfer belt  906  is secondary-transferred to the surface of the sheet  913 . The sheet  913  that carries an un-fixed toner image is conveyed to a fixing unit  911 , and is subjected to heat and pressure there so that the toner image is fixed on the surface of the sheet  913 . Thus, components that relate to the formation of the toner image on the sheet, such as the photoreceptor drum  901 , the intermediate transfer belt  906 , and the like, constitute an image forming unit. 
     A fixing unit  911  that uses an electromagnetic induction heating method will be described, with reference to  FIG. 2 . A fixing roller  92  includes a conductive heating element  921  with a thickness of, for example, lmm, and a non-conductive tube  922  that is stacked to cover the surface of the heating element  921 . The conductive heating element  921  functions as an electrically conductive layer. Inside the fixing roller  92 , an induction heating coil  91  is arranged in proximity to the conductive heating element  921 . The induction heating coil  91  generates a magnetic field by the application of a high-frequency AC current. An eddy current is generated in the conductive heating element  921  of the fixing roller  92  by this magnetic field, and as a result the conductive heating element  921  generates heat due to the eddy current. The induction heating coil  91  functions as a magnetic field generation unit that generates an eddy current in the conductive heating element  921  by generating a magnetic field (magnetic flux). A ferrite core  94  improves the heat generation efficiency by focusing the magnetic flux generated by the induction heating coil  91  on the conductive heating element  921 . A thermistor  95  is a temperature detecting element, which is in contact with the fixing roller  92  and measures the temperature of the fixing roller  92 . Thus, the fixing roller  92  includes an electrically conductive layer and a magnetic field generation unit that generates an eddy current in the electrically conductive layer by generating the magnetic field, and generates heat by the eddy current that flows therein. 
     A pressing roller  93  is arranged opposite to the fixing roller  92 , and is a roller that together with the fixing roller  92  forms a pressing part, which presses an un-fixed toner image to the sheet  913 . The surface of the pressing roller  93  is covered by a conductive tube  931 . Note that the fixing roller  92  rotates, following the pressing roller  93 . Note further that it is assumed here that an un-fixed toner image is formed on a first face of the sheet  913 , and either no un-fixed toner image is formed or a toner image that is already fixed is formed on a second face of the sheet  913 . Thus, of the two surfaces of the sheet  913 , the face on which an un-fixed toner image is formed contacts the fixing roller  92 , and the face on which no un-fixed toner image is formed contacts the pressing roller  93 . 
     The fixing temperature of the fixing unit  911  is changed, depending on the thickness, material, and the length of the sheet  913 . For example, the target temperature To of the fixing temperature may be 180° C. The electrical power that is applied to the induction heating coil  91  is increased or decreased so that the fixing temperature detected by the thermistor  95  is kept at the target temperature To. 
     One end of a feeding brush  96  is in electrical contact with the conductive heating element  921  of the fixing roller  92 . In other words, the conductive heating element  921  and the feeding brush  96  are electrically connected. The other end of the feeding brush  96  is connected to the anode of a first diode Dl, which is an example of a first rectification element. The cathode of the first diode Dl is connected to ground (grounding potential), such as a frame of the image forming apparatus  900 , or the like. The reason why the first diode Dl is connected in this direction is that electric charge that has the same polarity as the charged polarity of the un-fixed toner image is to be held at that position of the surface of the fixing roller  92  that contacts the un-fixed toner image. In other words, the toner that constitutes the un-fixed toner image is kept from adhering to the surface of the fixing roller  92  by exercising the coulomb force (repulsive force) between the un-fixed toner image and the fixing roller  92 . 
     One end of a feeding brush  97  is in electrical contact with the conductive tube  931  that covers the surface of the pressing roller  93 . The other end of the feeding brush  97  is connected to the cathode of a second diode D 2 , which is an example of a second rectification element. The anode of the second diode D 2  is connected to ground such as the frame, or the like. The reason why the second diode D 2  is connected in this direction is that electric charge that has the polarity opposite to the charged polarity of the un-fixed toner image is to be held at that position of the surface of the pressing roller  93  that does not contact the un-fixed toner image. In other words, the toner is held on the sheet  913 , and the toner that constitutes the un-fixed toner image is kept from adhering to the surface of the fixing roller  92  by exercising the coulomb force (attractive force) between the un-fixed toner image and the pressing roller  93 . 
       FIG. 3A  illustrates capacitance elements formed in the fixing unit  911 . C 1  is a stray capacitance generated due to the induction heating coil  91  being in proximity to the heating element  921 . C 2  is a capacitance element formed between the conductive heating element  921  and the conductive tube  931 . The non-conductive tube  922 , which covers the surface of the fixing roller  92 , functions as a dielectric material. 
       FIG. 3B  illustrates an equivalent circuit of the fixing unit  911 . V 92  is the potential of the conductive heating element  921  of the fixing roller  92 . Note that V 92  is the voltage between both ends of the first diode D 1 . V 93  represents the potential of the conductive tube  931  on the surface of the pressing roller  93 . V 93  is also the voltage between both ends of the second diode D 2 . 
     A method for applying electric power to the fixing unit  911  in the image forming apparatus  900  according to the first embodiment of the present invention will be described, with reference to the block diagram in  FIG. 4 . A power-supply apparatus  100 , which is connected to a commercial power supply  500 , is an apparatus that generates AC current for the induction heating coil  91  by converting the AC current from the commercial power supply  500 , and applies the AC current to the induction heating coil  91 . In other words, the power-supply apparatus  100  functions as an electric power supply unit that causes the induction heating coil  91  of the fixing roller  92  to generate a magnetic field by supplying electric power to the induction heating coil  91 . The AC current from the commercial power supply  500  is rectified by a diode bridge  101 , and is smoothed by a filter capacitor  102 . Further, the DC current that is output from the filter capacitor  102  is again converted to an AC current by a resonant circuit. The resonant circuit is formed with resonant capacitors  105  and  106 , and the induction heating coil  91 . A drive circuit  112  outputs drive signals  121  and  122 , and drives a first and a second switch element  103  and  104 . A control circuit  113  is connected to a current detection circuit  110  that detects an input current from the commercial power supply  500 , a voltage detection circuit  111  that detects an input voltage from the commercial power supply  500 , and a temperature detection circuit  114  that detects the fixing temperature using the thermistor  95 . An upper limit electric power Pmax is the maximum electric power on design, which is applicable to the induction heating coil  91 . The upper limit electric power Pmax and the target temperature To are set to the control circuit  113  by a CPU  1 . The control circuit  113  determines the pulse width of the drive signals  121  and  122  that are output from the drive circuit  112  so that the detection result of the temperature detection circuit  114  is equal to the target temperature To, and so that the current electric power P does not exceed the upper limit electric power Pmax, which is calculated by the detection results of the current detection circuit  110  and the voltage detection circuit  111 . The switch elements  103  and  104  alternately turn on and off according to the drive signals  121  and  122 , and supply high frequency current to the induction heating coil  91 . 
     Image Forming Process 
     With reference to  FIG. 5 , a basic image forming process will be described. In S 1001 , the CPU  1  controls a motor that drives the fixing roller  92 , and starts rotation of the fixing roller  92 . In S 1002 , the CPU  1  controls the power-supply apparatus  100 , and starts adjusting the fixing temperature (fixing temperature control). For example, the CPU  1  sets control parameters such as the target temperature To of the control circuit  113 . In S 1003 , the CPU  1  compares the detection value T of the fixing temperature with the target temperature To (for example: 180° C.), and waits for the detection value T to reach the target temperature To. When the detection value T reaches the target temperature To, the process advances to S 1004 . In S 1004 , the CPU  1  controls a motor, and starts rotation of the photoreceptor drum  901 , the conveyance roller, and the like. In S  1005 , the CPU  1  controls the charging bias of the primary charging roller  902 , and causes the surface of the photoreceptor drum  901  to be charged at positive and uniform potential. In S 1006 , the CPU  1  controls the laser unit  903 , and causes it to irradiate laser light on the surface of the photoreceptor drum  901 , which has been uniformly charged, and to form an electrostatic latent image thereon. In S  1007 , the CPU  1  controls the developing bias of the developing sleeve  904 , and the electrostatic latent image is developed on the photoreceptor drum  901  by toner, and as a result a toner image is formed. In S  1008 , the CPU  1  controls the primary transfer bias, and the toner image on the photoreceptor drum  901  is primary-transferred to the intermediate transfer belt  906 . In S 1009 , the CPU  1  drives a pickup roller that feeds a sheet  913  by controlling a motor, and the sheet  913  is conveyed from the cassette  910  to a conveyance path. In S  1010 , the CPU  1  controls the motor that drives the conveyance roller, which conveys the sheet  913 , so that the conveyance timing of the sheet  913  matches with the arriving timing of the toner image on the intermediate transfer belt  906 . The CPU  1  further controls the secondary transfer bias, and the toner image on the sheet  913  is secondary-transferred. In S 1011 , the CPU  1  controls the fixing unit  911 , and an un-fixed toner image on the sheet  913  is fixed. In S 1012 , the CPU  1  controls the motor that drives the conveyance roller, and the sheet  913  is discharged. In S 1013 , the CPU  1  ends adjustment of the fixing temperature. In S 1014 , the CPU  1  controls the motor to stop rotation of the fixing roller  92 . In S  1015 , the CPU  1  controls the motor to stop rotation of the photoreceptor drum  901  and other rollers, and ends the image forming operation. 
     Thus, the CPU  1  controls the power-supply apparatus  100  to supply electric power to the fixing unit  911  during the image formation, so that the fixing temperature of the fixing unit  911  is kept at a predetermined target temperature To. 
     Control of Fixing Temperature 
     With reference to the flowchart in FIG. 6 , a temperature control method of the fixing unit  911  during the image formation will be described. Control parameters such as the upper limit electric power Pmax and the target temperature To are set for the control circuit  113  by the CPU  1 . In the control of the fixing temperature, it is important to keep the fixing temperature at the target temperature To, and not to supply redundant electric power to the induction heating coil  91 . 
     In S 2001 , the control circuit  113  compares the current electric power P with the upper limit electric power Pmax, and determines whether the current electric power P exceeds the upper limit electric power Pmax or not. If the current electric power P exceeds the upper limit electric power Pmax, an increase of the electric power P is not allowed, and the process advances to S 2009 . If the current electric power P does not exceed the upper limit electric power Pmax, the process advances to S 2002 . 
     In S 2002 , the control circuit  113  compares the detection value T of the fixing temperature with the target temperature To, and determines whether the detection value T exceeds the target temperature To. If the detection value T exceeds the target temperature To, since the fixing temperature needs to be decreased, the process advances to S 2009 . On the other hand, if the detection value T does not exceed the target temperature To, the process advances to S 2003 . 
     In S 2003 , the control circuit  113  compares the detection value T of the fixing temperature with the target temperature To, and determines whether the detection value T is less than the target temperature To or not. If the detection value T is less than the target temperature To, since the fixing temperature needs to be increased, the process advances to S 2006 . On the other hand, if the detection value T is not less than the target temperature To, in other words, if the detection value T is equal to the target temperature To, since the fixing temperature does not need to be increased or decreased, the process advances to S 2004 . 
     In S 2004 , the control circuit  113  keeps the pulse width t of the drive signals  121  and  122 , which are output by the drive circuit  112 , at the current pulse width t. Thereafter, advancing to S 2005 , the control circuit  113  determines whether the discharge of the sheet  913  is completed or not. If the output is not completed, that is, since the sheet  913  enters into the fixing unit  911  from now, the temperature control needs to be continued. Thus the process returns to S 2001 . On the other hand, if the discharge of the sheet  913  is completed, the temperature control is ended. 
     Process when fixing temperature needs to be decreased 
     In S 2009 , the control circuit  113  determines if, when the pulse width t is decreased by a predetermined decrement value ta, the difference between the pulse width t and the decrement value ta (t-ta) becomes equal to or less than the minimum pulse width tmin. If the difference between the pulse width t and the decrement value ta (t-ta) is greater than the minimum pulse width tmin, the process advances to S 2010 . In S 2010 , the control circuit  113  decreases the pulse width t by the decrement value ta. On the other hand, if the difference between the pulse width t and the decrement value ta (t-ta) is equal to or less than the minimum pulse width tmin, the process advances to S 2011 . In S 2011 , the control circuit  113  sets the pulse width t to  0 . Accordingly, the pulse width t is prevented from becoming between 0 and the minimum pulse width tmin. The minimum pulse width tmin is decided by design. Thereafter, the process advances to S 2005 . 
     Process when fixing temperature needs to be increased 
     In S 2006 , the control circuit  113  determines if, when the pulse width t is increased by a predetermined increment value tb, the sum of the pulse width t and the increment value tb becomes equal to or more than the maximum pulse width tmax. If the sum of the pulse width t and the increment value tb is less than the maximum pulse width tmax, the process advances to S 2007 . In S 2007 , the control circuit  113  increases the pulse width t by the increment value tb. On the other hand, if the sum of the pulse width t and the increment value tb is equal to or more than the maximum pulse width tmax, the process advances to S 2008 . In S 2008 , the control circuit  113  sets the pulse width t to the maximum pulse width tmax, and then the process advances to S 2005 . Accordingly, the pulse width t is prevented from becoming longer than the maximum pulse width tmax. 
     Thus, the control circuit  113  increases and decreases the pulse width t of the drive signals  121  and  122  that the drive circuit  112  outputs, in order that the high frequency current flowing through the induction heating coil  91  is increased and decreased, so that the fixing temperature T is kept at the target temperature To. 
     Effect of Providing First and Second Diodes 
     As described above, in a conventional technique in which the first diode D 1  and the second diode D 2  are not provided, the polarity of the potential of the surface of the fixing roller  92  and the polarity of the potential of the surface of the pressing roller  93  respectively become the same as and opposite to the polarity of the charged potential of the toner, and as a result the offset is caused. For example, when the polarity of the potential of the surface of the fixing roller  92  becomes opposite to the polarity of the charged potential of the toner, the electrostatic holding force of the toner to the sheet  913  is weakened, and the toner tends to adhere to the fixing roller  92 . 
       FIG. 7  illustrates waveforms of the potential V 92  of the conductive heating element  921  of the fixing roller  92 , and the potential V 93  of the conductive tube  931  of the surface of the pressing roller  93 , respectively. As shown in  FIG. 7 , when the temperature control is started and high-frequency AC current flows through the induction heating coil  91 , the potential V 92  of the fixing roller  92  becomes negative, and the potential V 93  of the pressing roller  93  becomes positive. In the sheet feeding period thereafter, these polarities are kept this way. Thus, the electrostatic holding force of the toner to the sheet  913  is kept, and as a result the generation of the fixing offset can be reduced. 
     In this embodiment, an image forming unit in which the toner is negatively charged was described. The present invention can also be applied to an image forming unit in which the toner is positively charged. When an image forming unit in which the toner is positively charged is adopted, the same effect can be obtained by reversing the connecting direction of the first diode D 1  and the second diode D 2 , respectively. 
       FIG. 1  illustrates an image forming apparatus  900  that includes an image forming unit in which a single-color image is formed. The present invention can also be applied to an image forming apparatus that forms a multi-color image. In an image forming apparatus that forms a multi-color image, the image forming process is almost the same, and the offset may be generated in the fixing unit  911 . 
     Accordingly, in this embodiment, by connecting the first diode D 1  between the conductive heating element  921  of the fixing roller  92  and ground, electric charge with the same polarity as the charged polarity of the toner can be held on the surface of the fixing roller  92 . In other words, since the coulomb force (repulsive force) works between the electric charge on the surface of the fixing roller  92  and the toner on the sheet  913 , the toner tends not to adhere to the fixing roller  92 . Similarly, by connecting the second diode D 2  between the surface of the pressing roller  93  that does not contact an un-fixed toner image and ground, electric charge with the reverse polarity to the charged polarity of the toner can be held on the surface of the pressing roller  93 . Since the coulomb force (attractive force) works between the electric charge on the surface of the pressing roller  93  and the toner on the sheet  913 , the toner is attracted towards the pressing roller  93  through the sheet  913 , and tends not to adhere to the fixing roller  92 . In other words, since the toner is attracted to the direction where the sheet  913  is present, the toner tends not to leave the sheet  913 . Thus, since the polarity of electric charge that is induced by the electromagnetic induction is held at the polarity that is determined by the two diodes, the fixing offset can be suppressed without adding a new collector member or a charging capacitor. 
     Note that, although both the first diode D 1  and the second diode D 2  were provided in Embodiment 1, any one of these will suffice. In the case where only one of the first diode D 1  and the second diode D 2  is provided, although the effect to suppress the offset is reduced, when electric charge generated in the fixing unit  911  is small, only one of these diodes may be enough to suppress the offset. 
     In Embodiment 1, although the surface of the fixing roller  92  is configured by an electrically non-conductive layer (non-conductive tube  922 ) stacked on an electrically conductive layer (conductive heating element  921 ), the surface of the fixing roller  92  may be configured by an electrically conductive layer. 
     In Embodiment 1, although the first diode D 1  and the second diode D 2  were adopted as a rectification element, any element that has a rectification function can be adopted in place of the first diode D 1  and the second diode D 2 . For example, transistors may be adopted in place of the first diode D 1  and the second diode D 2 . 
     Embodiment 2 
     Embodiment 1 has the advantage that an offset can be suppressed by a relatively simple configuration, in which a first diode D 1  and a second diode D 2  were provided. Incidentally, when the potential generated by frictional electrification becomes too high, the voltage between the two ends of the first diode D 1  and the voltage between the two ends of the second diode D 2  may exceed the electrostatic breakdown voltage, which causes electrostatic breakdown. 
     Thus, this embodiment is characterized by a first variable resistor provided between a first diode Dl and ground to protect the first diode Dl from insulation breakdown. Furthermore, it is characterized by a second variable resistor provided between a second diode D 2  and ground to protect the second diode D 2  from insulation breakdown. Note that, by giving the same reference signs to constituent elements that were already described, explanation will be simplified. 
     With reference to  FIG. 8 , a fixing unit  911  in Embodiment 2 will be described. Here, although an example in which the first variable resistor and the second variable resistor are realized by transistors will be described, other circuit elements such as an authentic variable resistor may be utilized. 
     As shown in  FIG. 8 , a transistor  98  is serially connected between the cathode of the first diode D 1  and ground. The transistor  98  is an npn-type transistor. Similarly, a transistor  99  is serially connected between the anode of the second diode D 2  and ground. The transistor  99  is a pnp-type transistor. The types of the transistors  98  and  99  are selected according to the direction of the diodes. In other words, when the connection direction of the diode is reversed, the types of the transistors  98  and  99  are also changed. 
       FIG. 9A  illustrates capacitance elements and the transistors  98  and  99  in the fixing unit  911 .  FIG. 9B  illustrates an equivalent circuit of the fixing unit  911 . By adjusting the base current Ib 1  of the transistor  98 , the collector current Ic 1  can be adjusted. In other words, by adjusting the base current Ib 1 , the potential V 92  of the conductive heating element  921  of the fixing roller  92  can be adjusted. Similarly, by adjusting the base current Ib 2  of the transistor  99 , the collector current Ic 2  can be adjusted. In other words, by adjusting the base current Ib 2 , the potential V 93  of the conductive tube  931  on the surface of the pressing roller  93  can be adjusted. 
     With reference to  FIG. 10 , a power-supply apparatus  100  will be described. In the power-supply apparatus  100 , a potential control circuit  115  to control the potential of V 92  and V 93  is newly added. The potential control circuit  115  includes a voltage detection circuit to detect the potential V 92  and V 93 . The potential control circuit  115  controls the base current Ib 1  so that the potential V 92  does not exceed the insulation breakdown voltage of the first diode D 1 . Similarly, the potential control circuit  115  controls the base current Ib 2  so that the potential V 93  does not exceed the insulation breakdown voltage of the second diode D 2 . 
     Fixing Potential Control 
     Here, the control of the potential V 92  and V 93  is referred to as fixing potential control. Note that, the control flow is the same for the potential V 92  and V 93 , except that the polarity of the potential is reversed. Thus, the control of the potential V 93  will be described, with reference to  FIG. 11 . 
     In S 3001 , the potential control circuit  115  determines whether the fixing roller  92  is at a stop or not. Whether the fixing roller  92  is at a stop or not is determined by the control signal from the CPU  1 . When the fixing roller  92  starts rotation, the process advances to S 3002 . 
     In S 3002 , the potential control circuit  115  compares the detection value of the potential V 93  of the conductive tube  931  that is on the surface of the pressing roller  93  with the upper limit potential V 93 max, and determines whether the detection value of the potential V 93  exceeds the upper limit potential V 93 max or not. If the detection value of the potential V 93  exceeds the upper limit potential V 93 max, the potential V 93  needs to be decreased so that the insulation breakdown of the second diode D 2  can be prevented. Therefore, the process advances to S 3009 . In S 3009 , the potential control circuit  115  decreases the base current Ib 2  by a predetermined value I, and the process advances to S 3005 . On the other hand, if the detection value of the potential V 93  does not exceed the upper limit potential V 93 max, the process advances to S 3003 . 
     In S 3003 , the potential control circuit  115  compares the detection value of the potential V 93  with the upper limit potential V 93 max, and determines whether the detection value of the potential V 93  is less than the upper limit potential V 93 max or not. If the detection value of the potential V 93  is not less than the upper limit potential V 93 max, in other words, if the detection value of the potential V 93  is equal to the upper limit potential V 93 max, the process advances to S 3004 . In S 3004 , the potential control circuit  115  keeps the base current Ib 2  at the current value, and the process advances to S 3005 . On the other hand, if the detection value of the potential V 93  is less than the upper limit potential V 93 max, since the potential V 93  has room for increase, the process advances to S 3006 . 
     In S 3006 , the potential control circuit  115  determines if, when the base current Ib 2  is increased by I, the sum of the current base current Ib 2  and I becomes equal to or more than the maximum base current Ib 2 max. If the sum of the current base current Ib 2  and I is less than the maximum base current Ib 2 max, the process advances to S 3007 . In S 3007 , the potential control circuit  115  increases the base current Ib 2  by I. On the other hand, if the sum of the current base current Ib 2  and I becomes equal to or more than the maximum base current Ib 2 max, since the base current Ib 2  cannot be increased, the process advances to S 3008 . In S 3008 , the potential control circuit  115  sets the base current Ib 2  at the maximum base current Ib 2 max, and thereafter the process advances to S 3005 . 
     In S 3005 , the potential control circuit  115  determines whether the fixing roller  92  is rotating or not. If the fixing roller  92  is rotating, since the fixing potential needs to be controlled continuously, the process returns to S 3002 . On the other hand, if the fixing roller  92  is at a stop, the potential control circuit  115  ends the fixing potential control. 
     Thus, according to this embodiment, by providing a first variable resistor between the first diode D 1  and ground to protect the first diode D 1  from insulation breakdown, the insulation breakdown of the first diode D 1  is suppressed along with suppressing the offset. Similarly, by providing a second variable resistor between the second diode D 2  and ground to protect the second diode D 2  from insulation breakdown, the insulation breakdown of the second diode D 2  is suppressed along with suppressing the offset. Note that, as these variable resistors, although any type of resistor can be used as long as that can control the fixing potential, transistors can be adopted considering the ease of implementation. In this case, the potential control circuit  115  functions as a potential control unit that measures the fixing potential V 92  and V 93 , and controls the base potential of the transistors  98  and  99  so that each of potential V 92  and V 93  does not exceed the insulation breakdown voltage. Thus, according to Embodiment 2, in addition to the effect of Embodiment 1, further effect, which is to suppress the toner offset in the region where the insulation breakdown in the first diode D 1  and the second diode D 2  will not occur, can be obtained. Only one of the first diode D 1  and the second diode D 2  may be provided. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2012-174366, filed Aug. 6, 2012 which is hereby incorporated by reference herein in its entirety.