Patent Publication Number: US-11644766-B2

Title: Image forming apparatus that controls voltages to reduce image fogging

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a printer such as a laser printer or an LED printer and an image forming apparatus such as a digital copier using an electrophotographic system or an electrostatic recording system, and a heating fixation apparatus. 
     Description of the Related Art 
     Image forming apparatuses use a direct transfer system that directly transfers a toner image on the surface of a photosensitive drum onto a recording paper at a transfer nip formed between the surface of the photosensitive drum and a transfer roller. Further, some image forming apparatuses of the direct transfer system do not have a static elimination light unit for the purpose of reducing cost. In this case, since a drum surface potential (the surface potential of a photosensitive drum) is not initialized by static elimination light before charging, a difference in a transfer discharging state at a transfer nip directly exerts an influence upon the drum surface potential in the image forming apparatuses of the direct transfer system. Accordingly, impedance changes depending on the presence or absence of a recording paper at the transfer nip, and a discharging state at the transfer nip becomes different. Therefore, a drum surface potential after the recording paper has passed through the transfer nip (during paper non-passing) is different from a drum surface potential at a time at which the recording paper is passing through the transfer nip (during paper passing). Specifically, discharging directly occurs between a transfer roller and the photosensitive drum without the interposition of a recording paper during the paper non-passing, and the uniformity of the drum surface potential is lost due to the influence of the cell or fuzz of the transfer roller. Therefore, the drum surface potential becomes disrupted. 
     When the disrupted drum surface potential after transfer does not become uniform at next charging, the record of the disrupted drum surface potential exerts an influence upon developing. As a result, an image failure called “memory” is possibly caused on an image of a subsequent recording paper. In order to solve the memory, it is necessary to adjust the drum surface potential for a next image forming step. Japanese Patent Application Laid-open No. 2010-204322 describes a technology relating to the memory. As a general adjustment method, a method in which a charging voltage is switched between the paper passing region (the region that a recording paper passing through a transfer nip comes into contact) and the paper non-passing region (the region that the recording paper does not come into contact) of a photosensitive drum has been known. A charging voltage V1 for forming a surface potential (Vd1) of the paper passing region and a charging voltage V2 for forming a surface potential (Vd2) of the paper non-passing region are set. Then, (the absolute value of) the surface potential (Vd2) of the paper non-passing region is made lower than (the absolute value of) the surface potential (Vd1) of the paper passing region in advance, and a charging voltage V1 is applied to a charging member at the timing of next print. In this manner, it is possible to solve the memory according to a discharging amount. In the case of jumping development, an extremely small amount of toner flies due to a potential difference between a surface potential of the photosensitive drum and a developing potential of a developing roller at the moment at which the developing voltage is applied to the developing roller. Therefore, a developing voltage is preferably not applied to the developing roller until the timing at which the paper passing region faces the developing roller in terms of a toner consumption amount. However, the developing potential is not stabilized immediately after the developing voltage is applied. Therefore, it is necessary to apply the developing voltage immediately before the paper passing region faces the developing roller in expectation of the time at which the developing potential is stabilized. 
     A potential difference (Vback) between the surface potential of the photosensitive drum and the developing potential of the developing roller exerts an influence upon a fogging amount on the photosensitive drum during image formation. Note that the attachment of unnecessary toner to the photosensitive drum is called fogging and an amount of the unnecessary toner on the photosensitive drum is called a fogging amount.  FIG.  1    shows a general relationship between a potential difference (Vback) of monochrome toner and a fogging amount on a photosensitive drum. As shown in  FIG.  1   , the fogging amount on the photosensitive drum becomes large and toner having a normal charging polarity called ground fogging is developed on the photosensitive drum on a side where the potential difference (Vback) is small. Further, as shown in  FIG.  1   , the fogging amount on the photosensitive drum becomes large and toner having an inverted polarity (polarity opposite to the normal charging polarity) called reversal fogging is developed on the photosensitive drum on a side where the potential difference (Vback) is large. 
     The relationship between the potential difference (Vback) and the fogging amount changes depending on the characteristics of toner, and a potential difference called bottom at which the fogging amount is the smallest exists. The fogging amount of the paper passing region exerts a large influence upon a toner consumption amount, and the potential difference (Vback) is preferably set at the bottom in terms of the toner consumption amount. Note that in the present specification, the fact that potentials are large or small represents whether the potentials are large or small when their absolute values are compared with each other, and the fact that potentials are high or low represents whether the potentials are large or small when their absolute values are compared with each other. 
       FIG.  2    shows the output waveforms of respective potentials before and after the tip of the recording paper enters the transfer nip. As shown in  FIG.  2   , a potential difference (Vback 2 ) between the surface potential (Vd2) and a developing potential of the paper non-passing region becomes smaller than that of the paper passing region when the surface potential (Vd2) of the paper non-passing region is made smaller than the surface potential (Vd1) of the paper passing region in order to solve the memory. Note that the output timings of the respective potentials in  FIG.  2    are shown with their phases matching to a drum surface. In a charging member, a charging potential (Vpri 1 ) for forming the surface potential (Vd1) of the paper passing region and a charging potential (Vpri 2 ) for forming the surface potential (Vd2) of the paper non-passing region are formed. 
     It is shown by  FIG.  2    that when a potential difference (Vback 1 ) between the surface potential (Vd1) and a developing potential (Vdc) is set near the bottom, the potential difference (Vback 2 ) between the surface potential (Vd2) and the developing potential (Vdc) decreases by an amount of |Vd1−Vd2| and ground fogging occurs. The fogging of the paper non-passing region exerts an influence upon a toner consumption amount and causes stain on the transfer roller or the like. On the other hand, when the surface potential (Vd2) of the paper non-passing region is increased in order to increase the potential difference (Vback 2 ) so that fogging does not occur in the paper non-passing region, it is necessary to further increase the charging potential (Vpri 1 ) to solve the memory. When the charging potential (Vpri 1 ) is further increased, the surface potential (Vd1) of the paper passing region further increases, which results in an increase in a discharging amount at a transfer portion and may adversely affect transfer. Further, since the surface potential (Vd1) of the paper passing region increases, the potential difference (Vback 1 ) increases more than necessary. When the potential difference (Vback 1 ) is too large, a desired toner putting amount is not obtained, reversal fogging is likely to occur on a photosensitive drum, and a toner consumption amount is worsened. The present invention has been made in order to solve the above problems and has an object of providing a technology to reduce memory occurring at a transfer nip and reduce fogging. 
     SUMMARY OF THE INVENTION 
     In order to achieve the object described above, an image forming apparatus according to the present invention includes: an image bearing member that is rotatable; a charging member that charges a surface of the image bearing member at a charging portion; an exposure portion that exposes the image bearing member to form an electrostatic latent image on the surface of the image bearing member; a developer bearing member that bears a developer charged to have a normal polarity and supplies the developer to the electrostatic latent image formed on the surface of the image bearing member to form a developer image on the surface of the image bearing member; a transfer member that comes into contact with the image bearing member to form a nip and transfers the developer image onto a recording material conveyed to the nip; a charging voltage application portion that applies a charging voltage to the charging member; a developing voltage application portion that applies a developing voltage to the developer bearing member; a transfer voltage application portion that applies a transfer voltage to the transfer member; and a controller that controls the charging voltage application portion, the developing voltage application portion, and the transfer voltage application portion, wherein, when a region of the image bearing member that comes into contact with the recording material at the nip in a state in which the transfer voltage is applied to the transfer member and the recording material is held by the nip is assumed as a first region and a region of the image bearing member that does not come into contact with the recording material at the nip in a state in which the transfer voltage is applied to the transfer member and the recording material is not held by the nip is assumed as a second region, the controller controls to apply a first developing voltage to the developer bearing member when the first region in which a first surface potential has been formed at the charging portion faces the developer bearing member and apply a second developing voltage smaller than an absolute value of the first developing voltage to the developer bearing member when the second region in which a second surface potential has been formed at the charging portion faces the developer bearing member after the recording material passes through the nip. 
     In order to achieve the object described above, an image forming apparatus according to the present invention includes: an image bearing member that is rotatable; a charging member that charges a surface of the image bearing member at a charging portion; an exposure portion that exposes the image bearing member to form an electrostatic latent image on the surface of the image bearing member; a developer bearing member that bears a developer charged to have a normal polarity and supplies the developer to the electrostatic latent image formed on the surface of the image bearing member to form a developer image on the surface of the image bearing member; a transfer member that comes into contact with the image bearing member to form a nip and transfers the developer image onto a recording material conveyed to the nip; a charging voltage application portion that applies a charging voltage to the charging member; a developing voltage application portion that applies a developing voltage to the developer bearing member; a transfer voltage application portion that applies a transfer voltage to the transfer member; and a controller that controls the charging voltage application portion, the developing voltage application portion, and the transfer voltage application portion, wherein, when a region of the image bearing member that comes into contact with the recording material at the nip in a state in which the transfer voltage is applied to the transfer member and the recording material is held by the nip is assumed as a first region, a region of the image bearing member that does not come into contact with the recording material at the nip in a state in which the transfer voltage is applied to the transfer member and the recording material is not held by the nip is assumed as a second region, and a region of the image bearing member that is positioned between the first region and the second region and comes into contact with the recording material at the nip in a state in which the transfer voltage is applied to the transfer member and the recording material is held by the nip is assumed as a third region, the controller controls to apply a first developing voltage to the developer bearing member when the first region in which a first surface potential has been formed at the charging portion faces the developer bearing member and apply a second developing voltage smaller than an absolute value of the first developing voltage to the developer bearing member when the second region in which a second surface potential has been formed at the charging portion faces the developer bearing member after the recording material passes through the nip, a developing potential formed in the developer bearing member overshoots when the third region in which a third surface potential has been formed at the charging portion faces the developer bearing member and when the second developing voltage is switched to the first developing voltage, and an absolute value of the developing potential due to an overshoot of the developing potential is larger than an absolute value of the developing potential formed when the first developing voltage is applied to the developer bearing member. 
     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 showing a fogging amount of monochrome toner; 
         FIG.  2    is a diagram showing the output waveforms of respective potentials; 
         FIG.  3    is a diagram of the configuration of an image forming apparatus according to a first embodiment; 
         FIG.  4    is a diagram showing the relationship between the respective potentials according to the first embodiment; 
         FIG.  5    is a diagram showing a fogging amount of toner according to the first embodiment; 
         FIG.  6    is a diagram showing the output waveforms of the respective potentials according to the first embodiment; 
         FIG.  7    is a diagram showing waveforms formed when developing potentials rise; 
         FIG.  8    is a diagram showing the relationship between respective potentials according to a second embodiment; and 
         FIG.  9    is a diagram showing the output waveforms of the respective potentials according to the second embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. However, the dimensions, materials, shapes, relative arrangements, or the like of constituting components described in the embodiments should be appropriately changed according to the configurations, various conditions, or the like of an apparatus to which the invention is applied, and are not intended to limit the scope of the invention to the following embodiments. 
     First Embodiment 
     Image Forming Apparatus 
       FIG.  3    shows an image forming apparatus  100  according to a first embodiment, that is, the image forming apparatus  100  including a heating fixation apparatus and a printer control device according to the first embodiment. Note that  FIG.  3    is a vertical cross-sectional view showing the schematic configuration of a laser printer as an example of the image forming apparatus  100  according to the first embodiment. First, the configuration of the image forming apparatus  100  will be described in detail with reference to  FIG.  3   . The image forming apparatus  100  is, for example, a printer such as a laser printer or an LED printer, or an image forming apparatus such as a digital copier using an electrophotographic system or an electrostatic recording system. 
     The image forming apparatus  100  includes a drum-type electrophotographic photosensitive member (hereinafter described as a “photosensitive drum”)  1  serving as an image bearing member and a charging roller (charging member)  2  that charges the surface of the photosensitive drum  1 . The photosensitive drum  1  is one obtained by providing a photosensitive material such as an OPC (organic photoconductor), amorphous selenium, and amorphous silicon on a drum substrate on a cylinder formed of an aluminum alloy, nickel, or the like. The photosensitive drum  1  that is rotatable is rotated and driven at a prescribed process speed (peripheral speed) in an arrow R1 direction by a driving means (not shown). A charging portion on the surface of the photosensitive drum  1  is uniformly charged to have a prescribed polarity and potential by the charging roller  2 . Thus, a dark part potential VD is formed in the photosensitive drum  1 . 
     The charging roller  2  of the first embodiment is a contact charging roller and constituted by an electric conductor roller  2   a  such as a metal roller serving as a cored bar and a cylindrical conductive layer  2   b  formed on the outer peripheral surface of the electric conductor roller  2   a . The charging roller  2  is arranged parallel to the photosensitive drum  1  with both ends of the electric conductor roller  2   a  journaled in bearing members (not shown). Further, the charging roller  2  is brought into pressure contact with the photosensitive drum  1  by pressing means such as a spring (not shown) and rotates with the rotation of the photosensitive drum  1 . The image forming apparatus  100  has a charging high-voltage power supply (charging voltage application portion)  303  that applies a charging voltage to the charging roller  2 . 
     The surface of the photosensitive drum  1  after charging is exposed by a laser beam E for image formation from a laser scanner (exposure portion)  3 , and an electrostatic latent image is formed on the surface of the photosensitive drum  1 . The laser scanner  3  performs scanning exposure subjected to ON/OFF control according to image information in the longitudinal direction of the photosensitive drum  1  and eliminates charges from an exposure region to form an electrostatic latent image on the surface of the photosensitive drum  1 . That is, the photosensitive drum  1  after charging is exposed, and a light portion potential VL is formed in the photosensitive drum  1 . An electrostatic latent image formed by a dark part potential VD and a light portion potential VL of the photosensitive drum  1  is developed and visualized at a developing portion Ng formed by a developing apparatus (developing means)  4  and the photosensitive drum  1 . In the present embodiment, a jumping development method by which a gap is provided between the developing apparatus  4  and the photosensitive drum  1  is used. Besides the jumping development method, a two-component developing method, a contact developing method, or the like is used as a development method. Image exposure and reversal development may be combined together. When toner (developer) borne by a developing roller (developer bearing member)  41  is attached to the photosensitive drum  1  at the developing portion Ng, an electrostatic latent image described above is developed as a toner image (developer image). As described above, the developing roller  41  bears toner charged to have a normal polarity and supplies the toner to an electrostatic latent image formed on the surface of the photosensitive drum  1  to form a toner image on the surface of the photosensitive drum  1 . In the present embodiment, the normal polarity of the toner is a negative polarity. Note that the normal polarity of the toner may be a positive polarity. In this case, the polarity of a voltage applied to respective members is only required to be opposite to the polarity of the present embodiment. The image forming apparatus  100  has a developing high-voltage power supply (developing voltage application portion)  304  that applies a developing voltage to the developing roller  41 . 
     Recording papers (recording materials) P are accommodated in a paper feeding tray  101 . The image forming apparatus  100  includes a transfer roller (transfer member)  5 . The transfer roller  5  comes into contact with the photosensitive drum  1  to form a transfer nip portion (nip) Nt. Each of the recording papers P accommodated in the paper feeding tray  101  is individually fed by a paper feeding roller  102  and conveyed to the transfer nip portion Nt between the photosensitive drum  1  and the transfer roller  5  via conveyance rollers  103  or the like. At this time, the tip-end position of the recording paper P is detected by a top sensor  104 . The timing at which the tip end of the recording paper P reaches the transfer nip portion Nt is detected on the basis of a distance between the detecting position of the top sensor  104  and the position of the transfer nip portion Nt and the conveyance speed of the recording paper P. Further, the rear-end position of the recording paper P is detected by the top sensor  104 . The timing at which the rear end of the recording paper P reaches the transfer nip portion Nt is detected on the basis of a distance between the detecting position of the top sensor  104  and the position of the transfer nip portion Nt and the conveyance speed of the recording paper P. The image forming apparatus  100  has a transfer high-voltage power supply (transfer voltage application unit)  305  that applies a transfer voltage to the transfer roller  5 . When a transfer voltage is applied to the transfer roller  5 , a toner image on the photosensitive drum  1  is transferred onto the recording paper P that is fed and conveyed at prescribed timing in the manner described above. 
     The recording paper P onto which a toner image has been transferred is conveyed to the heating fixation apparatus (fixation means)  6 . When the recording paper P is heated and pressed while being held and conveyed by a fixation nip portion between a heating member  10  and a pressure roller  20  in the heating fixation apparatus  6 , the toner image is fixed onto the surface of the recording paper P. Then, the recording paper P is discharged by paper discharging rollers  106  onto a paper discharging tray  107  formed on the upper surface of the image forming apparatus  100 . Note that the presence or absence of the occurrence of a jam or the like is monitored during this time in such a manner that a paper discharging sensor  105  detects the timing at which the tip end and the rear end of the recording paper P pass therethrough. On the other hand, in the photosensitive drum  1  from which the toner image has been transferred, toner (untransferred toner) that has remained on the surface without being transferred onto the recording paper P is removed by a cleaning blade  71  of a cleaning apparatus (cleaning means)  7  and subjected to next image formation. By repeatedly performing the above operations, the image forming apparatus  100  is capable of performing image formation one after another. Note that the image forming apparatus  100  of the first embodiment is an example of an apparatus that has a resolution of 600 dpi, outputs  30  prints per minute (LTR longitudinal feed: a process speed of about 222 mm/s), and comes to the end of its life after outputting  100 , 000  prints. 
     Printer Control Apparatus 
     A control device (printer control device)  300  serving as a controller connects to a host computer  302  using a controller interface  301  to perform communication with the host computer  302 . The control device  300  controls a charging voltage applied to the charging roller  2  according to a detection result (detection information) of the top sensor  104 . By controlling the charging high-voltage power supply  303 , the control device  300  performs control to switch between a first charging voltage for forming a charging potential (Vpri 1 ) in the charging roller  2  and a second charging voltage for forming a charging potential (Vpri 2 ) in the charging roller  2 . The charging potential (Vpri 1 ) is an example of a first charging potential, and the charging potential (Vpri 2 ) is an example of a second charging potential. The absolute value of the charging potential (Vpri 1 ) is larger than that of the charging potential (Vpri 2 ). The first charging voltage and the second charging voltage have the same polarity, and the absolute value of the first charging voltage is larger than that of the second charging voltage. The first embodiment describes a case in which a charging potential and a charging voltage have a negative polarity, but it is assumed that the charging potential and the charging voltage have a positive polarity when the normal polarity of toner is a positive polarity as described above. Further, by controlling the transfer high-voltage power supply  305 , the control device  300  controls a transfer voltage applied to the transfer roller  5 . 
     In the first embodiment, a state in which the recording paper P exists at the transfer nip portion Nt will be called a paper passing state. The surface region of the photosensitive drum  1  has at least a first region and a second region. The first region of the surface region of the photosensitive drum  1  comes into contact with the recording paper P at the transfer nip portion Nt. In the first embodiment, the first region of the photosensitive drum  1  will be called a paper passing region. The paper passing region is the region of the photosensitive drum  1  that comes into contact with the recording paper P at the transfer nip portion Nt in a state in which a transfer voltage is applied to the transfer roller  5  and the recording paper P is held by the transfer nip portion Nt. A paper non-passing region is the region of the photosensitive drum  1  that does not come into contact with the recording paper P at the transfer nip portion Nt in a state in which a transfer voltage is applied to the transfer roller  5  and the recording paper P is not held by the transfer nip portion Nt. In the first embodiment, a state in which the recording paper P before reaching the transfer nip portion Nt does not exist at the transfer nip portion Nt will be called a paper non-passing state. The second region of the surface region of the photosensitive drum  1  does not come into contact with the recording paper P at the transfer nip portion Nt. In the first embodiment, the second region of the photosensitive drum  1  will be called a paper non-passing region. The paper non-passing region is positioned downstream of the paper passing region in the rotating direction of the photosensitive drum  1 . When a first charging voltage is applied to the charging roller  2  and the paper passing region is charged by the charging roller  2 , a surface potential (first surface potential) is formed in the paper passing region. When a second charging voltage is applied to the charging roller  2  and the paper non-passing region is charged by the charging roller  2 , a surface potential (second surface potential) is formed in the paper non-passing region. The paper non-passing region is positioned downstream of the paper passing region in the rotating direction of the photosensitive drum  1 . Therefore, the surface potential is formed in the paper passing region after the surface potential is formed in the paper non-passing region. 
     The control device  300  acquires information on the timing at which the paper passing region faces the developing roller  41  on the basis of a detection result of the top sensor  104 , a distance between the position of the transfer nip portion Nt and the position of the developing roller  41 , and the rotation speed of the photosensitive drum  1 . As the position of the developing roller  41 , the position on the surface of the developing roller  41  that is the closest to the photosensitive drum  1  may be used. The control device  300  may determine that the paper passing region faces the developing roller  41  when a distance between the paper passing region and the surface of the developing roller  41  is not more than a prescribed distance. Hereinafter, information on the timing at which the paper passing region faces the developing roller  41  will be expressed as information on first timing. 
     The control device  300  acquires information on the timing at which the paper non-passing region faces the developing roller  41  on the basis of a detection result of the top sensor  104 , a distance between the position of the transfer nip portion Nt and the position of the developing roller  41 , and the rotation speed of the photosensitive drum  1 . The control device  300  may determine that the paper non-passing region faces the developing roller  41  when a distance between the paper non-passing region and the surface of the developing roller  41  is not more than a prescribed distance. Hereinafter, information on the timing at which the paper non-passing region faces the developing roller  41  will be expressed as information on second timing. 
     The control device  300  acquires the surface potential (first surface potential) of the paper passing region and the surface potential (second surface potential) of the paper non-passing region. The control device  300  may calculate the surface potential of the paper passing region and the surface potential of the paper non-passing region on the basis of a charging voltage applied to the charging roller  2 . The control device  300  may correct the calculated surface potential of the paper passing region and the surface potential of the paper non-passing region according to the ambient (external environment) temperature and humidity of the image forming apparatus  100 . The control device  300  may measure the surface potential of the paper passing region and the surface potential of the paper non-passing region. The control device  300  controls a developing voltage applied to the developing roller  41  on the basis of the surface potential of the paper passing region, the surface potential of the paper non-passing region, information on the first timing, and information on the second timing. By controlling the developing high-voltage power supply  304 , the control device  300  performs control to switch between a first developing voltage for forming a developing potential (Vdc1) in the developing roller  41  and a second developing voltage for forming a developing potential (Vdc2) in the developing roller  41 . The developing potential (Vdc1) is an example of a first developing potential, and the developing potential (Vdc2) is an example of a second developing potential. 
     The control device  300  controls the developing high-voltage power supply  304  on the basis of information on the first timing so that a first developing voltage is applied to the developing roller  41  when the paper passing region in which a surface potential has been formed faces the developing roller  41 . When the paper passing region in which the surface potential has been formed faces the developing roller  41 , the developing high-voltage power supply  304  applies a first developing voltage to the developing roller  41 . The control device  300  controls the developing high-voltage power supply  304  so that a first developing voltage is applied to the developing roller  41  when the paper passing region in which a first surface potential has been formed faces the developing roller  41  in the charging portion on the surface of the photosensitive drum  1  after the recording paper has passed through the transfer nip portion Nt. Thus, a developing potential (Vdc1) is formed in the developing roller  41  when the paper passing region in which a surface potential has been formed faces the developing roller  41 . The control device  300  controls the developing high-voltage power supply  304  on the basis of information on the second timing so that a second developing voltage is applied to the developing roller  41  when the paper non-passing region in which a surface potential has been formed faces the developing roller  41 . When the paper non-passing region in which the surface potential has been formed faces the developing roller  41 , the developing high-voltage power supply  304  applies a second developing voltage to the developing roller  41 . The control device  300  performs control to apply the second developing voltage smaller than the absolute value of the first developing voltage to the developing roller  41  when the paper non-passing region in which a second surface potential has been formed in the charging portion on the surface of the photosensitive drum  1  faces the developing roller  41 . Thus, a developing potential (Vdc2) has been formed in the developing roller  41  when the paper non-passing region in which a surface potential has been formed faces the developing roller  41 . Since the paper non-passing region is positioned downstream of the paper passing region in the rotating direction of the photosensitive drum  1 , the developing potential (Vdc1) is formed in the developing roller  41  after the developing potential (Vdc2) is formed in the developing roller  41 . The control device  300  determines the value of the first developing voltage on the basis of the surface potential of the paper passing region, and determines the value of the second developing voltage on the basis of the surface potential of the paper non-passing region. The details of the value of the first developing voltage and the value of the second developing voltage will be described later. 
     The relationship between the potentials of the respective members in the first embodiment will be described using  FIG.  4   . 
     Charging Potential 
     Charging Potential to Paper Passing Region 
     As a first charging voltage, a DC voltage of −900 V is applied to the charging roller  2  from the charging high-voltage power supply  303 . Thus, for example, under an environment at a temperature of 25° C. and a relative humidity of 60%, the paper passing region is charged at a surface potential Vd1 of −400 V. As shown in  FIG.  4   , a charging potential (Vpri 1 ) of −900 V is formed in the charging roller  2  when a first charging voltage is applied to the charging roller  2 , and a surface potential (Vd1) of −400 V is formed in the paper passing region. 
     After the surface of the photosensitive drum  1  is charged by the charging roller  2 , the charging surface of the photosensitive drum  1  moves to the laser irradiation position of the laser scanner  3 , and the charging surface of the photosensitive drum  1  is exposed by the laser beam E for image formation. Thus, a light portion potential VL (image portion potential) developed by toner in a subsequent developing step is formed in the photosensitive drum  1 . The light portion potential VL is −100 V. 
     Charging Potential to Paper Non-Passing Region 
     As a second charging voltage, a DC voltage of −850 V is applied to the charging roller  2  from the charging high-voltage power supply  303 . Thus, for example, under an environment at a temperature of 25° C. and a relative humidity of 60%, the surface of the photosensitive drum  1  is charged at a surface potential Vd2 of −350 V. As shown in  FIG.  4   , a charging potential (Vpri 2 ) of −850 V is formed in the charging roller  2  when a second charging voltage is applied to the charging roller  2 , and a surface potential (Vd2) of −350 V is formed in the paper non-passing region. The absolute value of the surface potential (Vd1) of the paper passing region is larger than that of the surface potential (Vd2) of the paper non-passing region. 
     Developing Voltage 
     An AC voltage is applied to the developing roller  41  as a developing voltage. For example, when the developing roller  41  is arranged in a non-contact state with respect to the photosensitive drum  1  (jumping development method) like the configuration of the present embodiment, an AC voltage is applied to the developing roller  41 . In the first embodiment, an AC voltage having a duty of 50%, a frequency of 3 kHz, and a peak-to-peak voltage of 1750 V is applied from the developing high-voltage power supply  304  to the developing roller  41 . The AC voltage in the first embodiment is an actual value. By controlling the duty ratio and/or the peak-to-peak voltage of the AC voltage, the control device  300  controls a developing voltage applied to the developing roller  41 . Further, by superimposing a DC voltage on the AC voltage, the control device  300  may control the developing voltage applied to the developing roller  41 . 
     Developing Potential to Paper Passing Region 
     As shown in  FIG.  4   , a developing potential (Vdc1) of −320 V is formed in the developing roller  41  when a developing voltage (first developing voltage) of −320 V is applied to the developing roller  41 . Thus, the developing potential (Vdc1) of −320 V is formed in the developing roller  41  when the paper passing region faces the developing roller  41 . 
     Developing Potential to Paper Non-Passing Region 
     As shown in  FIG.  4   , a developing potential (Vdc2) of −270 V is formed in the developing roller  41  when a developing voltage (second developing voltage) of −270 V is applied to the developing roller  41 . Thus, the developing potential (Vdc2) of −270 V is formed in the developing roller  41  when the paper non-passing region faces the developing roller  41 . 
     The absolute value of the first developing voltage is larger than that of the second developing voltage. The control device  300  controls the developing high-voltage power supply  304  so that the absolute value of the first developing voltage becomes larger than that of the second developing voltage. The absolute value of the developing potential (Vdc1) is larger than that of the developing potential (Vdc2). The surface potentials (Vd1 and Vd2), the charging potentials (Vpri 1  and Vpri 2 ), and the developing potentials (Vdc1 and Vdc2) have the same polarity. The first and second charging voltages and the first and second developing voltages have the same polarity. The polarity of each of the surface potentials (Vd1 and Vd2) and the first and second developing voltages may be a normal polarity. In addition, the polarity of each of respective potentials and respective voltages may be a normal polarity. 
       FIG.  5    is a graph showing the relationship between a potential difference (Vback) between a surface potential (Vd) of the photosensitive drum  1  and a developing potential (Vdc) of the developing roller  41  and a fogging amount on the photosensitive drum  1 . In  FIG.  5   , a vertical axis shows the fogging amount on the photosensitive drum  1 , and a horizontal axis shows the potential difference (Vback) between the surface potential (Vd) of the photosensitive drum  1  and the developing potential (Vdc) of the developing roller  41 . Here, a method for measuring the fogging amount on the photosensitive drum  1  will be described. The following measurement method is only an example, and the fogging amount on the photosensitive drum  1  may be measured by another measurement method. First, the surface of the photosensitive drum  1  is taped up by a polyester adhesive tape, and fogging toner on the surface of the photosensitive drum  1  is peeled off by the adhesive tape. The peeled-off adhesive tape is affixed to a white plate, and the whiteness degree of the white plate to which the adhesive tape has been affixed is measured by a white photometer TS-6DS/A (manufactured by Tokyo Denshoku Co. Ltd.). Meanwhile, an adhesive tape is directly affixed to the white plate, and the whiteness degree of the white plate to which the adhesive tape has been affixed is also measured. The whiteness degree (first whiteness degree) of the adhesive tape (first adhesive tape) that has been affixed to the white plate after taping up the surface of the photosensitive drum  1  and the whiteness degree (second whiteness degree) of the adhesive tape (second adhesive tape) that has been directly affixed to the white plate are compared with each other. When the area of the first adhesive tape and the area of the second adhesive tape are the same, it is possible to calculate the first whiteness degree (%) by (the area of the first adhesive tape— the area of the toner in the first adhesive tape)/the area of the first adhesive tape. In this case, the second whiteness degree (%) is 100%. A difference between the first whiteness degree (%) and the second whiteness degree (%) is assumed as a fogging amount. Further, the fogging amount may be the ratio of the area of the toner in the first adhesive tape to the area of the first adhesive tape. As described above, the fogging amount on the photosensitive drum  1  may be the ratio of the area of toner in a prescribed region out of the surface region of the photosensitive drum  1  to the area of the prescribed region out of the surface region of the photosensitive drum  1 . 
     It is shown by  FIG.  5    that fogging is most favorable at about a potential difference (Vback) of 80 V in the first embodiment and becomes worse at a potential difference (Vback) of 80 V or smaller or 80 V or larger. The fogging has a range in which the quality of an image is allowable. In the first embodiment, a case in which the fogging amount is 3% is assumed as a limitation point at which the quality of an image is allowable. The fogging amount is measured in the manner described above. In the first embodiment, there is no problem in the quality of an image when the fogging amount is not more than 2%. When the potential difference (Vback) is at least 75 V and not more than 120 V, it is possible to obtain an image with favorable quality. 
     During image formation (paper passing state), respective potentials may be preferably set so that a potential difference (Vback 1 =|Vd1−Vdc1|) between a surface potential (Vd1) and a developing potential (Vdc1) of the paper passing region becomes at least 75 V and not more than 120 V. In the first embodiment, respective potentials are set so that a potential difference (Vback 1 ) serving as a first potential difference becomes 80 V. For example, when the surface potential (Vd1) of the paper passing region is −400 V during image formation, the developing potential (Vdc1) is set at −320 V. Thus, it is possible to reduce the fogging of the paper passing region during image formation. Further, during non-image formation (paper non-passing state), respective potentials may be preferably set so that a potential difference (Vback 2 =|Vd2−Vdc2|) between a surface potential (Vd2) and a developing potential (Vdc2) of the paper non-passing region becomes at least 75 V and not more than 120 V. In the first embodiment, respective potentials are set so that a potential difference (Vback 2 ) serving as a second potential difference becomes 80 V. For example, when the surface potential (Vd2) of the paper non-passing region is −350 V during non-image formation, the developing potential (Vdc2) is set at −270 V. Thus, it is possible to reduce the fogging of the paper passing region during image formation. 
     The control device  300  may control the charging high-voltage power supply  303  and the developing high-voltage power supply  304  so that a potential difference (Vback 1 ), at which the ratio of the area of toner in the paper passing region to the area of the paper passing region becomes not more than 2%, is formed. The control device  300  may control the charging high-voltage power supply  303  and the developing high-voltage power supply  304  so that a potential difference (Vback 1 ), at which the ratio of the area of toner in the paper passing region to the area of the paper passing region falls within a range in which the quality of an image is allowable, is formed. The control device  300  may control the charging high-voltage power supply  303  and the developing high-voltage power supply  304  so that a potential difference (Vback 2 ), at which the ratio of the area of toner in the paper non-passing region to the area of the paper non-passing region becomes not more than 2%, is formed. The control device  300  may control the charging high-voltage power supply  303  and the developing high-voltage power supply  304  so that a potential difference (Vback 2 ), at which the ratio of the area of toner in the paper non-passing region to the area of the paper non-passing region falls within a range in which the quality of an image is allowable, is formed. 
       FIG.  6    shows the waveforms of respective potentials in the first embodiment. In  FIG.  6   , a Y-axis (vertical axis) shows a potential. In  FIG.  6   , the waveforms of the respective potentials before and after the tip end of the recording paper P enters the transfer nip portion Nt are shown. The timings at which the respective potentials are output are shown with their phases matching to the surface of the photosensitive drum  1 . A coordinate 0 in an X-axis shows the timing at which the tip end of the recording paper P enters the transfer nip portion Nt. A charging potential (Vpri 2 ) is formed in the charging roller  2  when a second charging voltage is applied to the charging roller  2 , and a surface potential (Vd2) is formed in the paper non-passing region. The second charging voltage is applied to the charging roller  2  so that the surface potential (Vd2) is formed in the paper non-passing region. That is, the second charging voltage is applied to the charging roller  2  so that the surface potential (Vd2) is formed in the surface region (paper non-passing region) of the photosensitive drum  1  at the transfer nip portion Nt when the recording paper P does not exist at the transfer nip portion Nt. 
     A developing voltage is turned on 100 milliseconds before the recording paper P enters the transfer nip portion Nt. At this time, a developing potential (Vdc2) is formed in the developing roller  41  when a second developing voltage is applied to the developing roller  41 . Accordingly, a potential difference (Vback 2 =|Vd2−Vdc2|) between the surface potential (Vd2) of the paper non-passing region when the paper non-passing region faces the developing roller  41  and the developing potential (Vdc2) when the paper non-passing region faces the developing roller  41  becomes 80 V. Thus, the fogging of the paper non-passing region is reduced. Note that time in the X-axis (horizontal axis) of  FIG.  6    shows the timing at which the second developing voltage is applied to the developing roller  41  and does not show the timing at which the second developing voltage is switched to the first developing voltage. Further, the timing at which the second developing voltage is applied to the developing roller  41  is not limited to the timing 100 milliseconds before the recording paper P enters the transfer nip portion Nt but may be another timing. The second developing voltage may be applied to the developing roller  41  a prescribed time (for example, 100 milliseconds) after the timing at which the tip end of the paper non-passing region (the rear end of the paper passing region) on a downstream side in the rotating direction of the photosensitive drum  1  reaches a position facing the developing roller  41 . 
     The second charging voltage is switched to the first charging voltage so that a surface potential (Vd1) is formed in the paper passing region, and the first charging voltage is applied to the charging roller  2 . That is, the first charging voltage is applied to the charging roller  2  so that the surface potential (Vd1) is formed in the surface region (paper passing region) of the photosensitive drum  1  at the transfer nip portion Nt when the recording paper P exists at the transfer nip portion Nt. The first charging voltage is applied to the charging roller  2 , a charging potential (Vpri 1 ) is formed in the charging roller  2 , and the surface potential (Vd1) is formed in the paper passing region. Like this, the surface potential (Vd2) is switched to the surface potential (Vd1) according to the timing (X coordinate: 0) at which the charging potential (Vpri 2 ) is switched to the charging potential (Vpri 1 ) and the recording paper P enters the transfer nip portion Nt. 
     The control device  300  switches the second developing voltage to the first developing voltage at the timing at which the tip end of the paper passing region on the downstream side in the rotating direction of the photosensitive drum  1  reaches the position facing the developing roller  41 . Accordingly, a potential difference (Vback 1 =|Vd1−Vdc1|) between the surface potential (Vd1) of the paper passing region when the paper passing region faces the developing roller  41  and the developing potential (Vdc1) when the paper passing region faces the developing roller  41  becomes 80 V. Thus, the fogging of the paper passing region is reduced. The timing at which the second developing voltage is switched to the first developing voltage is not limited to the timing at which the tip end of the paper passing region reaches the position facing the developing roller  41 . The control device  300  may switch the second developing voltage to the first developing voltage before the timing at which the tip end of the paper passing region reaches the position facing the developing roller  41 . For example, the control device  300  may switch the second developing voltage to the first developing voltage 10 milliseconds before the timing at which the tip end of the paper passing region reaches the position facing the developing roller  41 . 
     Confirmation of Effect 
     The image forming apparatus  100  of the first embodiment switches between a first charging voltage for forming the surface potential (Vd1) of the paper passing region and a second charging voltage for forming the surface potential (Vd2) of the paper non-passing region. Further, the image forming apparatus  100  applies a second developing voltage to the developing roller  41  when the paper non-passing region faces the developing roller  41 . That is, the image forming apparatus  100  applies the second developing voltage to the developing roller  41  at prescribed timing after the surface potential (Vd2) is formed in the paper non-passing region. Then, the image forming apparatus  100  applies a first developing voltage to the developing roller  41  when the paper passing region faces the developing roller  41 . That is, the image forming apparatus  100  applies the first developing voltage to the developing roller  41  at prescribed timing after the surface potential (Vd2) is formed in the paper non-passing region. A developing potential (Vdc1) is formed in the developing roller  41  when the first developing voltage is applied to the developing roller  41 . Between the developing potential (Vdc1) and the surface potential (Vd1) of the paper passing region, a prescribed potential difference (Vback 1 ) is secured. The developing potential (Vdc2) is formed in the developing roller  41  when the second developing voltage is applied to the developing roller  41 . Between the developing potential (Vdc2) and the surface potential (Vd2) of the paper non-passing region, a prescribed potential difference (Vback 2 ) is secured. 
     As comparative example 1 and comparative example 2, image forming apparatuses that switch and apply any of first and second charging voltages to a charging roller  2  are used. The image forming apparatus of the comparative example 1 applies a first developing voltage to a developing roller  41  but does not apply a second developing voltage to the developing roller  41  when a paper passing region and a paper non-passing region face the developing roller  41 . The image forming apparatus of the comparative example 2 applies a second developing voltage to a developing roller  41  but does not apply a first developing voltage to the developing roller  41  when a paper passing region and a paper non-passing region face the developing roller  41 . 
     Experiment 1 
     As the experimental method of experiment 1, two A4 recording papers P are sequentially fed, and a fogging amount of the paper passing region and a fogging amount of the paper non-passing region are measured. A method for measuring the fogging amounts of the paper passing region and the paper non-passing region is the same as the method for measuring a fogging amount on the photosensitive drum  1 . Table 1 shows the results of the experiment 1. In table 1, a difference between a first whiteness degree (%) and a second whiteness degree (%) is shown as a fogging amount. Further, the fogging amount of the paper passing region may be the ratio of the area of toner in the paper passing region to the area of the paper passing region. The fogging amount of the paper non-passing region may be the ratio of the area of toner in the paper non-passing region to the area of the paper non-passing region. The lower the fogging amount of the paper passing region, the better the quality of an image is. Specifically, the fogging amount of the paper passing region is preferably not more than 3%. In the first embodiment, respective potentials are set so that the fogging amount of the paper passing region becomes not more than 3%. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Fogging amount of 
                 Fogging amount of 
               
               
                   
                 paper passing region 
                 paper non-passing region 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 First Embodiment 
                 0.5% 
                 0.5% 
               
               
                 Comparative Example 1 
                 0.5% 
                  10% 
               
               
                 Comparative Example 2 
                 3.1% 
                 0.5% 
               
               
                   
               
            
           
         
       
     
     In the comparative example 1, the fogging amount of the paper non-passing region is large since a potential difference (Vback) between the surface potential (Vd2) of the paper non-passing region and the developing potential (Vdc1) of the developing roller  41  is not properly secured. In the comparative example 2, the fogging amount of the paper passing region is large since a potential difference (Vback) between the surface potential (Vd1) of the paper passing region and the developing potential (Vdc2) of the developing roller  41  is not properly secured. On the other hand, it is possible to reduce the fogging amounts of the paper passing region and the paper non-passing region in the first embodiment since the potential differences (Vback 1  and Vback 2 ) are properly securable. 
     The above description shows an example in which an AC voltage is applied to the developing roller  41  as a developing voltage. However, a DC voltage may be applied to the developing roller  41  as a developing voltage. For example, when the developing roller  41  is arranged so as to be in contact with the photosensitive drum  1  (contact development method), a DC voltage is applied to the developing roller  41 . A DC voltage of −320 V may be applied as a first developing voltage from the developing high-voltage power supply  304  to the developing roller  41 . Thus, a developing potential (Vdc1) of −320 V is formed in the developing roller  41  when the paper passing region faces the developing roller  41 . A DC voltage of −270 V may be applied as a second developing voltage from the developing high-voltage power supply  304  to the developing roller  41 . Thus, a developing potential (Vdc2) of −270 V is formed in the developing roller  41  when the paper non-passing region faces the developing roller  41 . In this case, the DC voltage applied as a second developing voltage has the same polarity as that of the DC voltage applied as a first developing voltage. Further, the absolute value of the first developing voltage is larger than that of the second developing voltage. Further, a developing voltage obtained by superimposing an AC voltage on a DC voltage may be applied to the developing roller  41 . 
     Second Embodiment 
     An image forming apparatus  100  according to a second embodiment will be described. Note that since configurations other than configurations described below are similar to those of the first embodiment, their detailed descriptions will be omitted. 
     In the second embodiment, a state in which a recording paper P exists at a transfer nip portion Nt will be called a paper passing state. The surface region of a photosensitive drum  1  has at least a first region, a second region, and a third region. The first region and the third region of the surface region of the photosensitive drum  1  come into contact with the recording paper P at the transfer nip portion Nt. In the second embodiment, a region including the first region and the third region of the photosensitive drum  1  will be called a paper passing region, the first region of the photosensitive drum  1  will be called a first paper passing region, and the third region of the photosensitive drum  1  will be called a second paper passing region. In the second embodiment, a state in which the recording paper P before reaching the transfer nip portion Nt does not exist at the transfer nip portion Nt will be called a paper non-passing state. The second region of the surface region of the photosensitive drum  1  does not come into contact with the recording paper P at the transfer nip portion Nt. In the second embodiment, the second region of the photosensitive drum  1  will be called a paper non-passing region. The paper non-passing region is positioned downstream of the paper passing region in the rotating direction of the photosensitive drum  1 . The second paper passing region is positioned between the first paper passing region and the paper non-passing region. Accordingly, the first paper passing region is separated from the paper non-passing region, and the second paper passing region is continuous with the first paper passing region and the paper non-passing region. The second paper passing region is the region of the photosensitive drum  1  that is positioned between the first paper passing region and the paper non-passing region and does not come into contact with the recording paper P at the transfer nip portion Nt in a state in which a transfer voltage is applied to a transfer roller  5  and the recording paper P is held by the transfer nip portion Nt. 
     The image forming apparatus  100  of the second embodiment switches between a first charging voltage for forming a surface potential (Vd1) of the first paper passing region and a second charging voltage for forming a surface potential (Vd2) of the paper non-passing region. The image forming apparatus  100  applies a second developing voltage to a developing roller  41  when the paper non-passing region in which a surface potential has been formed faces the developing roller  41 , and then applies a first developing voltage to the developing roller  41  when the paper passing region in which a surface potential has been formed faces the developing roller  41 . In addition, the second embodiment is characterized in that a charging voltage with consideration given to an overshoot occurring during the switching of a developing voltage is used. Specifically, the second embodiment is characterized in that a third charging voltage for securing a developing potential overshooting at the switching timing of a developing voltage and a potential difference (Vback 3 ) serving as a third potential difference that does not cause fogging is used. 
     When a first charging voltage is applied to the charging roller  2  and the first paper passing region is charged by the charging roller  2 , a surface potential (first surface potential) is formed in the first paper passing region. When a second charging voltage is applied to the charging roller  2  and the paper non-passing region is charged by the charging roller  2 , a surface potential (second surface potential) is formed in the paper non-passing region. When a third charging voltage is applied to the charging roller  2  and the second paper passing region is charged by the charging roller  2 , a surface potential (third surface potential) is formed in the second paper passing region. The paper non-passing region is positioned downstream of the paper passing region in the rotating direction of the photosensitive drum  1 , and the second paper passing region is positioned between the first paper passing region and the paper non-passing region. Therefore, a surface potential is first formed in the paper non-passing region, and then a surface potential is formed in the second paper passing region. After that, a surface potential is formed in the first paper passing region. The absolute value of the surface potential of the first paper passing region is larger than that of the surface potential of the paper non-passing region. The absolute value of the surface potential of the second paper passing region is larger than those of the surface potentials of the first paper passing region and the paper non-passing region. 
     When a second developing voltage is switched to a first developing voltage at the timing at which the tip end of the paper passing region reaches a position facing the developing roller  41  like the first embodiment, a developing potential may overshoot before turning into a desired potential. An overshooting developing potential (Vdc3) becomes larger than a target developing potential (for example, Vdc1). In this case, a potential difference (Vback) between the surface potential (Vd1) of the paper passing region and the developing potential (Vdc3) becomes smaller than expected. Therefore, band-shaped toner is formed on the photosensitive drum  1  in some cases, and an image failure may occur. Note that although a charging potential overshoots during the switching of a charging voltage, the overshoot of the charging potential does not exert a large influence upon the surface potential of the photosensitive drum  1  due to the volume resistance of the charging roller  2 . 
       FIG.  7    shows waveforms formed when developing potentials rise. At the moment at which a second developing voltage is switched to a first developing voltage, a developing potential (Vdc1) overshoots and increases by about 30 V in the second embodiment. In this case, the second paper passing region in which a surface potential has been formed faces the developing roller  41 . The developing potential (Vdc1) overshoots when the second paper passing region in which the surface potential has been formed faces the developing roller  41  and when the second developing voltage is switched to the first developing voltage. As a result, a developing potential (Vdc3) is formed in the developing roller  41 . The developing potential (Vdc3) is an example of a third developing potential. The absolute value of the developing potential (Vdc3) is larger than those of the developing potentials (Vdc1) and (Vdc2). Note that the overshooting developing potential (Vdc3) gradually decreases and converges into the target developing potential (Vdc2) after a prescribed time (for example, after 50 milliseconds). 
     When the developing potential (Vdc1) overshoots, a potential difference (Vback 1 ) between the surface potential (Vd1) and the developing potential (Vdc1) instantaneously decreases down to 50 V. As in the first embodiment, when the second developing voltage is switched to the first developing voltage so that the potential difference (Vback 1 ) becomes 80 V, the band of fogging occurs on an image since the potential difference (Vback 1 ) instantaneously decreases. The relationship between the potential difference (Vback) between the surface potential (Vd) of the photosensitive drum  1  and the developing potential (Vdc) of the developing roller  41  and a fogging amount on the photosensitive drum  1  in the second embodiment is also shown in  FIG.  5   . Accordingly, like the first embodiment, the range of the potential difference (Vback 1 ) for obtaining an excellent-quality image with a fogging amount of not more than 2% becomes at least 105 V and not more than 120 V when the overshoot of the developing potential (Vdc1) is considered in the second embodiment. As described above, when an overshoot occurs, the range of the potential difference (Vback 1 ) where the overshoot does not occur narrows. On the other hand, a charging voltage is increased in advance at the timing at which the developing potential (Vdc1) overshoots in the second embodiment, whereby it is possible to reduce the band of fogging on an image. 
     The relationship between the potentials of respective members in the second embodiment will be described using  FIG.  8   . At the timing at which a second charging voltage is switched to a first charging voltage, a third charging voltage with consideration given to an overshoot is applied to the charging roller  2  for 50 milliseconds. Like this, the third charging voltage is applied to the charging roller  2  according to the timing at which a developing potential (Vdc1) overshoots. When the developing potential (Vdc1) overshoots and increases by 30 V, a third charging voltage is applied to the charging roller  2  so that a charging potential (Vpri 3 ) of the charging roller  2  becomes −930 V. For example, a DC voltage of −930 V is applied as a third charging voltage from a charging high-voltage power supply  303  to the charging roller  2 . The charging potential (Vpri 3 ) is an example of a third charging potential. The absolute value of the charging potential (Vpri 3 ) is larger than those of charging potentials (Vpri 1 ) and (Vpri 2 ). The absolute value of the third charging voltage is larger than those of the first and second charging voltages. 
     A charging potential (Vpri 3 ) of −930 V is higher than a charging potential (Vpri 2 ) of −900 V by 30 V toward a negative side. Thus, the surface potential (Vd3) of the photosensitive drum  1  at the timing at which an overshoot occurs becomes −430 V. A developing potential (Vdc3) at which an overshoot occurs is −350 V. Accordingly, a potential difference (Vback 3 ) between the surface potential (Vd3) of the photosensitive drum  1  and the developing potential (Vdc3) of the developing roller  41  at the timing at which an overshoot occurs becomes 80 V. Thus, it is possible to reduce the band of fogging on an image. The surface potentials (Vd1, Vd2, and Vd3), the charging potentials (Vpri 1 , Vpri 2 , and Vpri 3 ), and the developing potentials (Vdc1, Vdc2, and Vdc3) have the same polarity. The first, second, and third charging voltages and the first and second developing voltages have the same polarity. The polarity of each of the surface potentials (Vd1, Vd2, and Vd3) and the first and second developing voltages may be a normal polarity. In addition, the polarity of each of respective potentials and respective voltages may be a normal polarity. 
     Like the first embodiment, it is possible to obtain an excellent-quality image when a potential difference (Vback) is at least 75 V and not more than 120 V in the second embodiment. Accordingly, respective potentials are preferably set so that a potential difference (Vback 3 ) becomes at least 75 V and not more than 120 V. Note that the second embodiment describes a case in which a developing potential (Vdc1) overshoots and increases by about 30 V but an increase in the developing potential (Vdc1) is not limited to 30 V. An overshoot amount is different in some cases depending on the performance or the configuration of a developing high-voltage power supply  304 , or varies in some cases depending on a switching amount (|Vdc1−Vdc2|) of a developing potential. A charging potential (Vpri 3 ) with consideration given to an overshoot may be set depending on the performance or the configuration of the developing high-voltage power supply  304  and depending on a switching amount of a developing potential. The control device  300  may control the charging high-voltage power supply  303  and the developing high-voltage power supply  304  so that a potential difference (Vback 3 ), at which the ratio of the area of toner in the second paper passing region to the area of the second paper passing region becomes not more than 2%, is formed. The control device  300  may control the charging high-voltage power supply  303  and the developing high-voltage power supply  304  so that a potential difference (Vback 3 ), at which the ratio of the area of toner in the second paper passing region to the area of the second paper passing region falls within a range in which the quality of an image is allowable, is formed. 
       FIG.  9    shows the waveforms of respective potentials in the second embodiment. In  FIG.  9   , a Y-axis (vertical axis) shows a potential. In  FIG.  9   , the waveforms of the respective potentials before and after the tip end of the recording paper P enters the transfer nip portion Nt are shown. The timings at which the respective potentials are output are shown with their phases matching to the surface of the photosensitive drum  1 . A coordinate 0 in an X-axis shows the timing at which the tip end of the recording paper P enters the transfer nip portion Nt. A charging potential (Vpri 2 ) is formed in the charging roller  2  when a second charging voltage is applied to the charging roller  2 , and a surface potential (Vd2) is formed in the paper non-passing region. That is, the second charging voltage is applied to the charging roller  2  so that the surface potential (Vd2) is formed in the surface region (paper non-passing region) of the photosensitive drum  1  at the transfer nip portion Nt when the recording paper P does not exist at the transfer nip portion Nt. 
     A developing voltage is turned on 100 milliseconds before the recording paper P enters the transfer nip portion Nt. At this time, a developing potential (Vdc2) is formed in the developing roller  41  when a second developing voltage is applied to the developing roller  41 . Accordingly, a potential difference (Vback 2 =|Vd2−Vdc2|) between the surface potential (Vd2) of the paper non-passing region when the paper non-passing region faces the developing roller  41  and the developing potential (Vdc2) when the paper non-passing region faces the developing roller  41  becomes 80 V. Thus, the fogging of the paper non-passing region is reduced. Note that time in the X-axis (horizontal axis) of  FIG.  9    shows the timing at which the second developing voltage is applied to the developing roller  41  and does not show the timing at which the second developing voltage is switched to the first developing voltage. 
     The control device  300  switches the second developing voltage to the first developing voltage at the timing at which the tip end of the paper passing region faces the developing roller  41 . At this time, a developing potential (Vdc1) overshoots. That is, the developing potential (Vdc1) overshoots to a developing potential (Vdc3) higher than the developing potential (Vdc2) at the timing at which the second developing voltage is switched to the first developing voltage, and then converges into the developing potential (Vdc2). In view of this, a third charging voltage with consideration given to the occurrence of an overshoot is applied to the charging roller  2  at the timing at which the tip end of the paper passing region faces the developing roller  41 . Thus, a surface potential (Vd3) is formed in the second paper passing region (overshoot region) at the timing at which a developing potential (Vdc) overshoots. Accordingly, a potential difference (Vback 3 =|Vd3−Vdc3|) between the surface potential (Vd3) of the second paper passing region and the developing potential (Vdc3) of the developing roller  41  becomes 80 V at the timing at which the developing potential (Vdc) overshoots. In other words, a potential difference between the surface potential (Vd3) of the second paper passing region when the second paper passing region faces the developing roller  41  and the developing potential (Vdc3) when the second paper passing region faces the developing roller  41  becomes 80 V. Therefore, the fogging of the second paper passing region is reduced, and the band of fogging on an image is reduced. As a result, it is possible to reduce a toner consumption amount. 
     After that, the control device  300  switches the third charging voltage to a first charging voltage since the overshoot of the developing potential (Vdc1) converges. For example, the control device  300  switches the third charging voltage to the first charging voltage 50 milliseconds after the timing at which the tip end of the paper passing region on the downstream side of the photosensitive drum  1  faces the charging roller  2 . The timing at which the control device  300  switches the third charging voltage to the first charging voltage may be arbitrary timing. When the first charging voltage is applied to the charging roller  2 , a charging potential (Vpri 1 ) is formed in the charging roller  2  and a surface potential (Vd1) is formed in the first paper passing region. Accordingly, a potential difference (Vback 1 =|Vd1−Vdc1|) between the surface potential (Vd1) of the first paper passing region when the first paper passing region faces the developing roller  41  and the developing potential (Vdc1) when the first paper passing region faces the developing roller  41  becomes 80 V. Therefore, the fogging of the first paper passing region is reduced. As a result, it is possible to reduce a toner consumption amount. 
     According to the second embodiment, it is possible to retain a potential difference (Vback) at the timing at which an overshoot occurs at a value at which fogging does not occur even if the overshoot occurs when a developing potential (Vdc) is switched. Therefore, it is possible to reduce the band of fogging on an image due to the overshoot of a developing potential (Vdc). 
     In the above description, a potential difference between a charging potential (Vpri 1 ) and a charging potential (Vpri 3 ) and a potential difference between a developing potential (Vdc1) and a developing potential (Vdc3) are the same. However, the potential difference between the charging potential (Vpri 1 ) and the charging potential (Vpri 3 ) may be larger than the potential difference between the developing potential (Vdc1) and the developing potential (Vdc3) so long as fogging does not occur. That is, the potential difference between the charging potential (Vpri 1 ) and the charging potential (Vpri 3 ) may be at least the potential difference between the developing potential (Vdc1) and the developing potential (Vdc3). Further, the potential difference between the charging potential (Vpri 1 ) and the charging potential (Vpri 3 ) may be smaller than the potential difference between the developing potential (Vdc1) and the developing potential (Vdc3) so long as fogging does not occur. That is, the potential difference between the charging potential (Vpri 1 ) and the charging potential (Vpri 3 ) may be not more than the potential difference between the developing potential (Vdc1) and the developing potential (Vdc3). 
     In the first and second embodiments, a potential difference between a charging potential (Vpri 1 ) and a charging potential (Vpri 2 ) is set at 50 V. However, the potential difference between the charging potential (Vpri 1 ) and the charging potential (Vpri 2 ) may be set at a value other than 50 V when it is possible to reduce a memory. Accordingly, a difference between a first charging voltage and a second charging voltage may be set at a value other than 50 V when it is possible to reduce a memory. 
     In the first and second embodiments, a potential difference (50 V) between a charging potential (Vpri 1 ) and a charging potential (Vpri 2 ) is the same as a potential difference (50 V) between a developing potential (Vdc1) and a developing potential (Vdc2). When it is possible to reduce the fogging of the paper passing region and the paper non-passing region, the potential difference between the charging potential (Vpri 1 ) and the charging potential (Vpri 2 ) may be different from the potential difference between the developing potential (Vdc1) and the developing potential (Vdc2). For example, the potential difference between the charging potential (Vpri 1 ) and the charging potential (Vpri 2 ) may be larger than the potential difference between the developing potential (Vdc1) and the developing potential (Vdc2). For example, the potential difference between the charging potential (Vpri 1 ) and the charging potential (Vpri 2 ) may be smaller than the potential difference between the developing potential (Vdc1) and the developing potential (Vdc2). 
     When it is possible to reduce the fogging of the paper passing region and the paper non-passing region to such an extent that does not cause a practical problem, potential differences (Vback 1 ) and (Vback 2 ) may be set at values other than 80 V. For example, the potential differences (Vback 1 ) and (Vback 2 ) may be set so as to fall within a prescribed range. In this case, the control device  300  controls the charging high-voltage power supply  303  and the developing high-voltage power supply  304  so that the potential differences (Vback 1 ) and (Vback 2 ) fall within the prescribed range. The prescribed range is, for example, at least 75 V and not more than 120 V. The potential difference (Vback 1 ) may be the same as or different from the potential difference (Vback 2 ). When the potential differences (Vback 1 ) and (Vback 2 ) are at least 75 V and not more than 120 V, it is possible to reduce fogging amounts of the paper passing region and the paper non-passing region to not more than 2%. Accordingly, the control device  300  controls the potential difference (Vback 1 ) and the potential difference (Vback 2 ) so that fogging amounts of the paper passing region and the paper non-passing region become not more than 2% (prescribed value). As a result, the fogging of the paper passing region and the paper non-passing region is reduced to such an extent that does not cause a practical problem. 
     When it is possible to reduce the fogging of the second paper passing region to such an extent that does not cause a practical problem, a potential difference (Vback 3 ) may be set at a value other than 80 V. For example, the potential difference (Vback 3 ) may be set so as to fall within a prescribed range. In this case, the control device  300  controls the charging high-voltage power supply  303  and the developing high-voltage power supply  304  so that the potential difference (Vback 3 ) falls within the prescribed range. The prescribed range is, for example, at least 75 V and not more than 120 V. The potential difference (Vback 3 ) may be the same as the potential differences (Vback 1 ) and (Vback 2 ), may be different from the potential difference (Vback 1 ), or may be different from the potential difference (Vback 2 ). When the potential difference (Vback 3 ) is at least 75 V and not more than 120 V, it is possible to reduce a fogging amount of the second paper passing region to not more than 2%. Accordingly, the control device  300  controls the potential difference (Vback 3 ) so that a fogging amount of the second paper passing region becomes not more than 2% (prescribed value). As a result, the fogging of the second paper passing region is reduced to such an extent that does not cause a practical problem. The fogging amount of the second paper passing region may be the ratio of the area of toner in the second paper passing region to the area of the second paper passing region. 
     The number of switching times, a switching method, and switching timing for switching a charging voltage or a developing voltage may be arbitrarily set depending on the configuration of the image forming apparatus  100 . A roller charging system using a conductive roller (charging roller  2 ) is used in the first and second embodiments, but another charging system, for example, a corona charging system may be used. Even with another charging system, a similar effect is obtained. The image forming apparatus  100  is configured to include the cleaning apparatus  7  in the first and second embodiments, but a similar effect is obtained even with an image forming apparatus  100  of a cleaner-less system. 
     It is possible to reduce a memory occurring at a transfer nip and reduce fogging. 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. 2020-095756, filed on Jun. 1, 2020, which is hereby incorporated by reference herein in its entirety.