Abstract:
An image forming apparatus having image supporting members and developing devices provided for a plurality of colors respectively, and a developing bias source. The developing bias source includes DC power circuits provided for the colors respectively, an AC power circuit provided for use in common for all of the colors, and an element provided for at least one of the colors other than black. The developing bias source generates developing biases by superimposing an AC voltage output from the AC power circuit on DC voltages output from the DC power circuits and applies the developing biases to the respectively corresponding developing devices. The element maintains a constant voltage having a potential of an opposite polarity to a potential of charged toner used for the development. The developing bias source further superimposes the constant voltage on the developing bias for the at least one of the colors other than black.

Description:
This application claims benefit of priority to Japanese Patent Application No. 2014-055888 filed Mar. 19, 2014, the content of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus that develops a latent image on an image supporting member by applying a developing bias, which is a superimposed voltage of a DC voltage and an AC voltage, to a developing device. 
     2. Description of Related Art 
     As an image forming apparatus that develops a latent image on an image supporting member by applying a developing bias, which is a superimposed voltage of a DC voltage and an AC voltage, to a developing device, for example, an image forming apparatus disclosed in Japanese Patent Laid-Open Publication No. S61-166558 is known. Such an image forming apparatus (which will be hereinafter referred to as a conventional image forming apparatus) comprises image supporting members provided for a plurality of colors respectively, and developing devices provided for the plurality of colors respectively, and developing biases are applied to the respective developing devices for development of latent images formed on the image supporting members. The conventional image forming apparatus further comprises DC power circuits provided for the plurality of colors respectively to output DC components of the developing biases, and an AC power circuit for use in common to output AC components of the developing biases for all of the colors. 
     In the conventional image forming apparatus, however, the fact that the AC power circuit is used in common to output the AC components of the developing biases for all of the colors causes the following problem. Even in a monochromatic printing operation, toner movements from the developing devices for use in color printing to the corresponding image supporting members occur, which affects the picture quality of the developed image. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an image forming apparatus that reduces or prevents toner movements from the developing devices for use in color printing to the corresponding image supporting members although having an AC power circuit for use in common to output AC components of developing biases for all of the colors. 
     An image forming apparatus according to an embodiment of the present invention comprises: a plurality of image supporting members provided for a plurality of colors including black, respectively; a plurality of developing devices provided for the plurality of colors respectively and configured to develop latent images formed on the image supporting members respectively with charged toner while being impressed with developing biases; a developing bias source including a plurality of DC power circuits provided for the plurality of colors respectively and configured to output DC voltages, an AC power circuit for use in common for all of the colors and configured to output an AC voltage, and an element provided for at least one of the colors other than black and configured to maintain a constant voltage independently from the outputs from the DC power circuits, the developing bias source configured to generate the developing biases by superimposing the AC voltage output from the AC power circuit on the DC voltages output from the respective DC power circuits and to apply the developing biases to the respectively corresponding developing devices; wherein: the constant voltage has a potential of an opposite polarity to a potential of the charged toner; and the developing bias source further superimposes the constant voltage output from the element on at least one of the developing biases for the at least one of the colors other than black. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view illustrating the internal structure of an image forming apparatus. 
         FIG. 2  is a diagram illustrating an electric circuit of a developing bias source provided for an image forming apparatus according to a first embodiment and illustrating relation between the developing bias source and developing devices. 
         FIG. 3  is a diagram of an output voltage that is output from an AC power circuit of the developing bias source during a color printing operation. 
         FIG. 4  is a diagram of an output voltage that is output from a DC power circuit of the developing bias source during a color printing operation. 
         FIG. 5  is a diagram of an output voltage that is output from the developing bias source during a color printing operation, the output voltage being a superimposed voltage of the output voltage from the DC power circuit on the output voltage from the AC power circuit. 
         FIG. 6  is a diagram of a potential during a color printing operation at a node of the AC power circuit and the DC power circuit in an electric circuit of the developing bias source. 
         FIG. 7  is a diagram of a potential during a monochromatic printing operation at a node of the AC power circuit and the DC power circuit in the electric circuit of the developing bias source. 
         FIG. 8  is a block diagram illustrating relation among a control unit, the AC power circuit, the DC power circuit and a sheet ejection sensor. 
         FIG. 9  is a flowchart illustrating a control process of outputs from the power circuits of the developing bias source. 
         FIG. 10  is a diagram illustrating an electric circuit of a developing bias source provided for an image forming apparatus according to a second embodiment and illustrating relation between the developing bias source and developing devices. 
         FIG. 11  is a diagram illustrating an electric circuit of a developing bias source provided for an image forming apparatus according to a third embodiment and illustrating relation between the developing bias source and developing devices. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     General Structure of Image Forming Apparatus; See FIG.  1   
     An image forming apparatus  1 A according to a first embodiment is described below with reference to the drawings. In the drawings, the same members and parts are provided with the same reference marks, and the same descriptions will be omitted. 
     As illustrated in  FIG. 1 , the image forming apparatus  1 A according to the first embodiment is a tandem-type electrophotographic printer. The image forming apparatus  1 A comprises a control unit  4  configured to control the components and members of the image forming apparatus  1 A, imaging units  10  ( 10 Y,  10 M,  10 C and  10 K) configured to form toner images in colors of Y (yellow), M (magenta), C (cyan) and K (black) respectively, and an intermediate transfer unit  20 . 
     In each of the imaging units  10 , a charger  12 , a developing device  13  and other devices are arranged around a photoreceptor drum  11 . The photoreceptor drum  11  is exposed to light emitted from a laser scanning optical unit  14 , so that an electrostatic latent image is formed on the photoreceptor drum  11 , and negatively charged toner is moved from the developing device  13  to the photoreceptor drum  11  so as to develop the electrostatic latent image. In this way, toner images in the respective colors are formed. The intermediate transfer unit  20  includes an intermediate transfer belt  21  driven to rotate endlessly in a direction indicated by arrow W. By electric field applications from first transfer rollers  22  respectively opposed to the photoreceptor drums  11 , the toner images formed on the photoreceptor drums  11  are transferred onto the intermediate transfer belt  21  and combined thereon (first transfer), thereby forming into a composite toner image. Such an electrophotographic process is well known, and details are omitted. 
     In the lower section of the image forming apparatus body, an automatic sheet feeder unit  30  configured to feed sheets of a transfer substance (which will be referred to as sheets) one by one is provided. Each sheet is fed from a feed roller (not indicated in the drawings) to a nip portion between the intermediate transfer belt  21  and a second transfer roller  25  via a pair of timing rollers  31 , and by an electric field application from second transfer roller  25 , the toner image (composite color image) is transferred from the intermediate transfer belt  21  to the sheet. Thereafter, the sheet is fed to a fixing unit  35 , where toner is fixed on the sheet by heat, and then, the sheet is ejected to a tray section  2  on the upper surface of the apparatus body. 
     Structure of Developing Bias Source; See FIGS.  2 - 4   
     The image forming apparatus further comprises a developing bias source  40  as illustrated in  FIG. 2 . 
     The developing bias source  40  includes an AC power circuit  50  for use in common for the colors Y, M, C and K, DC power circuits  60  ( 60 Y,  60 M,  60 C and  60 K) provided for the colors Y, M, C and K respectively, four capacitors  56  provided for the colors Y, M, C and K respectively, resistors R 1  and R 2  provided for each of the colors Y, M, C and K, and zener diodes  70  ( 70 Y,  70 M,  70 C and  70 K) having a zener voltage of 200V and provided for the colors Y, M, C and K respectively. 
     In this embodiment, the AC power circuit  50  includes an AC power source  52  and an AC transformer  54 , and outputs an AC voltage AC (which will be hereinafter referred to as an AC output voltage) under control of the control unit  4 . Specifically, the AC power source  52  is connected to a primary side of the AC transformer  54 . The AC voltage from the AC power source  52  is raised by the AC transformer  54 , and is output as the AC output voltage AC of 1200 Vp-p as illustrated in  FIG. 3 . 
     In this embodiment, a secondary side of the AC transformer  54 , that is, the output port of the AC transformer  54  diverges into four, and the four ports are connected to respective one ends of the four capacitors  56  for the respective colors. Thereby, the AC output voltage AC is fed to the developing devices  13  via the capacitors  56 . Respective one ends of the resistors R 1  are connected to the other ends of the capacitors  56  so as to control the current flow to the developing devices  13 . 
     In this embodiment, each of the DC power circuits  60 Y,  60 M,  60 C and  60 K includes a DC power source  62  and a DC transformer  64 . Under control of the control unit  4 , DC voltages DCY, DCM, DCC and DCK of −400V (which will be hereinafter referred to as DC output voltages) as illustrated in  FIG. 4  are output from the DC power circuits  60 Y,  60 M,  60 C and  60 K respectively. In this regard, for the reasons that the toner characteristic varies from color to color and others, the potentials of the DC output voltages DC for the respective colors shall be individually adjusted by stabilization control. Therefore, the DC power circuits  60  are provided individually for the respective colors. Respective one ends of the resistors R 2  are connected to the output ports of the DC power circuits  60  so as to control the current flows to the developing devices  13 . 
     The anode terminals of the zener diodes  70 Y,  70 M,  70 C and  70 K are connected to the other ends of the resistors R 2 . In other words, the zener diodes  70 Y,  70 M,  70 C and  70 K are connected such that the directions from the respective DC circuits  60 Y,  60 M,  60 C and  60 K to the corresponding developing devices  13  are forward directions of the zener diodes  70 Y,  70 M,  70 C and  70 K. 
     The cathode terminals of the zener diodes  70 Y,  70 M,  70 C and  70 K are connected to nodes  42 Y,  42 M,  42 C and  42 K which the AC output voltages fed toward the developing devices  13  pass through respectively. At the nodes  42 Y,  42 M,  42 C and  42 K, the AC output voltage is superimposed on the DC output voltages DCY, DCM, DCC and DCK, and in this way, the developing biases  120 Y,  120 M,  120 C and  120 K for the respective colors are generated. 
     Details of Developing Biases; See FIGS.  2 ,  3 ,  5 - 7   
     For color printing, the DC power circuits  60 Y,  60 M,  60 C and  60 K output DC output voltages DCY, DCM, DCC and DCK of −400V. Simple superimposition of the AC output voltage AC of 1200 Vp-p on the DC output voltages DCY, DCM, DCC and DCK will result in AC output voltages AC having a maximum voltage of 200V and a minimum voltage of −1000V as illustrated in  FIG. 5 . In this embodiment, however, the zener diodes  70 Y,  70 M,  70 C and  70 K are provided between the respective output ports of the DC power circuits  60 Y,  60 M,  60 C and  60 K and the respective nodes  42 Y,  42 M,  42 C and  42 K. With this arrangement, the developing biases  120 Y,  120 M,  120 C and  120 K become AC voltages having a maximum voltage of 200V and a minimum voltage of −800V as illustrated in  FIG. 6 . 
     For monochromatic printing, on the other hand, it is not necessary to move toner from the developing devices  13  for the colors Y, M and C to the corresponding photoreceptor drums  11 , and the DC power sources  62  for the colors Y, M and C are turned off. Accordingly, the DC output voltages DCY, DCM and DCC are 0V. Meanwhile, since the AC power circuit  50  is used in common for the colors Y, M, C and K, an AC voltage of 1200 Vp-p as illustrated in  FIG. 3  is output from the AC power circuit  50  as the AC output voltage AC. In this embodiment, the zener diodes  70 Y,  70 M,  70 C and  70 K are provided between the respective output ports of the DC power circuits  60 Y,  60 M,  60 C and  60 K and the respective nodes  42 Y,  42 M,  42 C and  42 K. With the arrangement, AC voltages having a maximum voltage of 600V and a minimum voltage of −400V as illustrated in  FIG. 7  result as the developing biases  120 Y,  120 M and  120 C for the colors Y, M and C. Thus, because of the presence of the zener diodes  70 , the developing biases shift to the positive side, that is, in a direction opposite to the polarity (negative) of the toner. 
     Output Control of Developing Biases; See FIGS.  6 - 9   
     The image forming apparatus  1 A further comprises an operation panel  70  as indicated in  FIG. 8 . The control unit  4  includes a CPU  80  and a data storage section M. In the image forming apparatus  1 A, as illustrated in  FIG. 8 , the operation panel  70  receives an input from a user, and the data are sent to the CPU  80  in the control unit  4  that controls the components and the members of the image forming apparatus  1 A. On receiving the data, the CPU  80  controls the developing biases while referring to various values stored in the data storage section M of the control unit  4 . Details of the developing bias control carried out by the control unit  4  are described below. 
     The control is started by an input from a user. The input from a user is, for example, an input to request for a display of a print mode selection from several modes, such as a color print mode, a monochromatic print mode, etc. 
     As indicated in  FIG. 9 , at step S 1 , the control unit  4  stores a result of the print mode selection done by the user. 
     At step S 2 , the control unit  4  determines the output voltage from the AC power source  52  and the output voltages from the DC power sources  62  based on the data stored at step S 1 . Specifically, in a case where the color print mode is selected as the print mode, the output voltage from the AC power source  52  is determined such that the AC output voltage output from the AC power circuit  50  will be 1200 Vp-p, and the output voltages from the DC power sources  62  are determined such that the DC output voltages output from the DC power circuits  60  will be −400V. In a case where the monochromatic print mode is selected as the print mode, the output voltage from the AC power source  52  is determined such that the AC output voltage output from the AC power circuit  50  will be 1200 Vp-p, and the output voltages from the DC power sources  62  for the colors Y, M and C are determined as 0V. 
     At step S 3 , the control unit  4  determines whether a print request has been input from a user, that is, whether to start a print job. If a print job is to be started, the process goes to step S 4 , and if there is no print request input, the process stands by at step S 3 . 
     At step S 4 , the control unit  4  makes the AC power source and the DC power sources  62  start outputting voltages. Accordingly, in a case of color printing, the developing devices  13  are impressed with the developing biases as illustrated in  FIG. 6 , and in a case of monochromatic printing, the developing devices  13  for the colors Y, M and C are impressed with the developing biases as illustrated in  FIG. 7 . 
     At step S 5 , the control unit  4  determines whether the last sheet in the current print job has been ejected to the tray  2 . When it is determined that the last sheet has been ejected, the process goes to step S 6 . The process stays at step  5  until it is determined that the last sheet has been ejected. The determination of whether the last sheet has been ejected is based on a signal sent from a sheet ejection sensor  7 . 
     At step S 6 , the control section  4  makes the AC power source  52  and the DC power sources  62  stop outputting the voltages. With that, the process comes to the end. 
     Advantageous Effects 
     In the image forming apparatus  1 A, the developing bias source  40  further superimposes voltages from the zener diodes  70 Y,  70 M,  70 C and  70 K on the developing biases  120 Y,  120 M,  120 C and  120 K and applies the superimposed voltages to the developing devices  13 . With this arrangement, the developing biases  120 Y,  120 M,  120 C and  120 K shift to the positive side, that is, in a direction opposite to the polarity of the negatively-charged toner. Moreover, the offset does not depend on the outputs from the DC power circuits  60 Y,  60 M,  60 C and  60 K. Accordingly, in a case of monochromatic printing, that is, when the outputs from the DC power circuits  60 Y,  60 M and  60 C, which are for the colors only used for color printing, are 0V, the developing biases for the colors used for color printing have a positive polarity that is opposite to the polarity of the negatively-charged toner. Accordingly, at a time of monochromatic printing, the developing devices  13 C,  13 Y and  13 M for the colors used for color printing have a positive polarity that is opposite to the polarity of the toner. At a time of monochromatic printing, the surfaces of the photoreceptor drums  11  for use in color printing have a potential of 0V. Therefore, during monochromatic printing, the negatively-charged toner is attracted by the developing devices  13 C,  13 Y and  13 M that are charged positive in relation to the surface potential of the photoreceptor drums  11 . Hence, during monochromatic printing, the image forming apparatus  1 A reduces or prevents toner movements from the developing devices  13 C,  13 Y and  13 M to the photoreceptor drums  11  for use in color printing. 
     Since toner movements from the developing devices  13  to the photoreceptor drums  11  for use in color printing are prevented during monochromatic printing, situations that may degrade the picture quality, for example, white stripes on the photoreceptor drums, toner adhesion to the photoreceptor drums, etc. are avoided, and waste of toner is reduced. 
     Second Embodiment; See FIG.  10   
     An image forming apparatus  1 B according to a second embodiment is different from the image forming apparatus  1 A according to the first embodiment in the positions of the zener diodes  70 Y,  70 M,  70 C and  70 K in the electric circuit of the developing bias source  40 . Specifically, in the image forming apparatus  1 B, as illustrated in  FIG. 10 , the zener diodes  70 Y,  70 M,  70 C and  70 K are disposed between the respective nodes  42 Y,  42 M,  42 C and  42 K and the respective output ports of the developing bias source  40 . The structures of the other parts of the image forming apparatus  1 B and the function effects of the image forming apparatus  1 B are similar to those of the image forming apparatus  1 A according to the first embodiment. 
     Third Embodiment; See FIG.  11   
     An image forming apparatus  1 C according to a third embodiment is different from the image forming apparatus  1 A according to the first embodiment in the positions of the zener diodes  70 Y,  70 M,  70 C and  70 K in the electric circuit of the developing bias source  40 . Specifically, in the image forming apparatus  1 C, as illustrated in  FIG. 11 , the zener diodes  70 Y,  70 M,  70 C and  70 K are disposed between the respective nodes  42 Y,  42 M,  42 C and  42 K and the respective capacitors  56 . The structures of the other parts of the image forming apparatus  1 C and the function effects of the image forming apparatus  1 C are similar to those of the image forming apparatus  1 A according to the first embodiment. 
     Fourth Embodiment 
     An image forming apparatus  1 D according to a fourth embodiment is different from the image forming apparatus  1 A according to the first embodiment in that the zenor diode  70 K is not provided in a developing bias source  50  of the image forming apparatus  1 D. The structures of the other parts of the image forming apparatus  1 D and the function effects of the image forming apparatus  1 D are similar to those of the image forming apparatus  1 A according to the first embodiment. 
     Other Embodiments 
     The specific potentials of the voltages and the condition for a start of the developing bias control can be designed arbitrarily. 
     Although the present invention has been described in connection with the preferred embodiments above, it is to be noted that various changes and modifications are obvious to those who are skilled in the art. Such changes and modifications are to be understood as being within the scope of the invention.