Developing bias power unit for use in an image forming apparatus

Abstract: An image forming apparatus accommodates a developing bias power unit (10) for applying a bias voltage to a developing sleeve (4a) by the use of a DC power source (11) of which an output is on-off controlled. When the output of said DC power source (11) is activated, an electricity storage element (16) interposed between said DC power source (11) and said developing sleeve (4a) is electrically charged by an output voltage of said DC power source (11), and a first voltage is applied to said developing sleeve (4a). In contrast, when the output of said DC power source (11) is deactivated, a second voltage of reverse polarity is applied to said developing sleeve (4a) by electric charge held in said electricity storage element (16).

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates generally to an electrophotographic image forming apparatus such as, for example, a copier or a printer, and more particularly to a developing bias power unit for applying a bias voltage to a developing sleeve which confronts a photosensitive member and supplies it with toner. Description of the Prior Art Fig. 1 schematically depicts a conventional developing bias power unit 104 for use in a reversal-development type image forming apparatus 100. As shown in Fig. 2, the peripheral surface of a photosensitive member 101 is electrostatically charged at a predetermined voltage Vo, and the surface voltage of an area B on the photosensitive member 101 between respective image areas A and A′ on the neighboring pages (which area B is hereinafter referred to as an inter-image area) is reduced to Vo′ by illuminating the inter-image area B. Hereupon, at the inter-image area B between the image areas A and A′, a control for forcibly emitting a laser beam is carried out to regulate the intensity thereof, and as a result of such control, the surface potential of the inter-image area B is reduced to Vo′. On the other hand, in a developing unit 102 for supplying the photosensitive member 101 with toner, a developing sleeve 103 confronting the photosensitive member 101 is connected with the developing bias power unit 104. When the developing sleeve 103 confronts one of the image areas A and A′, a predetermined developing bias voltage V B is applied thereto by the developing bias power unit 104 to prevent the toner from adhering to a background area. When the developing sleeve 103 confronts the inter-image area B, no signal is output from the developing bias power unit 104 (developing bias voltage V B ′=0V), thereby preventing toner adhesion to the inter-image area B. However, the mere switching of the developing bias voltage V B ′ over to 0V at the time the developing sleeve 103 confronts the inter-image area B results in only a voltage difference Vo′ between the photosensitive member 101 and the developing sleeve 103. The voltage difference of this magnitude is not enough to prevent the toner adhesion to the inter-image area B. Particularly, in a so-called contact-transfer type image forming apparatus wherein toner is transferred on a transfer material while the transfer material is sandwiched between the photosensitive member 101 and a transfer roller and is conveyed by the rotation thereof, if the toner adheres to the inter-image area B, the problem arises that the toner may be transferred to the transfer roller, or the toner adhering to the transfer roller may be transferred to and spoils the rear surface of the transfer material. Prior to a printing operation, neither the developing sleeve 103 nor the photosensitive member 101 is electrostatically charged (0V), and there is no voltage difference therebetween. However, toner may adhere, more or less, to an uncharged area of the photosensitive member 101 that confronts the developing sleeve 103, because, prior to the printing operation, such an uncharged area is in contact with the toner at a nipping portion defined between the photosensitive member 101 and the developing sleeve 103. After the printing operation has been triggered and when the photosensitive member 101 starts rotating, an uncharged portion thereof lying between a charging region and a developing region is also in contact with the toner at the nipping portion. Because of this, even if there is no voltage difference between the photosensitive member 101 and the developing sleeve 103, the problem arises that the toner may adhere to such an uncharged portion to some extent. SUMMARY OF THE INVENTION The present invention has been developed to overcome the above-described disadvantages. It is accordingly an object of the present invention to provide an improved developing bias power unit capable of preventing undesired toner adhesion to an uncharged portion of a photosensitive member. In accomplishing the above and other objects, the developing bias power unit according to the present invention applies a bias voltage to a developing sleeve by the use of a DC power source of which an output is on-off controlled, and is characterized in that, when an output of the DC power source is activated, an electricity storage element interposed between the DC power source and the developing sleeve is electrically charged by an output voltage of the DC power source, and a first voltage is applied to the developing sleeve. The developing bias power unit according to the present invention is also characterized in that, when the output of the DC power source is deactivated, a second voltage of reverse polarity is applied to the developing sleeve by the electric charge held in the electricity storage element. The developing bias power unit having the above-described construction operates as follows. The output of the DC power source is activated for a given period of time before a photosensitive member or the developing sleeve starts rotating. At this moment, a predetermined developing bias voltage reduced by a first voltage divider is applied to the developing sleeve, and the electricity storage element is charged with electricity. When the DC power source is turned off, another developing bias voltage of reverse polarity is applied to the developing sleeve by electric charge held in the electricity storage element. Accordingly, even after the photosensitive member or the developing sleeve starts rotating, a sufficient developing bias voltage is held between the developing sleeve and un uncharged region on the photosensitive member, i.e., an area lying between a charging region and a developing region, thereby preventing undesired toner adhesion to the uncharged region. BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects and features of the present invention will become more apparent from the following description of a preferred embodiment thereof with reference to the accompanying drawings, throughout which like parts are designated by like reference numerals, and wherein: Fig. 1 is a schematic circuit diagram of a conventional developing bias power unit; Fig. 2 is a graph indicating a relationship between the surface voltage of a photosensitive member and the developing bias voltage in applications where the developing bias power unit of Fig. 1 is used; Fig. 3 is a schematic view of an image forming apparatus accommodating a developing bias power unit according to the present invention; Fig. 4 is a schematic circuit diagram of the developing bias power unit according to the present invention; Fig. 5 shows waveform charts indicating a relationship between the surface voltage of the photosensitive member or the developing bias voltage and a developing bias remote signal input to a bias power source and the like; Fig. 6 is a diagram similar to Fig. 4, but indicating a modification thereof; Fig. 7 is a schematic circuit diagram for changing the magnitude of the developing bias voltage; and Fig. 8 shows alternative waveform charts indicating a relationship among the bias remote signal, an output voltage from the bias power source, and the developing bias voltage. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings, there is schematically shown in Fig. 3 an image forming apparatus accommodating a developing bias power unit 10 according to the present invention. A photosensitive member 1 has a photosensitive layer formed on an outer periphery thereof and is driven by a main motor 20 to rotate in a direction shown by an arrow (a). A charger unit 2 electrostatically charges the photosensitive layer of the photosensitive member 1 at a predetermined voltage (Vo) at a region X (this region is hereinafter referred to as a charging region). An optical unit 3 emits a laser to expose the charged photosensitive member 1, thereby forming an electrostatic latent image on an image area A or A′ or reducing the voltage of an inter-image area B between these image areas A and A′. A developing unit 4 supplies the electrostatic latent image on the photosensitive member 1 with toner at a region Y (this region is hereinafter referred to as a developing region). At the developing region Y, a developing sleeve 4a accommodated in the developing unit 4 confronts the photosensitive member 1, and a predetermined developing bias voltage V B is applied thereto by the developing bias power unit 10. A transfer roller 5 is in pressure contact with the photosensitive member 1 at a region Z (this region is hereinafter referred to as a transfer region) and is driven by the rotation of the photosensitive member 1. A cleaning unit 6 collects unused toner from the photosensitive member 1. As shown in Fig. 4, the developing bias power unit 10 has a DC bias power source 11, and an output portion T1 of the DC bias power source 11 is grounded via a Zener diode 12, a diode 13, and a resistor 14. The Zener diode 12 serves as a first voltage divider, whereas the resistor 14 serves as a second voltage divider. The Zener diode 12 is connected at its output side T2 with the developing sleeve 4a. A capacitor 16 is connected in parallel with the Zener diode 12. In the image forming apparatus having the above-described construction, the photosensitive member 1 is rotated by the main motor 20 in the direction of the arrow (a), and the photosensitive layer thereof is charged at a predetermined voltage (Vo=-500V) by the charger unit 2 at the charging region X. Thereafter, application of a laser by the optical unit 3 results in a formation of an electrostatic latent image on each of the image regions A and A′ and a reduction of the voltage of the inter-image area B to a predetermined voltage. The electrostatic latent image is supplied with toner with negative electric charge from the developing sleeve 4a of the developing unit 4 at the developing region Y, and is made visible as a toner image. At the transfer region Z, the toner image is transferred on a transfer material (not shown), which is sandwiched between and is conveyed by the photosensitive member 1 and the transfer roller 5. After the toner of the toner image transferred on the transfer material has been fixed by a fixing unit (not shown), the transfer material is discharged on a suitable transfer material receiving unit (not shown). The cleaning unit 6 removes the remaining toner from the photosensitive member 1 after the photosensitive member 1 has passed the transfer region Z. As shown in Fig. 5, upon input of a print start signal, a developing bias remote signal output from a controller 18 is input to the bias power source 11 of the developing bias power unit 10. When the bias power source 11 receives the developing bias remote signal, the bias power source 11 outputs a DC current from the output portion T1 thereof at a voltage of -450V . The output voltage (-450V) is reduced by the Zener diode 12, and a developing bias voltage V B1 of -300V is applied to the developing sleeve 4a. The capacitor 16 is charged with electricity until a voltage equal to a voltage difference of 150V between input and output terminals of the Zener diode 12 is obtained. When the developing bias remote signal is deactivated, a developing bias voltage V B1 ′ of +150V of reverse polarity is applied to the developing sleeve 4a by the charged capacitor 16. The developing bias remote signal may be deactivated prior to the start of the main motor 20 or at the same timing as the latter. At the same time as the main motor 20 is turned on, a charging bias remote signal is activated, and the photosensitive layer passing the charging region X is charged at Vo=-500V. However, a region lying between the charging region X and the developing region Y is left uncharged (this region is hereinafter referred to as an uncharged region C). Accordingly, the uncharged region C, i.e., the region having a voltage of 0V initially advances into the developing region Y. At this moment, because the developing bias voltage V B1 ′=+150V of reverse polarity is applied to the developing sleeve 4a, the potential of the uncharged region C is 150V lower than that of the developing sleeve 4a. Because of this, toner with negative electric charge held on the developing sleeve 4a electrically repulses the photosensitive member 1, and never adheres to the uncharged region C of the photosensitive member 1. The Zener diode 12 and the diode 13 prevents leakage of electric charge held in the capacitor 16. When the bias power source 11 is turned off, the voltage of the developing sleeve 4a is gradually reduced. The reduction in voltage of the developing sleeve 4a holds considerably long because a time constant is about 8.3 seconds. Accordingly, the leakage of electric charge or the reduction in voltage of the developing sleeve 4a introduces no problems in preventing the toner adhesion to the photosensitive member 1. After the uncharged region C of the photosensitive member 1 has passed the developing region Y and when an image region A newly charged at Vo=-500V by the charger unit 2 reaches the developing region Y, the developing bias remote signal is activated, and a developing bias voltage V B2 of -300V is applied to the developing sleeve 4a, thereby preventing the toner adhesion to the background area. While the developing bias remote signal is on, the capacitor 16 is charged with electricity. After the image region A has passed the developing region Y and when the inter-image area B between respective image areas A and A′ on the neighboring pages reaches the developing region Y, the developing bias remote signal is deactivated. As a result, a developing bias voltage of V B2 ′=+150V is applied to the developing sleeve 4a in the presence of the electric charge in the capacitor 16. At this stage, the voltage of the inter-image area B on the photosensitive member 1 is reduced to Vo′=-100V by the application of the laser. Because the voltage of the inter-image area B is 250V lower than that of the developing sleeve 4a, the toner with negative electric charge held on the developing sleeve 4a never adheres to the inter-image area B on the photosensitive member 1. After the inter-image area B has passed the developing region Y and when the image region A′ of the next page reaches the developing region Y, the developing bias remote signal is activated, as is the case with the image region A. At this moment, the developing bias voltage V B2 is switched over to -300V, thereby preventing the toner adhesion to the background area. In the above-described embodiment, although the Zener diode 12 is used as the first voltage divider, a resistor 17 may be used in place thereof, as shown in Fig. 6. The diode 13 is not always required. Also in this case, while the developing bias remote signal is activated, each of the predetermined developing bias voltages V B1 and V B2 is applied to the developing sleeve 4a. Furthermore, while the developing bias remote signal is off, each of the developing bias voltages V B1 ′ and V B2 ′ of reverse polarity is applied to the developing sleeve 4a. In addition, because the time constant is sufficiently long, a reduction in developing bias voltage V B1 ′ or V B2 ′ at the time the developing bias signal is off introduces no problems. Although the developing bias voltages V B1 and V B2 are made identical before and after the main motor 20 is turned on, these voltages are not always required to be identical. A higher V B1 not only can shorten the time required for charging the capacitor 16, but also can reduce the waiting period for printing. However, if V B1 is far higher than V B2 , toner scatters and adheres to the photosensitive member 1 at the location where developer is in contact with the photosensitive member 1. In particular, if the scattering toner adheres to the photosensitive member 1 at a location downstream from the contact portion, it is likely that such toner is transferred to the transfer roller 5 or toner adhering thereto is transferred to the rear surface of a transfer material, thereby spoiling the transfer material. In contrast, if V B1 is far lower than V B2 , the charging of the capacitor 16 takes a lot of time, and therefore, the waiting period becomes long. In view of the above, it is necessary to determine the magnitude of each of the developing bias voltages V B1 and V B2 . Fig. 7 schematically depicts a circuit diagram for changing the magnitude of the developing bias voltages V B1 and V B2 . In Fig. 7, a D/A converter 19 is interposed between a controller (CPU) 18 and a developing bias power source 11. The controller 18 outputs an output voltage regulating signal to the D/A converter 19 and changes an output from the bias power source 11 based on the output voltage regulating signal. In the above-described embodiment, although the off-timing of the developing bias remote signal is determined based on the time when a driving operation of the main motor 20 is started, such determination may be based on the time when a driving operation of the developing sleeve 4a is started. In this case, the developing bias remote signal may be deactivated at or before this timing. It, however, is necessary to deactivate the remote signal of the developing bias voltage V B1 before the toner held on the developing sleeve 4a is moved relative to the photosensitive member 1 by the rotation of the photosensitive member 1 or the developing sleeve 4a. Fig. 8 depicts alternative waveform charts of the bias remote signal input to the bias power source 11, the output voltage therefrom, and the developing bias voltage. As shown by (a), the bias remote signal is a pulse signal periodically repeating on and off. When the photosensitive member 1 is driven by the main motor 20, the bias remote signal is input to the bias power source 11. When the bias remote signal is on, a voltage of V B =-600V is output from the bias power source 11, as shown by (b), and a developing bias voltage of V B ′=-450V is applied to the developing sleeve 4a by the operation of the Zener diode 12, as shown by (c). The capacitor 16 is charged with electricity by the application of a voltage equal to a voltage difference of 150V between input and output terminals of the Zener diode 12. When the bias remote signal is off, the output voltage V B of the bias power source 11 becomes OV. At this moment, a developing bias voltage of V B ′=+150V of reverse polarity is applied to the developing sleeve 4a by the charged capacitor 16. In short, a voltage of -450V and a voltage of +150V are alternately applied to the developing sleeve 4a, as shown by (c) in Fig. 8. Accordingly, an alternating bias voltage having an average voltage of Va=-150V and an amplitude of 600V is applied to the developing sleeve 4a. The application of such an alternating bias voltage results in an effective development. As is clear from the above, according to the developing bias power unit of the present invention, before the developing sleeve or the photosensitive member starts rotating, the capacitor is electrically charged. At the time the rotation of the developing sleeve or the photosensitive member is started, a developing bias voltage of reverse polarity is already applied to the developing sleeve 4a. Accordingly, even when an uncharged region of the photosensitive member lying between the charging region and the developing region passes the developing unit, the developing bias voltage of reverse polarity prevents toner from adhering to the uncharged region. Furthermore, even when an inter-image area on the photosensitive member between neighboring image areas passes the developing region, the developing bias voltage of reverse polarity is applied to the developing sleeve by the capacitor after the bias power source has been turned off. As a result, a sufficient voltage is held between the inter-image area and the developing sleeve, thereby reliably preventing the toner adhesion to the inter-image area. The developing bias power unit according to the present invention requires neither a developing bias power source capable of providing an output of positive polarity and that of negative polarity nor a mechanism for switching these outputs. Although the present invention has been fully described by way of examples with reference to the accompanying drawings, it is to be noted here that various changes and modifications will be apparent to those skilled in the art. Therefore, unless such changes and modifications otherwise depart from the scope of the present invention as claimed, they should be construed as being included therein.

Claims: A developing bias power unit (10) for use in an image forming apparatus, said developing bias power unit (10) applying a bias voltage to a developing sleeve (4a) positioned to confront a photosensitive member (1) by the use of a DC power source (11) of which an output is on-off controlled, said developing bias power unit (10) being characterized in that: when the output of said DC power source (11) is activated, an electricity storage element (16) interposed between said DC power source (11) and said developing sleeve (4a) is electrically charged by an output voltage of said DC power source (11), and a first voltage is applied to said developing sleeve (4a); and when the output of said DC power source (11) is deactivated, a second voltage of reverse polarity is applied to said developing sleeve (4a) by the electric charge held in said electricity storage element (16). The developing bias power unit (10) according to claim 1, wherein a first control means (18) is provided for controlling so that, when said developing sleeve (4a) confronts a first image region (A), said DC power source (11) is turned on, and when said developing sleeve (4a) confronts an area (B) between said first image region (A) and a second image region (A′), said DC power source (11) is turned off. The developing bias power unit (10) according to claim 1, wherein a second control means (18) is provided for controlling so that, prior to a driving operation of said image forming apparatus, said DC power source (11) is turned on for a given period of time, thereby electrically charging said electricity storage element (16), and after the driving operation of said image forming apparatus is started, said DC power source (11) is turned off and the electric charge held in said electricity storage element (16) is discharged therefrom. The developing bias power unit (10) according to claim 1, wherein a third control means (18) is provided for controlling so that the output voltage (V B1 ) of said DC power source (11) applied to said developing sleeve (4a) prior to a driving operation of said image forming apparatus differs from that (V B2 ) applied to said developing sleeve (4a) at the time said developing sleeve (4a) confronts a first image region (A) after the driving operation of said image forming apparatus is started. The developing bias power unit (10) according to claim 1, wherein said second voltage is equal to the voltage difference between said output voltage of said DC power source (11) and said first voltage. The developing bias power unit (10) according to claim 1, wherein said electricity storage element (16) comprises a capacitor. The developing bias power unit (10) according to claim 1, wherein said electricity storage element (16) is connected in parallel with a Zener diode (12) interposed between said DC power source (11) and said developing sleeve (4a), and an output side of said Zener diode (12) is grounded via a resistor (14). The developing bias power unit (10) according to claim 1, further comprising a fourth control means (18) for periodically activating and deactivating the output of said DC power source (11).

Inventors: GOTO HIROSHI C O MINOLTA CAMER, HARA KAZUYOSHI C O MINOLTA CAM, OKA TATEKI C O MINOLTA CAMERA, SAITO HITOSHI C O MINOLTA CAME, SAKURABA TAMOTSU C O MINOLTA C, TANAKA YASUO C O MINOLTA CAMER, UNO KOJI C O MINOLTA CAMERA K, GOTO, HIROSHI, C/O MINOLTA CAMERA K. K., HARA, KAZUYOSHI, C/O MINOLTA CAMERA K. K., OKA, TATEKI, C/O MINOLTA CAMERA K. K., SAITO, HITOSHI, C/O MINOLTA CAMERA K. K., SAKURABA, TAMOTSU, C/O MINOLTA CAMERA K. K., TANAKA, YASUO, C/O MINOLTA CAMERA K. K., UNO, KOJI, C/O MINOLTA CAMERA K. K.
Applicants: MINOLTA CAMERA KK, MINOLTA CAMERA KABUSHIKI KAISHA
Application year: 1992
Main classification: G03G15
IPC codes: G03G15