Image forming apparatus

Abstract

The present invention provides an image forming apparatus that suppresses the occurrence of density unevenness due to fluctuations in the output voltage of the AC component of the charging voltage. [Solution] An image forming apparatus comprising an image forming unit comprising a photoreceptor, a charging unit for charging the photoreceptor, a charging voltage application unit for applying a predetermined charging voltage to the charging unit, an exposure unit for forming an electrostatic latent image on the photoreceptor, a developing unit for supplying toner to the photoreceptor to form a toner image corresponding to the electrostatic latent image, a developing voltage application unit for applying a predetermined developing voltage to the developing unit, a transfer unit for transferring the toner image to a recording medium, and a fixing unit for heating and fixing the toner image to the recording medium, wherein the charging voltage application unit comprises an AC voltage output unit for outputting an AC voltage, a DC voltage output unit for outputting a DC voltage, and a superimposed voltage application unit for applying a charging voltage obtained by superimposing the AC voltage and the DC voltage to the charging unit, and the AC voltage output unit comprises an amplification circuit for amplifying and outputting the voltage of the AC voltage, and a bandpass filter for limiting the frequency of the AC voltage to a predetermined range of charging frequencies.

Description

Technical Field

[0006] , ,

[0001] This disclosure relates to an image forming apparatus, and more particularly to an image forming apparatus provided with a charging unit that charges a photoreceptor.

Background Art

[0002] Conventionally, in an image forming apparatus that applies a high voltage output in which an AC component is superimposed on a DC component to a contact charging unit, density unevenness may occur due to fluctuations in the output of the AC component.

[0003] In relation to such problems, conventionally, a frequency filter that attenuates a predetermined frequency component or a frequency component equal to or higher than the predetermined frequency is provided in a constant current circuit, and by setting the predetermined frequency to Vc / w or less, density unevenness due to deformation of the contact charging roller is prevented as much as possible, and a contact charging device for an image forming apparatus that improves image quality has been proposed (see, for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0007] This disclosure has been made in consideration of the circumstances described above, and its purpose is to provide an image forming apparatus that suppresses the occurrence of density unevenness due to fluctuations in the output voltage of the AC component of the charging voltage. [Means for solving the problem]

[0008] The image forming apparatus according to this disclosure comprises a photoreceptor, a charging unit for charging the photoreceptor, a charging voltage application unit for applying a predetermined charging voltage to the charging unit, an exposure unit for forming an electrostatic latent image on the photoreceptor, a developing unit for supplying toner to the photoreceptor to form a toner image corresponding to the electrostatic latent image, a developing voltage application unit for applying a predetermined developing voltage to the developing unit, a transfer unit for transferring the toner image to a recording medium, and a fixing unit for heating and fixing the toner image to the recording medium. The charging voltage application unit comprises an AC voltage output unit for outputting an AC voltage, a DC voltage output unit for outputting a DC voltage, and a superimposed voltage application unit for applying a charging voltage obtained by superimposing the AC voltage and the DC voltage to the charging unit. The AC voltage output unit comprises an amplification circuit for amplifying and outputting the voltage of the AC voltage, and a bandpass filter for limiting the frequency of the AC voltage to a predetermined range of charging frequencies. [Effects of the Invention]

[0009] According to this disclosure, an image forming apparatus can be realized that suppresses the occurrence of density unevenness due to fluctuations in the output voltage of the AC component of the charging voltage by providing a bandpass filter that limits the frequency of the AC voltage to a predetermined charging frequency range. [Brief explanation of the drawing]

[0010] [Figure 1] This is a perspective view showing the external appearance of a digital multifunction printer according to Embodiment 1 of this disclosure. [Figure 2] Figure 1 is a cross-sectional view showing the internal configuration of a digital multifunction printer. [Figure 3] Figure 1 is a block diagram showing the schematic configuration of a digital multifunction printer. [Figure 4] Figure 1 is an explanatory diagram showing the schematic configuration of the first visible image forming unit of the digital multifunction printer. [Figure 5] Figure 1 shows an example of the configuration of the charging voltage application section and the AC voltage output section of the digital multifunction printer. [Figure 6] Figure 5 is an explanatory diagram showing an example of the bandpass frequency range of the charged AC voltage at the output stage of the AC voltage. [Modes for carrying out the invention]

[0011] In this disclosure, "image forming apparatus" refers to a device that forms and outputs an image, such as a copier or multifunction printer that has a copying function, such as a printer that uses an electrophotographic method for image formation with toner, or an MFP (Multifunctional Peripheral) that also includes functions other than copying.

[0012] The "photoreceptor" is realized by photoreceptor drums 59, 65, 66, and 67. The "charged section" is realized by the charging roller 60. The "exposure section" is realized by the optical system unit 50. The "developing unit" is realized through the cooperation of developing units 61, 61a, 61b, 61c, 61d, a developing power supply 182, and a developing voltage application unit 172. The "transfer section" is realized through the cooperation of the primary transfer units 62, 62a, 62b, 62c, 62d, the secondary transfer unit 56, the transfer power supply 183, and the transfer voltage application section 173. The "amplification circuit" is realized through the cooperation of the comparator 17111 and the switching circuit 17112. The "band-pass filter" is realized by the low-pass filter 17113 and the high-pass filter 17114. Furthermore, a preferred embodiment of this disclosure will be described.

[0013] In the image forming apparatus according to this disclosure, the band-pass filter may be composed of a low-pass filter that allows only frequencies below a predetermined first frequency among the frequencies of the alternating voltage to pass through, and a high-pass filter that allows only frequencies above a predetermined second frequency to pass through.

[0014] The "first frequency" is set to a frequency that blocks high frequencies such as triangular waves (for example, 200 Hz) used as a reference voltage in the amplifier circuit. The "second frequency" is set to a frequency that blocks the 100 Hz / 120 Hz ripple of the commercial power supply frequency (50 Hz / 60 Hz).

[0015] In this way, by allowing only the frequency range used by the image forming apparatus to pass through the low-pass filter and the high-pass filter, the ripple of the commercial power supply frequency can be blocked, so that an image forming apparatus that suppresses the occurrence of density unevenness due to fluctuations in the output voltage of the AC component of the charging voltage can be realized.

[0016] In the image forming apparatus according to this disclosure, the range of the charging frequency may include the range from 1 kHz to 2 kHz.

[0017] In this way, while limiting the frequency of the alternating voltage to a predetermined range of charging frequencies, by providing a band-pass filter that allows the range of charging frequencies, which is the frequency range used by the image forming apparatus, to pass through up to 1 kHz to 2 kHz, an image forming apparatus that suppresses the occurrence of density unevenness due to fluctuations in the output voltage of the AC component of the charging voltage can be realized. <00000�3>

[0018] In the image forming apparatus according to this disclosure, the high-pass filter may be one in which the second frequency is set so as to cut off the 100 Hz / 120 Hz ripple of the commercial power frequency.

[0019] In this way, since the second frequency of the high-pass filter is set so as to cut off the 100 Hz / 120 Hz ripple of the commercial power frequency, the ripple of the commercial power frequency can be cut off, and an image forming apparatus that suppresses the occurrence of density unevenness due to fluctuations in the output voltage of the AC component of the charging voltage can be realized.

[0020] In the image forming apparatus according to this disclosure, the amplifier circuit may be a class D amplifier circuit.

[0021] In this way, in the AC voltage output section, by using a class D amplifier circuit with high power efficiency and low heat generation as the amplifier circuit, an image forming apparatus that suppresses the occurrence of density unevenness due to fluctuations in the output voltage of the AC component of the AC voltage and the charging voltage can be realized.

[0022] Hereinafter, this disclosure will be described in more detail with reference to the drawings. Note that the following description is illustrative in all respects and should not be construed as limiting this disclosure.

[0023] [Embodiment 1] <Configuration of Digital Multifunction Machine 1> Hereinafter, based on FIGS. 1 and 2, an overview of a digital multifunction machine 1 as an example of an image forming apparatus according to Embodiment 1 of this disclosure will be described.

[0024] FIG. 1 is a perspective view showing the appearance of the digital multifunction machine 1 according to Embodiment 1 of this disclosure. FIG. 2 is a cross-sectional view showing the internal configuration of the digital multifunction machine 1 in FIG. 1.

[0025] The digital multifunction machine 1 is a device that has a copying function, a scanner function, and a facsimile function, and digitally processes and outputs image data read from a document.

[0026] The digital multifunction printer 1 has copy, print, and fax functions as print modes, and the print function is selected by the control unit 10 (Figure 3) in response to operation input from the control unit 162 (Figure 3) or the reception of a print job from an external device such as a personal computer.

[0027] <Internal configuration of digital multifunction printer 1> In Figure 2, the digital multifunction printer 1 is a color multifunction printer and comprises an optical system unit 50, first to fourth visible image forming units 51 to 54, an intermediate transfer belt 55, a secondary transfer unit 56, a fuser unit 2, an internal paper feeding unit 57, a manual paper feeding unit 58, and an output tray 75.

[0028] The digital multifunction printer 1 forms a toner image using the first to fourth visible image forming units 51 to 54, the intermediate transfer belt 55, and the secondary transfer unit 56.

[0029] In the optical system unit 50, beams from four laser light sources 64 are arranged to reach four sets of photoreceptor drums 59, 65, 66, and 67.

[0030] The first visible image forming unit 51 includes a photoreceptor drum 59, a charging roller 60, an optical system unit 50, a developing unit 61a, and a primary transfer unit 62a.

[0031] The photosensitive drum 59, which serves as the image carrier, is surrounded by a charging roller 60, a developing unit 61a, and a cleaning unit 63.

[0032] These units form a toner image on the photoreceptor drum 59, and the toner image is transferred to the intermediate transfer belt 55.

[0033] The photoreceptor drum 59 is an image carrier on which a toner image is formed on its surface, and is supported so as to be rotatable around an axis. It includes a conductive substrate (preferably cylindrical, cylindrical, or thin film sheet, not shown) and a photoreceptor layer formed on the surface of the conductive substrate.

[0034] The photoreceptor drum 59 rotates in a counterclockwise direction toward the plane of the paper in Figure 2, for example, at a peripheral speed of 163 mm / s, by a photoreceptor drum drive gear (not shown) attached to the photoreceptor drum 59, which is engaged with a motor gear.

[0035] The primary transfer unit 62a is positioned to be pressed against the photoreceptor drum 59 via the intermediate transfer belt 55.

[0036] Furthermore, the second to fourth visible image forming units 52 to 54 have the same configuration as the first visible image forming unit 51, so their explanation will be omitted.

[0037] Each of the developing units 61a, 61b, 61c, and 61d in units 51 to 54 contains toners of the respective colors: black (K), cyan (C), magenta (M), and yellow (Y).

[0038] Hereafter, the developing units 61a, 61b, 61c, and 61d for each color may be referred to simply as developing unit 61. Furthermore, the primary transfer units 62a, 62b, 62c, and 62d of each color may be referred to simply as primary transfer unit 62.

[0039] The intermediate transfer belt 55 receives the toner images of each color, and the color toner images of each color are superimposed on its surface. The intermediate transfer belt 55 is driven and rotated by tension rollers 68 and 69. The secondary transfer unit 56 is positioned in contact with the tension roller 68 side of the intermediate transfer belt 55.

[0040] The secondary transfer unit 56 applies a high voltage with the opposite polarity to the charging polarity of the toner to the transfer area using a transfer roller 621a or the like, thereby transferring the color toner image formed on the intermediate transfer belt 55 to the recording medium that has been fed from the internal paper feeding unit 57 and the manual paper feeding unit 58 by the feeding rollers 73 and 74, respectively. Subsequently, the recording medium onto which the color toner image has been transferred is transported to the position of the fuser unit 2.

[0041] Furthermore, the waste toner box 70, which is in contact with the intermediate transfer belt 55 and located on the tension roller 69 side, collects the toner remaining on the surface of the intermediate transfer belt 55 after secondary transfer.

[0042] The fixing unit 2 is located downstream of the secondary transfer unit 56. The fixing unit 2 consists of a fixing belt 71 and a pressure roller 72. The pressure roller 72 is pressed against the fixing belt 71 at a predetermined pressure by a pressure mechanism (not shown). Furthermore, a paper output tray 75 is provided downstream of the fuser unit 2.

[0043] Next, the general configuration of the digital multifunction printer 1 will be explained based on Figure 3. Figure 3 is a block diagram showing the schematic configuration of the digital multifunction printer 1 shown in Figure 1.

[0044] As shown in Figure 3, the digital multifunction printer 1 comprises a control unit 10, an image data acquisition unit 11, an image forming unit 12, a storage unit 13, an image processing unit 14, a communication unit 15, an operation panel 16, a voltage application unit 17, and a power supply 18. The following describes each component of the digital multifunction printer 1.

[0045] The control unit 10 comprehensively controls the digital multifunction printer 1 and consists of at least one CPU (Central Processing Unit), at least one RAM (Random Access Memory), at least one ROM (Read-only memory), various interface circuits, and the like.

[0046] The control unit 10 monitors and controls all loads, including detection by each sensor, motor, clutch, and operation panel 16, in order to control the operation of the entire digital multifunction printer 1.

[0047] The image data acquisition unit 11 is the part that detects and reads documents placed on the document glass or documents transported from the document tray, and generates image data. It is also the part that acquires image data generated by an external information processing device (not shown) or a facsimile machine (not shown), etc.

[0048] The image forming unit 12 is the part that prints the image data generated by the image processing unit 14 onto paper.

[0049] The memory unit 13 is an element or storage medium that stores information and control programs necessary to realize the various functions of the digital multifunction device 1. For example, semiconductor elements such as RAM and ROM, storage media such as hard disks, flash memory units, and SSDs (Solid State Drives) are used. Furthermore, the data may be stored in different devices, such as a hard disk drive for the data storage area and a flash memory unit for the program storage area.

[0050] The image processing unit 14 is responsible for processing the image data input from the image data acquisition unit 11 into appropriate electrical signals based on the analysis results of print job commands acquired from user terminals, etc., via the communication unit 15, and print job commands input from the operation unit 162, so that the image data can be processed to be suitable for output such as enlargement and reduction.

[0051] The communication unit 15 is the part that communicates with computers, mobile information terminals, external information processing devices, facsimile machines, etc., via a network, and sends and receives various types of information such as emails and faxes with these external communication devices.

[0052] The operation panel 16 consists of a display panel made of a liquid crystal panel or the like, and a touch panel, such as a capacitive type, which is placed on top of the display panel and detects the position where a finger is touched, and includes a display unit 161 and an operation unit 162.

[0053] The display unit 161 is the part that displays various types of information. The display unit 161 is composed of, for example, a CRT display, a liquid crystal display, or an EL display, and is a display device such as a monitor or line display for displaying electronic data such as processing status by the operating system or application software. The control unit 10 displays the operation and status of the digital multifunction printer 1 through the display unit 161.

[0054] The control unit 162 is an interface for operating the digital multifunction printer 1 and also receives commands from the user, such as printing.

[0055] The voltage application unit 17 is the part that applies voltage and includes a charging voltage application unit 171, a developing voltage application unit 172, and a transfer voltage application unit 173.

[0056] The charging voltage application unit 171 applies a predetermined charging voltage to the charging roller 60. The developing voltage application unit 172 applies a predetermined developing voltage to the developing roller 611a. The transfer voltage application unit 173 applies a predetermined transfer voltage to the transfer roller 621a.

[0057] Power supply 18 supplies power to each part of the digital multifunction printer 1 from a commercial power supply (not shown). The power supply 18 includes a charging power supply 181, a developing power supply 182, and a transfer power supply 183.

[0058] <Operation of the first visible image forming unit 51 of the digital multifunction printer 1> Next, the operation of the first visible image forming unit 51 of the digital multifunction printer 1 will be explained based on Figure 4.

[0059] Figure 4 is an explanatory diagram showing the schematic configuration of the first visible image forming unit 51 of the digital multifunction printer 1 shown in Figure 1.

[0060] In the following explanation, the first visible image forming unit 51 will be used as an example, but the same applies to the other second to fourth visible image forming units 52 to 54.

[0061] In Figure 4, the photosensitive drum 59 rotates in the rotational direction R1 (counterclockwise toward the paper surface). The charging roller 60 rotates in the rotational direction R2 (clockwise towards the paper surface) in response to the rotation of the photosensitive drum 59. In Embodiment 1, the charging roller 60 is driven by the photoreceptor drum 59, but it does not have to be driven.

[0062] During image formation, the surface of the photoreceptor drum 59 is uniformly charged by the charging roller 60. In Embodiment 1, a contact-type charging roller system is employed to uniformly charge the surface of the photoreceptor drum 59 while minimizing ozone generation.

[0063] As shown in Figure 4, the charging roller 60 presses against the surface of the photoreceptor drum 59 with a predetermined pressure suitable for charging by a biasing force (not shown), such as a spring, and rotates in accordance with the rotation of the photoreceptor drum 59.

[0064] The charging voltage application unit 171 charges the surface of the photoreceptor drum 59 to a predetermined voltage (for example, -0V to -800V) by applying a predetermined charging voltage from the charging power supply 181 (high voltage power supply circuit) to the core metal 601 portion of the charging roller 60.

[0065] The optical system unit 50 exposes the surface of the charged photoreceptor drum 59 with a beam from the laser light source 64, thereby reducing the surface potential of the photoreceptor drum 59 after exposure to, for example, -100V or less, and forming an electrostatic latent image.

[0066] This disclosure is applicable to both the normal development method and the inversion development method, but Embodiment 1 will be explained using the inversion phenomenon method.

[0067] The laser light from the optical system unit 50 is irradiated onto the photoreceptor drum 59 through a polygon mirror and various lenses (not shown).

[0068] The laser light source 64 is controlled based on image information, and an electrostatic latent image corresponding to the image information is formed on the surface of the photoreceptor drum 59.

[0069] The developing unit 61a develops the electrostatic latent image on the photoreceptor drum 59 to form a toner image. The electrostatic latent image formed on the photoreceptor drum 59 is exposed by the developing unit 61a using a developer 612a containing toner and carrier, thereby forming a toner image.

[0070] As shown in Figure 4, the developing unit 61a is a developing unit provided opposite the photoreceptor drum 59, and the developing roller 611a, which serves as a developer carrier, is rotatably mounted on a rotation axis parallel to the rotation axis of the photoreceptor drum 59.

[0071] The developing unit 61a is a hollow container-like member made of, for example, a hard synthetic resin. The developing unit 61a contains a two-component developer containing toner and carrier as described above, but it may also contain a one-component developer containing only toner.

[0072] The developing roller 611a is a magnetic roller formed by arranging magnetic members with different polarities in a roughly alternating manner in the circumferential direction. The developing roller 611a attracts the developer 612a contained within the developing unit 61a by its magnetic force. The adsorbed developer 612a is restricted to a predetermined thickness by a developer regulating member (not shown) and is transported to the developing nip section where the developing roller 611a and the photoreceptor drum 59 are in close proximity.

[0073] The developing voltage application unit 172 charges the core metal 613a of the developing roller 611a with a predetermined voltage by applying a predetermined developing voltage from the developing power supply 182 (high voltage power supply circuit).

[0074] The developing voltage is set to a value lower than the surface potential of the unexposed area (-600V) of the photoreceptor drum 59, and higher than the surface potential of the exposed area (-100V).

[0075] As a result, toner charged with the same polarity as the charge polarity of the photoreceptor drum 59 (negative polarity in Embodiment 1) is adsorbed to the surface potential of the exposed area of ​​the photoreceptor drum 59, forming a toner image (inversion development).

[0076] On the other hand, since the surface potential of the unexposed area of ​​the photoreceptor drum 59 is lower than the development voltage of the developing roller 611a, toner adhesion is prevented.

[0077] The transfer roller 621a of the primary transfer unit 62a has a voltage applied to the core metal 622a portion from the transfer power supply 183 (high voltage power supply circuit) that is the opposite polarity to the toner, and the toner image developed on the photoreceptor drum 59 is transferred onto the intermediate transfer belt 55 in the transfer area where the primary transfer unit 62a and the photoreceptor drum 59 are in close proximity.

[0078] In Embodiment 1, the primary transfer unit 62a is configured using a transfer roller 621a, but it may also be a belt or a wire.

[0079] The other second to fourth visible image forming units 52 to 54 operate similarly, sequentially transferring the toner image onto the intermediate transfer belt 55.

[0080] The toner image on the intermediate transfer belt 55 is transported to the secondary transfer unit 56. As shown in Figure 2, the recording medium is supplied from the feed roller 73 of the internal paper feed unit 57 or the feed roller 74 of the manual feed unit 58 via the transport path RT1 or RT2, respectively. Then, a voltage with the opposite polarity to the toner is applied by the secondary transfer unit 56, and the toner image is transferred to the recording medium.

[0081] The recording medium bearing the toner image is transported to the fuser unit 2, where it is sufficiently heated by the fuser belt 71 and pressure roller 72. The unfixed toner image melts and solidifies on the recording medium, and is then discharged from the discharge roller ER to the paper output tray 75 via the transport path RT4.

[0082] In the case of double-sided printing, after the image formation on the surface of the recording medium is completed by passing through the fuser unit 2, the recording medium is inverted via the transport path RT3 to perform image formation on the back side.

[0083] After the transfer, the photoreceptor drum 59 will have residual toner that was not transferred to the intermediate transfer belt 55 adhering to it. This remaining toner is scraped off by a cleaning blade 63a attached to the cleaning unit 63 and collected as waste toner inside the cleaning unit 63.

[0084] The static elimination unit 76 removes the electric charge from the surface of the photoreceptor drum 59. The position of the static elimination unit 76 can be anywhere between the time of transfer and before charging.

[0085] The cleaning roller 77 is positioned opposite the charging roller 60 and cleans the surface of the charging roller 60.

[0086] <Configuration of the electrostatic voltage application unit and AC voltage output unit of the digital multifunction printer 1 according to Embodiment 1 of this disclosure> Next, the configuration of the charging voltage application unit and the AC voltage output unit of the digital multifunction printer 1 according to Embodiment 1 of this disclosure will be described based on Figure 5. Figure 5 shows an example of the configuration of the charging voltage application unit 171 and the AC voltage output unit 1711 of the digital multifunction printer 1 shown in Figure 1.

[0087] As shown in Figure 5(A), the charging voltage application unit 171 includes an AC voltage output unit 1711, a DC voltage output unit 1712, and a superimposed voltage application unit 1713.

[0088] The charging voltage application unit 171 applies a high voltage (AC voltage + DC voltage) which is obtained by superimposing a DC component voltage (DC voltage) onto an AC component voltage (AC voltage).

[0089] The AC voltage output section 1711 is a part that outputs an AC voltage while being controlled by a constant current circuit. The DC voltage output section 1712 is a part that outputs a DC voltage, which is controlled by a constant voltage circuit. The superimposed voltage application unit 1713 is the part that applies the AC voltage output from the AC voltage output unit 1711 to the charging roller 60 by superimposing the DC voltage (DC voltage + AC voltage) output from the DC voltage output unit 1712.

[0090] By applying these superimposed voltages, the surface potential of the photoreceptor drum 59 can be made uniform compared to the DC charging method in which only a DC voltage is applied to the charging roller 60.

[0091] As shown in Figure 5(B), the AC voltage output section 1711 includes a comparator 17111, a switching circuit 17112, a low-pass filter 17112, and a high-pass filter 17114. Furthermore, the comparator 17111 and the switching circuit 17112 constitute a Class D amplifier circuit, which functions as an amplification circuit.

[0092] The comparator 17111 uses the triangular wave voltage input from the negative terminal as the reference voltage value Vs, and compares it with the sinusoidal voltage value Vc input from the commercial voltage of the 24V line to the positive terminal. It outputs a 1-bit digital signal only when the sinusoidal voltage value Vc is higher than the reference voltage value Vs.

[0093] The digital signal, which has been modulated using PWM (Pulse Width Modulation) in this manner, is input to the switching circuit 17112, which then outputs a high-power AC voltage.

[0094] The frequency range is limited to a predetermined range by passing it through a low-pass filter 17113 and a high-pass filter 17114.

[0095] Figure 6 is an explanatory diagram showing an example of the bandpass frequency range of the charged AC voltage of the AC voltage output section 1711 of Figure 5.

[0096] As shown in Figure 6, the low-pass filter 17113 blocks the 200kHz triangular wave, while the high-pass filter 17114 blocks the 100Hz / 120Hz ripple of the commercial power frequency (50Hz / 60Hz). In other words, a cutoff frequency fc is set between the commercial power frequency ripple (100Hz / 120Hz) and the operating frequency range of the digital multifunction device 1 (1kHz~2kHz).

[0097] This allows only the operating frequency range (1kHz to 2kHz) of the digital multifunction printer 1 to be used. Therefore, it eliminates the need to add expensive components such as resistors to reduce commercial power frequency ripple or LDOs (Low Drop Outs) to suppress that ripple, as was done in the past.

[0098] In this way, by providing a bandpass filter (low-pass filter 17113 and high-pass filter 17114) that limits the frequency of the AC voltage to a predetermined charging frequency range, a digital multifunction device 1 can be realized that suppresses the occurrence of density unevenness due to fluctuations in the output voltage of the AC component of the charging voltage.

[0099] Preferred embodiments of this disclosure include combinations of any of the embodiments described above. In addition to the embodiments described above, various modifications of this disclosure are possible. These modifications should not be construed as being outside the scope of this disclosure. This disclosure should include the meaning of equivalents to the claims and all variations within that scope. [Explanation of Symbols]

[0100] 1: Digital multifunction printer, 2: Fusing unit, 10: Control unit, 11: Image data acquisition unit, 12: Image forming unit, 13: Storage unit, 14: Image processing unit, 15: Communication unit, 16: Operation panel, 17: Voltage application unit, 18: Power supply, 50: Optical system unit, 51: First visible image forming unit, 52: Second visible image forming unit, 53: Third visible image forming unit, 54: Fourth visible image forming unit, 55: Intermediate transfer belt, 56: Secondary transfer unit, 57: Internal paper feeding unit, 58: Manual paper feeding unit, 59, 65, 66, 67: Photoreceptor drum, 60: Charging roller, 61, 61a, 61b, 61c, 61d: Developing unit, 62, 62a, 62b, 62c, 62d: Primary transfer unit, 63: Cleaning unit, 63a: Cleaning blade, 64: Laser light source, 68, 69: Tension roller, 70: Waste toner box, 71: Fixing belt, 72: Pressure roller, 73, 74: Feeding roller, 75: Output tray, 76: Static elimination unit, 77: Cleaning roller, 161: Display unit, 162: Operation unit, 171: Charging voltage application unit, 172: Developing voltage application unit, 173: Transfer voltage application unit, 181: Charging power supply, 182: Developing power supply, 183: Transfer power supply, 601, 613a, 622a: Core metal, 611a: Developing roller, 612a: Developer, 621a: Transfer roller, 1711: AC voltage output unit, 1712: DC voltage output unit, 1713: Superimposed voltage application unit, 17111: Comparator, 17112: Switching circuit, 17113: Low-pass filter, 17114: High-pass filter, ER: Discharge roller, fc: Cutoff frequency, R1, R2: Rotation direction, RT1, RT2, RT3, RT4: Conveyor path

Claims

1. The image forming unit comprises a photoreceptor, a charging unit for charging the photoreceptor, a charging voltage application unit for applying a predetermined charging voltage to the charging unit, an exposure unit for forming an electrostatic latent image on the photoreceptor, a developing unit for supplying toner to the photoreceptor to form a toner image corresponding to the electrostatic latent image, a developing voltage application unit for applying a predetermined developing voltage to the developing unit, a transfer unit for transferring the toner image to a recording medium, and a fixing unit for heating and fixing the toner image to the recording medium. The charging voltage application unit comprises an AC voltage output unit that outputs an AC voltage, a DC voltage output unit that outputs a DC voltage, and a superimposed voltage application unit that applies a charging voltage obtained by superimposing the AC voltage and the DC voltage to the charging unit. The image forming apparatus is characterized in that the AC voltage output section comprises an amplification circuit that amplifies and outputs the voltage of the AC voltage, and a bandpass filter that limits the frequency of the AC voltage to a predetermined range of charging frequencies.

2. The image forming apparatus according to claim 1, wherein the bandpass filter comprises a low-pass filter that allows only frequencies of the AC voltage below a predetermined first frequency to pass through, and a high-pass filter that allows only frequencies above a predetermined second frequency to pass through.

3. The image forming apparatus according to claim 1, wherein the range of the charging frequency includes a range from 1 kHz to 2 kHz.

4. The image forming apparatus according to claim 2, wherein the high-pass filter has a second frequency set to block ripples of 100 Hz / 120 Hz of the commercial power supply frequency.

5. The image forming apparatus according to claim 1, wherein the amplification circuit is a Class D amplifier circuit.

Images