Image forming device

Abstract

To prevent uneven electrification of a photoconductor drum resulting from a transfer process and an image defect due to a remaining toner image.SOLUTION: An image forming apparatus comprises: a transfer roller 12 that is in contact with a photoconductor drum 1 to form a nip part and transfers a toner image on the photoconductor drum 1 to a recording material P; a transfer power supply device 50 including a rectification circuit part that outputs transfer voltage to the transfer roller 12; and a CPU 20 that controls the transfer power supply device 50. The rectification circuit part includes diodes D1, D2, D3, and, of a plurality of capacitors, a first capacitor group including capacitors connected in parallel without via the diodes D1, D2, D3, and a second capacitor group other than the first capacitor group. The capacity of the capacitor C2 in the second capacitor group is smaller than the capacities of capacitors C1, C3 in the first capacitor group.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus such as a copying machine, a printer, or a facsimile machine using an electrophotographic method. Regarding the device. [Background technology]

[0002] In an electrophotographic image forming apparatus, an electrostatic latent image is formed by contrasting a dark area potential (charge potential) VD on a photosensitive drum formed by charging with a charging unit with a light area potential VL on the photosensitive drum formed by exposing the dark area potential VD to light area potential VL on the photosensitive drum. The development contrast, which is the potential difference between the light area potential VL formed on the photosensitive drum and a development voltage applied to a developing roller carrying developer (toner), causes toner to migrate from the developing roller to the light area potential VL on the photosensitive drum. As a result, the toner adheres to the electrostatic latent image on the photosensitive drum, forming a toner image. The toner image formed on the photosensitive drum is transferred to a recording material by a transfer unit. A transfer roller is typically used as the transfer member used in the transfer unit. The transfer roller contacts the photosensitive drum to form a nip portion (hereinafter referred to as the transfer nip portion). As the transfer roller rotates in contact with the photosensitive drum, it nip-conveys the recording material and transfers the toner image on the photosensitive drum to the recording material. At this time, a transfer voltage of positive polarity, which is opposite to the negative polarity of the toner that forms the toner image, is applied to the transfer roller.

[0003] In electrophotographic image forming apparatuses, charging unevenness (charge history, potential unevenness) on the photosensitive drum resulting from the transfer process of a toner image from the photosensitive drum to a recording material can cause an image defect known as "memory," which manifests as density unevenness in the image formed on the recording material. As described above, in electrophotographic image forming apparatuses, the surface of the photosensitive drum becomes negative due to charging. Toner of the same polarity as the polarity of the photosensitive drum surface adheres to the light potential VL area, forming a toner image, which is then directly transferred to the recording material. In this configuration, for example, when the recording material peels off from the photosensitive drum at the transfer nip, a separation discharge can occur between the trailing edge of the recording material in the transport direction and the photosensitive drum, causing a positive charge of the same polarity as the transfer voltage to be transferred onto the photosensitive drum. If the amount of positive charge transferred to the photosensitive drum exceeds a certain amount, it cannot be removed by the charging device, resulting in a decrease in the absolute value of the dark potential VD of the area on the photosensitive drum where the separation discharge occurred, resulting in charging unevenness. As a result, the development contrast of that area is higher than that of other areas, resulting in a larger amount of toner adhering during development and a higher image density. In this way, uneven charging caused by the transfer process can cause horizontal black streaks called "memories" (areas of high density extending along the rotational axis of the photosensitive drum) to appear in the image that is subsequently formed.

[0004] For example, Patent Document 1 describes a method for suppressing the occurrence of "memory" by switching the transfer voltage in the non-image area of ​​the trailing edge of the recording material where no image is formed, i.e., by setting the transfer voltage to a voltage lower than that during transfer. By setting the transfer voltage to a voltage value lower than that during transfer, it is possible to suppress separation discharge that occurs between the trailing edge of the recording material and the photosensitive drum, and thus suppress the occurrence of "memory." [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-83812 Summary of the Invention [Problem to be solved by the invention]

[0006] However, in recent years, as printing speeds have increased due to improved productivity of image forming devices, the conveyance speed of recording materials within image forming devices has increased. Accordingly, the time it takes for the non-image portion of the trailing edge of the recording material to be conveyed through the transfer nip has also become shorter. Therefore, even if the transfer voltage is switched in the non-image portion of the trailing edge of the recording material, the transfer voltage may not fall (or decrease) sufficiently while the trailing edge of the recording material is conveyed through the transfer nip. As a result, the occurrence of peel discharge cannot be suppressed, and "memory" may occur. A power supply device that supplies the transfer voltage includes, for example, a transformer, a drive circuit for driving the transformer, and a rectifier circuit. The transformer boosts the voltage and rectifies and smooths it to generate a high-voltage DC transfer voltage. To suppress the occurrence of "memory," conventionally, the capacitance of the capacitor used in the rectifier circuit is reduced to speed up the fall of the transfer voltage.

[0007] However, although reducing the capacitance of the capacitor used in the rectifier circuit makes the transfer voltage fall faster, it also increases the ripple voltage of the transfer voltage.When the ripple voltage of the transfer voltage is large, depending on the image pattern and the environmental conditions such as temperature and humidity in which the image forming device is used, image defects such as "transfer defects" may occur, in which the toner image on the photosensitive drum remains on the photosensitive drum during transfer.

[0008] The present invention has been made under these circumstances, and has as its object to suppress image defects caused by uneven charging of the photosensitive drum due to the transfer process and by the toner image remaining on the photosensitive drum. [Means for solving the problem]

[0009] In order to solve the above-mentioned problems, the present invention has the following configuration.

[0010] (1) An image carrier; a transfer section that forms a nip portion with the image carrier and transfers a toner image formed on the image carrier to a recording material; and a power supply section that outputs a transfer voltage to the transfer section to transfer the toner image to the recording material, the power supply section including a transformer having a primary winding and a secondary winding, a switching section that performs a switching operation of a current flowing through the primary winding based on a drive signal, and a rectifier circuit section that rectifies and amplifies an AC voltage generated in the secondary winding of the transformer by the switching operation of the switching section and outputs the amplified voltage to the transfer section; a detection unit that is provided upstream of the transfer unit in a recording material conveyance path and detects the conveyed recording material; and a control means for outputting the drive signal to the switching unit to control the power supply unit so that a first transfer voltage is output from the power supply unit when the leading edge of the recording material reaches the nip portion, and a second transfer voltage lower than the first transfer voltage is output from the power supply unit when the trailing edge of the recording material reaches the nip portion, wherein the rectifier circuit unit includes a first capacitor group connected in series between the transformer and an output terminal that outputs a voltage to the transfer unit without passing through the diodes, and a second capacitor group that is the first capacitor group and is the second capacitor group of the plurality of capacitors, and the capacitance of the capacitors included in the second capacitor group is smaller than the capacitance of the capacitors included in the first capacitor group. When the detection means detects the leading edge of the recording material, the control means outputs the drive signal to the switching means at the timing when the leading edge of the recording material reaches the nip portion so that the rectifier circuit unit outputs the first transfer voltage by the time an image area of ​​the recording material, onto which the toner image on the image carrier is transferred, reaches the nip portion, and when the detection means detects the trailing edge of the recording material, stops outputting the drive signal at the timing when the trailing edge of the recording material reaches the nip portion so that the first transfer voltage output from the rectifier circuit unit is reduced to the second transfer voltage by the time the trailing edge of the recording material passes through the nip portion. An image forming apparatus characterized by: [Effects of the Invention]

[0011] According to the present invention, it is possible to suppress image defects caused by uneven charging of the photosensitive drum due to the transfer process and by the toner image remaining on the photosensitive drum. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a cross-sectional view showing a schematic configuration of an image forming apparatus according to an embodiment of the present invention. [Figure 2] FIG. 1 is a diagram illustrating a method for measuring the volume resistance of a transfer roller according to an embodiment of the present invention. [Figure 3] 1 is a schematic circuit diagram showing the circuit configuration of a transfer power supply device according to an embodiment of the present invention, and a diagram showing the output waveform of a transformer; [Figure 4] FIG. 10 is a diagram showing the correlation between transfer voltage and transfer efficiency in an embodiment. [Figure 5] 10 is a timing chart illustrating transfer voltage control according to an embodiment of the present invention; [Figure 6] FIG. 10 is a diagram illustrating a voltage waveform at the time of the falling edge of the transfer voltage in the embodiment. [Figure 7] FIG. 10 is a diagram illustrating a ripple voltage of a transfer voltage according to an embodiment of the present invention. [Figure 8] Schematic circuit diagram showing the circuit configuration of a transfer power supply device according to another embodiment. [Figure 9] Schematic circuit diagram showing the circuit configuration of a transfer power supply device according to another embodiment. [Figure 10] Schematic circuit diagram showing the circuit configuration of a transfer power supply device according to another embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. [Example]

[0014] [Configuration of image forming device] FIG. 1 is a cross-sectional view showing the schematic configuration of an electrophotographic image forming apparatus M to which the present invention is applied. The image forming apparatus M includes a photosensitive drum 1, which serves as an image carrier; a charging roller 2 that charges the surface of the photosensitive drum 1 with a uniform polarity and potential; and an exposure device 3 that exposes the surface of the photosensitive drum 1 to a light beam corresponding to image information to form an electrostatic latent image. The image forming apparatus M also includes a developing device 5 that develops the electrostatic latent image formed on the photosensitive drum 1 (image carrier). The developing device 5, which serves as a development unit, includes a developing toner container 5a, a developing roller 5b, and a developing blade 5c, and forms a toner image by attaching toner to the electrostatic latent image on the photosensitive drum 1. The image forming apparatus M also includes a transfer roller 12 that transfers the toner image formed on the photosensitive drum 1 to a recording material P, a waste toner container 4 that collects toner that is not transferred to the recording material P and remains on the surface of the photosensitive drum 1, and a cleaning member 4a. The photosensitive drum 1, the charging roller 2, the developing device 5, and the waste toner container 4 constitute the image forming unit. The image forming unit is integrated as a process cartridge, which is detachably mounted on the image forming apparatus M. Furthermore, in the process cartridge, the photosensitive drum 1 is rotatably supported by the image forming apparatus M, and is driven to rotate by a drive source (not shown) in the direction of arrow R1 (clockwise direction) in the drawing at a process speed of 250 mm / sec.

[0015] A paper feed cassette 7 containing recording material P such as paper is disposed below the image forming apparatus M. The image forming apparatus M includes, along a transport path for the recording material P, a paper feed roller 8 that feeds the recording material P from the paper feed cassette 7, a pair of transport rollers 9 that transport the recording material P fed by the paper feed roller 8, and a top sensor 10 that serves as a detection means for detecting the recording material P transported along the transport path. The image forming apparatus M also includes a pre-transfer guide 11 that guides the transported recording material P to a transfer roller 12 that serves as a transfer section, a transport guide 13 that guides the recording material P that has passed through the transfer roller 12 to a fixing device 14, and a fixing device 14 that fixes the toner image on the recording material P to the recording material P. The image forming apparatus M also includes a discharge roller 15 that discharges the recording material P that has passed through the fixing device 14 onto a discharge tray 16. A CPU 20, which serves as a control means, performs various controls, including the image formation operation of the image forming apparatus M.

[0016] [Image formation operation of image forming device] Next, the image forming operation of the image forming apparatus M will be described. When the CPU 20 receives a print job from an external device (not shown) instructing image formation on a recording material P, it starts controlling the image forming operation, as described below. The CPU 20 drives a drive source (not shown), and the surface of the photosensitive drum 1, which is rotated in the direction of arrow R1 by the drive source, is uniformly charged to a predetermined polarity and a predetermined potential by the charging roller 2. The charged photosensitive drum 1 is then irradiated with laser light L corresponding to the image information included in the print job by an exposure device 3, such as a laser optical system, and the exposed portion irradiated with the laser light L has its charge removed, forming an electrostatic latent image. The electrostatic latent image formed on the photosensitive drum 1 is developed by the developing device 5. The developing device 5 applies toner from a developing toner container 5a to the developing roller 5b, rotates the developing roller 5b in the direction r (counterclockwise) in the figure, and forms a toner layer imparted with a frictional charge by the developing blade 5c on the surface of the developing roller 5b. Then, by applying a development voltage to the development roller 5b, toner adheres to the electrostatic latent image on the photosensitive drum 1, thereby forming a toner image.

[0017] Meanwhile, recording material P is stored in a paper feed cassette 7 and is fed one sheet at a time from the paper feed cassette 7 by a paper feed roller 8, and the fed recording material P is conveyed along a conveyance path by a conveyance roller pair 9. As the recording material P is conveyed along the conveyance path, a top sensor 10 detects its leading edge in the conveyance direction and notifies the CPU 20. The CPU 20 controls the conveyance of the recording material P so that the timing at which the toner image on the photosensitive drum 1 moves to the transfer nip formed by the contact between the photosensitive drum 1 and the transfer roller 12 is synchronized with the timing at which the recording material P is conveyed to the transfer nip. Then, the recording material P conveyed along the conveyance path is conveyed to the transfer nip by a pre-transfer guide 11.

[0018] At the transfer nip, a transfer voltage of opposite polarity (positive polarity) to the toner charge polarity (negative polarity) is applied to the transfer roller 12 from a transfer power supply device 50, which serves as a transfer power supply, thereby transferring the toner image on the photosensitive drum 1 onto the recording material P. The recording material P with the transferred toner image is transported along a transport guide 13 to a fixing device 14. The fixing device 14 includes a fixing roller 14c with a built-in heater 14b that heats the toner image on the recording material P, and a pressure roller 14a that contacts the fixing roller 14c to form a nip and pressurizes the toner image on the recording material P. The fixing device 14 heats and pressurizes the unfixed toner image on the recording material P as it is conveyed through the nip, thereby fixing the toner image to the recording material P. The recording material P with the fixed toner image in the fixing device 14 is then discharged by discharge rollers 15 onto a discharge tray 16 on the top surface of the image forming apparatus M. On the other hand, toner (transfer residual toner) that is not transferred to the recording material P at the transfer nip and remains on the surface of the photosensitive drum 1 is removed by the cleaning member 4a and collected in the waste toner container 4. By repeating the above image forming operation, an image is formed on the recording material P.

[0019] The photosensitive drum 1 generally has a configuration in which a photosensitive material such as an OPC (organic photoconductor), amorphous selenium, amorphous silicon, etc. is provided on a drum-shaped (cylindrical) substrate (conductive substrate) made of aluminum, nickel, etc. The photosensitive drum 1 used in this embodiment is a negatively charged OPC photosensitive member with an outer diameter of 24 mm, and is configured with a photosensitive layer in which a charge generation layer and a charge transport layer are laminated in this order from the conductive substrate side on the surface of the conductive substrate made of an aluminum cylinder.

[0020] The charging roller 2 is composed of, for example, a conductive base shaft that also serves as an electrode to which a charging voltage is applied, and an elastic layer that cylindrically surrounds the outer circumferential surface of the conductive base shaft. The charging roller 2 used in this embodiment has an outer diameter of 10 mm, a core diameter of 5 mm, and an elastic layer thickness of 2.5 mm. The core is made of SUS, and the elastic layer is made of a mixed rubber material of NBR and epichlorohydrin.

[0021] The transfer roller 12 is composed of, for example, a conductive base shaft that also serves as an electrode to which a transfer voltage is applied, and an elastic layer that cylindrically surrounds the outer periphery of the base shaft. Semiconductive rubber materials such as EPDM, NBR, urethane rubber, epichlorohydrin, and silicone rubber are typically used for the elastic layer. The transfer roller 12 used in this embodiment has an outer diameter of 14 mm, a core diameter of 5 mm, an elastic layer thickness of 4.5 mm, and a hardness of 30° (Asker C hardness). The core is made of SUS, and the elastic layer is made of a mixed rubber material of NBR and epichlorohydrin. The contact pressure of the transfer roller 12 against the photosensitive drum 1 is 9.8 N (1 kgf).

[0022] In this embodiment, the surface of the photosensitive drum 1 is charged to a dark potential VD of −500 V by the charging roller 2, and the light potential VL after exposure by the exposure device 3 is roughly −100 V. A voltage of −1000 V is applied to the charging roller 2 from a charging power supply device (not shown) that generates a charging voltage. Furthermore, the toner is negatively charged by the developing device 5, and a developing voltage of −350 V is applied to the developing roller 5b. A positive voltage is applied to the transfer roller 12 from the transfer power supply device 50, and the toner image on the photosensitive drum 1 is transferred to the recording material P.

[0023] In this embodiment, negatively charged toner is used, but the present invention is not limited to this and can be similarly applied to the case where positively charged toner is used. When positively charged toner is used, the photosensitive drum 1 is positively charged by the charging roller 2, and a negative transfer voltage is applied to the transfer roller 12, whereby the toner image on the photosensitive drum 1 is transferred to the recording material P.

[0024] [Measurement of volume resistance of transfer roller] Next, a method for measuring the volume resistance of the transfer roller 12 will be described. FIG. 2 is a schematic diagram outlining the method for measuring the volume resistance of the transfer roller 12. The volume resistance of the transfer roller 12 is measured in an environment at a temperature of 23°C and a humidity of 50%. As shown in FIG. 2, both ends of the core 51 of the transfer roller 12 are pressed with a contact pressure of 4.9 N, so that the transfer roller 12 is pressed against a metal drum with a contact pressure of 9.8 N. Then, when a voltage V1 is applied to the core 51 of the transfer roller 12, the voltage Vref generated across the reference resistor Rref is measured using a digital multimeter (manufactured by FLUK). In this example, the applied voltage V1 to the core 51 is 2000 V, and the resistance of the reference resistor Rref is 1000 Ω. The voltage generated across the reference resistor Rref is measured 10 seconds after the voltage is applied to the core 51. Here, the average value of the voltage generated across the reference resistor Rref over a measurement time of 10 seconds is defined as voltage Vref, the value of the current flowing through the reference resistor Ref is defined as Iref, the voltage applied to the transfer roller 12 is defined as Vrol, and the current flowing through the transfer roller 12 is defined as Irol. Then, the volume resistance Rm of the transfer roller 12 can be calculated by the following (Equation 1). Volume resistance Rm = voltage Vrol / current Irol (Equation 1) Here, the voltage Vrol and the current Irol are calculated by the following (Equation 2) and (Equation 3). Voltage Vrol = Voltage V1 - Voltage Vref (Equation 2) Current Irol = Voltage Vref / Reference resistance Rref (Equation 3) Then, substituting (Equation 2) and (Equation 3) into (Equation 1), we obtain the following (Equation 4). Volume resistance Rm = (Voltage V1 - Voltage Vref) x Reference resistance Rref / Voltage Vref ...(Formula 4) From (Equation 4), the volume resistance Rm of the transfer roller 12 can be calculated based on the voltage Vref measured by the above-mentioned measurement method.

[0025] The volume resistivity Rm of the transfer roller 12 in this embodiment is 1.0×10 6 Ω~5.0×10 9 For example, when the volume resistance of the transfer roller 12 is 1.0×106 If the resistivity is smaller than Ω, the toner image formed on the photosensitive drum 1 may not be transferred sufficiently to the recording material P. The volume resistivity of ordinary paper used as the recording material P is 1.0×10 4 Ω m~1.0×10 13 Therefore, the volume resistance Rm of the transfer roller 12 is 1.0×10 6 If the volume resistivity Rm is smaller than 5.0×10 Ω, the transfer current does not easily flow from the transfer roller 12 to the recording material P during transfer. 9 If the volume resistivity Rm is greater than Ω, the transfer voltage required to pass the desired transfer current becomes too large, resulting in an increase in the cost of the transfer power supply device 50 that supplies the transfer voltage. 8 The transfer roller 12 is Ω.

[0026] [Configuration of transfer power supply] Next, the transfer power supply 50 that supplies the transfer voltage to the transfer roller 12 will be described. FIG. 3A is a circuit diagram showing the main circuit configuration of the transfer power supply 50 of this embodiment. In FIG. 3A, the transfer power supply 50 includes a transformer T having a primary winding and a secondary winding, and a field effect transistor (hereinafter referred to as FET) that is a switching unit that is switched by a drive signal output from the CPU 20. The transfer power supply 50 also includes a rectifier circuit (rectifier circuit unit) that rectifies the voltage induced on the secondary side of the transformer T and is composed of multiple diodes D1, D2, and D3, multiple capacitors C1, C2, and C3, and a resistor R1. One end of the secondary winding of the transformer T is connected to the anode terminal of the diode D1 (first diode) and one end of the capacitor C2 (second capacitor). The cathode terminal of the diode D1 is connected to the anode terminal of the diode D2 (second diode) and one end of the capacitors C1 and C3. The cathode terminal of diode D2 is connected to the anode terminal of diode D3 (third diode) and the other end of capacitor C2. The cathode terminal of diode D3 is connected to the other end of capacitor C3 (third capacitor), one end of resistor R1, and the output terminal. The other end of the secondary winding of transformer T is connected to the other end of capacitor C1 (first capacitor) and the other end of resistor R1.

[0027] In the transfer power supply device 50, a drive signal output from the CPU 20 drives the FET to repeatedly perform switching operations, thereby driving the transformer T. A positive DC voltage is generated by a rectifier circuit provided on the secondary side of the transformer T. The rectifier circuit provided on the secondary side of the transformer T is a voltage-tripling rectifier circuit that multiplies (amplifies) the voltage induced on the secondary side. Generally, when attempting to output a high voltage using a single rectifier circuit, measures such as covering the periphery of the transformer with a resin with high voltage resistance are required to prevent discharge and leakage within the transformer, resulting in a significant increase in costs. Therefore, using a voltage-doubler rectifier circuit, as in this embodiment, offers cost advantages.

[0028] [Transfer voltage output operation] Next, the operation of the transfer power supply device 50 when applying a transfer voltage to the transfer roller 12 during image formation will be described. Figure 3(b) is a diagram showing the voltage waveform of the AC voltage induced on the secondary side of the transformer T when the FET is repeatedly turned on and off by a drive signal output from the CPU 20 in the transfer power supply device 50, driving the transformer T. Figure 3(b) shows one cycle of the voltage waveform generated on the secondary winding side of the transformer T where the black circle indicating the start of winding is not marked in Figure 3(a), and the voltage is a square wave voltage waveform of +Vo and -Vo. In Figure 3(b), the vertical axis represents voltage (unit: V) and the horizontal axis represents time.

[0029] In FIG. 3(b), when the voltage is +Vo, diodes D1 and D3 are conductive, diode D2 is non-conductive, and capacitors C1 and C3 are charged. At this time, +Vo, which is a half-wave rectified voltage of the transformer output, is applied to capacitor C1, and +2Vo, which is a double rectified voltage of the transformer output, is applied to capacitor C3. As a result, a voltage of +3Vo is output to the output terminal of transfer power supply 50. On the other hand, when the voltage is -Vo, diode D2 is conductive, and diodes D1 and D3 are non-conductive. At this time, +2Vo, which is a double rectified voltage of the transformer output, is applied to capacitor C2.

[0030] The operation of the power supply circuit shown in Figure 3(a) is summarized below. As described above, when the AC voltage shown in Figure 3(b) is induced on the secondary side of the transformer T, a voltage of +Vo is applied to capacitor C1, +2Vo to capacitor C2, and +2Vo to capacitor C3, and capacitors C1 to C3 are charged. In this state, if a voltage of +Vo appears on the secondary winding side of the transformer T that is not marked with a black circle, the output voltage +Vo of the transformer T and the voltage of +2Vo due to the charge stored in capacitor C2 are combined, and +3Vo is output at the output terminal. On the other hand, if a voltage of -Vo appears on the secondary winding side of the transformer T that is not marked with a black circle, the voltage of +Vo due to the charge stored in capacitor C1 and the voltage of +2Vo due to the charge stored in capacitor C3 maintain +3Vo at the output terminal.

[0031] [Operation when transfer voltage output is stopped] Next, we will explain the behavior of the transfer power supply 50 when the application of the transfer voltage to the transfer roller 12 is stopped and the transfer voltage is dropped. First, when the drive signal output from the CPU 20 is stopped and the FET is turned off, voltage is no longer induced on the secondary side of the transformer T, and the charge stored in capacitors C1, C2, and C3 begins to discharge. Immediately after discharge begins, the charging voltage of capacitor C2 is smaller than the sum of the charging voltages of capacitors C1 and C3, so diode D3 does not become conductive. Therefore, capacitor C2 hardly discharges, and the discharge rate of the transfer voltage, which is the output voltage of the transfer power supply 50, is determined by the capacitance of the series-connected capacitors C1 and C3 and the resistance value of resistor R1.

[0032] As the discharge progresses, when the sum of the charging voltages of capacitors C1 and C3 becomes smaller than the charging voltage of capacitor C2, diode D3 becomes conductive and capacitor C2 begins to discharge. As a result, the capacitance value of the capacitors, which affects the discharge rate of the output voltage, increases by the capacitance of capacitor C2 in addition to the capacitance of capacitors C1 and C3, and the discharge rate slows down compared to before capacitor C2 began to discharge, slowing down the rate at which the transfer voltage decreases.

[0033] As the discharge progresses, when the charging voltage of capacitor C2 equals the sum of the charging voltages of capacitors C1 and C3, the discharge rates of capacitor C2, C1, and C3 become equal, and capacitor C1 or C3 completes discharging first. When capacitor C1 or C3 completes discharging, the capacitance of the capacitor related to the transfer voltage discharge rate changes from the capacitance when capacitors C1 and C3 are connected in series to the capacitance of capacitor C1 or C3 plus the capacitance of capacitor C2. As a result, the capacitance of the capacitor related to the transfer voltage discharge rate increases, further slowing the capacitor discharge rate and further slowing the rate at which the transfer voltage decreases. Whether capacitor C1 or C3 completes discharging first depends on the capacitance of each capacitor.

[0034] In this way, in the transfer power supply 50 of this embodiment, when the application of the transfer voltage to the transfer roller 12 is stopped and the transfer voltage is dropped, the discharge speed of the capacitors C1, C2, and C3 of the rectifier circuit changes twice depending on the charging voltage. The time until the discharge speed of the capacitors changes and the voltage at which the discharge speed of the capacitors changes can be changed by changing the capacitance value of each capacitor and the resistance value of resistor R1.

[0035] [Ripple voltage when transferring voltage is output] Next, we will explain the ripple voltage when the output voltage of the transformer T in the transfer power supply 50 of FIG. 3(a) is a square wave as shown in FIG. 3(b). First, when the output voltage of the transformer T is −Vo, diodes D1 and D3 are non-conductive, and only diode D2 is conductive. Because diode D3 is non-conductive, the capacitance value of the capacitors relative to the output voltage of the transfer power supply 50 is the capacitance value when capacitors C1 and C3 are connected in series. Therefore, when the output voltage of the transformer T is −Vo, the transfer voltage, which is the output voltage, decreases at a discharge rate determined by the capacitance value when capacitors C1 and C3 are connected in series, the resistance value of resistor R1, and the resistance value of the transfer roller 12. Next, when the output voltage of the transformer T is +Vo, diodes D1 and D3 are conductive, and diode D2 is non-conductive, so capacitors C1 and C3 are charged, and the transfer voltage applied to the transfer roller 12 increases. Next, when the output voltage of transformer T changes to -Vo, diodes D1 and D3 become non-conductive and diode D2 becomes conductive, causing the transfer voltage to drop again. By repeating this operation, a ripple voltage, which is the peak-to-peak voltage and the voltage difference between the maximum and minimum voltages in the transfer voltage, is generated in the transfer voltage applied to transfer roller 12.

[0036] As described above, the ripple voltage of the output voltage (transfer voltage) of the transfer power supply 50 shown in FIG. 3A is determined by the capacitances of the capacitors C1 and C3, which are connected in series between the transformer T and the transfer voltage output terminal without using multiple diodes. This series-connected set of capacitors is referred to as the first capacitor group. In other words, in this embodiment, the first capacitor group includes capacitors C1 and C3. On the other hand, the capacitance of capacitor C2 does not affect the ripple voltage. Therefore, by reducing the capacitance of capacitor C2, it is possible to terminate the application of the transfer voltage to the transfer roller 12 and shorten the fall time of the transfer voltage when the transfer voltage is lowered, without increasing the ripple voltage. Thus, the set of capacitors C1-C3 that are not included in the first capacitor group is referred to as the second capacitor group. In other words, in this embodiment, the second capacitor group includes capacitor C2. Therefore, in this embodiment, the circuit configuration of the transfer power supply 50 is such that the capacitances of capacitors C1, C2, and C3 are 300 pF, 50 pF, and 300 pF, respectively.

[0037] [Transfer void] As mentioned above, depending on the capacitance of the capacitor in the rectifier circuit of the transfer power supply device 50, the ripple voltage of the transfer voltage applied to the transfer roller 12 may exceed a predetermined voltage range. When the ripple voltage is large, the transfer voltage applied to the transfer roller 12 fluctuates, becoming higher or lower than the appropriate transfer voltage (first transfer voltage), which can result in an image defect known as a "transfer omission." Figure 4 illustrates the correlation between the transfer voltage applied to the transfer roller 12 from the transfer power supply device 50 and transfer efficiency. The vertical axis represents transfer efficiency (unit: %), and the horizontal axis represents transfer voltage (unit: V). Here, transfer efficiency is an index based on the difference in toner mass per unit area between before and after transfer of the toner image formed on the photosensitive drum 1 to the recording material P, and is defined by the following equation (5):

number

[0038] As shown in FIG. 4, transfer efficiency is maximized at an appropriate transfer voltage (Vmax in the figure). When the transfer voltage is lower than the transfer voltage Vmax that maximizes transfer efficiency, the transfer voltage required to transfer the toner image on the photosensitive drum 1 to the recording material P is insufficient, and toner that was not transferred to the recording material P remains on the photosensitive drum 1, resulting in reduced transfer efficiency. This phenomenon of reduced transfer efficiency due to insufficient transfer voltage is called "weak transfer loss." On the other hand, when the transfer voltage is higher than the transfer voltage Vmax that maximizes transfer efficiency, the polarity of the toner transferred to the recording material P at the transfer nip may reverse from negative to positive, resulting in retransfer of the toner onto the negative photosensitive drum 1. In this case, the retransferred toner remains on the photosensitive drum 1 after transfer, resulting in reduced transfer efficiency. This phenomenon of reduced transfer efficiency due to a transfer voltage higher than the appropriate transfer voltage Vmax is called "strong transfer loss." The above-described "weak transfer loss" and "strong transfer loss" are collectively referred to as "transfer loss." When "transfer loss" occurs, image defects such as missing toner images on the recording material P or density variations may occur. According to the study by the authors, when an image forming apparatus M equipped with the transfer power supply device 50 of this embodiment is used, it has been found that the occurrence of "missing transfer" can be suppressed by setting the ripple voltage to 30 V or less, which is within the predetermined voltage range.

[0039] [Transfer voltage control] Next, the control of the transfer voltage in the transfer power supply device 50 of this embodiment will be described. The CPU 20 calculates the timing at which the leading and trailing edges of the recording material P reach the transfer nip based on the timing at which the top sensor 10, located upstream of the transfer nip, detects the leading and trailing edges of the recording material P in the transport direction and the transport speed of the recording material P. In this embodiment, the photosensitive drum 1 is rotated at a peripheral speed of 250 mm / sec, and the recording material P is transported at roughly the same speed. Therefore, the CPU 20 calculates the required time for the leading edge of the recording material P to reach the transfer nip based on the timing at which the top sensor 10 detects the leading edge of the recording material P, the transport speed of the recording material P, and the distance from the top sensor 10 to the transfer nip. Similarly, the CPU 20 calculates the required time from the timing at which the top sensor 10 detects the trailing edge of the recording material P until the trailing edge of the recording material P reaches the transfer nip. The CPU 20 drives the transfer power supply device 50 and controls the transfer voltage based on the calculated timings at which the leading and trailing edges of the recording material P reach the transfer nip portion.

[0040] FIG. 5 is a diagram illustrating the control sequence of the transfer voltage of the transfer power supply device 50 by the CPU 20. FIG. 5 shows the state of the transfer voltage output from the transfer power supply device 50 when the recording material P is transported to the transfer nip portion. FIG. 5(a) shows two sheets of recording material P being transported to the transfer nip portion. In this embodiment, the areas 5 mm inward from the leading and trailing edges of the recording material P in the transport direction are designated as masked areas (non-image areas) where image formation is not performed, and the area further inward is designated as an image formable area. Similarly, for the edge sides of the recording material P perpendicular to the transport direction of the recording material P, the areas 5 mm inward from each edge are designated as masked areas (non-image areas) where image formation is not performed, and the area further inward is designated as an image formable area. FIG. 5(b) shows the period of transfer voltage control that controls the transfer voltage output from the transfer power supply device 50. In the figure, "OFF" indicates a period during which the CPU 20 does not control the output voltage of the transfer power supply device 50, and "ON" indicates a period during which the CPU 20 outputs a drive signal to FET1 of the transfer power supply device 50 to apply a transfer voltage to the transfer roller 12. As shown in FIG. 5(b), the signal turns "ON" when the leading edge of the recording material P reaches the transfer nip, and turns "OFF" when the trailing edge of the recording material P reaches the transfer nip. FIG. 5(c) shows the voltage value of the transfer voltage output from the transfer power supply device 50. "During transfer" indicates the transfer voltage output during the period during which the toner image on the photosensitive drum 1 is transferred to the recording material P, and "during non-transfer" indicates the transfer voltage during the period during which the toner image on the photosensitive drum 1 is not transferred to the recording material P. Note that the horizontal axes of FIGS. 5(a), (b), and (c) indicate time, and t1 to t8 indicate time (timing).

[0041] In this embodiment, the transfer voltage control is turned on when the leading edge of the recording material P reaches the transfer nip (times t1 and t5) (FIG. 5B). Then, the CPU 20 controls the transfer voltage output by the transfer power supply 50 to rise to a voltage value that allows the toner image on the photosensitive drum 1 to be transferred to the recording material P during the period until the non-image area at the leading edge of the recording material P reaches the transfer nip (times t2 and t6) (FIG. 5C). On the other hand, the transfer voltage control is turned off when the recording material P is 5 mm inside from the trailing edge and reaches the transfer nip (times t3 and t7) (FIG. 5B). Then, the CPU 20 controls the transfer voltage output by the transfer power supply 50 to fall to a transfer voltage value that does not cause the above-mentioned "memory" during the period until the trailing edge of the recording material P leaves (passes) the transfer nip (times t4 and t8) (FIG. 5C). In the image forming apparatus M of this embodiment, the process speed is 250 mm / sec, so it takes approximately 20 msec for the recording material P to move (be conveyed) 5 mm. In other words, 20 msec after the transfer voltage control is turned off, the trailing edge of the recording material P leaves (passes) the transfer nip. Therefore, in this embodiment, the transfer voltage must be lowered from the voltage used during transfer to a voltage that does not cause "memory" within 20 msec after the transfer voltage control is turned off (from the time when the recording material P reaches the transfer nip 5 mm inside from the trailing edge). According to the authors' investigation, when using the image forming apparatus M of this embodiment, it has been found that "memory" does not occur if the transfer voltage is lowered to approximately 150 V or less (second transfer voltage) when the trailing edge of the recording material P leaves the transfer nip.

[0042] [Effects of this Example] Next, an evaluation experiment conducted to confirm the effects of this embodiment will be described. In the evaluation experiment, in addition to the above-mentioned combinations of capacitances of capacitors C1, C2, and C3, the following combinations of capacitor capacitances different from those of this embodiment were also evaluated as comparative examples 1 and 2. In comparative example 1, the capacitances of capacitors C1, C2, and C3 were 300 pF, 300 pF, and 300 pF, respectively. On the other hand, in comparative example 2, the capacitances of capacitors C1, C2, and C3 were 300 pF, 300 pF, and 50 pF, respectively.

[0043] In the evaluation experiment, the image forming apparatus M was placed in an environment with a temperature of 23°C and a humidity of 50%, and the recording material P was LTR (letter) size with a basis weight of 75 g / m 2 A Vitality (manufactured by Xerox) was used. Under these conditions, images with a density of 40% were printed consecutively on two sheets of recording material P, and the presence or absence of "memory" in the image formed on the second sheet of recording material P due to separation discharge between the rear end of the first sheet of recording material P and the photosensitive drum 1 was confirmed. In addition, the transfer power supply device 50 was controlled so that the transfer voltage when the image area of ​​the recording material P (the leading and trailing ends of the recording material P, and the area 5 mm inward from the edge of the recording material on the side perpendicular to the conveyance direction) passed through the transfer nip was 2000 V.

[0044] Table 1 shows the results of an evaluation experiment conducted using the combinations of capacitors C1, C2, and C3 of the present embodiment and comparative examples 1 and 2. In Table 1, the vertical axis lists the experimental results for the present embodiment, comparative example 1, and comparative example 2, while the horizontal axis lists the following items: The horizontal axis lists the capacitances (pF) of capacitors C1, C2, and C3, the ripple voltage (V) in the transfer voltage, and the transfer voltage (V) when the first sheet of recording material P passed through the transfer nip. Furthermore, the horizontal axis of Table 1 also lists the presence or absence of image defects such as horizontal black streaks associated with "memory" and transfer voids caused by the ripple voltage in the transfer voltage. Regarding "memory," if an image defect occurred, it was evaluated as "poor," and if not, it was evaluated as "good." Similarly, if a "transfer void" occurred, it was evaluated as "poor," and if not, it was evaluated as "good." [Table 1]

[0045] FIG. 6 is a waveform diagram showing the falling edge of the transfer voltage when the transfer voltage control is "OFF" during the evaluation experiment described above. In FIG. 6, (a) shows the falling edge of the transfer voltage in this embodiment, (b) shows the falling edge of the transfer voltage in Comparative Example 1, and (c) shows the falling edge of the transfer voltage in Comparative Example 2. In FIGS. 6(a), (b), and (c), the vertical axis represents voltage, and the horizontal axis represents time, with ta in FIG. 6(a), tb in FIG. 6(b), and tc in FIG. 6(c) representing time (timing). The "Period 1," "Period 2," and "Period 3" on the horizontal axis in FIGS. 6(a), (b), and (c) represent the following periods: "Period 1" is the period from when the transfer voltage output from the transfer power supply 50 is stopped, the capacitors start discharging, and the charging voltage of the serially connected capacitors C1 and C3 becomes equal to the charging voltage of capacitor C2. "Period 2" is the period from when the charging voltage of the series-connected capacitors C1 and C3 becomes equal to the charging voltage of capacitor C2 until either capacitor C1 or capacitor C3 is discharged and the charging voltage becomes 0. "Period 3" is the period from when either capacitor C1 or capacitor C3 is discharged and the charging voltage becomes 0 until the charging voltage of the other capacitor and capacitor C2 are discharged and the charging voltage becomes 0.

[0046] 7 is a waveform diagram showing the state of ripple voltage in the transfer voltage applied to the transfer roller 12 from the transfer power supply device 50 during the period when the transfer voltage control was "ON" when the above-mentioned evaluation experiment was conducted. In FIG. 7, (a) shows the state of ripple voltage in this embodiment, (b) shows the state of ripple voltage in Comparative Example 1, and (c) shows the state of ripple voltage in Comparative Example 2. In FIGS. 7(a), (b), and (c), the vertical direction represents voltage and the horizontal direction represents time.

[0047] As shown in Table 1, the combination of capacitances of capacitors C1, C2, and C3 in this example did not result in horizontal black streaks associated with "memory," nor did "transfer gaps" due to ripple voltage in the transfer voltage occur. As shown in Table 1, the transfer voltage when the first sheet of recording material P passed through the transfer nip was 100 V, and the ripple voltage when the transfer voltage was applied to the transfer roller 12 was 11 V. In this example, by setting the capacitances of capacitors C1, C2, and C3 to 300 pF, respectively, the transfer voltage when the trailing edge of the recording material P passed through the transfer nip was rapidly reduced to 150 V or less, at which "memory" did not occur. Furthermore, by keeping the ripple voltage below 30 V, at which "transfer gaps" did not occur, the occurrence of transfer gaps was suppressed.

[0048] In this embodiment, the voltage waveform of the transfer voltage at its falling edge is as shown in FIG. 6(a). In this embodiment, the capacitance of capacitor C2, which is charged with the doubled rectified voltage in the rectifier circuit of the transfer power supply 50, is made smaller than that of capacitors C1 and C3, thereby increasing the discharge rate of capacitor C2 during period 2 compared to comparative examples 1 and 2. This results in a transfer voltage of 100 V at time ta, when the trailing edge of the recording material P exits the transfer nip. This allows the transfer voltage to drop to a voltage that does not cause "memory" by the time the trailing edge of the recording material P exits the transfer nip. Furthermore, as shown in FIG. 7(a), the ripple voltage of the transfer voltage in this embodiment is suppressed to 11 V because the capacitance of capacitor C3 is set to 300 pF. By suppressing the ripple voltage to 30 V or less, the occurrence of "missing transfer" is suppressed.

[0049] On the other hand, as shown in Table 1, the combination of capacitances of capacitors C1, C2, and C3 in Comparative Example 1 did not cause "transfer defects" due to the ripple voltage of the transfer voltage, but did cause horizontal black streaks due to "memory." As shown in Table 1, the transfer voltage was 400 V when the first recording material P passed through the transfer nip, and the ripple voltage was 11 V when the transfer voltage was applied to the transfer roller 12. In Comparative Example 1, the capacitances of capacitors C1, C2, and C3 were all 300 pF, which is the same and large. Therefore, as shown in Table 1, the transfer voltage only dropped to 400 V when the first recording material P passed through the transfer nip. The voltage waveform of the falling edge of the transfer voltage in Comparative Example 1 is as shown in Figure 6(b). As shown in Figure 6(b), the falling edge of the transfer voltage is gentler than in this example. As a result, the transfer voltage at time tb when the trailing edge of the recording material P leaves the transfer nip is 400V, and by the time the trailing edge of the recording material P leaves the transfer nip, the transfer voltage has not fallen to 150V or less, a voltage at which "memory" does not occur, and the occurrence of "memory" could not be suppressed. On the other hand, as shown in Figure 7(b), the ripple voltage of the transfer voltage in this embodiment is suppressed to 11V because the capacitance of capacitor C3 is 300pF, and the occurrence of "transfer omissions" is suppressed.

[0050] Furthermore, as shown in Table 1, the combination of capacitances of capacitors C1, C2, and C3 in Comparative Example 2 did not cause horizontal black streaks due to "memory," but did cause "transfer defects" due to the ripple voltage of the transfer voltage. As shown in Table 1, the transfer voltage was 100 V when the first sheet of recording material P left (passed) the transfer nip, and the ripple voltage was 35 V when the transfer voltage was applied to the transfer roller 12. The voltage waveform of the falling edge of the transfer voltage in Comparative Example 2 is shown in FIG. 6(c). As shown in FIG. 6(c), in Comparative Example 2, the capacitance of capacitor C3, which is charged with the doubled rectified voltage in the rectifier circuit of the transfer power supply device 50, is reduced, thereby increasing the capacitor discharge rate during period 1. As a result, the transfer voltage is 100 V at time tc when the trailing edge of the recording material P leaves the transfer nip, and the transfer voltage is reduced to 150 V or less, a voltage at which "memory" does not occur, by the time the trailing edge of the recording material P leaves the transfer nip. On the other hand, as shown in FIG. 7(c), the ripple voltage of the transfer voltage in this embodiment was 35V because the capacitance of the capacitor C3 was 50 pF, resulting in "missing transfer."

[0051] As described above, according to this embodiment, the occurrence of "memory" can be suppressed by quickly lowering the transfer voltage, and the occurrence of "transfer defects" can be suppressed by reducing the ripple voltage of the transfer voltage. As a result, image defects such as transfer defects and density unevenness can be suppressed, and good image formation without image defects can be performed.

[0052] In this embodiment, the configuration has been described in which the CPU 20 that controls the image forming apparatus M controls the transfer power supply device 50. For example, the transfer power supply device 50 may have a dedicated CPU that controls the transfer power supply device 50, and the dedicated CPU may control the output of the transfer voltage based on instructions from the CPU 20 of the image forming apparatus M.

[0053] As described above, according to this embodiment, it is possible to suppress image defects caused by uneven charging of the photosensitive drum due to the transfer process and by the toner image remaining on the photosensitive drum.

[0054] [Other Examples] In the above-described embodiment, a triple voltage rectifier circuit is used as the rectifier circuit of the positive polarity power supply circuit of the transfer power supply device 50, but the effects of the present invention are not limited to this. For example, the present invention can also be applied to a quadruple to sixfold voltage rectifier circuit, which can achieve the same effects as the triple voltage rectifier circuit.

[0055] [4x voltage rectifier circuit] FIG. 8 is a circuit diagram showing the main circuit configuration of a transfer power supply 50 having a quadruple voltage rectifier circuit. The transfer power supply 50 shown in FIG. 8 includes a transformer T having a primary winding and a secondary winding, and an FET that is switched by a drive signal output from the CPU 20. The transfer power supply 50 also includes a quadruple voltage rectifier circuit that rectifies the voltage induced on the secondary side of the transformer T, and the quadruple voltage rectifier circuit is composed of diodes D5, D6, D7, and D8 and capacitors C5, C6, C7, and C8. The circuit configuration in FIG. 8, excluding the quadruple voltage rectifier circuit, is the same as the circuit configuration of the transfer power supply 50 shown in FIG. 3(a) described above, and therefore will not be described here.

[0056] In Figure 8, one end of the secondary winding of the transformer T is connected to one end of a capacitor C5, and the other end of the capacitor C5 is connected to the cathode terminal of a diode D5, the anode terminal of a diode D6, and one end of a capacitor C7. The other end of the capacitor C7 is connected to the cathode terminal of the diode D7 and the anode terminal of a diode D8. The other end of the secondary winding of the transformer T is connected to the anode terminal of the diode D5 and one end of the capacitor C6. The other end of the capacitor C6 is connected to the cathode terminal of the diode D6, the anode terminal of the diode D7, and one end of the capacitor C8. The other end of the capacitor C8 is connected to the cathode terminal of the diode D8 and the output terminal.

[0057] 8, capacitor C5 is charged with +Vo, which is a half-wave rectified voltage of the output voltage of transformer T, and capacitors C6, C7, and C8 are each charged with +2Vo, which is a double rectified voltage of the output voltage of transformer T. As a result, a voltage of +4Vo is output from transfer power supply 50.

[0058] The ripple voltage of the output voltage (transfer voltage) of the transfer power supply 50 in Figure 8 is determined by the capacitance of the series-connected capacitors C6 and C8; the capacitance of capacitors C5 and C7 does not affect the ripple voltage. Therefore, by reducing the capacitance of capacitors C5 and C7, the application of the transfer voltage to the transfer roller 12 is terminated and the fall time of the transfer voltage when the transfer voltage is lowered can be shortened without increasing the ripple voltage, thereby suppressing the occurrence of "memory." Furthermore, by setting the capacitance of the series-connected capacitors C6 and C8 so that the ripple voltage is within a predetermined voltage range, the occurrence of "missing transfer" can be suppressed.

[0059] [5x voltage rectifier circuit] FIG. 9 is a circuit diagram showing the main circuit configuration of a transfer power supply 50 having a quintuple voltage rectifier circuit. The transfer power supply 50 shown in FIG. 9 includes a transformer T having a primary winding and a secondary winding, and an FET that is switched by a drive signal output from the CPU 20. The transfer power supply 50 also includes a quintuple voltage rectifier circuit that rectifies the voltage induced on the secondary side of the transformer T. The quintuple voltage rectifier circuit is composed of diodes D9, D10, D11, D12, and D13, and capacitors C9, C10, C11, C12, and C13. The circuit configuration in FIG. 9, excluding the quintuple voltage rectifier circuit, is the same as the circuit configuration of the transfer power supply 50 shown in FIG. 3(a) described above, and therefore will not be described here.

[0060] One end of the secondary winding of the transformer T is connected to the anode terminal of the diode D9 and one end of the capacitor C10. The cathode terminal of the diode D9 is connected to the anode terminal of the diode D10, one end of the capacitor C9, and one end of the capacitor C11. The cathode terminal of the diode D10 is connected to the anode terminal of the diode D11, the other end of the capacitor C10, and one end of the capacitor C12. The cathode terminal of the diode D11 is connected to the anode terminal of the diode D12, the other end of the capacitor C11, and one end of the capacitor C13. The cathode terminal of the diode D12 is connected to the anode terminal of the diode D13 and the other end of the capacitor C12. The cathode terminal of the diode D13 is connected to the other end of the capacitor C13 and the output terminal. The other end of the secondary winding of the transformer T is connected to the other end of the capacitor C9.

[0061] In Figure 9, capacitor C9 is charged with +Vo, which is the half-wave rectified voltage of the output voltage of transformer T, and capacitors C10, C11, C12, and C13 are each charged with +2Vo, which is a double rectified voltage of the output voltage of transformer T. As a result, a voltage of +5Vo is output from the positive power supply circuit.

[0062] The ripple voltage of the output voltage (transfer voltage) of the transfer power supply 50 in Figure 9 is determined by the capacitance of the series-connected capacitors C9, C11, and C13; the capacitance of capacitors C10 and C12 does not affect the ripple voltage. Therefore, by reducing the capacitance of capacitors C10 and C12, the application of the transfer voltage to the transfer roller 12 can be terminated without increasing the ripple voltage, and the fall time of the transfer voltage when it is lowered can be shortened, thereby suppressing the occurrence of "memory." Furthermore, by setting the capacitance of the series-connected capacitors C9, C11, and C13 so that the ripple voltage is within a predetermined voltage range, the occurrence of "missing transfer" can be suppressed.

[0063] [6x voltage rectifier circuit] FIG. 10 is a circuit diagram showing the main circuit configuration of a transfer power supply 50 having a hexavoltage rectifier circuit. The transfer power supply 50 shown in FIG. 10 includes a transformer T having a primary winding and a secondary winding, and an FET that is switched by a drive signal output from the CPU 20. The transfer power supply 50 also includes a hexavoltage rectifier circuit that rectifies the voltage induced on the secondary side of the transformer T. The hexavoltage rectifier circuit is composed of diodes D14, D15, D16, D17, D18, and D19, and capacitors C14, C15, C16, C17, C18, and C19. The circuit configuration in FIG. 10, excluding the hexavoltage rectifier circuit, is the same as the circuit configuration of the transfer power supply 50 shown in FIG. 3(a) described above, and therefore will not be described here.

[0064] 10, one end of the secondary winding of the transformer T is connected to one end of a capacitor C14, and the other end of the capacitor C14 is connected to the cathode terminal of a diode D14, the anode terminal of a diode D15, and one end of a capacitor C16. The other end of the capacitor C16 is connected to the cathode terminal of the diode D16, the anode terminal of a diode D17, and one end of a capacitor C18. The other end of the capacitor C18 is connected to the cathode terminal of the diode D18 and the anode terminal of a diode D19.

[0065] The other end of the secondary winding of transformer T is connected to the anode terminal of diode D14 and one end of capacitor C15. The other end of capacitor C15 is connected to the cathode terminal of diode D15, the anode terminal of diode D16, and one end of capacitor C17. The other end of capacitor C17 is connected to the cathode terminal of diode D17, the anode terminal of diode D18, and one end of capacitor C19. The other end of capacitor C19 is connected to the cathode terminal of diode D19 and the output terminal.

[0066] In Figure 10, capacitor C14 is charged with +Vo, which is the half-wave rectified voltage of the output voltage of transformer T, and capacitors C15, C16, C17, C18, and C19 are each charged with +2Vo, which is the double rectified voltage of the output voltage of transformer T. As a result, a voltage of +6Vo is output from the positive power supply circuit.

[0067] The ripple voltage of the output voltage (transfer voltage) of the transfer power supply 50 shown in FIG. 10 is determined by the capacitance of the series-connected capacitors C15, C17, and C19; the capacitance of capacitors C14, C16, and C18 does not affect the ripple voltage. Therefore, by reducing the capacitance of capacitors C14, C16, and C18, the application of the transfer voltage to the transfer roller 12 can be terminated and the fall time of the transfer voltage can be shortened without increasing the ripple voltage. This reduces the occurrence of "memory." Furthermore, by setting the capacitance of the series-connected capacitors C15, C17, and C19 so that the ripple voltage is within a predetermined voltage range, the occurrence of "missing transfer" can be reduced.

[0068] As explained above, the present invention can also be applied to rectifier circuits with four to six times the voltage. Among the capacitors constituting the n-times voltage rectifier circuit (where n is 3 or greater), the occurrence of "memory" can be suppressed by reducing the capacitance of the other capacitors, except for the capacitor connected in series between the transformer T and the output terminal of the transfer power supply 50 without a diode. Furthermore, by setting the capacitance of the capacitor connected in series between the transformer T and the output terminal of the transfer power supply 50 without a diode so that the ripple voltage falls within a predetermined voltage range, the occurrence of "missing transfer" can be suppressed.

[0069] As described above, according to the other embodiments, it is possible to suppress image defects caused by uneven charging of the photosensitive drum due to the transfer process and by the toner image remaining on the photosensitive drum.

[0070] In the above embodiment, a monochrome image forming apparatus M is described as an example in which a toner image is transferred from a photosensitive drum 1, which serves as an image carrier, to a recording material P. However, the present invention is not limited to this. The present invention may also be applied to a so-called tandem color image forming apparatus equipped with photosensitive drums corresponding to four colors of toner—yellow, magenta, cyan, and black—and an intermediate transfer belt. In this configuration, the toner images of each color are sequentially transferred from the photosensitive drums corresponding to each color to the intermediate transfer belt. The color toner images thus formed are transferred from the intermediate transfer belt to the recording material P, thereby forming a color toner image on the recording material P. The intermediate transfer belt, which serves as an image carrier, forms a nip with a secondary transfer roller, and the toner image is transferred by applying a transfer voltage to the secondary transfer roller. The transfer power supply 50 described above can be used as a device for outputting a transfer voltage to the secondary transfer roller. [Explanation of symbols]

[0071] 1 Photosensitive drum 12 Transfer roller 20 CPU 50 Transfer power supply unit C1, C2, C3 capacitors D1, D2, D3 diodes

Claims

1. an image carrier; and a transfer section that forms a nip portion with the image carrier and transfers a toner image formed on the image carrier to a recording material; a power supply unit that outputs a transfer voltage to the transfer unit in order to transfer the toner image onto a recording material, the power supply unit including: a transformer having a primary winding and a secondary winding; a switching unit that performs a switching operation of a current flowing through the primary winding based on a drive signal; and a rectifier circuit unit that rectifies and amplifies an AC voltage generated in the secondary winding of the transformer by the switching operation of the switching unit, and outputs the amplified voltage to the transfer unit; a detection unit that is provided upstream of the transfer unit in a recording material conveyance path and detects the conveyed recording material; a control means for outputting the drive signal to the switching means to control the power supply unit so that a first transfer voltage is output from the power supply unit when the leading edge of the recording material reaches the nip portion, and a second transfer voltage lower than the first transfer voltage is output from the power supply unit when the trailing edge of the recording material reaches the nip portion, the rectifier circuit unit includes a plurality of diodes and a plurality of capacitors, the plurality of capacitors include a first capacitor group connected in series between the transformer and an output terminal that outputs a voltage to the transfer unit without passing through the plurality of diodes, and a second capacitor group that is the plurality of capacitors excluding the first capacitor group, the capacitance of the capacitors included in the second capacitor group is smaller than the capacitance of the capacitors included in the first capacitor group; When the detection means detects the leading edge of the recording material, the control means outputs the drive signal to the switching unit at the timing when the leading edge of the recording material reaches the nip portion so that the rectifier circuit unit outputs the first transfer voltage by the time the image area of ​​the recording material, to which the toner image on the image carrier is transferred, reaches the nip portion, and when the detection means detects the trailing edge of the recording material, the control means stops outputting the drive signal at the timing when the trailing edge of the recording material reaches the nip portion so that the first transfer voltage output from the rectifier circuit unit is reduced to the second transfer voltage by the time the trailing edge of the recording material passes through the nip portion.

2. The rectifier circuit unit is a triple voltage rectifier circuit including a first diode, a second diode, a third diode, a first capacitor, a second capacitor, and a third capacitor; the first diode has an anode terminal connected to one end of the secondary winding of the transformer and a cathode terminal connected to the anode terminal of the second diode, the first capacitor, and one end of the third capacitor; a cathode terminal of the second diode connected to an anode terminal of the third diode and one end of the second capacitor; the third diode has a cathode terminal connected to the other end of the third capacitor and an output terminal that outputs a voltage to the transfer unit; the other end of the second capacitor is connected to one end of the secondary winding of the transformer; the other end of the first capacitor is connected to the other end of the secondary winding of the transformer; 2. The image forming apparatus according to claim 1, wherein the capacitance of the second capacitor is smaller than the capacitances of the first capacitor and the third capacitor.

3. The rectifier circuit unit is a quadruple voltage rectifier circuit including a first diode, a second diode, a third diode, and a fourth diode, and a first capacitor, a second capacitor, a third capacitor, and a fourth capacitor; the first capacitor has one end connected to one end of the secondary winding of the transformer and the other end connected to the cathode terminal of the first diode, the anode terminal of the second diode, and one end of the third capacitor; the other end of the third capacitor is connected to the cathode terminal of the third diode and the anode terminal of the fourth diode; an anode terminal of the first diode and one end of the second capacitor are connected to the other end of the secondary winding of the transformer; the other end of the second capacitor is connected to the cathode terminal of the second diode, the anode terminal of the third diode, and one end of the third capacitor; the other end of the third capacitor is connected to a cathode terminal of the fourth diode and an output terminal that outputs a voltage to the transfer unit, 2. The image forming apparatus according to claim 1, wherein the capacitances of the first capacitor and the third capacitor are smaller than the capacitances of the second capacitor and the fourth capacitor.

4. The rectifier circuit unit is a 5x voltage rectifier circuit including a first diode, a second diode, a third diode, a fourth diode, and a fifth diode, and a first capacitor, a second capacitor, a third capacitor, a fourth capacitor, and a fifth capacitor; an anode terminal of the first diode and one end of the second capacitor are connected to one end of the secondary winding of the transformer; the other end of the second capacitor is connected to the cathode terminal of the second diode, the anode terminal of the third diode, and one end of the fourth capacitor; the other end of the fourth capacitor is connected to the cathode terminal of the fourth diode and the anode terminal of the fifth diode; a cathode terminal of the first diode is connected to one end of the first capacitor, one end of the third capacitor, and an anode terminal of a second diode; the other end of the third capacitor is connected to the cathode terminal of the third diode, the anode terminal of the fourth diode, and one end of the fifth capacitor; a cathode terminal of the fifth diode is connected to the other end of the fifth capacitor and an output terminal that outputs a voltage to the transfer unit; the other end of the first capacitor is connected to the other end of the secondary winding of the transformer; 2. The image forming apparatus according to claim 1, wherein the capacitances of the second capacitor and the fourth capacitor are smaller than the capacitances of the first capacitor, the third capacitor and the fifth capacitor.

5. The rectifier circuit unit is a six-times voltage rectifier circuit including a first diode, a second diode, a third diode, a fourth diode, a fifth diode, and a sixth diode, and a first capacitor, a second capacitor, a third capacitor, a fourth capacitor, a fifth capacitor, and a sixth capacitor; one end of the first capacitor is connected to one end of the secondary winding of the transformer; the other end is connected to the cathode terminal of the first diode, the anode terminal of the second diode, and one end of the third capacitor; the other end of the third capacitor is connected to the cathode terminal of the third diode, the anode terminal of the fourth diode, and one end of a fifth capacitor; the other end of the fifth capacitor is connected to the cathode terminal of the fifth diode and the anode terminal of the sixth diode; an anode terminal of the first diode and one end of the second capacitor are connected to the other end of the secondary winding of the transformer; the other end of the second capacitor is connected to the cathode terminal of the second diode, the anode terminal of the third diode, and one end of the fourth capacitor; the other end of the fourth capacitor is connected to the cathode terminal of the fourth diode, the anode terminal of the fifth diode, and one end of the sixth capacitor; a cathode terminal of the sixth diode is connected to the other end of the sixth capacitor and an output terminal that outputs a voltage to the transfer unit; 2. The image forming apparatus according to claim 1, wherein the capacitances of the first capacitor, the third capacitor, and the fifth capacitor are smaller than the capacitances of the second capacitor, the fourth capacitor, and the sixth capacitor.

6. When the control means stops outputting the drive signal at the timing when the trailing edge of the recording material reaches the nip portion, the charged voltages of the first capacitor and the third capacitor are discharged in accordance with the capacitances of the first capacitor and the third capacitor, and the voltage output from the output terminal decreases, when the charging voltages of the first capacitor and the third capacitor become equal to the charging voltage of the second capacitor, the charging voltages of the first capacitor, the second capacitor, and the third capacitor are discharged according to the capacitances of the first capacitor, the second capacitor, and the third capacitor, and the voltage output from the output terminal decreases; 3. The image forming apparatus according to claim 2, wherein when the charging voltage of the first capacitor or the third capacitor is discharged, the charging voltage of the third capacitor or the first capacitor and the second capacitor is discharged depending on the capacitance of the third capacitor or the first capacitor and the second capacitor, and the voltage output from the output terminal decreases.

7. The transfer unit has a transfer roller that contacts the image carrier to form a nip portion and transfers the toner image on the image carrier to a recording material when the transfer voltage is applied, The volume resistance of the transfer roller is 1.0×10 ６ Ω ~ 5.0 x 10 ９ 2. The image forming apparatus according to claim 1, wherein the width is Ω.

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