Image forming device

Abstract

To prevent the occurrence of uneven electrification resulting from a transfer process.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 having a negative polarity power supply circuit, and a positive polarity power supply circuit that has a first rectification circuit part and superimposes a voltage output from the first rectification circuit part and a voltage output from the negative polarity power supply circuit with each other and outputs the superimposed voltages to the transfer roller 12; and a CPU 20 that controls the transfer power supply device 50. The first rectification circuit part has diodes D1, D2, D3 and capacitors C1, C2, C3. The capacity of the capacitor C1 charged with a half-wave rectification voltage of an AC voltage generated in secondary wiring of a transformer T1 is larger than the capacities of the capacitors C2, C3 charged with a voltage higher than the half-wave rectification voltage.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus using an electrophotographic method. [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" (high density areas 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 thereby suppress the occurrence of "memory."

[0005] On the other hand, even if the transfer voltage is set to 0 V (volts), it may not be possible to sufficiently suppress the occurrence of separation discharge between the trailing edge of the recording material and the photosensitive drum. This is because the photosensitive drum is charged to a dark potential VD. Depending on the thickness and resistance of the recording material, and environmental conditions such as the temperature and humidity in which the image forming apparatus is used, a potential difference that can cause separation discharge may occur between the recording material and the photosensitive drum. For this reason, a method is known in which a voltage opposite to that during transfer is applied to the transfer roller in the non-image area at the trailing edge of the recording material, thereby further reducing the potential difference between the trailing edge of the recording material and the photosensitive drum, thereby suppressing separation discharge. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-83812 Summary of the Invention [Problem to be solved by the invention]

[0007] However, in recent years, as the printing speed of image forming devices has increased due to improved productivity, the conveyance speed of the recording material inside the image forming device has also 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 (be reduced) 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. For this reason, in image forming devices with high printing speeds, it has been necessary to quickly lower the transfer voltage to a voltage that does not cause "memory."

[0008] However, when the transfer voltage is rapidly decreased, undershoot can cause the transfer voltage to drop below the desired level. When attempting to suppress the occurrence of "memory" by applying a negative transfer voltage (opposite polarity to that used during transfer) to the transfer roller, if the transfer voltage drops too much due to undershoot, the transfer roller will impart excessive negative charge to the photosensitive drum. As a result, when the charging unit next charges the photosensitive drum, charging unevenness can occur on the surface of the photosensitive drum, and the image formed on the photosensitive drum may subsequently develop "memory" (uneven density). As mentioned above, there are two types of "memory." To distinguish between the two types of "memory," hereafter, memory that occurs when the photosensitive drum is charged with the same polarity (positive polarity) as the transfer voltage due to peeling discharge will be referred to as "positive memory," and memory that occurs when a negative transfer voltage is applied to the transfer roller will be referred to as "negative memory."

[0009] The present invention has been made under these circumstances, and has as its object to suppress the occurrence of charging unevenness caused by the transfer process. [Means for solving the problem]

[0010] In order to solve the above-mentioned problems, the present invention has the following configuration.

[0011] (1) An image carrier; a transfer unit 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 transfer power supply unit that outputs a transfer voltage to the transfer unit to transfer the toner image to the recording material, the transfer power supply unit including a first power supply unit that outputs a positive voltage and a second power supply unit that outputs a negative voltage, wherein the first power supply unit includes a first transformer having a primary winding and a secondary winding, a first switching unit that performs a switching operation of a current flowing through the primary winding based on a drive signal, and a first rectification circuit unit that rectifies and amplifies an AC voltage generated in the secondary winding of the first transformer by the switching operation of the first switching unit and outputs the amplified voltage; the second power supply unit includes a second transformer having a primary winding and a secondary winding, a second switching unit that performs a switching operation of a current flowing through the primary winding based on a drive signal, and a second rectifier circuit unit that rectifies an AC voltage generated in the secondary winding of the second transformer by the switching operation of the second switching unit and outputs the rectified voltage;the transfer power supply unit that superimposes a voltage output from the first power supply unit and a voltage output from the second power supply unit and outputs the superimposed voltage to the transfer unit as the transfer voltage; 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 first switching unit to control the transfer power supply unit so that a first transfer voltage is output from the transfer 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 transfer power supply unit when the trailing edge of the recording material reaches the nip portion, wherein the first rectifier circuit unit includes a plurality of diodes and a plurality of capacitors, and the plurality of capacitors include a predetermined capacitor that is charged by a half-wave rectified voltage of an AC voltage generated in a secondary winding of the first transformer, and a capacitor that is charged by a voltage higher than the half-wave rectified voltage, and the capacitance of the predetermined capacitor is larger than the capacitance of the capacitor that is charged by the voltage higher than the half-wave rectified voltage. When the detection means detects the leading edge of the recording material, the control means outputs the drive signal to the first switching unit and the second switching unit at the timing when the leading edge of the recording material reaches the nip portion so that the first transfer voltage is output from the transfer power supply unit 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 to the first switching unit at the timing when the trailing edge of the recording material reaches the nip portion so that the first transfer voltage output from the transfer power supply 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]

[0013] According to the present invention, it is possible to suppress the occurrence of charging unevenness caused by the transfer process. [Brief explanation of the drawings]

[0014] [Figure 1] Schematic diagram showing the schematic configuration of an image forming apparatus according to first and second embodiments. [Figure 2] FIG. 1 is a diagram illustrating a method for measuring the volume resistance of the transfer rollers of Examples 1 and 2. [Figure 3] 1 is a schematic circuit diagram showing the circuit configuration of a transfer power supply device according to first and second embodiments, and a diagram showing the output voltage waveform of a transformer; [Figure 4] Timing chart for explaining transfer voltage control in the first and second embodiments [Figure 5] FIG. 10 is a diagram illustrating the voltage waveform at the time of the falling edge of the transfer voltage in the first and second embodiments. [Figure 6] Schematic circuit diagram showing the circuit configuration of a transfer power supply device according to another embodiment. [Figure 7] Schematic circuit diagram showing the circuit configuration of a transfer power supply device according to another embodiment. [Figure 8] Schematic circuit diagram showing the circuit configuration of a transfer power supply device according to another embodiment. [Figure 9] FIG. 10 is a diagram showing the correlation between transfer voltage and transfer efficiency in Example 2. [Figure 10] FIG. 10 is a diagram illustrating a ripple voltage of a transfer voltage according to a second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. [Example]

[0016] [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, which serves as a charging unit that charges the surface of the photosensitive drum 1 with a uniform polarity and potential; and an exposure device 3, which serves as an exposure unit 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 the developing 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.

[0017] 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.

[0018] [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.

[0019] 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.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] 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).

[0024] 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 negative power supply circuit (see FIG. 3(a)) of the transfer power supply device 50 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.

[0025] 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.

[0026] [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.

[0027] 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.

[0028] [Configuration of transfer power supply] Next, we will explain the transfer power supply 50 that supplies a transfer voltage to the transfer roller 12. Figure 3(a) is a circuit diagram showing the main circuit configuration of the transfer power supply 50 of this embodiment. The transfer power supply 50 shown in Figure 3(a) is composed of a positive power supply circuit that generates a positive voltage and a negative power supply circuit that generates a negative voltage. In the transfer power supply 50, the positive voltage output from the positive power supply circuit and the negative voltage output from the negative power supply circuit are superimposed and output from the output terminal as a transfer voltage and applied to the transfer roller 12, and the negative voltage output from the negative power supply circuit is applied to the charging roller 2.

[0029] The positive power supply circuit (first power supply unit) includes a transformer T1 (first transformer) having a primary winding and a secondary winding, and a field-effect transistor 1 (hereinafter referred to as FET1) that is a first switching unit that is switched by a drive signal output from the CPU 20. The positive power supply circuit also includes a rectifier circuit (first rectifier circuit unit) that rectifies the voltage induced on the secondary side of the transformer T1 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 T1 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 the diode D2 is connected to the anode terminal of the diode D3 (third diode) and the other end of the 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 T1 is connected to the other end of capacitor C1 (first capacitor) and the other end of resistor R1. In the positive power supply circuit, the transformer T1 is driven by a drive signal output from CPU 20, causing FET1 to repeatedly perform a switching operation, and a positive DC voltage is generated by a rectifier circuit provided on the secondary side of the transformer T1.

[0030] On the other hand, the negative power supply circuit (second power supply unit) includes a transformer T2 having a primary winding and a secondary winding, and a field-effect transistor 2 (hereinafter referred to as FET2) that is a second switching unit that is switched by a drive signal output from the CPU 20. The negative power supply circuit also includes a rectifier circuit (second rectifier circuit unit) composed of a diode D4 and a capacitor C4 that rectifies the voltage induced on the secondary side of the transformer T2 (second transformer). One end of the secondary winding of the transformer T2 is connected to the cathode terminal of the diode D4 (seventh diode), and the other end of the secondary winding of the transformer T2 is connected to one end of the capacitor C4 (seventh capacitor) and ground. The anode terminal of the diode D4 is connected to the other end of the capacitor C4, the other end of the capacitor C1 of the rectifier circuit (first rectifier circuit unit) of the positive power supply circuit, and the other end of the resistor R1. In the negative power supply circuit, a drive signal output from CPU 20 causes FET 2 to repeatedly perform switching operations, thereby driving transformer T2, and a negative DC voltage is generated by a rectifier circuit provided on the secondary side of transformer T2.

[0031] Here, the rectifier circuit of the negative power supply circuit is a half-wave rectifier circuit, while the rectifier circuit of the positive power supply circuit is a triple voltage rectifier circuit that multiplies (amplifies) the voltage induced on the secondary side. This is because the positive power supply circuit used during transfer needs to output a higher voltage than the negative power supply circuit. In addition, 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 inside the transformer, resulting in a significant increase in costs. Therefore, using a voltage doubler rectifier circuit, as in this embodiment, has cost advantages.

[0032] [Transfer voltage output operation] Next, we will explain the operation of the transfer power supply device 50 when applying a transfer voltage to the transfer roller 12 during image formation. Figure 3(b) is a diagram showing the voltage waveform of the AC voltage induced on the secondary side of the transformers T1 and T2 when the transformers T1 and T2 are driven by repeatedly turning on and off FETs 1 and 2 in response to a drive signal output from the CPU 20 in the transfer power supply device 50. Figure 3(b) shows one cycle of voltage waveform generated on the secondary winding side of the transformers T1 and T2 in Figure 3(a) where the black circles indicating the start of winding are not marked, 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.

[0033] First, in the positive power supply circuit, when the voltage shown in FIG. 3(b) is +Vo, diodes D1 and D3 are conductive and diode D2 is non-conductive, charging capacitors C1 and C3. At this time, +Vo, a half-wave rectified voltage that is the output voltage of transformer T1, is applied to capacitor C1, and +2Vo, a voltage that is double the output voltage of transformer T1, is applied to capacitor C3. As a result, a voltage of +3Vo is output from the positive power supply circuit. On the other hand, if the output voltage of transformer T2 in the negative power supply circuit is the same as that shown in FIG. 3(b), a voltage of +Vo is applied to capacitor C4, and the output voltage of the negative power supply circuit is -Vo. When a voltage of +3Vo is output from the positive power supply circuit and a voltage of -Vo is output from the negative power supply circuit, the transfer power supply device 50 outputs +2Vo, the sum of the output voltages of the positive and negative power supply circuits, as the transfer voltage.

[0034] In the positive power supply circuit, when the voltage shown in Figure 3(b) is -Vo, diode D2 is conductive and diodes D1 and D3 are non-conductive. At this time, capacitor C2 is charged with +2Vo, which is the double rectified voltage of the transformer output.

[0035] The operation of the positive-polarity 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 transformer T1, 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 transformer T1 that is not marked with a black circle, the output voltage +Vo of transformer T1 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 transformer T1 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.

[0036] [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. In this embodiment, to quickly drop the transfer voltage, the drive signal to FET2 in the negative power supply circuit is left on while the drive signal to FET1 in the positive power supply circuit is stopped. First, when the drive signal output from the CPU 20 to FET1 is stopped and FET1 is turned off, no voltage is induced on the secondary side of the transformer T1, and the charge stored in capacitors C1, C2, and C3 begins to discharge. Immediately after the 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 conduct. Therefore, capacitor C2 barely 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.

[0037] 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 of the capacitors that affect 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.

[0038] 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 value of the capacitor related to the transfer voltage discharge rate increases, further slowing the discharge rate of the capacitor 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.

[0039] As described above, in the transfer power supply device 50 of this embodiment, when the application of the transfer voltage to the transfer roller 12 is terminated and the transfer voltage is dropped, the discharge speed of the capacitors C1, C2, and C3 in the rectifier circuit of the positive power supply 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.

[0040] As described above, the negative power supply circuit in the transfer power supply device 50 of this embodiment also serves as a charging power supply circuit for applying a negative voltage to the charging roller 2. By sharing the power supply circuit in this way, it is possible to reduce the cost and size of the image forming apparatus. Note that the negative power supply circuit to be shared is not limited to the charging power supply circuit, and may be another power supply circuit used in the image forming apparatus M, such as a developing device.

[0041] [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 timings calculated in this way when the leading and trailing edges of the recording material P reach the transfer nip portion.

[0042] FIG. 4 is a diagram illustrating the control sequence of the transfer voltage of the transfer power supply device 50 by the CPU 20. FIG. 4 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. 4(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 ends of the recording material P in the transport direction are designated as mask areas (non-image areas) where no image is formed, and the areas further inward are designated as image areas where an image can be formed. 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 mask areas (non-image areas) where no image is formed, and the areas further inward are designated as image areas. FIG. 4(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 50, and "ON" indicates a period during which the CPU 20 outputs drive signals to FETs 1 and 2 of the transfer power supply 50 to apply a transfer voltage to the transfer roller 12. As shown in FIG. 4(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. 4(c) shows the voltage value of the transfer voltage output from the transfer power supply 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. 4(a), (b), and (c) indicate time, and t1 to t8 indicate time (timing).

[0043] 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. 4B). 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. 4C). 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. 4B). 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. 4C). 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 transported) 5 mm. That is, 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 reduced from the voltage used during transfer to a voltage that does not cause "positive 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 our research, when using the image forming apparatus M of this embodiment, "positive memory" does not occur if the transfer voltage is reduced to -100 V or less, which is the light area potential VL after exposure, when the trailing edge of the recording material P leaves the transfer nip. Furthermore, we have found that "negative memory" occurs if the transfer voltage is reduced to -500 V or less, which is the dark area potential VD after charging, when the trailing edge of the recording material P leaves the transfer nip. Therefore, when the trailing edge of the recording material P passes through the transfer nip, it is preferable to control the transfer voltage of the transfer power supply device 50 so that the transfer voltage is below the light area potential VL after exposure (below the second transfer voltage) and above the dark area potential VD (above the third transfer voltage).

[0044] [Effects of this Example] Next, an evaluation experiment conducted to confirm the effects of this embodiment will be described. In the evaluation experiment, the capacitances of capacitors C1, C2, and C3 in the positive power supply circuit of the transfer power supply device 50 in this embodiment were set to 300 pF, 50 pF, and 50 pF, respectively. In addition, as comparative examples 1 and 2, the following combinations of capacitor capacitances different from those in this embodiment were also evaluated. In comparative example 1, the capacitances of capacitors C1, C2, and C3 were set to 300 pF, 300 pF, and 300 pF, respectively. Meanwhile, in comparative example 2, the capacitances of capacitors C1, C2, and C3 were set to 50 pF, 50 pF, and 50 pF, respectively.

[0045] 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 printing was performed on the recording material P at a process speed of 250 mm / sec. 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, 25% density images were printed consecutively on two sheets of recording material P, and the presence or absence of "positive memory" in the image formed on the second sheet of recording material P, which is caused by discharge due to separation between the trailing edge of the first sheet of recording material P and the photosensitive drum 1, was confirmed. Similarly, the presence or absence of "negative memory" caused by excessive negative charge being applied from the transfer roller 12 to the photosensitive drum 1 was also confirmed. Furthermore, the transfer voltage when the image area of ​​the recording material P (the leading edge, trailing edge, and area 5 mm inward from the edge of the recording material perpendicular to the conveyance direction) passed through the transfer nip was controlled as follows: The output voltage of the positive power supply circuit in the transfer power supply device 50 was set to 3000 V, and the output voltage of the negative power supply circuit was set to -1000 V, and the transfer power supply device 50 was controlled so as to output the sum of the two voltages, 2000 V.

[0046] Table 1 shows the results of an evaluation experiment conducted on the combinations of capacitors C1, C2, and C3 of the present embodiment and comparative examples 1 and 2 described above. In Table 1, the vertical axis lists the experimental results of the present embodiment, comparative example 1, and comparative example 2, and the horizontal axis lists the following items. The horizontal axis of Table 1 lists the capacitances (unit: pF) of capacitors C1, C2, and C3, and the transfer voltage (unit: V) when the first sheet of recording material P passed through the transfer nip. Furthermore, the horizontal axis of Table 1 lists the presence or absence of horizontal black streak-like image defects associated with "positive memory" and image defects with uneven density associated with "negative memory." Regarding "positive memory" and "negative memory," the occurrence of image defects was evaluated as "×" (bad), and the absence of image defects was evaluated as "○" (good). [Table 1]

[0047] 5 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. 5, (a) shows the falling edge of the transfer voltage in this embodiment (Example 1), (b) shows Comparative Example 1, (c) shows Comparative Example 2, and (d) shows Example 2, which will be described later. In FIGS. 5(a), (b), (c), and (d), the vertical axis represents voltage, the horizontal axis represents time, and ta1 and ta2 in FIG. 5(a), tb1 and tb2 in FIG. 5(b), tc1 in FIG. 5(c), and td2 in FIG. 5(d) represent time (timing).

[0048] Furthermore, "Period 1," "Period 2," and "Period 3" on the horizontal axis of Figures 5(a), (b), (c), and (d) represent the following periods. Specifically, "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 series-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 zero. "Period 3" is the period from when either capacitor C1 or capacitor C3 is discharged and the charging voltage becomes zero, until the charging voltage of the other capacitor and the charging voltage of capacitor C2 are discharged and the charging voltage becomes zero. In Period 1, the discharge rate is determined by the capacitances of the series-connected capacitors C1 and C3 and resistor R1. In period 2, the capacitance of capacitor C2 is added compared to period 1, so the discharge rate is slightly slower than in period 1. In period 3, the discharge of capacitor C1 or capacitor C3 is completed, and the capacitance involved in the discharge becomes the capacitance when capacitor C1 or capacitor C3 and capacitor C2 are connected in parallel, so the discharge rate is even slower than in period 2.

[0049] As shown in Table 1, with the capacitance combination of capacitors C1, C2, and C3 in this embodiment, horizontal black streaks associated with "positive memory" did not occur, and neither did density unevenness associated with "negative memory." As shown in Table 1, the transfer voltage when the first sheet of recording material P passed through the transfer nip (time ta1 in FIG. 5A) was -300 V. In this embodiment, by setting the capacitances of capacitors C1, C2, and C3 to 300 pF, 50 pF, and 50 pF, the transfer voltage when the trailing edge of the recording material P passed through the transfer nip was rapidly reduced to -100 V or less, at which "positive memory" did not occur. Furthermore, the transfer voltage when the trailing edge of the recording material P passed through the transfer nip was maintained at -500 V or more, at which "negative memory" did not occur. This suppressed the occurrence of "memory" and produced a good image.

[0050] In this embodiment, the voltage waveform of the falling edge of the transfer voltage is as shown in Figure 5(a). In this embodiment, in the rectifier circuit of the positive power supply circuit of the transfer power supply device 50, the capacitance of capacitors C2 and C3, which are charged with a voltage rectified by double the output voltage of the transformer T1, is reduced, thereby increasing the discharge speed during periods 1 and 2. This allows the transfer voltage to fall to a voltage at which "positive memory" does not occur before the trailing edge of the recording material P leaves the transfer nip. Furthermore, by increasing the capacitance of capacitor C1, which is charged with the half-wave rectified voltage of the output voltage of the transformer T1, the falling speed of the transfer voltage during period 3 is slowed, thereby maintaining the transfer voltage at a voltage at which "negative memory" does not occur.

[0051] 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 density unevenness associated with "negative memory," but did cause horizontal black streaks associated with "positive memory." As shown in Table 1, the transfer voltage was 700 V when the first sheet of recording material P passed through the transfer nip (time tb1 in FIG. 5B). In Comparative Example 1, the capacitances of capacitors C1, C2, and C3 were set to 300 pF, 300 pF, and 300 pF, respectively. The capacitances of capacitors C2 and C3, which are charged with a voltage double the rectified voltage of the output voltage of transformer T1, were large. As a result, as shown in FIG. 5B, the discharge rate of the transfer voltage during periods 1 and 2 was slower than in this example shown in FIG. 5A. As a result, the transfer voltage could not be lowered to -100 V, a voltage at which "positive memory" would not occur, before the trailing edge of the recording material P passed through the transfer nip. As a result, although the occurrence of "negative memory" could be suppressed, the occurrence of "positive memory" could not be suppressed, resulting in the occurrence of horizontal black streaks.

[0052] Next, in Comparative Example 2, horizontal black streaks associated with "positive memory" did not occur, but density unevenness associated with "negative memory" occurred. As shown in Table 1, the transfer voltage was -700 V when the first sheet of recording material passed through the transfer nip (time tc1 in Figure 5(c)). In Comparative Example 2, the capacitances of capacitors C1, C2, and C3 were all 50 pF, and the capacitance of capacitor C1, which was charged by the half-wave rectified voltage of the output voltage of transformer T1, was smaller than in this example and Comparative Example 1. Therefore, as shown in Figure 5(c), the discharge rate of the capacitors during Period 3 was faster than in this example (Example 1), and the voltage dropped below -500 V, the threshold for "negative memory." Therefore, although "positive memory" was suppressed, density unevenness associated with "negative memory" occurred.

[0053] As described above, according to this embodiment, when the trailing edge of the recording material P leaves the transfer nip, the transfer power supply device 50 is controlled so that the transfer voltage is equal to or lower than the light area potential VL after exposure and equal to or higher than the dark area potential VD. This quickly reduces the transfer voltage, thereby suppressing the occurrence of "positive memory" on the photosensitive drum 1 and also suppressing the occurrence of "negative memory" due to undershoot. As a result, good image formation can be performed without image defects.

[0054] As described above, according to this embodiment, it is possible to suppress the occurrence of charging unevenness caused by the transfer process.

[0055] [Other Examples] In the first embodiment, a triple voltage rectifier circuit that outputs an odd-multiple voltage 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 rectifier circuit that outputs a quadruple to sixfold voltage, and can achieve the same effects as the triple voltage rectifier circuit.

[0056] [4x voltage rectifier circuit] FIG. 6 is a circuit diagram showing the main circuit configuration of a transfer power supply 50 having a quadruple voltage rectifier circuit that outputs an even-multiplied voltage. The transfer power supply 50 shown in FIG. 6 is composed of a positive power supply circuit that generates a positive voltage and a negative power supply circuit that generates a negative voltage. In FIG. 6, the quadruple voltage rectifier circuit that rectifies the voltage induced on the secondary side of the transformer T1 is composed of diodes D5, D6, D7, and D8 and capacitors C5, C6, C7, and C8. The circuit configuration in FIG. 6, 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.

[0057] In Figure 6, one end of the secondary winding of the transformer T1 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 T1 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.

[0058] In Figure 6, capacitor C5 is charged with +Vo, which is a half-wave rectified voltage of the output voltage of transformer T1, and capacitors C6, C7, and C8 are each charged with +2Vo, which is a double rectified voltage of the output voltage of transformer T1. This results in a voltage of +4Vo being output from the positive power supply circuit. By setting the capacitance of capacitors C6, C7, and C8, which are charged with the double rectified voltage of the output voltage of transformer T1, to be smaller than the capacitance of capacitor C5, the transfer voltage can be quickly reduced, thereby suppressing the occurrence of "positive memory" on photosensitive drum 1. Furthermore, by setting the capacitance of capacitor C5, which is charged with the half-wave rectified voltage of the output voltage of transformer T1, to be larger than the capacitance of capacitors C6, C7, and C8, the occurrence of "negative memory" due to undershoot can be suppressed.

[0059] [5x voltage rectifier circuit] FIG. 7 is a circuit diagram showing the main circuit configuration of a transfer power supply 50 having a 5x voltage rectifier circuit that outputs an odd-multiple voltage. The transfer power supply 50 shown in FIG. 7 is composed of a positive power supply circuit that generates a positive voltage and a negative power supply circuit that generates a negative voltage. In FIG. 7, the 5x voltage rectifier circuit that rectifies the voltage induced on the secondary side of the transformer T1 is composed of diodes D9, D10, D11, D12, and D13 and capacitors C9, C10, C11, C12, and C13. The circuit configuration in FIG. 7, excluding the 5x 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 T1 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 T1 is connected to the other end of the capacitor C9.

[0061] In Figure 7, capacitor C9 is charged with +Vo, which is the half-wave rectified voltage of the output voltage of transformer T1, and capacitors C10, C11, C12, and C13 are each charged with +2Vo, which is double the rectified voltage of the output voltage of transformer T1. This results in a voltage of +5Vo being output from the positive power supply circuit. By setting the capacitance of capacitors C10, C11, C12, and C13, which are charged with the double rectified voltage of the output voltage of transformer T1, to be smaller than the capacitance of capacitor C9, the transfer voltage can be quickly reduced, thereby suppressing the occurrence of "positive memory" on photosensitive drum 1. Furthermore, by setting the capacitance of capacitor C9, which is charged with the half-wave rectified voltage of the output voltage of transformer T1, to be larger than the capacitance of capacitors C10, C11, C12, and C13, the occurrence of "negative memory" due to undershoot can be suppressed.

[0062] [6x voltage rectifier circuit] FIG. 8 is a circuit diagram showing the main circuit configuration of a transfer power supply 50 having a sextuple voltage rectifier circuit that outputs an even-multiplied voltage. The transfer power supply 50 shown in FIG. 8 is composed of a positive power supply circuit that generates a positive voltage and a negative power supply circuit that generates a negative voltage. In FIG. 8, the sextuple voltage rectifier circuit that rectifies the voltage induced on the secondary side of the transformer T1 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. 8, excluding the sextuple 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.

[0063] 8, one end of the secondary winding of the transformer T1 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.

[0064] The other end of the secondary winding of transformer T1 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.

[0065] In Figure 8, capacitor C14 is charged with +Vo, which is the half-wave rectified voltage of the output voltage of transformer T1, and capacitors C15, C16, C17, C18, and C19 are each charged with +2Vo, which is double the rectified voltage of the output voltage of transformer T1. This results in a voltage of +6Vo being output from the positive power supply circuit. By setting the capacitance of capacitors C15, C16, C17, C18, and C19, which are charged with the double rectified voltage of the output voltage of transformer T1, smaller than the capacitance of capacitor C14, the transfer voltage drops quickly, thereby suppressing the occurrence of "positive memory" on photosensitive drum 1. Furthermore, by setting the capacitance of capacitor C14, which is charged with the half-wave rectified voltage of the output voltage of transformer T1, larger than the capacitance of capacitors C15, C16, C17, C18, and C19, the occurrence of "negative memory" due to undershoot can be suppressed.

[0066] As described above, the present invention can also be applied to rectifier circuits with a quadruple to sixfold voltage. Among the capacitors constituting the n-fold voltage rectifier circuit (where n is 3 or greater), the capacitance of the capacitor charged with the rectified voltage doubled by the output voltage of the transformer T1 can be reduced to suppress the occurrence of "positive memory" on the photosensitive drum 1. Furthermore, among the capacitors constituting the n-fold voltage rectifier circuit (where n is 3 or greater), the capacitance of the capacitor charged with the half-wave rectified voltage of the output voltage of the transformer T1 can be increased to suppress the occurrence of "negative memory" due to undershoot.

[0067] As described above, according to the other embodiments, it is possible to suppress the occurrence of charging unevenness caused by the transfer process. [Example]

[0068] In the first embodiment, a configuration was described in which the fall of the transfer voltage was accelerated by reducing the capacitance of the capacitor used in the transfer power circuit. Reducing the capacitance of the capacitor used in the transfer power circuit accelerates the fall of the transfer voltage, but the ripple voltage of the transfer voltage increases. When the ripple voltage of the transfer voltage is large, image defects such as "missing transfer" may occur depending on the image pattern and environmental conditions such as the temperature and humidity in which the image forming apparatus is used. In the second embodiment, a configuration is described in which the fall of the transfer voltage is accelerated and the ripple voltage of the transfer voltage is suppressed. The configurations of the image forming apparatus and the transfer power supply device in this embodiment are the same as those in the first embodiment, and the same devices and components are designated by the same reference numerals, and detailed descriptions thereof will be omitted.

[0069] [Transfer voltage ripple and transfer defects] As mentioned above, depending on the capacitance of the rectifier circuit of the positive power supply circuit of the transfer power supply device 50 shown in FIG. 3(a), the ripple voltage of the transfer voltage applied to the transfer roller 12 may become large. If the ripple voltage is large, the transfer voltage applied to the transfer roller 12 may fluctuate, becoming higher or lower than the appropriate transfer voltage (first transfer voltage), resulting in an image defect known as a "transfer omission." FIG. 9 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

[0070] As shown in FIG. 9, 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, reducing transfer efficiency. This phenomenon of reduced transfer efficiency due to insufficient transfer voltage is called "weak dropout." 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, reducing transfer efficiency. This phenomenon of reduced transfer efficiency due to a transfer voltage higher than the appropriate transfer voltage Vmax is called "strong dropout." The "weak dropout" and "strong dropout" described above are collectively referred to as "transfer dropout." When "transfer dropout" occurs, it can result in image defects such as missing toner images on the recording material P or density variations. 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.

[0071] [Transfer circuit of this embodiment] Next, we will explain the ripple voltage when the output voltage of the transformer T1 in the positive polarity power supply circuit of the transfer power supply device 50 shown in FIG. 3(a) is a square wave as shown in FIG. 3(b). First, when the output voltage of the transformer T1 is -Vo, diodes D1 and D3 are non-conductive, and only diode D2 is conductive. Because diode D3 is non-conductive, the capacitance of the capacitors relative to the output voltage of the positive polarity power supply circuit is the capacitance when capacitors C1 and C3 are connected in series. Therefore, when the output voltage of the transformer T1 is -Vo, the transfer voltage, which is the output voltage, decreases at a discharge rate determined by the capacitance of the series connection of capacitors C1 and C3, the resistance of resistor R1, and the resistance of the transfer roller 12. Next, when the output voltage of the transformer T1 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 T1 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 difference between the maximum and minimum voltages in the transfer voltage, is generated in the transfer voltage applied to transfer roller 12.

[0072] As described above, the ripple voltage of the output voltage (transfer voltage) in the positive polarity power supply circuit of FIG. 3(a) is determined by the capacitance of capacitors C1 and C3 when they are connected in series, and the capacitance of capacitor C2 does not affect the ripple voltage. Therefore, by reducing the capacitance of capacitor C2, it is possible to shorten the fall time of the transfer voltage when it is lowered after application of the transfer voltage to the transfer roller 12 is terminated without increasing the ripple voltage. Also, as described above, by increasing the capacitance of capacitor C1, which is charged by the half-wave rectified voltage of the output voltage of transformer T1, it is possible to suppress the occurrence of "negative memory" due to undershoot. Therefore, in this embodiment, the circuit configuration of transfer power supply device 50 is such that the capacitances of capacitors C1, C2, and C3 are 300 pF, 50 pF, and 300 pF, respectively.

[0073] Here, a set of capacitors C1 and C3 connected in series between the transformer T1 and the output terminal of the transfer voltage without using multiple diodes is defined as a first capacitor group. Of the multiple capacitors C1 to C3, a set of capacitors not included in the first capacitor group is defined as a second capacitor group. In the configuration of this embodiment, the capacitor C1, which is charged by the half-wave rectified voltage of the output voltage of the transformer T1, is included in the first capacitor group. However, depending on other circuit configurations described later, the capacitor charged by the half-wave rectified voltage of the output voltage of the transformer T1 may be included in the second capacitor group.

[0074] However, the configuration of this embodiment slows (lengthens) the fall time of the transfer voltage compared to the configuration described in embodiment 1, in which the capacitances of both capacitors C2 and C3 are small. Therefore, it is preferable to select an optimal capacitor capacitance configuration in consideration of the occurrence of "positive memory" and "transfer defects."

[0075] For example, if the process speed of the image forming apparatus M is slower than that of the configuration shown in Example 1 (250 mm / sec), the occurrence of "positive memory" can be suppressed even if the fall speed of the transfer voltage is correspondingly slower. In this way, if the process speed is slower than that of Example 1, the occurrence of "positive memory" can be suppressed even with the circuit configuration of this example described above, and the occurrence of "transfer voids" can also be suppressed by suppressing the ripple voltage.

[0076] Furthermore, the occurrence levels of "positive memory" and "transfer voids" vary depending on the thickness and resistance of the recording material P, and environmental conditions such as the temperature and humidity in which the image forming apparatus M is used. For example, if the occurrence level of "positive memory" as an image defect is slight and the occurrence level of "transfer voids" is high, it is preferable to use the circuit configuration of this embodiment rather than the circuit configuration of the first embodiment.

[0077] [Effects of this Example] Next, an evaluation experiment conducted to confirm the effects of this embodiment will be described. In the evaluation experiment, the capacitances of the capacitors C1, C2, and C3 in this embodiment (Example 2) of the positive power supply circuit of the transfer power supply device 50 were set to 300 pF, 50 pF, and 300 pF, respectively. In addition, for comparison with this embodiment, an evaluation experiment was also conducted on a combination of Example 1, in which the capacitances of the capacitors C1, C2, and C3 were 300 pF, 50 pF, and 50 pF, and Comparative Example 1, in which the capacitances of the capacitors C1, C2, and C3 were all 300 pF.

[0078] 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 printing was performed on the recording material P at a process speed of 160 mm / sec. 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, 25% density images were printed consecutively on two sheets of recording material P, and the presence or absence of "memory" (positive memory) in the image formed on the second sheet of recording material P, which was caused by separation discharge between the trailing edge of the first sheet of recording material P and the photosensitive drum 1, was confirmed. Similarly, the presence or absence of "transfer defects" caused by ripple voltage in the transfer voltage was also confirmed. Furthermore, the transfer voltage when the image area of ​​the recording material P (the leading edge, trailing edge, and area 5 mm inward from the edge of the recording material on the side perpendicular to the conveyance direction of the recording material P) passed through the transfer nip was controlled as follows: The output voltage of the positive power supply circuit in the transfer power supply device 50 was set to 3000 V, and the output voltage of the negative power supply circuit was set to -1000 V, and the transfer power supply device 50 was controlled so that the sum of the two voltages, 2000 V, was output.

[0079] Table 2 shows the results of an evaluation experiment conducted on the combinations of capacitors C1, C2, and C3 of the present embodiment (Example 2), Example 1, and Comparative Example 1 described above. In Table 2, the vertical axis lists the experimental results of the present embodiment, Example 1, and Comparative Example 1, and the horizontal axis lists the following items. The horizontal axis of Table 2 lists the capacitances (units: pF) of capacitors C1, C2, and C3, and the transfer voltage (units: V) when the first sheet of recording material P exited (passed) the transfer nip. Furthermore, the horizontal axis of Table 1 lists the presence or absence of image defects such as horizontal black streaks associated with "memory" and "transfer omissions." Regarding "memory," if an image defect occurred, it was evaluated as "x" (poor), and if not, it was evaluated as "o" (good). Similarly, if a "transfer omission" occurred, it was evaluated as "x" (poor), and if not, it was evaluated as "o" (good). [Table 2]

[0080] 10 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 50 during the period when the transfer voltage control was "ON" when the above-mentioned evaluation experiment was conducted. In FIG. 10, (a) shows the state of ripple voltage in this embodiment (Example 2), (b) shows Example 1, and (c) shows Comparative Example 1. In FIGS. 10(a), (b), and (c), the vertical direction indicates voltage and the horizontal direction indicates time.

[0081] As shown in Table 2, with the capacitance combination of capacitors C1, C2, and C3 in this example, horizontal black streaks associated with "memory" did not occur, and no "transfer gaps" due to ripple voltage in the transfer voltage did occur. As shown in Table 2, the transfer voltage was -100 V when the first sheet of recording material P left (passed) the transfer nip (time td2 in Figure 5(d)). In this example, by setting the capacitances of capacitors C1, C2, and C3 to 300 pF, 50 pF, and 300 pF, respectively, the transfer voltage when the trailing edge of the recording material P left the transfer nip was rapidly reduced to -100 V or below, 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.

[0082] The voltage waveform of the transfer voltage falling edge in this embodiment is shown in FIG. 5(d). In this embodiment, the capacitance of capacitor C3 in the positive power supply circuit is larger than in the configuration of Embodiment 1, so the fall speed of the transfer voltage, especially during Period 1, is slower than in Embodiment 1. However, the process speed of the image forming apparatus M in this embodiment is 160 mm / sec, which is slower than the process speed (250 mm / sec) of Embodiment 1. Therefore, the time it takes for the trailing edge of the recording material P to exit the transfer nip is longer than in Embodiment 1, so the transfer voltage can be lowered to a voltage that does not cause "positive memory." The ripple voltage of the transfer voltage in this embodiment is also shown in FIG. 10(a). In this embodiment, the capacitance of capacitor C3 is 300 pF, so the ripple voltage is suppressed to 16 V as described above, thereby preventing transfer defects.

[0083] On the other hand, as shown in Table 2, with the combination of capacitances of capacitors C1, C2, and C3 in Example 1, horizontal black streaks due to "memory" did not occur, but "transfer gaps" due to the ripple voltage of the transfer voltage occurred. In the transfer circuit configuration of Example 1, as shown in Table 2, the transfer voltage fell to -400 V when the trailing edge of the recording material P exited the transfer nip (time ta2 in Figure 5(a)). This prevented "memory" from occurring, but "transfer gaps" did occur. The voltage waveform of the falling transfer voltage in this example is shown in Figure 5(a). Because the rate of fall of the transfer voltage during Period 3 is slower, the transfer voltage when the trailing edge of the recording material P exited the transfer nip was maintained at a voltage that did not cause "negative memory." However, because the capacitance of capacitor C3 was set to 50 pF, the ripple voltage of the transfer voltage reached 52 V, as shown in Figure 10(b), resulting in "transfer gaps."

[0084] Furthermore, as shown in Table 2, the combination of the capacitances of capacitors C1, C2, and C3 in Comparative Example 1 did not cause "transfer defects" due to ripple voltage in the transfer voltage, but did cause horizontal black streaks associated with "memory" (positive memory). In Comparative Example 1, the capacitances of capacitors C1, C2, and C3 were all 300 pF, all of which were the same and large. Therefore, as shown in Table 2, the transfer voltage only dropped to 400 V when the first sheet of recording material P passed through the transfer nip (time tb2 in Figure 5(b)). This meant that "memory" (positive memory) could not be suppressed. In the circuit configuration of Comparative Example 1, the capacitance of capacitor C3 was 300 pF. Therefore, as shown in Figure 10(c), the ripple voltage was suppressed to 16 V, and "transfer defects" did not occur. However, as shown in FIG. 5B, the falling curve of the transfer voltage was gentle, and the transfer voltage did not drop completely when the trailing edge of the recording material P passed through the transfer nip, resulting in "memory."

[0085] As described above, according to this embodiment, by reducing the ripple voltage of the transfer voltage and speeding up the fall of the transfer voltage, it is possible to suppress the occurrence of "memory" and "transfer voids" and also to suppress the occurrence of "negative memory" due to undershoot, thereby obtaining a good image.

[0086] In the above embodiment, the capacitances of capacitors C1, C2, and C3 are set to 300 pF, 50 pF, and 300 pF, respectively, and C1 and C3 are set to the same capacitance, but this is not limiting. The capacitance of capacitor C3 may be smaller than that of capacitor C1. That is, the capacitance of capacitor C3 may be set to be equal to or smaller than that of capacitor C1. However, to suppress ripple voltage, the capacitance of capacitor C3 must be larger than that of capacitor C2. That is, the capacitances may be in the order of C1 > C3 > C2.

[0087] [Other Examples] In the second 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.

[0088] [4x voltage rectifier circuit] The capacitance of the capacitors for the output voltage of the positive power supply circuit in Figure 6 is the capacitance when capacitors C6 and C8 are connected in series. The ripple voltage of the output voltage (transfer voltage) is determined by the capacitance of capacitors C6 and C8 when connected in series, and the capacitance of capacitors C5 and C7 does not affect the ripple voltage. Therefore, by reducing the capacitance of capacitors C5 and C7, it is possible to shorten the fall time of the transfer voltage when it is lowered after application of the transfer voltage to transfer roller 12 has finished, without increasing the ripple voltage.

[0089] As mentioned above, in FIG. 6, the capacitance of capacitor C5, which is charged by the half-wave rectified voltage of the output voltage of transformer T1, is set to be larger than the capacitances of capacitors C6, C7, and C8, thereby suppressing the occurrence of "negative memory" due to undershoot. Therefore, the capacitance of capacitor C7 is set to be smaller than the capacitances of capacitors C5, C6, and C8. In summary, the capacitances of capacitors C5, C6, and C8 decrease in the order C5 > C6, and C8 > C7. The capacitances of C6 and C8 can be set to any value. As mentioned in the triple voltage rectifier circuit, the capacitances of C5, C6, and C8 may be set to be the same. In other words, the capacitances of capacitors C6 and C8 may be set to be equal to or smaller than the capacitance of capacitor C5.

[0090] As described above, by speeding up the fall of the transfer voltage while suppressing the ripple voltage of the transfer voltage, it is possible to suppress the occurrence of "memory" and "transfer defects" and also to suppress the occurrence of "negative memory" due to undershoot.

[0091] [5x voltage rectifier circuit] The capacitance of the capacitors for the output voltage of the positive power supply circuit in Figure 7 is the capacitance when capacitors C9, C11, and C13 are connected in series. The ripple voltage of the output voltage (transfer voltage) is determined by the capacitance of capacitors C9, C11, and C13 when connected in series, and the capacitance of capacitors C10 and C12 does not affect the ripple voltage. Therefore, by reducing the capacitance of capacitors C10 and C12, it is possible to shorten the fall time of the transfer voltage when it is lowered after application of the transfer voltage to transfer roller 12 has finished, without increasing the ripple voltage.

[0092] As described above, in FIG. 7, the capacitance of capacitor C9, which is charged by the half-wave rectified voltage of the output voltage of transformer T1, is set to be larger than the capacitances of capacitors C10, C11, C12, and C13. This prevents the occurrence of "negative memory" due to undershoot. The capacitances of capacitors C10 and C12 are set to be smaller than those of capacitors C9, C11, and C13. In summary, the capacitances decrease in the order C9 > C11, C13 > C10, C12. The relationship between the capacitances of C11 and C13 and C10 and C12 is not important. Furthermore, as described in the triple voltage rectifier circuit, the capacitances of C9, C11, and C13 may be set to be the same. In other words, capacitors C9, C11, and C13 are included in the first capacitor group described above. The capacitances of capacitors C11 and C13 other than capacitor C9 (other than the specified capacitors) may be set to be equal to or smaller than that of capacitor C9.

[0093] As described above, by speeding up the fall of the transfer voltage while suppressing the ripple voltage of the transfer voltage, it is possible to suppress the occurrence of "memory" and "transfer defects" and also to suppress the occurrence of "negative memory" due to undershoot.

[0094] [6x voltage rectifier circuit] The capacitance of the capacitors for the output voltage of the positive power supply circuit in Figure 8 is the capacitance when capacitors C15, C17, and C19 are connected in series. The ripple voltage of the output voltage (transfer voltage) is determined by the capacitance of capacitors C15, C17, and C19 connected in series, and 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, it is possible to shorten the fall time of the transfer voltage when it is lowered after application of the transfer voltage to transfer roller 12 has finished, without increasing the ripple voltage.

[0095] As mentioned above, in FIG. 8, the capacitance of capacitor C14, which is charged by the half-wave rectified voltage of the output voltage of transformer T1, is set to be larger than the capacitances of capacitors C15, C16, C17, C18, and C19. This prevents the occurrence of "negative memory" due to undershoot. Therefore, among capacitors C14, C16, and C18, the capacitances of capacitors C16 and C18 are set to be smaller than those of capacitors C14, C15, C17, and C19. In summary, the capacitances decrease in the order C14 > C15, C17, and C19 > C16 and C18. Note that the relationship between the capacitances of C15, C17, and C19, and between C16 and C18, is not important. Furthermore, as described in the triple voltage rectifier circuit, the capacitances of C14, C15, C17, and C19 may be set to be the same. That is, the capacitances of the capacitors C15, C17, and C19 may be set to be equal to or less than the capacitance of the capacitor C14.

[0096] As described above, by speeding up the fall of the transfer voltage while suppressing the ripple voltage of the transfer voltage, it is possible to suppress the occurrence of "memory" and "transfer defects" and also to suppress the occurrence of "negative memory" due to undershoot.

[0097] As described above, according to other embodiments, by speeding up the fall of the transfer voltage while suppressing the ripple voltage of the transfer voltage, it is possible to suppress the occurrence of "memory" and "transfer voids" and also to suppress the occurrence of "negative memory" due to undershoot, thereby obtaining a good image.

[0098] In the above-described first and second embodiments, 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.

[0099] As described above, according to this embodiment, it is possible to suppress the occurrence of charging unevenness caused by the transfer process.

[0100] 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]

[0101] 1 Photosensitive drum 12 Transfer roller 20 CPU 50 Transfer power supply 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 transfer power supply unit that outputs a transfer voltage to the transfer unit in order to transfer the toner image onto a recording material, the transfer power supply unit including a first power supply unit that outputs a voltage of positive polarity and a second power supply unit that outputs a voltage of negative polarity; the first power supply unit includes a first transformer having a primary winding and a secondary winding, a first switching unit that performs a switching operation of a current flowing through the primary winding based on a drive signal, and a first rectifier circuit unit that rectifies and amplifies an AC voltage generated in the secondary winding of the first transformer by the switching operation of the first switching unit and outputs the amplified voltage; the second power supply unit includes a second transformer having a primary winding and a secondary winding, a second switching unit that performs a switching operation of a current flowing through the primary winding based on a drive signal, and a second rectifier circuit unit that rectifies an AC voltage generated in the secondary winding of the second transformer by the switching operation of the second switching unit and outputs the rectified voltage; the transfer power supply unit that superimposes a voltage output from the first power supply unit and a voltage output from the second power supply unit and outputs the superimposed voltage to the transfer unit as the transfer voltage; 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 unit that outputs the drive signal to the first switching unit to control the transfer power supply unit so that a first transfer voltage is output from the transfer 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 transfer power supply unit when the trailing edge of the recording material reaches the nip portion, the first rectifier circuit unit includes a plurality of diodes and a plurality of capacitors, the plurality of capacitors include a predetermined capacitor that is charged by a half-wave rectified voltage of the AC voltage generated in the secondary winding of the first transformer, and a capacitor that is charged by a voltage higher than the half-wave rectified voltage, the capacitance of the predetermined capacitor is greater than the capacitance of a capacitor charged by a voltage higher than the half-wave rectified voltage; When the detection means detects the leading edge of the recording material, the control means outputs the drive signal to the first switching unit and the second switching unit at the timing when the leading edge of the recording material reaches the nip portion so that the first transfer voltage is output from the transfer power supply unit 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 to the first switching unit at the timing when the trailing edge of the recording material reaches the nip portion so that the first transfer voltage output from the transfer power supply 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 first rectifier circuit unit includes: 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 first 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 first transformer; the other end of the first capacitor is connected to the other end of the secondary winding of the first transformer; 2. The image forming apparatus according to claim 1, wherein the capacitance of the first capacitor is larger than the capacitances of the second capacitor and the third capacitor.

3. The first rectifier circuit unit includes: 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 first 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 first 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 capacitance of the first capacitor is greater than the capacitances of the second capacitor, the third capacitor, and the fourth capacitor.

4. The first rectifier circuit unit includes: 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 first 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 first transformer; 2. The image forming apparatus according to claim 1, wherein the capacitance of the first capacitor is greater than the capacitances of the second capacitor, the third capacitor, the fourth capacitor, and the fifth capacitor.

5. The first rectifier circuit unit includes: 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; the first capacitor has one end connected to one end of the secondary winding of the first 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, 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 first 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 capacitance of the first capacitor is greater than the capacitances of the second capacitor, the third capacitor, the fourth capacitor, the fifth capacitor, and the sixth capacitor.

6. the second rectifier circuit unit has a seventh diode and a seventh capacitor, the seventh diode has a cathode terminal connected to one end of the secondary winding of the second transformer and an anode terminal connected to one end of the seventh capacitor and the first rectifier circuit unit; 2. The image forming apparatus according to claim 1, wherein the other end of the seventh capacitor is connected to the other end of the secondary winding of the second transformer and to ground.

7. 2. The image forming apparatus according to claim 1, wherein the control means does not stop outputting the drive signal to the second switching unit when the trailing edge of the recording material reaches the nip portion, but stops outputting the drive signal to the second switching unit when the trailing edge of the recording material passes through the nip portion.

8. a charging unit having a charging roller that charges the surface of the image carrier to a uniform potential; 8. The image forming apparatus according to claim 7, wherein the charging roller charges the surface of the image carrier with a third transfer voltage lower than the second transfer voltage.

9. an exposure unit that irradiates a surface of the image carrier with a light beam to form an electrostatic latent image on the image carrier; 9. The image forming apparatus according to claim 8, wherein the second transfer voltage is a voltage on the surface of the image carrier irradiated with the light beam from the exposure unit.

10. 10. The image forming apparatus according to claim 9, wherein the second power supply applies a voltage to the charging roller for charging the image carrier.

11. 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 11. The image forming apparatus according to claim 10, wherein the width is Ω.

Images