Image forming apparatus and power supply device for the same

Abstract

To reduce an arrangement space and cost of a power supply circuit that generates primary transfer voltage.SOLUTION: An image forming apparatus has an intermediate transfer body having conductivity, a primary transfer member, detection means, an AC power supply, and a first DC power supply. The AC power supply supplies AC voltage to both the detection means and first DC power supply. The first DC power supply generates DC voltage with a first polarity from the AC voltage and applies it to the primary transfer member.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus that utilizes an electrophotographic system and a power supply device thereof. [Background technology]

[0002] An image forming apparatus transfers a toner image formed on a photoconductor to an intermediate transfer body (primary transfer), and then transfers the toner image from the intermediate transfer body to a sheet (secondary transfer). By applying a primary transfer voltage to the primary transfer roller, a current flows from the primary transfer roller to the photoconductor, facilitating the transfer of the toner image. Patent Document 1 describes a power supply circuit for generating the primary transfer voltage. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2016-004140 A Summary of the Invention [Problem to be solved by the invention]

[0004] Since the primary transfer voltage is high, the cost of the power supply circuit is high and the layout space thereof tends to be large. Therefore, an object of the present invention is to make it possible to reduce the cost and layout space of the power supply circuit that generates the primary transfer voltage. [Means for solving the problem]

[0005] The present invention relates to, for example, a toner container for storing toner; a photoreceptor on which a toner image is formed by the toner supplied from the toner container; an intermediate transfer body that rotates while being in contact with the photoreceptor and has electrical conductivity; a primary transfer member that is applied with a primary transfer voltage and transfers the toner image from the photoreceptor to the intermediate transfer member; a detection unit provided in the toner container and configured to detect information regarding a remaining amount of the toner contained in the toner container; an AC power source for supplying an AC voltage to the detection means; and a first DC power source that converts the AC voltage output from the AC power source to generate a DC voltage of a first polarity and applies the DC voltage of the first polarity to the primary transfer member. Effect of the Invention

[0006] According to the present invention, it is possible to reduce the cost and the layout space of the power supply circuit that generates the primary transfer voltage. [Brief description of the drawings]

[0007] [Figure 1] FIG. 1 is a schematic cross-sectional view of an image forming apparatus. [Diagram 2] FIG. 2 is a block diagram of a power supply device. [Diagram 3] Circuit diagram of positive and negative power supplies. [Figure 4] FIG. 4 is a circuit diagram of a toner detection circuit. [Diagram 5] Circuit diagram of AC power supply and auxiliary power supply (primary transfer power supply). [Figure 6] 4 is a graph showing the relationship between the duty ratio and the output voltage. [Figure 7] FIG. 4 is a diagram illustrating a power source for generating a primary transfer voltage. [Figure 8] FIG. 4 is a diagram showing time transitions of various signals and output voltages. [Figure 9] 5A and 5B are graphs showing the relationship between the duty ratio and the output voltage, and diagrams showing time transitions of various signals and the output voltage; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] Hereinafter, the embodiments will be described in detail with reference to the attached drawings. Note that the following embodiments do not limit the invention according to the claims. Although the embodiments describe a number of features, not all of these features are essential to the invention, and the features may be combined in any manner. Furthermore, in the attached drawings, the same reference numbers are used for the same or similar configurations, and duplicated descriptions are omitted.

[0009] <Example 1> (1) Structure of the image forming device FIG. 1 is a cross-sectional view of an image forming apparatus 101. A feeding unit 102 feeds a sheet P to a conveying path 111. An imaging unit 103 forms an image on an intermediate conveying belt 135 using black (K), cyan (C), magenta (M), and yellow (Y) toners. The intermediate conveying belt 135 moves in the direction of the arrow shown in FIG. 1. A transfer unit 104 transfers the toner image from the intermediate conveying belt 135 to the sheet P. A fixing unit 105 fixes the toner image on the sheet P. A discharge unit 106 discharges the sheet P to the outside of the image forming apparatus 101.

[0010] The feeding unit 102 has a feeding tray 121 and feeding rollers 122. The feeding tray 121 is a feeding container that stores sheets (printing media) to be printed. The feeding rollers 122 feed the sheets from the feeding tray 121 to the transport path 111 one by one.

[0011] The image forming unit 103 has four print stations corresponding to KCMY. Since each station has the same structure, the letters abcd at the end of the reference numerals are omitted.

[0012] The charging roller 132 is disposed so as to contact and face the photosensitive drum 131 with a certain gap formed therebetween. The surface of the photosensitive drum 131 is uniformly charged by a discharge phenomenon occurring in the gap between the charging roller 132 and the photosensitive drum 131 (charging process). The exposure device 130 outputs a laser beam corresponding to an image signal, exposes the surface of the photosensitive drum 131, and forms an electrostatic latent image (exposure process). The development roller 133 attaches toner supplied from the toner container 134 to the photosensitive drum 131, and forms a toner image from the electrostatic latent image (developing process). The toner container 134 has an electrode plate 138 and an electrode plate 139. By applying an AC voltage Vton_ac between the electrode plate 138 and the electrode plate 139, the electrostatic capacitance between the electrode plate 138 and the electrode plate 139 is detected. This electrostatic capacitance changes depending on the amount of toner present between the electrode plate 138 and the electrode plate 139. Therefore, the electrostatic capacitance is information related to the remaining amount of toner, and shows a correlation with the remaining amount of toner.

[0013] A high voltage (hereinafter, sometimes referred to as voltage) is applied to each component of the image forming unit 103 and the transfer unit 104. Specifically, the voltage is applied to the charging rollers 132a, 132b, 132c, and 132d, the developing rollers 133a, 133b, 133c, and 133d, the primary transfer rollers 136a, 136b, 136c, and 136d, and the secondary transfer roller 141. The image forming device 101 charges the toner to a negative polarity. Therefore, a negative voltage is applied to the charging rollers 132a, 132b, 132c, and 132d, and the developing rollers 133a, 133b, 133c, and 133d. On the other hand, a positive voltage is applied to the primary transfer rollers 136a, 136b, 136c, and 136d, and the secondary transfer roller 141.

[0014] The primary transfer roller 136 is disposed so as to contact the inner circumferential surface of the intermediate conveyor belt 135. The primary transfer roller 136 transfers a toner image from the photosensitive drum 131 to the intermediate conveyor belt 135. The primary transfer roller 136 has a metal core. A primary transfer voltage that promotes the transfer of the toner image is applied to this core. The primary transfer voltage is a high DC voltage. The photosensitive drum 131 is connected to GND (ground). An electric field is generated from the core of the primary transfer roller 136 to which a positive polarity primary transfer voltage is applied toward the photosensitive drum 131. This causes the negatively charged toner to move from the photosensitive drum 131 to the intermediate conveyor belt 135. In this way, the YMCK toner images are transferred in order to the intermediate conveyor belt 135, forming a full-color image.

[0015] The intermediate conveyor belt 135 is stretched around primary transfer rollers 136a, 136b, 136c, and 136d, an opposing roller 137, a tension roller 144, and a drive roller 145. The intermediate conveyor belt 135 is driven to rotate by the drive roller 145. A cleaning blade 142 cleans toner remaining on the intermediate conveyor belt 135. An image sensor 143 reads a test image formed on the intermediate conveyor belt 135. The read result can be used for color shift correction, gradation correction, adjustment of image formation conditions, and the like.

[0016] A secondary transfer voltage is applied to the core of the secondary transfer roller 141 of the transfer unit 104. As a result, the secondary transfer roller 141 transfers the toner image from the intermediate conveyor belt 135 to the sheet P. The secondary transfer voltage is a high DC voltage. The intermediate conveyor belt 135 is conductive, and the secondary transfer voltage applied to the secondary transfer roller 141 also acts on the primary transfer roller 136 via the intermediate conveyor belt 135. For example, the intermediate conveyor belt 135 can be formed by overlapping a plurality of thin films, and the inner layer of the plurality of thin films is formed of a conductive material and has a lower surface resistance than the outer layer. As a result, an electric field that promotes the transfer of the toner is generated between the intermediate conveyor belt 135 and the photosensitive drum 131.

[0017] The fixing unit 105 has a fixing roller 151 and a pressure roller 152. The fixing roller 151 and the pressure roller 152 supply heat and pressure to the sheet P, so that the toner image is fixed onto the sheet P.

[0018] The discharge section 106 includes discharge rollers 161a and 161b, and a discharge tray 162. The discharge rollers 161a and 161b discharge the sheet P onto the discharge tray 162.

[0019] (2) Voltage used in image forming equipment During image formation, the toner container 134a stirs the toner to charge it to a negative polarity, which is the normal charging polarity. Therefore, a positive voltage, which is the opposite polarity to the charging polarity, is applied to the primary transfer roller 136. Similarly, a positive voltage is also applied to the secondary transfer roller 141.

[0020] However, a part of the toner transferred to the intermediate conveyor belt 135 remains on the surface of the intermediate conveyor belt 135 without being transferred to the sheet P. In particular, since the test image is not transferred to the sheet P, a part of the test image adheres to the secondary transfer roller 141. This toner is scraped off by the cleaning blade 142. The scattered toner may be attracted to the positive voltage applied to the secondary transfer roller 141 and adhere to the surface of the secondary transfer roller 141. Therefore, when the image formation is completed, a negative voltage (cleaning voltage) is applied to the secondary transfer roller 141 (cleaning process). As a result, the toner adhered to the surface of the secondary transfer roller 141 returns to the surface of the intermediate conveyor belt 135. Then, the cleaning blade 142 removes the toner from the intermediate conveyor belt 135.

[0021] Also, discharge may occur between a certain photosensitive drum 131 and the toner carried on the surface of the intermediate conveyor belt 135, causing the toner to become positively charged. The positively charged toner is transported downstream by the intermediate conveyor belt 135. The positively charged toner is then attracted to another negatively charged photosensitive drum 131. For example, assume that discharge occurs on the magenta photosensitive drum 131c. In this case, the positively charged magenta toner adheres to the negatively charged cyan photosensitive drum 131b. Therefore, a negative voltage is applied to the primary transfer rollers 136a, 136b, 136c, and 136d. As a result, the positive toner adhered to the surfaces of the photosensitive drums 131a, 131b, 131c, and 131d is transferred again to the surface of the intermediate conveyor belt 135.

[0022] In either case, the toner returned to the surface of the intermediate transport belt 135 is scraped off by a cleaning blade 142. In addition, the application of a negative voltage to the primary transfer roller 136 and the secondary transfer roller 141 is performed by a special sequence (cleaning process) that is different from the print sequence.

[0023] (2)Power supply device 2 shows a power supply device 200 that supplies power to the image forming apparatus 101. The control unit 201 is a control circuit configured with a central processing unit (CPU) or an application specific integrated circuit (ASIC), etc. The AC power supply 230 is an AC power supply circuit that generates an AC voltage Vton_ac for detecting information on the remaining amount of toner. The toner detection circuit 250 has a capacitance detection circuit 251 that detects the electrostatic capacitance between the electrode plate 138 and the electrode plate 139, generates a detection signal indicating the electrostatic capacitance (remaining amount of toner), and outputs the detection signal to the TON_SNS terminal of the control unit 201.

[0024] The positive power supply 210 is a power supply circuit that generates a positive voltage Vtri2_P for image formation. The positive power supply 210 is connected to the secondary transfer roller 141 via an output resistor R17. The positive voltage Vtri2_P output from the positive power supply 210 is subject to a voltage drop by the output resistor R17 and is reduced to a secondary transfer voltage Vtri2. Therefore, the secondary transfer voltage Vtri2 is applied to the secondary transfer roller 141.

[0025] The negative power supply 220 is a power supply circuit that generates a negative voltage Vtri2_N for toner cleaning. The negative power supply 220 is connected in series to the positive power supply 210, and is connected to the secondary transfer roller 141 via output resistors R14 and R17. For ease of explanation, both the positive and negative voltages connected to the secondary transfer roller 141 are referred to as a secondary transfer voltage Vtri2.

[0026] The DC power supply 260 is an auxiliary power supply circuit that receives an AC voltage Vton_ac for toner detection, rectifies, smoothes, and boosts the AC voltage Vton_ac to generate a primary transfer voltage Vtri1, and applies the primary transfer voltage Vtri1 to the primary transfer roller 136. In the first embodiment, the secondary transfer voltage Vtri2 is applied to the secondary transfer roller 141, and a voltage Vtri2z is also applied to the primary transfer roller 136 via the intermediate conveyor belt 135. If this voltage Vtri2z is insufficient for the target voltage of the primary transfer voltage Vtri1z actually applied to the primary transfer roller 136, the DC power supply 260 also supplies the primary transfer voltage Vtri1 to the primary transfer roller 136. In this way, the primary transfer voltage Vtri1z actually applied to the primary transfer roller 136 is formed from the voltage Vtri2z and the primary transfer voltage Vtri1.

[0027] The control unit 201 has TON_SNS terminals (TON_SNS_Y terminal, TON_SNS_M terminal, TON_SNS_C terminal, and TON_SNS_K terminal). The TON_SNS_Y terminal is a terminal to which a detection signal indicating the remaining amount of yellow toner is input. The TON_SNS_M terminal is a terminal to which a detection signal indicating the remaining amount of magenta toner is input. The TON_SNS_C terminal is a terminal to which a detection signal indicating the remaining amount of cyan toner is input. The TON_SNS_K terminal is a terminal to which a detection signal indicating the remaining amount of black toner is input. The control unit 201 has a TON_TGT terminal that outputs a setting signal for setting a target voltage to the AC power supply 230, and a TON_CLK terminal that outputs a clock signal for operating the AC power supply 230. Here, the clock signal is a signal that repeats High and Low, and may be called a HI / Lo signal. The control unit 201 has a T1_TGT terminal that outputs a setting voltage for generating the primary transfer voltage Vtri1 to the DC power supply 260. The control unit 201 has a T2P_VSNS terminal to which a detection voltage proportional to the positive voltage Vtri2_P is input, and a T2P_CLK terminal to which a clock signal for operating the positive power supply 210 is output. The control unit 201 has a T2N_CLK terminal to which a clock signal for operating the negative power supply 220 is output. Furthermore, the control unit 201 has a T2_ISNS terminal to which a detection voltage proportional to the secondary transfer current is input, and a T2N_VSNS terminal to which a detection voltage proportional to the negative voltage Vtri2_N is input.

[0028] (2-1) Secondary transfer power supply 3 is a circuit diagram of a positive power supply 210 that supplies a positive voltage to the secondary transfer roller 141 and a negative power supply 220 that supplies a negative voltage to the secondary transfer roller 141. The positive power supply 210 is formed by cascading four rectifying and smoothing circuits formed by diodes D12 to D15 and capacitors C12 to C15.

[0029] The transformer T11 has a primary winding T11-1 and a secondary winding T11-2. The turns ratio between the primary winding T11-1 and the secondary winding T11-2 is 1:N. A power supply voltage V1 is connected to one terminal of the primary winding T11-1. A drain terminal of the FET11 is connected to the other terminal of the primary winding T11-1. FET is an abbreviation for field effect transistor. A resistor R12 is connected between the gate terminal and the source terminal of the FET11. The resistor R12 is a resistor for noise suppression. A resistor R13 is connected between the gate terminal of the FET11 and the T2P_CLK terminal of the control unit 201. The resistor R13 is a resistor for protecting the FET11. The source terminal of the FET11 is connected to ground (GND). The FET11 performs switching operation in response to a drive signal input to the source terminal.

[0030] A parallel circuit consisting of a capacitor C11 and a resistor R11 is connected to one terminal of the primary winding T11-1. The parallel circuit is connected to the cathode of a diode D11. The anode of the diode D11 is connected to the other terminal of the primary winding T11-1.

[0031] A four-stage rectifying and smoothing circuit consisting of diodes D12, D13, D14, and D15 and capacitors C12, C13, C14, and C15 is connected between the two terminals of the secondary winding T11-2. Each rectifying and smoothing circuit also functions as a boost circuit. In the first stage of the rectifying and smoothing circuit, the anode of the diode D12 is connected to one terminal of the secondary winding T11-2 of the transformer T11. The cathode of the diode D12 is connected to one terminal of the capacitor C12. The other terminal of the capacitor C12 is connected to the other terminal of the secondary winding T11-2.

[0032] In the second stage rectifying and smoothing circuit, the anode of the diode D13 is connected to one terminal of the capacitor C12. The cathode of the diode D13 is connected to one terminal of the capacitor C13. The other terminal of the capacitor C13 is connected to one terminal of the secondary winding T11-2.

[0033] In the third stage rectifying and smoothing circuit, the anode of the diode D14 is connected to one terminal of the capacitor C13. The cathode of the diode D14 is connected to one terminal of the capacitor C14. The other terminal of the capacitor C14 is connected to one terminal of the capacitor C12.

[0034] In the fourth stage rectifying and smoothing circuit, the anode of the diode D15 is connected to one terminal of the capacitor C14. The cathode of the diode D15 is connected to one terminal of the capacitor C15. The other terminal of the capacitor C15 is connected to one terminal of the capacitor C13.

[0035] Here, one terminal of the capacitor C15 is connected to the secondary transfer roller 141 via the output terminal 211. One terminal of the capacitor C15 is also connected to the inverting input terminal of the operational amplifier IC21 via a voltage dividing circuit consisting of resistors R15 and R16. The connection point between the resistors R15 and R16 is connected to the T2P_VSNS terminal of the control unit 201. In other words, the output voltage generated at the output terminal 211 is divided by the resistors R15 and R16 to become a detection voltage proportional to the output voltage, and is fed back to the control unit 201.

[0036] The positive power supply 210 operates as follows. When a Hi state signal is output from the T2P_CLK terminal of the control unit 201, the FET11 turns on and the drain voltage of the FET11 drops to approximately the GND potential. As a result, a forward voltage is applied across the primary winding T11-1 of the transformer T11, and an excitation current flows. In parallel with this, a forward voltage that is N times the forward voltage on the primary side is generated across the secondary winding T11-2, and the forward voltage is rectified and smoothed by the diode D12 and the capacitor C12.

[0037] Next, when the voltage output from the T2P_CLK terminal changes to the Lo state, the FET11 turns off and a flyback voltage is generated across the primary winding T11-1. In parallel, a flyback voltage N times that of the primary side is generated across the secondary winding T11-2. The voltage obtained by summing the flyback voltage generated in the secondary winding T11-2 and the voltage charged in the capacitor C12 is rectified and smoothed by the diode D13 and the capacitor C13. Here, the capacitor C11, the resistor R11, and the diode D11 act as a snubber to absorb the surge voltage generated by the leakage inductance of the primary winding T11-1.

[0038] Next, when the voltage output from the T2P_CLK terminal changes to the Hi state again, a forward voltage is generated across the secondary winding T11-2. The forward voltage generated in the secondary winding T11-2 and the voltage charged in the capacitor C13 are summed to become a total voltage. The total voltage is rectified by the diode D14 and smoothed by the capacitors C14 and C12.

[0039] Next, when the voltage output from the T2P_CLK terminal changes to the Lo state again, a flyback voltage is generated across the secondary winding T11-2. The total voltage of the flyback voltage generated in the secondary winding T11-2 and the voltages charged in the capacitors C12 and C14 is rectified by the voltage diode D15 and smoothed by the series connection capacitance of the capacitors C15 and C13. The voltage generated at one terminal of the capacitor C15 becomes a positive voltage Vtri2_P.

[0040] Here, the positive voltage Vtri2_P and the voltage at the inverting input terminal of the operational amplifier IC21 are divided by resistors R15 and R16, and the resulting value (detection voltage) is input to the T2P_VSNS terminal. The voltage at the inverting input terminal of the operational amplifier IC21 is the same as the voltage at the non-inverting input terminal, which is in a virtual short-circuit relationship (the voltage generated by dividing the power supply voltage V2 by resistors R24 and R25).

[0041] In the positive power supply 210, a positive voltage Vtri2_P is generated by repeatedly outputting a Hi / Lo signal from the T2P_CLK terminal. The value of the positive voltage Vtri2_P changes according to the Hi / Lo signal from the T2P_CLK terminal. The control unit 201 controls the Hi / Lo signal output from the T2P_CLK terminal so that the value input to the T2P_VSNS terminal becomes a desired value. In this embodiment, the length of time of the Hi signal is controlled while the time of the Lo signal output from the T2P_CLK terminal is fixed. However, this is only one example of a control method. As another control method, there is a method of controlling the duty ratio of the Hi signal while fixing the repetition period of the Hi signal and the Lo signal. Such a control signal or drive signal is sometimes called a PWM signal. PWM is an abbreviation of pulse width modulation.

[0042] The negative power supply 220 has a one-stage rectifying and smoothing circuit consisting of a diode D22 and a capacitor C22. The transformer T21 has a primary winding T21-1 and a secondary winding T21-2. The turn ratio between the primary winding T21-1 and the secondary winding T21-2 is 1:M. A power supply voltage V1 is applied to one terminal of the primary winding T21-1. The drain terminal of the FET21 is connected to the other terminal of the primary winding T21-1. A resistor R22 for noise countermeasures is connected between the gate terminal and the source terminal of the FET21. Furthermore, a protective resistor R23 is connected between the source terminal and the T2N_CLK terminal of the control unit 201. The source terminal of the FET21 is connected to GND. A parallel circuit consisting of a capacitor C21 and a resistor R21 and a diode D21 connected in series to the parallel circuit are connected between the two terminals of the primary winding T21-1.

[0043] A diode D22 and a capacitor C22 are connected between two terminals of the secondary winding T21-2. The cathode of the diode D22 is connected to one terminal of the secondary winding T21-2 of the transformer T21. The anode of the diode D22 is connected to one terminal of the capacitor C22. The other terminal of the capacitor C22 is connected to the other terminal of the secondary winding T21-2 and GND.

[0044] One terminal of the capacitor C22 is further connected to the cathode of a diode D23. The anode of the diode D23 is connected to one terminal of a secondary winding T11-2 of a transformer T11 of a positive power supply 210. The anode of the diode D23 is also connected to an inverting input terminal of an operational amplifier IC21 via a series circuit of a resistor R27 and a resistor R28. The connection point of the resistor R27 and the resistor R28 is connected to a T2N_VSNS terminal of a control unit 201. The negative voltage Vtri2_N generated by a negative power supply 220 is divided by the resistors R27 and R28 to become a detection voltage, which is fed back to the T2N_VSNS terminal.

[0045] The inverting input terminal of the operational amplifier IC21 is connected to GND via a capacitor C23. A resistor R26 and a capacitor C24 are connected in parallel between the inverting input terminal of the operational amplifier IC21 and the output terminal of the operational amplifier IC21. The output terminal of the operational amplifier IC21 is connected to the T2_ISNS terminal of the control unit 201. The connection point between the resistors R24 and R25 is connected to the non-inverting input terminal of the operational amplifier IC21. The resistors R24 and R25 divide the power supply voltage V2 and apply it to the non-inverting input terminal of the operational amplifier IC21. The operational amplifier IC21, the resistor R26, and the capacitor C24 form a current detection circuit that detects the current in the secondary transfer roller 141.

[0046] The negative power supply 220 operates as follows. When a Hi state signal is output from the T2N_CLK terminal of the control unit 201, the FET 21 turns on and the drain potential of the FET 21 drops to approximately the GND potential. This causes a voltage to be applied across the primary winding T21-1 of the transformer T21, causing an excitation current to flow. In parallel with this, a forward voltage that is M times the primary side forward voltage is generated across the secondary winding T21-2.

[0047] Next, when the voltage output from the T2N_CLK terminal changes to the Lo state, the FET21 turns off and a flyback voltage is generated across the primary winding T21-1. In parallel with this, a flyback voltage M times the primary flyback voltage is generated across the secondary winding T21-2. This flyback voltage is rectified and smoothed by the diode D22 and the capacitor C22. The capacitor C21, the resistor R21, and the diode D21 act as a snubber to absorb the surge voltage caused by the leakage inductance of the primary winding T21-1. The voltage at one terminal of the capacitor C22 becomes a negative voltage Vtri2_N, which is a negative secondary transfer voltage.

[0048] The negative voltage Vtri2_N is applied to the other terminal of the secondary winding T11-2 of the transformer T11 via a diode D23. The voltage of the anode of the diode D23 and the voltage of the inverting input terminal of the operational amplifier IC21 are divided by resistors R27 and R28, and the divided voltage is input to the T2N_VSNS terminal.

[0049] In the negative power supply 220, a Hi / Lo signal is repeatedly output from the T2N_CLK terminal, thereby generating a negative voltage Vtri2_N. The value of the negative voltage Vtri2_N changes according to the Hi / Lo signal from the T2N_CLK terminal. The control unit 201 controls the Hi / Lo signal output from the T2N_CLK terminal so that the value input to the T2N_VSNS terminal becomes a desired value. In this embodiment, the time of the Hi signal is controlled while the time of the Lo signal output from the T2N_CLK terminal is fixed.

[0050] Incidentally, the operational amplifier IC21 is provided to detect the value of the current supplied to the secondary transfer roller 141. The current supplied from one terminal of the capacitor C15 to the secondary transfer roller 141 flows to the frame (=GND) of the image forming apparatus 101, and returns from the output terminal of the operational amplifier IC21 to the other terminal of the capacitor C13 via resistors R26, R28, and R27. That is, the value obtained by converting the current flowing through the resistor R26 into a voltage is input to the T2_ISNS terminal.

[0051] As shown in FIG. 3, the positive power source 210 and the negative power source 220 are connected in series. The control unit 201 operates the positive power source 210 to generate a positive voltage Vtri2_P, which is a positive primary transfer voltage, and supplies it to the secondary transfer roller 141. The control unit 201 also operates the negative power source 220 to generate a negative voltage Vtri2_N and supplies it to the secondary transfer roller 141. As shown in FIG. 3, resistors R14 and R17 are interposed between the output terminal 221 and the secondary transfer roller 141. Therefore, the negative voltage Vtri2_N is divided by the combined resistance value of the resistors R14 and R17 and the resistance value of the secondary transfer roller 141, and is applied to the secondary transfer roller 141. The negative voltage Vtri2_N is a voltage for cleaning the toner, and is therefore applied during the pre-rotation, post-rotation, or between the sheets. The pre-rotation refers to the rotation of the intermediate conveyor belt 135 before a toner image is formed. The post-rotation means that the intermediate conveyor belt 135 rotates after a toner image is formed. The paper interval is the period between the timing when a toner image is formed on a preceding sheet P and the timing when a toner image is formed on a succeeding sheet P.

[0052] (2-2) Toner detection circuit The toner detection circuit 250 detects information on the remaining amount of toner filled in each of the toner containers 134a, 134b, 134c, and 134d. The dielectric constant of toner is greater than that of the air. Therefore, the capacitance between the electrode plates 138 and 139 changes depending on the amount of toner present between the electrode plates 138 and 139. The greater the amount of toner, the greater the capacitance. The smaller the amount of toner, the smaller the capacitance. Using this phenomenon, the control unit 201 calculates the remaining amount of toner held in the toner container 134 based on the capacitance detected by the toner detection circuit 250. When an AC voltage Vton_ac is applied to one electrode plate 138, an AC current flows through the other electrode plate 139. The capacitance detection circuit 251 detects this AC current and generates a detection signal indicating the capacitance.

[0053] FIG. 4 shows a circuit diagram of the capacitance detection circuit 251. The capacitance detection circuits 251a to 251d each have the same peak hold circuit. The anode of the diode D52 is connected to the electrode plate 139. The cathode of the diode D52 is connected to one end of the capacitor C51. The other end of the capacitor C51 is connected to GND. As a result, when the amplitude of the AC current is positive, the AC current charges the capacitor C51 via the diode D52. On the other hand, the cathode of the diode D51 is connected to the electrode plate 139. The anode of the diode D51 is connected to the other end of the capacitor C51. The resistor R51 is connected in parallel to the capacitor C51. The diode D51 and the resistor R51 discharge and reset the capacitor C51. One end of the capacitor C51 is connected to the TON_SNS terminal. That is, the peak voltage of the AC voltage generated between the electrode plate 138 and the electrode plate 139 is applied to the TON_SNS terminal. The control unit 201 obtains the peak voltage value by analog-to-digital conversion of the peak voltage, and calculates the capacitance from the peak voltage value.

[0054] (2-3) AC power supply and DC power supply (2-3-1) Circuit configuration of AC power supply FIG. 5 is a circuit diagram of the AC power supply 230 and the DC power supply 260. The AC power supply 230 has a transformer T31. The transformer T31 has a primary winding T31-1 and a secondary winding T31-2. The turn ratio of the primary winding T31-1 and the secondary winding T31-2 is L. One terminal of the primary winding T31-1 is connected to one terminal of the capacitor C31. The other terminal of the primary winding T31-1 is connected to GND. The other terminal of the capacitor C31 is connected to the emitter terminal of the NPN type transistor Tr31 and the emitter terminal of the PNP type transistor Tr32. The collector terminal of the transistor Tr31 is connected to the power supply voltage V1. The collector terminal of the transistor Tr32 is connected to GND. A resistor R34, a diode D31, a diode D32, and a resistor R35 are connected in series between the power supply voltage V1 and GND. The base terminal of the transistor Tr31 is connected between the resistor R34 and the anode of the diode D31 via a resistor R36. The base terminal of the transistor Tr32 is connected between the cathode of the diode D32 and a resistor R35 via a resistor R37. The resistors R36 and R37 are protective resistors. The connection point between the diodes D31 and D32 is connected to the drain terminal of the FET31 and is also connected to the output terminal of the operational amplifier IC31 via a resistor R33. The gate terminal of the FET31 is connected to the TON_CLK terminal of the control unit 201 via the resistor R31. A resistor R32 is connected between the gate terminal and the source terminal of the FET31 to reduce noise.

[0055] A capacitor C32 is connected between two terminals of the secondary winding T31-2 of the transformer T31. One terminal of the secondary winding T31-2 is connected to one of the electrode plates 138a, 138b, 138c, and 138d. The other terminal of the secondary winding T31-2 is connected to GND.

[0056] One terminal of the secondary winding T31-2 is also connected to the anode terminal of a diode D41. The cathode terminal of the diode D41 is connected to GND via a capacitor C44. The cathode terminal of the diode D41 is further connected to an inverting input terminal of an operational amplifier IC31 via a resistor R46. The inverting input terminal of the operational amplifier IC31 is connected to GND via a resistor R45. Furthermore, the inverting input terminal is connected to an output terminal of the operational amplifier IC31 via a parallel connection circuit of a resistor R44 and a capacitor C43. The non-inverting input terminal of the operational amplifier IC31 is connected to GND via a capacitor C41. Furthermore, the non-inverting input terminal is connected to a drain terminal of a FET41 via a resistor R44. The drain terminal of the FET41 is connected to a power supply voltage V2 via a resistor R43. The gate terminal of the FET41 is connected to a TON_TGT terminal of the control unit 201 via a resistor R41. A resistor R42 is connected between the gate terminal and the source terminal of the FET41.

[0057] (2-3-2) AC power supply operation When the TON_CLK terminal of the control unit 201 outputs a signal in the Hi state, the AC power supply 230 does not output the AC voltage Vton_ac. In this state, the FET31 maintains the ON state, and the drain voltage of the FET31 has substantially the same potential as the GND. As a result, a current flows from the power supply voltage V1 to the GND via the resistor R34, the diode D31, and the FET31, and the voltage of the anode terminal of the diode D31 becomes the forward voltage Vf of the diode D31. Here, if the Vf of the diode D31 is equal to or lower than the Vf between the base and emitter of the transistor Tr31, no current flows into the base terminal of the transistor Tr31. Therefore, the transistor Tr31 is turned off. If the Vf of the diode D31 is larger than the Vf between the base and emitter of the transistor Tr31, the transistor Tr31 is turned on until the voltage of the base terminal of the transistor Tr31 becomes the same potential as the Vf of the diode D31. In either case, the potential difference between both ends of the capacitor C31 becomes substantially zero. Then, the transistor Tr32 is turned off.

[0058] When the TON_CLK terminal of the control unit 201 outputs a Lo signal, the FET31 turns off and the voltage of the drain terminal of the FET31 rises. As a result, a current flows from the power supply voltage V1 to the base terminal of the transistor Tr31 via the resistors R34 and R36. This turns on the transistor Tr31 and a current flows from the power supply voltage V1 to the capacitor C31 and the primary winding T31-1 via the transistor Tr31. This current charges the capacitor C31 and a voltage is generated across the capacitor C31. Here, a voltage is generated between one terminal and the other terminal of the primary winding T31-1, and the terminal connected to the capacitor C31 has a positive polarity.

[0059] After that, when the TON_CLK terminal of the control unit 201 outputs a Hi signal again, the FET31 turns on and the transistor Tr31 turns off. Here, the capacitor C31 is charged, so the charge stored in the capacitor C31 flows from the emitter terminal to the base terminal of the transistor Tr32, and the transistor Tr32 turns on. As a result, a discharge current flows from the capacitor C31 through the transistor Tr32, GND, and the primary winding T31-1, returning to the capacitor C31. At this time, a voltage is generated between one terminal and the other terminal of the primary winding T31-1, but the terminal connected to the capacitor C31 becomes negative polarity.

[0060] In this way, the TON_CLK terminal of the control unit 201 repeatedly outputs a Hi / Lo signal (clock signal), so that an AC current is generated in the capacitor C31 and the primary winding T31-1, and an AC voltage is generated between the two terminals of the primary winding T31-1. Accordingly, an AC voltage is generated in the secondary winding T31-2, which is the AC voltage generated between the two terminals of the primary winding T31-1 multiplied by the winding ratio L. The AC voltage Vton_ac generated in the secondary winding T31-2 is supplied to one of the electrode plates 138a, 138b, 138c, and 138d. The voltage across the capacitor C31 is stable at about half the drain voltage of the FET31 when the FET31 is in the off state. The peak-to-peak value of the AC voltage generated between the terminals of the primary winding T31-1 is almost equal to the drain voltage of the FET31 when the FET31 is in the off state.

[0061] The AC voltage Vton_ac is peak-held by the diode D41 and the capacitor C44. The peak-held voltage is divided by the resistors R46 and R45 and input to the inverting input terminal of the operational amplifier IC31. The FET41 is turned on / off by the Hi / Lo signal output from the TON_TGT terminal of the control unit 201, and the voltage rectified and smoothed by the resistor R44 and the capacitor C41 is input to the non-inverting input terminal of the operational amplifier IC31. When the voltage of the inverting input terminal is smaller than the voltage of the non-inverting input terminal, the operational amplifier IC31 increases the voltage of the output terminal. This increases the AC voltage between the two terminals of the primary winding T31-1 and the AC voltage Vton_ac. When the voltage of the inverting input terminal is larger than the voltage of the non-inverting input terminal, the operational amplifier IC31 decreases the voltage of the output terminal. This decreases the AC voltage between the two terminals of the primary winding T31-1 and the AC voltage Vton_ac. Such a feedback operation causes the voltage at the inverting input terminal to approach the voltage at the non-inverting input terminal.

[0062] In this way, the control unit 201 controls the Hi / Lo signal output from the TON_TGT terminal, thereby controlling the voltage value of the AC voltage Vton_ac supplied to one of the electrode plates 138a, 138b, 138c, 138d. In the first embodiment, the frequency of the Hi / Lo signal output from the TON_TGT terminal is fixed, and the ratio of the time during which the Hi signal is output per period (duty ratio) is controlled.

[0063] FIG. 6A is a graph showing the relationship between the duty ratio of the TON_TGT terminal and the AC voltage Vton_ac. When the duty ratio is 0%, the AC voltage Vton_ac is a maximum value (e.g., 260 Vpp). When the duty ratio is 100%, the AC voltage Vton_ac is a minimum value (e.g., 0 Vpp).

[0064] (2-3-3) DC power supply circuit configuration A positive polarity primary transfer voltage Vtri1 generated by a DC power supply 260 is applied to the primary transfer rollers 136a, 136b, 136c, and 136d. In the first embodiment, as shown in Fig. 5, the DC power supply 260 generates a positive voltage by boosting the AC voltage Vton_ac using a four-stage rectifying and smoothing circuit (boosting circuit). The number of stages of the rectifying and smoothing circuit may be determined based on the required level of the primary transfer voltage Vtri1 and the level of the input AC voltage Vton_ac.

[0065] According to FIG. 5, a multi-stage rectifying and smoothing circuit is connected between two terminals of the secondary winding T31-2 of the transformer T31. Such a multi-stage rectifying and smoothing circuit is called a Cockcroft-Walton circuit. The first stage rectifying and smoothing circuit is composed of a capacitor C61 and a diode D61. The second stage rectifying and smoothing circuit is composed of a capacitor C62 and a diode D62. The third stage rectifying and smoothing circuit is composed of a capacitor C63 and a diode D63. The fourth stage rectifying and smoothing circuit is composed of a capacitor C64 and a diode D64.

[0066] The connection point between the capacitor C64 and the cathode of the diode D64 is connected to the primary transfer rollers 136a, 136b, 136c, and 136d and the collector terminal of the transistor Tr61 via the resistor R61. The emitter terminal of the transistor Tr61 is connected to GND. The base terminal of the transistor Tr61 is connected to the output terminal of the operational amplifier IC61. The collector terminal of the transistor Tr61 is connected to GND via the resistors R63 and R64. The connection point between the resistors R63 and R64 is connected to the non-inverting input terminal of the operational amplifier IC61. A parallel circuit consisting of the resistor R62 and the capacitor C65 is connected between the output terminal and the inverting input terminal of the operational amplifier IC61. One end of the capacitor C66 and one end of the resistor R68 are connected to the inverting input terminal of the operational amplifier IC61. The other end of the capacitor C66 is connected to GND. The other end of the resistor R68 is connected to the drain terminal of the FET61. The drain terminal of the FET 61 is connected to the power supply voltage V2 via a resistor R67. The gate terminal of the FET 61 is connected to the T1_TGN terminal of the control unit 201 via a resistor R65. A resistor R66 is connected between the gate terminal and the source terminal of the FET 61.

[0067] (2-3-4) Auxiliary power supply operation The AC voltage Vton_ac is boosted by a four-stage rectifying and smoothing circuit to generate a base voltage Vtri1_base. The base voltage Vtri1_base is approximately four times the peak-to-peak voltage of the AC voltage Vtri_ac. The base voltage Vtri1_base is subjected to a voltage drop by resistor R61 to become the primary transfer voltage Vtri1. The current flowing through resistor R61 is the sum of the current flowing through primary transfer rollers 136a, 136b, 136c, and 136d and the current flowing through transistor Tr61 and resistor R63. In order to control the primary transfer voltage Vtri1 to a predetermined value, it is necessary to control the current flowing through resistor R61.

[0068] When the T1_TGT terminal of the control unit 201 outputs a Hi / Lo signal, the FET 61 is turned on / off. As a result, the voltage rectified and smoothed by the resistor R68 and the capacitor C66 is input to the inverting input terminal of the operational amplifier IC61. The primary transfer voltage Vtri1 is divided by the resistors R63 and R64. This divided voltage becomes the voltage of the non-inverting input terminal of the operational amplifier IC61. If the divided voltage is greater than the voltage of the inverting input terminal (the set voltage corresponding to the target voltage), the voltage of the output terminal of the operational amplifier IC61 increases, and the current flowing through the transistor Tr61 also increases. The current flowing through the resistor R61 also increases. As a result, the voltage drop caused by the resistor R61 that the main voltage Vtri1_base receives increases, and the primary transfer voltage Vtri1 decreases. Conversely, if the voltage of the non-inverting input terminal is smaller than the voltage of the inverting input terminal, feedback is applied so that the primary transfer voltage Vtri increases. As a result, the primary transfer voltage Vtri1 is maintained at the target voltage.

[0069] In this way, the controller 201 controls the Hi / Lo signal output from the T1_TGT terminal to control the primary transfer voltage Vtri1 supplied to the primary transfer rollers 136a, 136b, 136c, and 136d. In the first embodiment, the frequency of the Hi / Lo signal output from the T1_TGT terminal is fixed, and the time ratio (duty ratio) at which the Hi signal is output is controlled.

[0070] FIG. 6B is a graph showing the relationship between the duty ratio of the signal output from the T1_TGT terminal and the primary transfer voltage Vtri1. When the duty ratio is 100%, the primary transfer voltage Vtri1 is 0V. As the duty ratio decreases, the primary transfer voltage Vtri1 increases. Eventually, a control region is reached where the primary transfer voltage Vtri1 does not change even if the duty ratio decreases. The primary transfer voltage Vtri1 at this time is defined as Vtri1(max). Vtri1(max) depends on the load on the primary transfer roller 136. The greater the load on the primary transfer roller 136, the smaller Vtri1(max).

[0071] Incidentally, the inner surface layer of the intermediate conveying belt 135 in the first embodiment is conductive. When a positive voltage Vtri2_P is applied to the secondary transfer roller 141, a current flows from the secondary transfer roller 141 through the intermediate conveying belt 135 to the primary transfer rollers 136a, 136b, 136c, and 136d and the DC power source 260. As a result, a voltage is applied to the primary transfer rollers 136a, 136b, 136c, and 136d. In other words, the positive voltage Vtri2_P also functions as an auxiliary power source for the primary transfer voltage Vtri1.

[0072] In the first embodiment, the DC power supply 260 does not have a dedicated AC power supply, but uses the AC power supply 230 for the toner detection circuit 250. That is, the AC power supply 230 is shared by the toner detection circuit 250 and the DC power supply 260 that generates the primary transfer voltage Vtri1. This makes it possible to reduce the cost and layout space of the power supply circuit that generates the primary transfer voltage Vtri1. That is, the circuit scale of the power supply circuit is reduced, and the size of the image forming apparatus 101 can also be reduced.

[0073] <Example 2> In the second embodiment, the operation of the AC power supply 230 changes depending on whether the positive power supply 210 generates the positive voltage Vtri2_P. Other points in the second embodiment are common to the first embodiment. Therefore, the description of the first embodiment is used for the common points.

[0074] 7A, the power source of the primary transfer voltage Vtri1z is composed of an AC power source 230 and a positive power source 210.

[0075] FIG. 7B is a graph showing the relationship between the duty ratio and the AC voltage Vton_ac. Considering the power supply from the positive power source 210, the AC voltage Vton_ac required as the power source of the primary transfer voltage Vtri1z is, for example, 60 Vpp. This corresponds to a duty ratio of 77% at the TON_TGT terminal. On the other hand, the AC voltage Vton_ac required for the capacitance detection is, for example, 150 Vpp. This corresponds to a duty ratio of 52% at the TON_TGT terminal. Therefore, the AC voltage Vton_ac required as the power source of the primary transfer voltage Vtri1z is lower than the AC voltage Vton_ac required for the capacitance detection. Therefore, at the timing when the control unit 201 is not performing the capacitance detection, the AC power source 230 may generate the AC voltage Vton_ac of 60 Vpp.

[0076] (3-1) Signal and voltage transitions over time when capacitive sensing is performed 8(A) and 8(B) show the time evolution of various signals and output voltages associated with the positive power supply 210, the AC power supply 230, and the DC power supply 260 when imaging begins. In particular, FIG. 8(A) shows the time evolution when capacitive sensing is performed.

[0077] At time t1, the control unit 201 sets the TON_TGT terminal and the T1_TGT terminal. The duty ratio of the signal output from the TON_TGT terminal is 52%, which corresponds to 150 Vpp of the AC voltage Vton_ac. Also, as shown in FIG. 7C, in order to set the primary transfer voltage Vtri1 to 210 V, the duty ratio of the signal output from the T1_TGT terminal is set to 65%.

[0078] At time t2, the control unit 201 starts outputting the Hi / Lo signal from the TON_CLK terminal. This causes the AC voltage Vton_ac to become 150 Vpp. The primary transfer voltage Vtri1 increases to 210 V.

[0079] At time t3, the control unit 201 starts outputting a Hi / Lo signal from the T2P_CLK terminal, causing the positive voltage Vtri2_P to rise to a desired value.

[0080] At time t4, the control unit 201 starts the fall process. That is, the control unit 201 stops outputting the signal from the TON_CLK terminal. This causes the primary transfer voltage Vtri1 to fall to 0V.

[0081] At time t5, the control unit 201 stops outputting the signal from the T2P_CLK terminal, causing the positive voltage Vtri2_P to drop to 0 V.

[0082] (3-2) Signal and voltage transitions over time when capacitive sensing is not performed Fig. 8(B) shows the transition over time when the electrostatic capacitance detection is not performed. In Fig. 8(B), the parts that are different from Fig. 8(A) are drawn in bold lines.

[0083] At time t11, the control unit 201 sets the TON_TGT terminal and the T1_TGT terminal. The duty ratio of the signal output from the TON_TGT terminal is set to 77%, which corresponds to 60 Vpp of the AC voltage Vton_ac. The duty ratio of the T1_TGT terminal is set to 65%, which corresponds to 210 V of the primary transfer voltage Vtri1.

[0084] At time t12, the control unit 201 starts outputting a Hi / Lo signal from the TON_CLK terminal. This causes the AC voltage Vton_ac to rise to 60 Vpp. At this point, the AC voltage Vton_ac is insufficient, so the primary transfer voltage Vtri1 does not reach 210 V set at the T1_TGT terminal.

[0085] At time t13, the control unit 201 starts outputting a Hi / Lo signal from the T2P_CLK terminal. This causes the positive voltage Vtri2_P to rise to a desired value. With the assistance of the positive voltage Vtri2_P, the primary transfer voltage Vtri1 (more precisely, the primary transfer voltage Vtri1z) reaches the target voltage of 210V.

[0086] At time t14, the control unit 201 starts the fall process. That is, the control unit 201 stops outputting the signal from the TON_CLK terminal. This causes the primary transfer voltage Vtri1 to fall to 0 V.

[0087] At time t15, the control unit 201 stops outputting the signal from the T2P_CLK terminal, causing the positive voltage Vtri2_P to drop to 0 V.

[0088] In the second embodiment, the duty ratio of the TON_TGT terminal when the capacitance detection is performed is assumed to be 77% (Vton_ac=60Vpp), but this is merely an example. For example, depending on the materials of the intermediate conveyor belt 135 and the secondary transfer roller 141 and the configuration of the image forming apparatus 101, the duty ratio of the TON_TGT terminal may be set to 100% (corresponding to Vton_ac=0Vpp). That is, there may be cases where the AC voltage Vton_ac does not need to be output. The state in which the AC voltage Vton_ac is not output may be expressed as a state in which the AC power supply 230 is stopped. The state in which the AC voltage Vton_ac is output may be expressed as a state in which the AC power supply 230 is operating.

[0089] According to the second embodiment, when the control unit 201 is performing capacitance detection, the output of the AC power supply 230 is increased. That is, when the control unit 201 is not performing capacitance detection, the output of the AC power supply 230 is reduced. This reduces the power loss in the AC power supply 230. The time for performing capacitance detection is about several tens of seconds, which is sufficiently short compared to the time for which the image forming apparatus 101 is performing image formation. Therefore, the rating of the element in the second embodiment is lower than the rating of the element for constantly operating the AC power supply 230. That is, it is possible to reduce the cost of the power supply device of the image forming apparatus 101.

[0090] <Example 3> The third embodiment is different from the first embodiment in that the operation of the AC power supply 230 is changed depending on whether the negative power supply 220 generates the negative voltage Vtri2_N. In the third embodiment, only the points different from the first embodiment will be described, and the explanation of the first embodiment will be cited for the points equivalent to those in the first embodiment.

[0091] The positive voltage Vtri2_P also functions as an auxiliary power source for the primary transfer voltage Vtri1z. When the secondary transfer voltage Vtri2 is negative, the negative voltage Vtri2_N takes away the power of the primary transfer voltage Vtri1z. Therefore, when the secondary transfer voltage Vtri2 is negative, the AC power source 230 needs to make the AC voltage Vton_ac larger than normal in order to obtain the primary transfer voltage Vtri1z.

[0092] 9A is a graph showing the relationship between the duty ratio and the AC voltage Vton_ac. When the secondary transfer voltage Vtri2 is negative, the AC voltage Vton_ac required as a power source for the primary transfer voltage Vtri1z is, for example, 200 Vpp. This corresponds to a duty ratio of the TON_TGT terminal being 23%. 200 Vpp exceeds the AC voltage Vton_ac of 150 Vpp (duty ratio of the TON_TGT terminal=52%) required for capacitance detection. Therefore, when the secondary transfer voltage Vtri2 is negative, the AC power supply 230 generates an AC voltage Vton_ac of 200 Vpp in order to maintain the primary transfer voltage Vtri1z at the target voltage.

[0093] Fig. 9B is a graph showing the time transition of the signal and the voltage. In Fig. 9B, the parts different from Fig. 8A are drawn with thick lines. When the control unit 201 performs color misregistration correction or when the cleaning process is performed, the negative secondary transfer voltage Vtri2 is required.

[0094] At time t21, the control unit 201 sets the TON_TGT terminal and the T1_TGT terminal. The duty ratio of the TON_TGT terminal is set to 23%. This corresponds to the AC voltage Vton_ac being 200 Vpp. The duty ratio of the T1_TGT terminal is set to 65%. This corresponds to the primary transfer voltage Vtri1 being 210 V.

[0095] At time t22, the control unit 201 starts outputting a Hi / Lo signal from the TON_CLK terminal. This causes the AC voltage Vton_ac to rise to 200 Vpp. The primary transfer voltage Vtri1 rises, and the primary transfer voltage Vtri1z that actually acts on the primary transfer roller 136 rises to 210 V.

[0096] At time t23, the controller 201 starts outputting the Hi / Lo signal from the T2N_CLK terminal, causing the secondary transfer voltage Vtri2 to reach a desired value.

[0097] At time t24, the control unit 201 starts the falling process. That is, the control unit 201 stops outputting the signal from the TON_CLK terminal. This causes the primary transfer voltage Vtri1 to drop to 0 V.

[0098] At time t25, the controller 201 stops outputting the signal from the T2N_CLK terminal, causing the secondary transfer voltage Vtri2 to return to 0 V.

[0099] According to the third embodiment, when the secondary transfer voltage Vtri2 is negative, the output of the AC power supply 230 is increased (Vton_ac=200Vpp). On the other hand, when the secondary transfer voltage Vtri2 is positive, the output of the AC power supply 230 is decreased (Vton_ac=150Vpp). This reduces the power loss in the AC power supply 230. Even when the secondary transfer voltage Vtri2 is negative, the primary transfer voltage Vtri1 is maintained at the target voltage, and the primary transfer is possible. The implementation time of the color misregistration correction is sufficiently short with respect to the image formation time. Therefore, compared with the circuit conditions required to constantly output the AC voltage Vton_ac at 200Vpp, in the third embodiment, the rating of the circuit elements can be reduced. That is, the cost of the power supply device of the image forming apparatus 101 can be reduced. In the third embodiment, during the period when the capacitance detection is required, the AC voltage Vton_ac is decreased to 60Vpp.

[0100] <Technical ideas derived from examples> [Item 1] a toner container for storing toner; a photoreceptor on which a toner image is formed by the toner supplied from the toner container; an intermediate transfer body that rotates while being in contact with the photoreceptor and has electrical conductivity; a primary transfer member that is applied with a primary transfer voltage and transfers the toner image from the photoreceptor to the intermediate transfer member; a detection unit provided in the toner container and configured to detect information regarding a remaining amount of the toner contained in the toner container; an AC power source for supplying an AC voltage to the detection means; a first DC power source that converts the AC voltage output from the AC power source to generate a DC voltage of a first polarity and applies the DC voltage of the first polarity to the primary transfer member; An image forming apparatus comprising:

[0101] The photosensitive drum 131 is an example of a photosensitive body. The intermediate conveyor belt 135 is an example of an intermediate transfer body. The primary transfer roller is an example of a primary transfer member. The electrode plate 138 and the electrode plate 139 are an example of a detection means. The DC power supply 260 is an example of a first DC power supply. In this way, the detection means and the first DC power supply share one AC power supply, so that the cost and arrangement space of the power supply circuit that generates the primary transfer voltage can be reduced. The first polarity and the second polarity are determined according to the charge polarity of the toner. If the charge polarity of the toner is negative, the first polarity is positive and the second polarity is negative. On the other hand, if the charge polarity of the toner is positive, the first polarity is negative and the second polarity is positive.

[0102] [Item 2] a secondary transfer member that rotates while in contact with the intermediate transfer body and that is applied with a secondary transfer voltage to transfer the toner image from the intermediate transfer body to a sheet; a second DC power source that generates a second DC voltage of the first polarity and applies the second DC voltage of the first polarity to the secondary transfer member as the secondary transfer voltage, The primary transfer voltage is a DC voltage of the first polarity applied from the first DC power source to the primary transfer member; a DC voltage generated by the second DC voltage of the first polarity being subjected to a voltage drop by the intermediate transfer body in a route from the second DC power source to the primary transfer member via the secondary transfer member and the intermediate transfer body; 2. The image forming apparatus according to item 1,

[0103] The secondary transfer roller 141 is an example of a secondary transfer member. The positive power source 210 is an example of a second DC power source. Here, the primary transfer voltage Vtri1z actually applied to the primary transfer member is formed from a DC voltage (Vtri1) of a first polarity and a DC voltage (Vtri2z) generated by subjecting a second DC voltage (Vtri2_P) of the first polarity to a voltage drop caused by the intermediate transfer body. In this way, when the primary transfer voltage is negative, a circuit that generates the secondary transfer voltage is used. This makes it possible to reduce the cost and arrangement space of the power supply circuit that generates the primary transfer voltage.

[0104] [Item 3] The power supply may further include a control unit for controlling the power supply. The control means When the detection unit detects information regarding the remaining amount of toner, the AC power source is controlled so that the AC voltage becomes a first target voltage; When the detection unit does not detect information regarding the remaining amount of toner, the AC power source is controlled so that the AC voltage becomes a second target voltage that is lower than the first target voltage. 3. The image forming apparatus according to item 2, wherein the primary transfer voltage is maintained constant.

[0105] The control unit 201 is an example of a control means. As shown in FIG. 7B, the first target voltage may be, for example, 150 Vpp. The second target voltage may be, for example, 60 Vpp. Here, since the time during which the first target voltage is employed is shorter than the time during which the second target voltage is employed, it is possible to lower the ratings of the circuit elements required to configure the AC power supply. In other words, it will be possible to further reduce the cost of the power supply circuit.

[0106] [Item 4] The power supply may further include a control unit for controlling the power supply. The control means When the detection unit detects information regarding the remaining amount of toner, the AC power source is controlled so that the AC voltage becomes a first target voltage; When the detection means does not detect the information regarding the remaining amount of toner, the AC power source is controlled to stop outputting the AC voltage. 3. The image forming apparatus according to item 2, wherein the primary transfer voltage is maintained constant.

[0107] As described in the second embodiment, it may be possible to cover the primary transfer voltage Vtri1z only with the DC voltage (Vtri2z) resulting from the positive voltage Vtri2_P output from the positive power supply 210. In this case, the control unit 201 may stop the DC power supply 260 by stopping the AC power supply 230. This will reduce power loss.

[0108] [Item 5] a cleaning member for cleaning the toner remaining on the intermediate transfer body; a third DC power source connected in series to the second DC power source, generating a third DC voltage of a second polarity opposite to the first polarity, and applying the third DC voltage of the second polarity to the secondary transfer member as the secondary transfer voltage; 5. The image forming apparatus according to claim 3, wherein in a cleaning step of returning toner adhering to the secondary transfer member to the intermediate transfer body and removing the toner from the intermediate transfer body with the cleaning member to clean the secondary transfer member, the control unit stops the second DC power supply, operates the third DC power supply, and controls the AC power supply so that the AC voltage becomes a third target voltage higher than the first target voltage.

[0109] The cleaning blade 142 is an example of a cleaning member. The negative power source 220 is an example of a third DC power source. The polarity of the negative voltage Vtri2_N output in the cleaning process is opposite to the polarity of the primary transfer voltage Vtri1z. Therefore, the AC voltage is controlled to a third target voltage (e.g., 200 Vpp). This maintains the primary transfer voltage Vtri1z at the target value, and maintains the primary transfer performance of the toner image.

[0110] [Item 6] The test image formed on the intermediate transfer body is read by a reading means. 6. The image forming apparatus according to claim 5, wherein the cleaning step includes cleaning a portion of the test image adhering to the secondary transfer member.

[0111] The image sensor 143 is an example of a reading unit. As described above, since the test image formed by the color shift correction is not transferred to the sheet P, a part of the test image may adhere to the secondary transfer member. Therefore, a negative voltage Vtri2_N is required to clean the test image.

[0112] [Item 7] The detection means is a first electrode plate disposed inside the toner container and to which the AC voltage is applied; a second electrode plate disposed inside the toner container; an acquisition circuit that acquires an AC current generated in the second electrode plate in accordance with information regarding the remaining amount of toner by applying the AC voltage to the first electrode plate; 7. The image forming apparatus according to claim 1, further comprising:

[0113] 4, capacitance detection circuits 251a-251d are an example of an acquisition circuit. That is, the capacitance changes according to the amount of toner present between electrode plate 138 and electrode plate 139, and the AC current generated in electrode plate 139 changes. That is, capacitance detection circuits 251a-251d acquire the AC current, and thus the capacitance can be measured based on the AC current.

[0114] [Item 8] Item 7, the image forming apparatus, wherein the acquisition circuit is a peak hold circuit. As shown in FIG. 4, the capacitance detection circuits 251a to 251d may be realized by peak hold circuits. This allows the capacitance detection circuits 251a to 251d to be realized by relatively simple circuits. Note that the peak hold circuits may employ a higher performance circuit configuration, such as an operational amplifier.

[0115] [Item 9] 9. The image forming apparatus according to any one of items 1 to 8, wherein the first DC power supply is a boost circuit including a rectifying element that rectifies the AC voltage output from the AC power supply, and a capacitive element that smoothes a pulsating current generated by the rectifying element to generate a DC current.

[0116] 5, the DC power supply 260 may be realized by a boost circuit (rectifying and smoothing circuit) including diodes D61 to D64 and capacitors C61 to C64. Since the primary transfer voltage Vtri1 is a high voltage, a boost circuit is necessary.

[0117] [Item 10] 10. The image forming apparatus according to item 9, wherein the boost circuit is a Cockcroft-Walton circuit.

[0118] The Cockcroft-Walton circuit is capable of boosting the voltage with a relatively small number of circuit elements. Therefore, by adopting the Cockcroft-Walton circuit in the DC power supply 260, it is possible to reduce the cost of the DC power supply 260.

[0119] [Item 11] a resistor connected between the boost circuit and the primary transfer member; A current control circuit for controlling a current flowing through the resistor, 11. The image forming apparatus according to claim 9, wherein the DC voltage of the first polarity is adjusted by the current control circuit controlling the current flowing through the resistor.

[0120] The control unit 201 and operational amplifier IC61 that control the current flowing through resistor R61 are an example of a current control circuit.

[0121] [Item 12] an AC power source provided in the toner container and configured to supply an AC voltage to a detection unit that detects information regarding the remaining amount of toner contained in the toner container; a first DC power source that converts the AC voltage output from the AC power source to generate a DC voltage of a first polarity and applies the DC voltage of the first polarity to a primary transfer member; A power supply device having

[0122] [Item 13] a second DC power source that generates a second DC voltage of the first polarity and applies the second DC voltage of the first polarity to a secondary transfer member as a secondary transfer voltage; The primary transfer voltage applied to the primary transfer member is a DC voltage of the first polarity applied from the first DC power source to the primary transfer member; a DC voltage generated by the second DC voltage of the first polarity being subjected to a voltage drop by the intermediate transfer body in a route from the second DC power source to the primary transfer member via the secondary transfer member and the intermediate transfer body; Item 13. The power supply of item 12, formed from

[0123] The invention is not limited to the above-described embodiments, and various modifications and variations are possible without departing from the spirit and scope of the invention. Accordingly, the following claims are appended to apprise the public of the scope of the invention. [Explanation of symbols]

[0124] 134: toner container, 131: photoconductor, 135: intermediate transport belt, 136: primary transfer roller, 141: secondary transfer roller, 138, 139: electrode plates, 230: AC power source, 260: auxiliary power source, 210: positive power source, 220: negative power source

Claims

1. a toner container for storing toner; a photoreceptor on which a toner image is formed by the toner supplied from the toner container; an intermediate transfer body that rotates while being in contact with the photoreceptor and has electrical conductivity; a primary transfer member that is applied with a primary transfer voltage and transfers the toner image from the photoreceptor to the intermediate transfer member; a detection unit provided in the toner container and configured to detect information regarding a remaining amount of the toner contained in the toner container; an AC power source for supplying an AC voltage to the detection means; a first DC power source that converts the AC voltage output from the AC power source to generate a DC voltage of a first polarity and applies the DC voltage of the first polarity to the primary transfer member; An image forming apparatus comprising:

2. a secondary transfer member that rotates while in contact with the intermediate transfer body and that is applied with a secondary transfer voltage to transfer the toner image from the intermediate transfer body to a sheet; a second DC power source that generates a second DC voltage of the first polarity and applies the second DC voltage of the first polarity to the secondary transfer member as the secondary transfer voltage, The primary transfer voltage is a DC voltage of the first polarity applied from the first DC power source to the primary transfer member; a DC voltage generated by the second DC voltage of the first polarity being subjected to a voltage drop by the intermediate transfer body in a route from the second DC power source to the primary transfer member via the secondary transfer member and the intermediate transfer body; The image forming apparatus of claim 1 .

3. The power supply may further include a control unit for controlling the power supply. The control means When the detection unit detects information regarding the remaining amount of toner, the AC power source is controlled so that the AC voltage becomes a first target voltage; When the detection unit does not detect information regarding the remaining amount of toner, the AC power source is controlled so that the AC voltage becomes a second target voltage that is lower than the first target voltage. The image forming apparatus according to claim 2 , wherein the primary transfer voltage is maintained constant by controlling the primary transfer voltage.

4. The power supply may further include a control unit for controlling the power supply. The control means When the detection unit detects information regarding the remaining amount of toner, the AC power source is controlled so that the AC voltage becomes a first target voltage; When the detection means does not detect the information regarding the remaining amount of toner, the AC power source is controlled to stop outputting the AC voltage. The image forming apparatus according to claim 2 , wherein the primary transfer voltage is maintained constant by controlling the primary transfer voltage.

5. a cleaning member for cleaning the toner remaining on the intermediate transfer body; a third DC power source connected in series to the second DC power source, generating a third DC voltage of a second polarity opposite to the first polarity, and applying the third DC voltage of the second polarity to the secondary transfer member as the secondary transfer voltage; 5. The image forming apparatus according to claim 3, wherein the control means stops the second DC power supply, operates the third DC power supply, and controls the AC power supply so that the AC voltage becomes a third target voltage higher than the first target voltage, during a cleaning process in which toner adhering to the secondary transfer member is returned to the intermediate transfer body and the toner is removed from the intermediate transfer body by the cleaning member to clean the secondary transfer member.

6. The test image formed on the intermediate transfer body is read by a reading means. The image forming apparatus according to claim 5 , wherein the cleaning step includes cleaning a portion of the test image adhering to the secondary transfer member.

7. The detection means is a first electrode plate disposed inside the toner container and to which the AC voltage is applied; a second electrode plate disposed inside the toner container; an acquisition circuit that acquires an AC current generated in the second electrode plate in accordance with information regarding the remaining amount of toner by applying the AC voltage to the first electrode plate; The image forming apparatus according to claim 1 .

8. The image forming apparatus according to claim 7 , wherein the acquisition circuit is a peak hold circuit.

9. 2. The image forming apparatus according to claim 1, wherein the first DC power supply is a boost circuit including a rectifying element that rectifies the AC voltage output from the AC power supply, and a capacitive element that smoothes a pulsating current generated by the rectifying element to generate a DC current.

10. 10. The image forming apparatus according to claim 9, wherein the boost circuit is a Cockcroft-Walton circuit.

11. a resistor connected between the boost circuit and the primary transfer member; A current control circuit for controlling a current flowing through the resistor, 10. The image forming apparatus according to claim 9, wherein the DC voltage of the first polarity is adjusted by the current control circuit controlling the current flowing through the resistor.

12. an AC power source provided in the toner container and configured to supply an AC voltage to a detection unit that detects information regarding the remaining amount of toner contained in the toner container; a first DC power source that converts the AC voltage output from the AC power source to generate a DC voltage of a first polarity and applies the DC voltage of the first polarity to a primary transfer member; A power supply device having

13. a second DC power source that generates a second DC voltage of the first polarity and applies the second DC voltage of the first polarity to a secondary transfer member; The primary transfer voltage applied to the primary transfer member is a DC voltage of the first polarity applied from the first DC power source to the primary transfer member; a DC voltage generated by the second DC voltage of the first polarity being subjected to a voltage drop by the intermediate transfer body in a route from the second DC power source to the primary transfer member via the secondary transfer member and the intermediate transfer body; 13. The power supply of claim 12 formed from

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