Image forming device

Abstract

To achieve both reduction of overshoot and attainment of a target value of an amplitude value of an AC voltage.SOLUTION: A power supply device has an AC voltage output circuit that outputs, to developing means, an AC voltage having a difference between its positive and negative amplitude values, and a resistance circuit that is arranged between the AC voltage output circuit and the developing means. Switching means switches a resistance value of the resistance circuit according to a polarity of a current flowing in the resistance circuit upon application of the AC voltage to the resistance circuit.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus. [Background technology]

[0002] An electrophotographic developing device develops an electrostatic latent image made of toner by applying a developing voltage to a developing sleeve. In general, the developing voltage is generated by superimposing an AC voltage (square wave) on a DC voltage. It is known that image quality improves when the positive amplitude (Vp+) and the negative amplitude (Vp-) of the AC voltage are different values. Such an AC voltage waveform is called a biased duty waveform. Vp+:|Vp-| is called a biased duty ratio, and 4:6 to 3:7 are adopted. The inverse ratio of the biased duty ratio is adopted as the ratio between the application time ta of Vp+ and the application time tb of Vp-. For example, when Vp+:|Vp-| is 4:6, ta:tb is 6:4. This makes it possible to output an AC voltage without fluctuating the DC voltage. Furthermore, intermittently outputting a square wave (for example, outputting a square wave for two periods and stopping the output of the square wave for the next three periods, and repeating this) also improves image quality (Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2009-033815 A Summary of the Invention [Problem to be solved by the invention]

[0004] However, metal foreign matter can get between the photosensitive drum and the developing unit. In this case, a discharge phenomenon occurs, and an abnormal image called a ring mark can appear. The higher the peak value of the AC voltage, the more likely the ring mark will appear. One method to reduce the overshoot is to place a damping resistor in the output section of the developing voltage.

[0005] However, when the AC voltage has a large bias in the duty ratio (e.g., 2:8), the application time tb of Vp- becomes shorter. This makes it difficult to lower Vp- to the target voltage. If the resistance value of the damping resistor is lowered to solve this problem, an overshoot will occur in Vp+. If the resistance value of the damping resistor is increased to suppress the overshoot of Vp+, Vp- will not be able to reach the target voltage. In other words, there are two opposing issues. Therefore, the present invention aims to achieve both the reduction of overshoot and the achievement of the amplitude value of the AC voltage to reach the target value. [Means for solving the problem]

[0006] The present invention relates to, for example, A photoconductor; A charging means for uniformly charging the photoconductor; an exposure means for forming an electrostatic latent image by exposing the charged photoconductor; a developing unit for developing the electrostatic latent image with a toner to form a toner image; a transfer means for transferring the toner image to a sheet; a power source that generates a development voltage applied to the developing means; The power source includes: Transformer and a drive circuit provided on a primary side of the transformer for outputting AC voltages having different positive and negative amplitude values ​​as the developing voltage to the developing means; a resistor circuit provided on a secondary side of the transformer, the resistor circuit having a resistance value that changes depending on a polarity of the AC voltage generated on the secondary side of the transformer by an operation of the drive circuit, The resistor circuit includes: The at least one resistor element and a rectifying element connected in parallel to the at least one resistor element are included. When the polarity of the AC voltage is a first polarity, the resistance value becomes a first value, and when the polarity of the AC voltage is a second polarity, the resistance value becomes a second value smaller than the first value. When the polarity of the AC voltage is the second polarity, a current flows through the rectifying element, and when the polarity of the AC voltage is the first polarity, a current does not flow through the rectifying element. The present invention provides an image forming apparatus characterized by being configured as above. Effect of the Invention

[0007] According to the present invention, it is possible to achieve both a reduction in overshoot and an achievement of a target value for the amplitude value of the AC voltage. [Brief description of the drawings]

[0008] [Figure 1] FIG. 1 is a diagram illustrating an image forming apparatus. [Diagram 2] Diagram explaining overshoot and insufficient fall of development voltage [Diagram 3] Diagram explaining the power supply unit [Figure 4] Diagram explaining a damping resistor circuit with variable resistance [Diagram 5] FIG. 1 is a diagram for explaining a drive signal, a development voltage, and an output current of an AC generating circuit. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] 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.

[0010] <Image forming device> FIG. 1 is a diagram showing a schematic configuration of an image forming apparatus 100 equipped with a power supply device 10. The image forming apparatus 100 has four image forming stations for forming a color image using toners of four colors, yellow, magenta, cyan, and black. In FIG. 1, the letters a to d added after the reference numerals indicate yellow, magenta, cyan, and black, respectively. Note that since there is no difference in the configuration of each image forming station, the letters a to d will be omitted in the following description.

[0011] The photoconductor 1 is a drum-shaped image carrier. The charger 2 uniformly charges the surface of the photoconductor 1. A charging voltage generated by a power supply 10 is applied to the charger 2. The exposure device 3 irradiates the uniformly charged surface of the photoconductor 1 with laser light according to image information to form an electrostatic latent image. The rotary polygon mirror of the exposure device 3 is driven by a scanner motor and scans the photoconductor with the laser light. The developer 4 attaches toner to the electrostatic latent image to develop it and form a toner image. The developer 4 has a developing sleeve. A high-voltage developing voltage is applied to the developing sleeve to promote development. This developing voltage is generated by a power supply 10. The primary transfer roller 6 transfers the toner image carried by the photoconductor 1 to the intermediate transfer belt 5. A high-voltage primary transfer voltage to promote primary transfer may be applied to the primary transfer roller 6 from the power supply 10. The sheet P is fed to the conveying path by a paper feed roller 9. The secondary transfer roller 7 transfers the toner image carried by the intermediate transfer belt 5 to the sheet P. The fixing device 8 applies heat and pressure to the toner image transferred onto the sheet P to fix it.

[0012] <Issues with developing voltage> FIG. 2 is a diagram for explaining the problems of the conventional technology. As shown in FIG. 2, the development voltage is generated by superimposing an AC component and a DC component. VRL indicates the development voltage when the resistance value of the damping resistor is relatively low. When the development voltage is increased, an overshoot occurs in the positive amplitude VP+. VRH indicates the development voltage when the resistance value of the damping resistor is relatively high. It can be seen that when the development voltage is increased, the negative amplitude VP- does not fall to the target value. In this way, when the resistance value of the damping resistor is constant, an overshoot occurs or the negative amplitude VP- does not fall to the target value.

[0013] <Power supply> FIG. 3 shows a developing voltage generating circuit included in the power supply device 10. The controller 300 sets command values ​​of a positive amplitude Vp+ and a negative amplitude Vp- to be applied to the developer 4 in the control circuit 301. The control circuit 301 drives the AC generating circuit 310 and the voltage generating circuits 330 and 340 according to the command values ​​from the controller 300. Vin is a power supply that supplies an operating voltage (e.g., 24 V) to the power supply device 10. The AC generating circuit 310 is a circuit that generates an AC component of the developing voltage. For example, the AC generating circuit 310 may be an H-bridge circuit configured with switch elements Q1, Q2, Q3, and Q4. The DC power supply Vdc is a circuit that generates a DC component of the developing voltage.

[0014] As shown in Fig. 3, the load of the H-bridge circuit is the primary winding of the transformer T1 and the capacitor C1. The switch elements Q1, Q2, Q3, and Q4 are, for example, field effect transistors (FETs). One end of the capacitor C1 is connected to one end Tb of the transformer T1. The capacitor C1 absorbs the imbalance between the positive and negative amplitudes of the AC component.

[0015] A voltage Va generated by a voltage generating circuit 330 is applied to the drain of the switching element Q1. A gate of the switching element Q1 is connected to a control circuit 301. A source of the switching element Q1 is connected to the other end Ta of the primary winding of the transformer T1 and to the drain of the switching element Q2. The switching element Q1 turns on and off the voltage applied to the other end Ta of the primary winding in accordance with a drive signal output from the control circuit 301.

[0016] The drain of the switch element Q2 is connected to the other end Ta of the transformer T1 and the source of the switch element Q1. The source of the switch element Q2 is connected to ground. The gate of the switch element Q2 is connected to the control circuit 301. The switch element Q2 can forcibly switch the potential of the other end Ta of the transformer T1 to the ground potential.

[0017] The source of the switch element Q3 is connected to the other end of the capacitor C1 and the drain of the switch element Q4. That is, the source of the switch element Q3 is connected to one end Tb of the transformer via the capacitor C1. The drain of the switch element Q3 is applied with a voltage Vb from a voltage generating circuit 340. The gate of the switch element Q3 is connected to a control circuit 301. The switch element Q3 turns on and off the voltage applied to one end Tb of the primary winding of the transformer T1.

[0018] The drain of the switch element Q4 is connected to the source of the switch element Q3 and the other end of the capacitor C1. The source of the switch element Q4 is connected to ground. The gate of the switch element Q4 is connected to the control circuit 301. The switch element Q4 can forcibly switch the potential of the other end Tb of the primary winding of the transformer T1 to the ground potential.

[0019] The voltage generating circuit 330 has a transistor Q5 and a capacitor C2. The collector of the transistor Q5 is connected to the power supply Vin. The emitter of the transistor Q5 is connected to the positive terminal of the capacitor C2 and the drain of the switch element Q1. The negative terminal of the capacitor C2 is grounded. The base of the transistor Q5 is connected to the control circuit 301. The control circuit 301 drives the transistor Q5 to maintain the voltage Va at the target voltage. In other words, the transistor Q5 functions as a voltage control element. The capacitor C2 is an electrolytic capacitor that stabilizes the voltage Va.

[0020] The voltage generating circuit 340 has a transistor Q6 and a capacitor C3. The collector of the transistor Q6 is connected to the power supply Vin. The emitter of the transistor Q6 is connected to the positive terminal of the capacitor C3 and the drain of the switch element Q3. The negative terminal of the capacitor C3 is grounded. The base of the transistor Q6 is connected to the control circuit 301. The control circuit 301 drives the transistor Q6 to maintain the voltage Vb at the target voltage. In other words, the transistor Q6 functions as a voltage control element. The capacitor C3 is an electrolytic capacitor that stabilizes the voltage Vb.

[0021] One end of the secondary winding of the transformer T1 is connected to an input part of the damping resistance circuit 320. The other end of the secondary winding of the transformer T1 is connected to a DC power supply Vdc. The damping resistance circuit 320 is a variable resistance circuit. The output part of the damping resistance circuit 320 is connected to the developer 4, which is a capacitive load. Since there is a gap between the developer 4 and the photoconductor 1, this gap generates a capacitance. Therefore, the developer 4 is called a capacitive load. As shown in FIG. 3, the damping resistance circuit 320 has damping resistors R1 to R3 and a high-voltage diode D1.

[0022] The control circuit 301 controls the on / off of the switching elements Q1 to Q4 and the base voltages of the transistors Q5 and Q6 independently. The control circuit 301 controls the base voltage of the transistor Q5 so that the voltage Va that satisfies n2*Va=n1*Vp+ becomes the potential of the capacitor C2 in response to the command values ​​(Vp+, Vp-). Here, n1 is the number of turns of the primary winding of the transformer T1. n2 is the number of turns of the secondary winding of the transformer T1. The control circuit 301 controls the base voltage of the transistor Q6 so that the voltage Vb that satisfies n2*Vb=n1*Vp- becomes the potential of the capacitor C3. The control circuit 301 derives the application times ta and tb so as to satisfy the following formula. Va:Vb(=Vp+:|Vp-|) = tb:ta····(1) t = ta + tb (2) Here, t is the period of the AC component. For example, the control circuit 301 determines the application times ta and tb so that the ratio of the voltage Va to the voltage Vb coincides with the ratio of the application time tb to the application time ta. During the application time ta, a voltage is applied to the primary winding of the transformer T1 so that a current flows from one end Tb to the other end Ta of the primary winding. During the application time tb, a voltage is applied to the primary winding of the transformer T1 so that a current flows from the other end Ta to one end Tb of the primary winding. The control circuit 301 applies an ON signal to each gate of the switch elements Q2 and Q3 for the application time ta. As a result, the switch elements Q2 and Q3 are conductive during the application time ta. An OFF signal is applied to each gate of the switch elements Q1 and Q4 for the application time ta. As a result, the switch elements Q1 and Q4 are not conductive during the application time ta. Meanwhile, the control circuit 301 applies an ON signal to each gate of the switch elements Q1 and Q4 for the application time tb. As a result, during application time tb, switch elements Q1 and Q4 are conductive. An OFF signal is sent to each gate of switch elements Q2 and Q3 for application time tb. As a result, switch elements Q2 and Q3 are not conductive during application time tb. As a result, the waveform of the AC component becomes the target waveform.

[0023] The control circuit 301 supplies an ON signal to each gate of the switching elements Q1 and Q3 for the blank time tblank. Meanwhile, the control circuit 301 supplies an OFF signal to each gate of the switching elements Q2 and Q4 for the blank time tblank. As a result, the switching elements Q1 and Q3 are conductive, the switching elements Q2 and Q4 are not conductive, and the output voltage on the secondary side of the transformer T1 becomes 0V. As a result, a "blank pulse period" in which no AC component is applied to the developer 4 is generated. The period in which the AC component is applied to the developer 4 may be called a square wave period.

[0024] In this embodiment, the square wave period and the blank pulse period are alternately repeated. In this case, when the blank pulse period is switched to the square wave period, it is difficult to form the waveform in the square wave period into a predetermined waveform. Therefore, in this embodiment, the potential difference between the voltage Vb and the voltage Va is steadily applied to the capacitor C1 during the blank time tblank. The waveform of the square wave of this embodiment is equivalent to that of the comparative example that always outputs a square wave without providing a blank pulse period.

[0025] As an example, assume that Vp+=200V, Vp-=-800V, n2 / n1=40, and t=100us. The unit us indicates microseconds. Furthermore, assume that Va=5V, Vb=20V, ta=80us, tb=20us, and tblank=300us. In this case, a square wave with a partial duty ratio of 2:8 is output for two periods (200us), and then three periods (300us) are set as the blank time tblank.

[0026] <Damping resistor circuit> As shown in FIG. 3, the output terminal of the transformer T1 is connected to the damping resistor circuit 320. In the damping resistor circuit 320, the damping resistors R1, R2, and R3 form a series circuit. The anode of the high-voltage diode D1 is connected to the connection point between the damping resistor R2 and the damping resistor R3. The cathode of the high-voltage diode D1 is connected to the connection point between the damping resistor R1 and the damping resistor R2. As a result, when the amplitude of the developing voltage is positive, a current Iab flows in the direction from point Pa to point Pb. At this time, the potential of the cathode of the high-voltage diode D1 becomes higher than the potential of the anode. Therefore, no current flows to the high-voltage diode D1. On the other hand, when the amplitude of the developing voltage is negative, a current Iba flows from point Pb to point Pa. When the voltage generated across the damping resistor R2 by the current Iba becomes equal to or higher than the forward voltage of the high-voltage diode D1, the current Iba flows to the high-voltage diode D1. The current flowing to the damping resistor R2 no longer increases. Therefore, when the amplitude of the developing voltage is negative, the output current flows through a current path formed by the damping resistor R1, the high-voltage diode D1, and the damping resistor R3. When the amplitude of the developing voltage is positive, the resistance value of the damping resistor circuit 320 is R1+R2+R3. When the amplitude of the developing voltage is negative, the resistance value of the damping resistor circuit 320 is R1+R3.

[0027] Here, it is assumed that R1=R2=R3=10 kΩ. When the developing voltage is positive, the resistance value of the damping resistance circuit 320 is 30 kΩ. When the developing voltage is negative, the resistance value of the damping resistance circuit 320 is 20 kΩ. In other words, the damping resistance circuit 320 is a variable resistance circuit whose resistance value changes depending on the polarity of the developing voltage.

[0028] In this embodiment, the high-voltage diode D1 is connected in parallel to the damping resistor R2. The high-voltage diode D1 may be connected in another position. According to FIG. 4(A), the high-voltage diode D1 is connected in parallel to the damping resistor R1. According to FIG. 4(B), the high-voltage diode D1 is connected in parallel to the damping resistor R3. Incidentally, the damping resistors R1, R2, and R3 each generate heat. This is because the damping resistors R1, R2, and R3 convert the ringing of the waveform into heat to suppress the ringing. The damping resistor R2 receives heat from the damping resistors R1 and R3, so the temperature of the damping resistor R2 becomes higher than the temperatures of the damping resistors R1 and R3. Therefore, when the high-voltage diode D1 is connected in parallel to the damping resistor R2, it is possible to reduce the temperature of the damping resistor R2.

[0029] As shown in FIG. 4C, the high-voltage diode D1 may be replaced with a switch element SW. In this case, the control circuit 301 acquires the polarity of the developing voltage and outputs a signal to the switch element SW to turn the switch element SW on and off according to the polarity. The switch element SW may be a relay or a semiconductor switch. Note that, in order to input the polarity of the developing voltage to the control circuit 301, a voltage dividing resistor that divides the developing voltage and converts it into a detection voltage, and a photocoupler that transmits the detection voltage to the control circuit 301 will be required. In comparison, the high-voltage diode D1 can reduce the number of parts and the processing load of the control circuit 301.

[0030] In this embodiment, three 10 kΩ resistors are used as the damping resistors. However, this is merely an example. As shown in FIG. 4D, a series circuit having four damping resistors R11, R12, R14, and R15, each having a resistance value of 5 kΩ, and a damping resistor R13 having a resistance value of 10 kΩ may be used. In FIG. 4D, a high-voltage diode D1 is connected in parallel to the damping resistor R13. As described above, the circuit configuration of the damping resistor circuit 320 is arbitrary as long as the resistance value of the damping resistor circuit 320 can be changed according to the polarity of the development voltage.

[0031] <Timing chart> Fig. 5 shows the timing when the switch elements Q1, Q2, Q3, and Q4 are turned ON, the waveform of the developing voltage, and the waveform of the output current in this embodiment. In Fig. 5, the dashed line in the developing voltage is the waveform at point Pa shown in Fig. 3. Similarly, the solid line in the developing voltage is the waveform at point Pb shown in Fig. 3. In other words, the solid line in the developing voltage shows the effect of the damping resistors R1 to R3.

[0032] Time t0 is the timing before the development voltage is output. At this time, the switch elements Q1 and Q3 are both ON. The switch elements Q2 and Q4 are both OFF. The development voltage is a voltage Vdc that is a DC component.

[0033] At time t1, the switch element Q1 is switched from ON to OFF, and the switch element Q2 is switched from OFF to ON. As a result, the development voltage contains an AC component, and the amplitude rises to Vp+. The application time ta is from time t1 to time t2. During the application time ta, the resistance value of the damping resistance circuit 320 is R1+R2+R3=30 kΩ. This suppresses the overshoot of Vp+.

[0034] At time t2, the switch elements Q1 and Q4 are switched from OFF to ON. The switch elements Q2 and Q3 are switched from ON to OFF. As a result, the developing voltage falls to a negative amplitude Vp-. The application time tb is from time t2 to time t3. At application time tb, the resistance value of the damping resistor circuit 320 becomes 20 kΩ, which is R1+R3. That is, since the resistance value at application time tb is lower than the resistance value at application time ta, the fall (gradient) of Vp- becomes steeper compared to the rise (gradient) of Vp+. In this embodiment, the AC component is output over two periods (the period from time t1 to time t5). Therefore, at time t3, the switch elements Q2 and Q3 are switched from OFF to ON, and the switch elements Q1 and Q4 are switched from ON to OFF. As a result, the resistance value of the damping resistor circuit 320 is returned to 30 kΩ. The developing voltage rises to Vp+. At time t4, the switch elements Q1 and Q4 are switched from OFF to ON, and the switch elements Q2 and Q3 are switched from ON to OFF. The resistance value of the damping resistance circuit 320 is switched to 20 kΩ. The developing voltage falls to Vp-.

[0035] The period from time t5 to time t6 is a blank time tblank. Therefore, the switch element Q1 remains ON, the switch element Q3 is switched from OFF to ON, the switch element Q2 remains OFF, and the switch element Q4 is switched from ON to OFF. As a result, the developing voltage is maintained at Vdc. The resistance value of the damping resistance circuit 320 becomes 30 kΩ. After that, the period from time t6 to time t7 is again the application time ta.

[0036] In this embodiment, the resistance value of the damping resistance circuit 320 changes depending on the polarity of the developing voltage, so that the developing voltage can be sufficiently controlled to the target voltage while reducing overshoot.

[0037] In this embodiment, since it is assumed that |Vp-| > |Vp+|, the resistance value of the damping resistance circuit 320 is reduced during the period when the developing voltage is negative compared to the period when the developing voltage is positive. When |Vp-| < |Vp+|, the resistance value of the damping resistance circuit 320 may be reduced during the period when the developing voltage is positive compared to the period when the developing voltage is negative.

[0038] As shown in FIG. 5, in this embodiment, a biased duty type blank pulse is adopted as the waveform of the AC component. However, the technical idea of this embodiment may also be applied to a biased duty type without a blank pulse. As an example of the AC generation circuit, an H-bridge circuit has been described. Any circuit that can generate an AC voltage, such as a push-pull circuit, can be similarly adopted.

[0039] The invention is not limited to the above embodiments, and various changes and modifications can be made without departing from the spirit and scope of the invention. Therefore, claims are appended to disclose the scope of the invention.

[0040] <Technical idea derived from the embodiment> [Viewpoint 1] As shown in FIG. 1, the charger 2 is an example of a charging means for uniformly charging the photoconductor. The exposure device 3 is an example of an exposure means for forming an electrostatic latent image by exposing the photoconductor. The developer 4 is an example of a developing means for developing the electrostatic latent image with toner to form a toner image. The primary transfer roller 6, the intermediate transfer belt 5, and the secondary transfer roller 7 are examples of a transfer means for transferring the toner image to a sheet. The power supply device 10 is an example of a power supply for generating a developing voltage applied to the developing means. As shown in FIG. 3, the transformer T1 is an example of an AC voltage output circuit (e.g., AC generating circuit 310, transformer T1) that outputs AC voltages with different positive and negative amplitude values ​​to the developing means. The damping resistance circuit 320 is an example of a resistance circuit arranged between the AC voltage output circuit and the developing means. The high-voltage diode D1 and the switch element SW are examples of a switching means for switching the resistance value of the resistance circuit depending on the polarity of the current flowing through the resistance circuit when the AC voltage is applied to the resistance circuit. This achieves both a reduction in overshoot and an amplitude value of the AC voltage reaching a target value.

[0041] [Point 2] 3 and 4(A) to 4(D), the resistance circuit has at least one resistance element. The switching means may have a rectifying element (e.g., a high-voltage diode D1 or a semiconductor switch) connected in parallel to the at least one resistance element.

[0042] [Point 3] The resistor circuit may include a first resistor or a first group of resistors (e.g., R1, R3, R11, R12, R14, R15) and a second resistor or a second group of resistors (e.g., R2, R13) connected in series to the first resistor or the first group of resistors. The switching means may include a rectifying element connected in parallel to the second resistor or the second group of resistors (e.g., R2, R13).

[0043] [Point 4] 3 and 4(A) to 4(C), the resistor circuit may include a first resistor, a second resistor, and a third resistor connected in series, and the switching means may include a rectifying element connected in parallel to at least one of the first resistor, the second resistor, and the third resistor.

[0044] [Point 5] As shown in Figures 3 and 4(A) to 4(C), the second resistor may be connected between the first resistor and the third resistor, and as shown in Figures 3 and 4(C), the rectifying element may be connected in parallel to the second resistor.

[0045] [Point 6] The rectifying element may be configured to pass a current when a polarity of a current flowing through the resistive circuit is a first polarity. The rectifying element may be configured to not pass a current when a polarity of a current flowing through the resistive circuit is a second polarity.

[0046] [Points 7 and 8] The resistance value of the resistor circuit may be a first resistance value (e.g., 20 kΩ) when the polarity of the current flowing through the resistor circuit is a first polarity (e.g., negative polarity). On the other hand, when the polarity of the current flowing through the resistor circuit is a second polarity (e.g., positive polarity), the resistance value may be a second resistance value (e.g., 30 kΩ) larger than the first resistance value. Note that these resistance values ​​may be any value that can realize a sufficient fall of the developing voltage while reducing the overshoot. In the above embodiment, it is assumed that an overshoot is likely to occur when the polarity of the current is positive. However, the present invention is also applicable to cases where it is assumed that an overshoot is likely to occur when the polarity of the current is negative. In other words, it is sufficient that the resistance value is controlled to be large when the polarity of the current is negative, and the resistance value is controlled to be small when the polarity of the current is positive.

[0047] [Points 9-11] The rectifying element may be a diode. The rectifying element may be a switching element that switches between ON and OFF depending on the polarity of the AC voltage. The switching element may be a transistor such as an FET. The switching element may be a relay.

[0048] [Point 12] The AC generating circuit 310 is an example of a generating circuit that generates an AC voltage. The AC voltage may be a square wave.

[0049] [Points 13-17] The positive amplitude of the developing voltage may be different from the negative amplitude of the developing voltage. The negative amplitude of the developing voltage may be greater than the positive amplitude of the developing voltage. In one cycle of the developing voltage, the length of the period during which the developing voltage is positive may be different from the length of the period during which the developing voltage is negative. The length of the period during which the developing voltage is negative may be shorter than the length of the period during which the developing voltage is positive. These conditions may improve the quality of the toner image.

[0050] [Points 17, 18] The control circuit 301 may function as an output control circuit that outputs a developing voltage in a first period and does not output a developing voltage in a second period. This is as described with respect to the blank time tblank. The generating circuit may include an H-bridge circuit, but may also include a push-pull circuit or the like. The AC voltage output circuit may also include a transformer and an H-bridge circuit connected to the transformer.

[0051] [Point 19] As shown in FIG. 3, the H-bridge circuit may be configured with a first switch element, a second switch element, a third switch element, and a fourth switch element. A first terminal of a primary winding of the transformer T1 may be connected to a midpoint between the first switch element and the third switch element. A second terminal of a primary winding of the transformer T1 may be connected to a midpoint between the second switch element and the fourth switch element. When a first voltage (e.g., Va) is applied to the primary winding, the first switch element and the fourth switch element are turned on, and the second switch element and the third switch element are turned off. When a second voltage (e.g., Vb) is applied to the primary winding, the first switch element and the fourth switch element are turned off, and the second switch element and the third switch element are turned on.

[0052] [Point 20] The power supply Vin is an example of a DC voltage source that supplies a DC voltage. The voltage generation circuit 330 is an example of a first generator that generates a first voltage from a DC voltage. The voltage generation circuit 340 is an example of a second generator that generates a second voltage from a DC voltage. The first generator may include a first transistor (e.g., Q5) and a first capacitor (e.g., C2). The first transistor controls the voltage generated across the first capacitor to a first voltage by controlling the time during which a DC voltage is applied to the first capacitor. The second generator may include a second transistor (e.g., Q6) and a second capacitor (e.g., C3). The second transistor controls the voltage generated across the second capacitor to a second voltage by controlling the time during which a DC voltage is applied to the second capacitor.

[0053] [Point 21] The DC power supply Vdc is an example of a DC voltage output circuit that generates a DC voltage. The AC generating circuit 310 (AC voltage output circuit) may be configured to generate a development voltage by superimposing an AC voltage on a DC voltage.

[0054] [others] The power supply device 10 includes a transformer that outputs AC voltages with different positive and negative amplitude values ​​to a capacitive load, and a resistive circuit disposed between the transformer and the capacitive load. In particular, the power supply device 10 may include a switching means that switches the resistance value of the resistive circuit depending on the polarity of a current that flows through the resistive circuit when an AC voltage is applied to the resistive circuit. The power supply device 10 may be used as a power source for electronic devices other than an image forming device. [Explanation of symbols]

[0055] 1: photoconductor, 2: charger, 3: exposure device, 4: developer, 5: intermediate transfer belt, 10: power supply, T1: transformer, 320: damping resistor circuit, D1: high voltage diode

Claims

1. A photoconductor; A charging means for uniformly charging the photoconductor; an exposure means for forming an electrostatic latent image by exposing the charged photoconductor; a developing unit for developing the electrostatic latent image with a toner to form a toner image; a transfer means for transferring the toner image to a sheet; a power source that generates a development voltage applied to the developing means; The power source includes: Transformer and a drive circuit provided on a primary side of the transformer for outputting AC voltages having different positive and negative amplitude values ​​as the developing voltage to the developing means; a resistor circuit provided on a secondary side of the transformer, the resistor circuit having a resistance value that changes depending on a polarity of the AC voltage generated on the secondary side of the transformer by an operation of the drive circuit, the resistance circuit has at least one resistance element and a rectifying element connected in parallel to the at least one resistance element, and is configured such that when the polarity of the AC voltage is a first polarity, the resistance value becomes a first value, and when the polarity of the AC voltage is a second polarity, the resistance value becomes a second value smaller than the first value, and when the polarity of the AC voltage is the second polarity, a current flows through the rectifying element, and when the polarity of the AC voltage is the first polarity, no current flows through the rectifying element, characterized in that

2. 2. The image forming apparatus according to claim 1, wherein the resistance circuit has a first resistor and a second resistor connected in series to the first resistor, and the rectifying element is connected in parallel to the second resistor, which is the at least one resistance element.

3. 2. The image forming apparatus according to claim 1, wherein the resistor circuit has a first resistor, a second resistor, and a third resistor connected in series, and the rectifying element is connected in parallel to at least one resistor among the first resistor, the second resistor, and the third resistor, which is the at least one resistive element.

4. the second resistor is connected between the first resistor and the third resistor; 4. The image forming apparatus according to claim 3, wherein the rectifying element is connected in parallel to the second resistor which is the at least one resistive element.

5. 2. The image forming apparatus according to claim 1, wherein the first polarity is a positive polarity, and the second polarity is a negative polarity.

6. 2. The image forming apparatus according to claim 1, wherein the rectifying element is a diode.

7. 2. The image forming apparatus according to claim 1, wherein the rectifying element is a switching element that is switched between ON and OFF in accordance with the polarity of the AC voltage.

8. 8. The image forming apparatus according to claim 7, wherein the switching element is a transistor.

9. 8. The image forming apparatus according to claim 1, wherein the AC voltage has a square wave.

10. 2. The image forming apparatus according to claim 1, wherein the negative amplitude of the developing voltage is greater than the positive amplitude of the developing voltage.

11. 11. The image forming apparatus according to claim 9, wherein, in one cycle of the developing voltage, a length of a period during which the developing voltage is positive is different from a length of a period during which the developing voltage is negative.

12. 12. The image forming apparatus according to claim 11, wherein the length of the period in which the developing voltage is negative is shorter than the length of the period in which the developing voltage is positive.

13. 13. The image forming apparatus according to claim 9, wherein the drive circuit drives the transformer so as to output the AC voltage to a secondary side of the transformer during a first period and not to output the AC voltage to the secondary side of the transformer during a second period.

14. 14. The image forming apparatus according to claim 9, wherein the drive circuit includes an H-bridge circuit.

15. the H-bridge circuit is composed of a first switch element, a second switch element, a third switch element, and a fourth switch element, a first terminal of a primary winding of the transformer is connected to a midpoint between a first switch element and a third switch element; a second terminal of the primary winding of the transformer is connected to a midpoint between a second switch element and a fourth switch element; When a first voltage is applied to the primary winding, the first switch element and the fourth switch element are turned on, and the second switch element and the third switch element are turned off, 15. The image forming apparatus according to claim 14, wherein when a second voltage is applied to the primary winding, the first switch element and the fourth switch element are turned off, and the second switch element and the third switch element are turned on.

16. A DC voltage source for supplying a DC voltage; a first generator that generates the first voltage from the DC voltage; a second generator for generating the second voltage from the DC voltage; the first generator includes a first transistor and a first capacitor, and the first transistor controls a time during which the DC voltage is applied to the first capacitor to control a voltage generated across the first capacitor to the first voltage; 16. The image forming apparatus according to claim 15, wherein the second generator has a second transistor and a second capacitor, and the second transistor is configured to control a voltage generated across the second capacitor to the second voltage by controlling a time for which the DC voltage is applied to the second capacitor.

17. the power supply further includes a DC voltage output circuit provided on a secondary side of the transformer and generating a DC voltage; 17. The image forming apparatus according to claim 1, wherein the power source is configured to generate the developing voltage by superimposing the AC voltage on the DC voltage.

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