JP7809508B2 - Power supply device and image forming apparatus - Google Patents

Description

The present invention relates to a power supply device and an image forming device.

Electrophotographic image forming devices use various high voltages to promote the formation and transfer of toner images. The photosensitive drum is uniformly charged by a charging voltage, toner is attached to the photosensitive drum by a developing voltage, and the toner image is transferred to a sheet by a transfer voltage. Patent Document 1 describes a power supply circuit that generates these high voltages.

U.S. Patent No. 7,877,036

The developing device develops the electrostatic latent image with toner to form a toner image. The developing device has a developing roller (sleeve) facing the photosensitive drum and a supply roller facing the developing roller, supplying toner to the developing roller. The potential difference between the developing roller and the supply roller allows toner to adhere from the supply roller to the developing roller. While uniform resistance between the supply roller and the developing roller is expected, uneven toner adhesion can actually result in localized low resistance areas between the supply roller and the developing roller. Higher current flows through these low-resistance areas, shortening the roller's lifespan. This problem can be particularly exacerbated when ionic conductive materials are used as the surface material of these rollers. This is because high current flow accelerates electrolysis of the ionic conductive material, accelerating its wear. As a result, uneven density can occur in the toner image. Furthermore, to prevent toner from adhering to the backside of the sheet and causing contamination, an adhesion-suppressing voltage is sometimes applied to the developing roller. This increases the potential difference, necessitating measures to protect the roller surface. Therefore, the present invention aims to reduce unnecessary toner adhesion to the back surface of the sheet while extending the life of components involved in development.

The present invention is, for example,
a rotatable image carrier;
a developing member disposed opposite the image carrier and configured to develop an electrostatic latent image formed on the image carrier with toner to form a toner image;
a supply member disposed opposite to the developing member and supplying toner to the developing member;
a transfer unit that transfers the toner image formed on the image carrier to a transfer target;
a first power source that generates a development voltage of a first polarity or a second polarity opposite to the first polarity, the first polarity being applied to the development member;
a second power source for generating an accelerating voltage applied to the supply member to accelerate the supply of toner from the supply member to the developing member;
a control unit that controls the first power source and the second power source,
The control unit
controlling the first power source so that a developing voltage of the first polarity is applied to the developing member during a first period in which a toner image is not formed on the image carrier;
controlling the first power source so that a developing voltage of the second polarity is applied to the developing member during a second period in which the electrostatic latent image formed on the image carrier is developed with toner;
controlling the first power source and the second power source so that a potential difference between the developing member and the supply member in a third period, which is different from the first period and in which a toner image is not formed on the image carrier, is smaller than an absolute value of the potential difference between the developing member and the supply member in the first period and is also smaller than an absolute value of the potential difference between the developing member and the supply member in the second period;
The image forming apparatus is provided such that the first period is a period after the second period, and the third period is a period after the first period .

This invention makes it possible to reduce unnecessary toner adhesion to the back surface of the sheet while extending the life of components involved in development.

FIG. 1 is a schematic cross-sectional view of an image forming apparatus. FIG. 5A and 5B are diagrams illustrating the cause of toner being transferred to the rear surface of a sheet. 10A and 10B are diagrams illustrating a method for reducing the transfer of toner to the rear surface of a sheet. FIG. 1 is a circuit diagram of a portion of a power supply unit. FIG. 2 is a diagram illustrating a power supply device. FIG. 1 is a circuit diagram of a portion of a power supply unit. 4 is a timing chart illustrating changes in control signals, voltages, and potential differences. FIG. 1 is a circuit diagram of a portion of a power supply unit. FIG. 1 is a circuit diagram of a portion of a power supply unit. 4 is a timing chart illustrating changes in control signals, voltages, and potential differences. 4 is a timing chart illustrating changes in control signals, voltages, and potential differences. FIG. 1 is a circuit diagram of a portion of a power supply unit. 4 is a timing chart illustrating changes in control signals, voltages, and potential differences. FIG. 1 is a circuit diagram of a portion of a power supply unit.

The following describes the embodiments in detail with reference to the attached drawings. Note that the following embodiments do not limit the scope of the claimed invention. While the embodiments describe multiple features, not all of these features are necessarily essential to the invention, and multiple features may be combined in any desired manner. Furthermore, in the attached drawings, the same reference numbers are used to designate identical or similar components, and redundant explanations will be omitted.

Example 1
[Image forming apparatus]
FIG. 1 shows an electrophotographic image forming apparatus 1. A paper feed cassette 2 and a paper feed tray 3 are storage means for storing sheets P. Paper feed rollers 4a and 4b are supply means for sending sheets P to a transport path and supplying them to an image forming unit 7. The transport path is provided with a pair of transport rollers 5 and a pair of registration rollers 6 for transporting sheets P. The image forming unit 7 is provided with a photosensitive drum 11 that carries an electrostatic latent image or a toner image. The photosensitive drum 11 is driven to rotate by a motor M1. A charging voltage Vpr is applied to a charging roller 12 from a power supply 10, uniformly charging the surface of the photosensitive drum 11. An exposure device 13 has a semiconductor laser, modulates laser light with an image signal corresponding to an input image, and scans the laser light across the photosensitive drum 11. This forms an electrostatic latent image. The exposure device 13 may also be called an optical scanning device because it scans light using a rotating polygonal mirror. The developing roller 15 receives a developing voltage Vdc from the power supply 10 and develops the electrostatic latent image with toner to form a toner image. The transfer roller 16 receives a transfer voltage Vtr from the power supply 10 and transfers the toner image conveyed by the photosensitive drum 11 onto the sheet P. The fixing unit 20 applies heat and pressure to the toner image transferred to the sheet P while conveying the sheet P, fixing the toner image to the sheet P. The pressure roller 22 is biased so as to contact the fixing film 24. The heater 23 contacts the inner surface of the cylindrical fixing film 24 and heats the fixing temperature of the fixing film 24 to a target temperature. The sheet discharge roller 29 discharges the sheet P on which the toner image has been fixed by the fixing unit 20.

Figure 2 is a cross-sectional view of the image forming unit. The power supply device 10 has a charging power supply VPR, a development power supply VRS, and a transfer power supply VTR. The charging power supply VPR generates a charging voltage Vpr and applies it to the charging roller 12. The charging voltage Vpr is, for example, approximately -1000 V. The charging roller 12 charges the surface potential of the photosensitive drum 11 to a dark potential Vd. The dark potential Vd is, for example, approximately -700 V. The photosensitive drum 11 rotates clockwise and is irradiated with laser light X. When irradiated with laser light X, the surface potential of the photosensitive drum 11 changes to a light potential Vl. The light potential Vl is, for example, approximately -100 V. A developing voltage Vdc generated by the developing power supply VRS and resistor R0 is applied to the developing roller 15. Resistor R0 is a shunt resistor. The developing voltage Vdc is, for example, approximately -400 V. Inside the developing device 18, the supply roller 17 is disposed opposite the developing roller 15 and abuts against the developing roller 15. The developing roller 15 rotates in the opposite direction to the supply roller 17. A toner supply bias Vrs is applied to the supply roller 17. The toner supply bias Vrs is, for example, approximately −600 V. When the developing voltage Vdc is higher than the toner supply bias Vrs, negatively charged powder toner t adheres to the surface of the developing roller 15. The blade 19 abuts against the developing roller 15. The blade 19 is connected in parallel with the supply roller 17 and is applied with a toner supply bias Vrs (blade bias Vbl). The blade 19 regulates the thickness of the layer of toner t adhering to the developing roller 15 and charges the toner t. In this way, the supply roller 17 and the blade 19 supply an appropriate amount of toner t from the toner cartridge in the developing device 18 to the developing roller 15. As the photosensitive drum 11 continues to rotate, the electrostatic latent image comes into contact with the developing roller 15. As toner t is supplied from the supply roller 17, the toner t adheres to the periphery of the developing roller 15. When the light area potential Vl (e.g., -100 V) is higher than the developing voltage Vdc (e.g., -400 V), the toner t adheres to the surface (electrostatic latent image) of the photosensitive drum 11. This forms a toner image. As the photosensitive drum 11 continues to rotate, the toner image arrives at the transfer nip formed by the photosensitive drum 11 and the transfer roller 16. At the transfer nip, the toner image is transferred to the first surface (front surface) of the sheet P. A transfer voltage Vtr is applied to the transfer roller 16 from a transfer power source VTR. The transfer voltage Vtr is, for example, about +1000 V. Because the transfer voltage Vtr (e.g., +1000 V) is higher than the light area potential Vl (e.g., -100 V), the toner t peels off from the photosensitive drum 11 and adheres to the sheet P.

[Toner stain prevention]
As shown in Figure 3, when the photosensitive drum 11 starts to rotate, the surface potential of the sector 14 remains at 0 V and reaches the developing roller 15. If the developing voltage Vdc is a lower (negative) potential than 0 V, toner t adheres to the photosensitive drum 11. As shown in Figure 3, the photosensitive drum 11 carries the toner t to the transfer roller 16, and the toner t adheres to the transfer roller 16. When the sheet P arrives at the transfer nip, the toner t adheres to the second surface (back surface) of the sheet P from the transfer roller 16, causing the sheet P to become dirty.

As shown in Figure 4, the bias circuit 200 applies a positive development voltage Vdc to the development roller 15 so that the potential of the development roller 15 is higher than 0V when the photosensitive drum 11 starts to rotate. The bias circuit 200 receives a reference voltage Va from a reference voltage source VA and generates the positive development voltage Vdc. This makes it difficult for toner t to adhere from the development roller 15 to the photosensitive drum 11.

In this way, the power supply device 10 has a power supply that generates a negative polarity development voltage Vdc and a power supply that generates a positive polarity development voltage Vdc. This makes it possible to improve development characteristics while reducing toner contamination on the sheet P.

[Bias circuit]
FIG. 5 shows the bias circuit 200. A reference voltage source VA in the power supply 10 supplies a specified voltage Va (e.g., +24 V). The specified voltage Va is applied to the emitter of transistor Tr1. The collector of transistor Tr1 is connected to the emitter of transistor Tr2. The collector of transistor Tr2 is connected to one end of resistor R0 and the developing roller 15. The other end of resistor R0 is connected to the blade 19, the supply roller 17, and the negative terminal of the developing power supply VRS. The CPU 300 is a central processing unit that controls the power supply 10 and may be mounted outside the bias circuit 200. The CPU 300 receives a specified voltage Vb (e.g., +3.3 V) from the power circuit of the power supply 10. The CPU 300 outputs a PWM signal from the output terminal TGT to set the developing voltage Vdc to a target voltage. PWM stands for pulse width modulation. A PWM signal is a type of rectangular wave. The CPU 300 sets the target voltage by adjusting the duty ratio of the PWM signal. The PWM signal is smoothed by a smoothing circuit formed by resistor R14 and capacitor C2 and converted into a set voltage Vtgt. The set voltage Vtgt is proportional to the target voltage. The set voltage Vtgt is input to the inverting input terminal V- of the operational amplifier OP1. A detection voltage Vsns proportional to the development voltage Vdc is input to the non-inverting input terminal V+ of the operational amplifier OP1. Resistors R12 and R13 form a voltage divider circuit. The development voltage Vdc is divided by this voltage divider circuit and converted into a detection voltage Vsns proportional to the development voltage Vdc. A capacitor C1 is connected between the non-inverting input terminal V+ and the output terminal of the operational amplifier OP1. The output terminal of the operational amplifier OP1 is connected to the base of transistor Tr1 via current-limiting resistor R11. A resistor R10 is connected between the emitter and base of transistor Tr1 to prevent malfunction. Resistors R1 and R2 are connected in series to form a voltage divider circuit. The resistance values of resistors R1 and R2 may be the same. This voltage divider circuit is connected between the base of transistor Tr1 and the output terminal of the development voltage Vdc. Therefore, a voltage proportional to the development voltage Vdc is applied to the base of transistor Tr2. The output terminal of operational amplifier OP1 is connected to the base of transistor Tr2 via resistor R15 and diode D0. In other words, the output terminal of operational amplifier OP1 is connected to one end of resistor R15. The other end of resistor R15 is connected to the cathode of diode D0. The anode of diode D0 is connected to the base of transistor Tr2.

In Figure 5, the dashed line indicates the shunt current Is that flows from the specified voltage source VA to ground via the bias circuit 200. When transistors Tr1 and Tr2 are turned ON, the shunt current Is flows through resistor R0 via transistors Tr1 and Tr2. The CPU 300 controls the development voltage Vdc by appropriately increasing or decreasing the magnitude of the shunt current Is.

Vdc = Vrs + R0×Is...(1)
Here, when the detection voltage Vsns is higher than the set voltage Vtgt, the output voltage Vo of the operational amplifier OP1 increases. This reduces the base current of the transistor Tr1, and the collector current (= shunt current Is) of the transistor Tr1 decreases. When the shunt current Is decreases, the development voltage Vdc decreases according to equation (1). As a result, the detection voltage Vsns also decreases.

On the other hand, if the detection voltage Vsns is lower than the set voltage Vtgt, the output voltage Vo decreases. This increases the base current of transistor Tr1, and the collector current (shunt current Is) of transistor Tr1 increases. When the shunt current Is increases, according to equation (1), the development voltage Vdc increases, and the detection voltage Vsns also increases. In this way, negative feedback is applied to the operational amplifier OP1, and the detection voltage Vsns becomes approximately equal to the set voltage Vtgt. As a result, the development voltage Vdc is maintained at the target voltage.

The memory 301 may store in advance a table showing the relationship between the duty ratio of the PWM signal (i.e., the set voltage Vtgt) and the development voltage Vdc. By referencing the table, the CPU 300 determines the set voltage Vtgt (i.e., the duty ratio of the PWM signal) that corresponds to the target voltage of the development voltage Vdc. Here, the higher the duty ratio (i.e., the set voltage Vtgt), the higher the development voltage Vdc.

The voltage applied to the base of transistor Tr1 is the specified voltage Va minus the base-emitter voltage of transistor Tr1. The base-emitter voltage is, for example, 0.6 V, which is a very low voltage. Therefore, the base-emitter voltage can be considered to be approximately zero. Therefore, the specified voltage Va is applied to one end of resistor R2 (the upper terminal in Figure 5). The development voltage Vdc is applied to the other end of resistor R1 (the lower terminal in Figure 5). The base of transistor Tr2 is connected to the voltage division point of resistors R1 and R2. Therefore, an intermediate voltage between the specified voltage Va and the development voltage Vdc is applied to the base of transistor Tr2. As a result, the emitter-collector voltage of transistor Tr1 and the emitter-collector voltage of transistor Tr2 are approximately equal. The negative development voltage Vdc is generated in this way.

On the other hand, the positive polarity developing voltage Vdc is generated from the specified voltage Va according to the following sequence.
(S1) The CPU 300 sets the duty ratio of the PWM signal output from the output terminal TGT to the maximum (always at H level).
(S2) The output voltage Vo of the operational amplifier OP1 becomes the minimum value (approximately 0 V).
(S3) The base current Ib1 of the transistor Tr1 increases, and the transistor Tr1 becomes saturated.
(S4) The collector voltage Vc1 of the transistor Tr1 becomes Va-Vce1.

Vc1 = Va - Vce1...(2)
(S5) A base current Ib2 flows through the transistor Tr2, and the base voltage Vb2 of the transistor Tr2 becomes Vc1-Vbe2.

Vb2 = Vc1 - Vbe2...(3)
(S6) By eliminating Vc1 from equations (2) and (3), equation (4) is obtained.

Vb2 = Va - Vce1 - Vbe2 ... (4)
Generally, Vce1 is about 0.3V and Vbe2 is about 0.6V. The specified voltage Va is about +24V. Substituting these values into equation (4), Vb2 ≈ 23.1V. Therefore, a forward voltage is applied to diode D0, and base current Ib2 flows through the route Tr2 → D0 → R15 → OP1. Base Ib2 is the base current of transistor Tr2. In this way, the base current Ib2 of transistor Tr2 increases, and transistor Tr2 becomes saturated.
(S7) At this time, the developing voltage Vdc is expressed by the following formula:

Vdc = Va - Vce1 - Vce2 ... (5)
Generally, the collector-emitter voltage Vce of a transistor is about 0.3 V. Therefore, Vce1 is about 0.3 V, and Vce2 is also about 0.3 V. The specified voltage Va is about +24 V. Therefore, Vdc ≈ +23.4 V, which is applied to the developing roller 15 as a positive DC bias.

In this way, even when the photosensitive drum 11 starts to rotate and areas of the surface of the photosensitive drum 11 that are not negatively charged (areas at 0 V) reach the developing roller 15, the potential of the developing roller 15 is positive, making it difficult for toner to adhere to the areas.

[Extended roller life]
Ion-conductive materials (ion-conductive materials) may be used as the surface materials of the developing roller 15 and the supply roller 17. In this case, if a large potential difference exists between the developing roller 15 and the supply roller 17, electrolysis occurs at the contact surfaces of both the developing roller 15 and the supply roller 17. As a result, the lifespans of the developing roller 15 and the supply roller 17 may be shortened. Furthermore, if the developing roller 15 and the supply roller 17 are used after their lifespans have expired, the development performance may be reduced below the performance expected in the design, resulting in uneven density in some parts of the toner image. The operating states of the image forming apparatus 1 generally include a pre-rotation state, an image forming state, a post-rotation state, and a standby state. The pre-rotation state is the operating state from when the photosensitive drum 11 starts to rotate until it is ready to form an image. The image forming state is the operating state in which the photosensitive drum 1 forms a toner image according to the electrophotographic process. The post-rotation state is the operating state in which remaining toner on the photosensitive drum 11 is cleaned. The standby state is an operating state in which the photosensitive drum 11, the developing roller 15, and the supply roller 17 are stopped. If the developing roller 15 and the supply roller 17 are rotating, the ion conductive material is only slightly consumed even if a large potential difference occurs between the developing roller 15 and the supply roller 17. However, in the standby state, the developing roller 15 and the supply roller 17 are stopped, so the ion conductive material is likely to be consumed by electrolysis. Therefore, it is particularly necessary to appropriately control the potential difference between the developing roller 15 and the supply roller 17 in the standby state.

(Addition of a boost circuit)
FIG. 6 shows that a boost circuit 600 is added to the power supply device 10 to reduce unwanted toner adhesion. Setting the development voltage Vdc to as high a positive voltage as possible is effective in reducing unwanted toner t adhesion to the back surface of the sheet P. The boost circuit 600 boosts the specified voltage Va (e.g., +24 V) to generate an output voltage Ve (e.g., +150 V) higher than the specified voltage Va and supplies it to the bias circuit 200. This enables the bias circuit 200 to generate a positive and higher development voltage Vdc. It can be seen from equation (5) that if a higher output voltage Ve is adopted instead of the specified voltage Va, the development voltage Vdc increases.

Figure 7 shows the power supply unit 10 having a boost circuit 600. Components already described are assigned the same reference numerals, and their descriptions are incorporated herein by reference. The boost circuit 600 boosts the specified voltage Va supplied from a reference voltage source VA to an output voltage Ve. One end of the coil L1 is connected to the reference voltage source VA, which supplies the specified voltage Va. The reference voltage source VB is a power supply circuit that generates a specified voltage Vb. The specified voltage Vb is used as the operating voltage for the CPU 300. The reference voltage source VC is a power supply circuit that generates a specified voltage Vc. The specified voltage Vc is applied to one end of resistor R12 instead of the specified voltage Vb. In other words, the specified voltage Vc is used to generate the detection voltage Vsns.

The other end of coil L1 is connected to the drain of MOS (metal oxide semiconductor) transistor Tr5. The gate of transistor Tr5 is connected to CPU 300 and receives clock signal CLK1. The source of transistor Tr5 is grounded. Transistor Tr5 switches in accordance with clock signal CLK1, boosting the voltage applied to coil L1. The other end of coil L1 is connected to the anode of diode D1, rectifying current Ia. The cathode of diode D1 is connected to one end of capacitor C3 and the emitter of transistor Tr1. The other end of capacitor C3 is grounded. Current Ia is smoothed by capacitor C3. The voltage across capacitor C3 is output voltage Ve. In other words, the specified voltage Va is boosted to output voltage Ve. Output voltage Ve is a voltage (e.g., +150 V) that is sufficient to prevent toner adhesion.

The development power supply VRS includes a step-up transformer T1, a control circuit 701, a diode D2, a capacitor C4, and a feedback circuit 702. The control circuit 701 receives a clock signal CLK2 from the CPU 300 and a feedback voltage Vfb from the feedback circuit 702. The feedback circuit 702 detects the toner supply bias Vrs and generates the feedback voltage Vfb. The output of the control circuit 701 is determined by the clock signal CLK2 and the feedback voltage Vfb. The control circuit 701 may also include a switching circuit for driving the step-up transformer T1. The voltage generated on the secondary side of the step-up transformer T1 is rectified by the diode D2 and smoothed by the capacitor C4 to become a DC (direct current) voltage. This DC voltage is applied to the supply roller 17 as the toner supply bias Vrs.

(At the start of rotation (pre-rotation state))
At the start of rotation, the power supply device 10 generates the developing voltage Vdc as a positive voltage to suppress unwanted adhesion of toner. The operation of the power supply device 10 to generate the positive developing voltage Vdc has already been described.

When rotation begins, a clock signal CLK1 is input from the CPU 300 to transistor Tr5 of the boost circuit 600. This causes the boost circuit 600 to boost the specified voltage Va to generate an output voltage Ve (e.g., +150 V). In the bias circuit 200, the detection voltage Vsns is supplied to the inverting input terminal V- of the operational amplifier OP1. Meanwhile, the set voltage Vtgt is supplied to the non-inverting input terminal V+ of OP1. Therefore, the lower the duty ratio of the PWM signal (set voltage Vtgt), the greater the absolute value of the development voltage Vdc (negative polarity). A protection circuit 703 may be added to avoid requiring the output terminal of the operational amplifier OP1 to withstand high voltages. The protection circuit 703 includes transistor Tr4 and resistors R16 and R17. Resistors R16 and R17 are used to ensure stable operation of transistor Tr4. Without protection circuit 703, the base currents of transistors Tr1 and Tr2 flow into operational amplifier OP1 as sink currents. On the other hand, with protection circuit 703, the base currents of transistors Tr1 and Tr2 flow as collector currents of transistor Tr4 within protection circuit 703. This protects the output terminal of operational amplifier OP1. Note that when protection circuit 703 is added, the logic of the output terminal of operational amplifier OP1 must be inverted. For this reason, in Figure 7, the detection voltage Vsns is input to the inverting input terminal of operational amplifier OP1, and the set voltage Vtgt is input to the non-inverting input terminal of operational amplifier OP1.

When the developing roller 15 starts to rotate, a developing voltage Vdc of +150 V is applied to it. Meanwhile, a toner supply bias Vrs of -50 V is applied to the supply roller 17. Therefore, the potential difference between the developing roller 15 and the supply roller 17 is approximately 200 V. However, because the developing roller 15 and the supply roller 17 are rotating, wear due to electrolysis is minimal.

(Normal operation (image formation state))
In normal operation, the power supply device 10 outputs a negative voltage (for example, −400 V) as the development voltage Vdc, which promotes development using toner.

During normal operation, the boost circuit 600 is stopped, and the output voltage Ve supplied to the bias circuit 200 is approximately equal to the specified voltage Va. Therefore, the development voltage Vdc is controlled to a negative voltage.

During normal operation, a development voltage Vdc of approximately -400 V is applied to the developing roller 15. A toner supply bias Vrs of approximately -600 V is applied to the supply roller 17. Therefore, the potential difference between the developing roller 15 and the supply roller 17 is approximately 200 V. However, because the developing roller 15 and the supply roller 17 are both rotating, wear due to electrolysis is minimal.

(rear rotation state)
In the rear rotation state, it is necessary to reduce the adhesion of unnecessary toner, just as in the front rotation state. Therefore, the development voltage Vdc in the rear rotation state is equal to the development voltage Vdc in the front rotation state. Therefore, the operation of the power supply device 10 in the rear rotation state is the same as the operation of the power supply device 10 in the front rotation state.

(standby state)
In the standby state, the developing roller 15 and the supply roller 17 are both stopped. Therefore, it is necessary to reduce wear due to electrolysis. The CPU 300 suppresses electrolysis by controlling the potential difference between the developing voltage Vdc and the toner supply bias Vrs.

8 is a timing chart showing changes in signals and voltages when the image forming apparatus 1 transitions from the post-rotation state to the standby state. The post-rotation period is the period during which the post-rotation state continues. The standby period is the period during which the standby state continues. When cleaning of the photosensitive drum 11 and the like is completed during the post-rotation period, the operating state of the image forming apparatus 1 transitions to the standby state.
(S1) The CPU 300 turns off the clock signal CLK1 (always in the L state).
(S2) When the clock signal CLK1 turns OFF, the booster circuit 600 stops. As a result, the output voltage Ve applied to the emitter of the transistor Tr1 becomes approximately equal to Va.
(S3) The CPU 300 turns off the clock signal CLK2 (always in the L state).
(S4) Since the clock signal CLK2 is turned OFF, the toner supply bias Vrs is calculated by the following formula.

Here, RT1 is the resistance component of the transformer T1, and VfD2 is the forward voltage of the diode D2. Assume that Ve is +24 V, Vce1 and Vce2 are 0.3 V, RT1 is 1 kΩ, R0 is 2 MΩ, and VfD1 is 2.5 V. In this case, the toner supply bias Vrs is approximately 3 V.
(S5) The CPU 300 sets the duty ratio of the PWM signal for generating the set voltage TGT to the minimum value (always in the L state).
(S6) As a result, the voltage at the non-inverting input terminal V(+) of the operational amplifier OP1 drops, and the output voltage Vo of the operational amplifier OP1 becomes approximately 0V.
(S7) The base current of the transistor Tr4 of the protection circuit 703 decreases, and the transistor Tr4 enters a non-saturated state.
(S8) The base current of transistor Tr1 decreases, and transistor Tr1 enters a non-saturated state. In this way, due to the presence of protection circuit 703, the state of transistor Tr1 when output voltage Vo is approximately 0 V is logically inverted.
(S9) The base current of transistor Tr2 decreases, causing transistor Tr2 to enter a non-saturated state. Due to the presence of protection circuit 703, the state of transistor Tr2 when output voltage Vo is approximately 0 V is logically inverted, just like transistor Tr1.
(S10) The developing voltage Vdc is calculated by the following formula:

Here, it is assumed that Ve is +24 V, RT1 is 1 kΩ, R0 is 2 MΩ, R10 is 10 kΩ, R1 is 15 MΩ, R2 is 15 MΩ, and VfD2 is 2.5 V. In this case, the development voltage Vdc is approximately 4 V.

According to Example 1, a power supply device 10 is provided that can output a negative development voltage Vdc in the image formation state and a positive development voltage Vdc in the pre-rotation state and post-rotation state. This reduces toner contamination on the back surface of the sheet P caused by the adhesion of unwanted toner. Furthermore, in the standby state, the potential difference between the development voltage Vdc and the toner supply bias Vrs can be set to approximately 1 V. This makes it less likely that electrolysis will occur in the development roller 15 and the supply roller 17, thereby achieving a longer life for the development roller 15 and the supply roller 17. As a result, uneven density in the toner image caused by wear of the development roller 15 and the supply roller 17 is less likely to occur.

In the first embodiment, the CPU 300 appropriately controls the boost circuit 600 and bias circuit 200 to generate a positive polarity development voltage Vdc and a negative polarity development voltage Vdc. While specific circuit diagrams are shown for the boost circuit 600 and bias circuit 200, this is merely an example. For example, as shown in FIG. 9, a power supply (e.g., voltage source VDC) that generates the development voltage Vdc and a power supply that generates the toner supply bias Vrs may be provided independently. In this case, the CPU 300 controls the boost circuit 600, bias circuit 200, and development power supply VRS so that the development voltage Vdc and the toner supply bias Vrs are equal. This reduces the likelihood of electrolysis occurring during standby, thereby extending the life of the development roller 15 and supply roller 17.

Example 2
In the first embodiment, the CPU 300 controls the development voltage Vdc and the toner supply bias Vrs, thereby reducing the potential difference between the development roller 15 and the supply roller 17. However, in the circuit configuration described in the first embodiment, a small potential difference may occur between the development roller 15 and the supply roller 17. Therefore, in the second embodiment, the CPU 300 stops the specified voltage source VA, which supplies the specified voltage Va to the boost circuit 600. As a result, the potential difference between the development roller 15 and the supply roller 17 is controlled to a smaller value (approximately 0 V).

Figure 10 is a circuit diagram showing a power supply device 10 of Example 2. Descriptions of matters common to Example 1 will be omitted. In Example 2, the regulated voltage source VA includes a reference voltage source V0 that generates a reference voltage from AC power supplied from a commercial power source. Furthermore, the regulated voltage source VA includes transistors Tr6, Tr7, capacitor C5, and resistors R21, R22, and R23. Transistor Tr6 is a MOS-type field-effect transistor (FET). One end of resistor R21, one end of capacitor C5, and the source of transistor Tr6 are connected to the positive terminal of reference voltage source V0. The drain of transistor Tr6 outputs a regulated voltage Va. The other end of resistor R21 and the other end of capacitor C5 are connected to a control terminal VAON of the CPU 300. Furthermore, the control terminal VAON is connected to the base of transistor Tr7 via resistor R22. A resistor R23 is connected between the emitter and base of transistor Tr7 to stabilize the operation of transistor Tr7. The collector of transistor Tr7 is connected to the gate of transistor Tr6. The CPU 300 turns the control signal VAON on and off to turn the specified voltage source VA on and off. In other words, the specified voltage Va is output or stopped.

[Post-rotation state and standby state]
FIG. 11 shows that the potential difference between the developing roller 15 and the supply roller 17 is reduced by turning off the output of the specified voltage source VA.
(S1) The CPU 300 turns off the clock signal CLK1 (always in the L state).
(S2) When the clock signal CLK1 turns OFF, the booster circuit 600 stops. As a result, the output voltage Ve applied to the emitter of the transistor Tr1 becomes approximately equal to Va.
(S3) The CPU 300 turns off the clock signal CLK2 (always in the L state).
(S4) Since the clock signal CLK2 is turned OFF, the toner supply bias Vrs becomes the value calculated by the formula (6).
(S5) The CPU 300 switches the control signal VAON from ON to OFF.
(S6) The base current of the transistor Tr7 decreases, and the transistor Tr7 enters a non-saturated state.
(S7) The transistor Tr6 is turned from ON to OFF. As a result, the connection between the specified voltage source VA and the booster circuit 600 is cut off, and the specified voltage Va becomes approximately 0V.
(S8) The development voltage Vdc is calculated from equation (8).

As an example, it is assumed that the specified voltage Vc is +5 V, R12 is 300 kΩ, R13 is 30 MΩ, and R0 is 2 MΩ. In this case, the development voltage Vdc is calculated to be approximately 0.3 V from equation (8).
(S9) The CPU 300 turns off the clock signal CLK2 (always at L state), so that the toner supply bias Vrs is set to a value within the range from approximately 0 V to the developing voltage Vdc.
(S10) The potential difference between the development voltage Vdc and the toner supply bias Vrs is approximately 0.3 V at maximum. Generally, the specified voltage Vc used in the feedback circuit is lower than the specified voltage Va used to output a high voltage. Therefore, by turning off the output of the specified voltage source VA, it is possible to further reduce the potential difference between the development roller 15 and the supply roller 17 more easily.

The circuit configuration of the specified voltage source VC, which outputs the specified voltage Vc, may be the same as that of the specified voltage source VA. In other words, the specified voltage source VC may also include a load switch formed by transistors Tr6 and Tr7. This allows the CPU 300 to stop the specified voltage source VC and control the specified voltage Vc to approximately 0 V.

Figure 12 shows the signal and voltage changes in a circuit configuration in which it is possible to turn off the specified voltage source VC in addition to the specified voltage source VA. Comparing Figure 12 with Figure 11, turning off the load switch of the specified voltage source VC disconnects the specified voltage source VC from one end of resistor R12. From equation (8), when the specified voltage Vc becomes approximately 0 V, the development voltage Vdc also becomes approximately 0 V. Therefore, the toner supply bias Vrs also becomes approximately 0 V, and the potential difference between the development roller 15 and the supply roller 17 also becomes approximately 0 V.

In this way, Example 2 achieves the same effects as Example 1. Furthermore, compared to Example 1, Example 2 makes it possible to further reduce the potential difference between the development voltage Vdc and the toner supply bias Vrs. For example, it becomes possible to control the potential difference between the development roller 15 and the supply roller 17 to approximately 0 V. As a result, Example 2 makes it possible to further extend the life of the development roller 15 and the supply roller 17. It will also likely reduce density unevenness in the toner image.

In the second embodiment, transistors Tr6 and Tr7 are used to stop the voltage output function of the specified voltage power supplies VA and VC, but this circuit configuration is merely an example. Other switching elements, such as relays that turn on/off the voltage output terminals of the specified voltage power supplies VA and VC, may also be used. Alternatively, as indicated by the dashed dotted line in Figure 10, the CPU 300 may stop the voltage output function of the specified voltage power supplies VA and VC by controlling the output of the reference voltage source V0 to 0 V.

Example 3
In the second embodiment, the potential difference between the developing roller 15 and the supply roller 17 can be set to approximately 0 V. This is achieved by lowering the voltage of the developing roller 15 and the voltage of the supply roller 17 to approximately 0 V. This increases the time required to transition from the standby state to the pre-rotation state. In other words, the time required to raise the developing voltage Vdc to the target voltage increases. Therefore, in the third embodiment, a method that is more advantageous than that of the second embodiment is proposed for the time required from the issuance of a print command to the completion of printing.

In Example 3, as shown in FIG. 13, a switch 1301 is inserted in a power supply line 1300 that connects the bias circuit 200 and the developing roller 15 and supplies the developing voltage Vdc to the developing roller 15. The switch 1301 is a disconnecting circuit or disconnecting element that connects or disconnects the bias circuit 200 and the developing roller 15 in accordance with a switching signal SW output from the CPU 300. The switch 1301 can be realized by a relay, a semiconductor switch, or the like.

[Post-rotation period and standby period]
In the third embodiment, the potential difference between the developing roller 15 and the supply roller 17 can be set to approximately 0 V by disconnecting the bias circuit 200 from the developing roller 15. This makes it possible to reduce wear of the developing roller 15 and the supply roller 17 due to electrolysis.

FIG. 14 shows signal and voltage changes in a circuit configuration using switch 1301. When image formation is completed, the operating state transitions to a post-rotation state. Furthermore, the operating state transitions from the post-rotation state to a standby state. In the pre-rotation state, image formation state, and post-rotation state described above, switch 1301 is controlled to be on. Other signal and voltage changes are common to those described in Example 1 or Example 2. Furthermore, in Example 3, the initial operation for transitioning from the post-rotation state to the standby state is common to Example 1. Therefore, the following will describe differences from Example 1.
(S1) The CPU 300 switches the switch signal SW from ON to OFF. The switch signal SW is a control signal for turning the switch 1301 ON (connected state)/OFF (disconnected state).
(S2) The switch 1301 is switched from ON to OFF, and the bias circuit 200 and the developing roller 15 change from a conductive state to a non-conductive state.
(S3) The CPU 300 turns off the clock signal CLK2 (always at L state). This stops the development power supply VRS, and the toner supply bias Vrs becomes approximately 0V.
(S4) The potential difference between the development voltage Vdc and the toner supply bias Vrs becomes approximately 0V.

At this time, the output voltage Ve has already risen to the target voltage of +24V. Therefore, when a print command is input, the CPU 300 switches the switch 1301 from OFF to ON. This allows the development voltage Vdc to be quickly applied to the development roller 15.

In this way, by electrically disconnecting the bias circuit 200 and the developing roller 15 with the switch 1301, the potential difference between the developing voltage Vdc and the toner supply bias Vrs can be controlled to approximately 0 V. Because the potential difference between the developing roller 15 and the supply roller 17 is approximately 0 V, wear due to electrolysis can be significantly reduced. Furthermore, deterioration in the quality of toner images due to wear on the developing roller 15 is less likely to occur. Furthermore, because a highly positive voltage is applied to the developing roller 15 in the standby state, the back surface of the sheet is less likely to become soiled with unnecessary toner.

In Example 3, the bias circuit 200 and the developing roller 15 are disconnected by a switch 1301 that can be controlled by a control signal from the CPU 300. However, the control signal for controlling the ON/OFF of the switch 1301 does not have to be output from the CPU 300. For example, the switch 1301 may be configured to switch from ON to OFF when the output voltage Ve drops from 150 V to 24 V. Alternatively, the switch 1301 may be configured to switch from ON to OFF when the toner supply bias Vrs rises from -50 V to 0 V. In this case, there is no need to provide an output port in the CPU 300 for outputting the switch signal SW, which may reduce costs.

As shown in Figure 9, the technical concept of Example 3 can be applied when the output terminal for the development voltage Vdc and the output terminal for the toner supply bias Vrs are physically separated. That is, as shown in Figure 15, switch 1501 may be inserted in the power supply line connecting the development roller 15 to the bias circuit 200 and power supply VDC. Also, switch 1502 may be inserted in the power supply line connecting the toner supply bias 17 to the development power supply VRS. When transitioning from the post-rotation state to the standby state, CPU 300 switches switches 1501 and 1502 from on to off using switch signal SW. As a result, the potential difference between the development roller 15 and supply roller 17 may be controlled to approximately 0 V.

<Treatment concepts derived from practical examples>
[Points 1, 20]
The photosensitive drum 11 is an example of a rotatable image carrier. The developing roller 15 is disposed opposite the image carrier and is an example of a developing member that develops an electrostatic latent image formed on the image carrier with toner to form a toner image. The supply roller 17 is disposed opposite the developing member and is an example of a supply member that supplies toner to the developing member. The transfer roller 16 is an example of a transfer unit that transfers the toner image formed on the image carrier to a transfer medium. The transfer medium is, for example, a sheet or an intermediate transfer medium (e.g., an intermediate transfer belt). The boost circuit 600, the bias circuit 200, and the resistor R0 function as a first power source that generates a development voltage of a first polarity or a second polarity that is applied to the developing member. The second polarity is opposite to the first polarity. The development power source VRS is an example of a second power source that is applied to the supply member and generates an acceleration voltage that accelerates the supply of toner from the supply member to the developing member. The CPU 300 is an example of a control unit that controls the first power source and the second power source. The CPU 300 controls the first power source so that a development voltage of a first polarity (e.g., +150 V) is applied to the developing member during a first period. The first period is, for example, a preparatory period (e.g., a pre-rotation period or a post-rotation period) for suppressing unwanted toner adhesion to the image carrier. The CPU 300 controls the first power source so that a development voltage of a second polarity is applied to the developing member during a second period in which the electrostatic latent image formed on the image carrier is developed with toner. The second period may be referred to as, for example, a development period. A third period different from the first period exists. Note that the third period may also be different from the second period. The third period may be, for example, a standby period in which a toner image is not formed on the image carrier. The CPU 300 controls the first power source and the second power source so that the potential difference between the developing member and the supply member during the third period is smaller than the potential difference between the developing member and the supply member during the first and second periods. This makes it possible to reduce unwanted toner adhesion to the back surface of the sheet while extending the life of the components involved in development. The power supply device 10 is an example of a power supply device that supplies power to an image forming apparatus.

[Point 2]
The control unit may control the first power source and the second power source so that the potential difference between the developing member and the supply member is equal to or less than a predetermined potential difference that can reduce electrolysis of the developing member and the supply member during the third period, thereby reducing unnecessary toner adhesion to the back surface of the sheet and extending the life of the members involved in development.

[Point 3]
For example, the first period (e.g., the period during which the rotation state is reached) may be a period following the second period, and the third period may be a period following the first period. Note that a fourth period (e.g., the period during which the rotation state is reached) similar to the first period may also be present before the second period.

[Point 4]
The absolute value of the potential difference between the developing member and the supply member during the third period is smaller than the absolute value of the potential difference between the developing member and the supply member during the second period. During the second period, both the developing member and the supply member are rotating, so electrolytic wear is unlikely to occur. On the other hand, during the third period, both the developing member and the supply member are not rotating, so electrolytic wear is likely to occur. Therefore, by making the potential difference during the third period smaller than the potential difference during the second period, the life of the components involved in development will be extended.

[Point 5]
At least one of the surface members of the developing member and the supply member may contain an ionically conductive material. When a large potential difference is applied between two opposing ionically conductive members, electrolysis occurs, causing the contact surfaces of the two ionically conductive members to wear away. Thus, the present invention will be useful when a material that undergoes electrolysis, such as an ionically conductive member, is used as the surface member.

[Point 6]
The specified voltage source VA is an example of a first generation circuit that generates a specified voltage of a first polarity (e.g., Va = +24 V). The boost circuit 600 is an example of a boost circuit that is controlled by the control unit and boosts the specified voltage to a voltage of a first polarity (e.g., Ve = +150 V). The bias circuit 200 is an example of a second generation circuit connected to the boost circuit. The second generation circuit outputs a development voltage of a first polarity in a first period. The second generation circuit outputs a development voltage of a second polarity in a second period. The output of the second generation circuit may be controlled by the control unit in a third period so that the potential difference between the developing member and the supply member is equal to or less than a predetermined potential difference.

[Point 7]
Resistor R0 is an example of a resistor connecting the supply member and the developing member. During the second period, the second generation circuit controls the current flowing from the boost circuit to the resistor via the second generation circuit. This causes the second generation circuit to generate a developing voltage of a second polarity. During the first period, the second generation circuit suppresses the current flowing from the boost circuit to the resistor via the second generation circuit. This causes the second generation circuit to generate a developing voltage of a first polarity. During the third period, the second generation circuit reduces the developing voltage to a predetermined voltage or lower.

[Point 8]
The boost circuit 600 may stop boosting the specified voltage during the third period, thereby decreasing the development voltage.

[Point 9]
The second power supply may control the absolute value of the acceleration voltage to a first value (e.g., −50 V) during a first period, a second value (e.g., −600 V) greater than the first value during a second period, and a third value (e.g., +3 V) less than the first value during a third period.

[Points 10 and 11]
The control unit may control the first generation circuit to make the development voltage approach 0 V (e.g., 0 to 4 V (0.3 V)) during the third period. The control unit may control the second power supply to make the output voltage of the second power supply approach 0 V (e.g., 0 to 3 V (0.3 V)) during the third period.

[Points 12 to 15]
The control unit may stop the first power supply during the third period. This will cause the development voltage to approach 0 V. The control unit may stop the boost circuit during the third period. This will cause the development voltage to approach 0 V. The control unit may stop the first power supply by stopping the first generation circuit during the third period. Furthermore, the control unit may stop the first power supply by stopping the first generation circuit and the second generation circuit during the third period. The control unit may stop the second power supply during the third period.

[Points 16-18]
Switches 1301 and 1501 are examples of a first cutoff circuit that cuts off the development voltage from the first power source to the development member during the third period. As shown in FIG. 13 , a first cutoff circuit that cuts off the development voltage from the first power source to the development member during the third period may be provided between the second generation circuit and the resistor. Switch 1502 is an example of a second cutoff circuit that cuts off the acceleration voltage from the second power source to the supply member during the third period. In this case, since the first power source and the second power source can be kept operating, it is possible to immediately transition to the pre-rotation state when the third period ends. This means that the user's waiting time will be reduced.

[Point 19]
As illustrated in FIGS. 9 and 15, the first power source and the second power source may be independent.

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. Therefore, the following claims are appended to clarify the scope of the invention.

10: Power supply unit, 11: Photosensitive drum, 15: Development roller, 17: Toner supply roller

Claims (19)

a rotatable image carrier;
a developing member disposed opposite the image carrier and configured to develop an electrostatic latent image formed on the image carrier with toner to form a toner image;
a supply member disposed opposite to the developing member and supplying toner to the developing member;
a transfer unit that transfers the toner image formed on the image carrier to a transfer target;
a first power source that generates a development voltage of a first polarity or a second polarity opposite to the first polarity, the first polarity being applied to the development member;
a second power source for generating an accelerating voltage applied to the supply member to accelerate the supply of toner from the supply member to the developing member;
a control unit that controls the first power source and the second power source,
The control unit
controlling the first power source so that a developing voltage of the first polarity is applied to the developing member during a first period in which a toner image is not formed on the image carrier;
controlling the first power source so that a developing voltage of the second polarity is applied to the developing member during a second period in which the electrostatic latent image formed on the image carrier is developed with toner;
controlling the first power source and the second power source so that a potential difference between the developing member and the supply member in a third period, which is different from the first period and in which a toner image is not formed on the image carrier, is smaller than an absolute value of the potential difference between the developing member and the supply member in the first period and is also smaller than an absolute value of the potential difference between the developing member and the supply member in the second period;
The image forming apparatus, wherein the first period is a period after the second period, and the third period is a period after the first period . The image forming apparatus of claim 1, wherein the control unit controls the first power source and the second power source so that, during the third period, the potential difference between the developing member and the supply member is equal to or less than a predetermined potential difference that can reduce the occurrence of electrolysis of the developing member and the supply member. 3. The image forming apparatus according to claim 1 , wherein a surface member of at least one of said developing member and said supplying member has an ion conductive material. The first power source is
a first generating circuit that generates the specified voltage of the first polarity;
a boost circuit controlled by the control unit and configured to boost the specified voltage to a voltage of the first polarity;
3. The image forming apparatus according to claim 2, further comprising: a second generating circuit connected to the boost circuit, which outputs a developing voltage of the first polarity during the first period, outputs a developing voltage of the second polarity during the second period, and whose output is controlled by the control unit so that a potential difference between the developing member and the supply member is equal to or less than the predetermined potential difference during the third period. a resistor connecting the supply member and the developing member;
5. The image forming apparatus according to claim 4, wherein the second generation circuit generates the developing voltage of the second polarity by controlling the current flowing from the boost circuit to the resistor via the second generation circuit during the second period, generates the developing voltage of the first polarity by suppressing the current flowing from the boost circuit to the resistor via the second generation circuit during the first period, and reduces the developing voltage to a predetermined voltage or less during the third period. 6. The image forming apparatus according to claim 4 , wherein the boost circuit stops boosting the specified voltage during the third period. 6. The image forming apparatus according to claim 1, wherein the second power source controls the absolute value of the acceleration voltage to a first value during the first period, to a second value greater than the first value during the second period, and to a third value less than the first value during the third period. The image forming apparatus according to claim 4 , wherein the control unit controls the first generation circuit so as to bring the development voltage closer to 0 V during the third period. The image forming apparatus according to claim 7 , wherein the control unit controls the second power source so that the output voltage of the second power source approaches 0 V during the third period. The image forming apparatus according to claim 1 , wherein the control unit stops the first power source during the third period. The image forming apparatus according to claim 4 , wherein the control unit stops the first power source by stopping the booster circuit during the third period. The image forming apparatus according to claim 4 , wherein the control unit stops the first power supply by stopping the first generation circuit during the third period. The image forming apparatus according to claim 4 , wherein the control unit stops the first power source by stopping the first generation circuit and the second generation circuit during the third period. The image forming apparatus according to claim 9 , wherein the control unit stops the second power source during the third period. The image forming apparatus of claim 1, further comprising a first cutoff circuit that cuts off the development voltage from the first power source to the development member during the third period. 6. The image forming apparatus according to claim 5 , wherein a first cutoff circuit that cuts off the development voltage from the first power source to the development member during the third period is provided between the second generation circuit and the resistor. 17. The image forming apparatus according to claim 15 , further comprising a second cutoff circuit that cuts off the acceleration voltage from the second power source to the supply member during the third period. The image forming apparatus according to claim 1 , wherein the first power source and the second power source are independent of each other. A power supply device for supplying power to an image forming apparatus having a rotatable image carrier, a developing member disposed opposite the image carrier and configured to develop an electrostatic latent image formed on the image carrier with toner to form a toner image, a supply member disposed opposite the developing member and configured to supply toner to the developing member, and a transfer unit configured to transfer the toner image formed on the image carrier to a transfer target,
a first power source that generates a development voltage of a first polarity or a second polarity opposite to the first polarity, the first polarity being applied to the development member;
a second power source for generating an accelerating voltage applied to the supply member to accelerate the supply of toner from the supply member to the developing member;
a control unit that controls the first power source and the second power source,
The control unit
controlling the first power source so that a developing voltage of the first polarity is applied to the developing member during a first period in which a toner image is not formed on the image carrier;
controlling the first power source so that a developing voltage of the second polarity is applied to the developing member during a second period in which the electrostatic latent image formed on the image carrier is developed with toner;
a power supply device that controls the first power supply and the second power supply so that the potential difference between the developing member and the supply member during a third period, which is different from the first period and in which a toner image is not formed on the image carrier, is smaller than the absolute value of the potential difference between the developing member and the supply member during the first period and is also smaller than the absolute value of the potential difference between the developing member and the supply member during the second period.