Image forming apparatus

An image forming apparatus includes a power supply circuit, at least one motor configured to receive electric power from the power supply circuit, a photosensitive body configured to be rotated by the motor for forming an image on a sheet, a charger configured to charge the photosensitive body, a high voltage generation circuit configured to receive electric power from the power supply circuit and generate a high voltage applied to the charger, and a control device. The control device is configured to determine whether a motor activation condition to activate the motor is satisfied, determine whether a limitation condition to limit a peak of a current output from the power supply circuit is satisfied, and regulate the current flowing through the high-voltage generation circuit if the motor activation condition and the limitation condition are satisfied.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority from Japanese Patent Application No. 2012-205917 filed on Sep. 19, 2012. The entire content of this priority application is incorporated herein by reference.

TECHNICAL FIELD

The disclosure relates to an image forming apparatus.

BACKGROUND

It is well known that a current value detection circuit for overcurrent protection is provided at a primary side of a switching power supply circuit.

SUMMARY

An image forming apparatus disclosed herein includes a power supply circuit, at least one motor configured to receive electric power from the power supply circuit, a photosensitive body configured to be rotated by the motor for forming an image on a sheet, a charger configured to charge the photosensitive body, a high-voltage generation circuit configured to receive electric power from the power supply circuit and generate a high voltage applied to the charger, and a control device. The control device is configured to determine whether a motor activation condition to activate the motor is satisfied, determine whether a limitation condition to limit a peak of a current output from the power supply circuit is satisfied, and regulate the current flowing through the high-voltage generation circuit if the motor activation condition and the limitation condition are satisfied, whereby the peak of the current output from the power supply circuit is limited.

DETAILED DESCRIPTION

A first illustrative aspect will be described with reference toFIGS. 1 to 7.

1. Overall Configuration of the Printer

In the following explanation, each alphabet B, Y, M, or C, which indicates black, yellow, magenta, or cyan, respectively, is added to a reference numeral if a member indicated by the reference numeral is distinguished by color. If the member is not distinguished by color, such an alphabet is not added.

As illustrated inFIG. 1, a printer1(an example of an image forming apparatus) includes a feeder3, an image forming unit5, a conveying mechanism7, a fusing unit9, a belt cleaning mechanism20, a conveying roller11, and a registration roller12. The feeder3is located at a bottom of the printer1. The feeder3includes a tray17for holding sheets (such as papers and OHP sheets) and a pickup roller19. The sheets15in the tray17are each fed out by the pickup roller19and sent to the conveying mechanism7through the conveying roller11and the registration roller12.

The conveying mechanism7is configured to convey the sheet15and located at an upper side of the feeder3in the printer1. The conveying mechanism7includes a driving roller31, a driven roller32, and a belt34. The belt34is arranged to bridge the driving roller31and the driven roller32. Upon rotation of the driving roller31, a part of the belt34that faces photosensitive drums41B,41Y,41M,41C is moved from a right side to a left side inFIG. 1. The photosensitive drums41are included in the image forming unit5and will be described later. In this configuration, the sheet15sent by the registration roller12is passed under the image forming unit5.

The belt34is provided with four transfer rollers33B,33Y,33M,33C. The transfer rollers33are positioned to face the photosensitive drums41B,41Y,41M,41C with the belt34located therebetween.

The image forming unit5includes four process units40B,40Y,40M,40C and four exposure units49B,49Y,49M,49C each arranged to correspond to the respective process units40B,40Y,40M,40C. The process units40B,40Y,40M,40C are arranged in a line along a direction in which the sheet15is sent (a right-left direction inFIG. 1).

The process units40B,40Y,40M,40C have the same configuration. The process unit40B is illustrated inFIG. 2as an example. The process unit40B includes a photosensitive drum41B (an example of a photosensitive body), a toner case43that houses toner as developer (for example, positively charged nonmagnetic one-component toner), a developing roller45(an example of a developing unit), a feed roller46, and a charger50B. Each process unit40B,40Y,40M,40C includes the photosensitive drum41and the charger50for corresponding color.

The photosensitive drum41B,41Y,41M,41C each include an aluminum substrate and a positively charged photosensitive layer arranged on the substrate, for example. The substrate is connected to a ground of the printer1.

The developing roller45and the feed roller46that is configured to feed the toner from the toner case43are located at a lower side of the toner case43so as to face each other. When the toner passes between the developing roller45and the feed roller46, the developing roller45positively charges the toner and supplies onto each photosensitive drum41B,41Y,41M,41C to form uniform thin toner layer thereon. Accordingly, the developing roller45develops an electrostatic latent image on the photosensitive drum41.

The development roller45is movable between a contact position where the development roller45comes in close contact with the photosensitive drum41and a separated position where the development roller45is away from the photosensitive drum41by a displacement unit70. An example of the displacement unit70is a separating and pressing mechanism disclosed in JP-A-2008-58629. The displacement unit70will be described in more detail later.

Each charger50B,50Y,50M,50C is a scorotron charger and, as illustrated inFIG. 2, includes a shield case51, a wire53, and a grid electrode55made of metal. The shield case51has a square tube shape elongated along a rotation axis of the photosensitive drum41. The shield case51has a discharge opening52that opens toward the photosensitive drum41.

The wire53is a tungsten wire, for example. The wire53is arranged in the shield case51so as to extend along a rotation axis direction of the photosensitive drum41. A high voltage is applied to the wire53by a high-voltage generation circuit150, which will be described later. The application of the high voltage to the wire53induces a corona discharge in the shield case51. The corona discharge generates ions and the ions exit from the discharge opening52toward the photosensitive drum41resulting in a flow of discharging current from the charger50to the photosensitive drum41. As a result, a surface of the photosensitive drum41is uniformly and positively charged. A discharge-starting voltage at which the discharge from the wire53starts is about 5 kV. In the image formation, a voltage of about 6.3 kV, which is higher than the discharge-starting voltage, is applied to the wire53to stabilize the discharge current at a level higher than a target level.

The grid electrode55having a plate like shape with slits or through holes is attached to the shield case51to cover the discharge opening52. A voltage is applied to the grid electrode55. The charge voltage of the photosensitive drum41can be controlled by the voltage applied to the grid electrode55.

The exposure units49B,49Y,49M,49C are each include light emitting elements (such as LEDs) arranged in a line along the rotation axis of each photosensitive drum41B,41Y,41M,41C. The light emitting elements emit light according to print data that is sent by an external device, and thus an electrostatic latent image is developed on the surface of each photosensitive drum41B,41Y,41M,41C.

The image forming process executed by the printer1having the above-described configuration will be briefly explained. Upon receiving a print data, the printer1starts a printing process. At first, each charger50B,50Y,50M,50C uniformly and positively charges the surface of each photosensitive drum41B,41Y,41M,41C. Then, each exposure unit49B,49Y,49M,49C applies light onto each photosensitive drum41B,41Y,41M,41C. Accordingly, a predetermined electrostatic latent image for the image data is developed on the surface of each photosensitive drum41B,41Y,41M,41C. That is, the surface of each photosensitive drum41B,41Y,41M,41C that is uniformly and positively charged has a lower potential at a part to which the light is applied.

Then, the developing roller45is rotated to supply the positively charged toner that is held on the developing roller45to the electrostatic latent image formed on the surface of each photosensitive drum41B,41Y,41M,41C. This converts the electrostatic latent image on each photosensitive drum41B,41Y,41M,41C into a visible image, and thus a toner image developed through a reversal development is held on the surface of each photosensitive drum41B,41Y,41M,41C.

Concurrently with the above formation process of the toner image, a conveying process is executed to convey the sheet15. Specifically, the pickup roller19is turned to send the sheets15to a sheet conveying path Y one by one. The sheets15sent to the sheet conveying path Y is conveyed to a transfer position by the conveying roller11and the belt34. At the transfer position, a toner image on the photosensitive drum41is brought into contact with the transfer roller33.

The toner image (a developer image) in each color on the surface of each photosensitive drum41is sequentially transferred onto the surface of the sheet15and superposed with each other by a transfer bias applied to each transfer roller33. Thus, the toner image (the developer image) in color is formed on the sheet15. Subsequently, the transferred toner image (the developer image) is thermally fused onto the sheet15when the sheet15is passed through the fusing unit9arranged at a rear side of the belt34, which is a left side inFIG. 1. Then, the sheet15is ejected onto a discharge tray60.

2. Electrical Configuration of the Printer1

The electrical configuration of the printer1will be explained with reference toFIG. 3. The printer1includes the displacement unit70, the high-voltage generation circuit150, a first motor driving circuit91, a second motor driving circuit95, a first motor93, a second motor97, an operation unit61, a display63, a temperature sensor65, a print counter67, a network interface75, and a control device80. The control device80is an example of a control device. The temperature sensor65is an example of a sensor. The print counter67is an example of a counter.

The displacement unit70is configured to move the developing roller45between the contact position where the developing roller45comes in close contact with the photosensitive drum41and the separated position where the developing roller45is away from the photosensitive drum41. The high-voltage generation circuit150is configured to receive power supplied by a low-voltage power supply circuit100and to generate a high voltage to be applied to each charger50B,50Y,50M,50C. The low-voltage power supply circuit100will be described later. The high-voltage generation circuit150is a self-excited flyback converter, for example. The control device80inputs a PWM signal S3to the high-voltage generation circuit150(seeFIG. 4). When the PWM signal S3is input, the high-voltage generation circuit150activates and outputs a voltage (high voltage) corresponding to a PWM value of the PWM signal S3. Examples of the high-voltage generation circuit150include a “power supply10” that is disclosed in JP-A-2011-75871 and a “voltage applying circuit200” that is disclosed in JP-A-2012-32532.

The first motor93is a driving power source for the conveying mechanism7and the photosensitive drum41B for black. The second motor97is a driving power source for each photosensitive drum41Y,41M,41C for yellow, magenta, and cyan. The first motor driving circuit91is a circuit for controlling a motor current that is supplied to the first motor93. The second motor driving circuit95is a circuit for controlling a motor current that is supplied to the second motor97.

The operation unit61includes buttons. A user can input instructions to the printer1through the operation unit61for various printer operations such as printing images on a sheet15. The display63includes a liquid crystal display and a lamp, for example. The display63is configured to display various setting screens and an operating status. The temperature sensor65is arranged in the printer1and is configured to measure a temperature inside the printer1. The print counter67is configured to count the accumulated number of the printed sheets15. The number in the counter is incremented by one as one sheet15is printed. The network interface75is connected to an information terminal device such as a PC or a FAX via a communication line NT that enables the communication therebetween.

The control device80is configured to control the components included in the printer1. The control device80includes a CPU81, a ROM83, a RAM84, a non-volatile NVRAM85, and a timer87for measuring time. The ROM83stores various programs for controlling the printer1. The RAM84and the NVRAM85are configured to store various data. When the control device80receives a print job from the information terminal device, the CPU81included in the control device80executes the printing process to print the image on the sheet15based on the print data of the print job.

3. A Power Supply Configuration of the Printer1

A power supply configuration of the printer1will be explained with reference toFIG. 4.

The printer1includes the low-voltage power supply circuit100. The low-voltage power supply circuit100is configured to convert an AC voltage input from an AC power supply130into a DC voltage and output the DC voltage. The output voltage of the low-voltage power supply circuit100is DC 24 V. A power-supply voltage of 24V is applied to the high-voltage generation circuit150by the low-voltage power supply circuit100. Further, the first motor driving circuit91and the second motor driving circuit95are connected to the low-voltage power supply circuit100such that the power-supply voltage of 24 V is applied to the first motor93and the second motor97by the low-voltage power supply circuit100. A DC-DC converter77is connected between the low-voltage power supply circuit100and the control device80. The DC-DC converter77is configured to reduce the voltage of 24 V output by the low-voltage power supply circuit100to 5 V such that the power-supply voltage of 5 V is applied to the control device80.

The control device80is connected to the first and second motor driving circuits91,95via signal lines. The control device80inputs control signals S1, S2to the first motor driving circuit91and the second motor driving circuit95, respectively, to control the first motor activation timing t1and the second motor activation timing t2(seeFIG. 7). The first motor activation timing is timing to activate the first motor93, i.e., to start feeding the motor current (starting current) to the first motor93. The second motor activation timing t1, t2is timing to activate the second motor97, i.e., to start feeding the motor current (starting current) to the second motor97.

In addition, the control device80is connected to the high-voltage generation circuit150via a signal line. The control device80inputs a control signal (PWM signal) S3to the high-voltage generation circuit150to control the high-voltage generation circuit activation timing t3(seeFIG. 7) and the set voltage (set value of the output voltage) of the high-voltage generation circuit150. The high-voltage generation circuit activation timing is a timing to activate the high-voltage generation circuit150. InFIG. 4, the bold lines indicate the power supply lines and the one-dotted chain lines indicate the signal lines. The low-voltage power supply circuit100is an example of a power supply circuit.

4. Configuration of the Low-Voltage Power Supply Circuit100

The configuration of the low-voltage power supply circuit100will be explained with reference toFIG. 5. The low-voltage power supply circuit100is a switching power supply circuit that includes an insulation transformer101having a primary coil N1, a secondary coil N2, and an auxiliary coil N3. The low-voltage power supply circuit100includes a bridge diode D1for rectification, a capacitor C1for smoothing, a FET (field-effect transistor)103, a control IC105for switching control of the FET103, a current detection resistor R, and a voltage generation circuit107, at a primary side thereof The voltage generation circuit170is configured to generate a power-supply voltage for the control IC105by the voltage induced on the auxiliary coil N3of the insulation transformer101. The control IC105includes an output port P1and an input port P2. Further, the low-voltage power supply circuit100includes a rectifying and smoothing circuit110at a secondary side thereof The rectifying and smoothing circuit110includes a diode D2and a capacitor C2.

An AC voltage from the AC power130is rectified by the diode bridge D1and then smoothed by the capacitor C1. Then, the voltage obtained by rectifying and smoothing the AC voltage from the AC power supply130is applied to the primary coil N1of the insulation transformer101.

The FET103is a N-channel MOSFET, and a drain D thereof is connected to the primary coil N1of the insulation transformer101and a source S thereof is connected to the ground via the current detection resistor R. ON-OFF signals (PWM signal) are transmitted from the output port P1of the control IC105to a gate G of the FET103to turn on and off the FET103. Accordingly, the primary side of the insulation transformer101repeatedly turns on and off, and the voltage is induced in the secondary coil N2of the insulation transformer101.

The voltage induced in the secondary coil N2of the insulation transformer101is rectified and smoothed by the rectifying and smoothing circuit110, and then output. The output voltage of the low-voltage power supply circuit100is DC 24 V. The power-supply voltage of 24 V is applied to the electric components such as the high-voltage generation circuit150and the motors93,97connected to the low-voltage power supply circuit100.

The low-voltage power supply circuit100has an overcurrent protection implemented by an overcurrent protection circuit U so that the output current does not exceed a predetermined upper limit. Specifically, the current detection resistor R connected between the source S of the FET103and the ground is configured to detect a primary current I1flowing in the primary side of the insulation transformer103. The current detection resistor R is an example of a current detecting element.

A connection point (a) between the current detection resistor R and the source S of the FET103is connected to the input port P2of the control IC105. The control IC105is configured to detect a level of the voltage input to the input port P2, and thus the amount of the primary current I1is detected.

The control IC105determines whether the detected primary current I1is within a limit. If the primary current I1exceeds the limit, the input of the ON-OFF signal to the FET103is stopped. The current detection resistor R and the control IC105at the primary side of the insulation transformer101configure the overcurrent protection circuit U. When the primary current I1exceeds the limit, the low-voltage power supply circuit100is shut down to protect the low-voltage power supply circuit100from overcurrent.

5. Malfunction of the Overcurrent Protection Circuit U

As described above, in the printer1, the current detection resistor R and the control IC105, which configure the overcurrent protection circuit U, are located at the primary side of the low-voltage power supply circuit100. Namely, a series of steps from the detection of the overcurrent to the shutdown of the circuit is completed at the primary side. With this configuration, the number of components can be reduced compared to a low-voltage power supply circuit that includes the current detection resistor R at the secondary side to detect the secondary current, and thus the substrate of the low-voltage power supply circuit100can be downsized. If the current detection resistor R is included at the secondary side, the number of components increases, because a photo coupler or some components may be required to send the result of the detection to the control IC105arranged at the primary side.

The primary current I1and the secondary current I2of the insulation transformer101are substantially in a proportional relationship. However, the secondary current I2and an output current Io of the low-voltage power supply circuit100may vary with respect to the primary current I1due to leakage flux or other factors. Due to the variation in current, in the overcurrent protection process to shut down the low-voltage power supply circuit100based on the detected primary current I1, the overcurrent protection circuit U may be activated based on the detected primary current I1having a value that is different from a set value to activate the overcurrent protection circuit U. For example, if the limit of the primary current I1is set to shut down the low-voltage power supply circuit100when the output current Io is X (A), the overcurrent protection circuit U may be activated to shut down the low-voltage power supply circuit even if the value of the primary current I1is smaller than X (A) by a variation α.

In the printer1according to this illustrative aspect, the control device80is configured to regulate the current to be supplied to the high-voltage generation circuit150after the activation of the motors93,97. That is, the control device80executes a peak control process to limit the peak of the output current of the low-voltage power supply circuit100. More specifically described, the control device80is configured to activate the high-voltage generation circuit150later than the motors93,97so that the starting current starts flowing through the high-voltage generation circuit150later than through the motors93,97. The starting current of each motor93,97is a motor current that is supplied by the low-voltage power supply circuit100to each motor93,97at the activation of the motors93,97. The starting current of the high-voltage generation circuit150is a current that is supplied by the low-voltage power supply circuit100to the high-voltage generation circuit150at the activation of the high-voltage generation circuit150.

In the first illustrative aspect, the peak control process is executed only if a condition that shows an indication of an increase in the output current Iofrom the low-voltage power supply circuit100is satisfied. This is because a malfunction of the overcurrent protection circuit U is likely to occur if an increase in the output current Iois present.

For example, the output current Io from the low-voltage power supply circuit100is likely to increase when the temperature measured by the temperature sensor65(the temperature in the printer1) is lower than a predetermined temperature (for example, 10° C.) and the number of printed sheets15counted by the print counter67exceeds a predetermined number (for example, 40,000). In the first illustrative aspect, if the above two conditions are satisfied, the peak control process is executed.

The output current Io is likely to increase when the temperature measured by the temperature sensor65is lower than the predetermined temperature, because a larger amount of the motor current is required at a low temperature to increase a torque to a predetermined torque according to characteristics of the motor. Further, the output current Io is likely to increase when the number of printed sheet15counted by the print counter67exceeds the predetermined number, because the load on the motor increases as the mechanical loss increases.

6. Execution Sequence of the Peak Control Process

Next, the execution sequence of the peak control process executed by the control device80will be explained with reference toFIG. 6. The control device80executes the execution sequence of the peak control process indicated inFIG. 6upon receiving the print job from the information terminal device.

Upon receiving the print job, the control device80determines if a motor activation condition to activate the motors93,97is satisfied. The motor activation condition is satisfied when the control device80receives a motor activation signal. If the motor activation condition is not satisfied, the process returns to the start. If the motor activation condition is satisfied, the control device80causes the displacement unit70to move the development rollers45for each color away from the corresponding photosensitive drums41(S10). Then, the control device80receives the detected value from the temperature sensor65and determines the temperature measured by the temperature sensor65(S20). Subsequently, the control device80determines whether the temperature measured by the temperature sensor65is lower than a predetermined temperature (for example, 10° C.) (S30).

If the temperature measured by the temperature sensor65is equal to or higher than the predetermined temperature (S30: NO), the control device80activates the first motor93(S50). Specifically, the control device80inputs the control signal51to the first motor driving circuit91to activate the first motor93. Upon receiving the control signal51, the first motor driving circuit91enables supply of current to the first motor93. Thus, the starting current starts flowing through the first motor93and the first motor starts rotating.

The control device80activates the second motor97(S60). Specifically, the control device80inputs the control signal S2to the second motor driving circuit95to activate the second motor97. Upon receiving the control signal S2, the second motor driving circuit95enables supply of current to the second motor97. Thus, the staring current starts flowing through the second motor97and the second motor97starts rotating.

The control device80activates the high-voltage generation circuit150(S70). Specifically, the control device80inputs the PWM signal S3to the high-voltage generation circuit150to activate the high-voltage generation circuit150. The control device80inputs the PWM signal S3at a PWM value corresponding to a first voltage that is the set voltage for the image formation (for example, 6.3 kV). Accordingly, at the activation, the set voltage of the high-voltage generation circuit150is equal to the first voltage (for example, 6.3 kV).

The control device80activates the first motor93(S50), the second motor97(S60), and the high-voltage generation circuit150(S70) without time delay. Specifically, the control device80inputs the control signal51to the first motor driving circuit91, inputs the control signal S2to the second motor driving circuit95immediately after the input of the control signal51, and inputs the PWM signal S3to the high-voltage generation circuit150immediately after the input of the control signal S2. Accordingly, the starting current starts to be supplied to each of the first motor93, the second motor97, and the high-voltage generation circuit150without time interval. Thus, the output current Io in the low-voltage power supply circuit100becomes relatively high.

When the high-voltage generation circuit150activates simultaneously with the first motor93and the second motor97, the rotation of the photosensitive drums41B,41Y,41M,41C by the activation of the motors93,97and the discharge of the chargers50B,50Y,50M,50C start at the same time. That is, the photosensitive drums41B,41Y,41M,41C are charged immediately after the rotation started.

When the entire circumferences of the photosensitive drums41B,41Y,41M,41C are charged after being rotated by 360 degrees, for example, the control device80activates the displacement unit70to move the development rollers45for each color that are located away from the photosensitive drums41B,41Y,41M,41C to be in close contact with the photosensitive drums41B,41Y,41M,41C (S130). Then, the control device80executes a printing process to print the print data of the received print job on the sheet15(S140). When the printing process is completed, the printing sequence is terminated.

If the temperature measured by the temperature sensor65is lower than the predetermined temperature (S30: YES), the control device80further determines whether the number of sheets15counted by the print counter67exceeds the predetermined number (for example, 40,000) (S40). If the number of printed sheets is equal to or smaller than the predetermined number (S40: NO), the process proceeds to step S50.

Step S50is the same step as the step that is executed by the control device80when the temperature measured by the temperature sensor65is lower than the predetermined temperature (S30: NO). Accordingly, like the above, the first motor93, the second motor97, and the high-voltage generation circuit150activate (S50, S60, and S70) without time interval.

As a result, the rotation of the photosensitive drums41B,41Y,41M,41C by the activation of the first and second motors93,97and the discharge of the chargers50B,50Y,50M,50C start at substantially the same time. Accordingly, the photosensitive drums41B,41Y,41M,41C are charged immediately after the rotation started.

When the entire circumferences of the photosensitive drums41B,41Y,41M,41C are charged after being rotated by 360 degrees, for example, the control device80activates the displacement unit70to move the development roller45for each color to be in close contact with each photosensitive drum41B,41Y,41M,41C (S130). Then, the control device80executes a printing process to print the print data of the received print job on the sheet15(S140). When the printing process is completed, the printing sequence is terminated.

Next, if the temperature measured by the temperature sensor67is lower than the predetermined temperature (S30: YES) and the number of printed sheets15exceeds the predetermined number (S40: YES), the peak control process is performed. The peak control process according to the first illustrative aspect includes steps S80, S90, S100, S110, and S120.

In the peak control process, the control device80activates the first motor93(S80). Specifically, the control device80inputs the control signal Si to the first motor driving circuit91to activate the first motor93. Upon receiving the control signal S1, the first motor driving circuit91enables supply of current to the first motor93. Thus, the starting current starts flowing through the first motor93and the first motor93starts rotating. By the rotation of the first motor93, the photosensitive drum41B for black starts rotating. After the activation of the first motor93, the control device80executes a waiting process to wait for a period T1(for example, 300 ms) (S90). The period T1is measured by the timer87of the control device80.

After the period T1, the control device80activates the second motor97(S100). Specifically, the control device80inputs the control signal S2to the second motor driving circuit95to activate the second motor97(S100). Upon receiving the control signal S2, the second motor driving circuit95enables supply of current to the second motor97. Accordingly, as indicated inFIG. 7, the starting current starts flowing through the second motor97and the second motor97starts rotating at the second motor activation timing t2at which the period T1has elapsed since the first motor activation timing1.

By the rotation of the second motor97, the photosensitive drums41Y,41M,41C for yellow, magenta, and cyan are started to rotate. After the activation of the second motor97, the control device80executes a waiting process to wait for a period T2(for example, 50 ms) (S110). The period T2is measured by the timer87of the control device80.

When the period T2has elapsed since the activation of the second motor97, the control device80activates the high-voltage generation circuit150(S120). Specifically, the control device80inputs the PWM signal S3to the high-voltage generation circuit150to activate the high-voltage generation circuit150. At this time, the control device80inputs the PWM signal S3at the PWM value corresponding to the first voltage (for example, 6.3 kV) that is the set voltage for the image formation. Accordingly, at the time of activation of the high-voltage generation circuit150, the set voltage for the high-voltage generation circuit150is equal to the first voltage (for example, 6.3 kV).

As illustrated inFIG. 7, at the high-voltage generation circuit activation timing t3at which the period T2has elapsed since the second motor activation timing t2, the high-voltage generation circuit150activates and the starting current starts flowing through the high-voltage generation circuit150. Then, by the activation of the high-voltage generation circuit150, the chargers50B,50Y,50M,50C start to discharge. Thus, at the high-voltage generation circuit activation timing t3, the photosensitive drum41B that starts to rotate at the first motor activation timing t1and the photosensitive drums41Y,41M,41C that start to rotate at the second motor activation timing t2start to be charged.

As described above, if the temperature in the printer1is lower than the predetermined temperature and the number of printed sheets counted by the print counter67exceeds the predetermined number (for example, 40,000), the first motor93, the second motor97, and the high-voltage generation circuit150activate at different times.

Then, when the entire circumference of each photosensitive drum41B,41Y,41M,41C is charged, the control device80activates the displacement unit70to move the development rollers45for each color to be in close contact with the photosensitive drums41B,41Y,41M,41C (S130). Subsequently, the control device80executes a printing process to pint the print data of the received print job on the sheet15(S140). When the printing process is completed, the printing sequence is terminated.

In the peak control process of the printer1, the control device80activates the high-voltage generation circuit150later than the motors93,97. That is, the staring current is to be supplied to the high-voltage generation circuit150later than to the motors93,97. With this configuration, compared to the case where the starting current start to be supplied to the high-voltage generation circuit150and the motors93,97without time interval, the peak of the output current Io of the low-voltage power supply circuit100can be limited. Accordingly, the malfunction of the overcurrent protection circuit U due to the variations in the current is less likely to occur.

In the printer1, the set voltage for activation of the high-voltage generation circuit150is equal to the first voltage that is the set voltage for image formation. With this configuration, the set voltage of the high-voltage generation circuit150does not need to be changed after its activation, and thus the control device80can control the high-voltage generation circuit150in a simple way.

In the printer1, the peak control process to limit the peak of the output current Io of the low-voltage power supply circuit100is executed only when the temperature in the printer1is lower than the predetermined temperature and the number of printed sheets counted by the printer counter67exceeds the predetermined number. With this configuration, the peak control process can be executed less frequently.

Further, in the printer1, during the charging of the photosensitive drums41by the chargers50, the photosensitive drums41are located away from the development rollers45. With this configuration, the toner, which is supplied through the development rollers45, is hardly attached to the photosensitive drums41during the charging. Accordingly, the quality of the printed image can be maintained at a high level.

The second illustrative aspect of the present invention will be explained with reference toFIG. 8toFIG. 10.

In the peak control process according to the first illustrative aspect, the control device80activates the high-voltage generation circuit150later than the motors93,97so that the supply of the starting current to the high-voltage generation circuit150starts later than the start of the starting current supply to the motor93,97.

In the second illustrative aspect, the way of limiting the peak is different from that of the first illustrative aspect. Specifically, the voltage of the high-voltage generation circuit150is set to be a second voltage (for example, about 5.5 kV) that is lower than the first voltage (for example, about 6.3 kV) for the image formation. The lower the set voltage of the high-voltage generation circuit150, the lower the starting current of the high-voltage generation circuit150. Thus, if the set voltage of the high-voltage generation circuit150is lowered, the amount of the starting current of the high-voltage generation circuit150that is supplied at the same time with the starting current of the motor93,97is reduced. Accordingly, like the first illustrative aspect, the peak of the output current Io of the low-voltage power supply circuit100can be limited.

Hereinafter, the execution sequence of the peak control process according to the second illustrative aspect will be explained with reference toFIG. 8. Like the first illustrative aspect, in the peak control process of the second illustrative aspect, the control device80determines whether the temperature measured by the temperature sensor65is lower than the predetermined temperature (S30) and whether the number of printed sheets counted by the print counter67exceeds the predetermined number (S40). The peak control process is executed if the temperature measured by the temperature sensor65is lower than the predetermined temperature and the number of printed sheets counted by the print counter67exceeds the predetermined number (for example, 40,000), i.e., YES in both S30and S40.

The peak control process according to the second illustrative aspect includes S80, S100, S121, S123, and S125. The control device80activates the first motor93(S80). The control device80inputs the control signal51to the first motor driving circuit91to activate the first motor93. Upon receiving the control signal51, the first motor driving circuit91enables supply of the current to the first motor93. Thus, the starting current starts flowing through the first motor93and the first motor93start rotating.

The control device80activates the second motor (S100). The control device80inputs the control signal S2to the second motor driving circuit95to activate the second motor97. Upon receiving the control signal S2, the second motor driving circuit91enables supply of current to the second motor97. Thus, the starting current starts flowing through the second motor97and the second motor97start rotating.

The control device80activates the high-voltage generation circuit150(S121). The control device80inputs the PWM signal S3to the high-voltage generation circuit150to activate the high-voltage generation circuit150.

The control device80activates the first motor93(S80), the second motor97(S100), and the high-voltage generation circuit150(S121) without time delay. Specifically, the control device80inputs the control signal51to the first motor driving circuit91, inputs the control signal S2to the second motor driving circuit95immediately after the input of the control signal51, and inputs the PWM signal S3to the high-voltage generation circuit150immediately after the input of the control signal S2. Accordingly, the first motor93, the second motor97, and the high-voltage generation circuit150activate without time interval.

In step S121, the control device80inputs the PWM signal S3to the high-voltage generation circuit150at the PWM value corresponding to the second voltage that is lower than the first voltage, which is the set voltage for the image formation. Thus, by the control device80, the set voltage for the activation of the high-voltage generation circuit150is controlled to be the second voltage that is lower than the first voltage. Accordingly, the amount of the starting current of the high-voltage generation circuit150that is supplied at the same time with the starting current of the motors93,97is reduced, and thus the peak of the output voltage To of the low-voltage power supply circuit100is limited like the first illustrative aspect.

The second voltage may be larger than a discharge starting voltage at which the discharge from the wire53of the charger50starts (for example, 5 kV). With this configuration, the charging of the photosensitive drums41can be started immediately after the rotation thereof started, and thus the toner in the air is hardly attached to the rotating photosensitive drums41.

After the activation of the high-voltage generation circuit150, the control device80executes a waiting process to wait for a period T3(S123). When the period T3has elapsed since the activation of the high-voltage generation circuit150, the control device80switches the set voltage of the high-voltage generation circuit150from the second voltage to the first voltage by changing the PWM value of the PWM signal S3(S125). As indicated inFIG. 9, the set voltage of the high-voltage generation circuit150is switched to the first voltage (for example, 6.3 kV) at switch timing t5at which the period T3has elapsed since an activation timing t4. The activation timing t4is timing to activate the high-voltage generation circuit150. The period T3is measured by the timer87of the control device80.

The period T3is set to be longer than a stabilization time Tm for stabilizatng the starting current Im of each motor93,97(seeFIG. 10). Specifically, if the first motor93and the second motor97take different stabilization times to stabilize the starting current, the period T3is set based on longer one of the stabilization times. The longer one of the stabilization times is 300 ms, for example. In such a case, the period T3is set at 300 ms that is the longer one of the stabilization times Tm.

Since the period T3is set to be longer than the stabilization time Tm, the set voltage of the high-voltage generation circuit150is switched from the second voltage to the first voltage after the stabilization of the starting current Im of each motor93,97. By setting the period T3as above, a large peak is less likely to be present compared to the case where the set voltage is switched before the stabilization of the starting current Im of each motor93,97. The amount of the output current Io of the low-voltage power supply circuit100is reduced.

The stabilization of the starting current Im of each motor93,97means “a response is in a predetermined acceptable range E, for example, in a range of 5% above and below the target value”. Further, “the stabilization time Tm” is duration of time required to stabilize the starting current Im from a start of supply of the starting current Im (seeFIG. 10). The stabilization time Tm of the starting current Im can be calculated from a circuit constant of each motor93,97or each motor driving circuit91,95. In the second illustrative aspect, the stabilization time Tm is calculated using the circuit constant. Other than the above, the stabilization time Tm of the starting current Im may be obtained using an actual measured value obtained by a test circuit.

After switching the set voltage from the second voltage to the first voltage, the control device80executes the printing process to print the print data of the received print job on the sheet15(S140). When the printing process is completed, the printing sequence is terminated.

In the peak control process according to the second illustrative aspect, when the motor activation condition to activate the motors93,97is satisfied, the set voltage of the high-voltage generation circuit150is equal to be the second voltage that is lower than the first voltage for the image formation. The lower the set voltage of the high-voltage generation circuit150, the lower the starting current of the high-voltage generation circuit150. Thus, the amount of the staring current of the high-voltage generation circuit150that is supplied at the same time with the starting current of the motors93,97is reduced. Accordingly, like the first illustrative aspect, the peak of the output voltage To of the low-voltage power supply circuit100is limited.

In the peak control process according to the second illustrative aspect (S80, S100, S121, S123, S125), the control device80activates the motors93,97and the high-voltage generation circuit150without time delay. With this configuration, the charging of the photosensitive drums41starts immediately after the photosensitive drums41starts rotating, and thus the toner in the air is hardly attached to the photosensitive drums41.

The present invention is not limited to the illustrative aspects described above with reference to the drawings, and may include the following various illustrative aspects in the technical scope of the invention.

(1) In the above first and second illustrative aspects, the control device80includes one CPU81, the ROM83, the NVRAM85, and the like. However, the number of CPUs81may be two or more. Further, the control device80may include the CPU81and a hard circuit such as an ASIC or may only include a hard circuit.

(2) In the above first and second illustrative aspects, the printer1includes the first motor93and the second motor97. However, the printer1may include only one motor or more than two motors.

(3) In the above first and second illustrative aspect, the peak control process is executed if the temperature measured by the temperature sensor65is lower than the predetermined temperature and the number of printed sheets15counted by the print counter67exceeds the predetermined number. However, the peak control process may be executed when one of the above conditions that shows an indication of increase in the output current Io from the low-voltage power supply circuit100, is satisfied. Namely, the peak control process may be executed if the temperature measured by the temperature sensor65is lower than the predetermined temperature or if the number of printed sheets15counted by the print counter exceeds the predetermined number.

(4) In the peak control process according to the first illustrative aspect, the motors93,97and the high voltage generation circuit100may activate in any order as long as there is the time interval between the activations.

(5) In the peak control process according to the above second illustrative aspect, the motors93,97and the high-voltage generation circuit150activate without time delay. However, the high-voltage generation circuit150may activate before the activation of the motors93,97as long as the set voltage of the high-voltage generation circuit150is equal to the second voltage that is lower than the first voltage for the image formation.

In a known power supply circuit including an overcurrent protection circuit at a primary side thereof, a series of steps from the detection of an overcurrent to the shutdown of the circuit can be completed at the primary side. This can reduce the number of components of the power supply circuit compared to a power supply circuit including an overcurrent protection circuit at a secondary side thereof. Accordingly, the power supply circuit including the overcurrent protection circuit at the primary side can include a smaller substrate, for example. However, malfunction may occur in the overcurrent protection circuit of such a known power supply circuit due to variations in a secondary current with respect to a primary current. Thus, a peak of an output current of the power supply circuit may be required to be limited. According to the technology described in the above illustrative aspects, the peak of the output current is limited and thus an improper operation of the overcurrent protection circuit is less likely to occur.