Power supply device and image forming device

Abstract

To reduce manufacturing cost of a power supply device.SOLUTION: Control means outputs a first pulse signal of a duty ratio corresponding to target output voltage and a second pulse signal of a prescribed frequency. Generation means generates driving voltage so that output voltage approaches target output voltage based on the first pulse signal and feedback information related to output voltage from a power supply device target output voltage. Driving means switches driving voltage according to the second pulse signal and generates switching voltage. A piezoelectric transformer boosts switching voltage and generates AC voltage. Detection means detects driving voltage and outputs voltage information indicative of driving voltage to control means. Control means is configured to generate the second pulse signal of the frequency by specifying the frequency at which driving voltage becomes the lowest by sweeping the frequency of the second pulse signal and changing driving voltage or reading the previously specified frequency from storage means.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to a power supply device and an image forming apparatus equipped with the same. [Background technology]

[0002] An image forming apparatus employing an electrophotographic method requires a high-voltage power supply that generates multiple high voltages (e.g., charging voltage, developing voltage, and transfer voltage). The high-voltage power supply generates the high voltages using an electromagnetic transformer or a piezoelectric transformer. The piezoelectric transformer contributes to miniaturizing the high-voltage power supply. Patent Document 1 proposes using a general-purpose microcomputer to control the piezoelectric transformer. Patent Document 1 also proposes that the drive frequency of the piezoelectric transformer is fixed, and the input voltage is controlled to increase the resolution of the output voltage. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 5864383 Summary of the Invention [Problem to be solved by the invention]

[0004] However, in order to obtain the maximum output voltage required for the power supply device, expensive electronic components with high rated voltages and high rated currents are required, which makes the power supply device expensive. Therefore, an object of the present invention is to reduce the manufacturing costs of the power supply device. [Means for solving the problem]

[0005] The present invention relates to, for example, 1. A power supply device for generating a predetermined output voltage, comprising: a control means for outputting a first pulse signal having a duty ratio corresponding to a target output voltage that is a target for the predetermined output voltage, and a second pulse signal having a predetermined frequency; a generating means for generating a drive voltage based on the first pulse signal and feedback information on an output voltage from the power supply device so that the output voltage approaches the target output voltage; a driving means for switching the driving voltage in response to the second pulse signal to generate a switching voltage; a piezoelectric transformer that boosts the switching voltage to generate an AC voltage; a rectifying means for rectifying the AC voltage to generate the output voltage; a feedback means for generating the feedback information for feeding back the output voltage to the drive voltage; a detection means for detecting the drive voltage and outputting voltage information indicating the drive voltage to the control means; The control means is configured to sweep the frequency of the second pulse signal to change the drive voltage and thereby identify the frequency at which the drive voltage is lowest, or to read out the pre-identified frequency from a memory means and thereby generate the second pulse signal of that frequency. Effect of the Invention

[0006] According to the present invention, it is possible to reduce the manufacturing costs of the power supply device. [Brief description of the drawings]

[0007] [Figure 1] FIG. 1 is a diagram illustrating an image forming apparatus. [Diagram 2] Block diagram showing the controller involved in image formation [Diagram 3] Diagram explaining the characteristics of a piezoelectric transformer [Figure 4] Circuit diagram showing the power supply unit [Diagram 5] FIG. 1 is a diagram illustrating the relationship between the drive frequency and the output voltage or drive voltage. [Figure 6] FIG. 1 is a diagram for explaining various waveforms in the first embodiment. [Figure 7] FIG. 1 is a diagram for explaining the driving frequency and various measurement results in Example 1. [Figure 8] Flowchart showing a control method according to the second embodiment DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

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

[0009] <Example 1> (1) Image forming device As shown in FIG. 1, the image forming apparatus 100 is a copying machine, a multifunction machine, a facsimile machine, a printer, or the like that utilizes an electrophotographic method. The image forming apparatus 100 is a so-called tandem type printer having a plurality of image forming stations 10a to 10d. The image forming station 10a forms a yellow toner image. The image forming station 10b forms a magenta toner image. The image forming station 10c forms a cyan toner image. The image forming station 10d forms a black toner image. In this manner, the letters abcd affixed to the end of the reference symbols correspond to the toner colors yellow (Y), magenta (M), cyan (C), and black (Bk), respectively. When matters common to the four colors are described, the letters abcd are omitted from the reference symbols. Note that this embodiment is also applicable to an image forming apparatus that forms a monochrome image.

[0010] The image forming station 10 includes a photoconductor drum 1, a charging roller 2, a developing device 3, and a drum cleaner 4. The photoconductor drum 1 is an image carrier that is driven to rotate at a predetermined peripheral speed (process speed) in the direction of the arrow and rotates while carrying an electrostatic latent image and a toner image (developer image). A charging voltage is applied to the charging roller 2 from a charging power source 21, and the surface of the photoconductor drum 1 is uniformly charged. As an example, it is assumed that the surface of the photoconductor drum 1 is negatively charged. The exposure device 5 exposes the surface of the photoconductor drum 1 according to image data to form an electrostatic latent image. The developing device 3 develops the electrostatic latent image using toner contained in a storage container to form a toner image. The toner is also negatively charged. A developing voltage is applied to the developing sleeve (developing roller) of the developing device 3 from a developing power source 22. A primary transfer voltage is applied to the primary transfer member 6 from a primary transfer power source 23, and the toner image is transferred from the photoconductor drum 1 to the transfer conveyor belt 7. The drum cleaner 4 is a member for collecting toner remaining on the photosensitive drum 1. The drum cleaner 4 includes, for example, a cleaning blade that is a cleaning member that contacts the photosensitive drum 1a, and a waste toner box that stores the toner collected by the cleaning blade.

[0011] The transfer conveyor belt 7 is an endless belt, and is stretched around tension rollers 71, 72, and 73 as support members. The transfer conveyor belt 7 may also be called an intermediate transfer belt. The transfer conveyor belt 7 is rotated while in contact with the photosensitive drums 1a to 1d. The yellow toner image formed on the photosensitive drum 1a is transferred onto the transfer conveyor belt 7 while passing through a contact portion between the photosensitive drum 1a and the transfer conveyor belt 7 (hereinafter referred to as a primary transfer nip). Similarly, a magenta toner image, a cyan toner image, and a black toner image are transferred onto the transfer conveyor belt 7 in order, and a full-color image is formed.

[0012] A feed roller 9 feeds a sheet P one by one from a plurality of sheets P stored in a sheet cassette 11. The four-color toner image on the transfer conveyor belt 7 is transferred to the surface of the sheet P (secondary transfer) as it passes through a secondary transfer nip formed by the transfer conveyor belt 7 and secondary transfer roller 8. The secondary transfer roller 8 is rotated in response to the rotation of the transfer conveyor belt 7. A secondary transfer voltage (e.g., 1800V to 2300V) is applied to the secondary transfer roller 8 from a secondary transfer power source 24.

[0013] Toner remaining on the transfer / transport belt 7 after the secondary transfer (residual toner) is charged by the cleaning brush 13. As a result, the residual toner is transferred back from the transfer / transport belt 7 to the photosensitive drum 1a, and is cleaned and removed by the drum cleaner 4a.

[0014] Thereafter, the sheet P carrying the four-color toner image is conveyed to the fixing device 12. The fixing device 12 applies heat and pressure to the toner image and the sheet P, melting and mixing the four color toners. This fixes the toner image onto the sheet P. Thereafter, the sheet P is discharged outside the image forming apparatus 100.

[0015] (2) Overall control of the image forming device FIG. 2 shows a controller 200 that controls the image forming apparatus 100. The controller 200 has a CPU 201. The CPU 201 incorporates a ROM 203 and a RAM 204. CPU is an abbreviation for central processing unit. ROM is an abbreviation for read-only memory. RAM is an abbreviation for random access memory. The CPU 201 comprehensively controls the exposure control unit 205, the charging control unit 206, the development control unit 207, the primary transfer control unit 208, and the secondary transfer control unit 209 according to a control program stored in the ROM 203. The ROM 203 may store an environment table and various tables for transfer control. The CPU 201 identifies the temperature and humidity by checking the detection result of the environment sensor 210 against the environment table. The CPU 201 may adjust the charging voltage, the developing voltage, the primary transfer voltage, and the secondary transfer voltage by referring to the control table based on the temperature and humidity. The RAM 204 temporarily holds control data and is used as a work area for arithmetic processing associated with the control.

[0016] The charging control unit 206 controls the charging voltage output from the charging power supply 21. The development control unit 207 controls the development voltage output from the development power supply 22. The primary transfer control unit 208 controls the primary transfer voltage output from the primary transfer power supply 23 based on a current value detected by a current detection circuit (not shown). The secondary transfer control unit 209 controls the secondary transfer voltage output from the secondary transfer power supply 24 based on a current value detected by a current detection circuit (not shown).

[0017] The external device 215 is a host computer that transmits image information and a print command. The video controller 216 receives the image information and the print command from the external device 215, generates an image signal, and transmits it to the controller 200. The controller 200 controls the exposure control unit 205, the charging control unit 206, the development control unit 207, the primary transfer control unit 208, and the secondary transfer control unit 209 in response to the image signal, and causes the image forming apparatus 100 to perform image formation.

[0018] The ROM 203 and the RAM 204 may be provided outside the CPU 201. A NVRAM 202 may be connected to the CPU 201 as a non-volatile memory.

[0019] (3) Power supply device (3-1) Characteristics of piezoelectric transformers The charging power supply 21, the developing power supply 22, the primary transfer power supply 23, and the secondary transfer power supply 24 all output high voltages and are therefore referred to as high voltage power supplies. Since the power supply circuits of the charging power supply 21, the developing power supply 22, the primary transfer power supply 23, and the secondary transfer power supply 24 are almost the same, the charging power supply 21 will be described below as a representative. In other words, the description of the charging power supply 21 can also be applied to the developing power supply 22, the primary transfer power supply 23, and the secondary transfer power supply 24.

[0020] The charging power supply 21 generates a charging voltage that determines the amount of toner carried. Therefore, the accuracy of the output voltage of the charging power supply 21 directly affects the density of the image. Therefore, the charging power supply 21 is required to have high output voltage accuracy.

[0021] Figure 3 shows the characteristics of the output voltage of a piezoelectric transformer versus the drive frequency. The horizontal axis represents the drive frequency, and the vertical axis represents the output voltage. Generally, the output voltage of a piezoelectric transformer is maximum at the resonance frequency f0 as shown in Figure 3. One method of controlling a piezoelectric transformer that utilizes this characteristic is frequency control, which controls the drive frequency of the piezoelectric transformer. When increasing the output voltage from V1 to V2 through frequency control, the drive frequency is increased from f1 to f2 (f1>f2). However, to achieve high resolution for the output voltage, it is necessary to control the drive frequency continuously. This requires dedicated analog ICs and other generally expensive electronic components.

[0022] In contrast, by using a general-purpose microcomputer to control the piezoelectric transformer as in this embodiment, it is possible to reduce the cost of the power supply device. However, there is a limit to the resolution of the switching frequency of the general-purpose microcomputer. Therefore, it may be impossible to achieve the desired output voltage precision. In particular, high output voltage precision is required for the charging power supply 21. Therefore, input voltage control is performed to increase the resolution of the output voltage. Input voltage control is a method of controlling the output voltage by adjusting the voltage input to the piezoelectric transformer while maintaining the driving frequency of the piezoelectric transformer constant. The details of the circuit operation in this case will be described later.

[0023] (3-2) Circuit diagram 4 is a circuit diagram of a power supply device 400 which is any one of the charging power supply 21, the developing power supply 22, the primary transfer power supply 23, and the secondary transfer power supply 24. As shown in Fig. 4, an IC 401 monitors an input voltage (driving voltage Vdrv) input to a driving circuit 402 which drives a piezoelectric transformer 407, and drives the piezoelectric transformer 407 at a resonance frequency f0 of the piezoelectric transformer 407 based on the monitoring result. The IC 401 may be implemented in any one of the charging control unit 206, the developing control unit 207, the primary transfer control unit 208, and the secondary transfer control unit 209.

[0024] The IC401 is a general-purpose microcomputer that controls the output of the power supply device 400. The IC401 has at least output ports 411 and 412 and an input port 413. The output port 411 outputs a pulse width modulation signal (hereinafter referred to as a "PWM signal") as a control signal. The output port 412 outputs a pulse frequency modulation signal (hereinafter referred to as a "PFM signal") as a pulse signal to the drive circuit 402. Here, it is assumed that the pulse signal is a PFM signal, but this is merely an example. The pulse signal may be any signal with a variable frequency. The input port 413 is an input port having an analog-to-digital conversion circuit (ADC).

[0025] The drive circuit 402 is composed of a field effect transistor FET1, an inductor L1, and a capacitor C6. The gate of the field effect transistor FET1 is connected to an output port 412. The source of the field effect transistor FET1 is connected to the frame ground, one input electrode of the piezoelectric transformer 407, and one end of the capacitor C6. The drain of the field effect transistor FET1 is connected to one end of the inductor L1, the other input electrode of the piezoelectric transformer 407, and the other end of the capacitor C6. The other end of the inductor L1 is connected to the capacitor C20, one end of the resistor R21, and the emitter of the transistor Tr1.

[0026] The driving circuit 402 is a circuit that drives the piezoelectric transformer 407 based on the frequency of the PFM signal input from the IC 401, thereby causing the piezoelectric transformer 407 to output a predetermined AC voltage. The piezoelectric transformer 407 is an element that increases or boosts the AC voltage generated by the field effect transistor FET1 of the driving circuit 402.

[0027] The output electrode of the piezoelectric transformer 407 is connected to the rectifying and smoothing circuit 403. The rectifying and smoothing circuit 403 includes diodes D8 and D9, and a capacitor C10. The anode of the diode D8 and the cathode of the diode D9 are connected to the output electrode of the piezoelectric transformer 407. The cathode of the diode D8 is connected to the frame ground and one end of the capacitor C10. The anode of the diode D9 is connected to the load 410 and the other end of the capacitor C10. The AC voltage generated at the output electrode of the piezoelectric transformer 407 is rectified by the diodes D8 and D9, and smoothed by the capacitor C10. This generates a negative polarity DC high voltage (output voltage Vout). The load 410 is the charging roller 2, the developing roller of the developing device 3, the primary transfer member 6, or the secondary transfer roller 8.

[0028] The output voltage Vout is divided by a voltage divider circuit formed by resistors R11, R12, and R13. This generates a feedback voltage Vfd that is correlated with the output voltage Vout. The feedback voltage Vfd is input to the voltage adjustment circuit 404.

[0029] The voltage adjustment circuit 404 generates a drive voltage Vdrv from the feedback voltage Vfd, the PWM signal, and the power supply voltage of +24V. The voltage adjustment circuit 404 is composed of an operational amplifier OP1, resistors R15 and R16, capacitors C17 and C18, and a transistor Tr1. One end of the resistor R15 receives the feedback voltage. The other end of the resistor R15 is connected to the non-inverting input terminal of the operational amplifier OP1. The resistor R15 functions as a protective resistor. One end of the resistor R16 is connected to the output port 411. The other end of the resistor R16 is connected to the inverting input terminal of the operational amplifier OP1, one end of the capacitor C18, and one end of the capacitor C17. The other end of the capacitor C18 is connected to the output terminal of the operational amplifier OP1 and the base of the transistor Tr1. The other end of the capacitor C17 is connected to the frame ground. The resistor R16 and the capacitor C17 form a kind of low-pass filter that rectifies and smoothes the PWM signal to generate a DC voltage. This DC voltage is proportional to the target value of the output voltage Vout (target output voltage Vtar). This DC voltage may be called a "control voltage." IC401 changes the voltage value of the control voltage by changing the duty ratio of the PWM signal. As an example, the control signal of the power supply device 400 is assumed to be a PWM signal, but any signal with a variable duty ratio can be used. The control voltage is assumed to be a DC voltage generated by rectifying and smoothing the PWM signal. However, this is only an example. The control signal output from the output port 411 of IC401 can be any DC voltage with a variable voltage value. Capacitor C18 is connected as an element for adjusting the responsiveness of the operational amplifier OP1.

[0030] A power supply voltage of +24 V is applied to the collector of the transistor Tr1. A drive voltage Vdrv to the drive circuit 402 is generated at the emitter of the transistor Tr1.

[0031] Vdrv = +24 - Vce (Eq1) Here, Vce is the collector-emitter voltage of the transistor Tr1. The collector-emitter voltage Vce of the transistor Tr1 is controlled by the operational amplifier OP1 so that the control voltage input to the inverting input terminal of the operational amplifier OP1 is equal to the feedback voltage input to the non-inverting input terminal of the operational amplifier OP1. The capacitor C20 is connected for stabilizing the generated drive voltage Vdrv. The other end of the capacitor C20 is connected to the frame ground. The drive voltage Vdrv thus generated is divided by a voltage divider circuit formed by resistors R21 and R22, and input to the input port 413 of the IC401. One end of the resistor R21 is connected to the emitter of the transistor Tr1, the other end of the inductor L1, and one end of the capacitor C20. The other end of the resistor R21 is connected to one end of the resistor R22 and the input port 413. The other end of the resistor R22 is connected to the frame ground.

[0032] In order to control the output voltage Vout to the target output voltage Vtar, a control voltage corresponding to the target output voltage Vtar is input to the inverting input terminal of the operational amplifier OP1. Specifically, the IC401 outputs a PWM signal having a duty ratio corresponding to the target output voltage Vtar from the output port 411. For example, the IC401 may have an internal memory that stores a control table that converts the voltage value of the target output voltage Vtar into a duty ratio. The IC401 may determine the duty ratio corresponding to the target output voltage Vtar set by the CPU201 by referring to the control table. Note that the process of determining the duty ratio may be executed by the CPU201. Not limited to this, the calculation process executed by the IC401 may be executed by the CPU201.

[0033] The IC 401 drives the piezoelectric transformer 407 by outputting a PFM signal having a certain frequency. The voltage adjustment circuit 404 adjusts the drive voltage Vfrv so that the output voltage Vout coincides with the target output voltage Vtar. However, the power supply device 400 can output the target output voltage Vtar only when the frequency of the PFM signal is within a certain frequency range. In this embodiment, the frequency of the PFM signal is set to the resonant frequency f0 of the piezoelectric transformer 407. In other words, the resonant frequency f0 is included in the certain frequency range.

[0034] FIG. 5(A) shows the relationship between the output voltage Vout and the drive frequency of the piezoelectric transformer 407. The horizontal axis shows the drive frequency. The vertical axis shows the output voltage Vout. FIG. 5(B) shows the relationship between the drive voltage Vdrv and the drive frequency of the piezoelectric transformer 407. The horizontal axis shows the drive frequency. The vertical axis shows the drive voltage Vdrv. Here, the target output voltage Vtar is assumed to be −1200V.

[0035] In this example, when the frequency of the PFM signal (drive frequency f) is higher than 170 kHz, even though the drive voltage Vdrv is controlled to its maximum value, the output voltage Vout does not reach -1200 V. On the other hand, when the frequency of the PFM signal is set to 170 kHz or lower, the voltage adjustment circuit 404 adjusts the drive voltage Vdrv so that the output voltage Vout can reach -1200 V.

[0036] Whether the output voltage Vout can reach the target output voltage Vtar depends on the combination of the drive voltage Vdrv and the drive frequency f of the piezoelectric transformer 407. The frequency of the PFM signal when the voltage input to the input port 413 of the IC 401 is the lowest is the resonant frequency f0 of the piezoelectric transformer 407. In this example, the resonant frequency f0 of the piezoelectric transformer 407 is 165 kHz. When the piezoelectric transformer 407 is driven at the resonant frequency f0, the energy conversion efficiency of the piezoelectric transformer 407 is maximized. Even if the same target output voltage Vtar (=-1200V) is set, the operation of the drive circuit 402 differs between when the drive frequency f is off the resonant frequency f0 (e.g., 170 kHz) and when the drive frequency f is the resonant frequency f0 (=165 kHz).

[0037] 6(A) to 6(E) show various waveforms when the target output voltage Vtar (=-1200V) is the same and the drive frequency f is different. Fig. 6(A) shows the waveform of the voltage Vgs of the PFM signal input between the gate and source of the field effect transistor FET1. The horizontal axis represents time, and the vertical axis represents voltage.

[0038] FIG. 6(B) shows the drain-source voltage Vds of the field effect transistor FET1 when the drive frequency f is 170 kHz, which is deviated from the resonant frequency f0. The horizontal axis shows time. The vertical axis shows voltage. According to FIG. 6(B), the maximum value of the drain-source voltage Vds is 74 V. FIG. 6(B) shows the current IL flowing through the inductor L1 when the drive frequency f is 170 kHz, which is deviated from the resonant frequency f0. The horizontal axis shows time. The vertical axis shows current. The maximum value of the current IL is 127 mA. The temperature increase of the piezoelectric transformer 407 at this time was Δ20°C.

[0039] FIG. 6(D) shows the drain-source voltage Vds of the field effect transistor FET1 when the drive frequency f is the resonant frequency f0 (=165 kHz). The horizontal axis represents time. The vertical axis represents voltage. According to FIG. 6(D), the maximum value of the drain-source voltage Vds is 35 V. FIG. 6(E) shows the current IL flowing through the inductor L1 when the drive frequency f is the resonant frequency f0. The horizontal axis represents time. The vertical axis represents current. The maximum value of the current IL is 68 mA. The temperature increase of the piezoelectric transformer 407 at this time was Δ5°C.

[0040] FIG. 7(A) shows the relationship between the drive frequency f and the drain-source voltage Vds of the field effect transistor FET1 of the drive circuit 402. According to this, when the drive frequency f is the resonant frequency f0, the drain-source voltage Vds is at a minimum. FIG. 7(B) shows the relationship between the drive frequency f and the current flowing through the inductor L1. According to this, when the drive frequency f is the resonant frequency f0, the current IL flowing through the inductor L1 is at a minimum. FIG. 7(C) shows the temperature of the piezoelectric transformer 407. According to this, when the drive frequency f is the resonant frequency f0, the temperature is at a minimum.

[0041] 7(A) and 7(B), it can be seen that when the drive frequency f matches the resonant frequency f0 of the piezoelectric transformer 407, the drain-source voltage Vds, the current IL flowing through the inductor L1, and the temperature of the piezoelectric transformer 407 all reach their minimum values. Therefore, when the drive frequency f matches the resonant frequency f0 of the piezoelectric transformer 407, electronic components with low rated voltages and rated currents can be easily used as the field effect transistor FET1 and the inductor L1. Electronic components with low rated voltages and rated currents are generally inexpensive. This allows the manufacturing cost of the power supply device 400 to be reduced.

[0042] The resonant frequency f0 of the piezoelectric transformer 407 has manufacturing variations (individual differences). Therefore, it is necessary for the IC 401 to specify the resonant frequency f0. According to this embodiment, a voltage generated by dividing the drive voltage Vdrv is input to the input port 413. This allows the IC 401 to monitor the drive voltage Vdrv. That is, the IC 401 can output a PWM signal with a duty ratio corresponding to a certain target output voltage Vtar, while monitoring the drive voltage Vdrv while changing the frequency of the PFM signal. As shown in FIG. 5B, the frequency of the PFM signal (drive frequency f) when the drive voltage Vdrv is at its smallest coincides with the resonant frequency f0 of the piezoelectric transformer 407. Therefore, the IC 401 can specify the resonant frequency f0 by monitoring the drive voltage Vdrv.

[0043] When the image forming apparatus 100 or the power supply device 400 is shipped from the factory, they undergo a shipping inspection. At that time, the IC 401 outputs a PWM signal corresponding to a specific target output voltage Vtar (e.g., −1200V) and monitors the driving voltage Vtar while sweeping the frequency of the PFM signal. When the power supply device 400 is the charging power supply 21, the target output voltage Vtar is set to the charging voltage (e.g., −1200V), but this is only one example. As long as the voltage of each element constituting the power supply device 400 is equal to or lower than the rated voltage and the current of each element is equal to or lower than the rated current, the target output voltage Vtar may be another voltage value. At this time, the inspection tool is connected to the controller 200, and the frequency (driving frequency f) of the PFM signal when the driving voltage Vdrv is the smallest is stored in the NVRAM 202. When the image forming apparatus 100 is started by a user, the CPU 201 reads out the frequency from the NVRAM 202 and sets it in the power supply device 400. The IC 401 of the power supply device 400 outputs a PFM signal at a frequency set by the CPU 201 .

[0044] According to this embodiment, the drain-source voltage Vds of the field effect transistor FET1 of the drive circuit 402 and the current IL flowing through the inductor L1 are both reduced. Therefore, electronic components with low rated voltage and rated current can be used as the field effect transistor FET1 and the inductor L1. In other words, cheaper electronic components can be used. As a result, the manufacturing costs of the power supply device 400 and the image forming apparatus 100 are reduced.

[0045] Incidentally, instead of adopting electronic components with low rated voltage and rated current, the maximum output voltage of the power supply device 400 may be increased. In general, the higher the output voltage Vout, the greater the drain-source voltage Vds and the current IL flowing through the inductor L1. Therefore, the rated voltage and rated current of the electronic components may become the governing factor and limit the maximum output voltage. In this case, by driving the piezoelectric transformer 407 at the resonant frequency f0, the margin for the rated voltage and rated current becomes large. Therefore, it is also possible to increase the maximum output voltage of the power supply device 400.

[0046] <Example 2> In the first embodiment, the resonance frequency f0 is measured at the time of shipment from the factory and stored in the NVRAM 202. On the other hand, the second embodiment relates to a method for dynamically detecting the resonance frequency f0. The resonance frequency f0 of the piezoelectric transformer 407 may have temperature dependency. In this case, the resonance frequency f0 may change depending on the environment in which the image forming apparatus 100 is installed. In the second embodiment, even in such a case, the resonance frequency f0 is more accurately identified. For the technical matters of the second embodiment that are common to the first embodiment, the explanation of the first embodiment is used.

[0047] In the second embodiment, a first target output voltage Vtar1 and a second target output voltage Vtar2 are used as the target output voltage Vtar. The first target output voltage Vtar1 is a target output voltage set to detect the resonance frequency f0 of the piezoelectric transformer 407. The first target output voltage Vtar1 is set so that the voltage applied to each element constituting the power supply device 400 is equal to or lower than the rated voltage and the current flowing through each element is equal to or lower than the rated current when searching for the resonance frequency f0 of the piezoelectric transformer 407. The second target output voltage Vtar2 is a target output voltage that the power supply device 400 ultimately wants to output. In other words, the second target output voltage Vtar2 is a voltage (e.g., charging voltage, developing voltage, primary transfer voltage, secondary transfer voltage) required by the load 410. As long as there is no problem with the rated voltage and rated current of each element, the first target output voltage Vtar1 and the second target output voltage Vtar2 may be equal to each other.

[0048] 8 is a flowchart showing a method for controlling the output voltage in the second embodiment. This control method is executed by CPU 201 in cooperation with IC 401, but may be executed by IC 401 alone. In the latter case, IC 401 has an arithmetic circuit such as CPU 201 and a memory such as RAM 204. This control method can be executed, for example, when image forming apparatus 100 is instructed by a user to form an image.

[0049] In S801, the CPU 201 sets the first target output voltage Vtar1 in the IC 401. In this embodiment, the first target output voltage Vtar is set so that the voltage applied to each element constituting the power supply device 400 is equal to or lower than the rated voltage, and the current flowing through each element is equal to or lower than the rated current. The first target output voltage Vtar1 may be stored in the ROM 203. When the second target output voltage Vtar2 is −1200V, the first target output voltage Vtar1 may be set to −600V. The IC 401 determines a duty ratio corresponding to −600V, and outputs a PWM signal of the determined duty ratio from the output port 411.

[0050] In S802, the CPU 201 commands the IC 401, and the IC 401 outputs a PFM signal at a predetermined sweep start frequency f1. The sweep start frequency f1 is set so as to be higher than the resonance frequency f0 of the piezoelectric transformer 407 even if the resonance frequency f0 of the piezoelectric transformer 407 varies. For example, the sweep start frequency f1 is set to be equal to or higher than the sum of the nominal resonance frequency f0 and its maximum variation. The sweep start frequency f1 may be stored in the ROM 203.

[0051] In S803, the CPU 201 waits for the output voltage Vout to stabilize. For example, the CPU 201 may count a predetermined waiting time using a timer or counter circuit. Alternatively, the CPU 201 may have the IC 401 monitor the drive voltage Vdrv and wait for the amount of fluctuation in the drive voltage Vdrv to become sufficiently small. This is because when the output voltage Vout stabilizes, the drive voltage Vdrv also stabilizes.

[0052] In step S804, the CPU 201 detects the drive voltage Vdrv from the voltage input to the input port 413 of the IC 401, and stores the detection result in the variable V f Store in several V f is stored in RAM 204.

[0053] In S805, the CPU 201 instructs the IC 401 to lower the frequency (drive frequency f) of the PFM signal by about one step (frequency df). The IC 401 can change the frequency by one step, where one step may correspond to the resolution of the settable frequency. The IC 401 outputs a PFM signal having a frequency that is one step lower from the output port 412.

[0054] In S806, the CPU 201 waits for the output voltage Vout to stabilize. The method for determining whether the output voltage Vout is stable is as described in S803.

[0055] In step S807, the CPU 201 detects the drive voltage Vdrv from the voltage input to the input port 413 of the IC 401, and stores the detection result in the variable V f-1 Store in variable Vf-1 is stored in RAM 204.

[0056] CPU201 in S808 is V f -V f-1 It is determined whether V is less than 0. As shown in FIG. 5B, when the driving frequency f is higher than the resonance frequency f0, V f -V f-1 On the other hand, when the driving frequency f is lower than the resonant frequency f0, V f -V f-1 becomes negative. Therefore, V f -V f-1 The drive frequency f at which the sign of V changes is searched for. f -V f-1 If the variable V is 0 or a positive value, the CPU 201 proceeds from S808 to S821. f-1 V f Then, proceed to S805. f -V f-1 If the value is negative, the CPU 201 proceeds from step S808 to step S809.

[0057] In S809, the CPU 201 instructs the IC 401 to increase the frequency of the PFM signal by about one step (frequency df), thereby setting the drive frequency f to near the resonance frequency f0.

[0058] In S810, the CPU 201 sets the second target output voltage Vtar2 in the IC 401. The IC 401 determines a duty ratio corresponding to the second target output voltage Vtar2, and outputs from the output port 411 a PWM signal having the determined duty ratio.

[0059] In S811, the CPU 201 waits for the output voltage Vout to stabilize. The method for determining whether the output voltage Vout is stable is as described in S803.

[0060] According to the second embodiment, even if there is manufacturing variation in the resonance frequency f0 of the piezoelectric transformer 407, the frequency of the PFM signal coincides with or is close to the resonance frequency f0 of the piezoelectric transformer 407. That is, in the second embodiment, as in the first embodiment, electronic components with generally low cost rated voltages and rated currents can be adopted, and the manufacturing costs of the power supply device 400 and the image forming apparatus 100 are reduced. Furthermore, the second embodiment does not require the NVRAM 202 for storing the resonance frequency f0 of the piezoelectric transformer 407 measured at the time of shipment from the factory, that is, the manufacturing costs of the power supply device 400 are further reduced. As described in the first embodiment, the process of storing the frequency of the PFM signal when the drive voltage Vdrv is the smallest in the NVRAM 202 using an inspection tool at the time of shipment from the factory is not required.

[0061] In the second embodiment, the resonant frequency f0 for detecting the resonant frequency f0 of the piezoelectric transformer 407 is different from the second target output voltage Vtar2 required for the load 410. However, the second target output voltage Vtar2 may be used as the first target output voltage Vtar1 by adjusting the sweep start frequency f. In this case, even if there is variation in each element constituting the power supply device 400, a sweep start frequency f is required such that the voltage applied to each element is equal to or lower than the rated voltage and the current flowing through each element is equal to or lower than the rated current. In this case, in S801, the second target output voltage Vtar2 is set as the target output voltage Vatr instead of the first target output voltage Vtar1. Furthermore, S810 and S811 are omitted.

[0062] Thus, according to the second embodiment, the NVRAM 202 for storing the resonant frequency f0 of the piezoelectric transformer 407 in advance is not required. In addition, the drain-source voltage Vds of the field effect transistor FET1 of the drive circuit 402 and the current flowing through the inductor L1 can be reduced. Therefore, generally low-cost parts can be adopted for the field effect transistor FET1 and the inductor L1. As a result, the manufacturing costs of the power supply device 400 and the image forming apparatus 100 are reduced.

[0063] <Technical ideas derived from examples> (Item 1) 1. A power supply device for generating a predetermined output voltage, comprising: a control means for outputting a first pulse signal having a duty ratio corresponding to a target output voltage that is a target for the predetermined output voltage, and a second pulse signal having a predetermined frequency; a generating means for generating a drive voltage based on the first pulse signal and feedback information on an output voltage from the power supply device so that the output voltage approaches the target output voltage; a driving means for switching the driving voltage in response to the second pulse signal to generate a switching voltage; a piezoelectric transformer that boosts the switching voltage to generate an AC voltage; a rectifying means for rectifying the AC voltage to generate the output voltage; a feedback means for generating the feedback information for feeding back the output voltage to the drive voltage; a detection means for detecting the drive voltage and outputting voltage information indicating the drive voltage to the control means; The control means is configured to sweep the frequency of the second pulse signal to change the drive voltage, thereby identifying a frequency at which the drive voltage is lowest, or to read out a pre-identified frequency from a memory means, thereby generating the second pulse signal of that frequency.

[0064] The CPU 201 and the IC 401 are examples of a control means. The voltage adjustment circuit 404 is an example of a generation means. The drive circuit 402 is an example of a drive means. The rectification smoothing circuit 403 is an example of a rectification means. The resistors R11, R12, and R13 are examples of a feedback means. The resistors R21, R22, and the input port 413 are examples of a detection means. As described in the second embodiment, the CPU 201 and the IC 401 may specify a frequency (e.g., a resonant frequency f0) at which the drive voltage Vdrv is the lowest by sweeping the frequency f of the second pulse signal to change the drive voltage Vdrv. As described in the first embodiment, the CPU 201 and the IC 401 may generate a second pulse signal of a predetermined frequency by reading the frequency from a storage means (e.g., the NVRAM 202). This allows inexpensive electronic components to be used as electronic components constituting the power supply device 400, thereby reducing the manufacturing cost of the power supply device 400. (Item 2) 2. The power supply device according to item 1, wherein the frequency at which the drive voltage is lowest is equal to a resonant frequency of the piezoelectric transformer.

[0065] As described with reference to Figures 5(A) and 5(B), when the drive frequency f matches the resonant frequency f0, the drive voltage Vdrv is at its lowest. However, it is not necessary for the drive frequency f to match the resonant frequency f0 completely. This is because the effects targeted in the first and second embodiments are achieved as long as the difference between the drive frequency f and the resonant frequency f0 is sufficiently small. (Item 3) The control means Detecting the driving voltage every time the frequency of the second pulse signal is changed by about one step; A difference between the drive voltage detected earlier and the drive voltage detected later is calculated. Searching for a frequency at which the difference changes sign; 3. The power supply device according to item 1 or 2, wherein the frequency before the searched frequency is changed by about one step is identified as the frequency at which the drive voltage is the lowest.

[0066] As explained with respect to FIG. f and Vf-1 corresponds to the drive voltage detected earlier and the drive voltage detected later. As exemplified in S808, a drive frequency f at which the sign of the difference between them changes may be searched for. As exemplified in S809, a frequency before the searched frequency is changed by about one step may be specified as the frequency at which the drive voltage is the lowest. This will efficiently specify a frequency close to the resonant frequency f0 of the piezoelectric transformer 407. (Item 4) The control means Detecting the driving voltage every time the frequency of the second pulse signal is reduced by about one step; A difference between the drive voltage detected earlier and the drive voltage detected later is calculated. Searching for a frequency at which the sign of the difference changes from positive to negative; 4. The power supply device according to any one of items 1 to 3, wherein a frequency obtained by adding one step to the searched frequency is specified as the frequency at which the drive voltage is the lowest.

[0067] As illustrated in FIG. f and V f-1 A frequency at which the sign of the difference between the positive and negative frequencies may be searched for. As suggested in S809, a frequency obtained by adding a frequency df corresponding to one step to the searched frequency may be specified as the frequency at which the drive voltage Vdrv is the lowest. (Item 5) 5. The power supply device according to item 3 or 4, wherein an initial frequency used in the sweep is set to a frequency equal to or higher than the sum of the nominal resonant frequency of the piezoelectric transformer and the maximum variation of the resonant frequency.

[0068] As described in relation to S802, the drive frequency f1, which is the sweep start frequency, may be set to a frequency equal to or greater than the sum of the nominal resonant frequency f0 of the piezoelectric transformer 407 and the maximum variation in the resonant frequency f0. This will enable the frequency at which the drive voltage Vdrv is the lowest to be efficiently identified. (Item 6) the first pulse signal is a pulse width modulated (PWM) signal; 2. The power supply device of claim 1, wherein the second pulse signal is a pulse frequency modulated (PFM) signal.

[0069] These are just some examples of pulse signals: other types of pulse signals may be employed. (Item 7) 7. The power supply device according to any one of items 1 to 6, wherein the control means executes a specification process for specifying a frequency at which the drive voltage is lowest prior to starting the supply of the output voltage to a load.

[0070] As described in the second embodiment, a characterization process may be performed prior to providing an output voltage to the load 410. The characterization process may be referred to as a frequency sweep and search process. (Item 8) 8. The power supply device according to item 7, wherein a first target output voltage set as the target output voltage in the specification process is a voltage lower than a second target output voltage for the output voltage output to the load.

[0071] The first target output voltage Vtar1 may be lower than the second target output voltage Vtar2. If each element of the power supply device 400 has sufficient performance for the second target output voltage Vtar2, each element also has sufficient performance for the first target output voltage Vtar1. In other words, during the frequency sweep and search process, the voltage applied to each element will not exceed the rated voltage, and the current flowing through each element will not exceed the rated current. (Item 9) 9. The power supply device of claim 8, wherein the first target output voltage is less than or equal to half of the second target output voltage.

[0072] When the second target output voltage Vtar2 is −1200 V, the first target output voltage Vtar1 may be set to −600 V. In the first and second embodiments, the concept of high and low voltages is obtained by comparing the absolute values ​​of voltages. (Item 10) 10. The power supply device according to item 8 or 9, wherein the control means switches the target output voltage from the first target output voltage to the second target output voltage when the identification process is completed.

[0073] As shown in S810, when the frequency is specified, the target output voltage Vtar is switched from the first target output voltage Vtar1 to the second target output voltage Vtar2. This makes it possible to efficiently obtain the output voltage Vout equivalent to the second target output voltage Vtar2. (Item 11) 8. The power supply device according to item 7, wherein a first target output voltage set as the target output voltage in the specification process is equal to a second target output voltage for the output voltage output to the load.

[0074] As mentioned in the second embodiment, the first target output voltage Vtar1 and the second target output voltage Vtar2 may be equal to each other. This will reduce the time required from the start of control of the output voltage Vout until the output voltage Vout stabilizes. (Item 12) 2. The power supply device according to item 1, wherein the storage means is a non-volatile memory that stores the pre-specified frequency obtained when the power supply device is shipped from a factory.

[0075] As described in the first embodiment, a pre-specified frequency obtained when the power supply device 400 is shipped from the factory may be stored in the NVRAM 202. This will shorten the time required for the output voltage Vout to stabilize after control of the output voltage Vout starts. (Item 13) The generating means includes: a protective resistor having one end connected to the feedback means; an operational amplifier having a non-inverting input terminal connected to the other end of the protective resistor, an inverting input terminal connected to a conversion circuit that converts the first pulse signal into a DC voltage and outputs the DC voltage, and an output terminal that outputs an adjustment voltage corresponding to a difference between the feedback information input to the non-inverting input terminal via the protective resistor and the DC voltage input to the inverting input terminal; a capacitor having one end connected to the output terminal of the operational amplifier and the other end connected to the inverting input terminal; a transistor having a collector connected to a power supply voltage, a base connected to the output terminal of the operational amplifier, and an emitter for outputting the drive voltage; 13. The power supply device according to any one of items 1 to 12, comprising:

[0076] Resistor R15 is an example of a protective resistor. Resistor R16 and capacitor C17 are an example of a conversion circuit. (Item 14) The detection means is a voltage dividing circuit that divides the drive voltage; an AD converter that performs analog-to-digital conversion on an output of the voltage divider circuit to generate the voltage information; 14. The power supply device according to any one of items 1 to 13, comprising:

[0077] The resistors R21 and R22 are an example of a voltage divider circuit. The input port 413 is an example of an AD converter. (Item 15) The driving means is an inductor having one end to which the drive voltage is applied; a field effect transistor having a drain connected to the other end of the inductor and a first input electrode of the piezoelectric transformer, a gate to which the second pulse signal is input, and a source connected to a frame ground and a second input electrode of the piezoelectric transformer; a capacitor connected between the drain and the source; 15. The power supply device according to any one of items 1 to 14, comprising:

[0078] Capacitor C6 is an example of a capacitor. (Item 16) 16. The power supply device according to any one of items 1 to 15, wherein the rectifying means further includes a rectifying and smoothing circuit connected between a load of the power supply device and an output electrode of the piezoelectric transformer, and rectifying and smoothing the AC voltage generated at the output electrode of the piezoelectric transformer to generate the output voltage. (Item 17) 17. The power supply device according to any one of items 1 to 16, wherein the feedback means includes a plurality of resistive elements that divide the output voltage.

[0079] The resistors R11, R12, and R13 are examples of a plurality of resistive elements. (Item 18) an image forming means for forming an image on a sheet; A power supply device for supplying a high voltage to the image forming means, the power supply device being described in any one of items 1 to 17; An image forming apparatus comprising:

[0080] The image forming stations 10a to 10d and the secondary transfer roller 8 are an example of an image forming unit. The charging power supply 21, the developing power supply 22, the primary transfer power supply 23, and the secondary transfer power supply 24 are an example of a power supply device 400. (Item 19) The image forming means includes: A photoconductor; a charging means for uniformly charging the surface of the photoconductor using a charging voltage; an exposure unit for exposing the surface of the photoreceptor to light to form an electrostatic latent image; a developing means for developing the electrostatic latent image using a developing voltage to form a toner image; a transfer means for transferring the toner image to an intermediate transfer body or the sheet using a transfer voltage; 20. The image forming apparatus according to claim 18, wherein the power supply generates the charging voltage, the developing voltage, or the transfer voltage.

[0081] The primary transfer member 6 and the secondary transfer roller 8 are an example of a transfer means. The primary transfer member 6 may be any of a brush, a blade, or a roller.

[0082] The invention is not limited to the above-described embodiments, and various modifications and variations are possible without departing from the spirit and scope of the invention. Accordingly, the following claims are appended to apprise the public of the scope of the invention. [Explanation of symbols]

[0083] 400: power supply unit, 201: CPU, 401: IC, 404: voltage adjustment circuit, 402: drive circuit, 407: piezoelectric transformer, 403: rectification smoothing circuit, R11, R12, R13, R21, R22: resistors, 413: input port

Claims

1. 1. A power supply device for generating a predetermined output voltage, comprising: a control means for outputting a first pulse signal having a duty ratio corresponding to a target output voltage that is a target for the predetermined output voltage, and a second pulse signal having a predetermined frequency; a generating means for generating a drive voltage based on the first pulse signal and feedback information on an output voltage from the power supply device so that the output voltage approaches the target output voltage; a driving means for switching the driving voltage in response to the second pulse signal to generate a switching voltage; a piezoelectric transformer that boosts the switching voltage to generate an AC voltage; a rectifying means for rectifying the AC voltage to generate the output voltage; a feedback means for generating the feedback information for feeding back the output voltage to the drive voltage; a detection means for detecting the drive voltage and outputting voltage information indicating the drive voltage to the control means; The control means is configured to sweep the frequency of the second pulse signal to change the drive voltage, thereby identifying a frequency at which the drive voltage is lowest, or to read out a pre-identified frequency from a memory means, thereby generating the second pulse signal of that frequency.

2. 2. The power supply device according to claim 1, wherein the frequency at which the drive voltage is lowest is equal to a resonance frequency of the piezoelectric transformer.

3. The control means Detecting the driving voltage every time the frequency of the second pulse signal is changed by about one step; A difference between the drive voltage detected earlier and the drive voltage detected later is calculated. Searching for a frequency at which the difference changes sign; 2. The power supply device according to claim 1, wherein a frequency before the searched frequency is changed by about one step is specified as the frequency at which the drive voltage is the lowest.

4. The control means Detecting the driving voltage every time the frequency of the second pulse signal is reduced by about one step; A difference between the drive voltage detected earlier and the drive voltage detected later is calculated. Searching for a frequency at which the sign of the difference changes from positive to negative; The power supply device according to claim 1 , wherein a frequency obtained by adding one step to the searched frequency is specified as the frequency at which the drive voltage is the lowest.

5. 5. The power supply device according to claim 4, wherein the initial frequency used in the sweep is set to a frequency equal to or higher than the sum of a nominal resonant frequency of the piezoelectric transformer and a maximum variation in the resonant frequency.

6. the first pulse signal is a pulse width modulated (PWM) signal; 2. The power supply of claim 1, wherein the second pulse signal is a pulse frequency modulated (PFM) signal.

7. 2. The power supply device according to claim 1, wherein the control means executes a specification process for specifying a frequency at which the drive voltage becomes the lowest, prior to starting the supply of the output voltage to a load.

8. The power supply device according to claim 7 , wherein a first target output voltage set as the target output voltage in the specification process is a voltage lower than a second target output voltage for the output voltage output to the load.

9. 9. The power supply device of claim 8, wherein the first target output voltage is equal to or less than half of the second target output voltage.

10. The power supply device according to claim 8 , wherein the control means switches the target output voltage from the first target output voltage to the second target output voltage when the specifying process is completed.

11. The power supply device according to claim 7 , wherein a first target output voltage set as the target output voltage in the specification process is equal to a second target output voltage for the output voltage output to the load.

12. 2. The power supply device according to claim 1, wherein said storage means is a non-volatile memory that stores said pre-specified frequency obtained when said power supply device is shipped from a factory.

13. The generating means includes: a protective resistor having one end connected to the feedback means; an operational amplifier having a non-inverting input terminal connected to the other end of the protective resistor, an inverting input terminal connected to a conversion circuit that converts the first pulse signal into a DC voltage and outputs the DC voltage, and an output terminal that outputs an adjustment voltage corresponding to a difference between the feedback information input to the non-inverting input terminal via the protective resistor and the DC voltage input to the inverting input terminal; a capacitor having one end connected to the output terminal of the operational amplifier and the other end connected to the inverting input terminal; a transistor having a collector connected to a power supply voltage, a base connected to the output terminal of the operational amplifier, and an emitter for outputting the drive voltage; 2. The power supply of claim 1, further comprising:

14. The detection means is a voltage dividing circuit that divides the drive voltage; an AD converter that performs analog-to-digital conversion on an output of the voltage divider circuit to generate the voltage information; 2. The power supply of claim 1, further comprising:

15. The driving means is an inductor having one end to which the drive voltage is applied; a field effect transistor having a drain connected to the other end of the inductor and a first input electrode of the piezoelectric transformer, a gate to which the second pulse signal is input, and a source connected to a frame ground and a second input electrode of the piezoelectric transformer; a capacitor connected between the drain and the source; 2. The power supply of claim 1, further comprising:

16. 2. The power supply device according to claim 1, further comprising a rectifying and smoothing circuit connected between a load of the power supply device and an output electrode of the piezoelectric transformer, for rectifying and smoothing the AC voltage generated at the output electrode of the piezoelectric transformer to generate the output voltage.

17. 2. The power supply device according to claim 1, wherein said feedback means includes a plurality of resistive elements which divide said output voltage.

18. an image forming means for forming an image on a sheet; A power supply device for supplying a high voltage to the image forming unit, comprising: the power supply device according to any one of claims 1 to 17; An image forming apparatus comprising:

19. The image forming means includes: A photoconductor; a charging means for uniformly charging the surface of the photoconductor using a charging voltage; an exposure unit for exposing the surface of the photoreceptor to light to form an electrostatic latent image; a developing means for developing the electrostatic latent image using a developing voltage to form a toner image; a transfer means for transferring the toner image to an intermediate transfer body or the sheet using a transfer voltage; The image forming apparatus according to claim 18 , wherein the power supply generates the charging voltage, the developing voltage, or the transfer voltage.

Images