Power supply device and image forming apparatus

Abstract

To reduce the occurrence of ripple in output voltage and power consumption in a power supply device that can change the output voltage.SOLUTION: When increasing output voltage, if a target voltage is lower than a first threshold, a control unit outputs a control signal for a duty D1, and when the target voltage is switched to be equal to or higher than the first threshold, outputs a control signal for a duty D2 that is larger than the duty D1. When decreasing output voltage, if the target voltage is equal to or higher than a second threshold, the control unit outputs the control signal for the duty D2, and when the target voltage is switched to be equal to or lower than the second threshold, outputs the control signal for the duty D1. The first threshold and the second threshold are different from each other.SELECTED DRAWING: Figure 4

Description

Technical Field

[0001] The present invention relates to a power supply device and an image forming apparatus, and more particularly to a control method for a high-voltage power supply in an electrophotographic or electrostatic recording type image forming apparatus such as a copying machine or a printer.

Background Art

[0002] Conventionally, a high-voltage power supply including a transformer and a switching element that is turned on or off by a drive pulse signal has been proposed. Regarding the high-voltage power supply, a method of controlling the output voltage of the transformer, the frequency of the drive pulse signal, and the duty of the drive pulse signal in association with each other has been proposed (see, for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the conventional circuit, the duty of the drive pulse signal was selected according to the range of the output voltage. Therefore, when the range of the determined output voltage is exceeded, the duty of the drive pulse signal is switched. That is, when changing the output voltage, if the range of the determined output voltage is exceeded, it is necessary to switch the frequency or duty of the drive pulse signal. For this reason, when these are switched, there is a risk that a ripple may occur in the output voltage or the power of a switching element that is turned on or off by the drive pulse signal, for example, a field effect transistor may increase. Therefore, when the high-voltage power supply is mounted on an image forming apparatus, it may be desirable to avoid switching the duty of the drive pulse signal when changing the output voltage within one job.

[0005] The present invention has been made under such circumstances, and an object thereof is to reduce the generation of ripple in the output voltage and the power consumption in a power supply device capable of changing the output voltage.

Means for Solving the Problems

[0006] In order to solve the above-described problems, the present invention includes the following configuration. (1) A transformer having a primary winding and a secondary winding, a switching element connected in series to the primary winding and turned on or off in response to an input control signal, control means for outputting the control signal having a predetermined duty to the switching element, and stop means for stopping the input of the control signal to the switching element so that the output voltage output from the secondary side of the transformer approaches a target voltage, the control means being a power supply device capable of changing the output voltage by switching the target voltage, and when the control means increases the output voltage, the control signal having a first duty is output in a state where the target voltage is lower than a first threshold value, and when the target voltage switches to a state equal to or higher than the first threshold value, the control signal having a second duty larger than the first duty is output, and when the control means decreases the output voltage, the control signal having the second duty is output in a state where the target voltage is equal to or higher than a second threshold value, and when the target voltage switches to a state lower than the second threshold value, the control signal having the first duty is output. The absolute value of the second threshold is greater than the absolute value of the first threshold A power supply device characterized by the above. (2) A power supply device comprising a transformer having a primary winding and a secondary winding, a switching element connected in series to the primary winding and turning on or off according to an input control signal, control means for outputting the control signal of a predetermined frequency to the switching element, and stop means for stopping the input of the control signal to the switching element so that the output voltage output from the secondary side of the transformer approaches a target voltage. The control means can change the output voltage by switching the target voltage. When increasing the output voltage, the control means outputs the control signal of a first frequency in a state where the target voltage is lower than a first threshold value, and outputs a control signal of a second frequency higher than the first frequency when the target voltage switches to a state equal to or higher than the first threshold value. When decreasing the output voltage, the control means outputs the control signal of the second frequency in a state where the target voltage is equal to or higher than a second threshold value, and outputs the control signal of the first frequency when the target voltage switches to a state lower than the second threshold value. The absolute value of the second threshold is greater than the absolute value of the first threshold A power supply device characterized by the above. (3) An image forming apparatus comprising image forming means for forming an image on a recording medium and the power supply device according to (1) or (2).

Effects of the Invention

[0007] According to the present invention, in a power supply device capable of changing an output voltage, it is possible to reduce the generation of ripple of the output voltage and power consumption.

Brief Description of the Drawings

[0008]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Embodiments for Carrying Out the Invention

[0009] Hereinafter, embodiments for carrying out the present invention will be described in detail with reference to the drawings by way of examples.

Examples

[0010] In Example 1, the threshold value Vth1 of the output voltage during the sequence of increasing the voltage and the threshold value Vth2 of the output voltage during the sequence of decreasing the voltage are set to different values. A method for selecting the duty and frequency of an optimal drive signal (S202 described later) according to the initial output voltage will be described.

[0011] [Configuration of Image Forming Apparatus] FIG. 1 is a configuration diagram showing an image forming apparatus of a tandem type (4-drum system) employing an intermediate transfer belt. Each component of the image forming apparatus 1 is as follows. The paper feed cassette 2 is a cassette for storing a sheet P which is a recording medium. The image forming control unit (hereinafter referred to as the control unit), which is a control means, 3 controls the image forming operation of the image forming apparatus 1. The paper feed roller 4 is a roller for feeding the sheet P from the paper feed cassette 2, and the conveying roller 5 is a roller for conveying the fed sheet P. The photosensitive drums 8Y, 8M, 8C, 8K, which are image carriers, carry developers of each color of yellow, magenta, cyan, and black. Note that the subscripts Y, M, C, K of the reference numerals represent each color, and the subscripts are omitted unless explaining a member of a specific color.

[0012] The charging roller 9 is a roller for uniformly charging the photosensitive drum 8 to a predetermined potential. The optical unit 10 irradiates a laser beam corresponding to the image data of each color onto the photosensitive drum 8 charged by the charging roller 9 to form an electrostatic latent image. The developing device 11 visualizes the electrostatic latent image formed on the photosensitive drum 8 to form a toner image. The developing roller 12 is a roller for sending out the developer in the developing device 11 to the portion facing the photosensitive drum 8. The developing blade 13, which is a developing regulation member, is a member for regulating the toner on the developing roller 12 into a thin layer. The supply roller 14 is a roller for supplying toner to the developing roller 12. Also, it has a function of stripping off the excessively supplied toner by switching (changing) the voltage applied to the supply roller 14. The voltage applied to the charging roller 9 is called the charging voltage, the voltage applied to the developing roller 12 is called the developing roller voltage, the voltage applied to the developing blade 13 is called the developing blade voltage, and the voltage applied to the supply roller 14 is called the supply roller voltage.

[0013] The primary transfer roller 16 is a roller for transferring the toner image formed on the photosensitive drum 8 (hereinafter referred to as primary transfer). The intermediate transfer belt 17 is a belt for carrying the toner image transferred in the primary transfer. The driving roller 18 drives the intermediate transfer belt 17. The secondary transfer roller 19 is a roller for transferring the toner image transferred onto the intermediate transfer belt 17 to the sheet P (hereinafter referred to as secondary transfer), and the secondary transfer opposing roller 20 is a roller opposing the secondary transfer roller 19. The fixing unit 21 melts and fixes the unfixed toner image transferred to the sheet P while conveying the sheet P. The discharge roller 22 discharges the sheet P on which fixing has been performed by the fixing unit 21 to the tray 23.

[0014] Next, the image forming operation of the image forming apparatus 1 will be described. When print data including a print command, image information, etc. is input from a host computer (not shown) or the like to the control unit 3, the image forming apparatus 1 starts a printing operation, and the sheet P is fed from the paper feed cassette 2 by the paper feed roller 4 and sent out onto the conveyance path. Along with the operation of feeding the sheet P, the photosensitive drum 8 is charged to a certain potential by the charging roller 9. In accordance with the input print data, the optical unit 10 performs exposure scanning on the surface of the charged photosensitive drum 8 with a laser beam to form an electrostatic latent image. Development is performed by the developing unit 11, the developing roller 12, the developing blade 13, and the supply roller 14 in order to visualize the formed electrostatic latent image. The electrostatic latent image formed on the surface of the photosensitive drum 8 is developed as a toner image in each color by the developing unit 11. The photosensitive drum 8 is in contact with the intermediate transfer belt 17 and rotates in synchronization with the rotation of the intermediate transfer belt 17. The developed toner images of each color are sequentially multi-transferred onto the intermediate transfer belt 17 by the primary transfer rollers 16Y, 16M, 16C, and 16K to form a color toner image. Then, the color toner image is secondarily transferred onto the sheet P by the secondary transfer roller 19 and the secondary transfer counter roller 20. The toner image transferred onto the sheet P is fixed by a fixing unit 21 composed of a fixing roller or the like. The fixed sheet P is discharged to the tray 23 by the discharge roller 22, and the image forming operation is completed. Note that the members that contribute to forming an image on the sheet P function as image forming means.

[0015] [Power supply device] FIG. 2(a) is a diagram showing an example of a circuit of a high voltage power supply unit 200 which is the power supply device of Example 1. The high voltage power supply unit 200 includes a transformer 201, an Nch-type field effect transistor (hereinafter referred to as FET) 203, a comparator 204, an electrolytic capacitor 205, a ceramic capacitor 206, a diode 207, and fixed resistors 208 to 211. The transformer 201 has a primary winding 201a and a secondary winding 201b. V1 indicates an input voltage, and V2 indicates a reference voltage.

[0016] Next, the operation of the high-voltage power supply unit 200 will be described with reference to FIG. 2(a). A FET 203, which is a switching element, is connected in series to the primary winding 201a of the transformer 201. A drive signal S202, which is a control signal, is a pulse signal sent from the control unit 3 and is input to the gate terminal of the FET 203 to drive the transformer 201 by the FET 203. When the drive signal S202 is input to the gate terminal of the FET 203, the FET 203 performs a switching operation of repeating on or off (hereinafter referred to as on / off) according to the duty and frequency of the drive signal S202. By turning the FET 203 on / off, a DC voltage across the electrolytic capacitor 205 is applied as a pulsed waveform to the primary winding 201a of the transformer 201. As a result, a negative voltage is generated by the secondary winding 201b of the transformer 201, the ceramic capacitor 206, and the diode 207. That is, a pulsed voltage of the same period that has been boosted is output from the secondary side of the transformer 201. The pulsed voltage output from the secondary side of the transformer 201 is rectified and smoothed by a rectifying and smoothing circuit composed of the diode 207 and the ceramic capacitor 206. Then, a high-voltage DC voltage VA, which is the output voltage of the high-voltage power supply unit 200 (hereinafter simply referred to as the output voltage), appears across the ceramic capacitor 206.

[0017] The resistor 208 is a bleeder resistor for obtaining a stable output voltage. The input signal S201 is a signal for adjusting the DC voltage VA sent from the control unit 3, and is a DC voltage corresponding to the DC voltage VA, hereinafter referred to as the target voltage signal S201. The target voltage signal S201 is input to the - terminal (inverting input terminal) of the comparator 204. To the + terminal (non-inverting input terminal) of the comparator 204, a voltage (hereinafter referred to as the feedback voltage) obtained by dividing the DC voltage VA by the resistors 209, reference voltage V2, resistor 210, and resistor 211 is input. The output terminal of the comparator 204 is connected to the gate terminal of the FET 203. When the output voltage increases in absolute value and the voltage input to the + terminal of the comparator 204 becomes smaller than the target voltage input to the - terminal, the voltage at the output terminal of the comparator 204 becomes a low level, and the gate voltage of the FET 203 becomes 0V.

[0018] In this way, during a predetermined period in which the voltage at the + terminal of the comparator 204 is smaller than the target voltage at the - terminal, the state in which the output of the drive signal S202 is stopped is referred to as the "state in which the drive signal S202 is thinned out". When the drive signal S202 is thinned out, the drive of the transformer 201 stops, so the output voltage decreases in absolute value. In this way, when the feedback voltage (+ terminal) of the comparator 204 is lower than the target voltage signal S201 (- terminal), the comparator 204 stops the drive of the transformer 201 by the FET 203 regardless of the drive signal S202. After that, when the feedback voltage (+ terminal) of the comparator 204 becomes larger than the target voltage signal S201 (- terminal), the FET 203 resumes the on / off (switching operation) by the drive signal S202, and the output voltage increases again in absolute value. By repeating such an operation, the output voltage is maintained at the target voltage. The comparator 204 functions as a stopping means for stopping the input of the drive signal S202 to the FET 203 when the output voltage output from the secondary side of the transformer 201 exceeds the target voltage.

[0019] Since the high-voltage power supply unit 200 in Fig. 2(a) has such circuit operation, inevitably, a ripple is likely to occur in the output voltage (DC voltage VA). Furthermore, the ripple of the output voltage is correlated with the target output voltage (the above-mentioned target voltage, hereinafter referred to as the target output voltage). Fig. 2(b) shows the output characteristics of the high-voltage power supply unit 200. In Fig. 2(b), the horizontal axis represents the on-duty of the drive signal S202, and the vertical axis represents the output voltage (-) of the high-voltage power supply unit 200. Note that the duty of the drive signal S202 may be the off-duty, and in this case, the correlation is reversed. Here, as shown in Fig. 2(b), the output characteristics of the high-voltage power supply unit 200 are such that the more the on-duty of the drive signal S202 increases, the larger the negative output voltage becomes, in other words, the larger the absolute value of the DC voltage VA becomes. f1, R, Lmax, V(a), and V(b) will be described later.

[0020] [Output waveform of high-voltage power supply unit 200] Figs. 3(a) and (b) show the output waveforms of the high-voltage power supply unit 200. The high-voltage power supply unit 200 can change the DC voltage VA by switching the target output voltage. Fig. 3(a) shows each waveform when the target output voltage is V(a), and (b) shows each waveform when the target output voltage is V(b). Also, the relationship between the target output voltage V(a) and the target output voltage V(b) is defined by the following equation (1). |V(b)| < |V(a)| ··· (1) In Figs. 3(a) and (b), with the horizontal axis being time, (i) shows the waveform of the drive signal S202, and (ii) shows the output voltage (DC voltage VA) of the high-voltage power supply unit 200. Note that in (ii), the target output voltage (V(a) or V(b)) is shown by a dashed line.

[0021] From the output characteristics shown in Fig. 2(b), when the frequency f of the drive signal S202 is f1 and the duty ratio is D, the maximum output voltage of the high-voltage power supply unit 200 is VLmax. Here, in Fig. 2(b), the load impedance is R, and the transformer 201 with an L value of Lmax is used. When the target output voltage V(a) is close to the maximum output voltage VLmax, as shown in the waveform of Fig. 3(a), since the decimation of the drive signal S202 is small, the ripple of the output voltage is small. On the other hand, when the target output voltage is V(b) lower than the target output voltage V(a), as shown in the waveform of Fig. 3(b), the decimation of the drive signal S202 becomes larger compared to Fig. 3(a), and the ripple of the output voltage becomes larger.

[0022] That is, in order to suppress the ripple of the output voltage, the frequency f and duty ratio D of the drive signal S202 may be adjusted to reduce the difference between the output voltage of the high-voltage power supply unit 200 and the target output voltage. Ideally, the output voltage and the target output voltage should be made equal, and the drive signal S202 should be driven in a state where it is not decimated at all.

[0023] [For the case of generating the charging voltage] Next, a sequence when the high-voltage power supply unit 200 is applied to the charging voltage will be described. When the output voltage is used as the charging voltage, the charging voltage is usually output as a fixed voltage. However, in order to follow, for example, the density change of an image due to temperature rise that occurs during continuous printing, the output voltage may be changed according to a plurality of conditions such as the number of printed sheets and temperature. However, when switching the on-duty of the drive signal S202 in a state where the output voltage is applied during one job of continuous printing, the following problems occur. That is, a ripple occurs in the output voltage when switching, or the power consumption of the FET 203 turned on and off by the drive signal S202 increases. Therefore, it is desirable to avoid changing the duty of the drive signal S202 within one job. For this reason, the control unit 3 changes the output voltage using one duty without switching the duty as long as the ripple of the output voltage is within a range smaller than the allowable output ripple Y described later. As described above, in order to change the output voltage using one duty, it is performed by adjusting the sampling amount of the drive signal S202 by the comparator 204. Thereby, the output voltage can be made closer to the target voltage.

[0024] FIG. 4(a), (b), and (c) show diagrams showing the relationship between the output voltage (-) on the horizontal axis and the ripple of the output voltage on the vertical axis. The operation when the on-duty of the drive signal S202 is switched at a predetermined output voltage will be described. Here, the threshold value of the output voltage for setting the duty is defined as the threshold value Vth1. As an example, the initial output voltage (hereinafter referred to as the initial output voltage) Vi1 is -990V, the initial output voltage Vi2 is -1010V, and the threshold value Vth1 is -1000V. The characteristics when the duty of the drive signal S202 is D1 are indicated by a broken line, and the characteristics when it is D2 are indicated by a solid line.

[0025] When the duty is the first duty D1, the limit of the output voltage is the output voltage V(D1). On the other hand, when the duty is the second duty D2, the limit of the output voltage is the output voltage V(D2), and the respective duties have the relationship D1 < D2. The absolute value of the maximum value (V(D2)) of the output voltage output according to the duty D2 is larger than the absolute value of the maximum value (V(D1)) of the output voltage output according to the duty D1. As an example, let the output voltage V(D1) be -1050V and the output voltage V(D2) be -1200V. For both duties D1 and D2, the larger the output voltage (in absolute value), the smaller the ripple. Also, the ripple (Y in Fig. 4(a)) when a predetermined output voltage (for example, -1000V) is output according to the duty D2 is larger than the ripple Y1 when substantially the same output voltage as the predetermined output voltage is output according to the duty D1 (Y > Y1).

[0026] (When the output voltage is made larger than the initial output voltage) Using Fig. 4(a), a sequence in which the initial output voltage Vi1 is -990V and the output voltage is increased (in absolute value) will be described. The allowable ripple of the output voltage (hereinafter referred to as the allowable output ripple) is indicated by Y and shown by a dotted line. If the ripple of the output voltage becomes larger than the allowable output voltage ripple Y, the output voltage may not stabilize and image defects may occur. The threshold value Vth1 is the output voltage when the FET203 is turned on or off with the duty D2 of the drive signal S202 such that the ripple of the output voltage is the maximum allowable ripple, that is, the allowable output voltage ripple Y. Therefore, when the initial output voltage Vi1 is -990V, since the generated ripple exceeds the allowable output voltage ripple Y, the duty of the drive signal S202 cannot be driven with D2. Since the initial output voltage Vi1 (-990V) is smaller than the threshold value Vth1 (-1000V), the duty of the drive signal S202 is driven with D1. In this case, the generated ripple becomes equal to or less than the allowable output voltage ripple Y. From here, the target voltage signal S201 is changed (switched), the output voltage is increased, and it can be changed up to the output voltage V(D1) (-1050V), which is the limit of the output voltage of the duty D1 of the drive signal S202.

[0027] That is, when increasing the output voltage from the initial output voltage Vi1, in order to maintain the generated ripple below the allowable output voltage ripple Y, the control unit 3 can change within the range from -990V to -1050V while keeping the duty D1. When the initial output voltage is equal to or higher than the threshold value Vth1 (equal to or higher than the threshold value) and the output voltage is increased, the output voltage will be increased while keeping the duty D2. The threshold value Vth1 in FIG. 4(a) can also be said to be a threshold value for setting the initial duty according to the initial output voltage.

[0028] (When the output voltage is made smaller than the initial output voltage) Next, with reference to FIG. 4(b), a sequence in which the initial output voltage Vi2 is -1010V and the output voltage is decreased (in absolute value) will be described. Since the initial output voltage Vi2 (-1010V) is higher than the threshold value Vth1 (-1000V), first, the duty of the drive signal S202 is driven at D2. From here, the target voltage signal S201 is changed to decrease the output voltage, and it can be made possible to change the output voltage to -1000V, which is the allowable output voltage ripple Y of the duty D2 of the drive signal S202. However, when the control unit 3 changes the output voltage, if the output voltage becomes lower than the threshold value Vth1, the ripple generated while keeping the duty D2 will exceed the allowable output voltage ripple Y. Therefore, the duty is switched from D2 to D1 at the threshold value Vth1.

[0029] Then, in FIG. 4(b), the range in which the output voltage can be changed at the duty D2 can only be set to a mere 10 V from the initial output voltage Vi2 (-1010 V) to the allowable output voltage ripple Y. That is, the variable range of the output voltage is only 10 V. Therefore, in the sequence of decreasing the output voltage, the duty of the drive signal S202 is switched at a threshold Vth2 (Vth2 > Vth1) whose (absolute value) is larger than the threshold Vth1. That is, the control unit 3 sets different predetermined thresholds depending on whether the output voltage is increased from the initial output voltage or decreased from the initial output voltage. With reference to FIG. 4(c), the sequence of decreasing the output voltage with the initial output voltage Vi2 being -1010 V in the first embodiment will be described. In FIG. 4(c), the output voltage V(D1) (-1050 V), which is the limit of the output voltage at the duty D1 of the drive signal S202, is set as the threshold Vth2.

[0030] Since the initial output voltage Vi2 (-1010 V) is smaller than the threshold Vth2 (-1050 V) (Vi2 < Vth2 (= V(D1))), the drive signal S202 is driven at the duty D1. From here, by changing the target voltage signal S201, the output voltage can be decreased until it reaches the output voltage VY(D1) that is the allowable output voltage ripple Y at the duty D1 of the drive signal S202. At this time, the range in which the output voltage can be changed is the difference between the initial output voltage Vi2 and the output voltage VY(D1), which is larger than 10 V in the example of FIG. 4(b) (|Vi2| - |VY(D1)| > |Vi2| - |Vth1|).

[0031] Thus, in the sequence of decreasing the output voltage, the duty is switched using a second threshold value Vth2 (> Vth1), which is different from the first threshold value Vth1 of the output voltage in the sequence of increasing the output voltage. By setting the duty according to the initial output voltage and the threshold value Vth2, in FIG. 4(b), the output voltage could only be changed within a range of 10V, whereas in FIG. 4(c), the following occurs. That is, the output voltage can be greatly changed with a single duty without switching the duty. When the initial output voltage is greater than the threshold value Vth2 and the output voltage is to be decreased, first, it operates with duty D2, and the duty is switched from duty D2 to duty D1 at the threshold value Vth2 to decrease the output voltage.

[0032] As described above, the threshold value Vth1 during the sequence of increasing the voltage and the threshold value Vth2 during the sequence of decreasing the voltage are set to different values. Thereby, according to the initial output voltage, the duty of the optimal drive signal S202 can be selected respectively in the sequence of increasing the output voltage and the sequence of decreasing the output voltage.

[0033] [Regarding the switching of the frequency] Also, when the frequency is switched, the operation is the same as the operation when the duty is switched. FIGS. 5(a), (b), and (c) show diagrams showing the relationship between the output voltage ( - ) on the horizontal axis and the ripple of the output voltage on the vertical axis. The operation when the frequency of the drive signal S202 is switched at a predetermined output voltage will be described.

[0034] The limit of the output voltage at the frequency f1 which is the first frequency is the output voltage V(f1). On the other hand, the limit of the output voltage at the frequency f2 which is the second frequency is the output voltage V(f2), and the respective frequencies have the relationship of f1 < f2. The absolute value of the maximum value (V(f2)) of the output voltage output according to the frequency f2 is larger than the absolute value of the maximum value (V(f1)) of the output voltage output according to the frequency f1. As an example, let the output voltage V(f1) be -1050V and the output voltage V(f2) be -1200V. For both frequencies f1 and f2, the larger the output voltage (in absolute value), the smaller the ripple. Also, the ripple (Yf in FIG. 5(a)) when a predetermined output voltage (for example, -1000V) is output according to the frequency f2 is larger than the ripple Yf1 when substantially the same output voltage as the predetermined output voltage is output according to the frequency f1 (Yf > Yf1).

[0035] As shown in FIGS. 5(b) and 5(c), when the frequency is switched, similar to when the duty is switched, the threshold value Vth1 in the sequence of increasing the voltage and the threshold value Vth2 in the sequence of decreasing the voltage are set to different values. Thereby, according to the initial output voltage, in the sequence of increasing the output voltage and the sequence of decreasing the output voltage, the optimal frequency of the drive signal S202 can be selected respectively.

[0036] Also, in the first embodiment, the charging voltage was described as an example, but it is not limited to the charging voltage. Further, the image forming apparatus 1 to which the high voltage power supply unit 200 of the first embodiment can be applied is not limited to the configuration shown in FIG. 1. Furthermore, the control means for outputting the drive signal to the FET 203 may not be the control unit 3, and may be control means included in the high voltage power supply unit 200 separately from the control unit 3. As described above, according to the first embodiment, in a power supply device capable of changing the output voltage, it is possible to reduce the generation of ripple and power consumption of the output voltage.

Embodiment

[0037] In Embodiment 1, regardless of the load of the output voltage (DC voltage VA), a threshold value of the output voltage for switching the duty of the drive signal S202 is set according to whether the sequence increases or decreases the output voltage from the initial output voltage. It is assumed that the load of the image forming apparatus 1 has no influence. For example, in the case of the charging process, the load of the photosensitive drum 8 is determined, and in the case of the transfer process, the load of the primary transfer roller 16 or the like is determined. However, since the resistance value of the load changes due to component variations, environmental temperature and humidity, the moisture absorption state of the paper (sheet P), etc., a design considering such variations in the resistance value of the load is necessary. Therefore, in Embodiment 2, a configuration is provided for switching the first threshold value and / or the second threshold value. That is, a method will be described in which the load current is monitored using the load current detection circuit, and the drive signal S202 is set to appropriate drive conditions by changing the threshold value of the output voltage.

[0038] [Power supply device] FIG. 6 shows an example of the circuit of the high voltage power supply unit 500 which is the power supply device of Embodiment 2. Since the same reference numerals, the operations of the transformer 201 and the comparator 204, and the image forming apparatus 1 are the same as those in Embodiment 1, the description thereof will be omitted, and the load current detection circuit 501 which is a detection means for detecting the current flowing through the load will be described. In Embodiment 2, the control unit 3 varies the second threshold value based on the detection result by the load current detection circuit 501. The load current detection circuit 501 includes an operational amplifier 504 and resistors 505 to 508.

[0039] The current flowing through the secondary side of the transformer 201 is supplied from the high-voltage power supply unit 500 to the load of the image forming apparatus 1, passes through the resistor 508 and the resistor 507 from the GND (not shown) of the operational amplifier 504, and returns to the transformer 201. Therefore, the load current can be calculated from the voltage difference across both ends of the resistor 507. The - terminal of the operational amplifier 504 has the same voltage as the + terminal of the operational amplifier 504. The + terminal of the operational amplifier 504 is known because it is determined by the reference voltage V2, the resistor 505, and the resistor 506. The load current detection circuit 501 outputs a current detection signal S203, and the current detection signal S203 is input to the AD port of the control unit 3. Therefore, the control unit 3 can calculate the load current from the known voltage and the voltage difference across both ends of the resistor 507.

[0040] By the way, as described in the first embodiment, the DC voltage VA, which is the output voltage generated by driving the transformer 201 by the switching operation of the FET 203, is supplied to the resistor 209, the resistor 208, and the load. Therefore, when the load current increases or decreases, it is controlled by the comparator 204 so that the power supplied from the transformer 201 increases or decreases, and the thinning of the drive signal S202 by the comparator 204 increases or decreases. Since the thinning by the comparator 204 increases or decreases according to the load current, when the load current changes greatly, the relationship between the output voltage and the ripple of the output voltage also changes. For example, in the case of the photosensitive drum 8 to which the charging voltage is applied, the load current is large at the initial stage because the film thickness formed on the surface of the photosensitive drum 8 is thick, but as the use progresses and the surface of the photosensitive drum 8 is worn away, the film thickness becomes thin and the load current becomes small. These currents can be calculated before the image forming process (for example, at the time of starting the image forming apparatus).

[0041] [Relationship between Load Current and Ripple] Next, a sequence for reducing the output voltage will be described with reference to FIGS. 7(a) and 7(b). When the load current is large in FIG. 7(a) and when the load current is small in FIG. 7(b), the relationships between the output voltage (-) and the ripple are shown for the case of duty D1 (broken line) and the case of duty D2 (solid line) in the same manner as in the first embodiment. Note that the graph in FIG. 7 is the same graph as in FIG. 4. When the load current detected by the load current detection circuit 501 is large, as shown in FIG. 7(a), the ripple when a predetermined output voltage Vα is output at duty D1 is Ya. When the load current detected by the load current detection circuit 501 is small, as shown in FIG. 7(b), the ripple when a predetermined output voltage Vα is output at duty D1 is Yb. Thus, when the same duty and the same output voltage are used, the ripple of the output voltage becomes smaller as the load current is larger (Ya < Yb).

[0042] [When reducing the output voltage from the initial output voltage] Next, a sequence for reducing the output voltage from the initial output voltage Vi3 will be described. Let the threshold value in the sequence for increasing the output voltage when the load current is large be Vth1, and the threshold value in the sequence for decreasing the output voltage be Vth2. Also, let the threshold value in the sequence for increasing the output voltage when the load current is small be Vth3, and the threshold value in the sequence for decreasing the output voltage be Vth4. The same applies to FIG. 8. As an example, let the initial output voltage Vi3 be -1100V, the threshold value Vth1 be -1000V, and the threshold value Vth2 (|Vth2| > |Vth1|) be -1050V. Also, let the threshold value Vth3 be -1075V (|Vth3| > |Vth2|), and the threshold value Vth4 be -1175V (|Vth4| > |Vth3|). In the following description, when the load current is large, it means, for example, when the load current is larger than a predetermined value, and when the load current is small, it means, for example, when the load current is equal to or less than the predetermined value.

[0043] (When the load current is large) When the load current is large, as shown in Fig. 7(a), since the absolute value of the initial output voltage Vi3 (-1100V) is larger than the threshold voltage Vth2 (-1050V), it operates with duty ratio D2. From this point, the target voltage signal S201 is changed to decrease the output voltage. When the threshold voltage Vth2 is reached, the duty ratio is switched from D2 to D1 to further decrease the output voltage. Note that the output voltage may also be decreased while maintaining the duty ratio D2 without switching at the threshold voltage Vth2. In this case, the output voltage can be changed to the allowable output voltage ripple Y of the duty ratio D2 of the drive signal S202, i.e., VY(D2). When the initial output voltage is smaller than the threshold voltage Vth2 and the output voltage is to be decreased, it operates with duty ratio D1 and the output voltage is decreased as it is.

[0044] (When the load current is small) When the load current is small, the characteristics are as shown in Fig. 7(b). Specifically, when the load current is small, the output voltage that becomes the allowable output voltage ripple Y when the duty of the drive signal S202 is D2 is, for example, -1075V. Since the initial output voltage Vi3 is larger than the threshold Vth2 (-1050V), when operating with the duty D2, only 25V can be changed up to -1075V (=Vth3) which is the allowable output voltage ripple Y of the duty D2. That is, when the load current is small, when the duty is D2, the variable range of the output voltage is only 25V. Therefore, in the second embodiment, when the load current is small, as shown in Fig. 7(b), the duty is switched with the threshold Vth4 (-1175V) as the boundary. That is, since the initial output voltage Vi3 (-1100V) is smaller than the threshold Vth4 (-1175V), it operates with the duty D1. Thereby, a large voltage variation up to the voltage VY(D1) which becomes the allowable output voltage ripple Y of the duty D1 when the load current is small can be achieved. At this time, the range in which the output voltage can be changed is the difference between the initial output voltage Vi3 and the output voltage VY(D1), which is larger than 25V (|Vi3| - |VY(D1)| > |Vi3| - |Vth3|). When the initial output voltage is larger than the threshold Vth4 and the output voltage is decreased, first, it operates with the duty D2, and the duty is switched from D2 to D1 at the threshold Vth4 to decrease the output voltage.

[0045] As described above, when the control unit 3 decreases the output voltage from the initial output voltage, the following control is performed based on the detection result of the load current detection circuit 501. That is, when the current detected by the load current detection circuit 501 is equal to or less than a predetermined value, the control unit 3 sets the threshold for switching the duty to the threshold Vth4 which is the third threshold. Here, the threshold Vth4 is larger than the threshold Vth2 which is the second threshold for switching the duty when the current detected by the load current detection circuit 501 is larger than the predetermined value.

[0046] [When increasing the output voltage from the initial output voltage] Next, the sequence for increasing the output voltage in FIGS. 8(a) and 8(b) will be described. When the load current is large in FIG. 8(a) and when the load current is small in FIG. 8(b), the relationships between the output voltage and the ripple are shown for the case of duty D1 (dashed line) and the case of duty D2 (solid line) in the same manner as in Example 1. Next, the sequence for increasing the output voltage from the initial output voltage Vi4 will be described. As an example, the initial output voltage Vi4 is set to -1025V.

[0047] (When the load current is large) When the load current is large, as shown in FIG. 8(a), since the initial output voltage Vi4 (-1025V) is greater than the threshold voltage Vth1 (-1000V), it operates with duty D2. From here, the target voltage signal S201 is changed to increase the output voltage, and it can be changed up to the output voltage at which the output capacity limit of the duty D2 of the drive signal S202 is reached. When the initial output voltage is less than the threshold voltage Vth1 and the output voltage is to be increased, first, it operates with duty D1, and when it reaches the threshold voltage Vth1, it switches from duty D1 to duty D2 to decrease the output voltage. Also, even if the output voltage is increased while remaining at duty D1, since the generated ripple maintains below the allowable output voltage ripple Y, it is not necessary to switch to duty D2.

[0048] (When the load current is small) When the load current is small, the characteristics are as shown in Fig. 8(b). Since the initial output voltage Vi4 is greater than the threshold voltage Vth1 (-1000 V), if it is operated at duty D2, the ripple of the output voltage will exceed the allowable output voltage ripple Y of duty D2, and there is a risk of image defects. Therefore, in the second embodiment, when the load current is small, as shown in Fig. 8(b), the duty is switched at the boundary of the threshold voltage Vth3 (-1075 V) (> Vth1). That is, since the initial output voltage Vi4 (-1025 V) is smaller than the threshold voltage Vth3 (-1075 V), it is operated at duty D1. Then, as the output voltage is increased, when it reaches the threshold voltage Vth3, it is switched from duty D1 to duty D2, and the output voltage can be changed up to the output limit voltage of duty D2. Note that, without switching the duty at the threshold voltage Vth3, it may be changed up to the output limit voltage of duty D1 when the load current is small while remaining at duty D1, and the voltage can be changed over a large range.

[0049] As described above, when the control unit 3 increases the output voltage from the initial output voltage, the following control is performed based on the detection result of the load current detection circuit 501. That is, when the current detected by the load current detection circuit 501 is equal to or less than a predetermined value, the control unit 3 sets the threshold value for switching the duty to the threshold voltage Vth3, which is the fourth threshold value. Here, the threshold voltage Vth3 is greater than the threshold voltage Vth1, which is the first threshold value for switching the duty when the current detected by the load current detection circuit 501 is greater than the predetermined value.

[0050] As described above, in addition to the sequence of increasing or decreasing the output voltage from the initial output voltage, the threshold value for switching the duty of the drive signal S202 is changed according to the load current. Thereby, even when the load current changes significantly, it is possible to set the duty of the respective optimal drive signals S202.

[0051] [Regarding the switching of the frequency] Also, when the frequency is switched, the operation is the same as that when the duty is switched. Figures 9(a)(b) and 10(a)(b) show diagrams indicating the relationship between the output voltage (-) on the horizontal axis and the ripple of the output voltage on the vertical axis.

[0052] [When decreasing the output voltage from the initial output voltage] Next, the sequence of decreasing the output voltage from the initial output voltage Vi3 will be explained using Figures 9(a)(b). Figure 9(a) shows the relationship between the output voltage (-) and the ripple when the load current is large, and Figure 9(b) shows the relationship between the output voltage (-) and the ripple when the load current is small, for both the case of frequency f1 (dashed line) and frequency f2 (solid line).

[0053] (When the load current is large) When the load current is large, as shown in Figure 9(a), since the absolute value of the initial output voltage Vi3 (-1100V) is larger than the threshold value Vth2 (-1050V), it operates at frequency f2. From here, the target voltage signal S201 is changed to decrease the output voltage. When the threshold value Vth2 is reached, the frequency is switched from f2 to f1, and the output voltage continues to be decreased. Note that it is also possible to decrease the output voltage while remaining at frequency f2 without switching the frequency at the threshold value Vth2. In this case, the output voltage can be changed to the allowable output voltage ripple Yf of the frequency f2 of the drive signal S202, which is the output voltage VY(f2). When the initial output voltage is smaller than the threshold value Vth2 and the output voltage is being decreased, it operates at frequency f1 and the output voltage is decreased as it is.

[0054] (When the load current is small) When the load current is small, the characteristics are as shown in Fig. 9(b). Specifically, when the load current is small, the output voltage that becomes the allowable output voltage ripple Yf when the frequency of the drive signal S202 is f2 is, for example, -1075V. Since the initial output voltage Vi3 is larger than the threshold value Vth2 (-1050V), when operating at the frequency f2, only 25V can be changed up to -1075V (=Vth3), which is the allowable output voltage ripple Yf at the frequency f2. That is, when the load current is small and the frequency is f2, the variable range of the output voltage is only 25V. Therefore, when the load current is small, as shown in Fig. 9(b), the frequency is switched with the threshold value Vth4 (-1175V) as the boundary. That is, since the initial output voltage Vi3 (-1100V) is smaller than the threshold value Vth4 (-1175V), it operates at the frequency f1. As a result, a large voltage variation up to the voltage VY(f1) that becomes the allowable output voltage ripple Yf at the frequency f1 when the load current is small can be achieved. At this time, the range in which the output voltage can be changed is the difference between the initial output voltage Vi3 and the output voltage VY(f1), which is larger than 25V (|Vi3| - |VY(f1)| > |Vi3| - |Vth3|). When the initial output voltage is larger than the threshold value Vth4 and the output voltage is decreased, first, it operates at the frequency f2, and at the threshold value Vth4, the frequency is switched from f2 to f1 to decrease the output voltage.

[0055] As described above, when the control unit 3 decreases the output voltage from the initial output voltage, the following control is performed based on the detection result of the load current detection circuit 501. That is, when the current detected by the load current detection circuit 501 is equal to or less than a predetermined value, the control unit 3 sets the threshold value for switching the frequency to the threshold value Vth4, which is the third threshold value. Here, the threshold value Vth4 is larger than the threshold value Vth2, which is the second threshold value for switching the frequency when the current detected by the load current detection circuit 501 is larger than the predetermined value. In Fig. 9, Yfa and Yfb correspond to Ya and Yb in Fig. 7, and the description thereof is omitted.

[0056] [When increasing the output voltage from the initial output voltage] Next, the sequence for increasing the output voltage in FIGS. 10(a) and 10(b) will be described. In FIG. 10(a), when the load current is large, and in FIG. 10(b), when the load current is small, the relationships between the output voltage and the ripple are shown for the frequency f1 (dashed line) and the frequency f2 (solid line), respectively. Next, the sequence for increasing the output voltage from the initial output voltage Vi4 will be described. As an example, the initial output voltage Vi4 is set to -1025V.

[0057] (When the load current is large) When the load current is large, as shown in FIG. 10(a), since the initial output voltage Vi4 (-1025V) is greater than the threshold voltage Vth1 (-1000V), it operates at the frequency f2. From here, the target voltage signal S201 is changed to increase the output voltage, and it can be changed up to the output voltage at which the output capacity limit of the frequency f2 of the drive signal S202 is reached. Note that when the initial output voltage is less than the threshold voltage Vth1 and the output voltage is increased, first, it operates at the frequency f1, and when the threshold voltage Vth1 is reached, it switches from the frequency f1 to the frequency f2 to decrease the output voltage. Also, even if the output voltage is increased while remaining at the frequency f1, since the generated ripple maintains below the allowable output voltage ripple Yf, it is not necessary to switch to the frequency f2.

[0058] (When the load current is small) When the load current is small, the characteristics are as shown in Fig. 10(b). Since the initial output voltage Vi4 is greater than the threshold voltage Vth1 (-1000V), if it operates at frequency f2, the ripple of the output voltage will exceed the allowable output voltage ripple Yf of frequency f2, and there is a risk of image defects. Therefore, when the load current is small, as shown in Fig. 10(b), the frequency is switched with the threshold voltage Vth3 (-1075V) (>Vth1) as the boundary. That is, since the initial output voltage Vi4 (-1025V) is smaller than the threshold voltage Vth3 (-1075V), it operates at frequency f1. Then, as the output voltage is increased, when it reaches the threshold voltage Vth3, the frequency is switched from f1 to f2, and the output voltage can be changed up to the output limit voltage of frequency f2. Note that it is also possible to change to the output limit voltage of frequency f1 when the load current is small without switching the frequency at the threshold voltage Vth3 and keep the frequency at f1, and the voltage can be changed over a large range.

[0059] As described above, when the control unit 3 increases the output voltage from the initial output voltage, it performs the following control based on the detection result of the load current detection circuit 501. That is, when the current detected by the load current detection circuit 501 is equal to or less than a predetermined value, the control unit 3 sets the threshold value for switching the frequency to the threshold voltage Vth3, which is the fourth threshold value. Here, the threshold voltage Vth3 is greater than the threshold voltage Vth1, which is the first threshold value for switching the frequency when the current detected by the load current detection circuit 501 is greater than the predetermined value.

[0060] As described above, in addition to the sequence of increasing or decreasing the output voltage from the initial output voltage, the threshold value for switching the frequency of the drive signal S202 is changed according to the load current. Thereby, even when the load current changes greatly, it is possible to set the optimal frequency of the drive signal S202 respectively.

[0061] Also, as shown in FIG. 6, a non-volatile memory 510, which is a storage unit capable of communicating with the control unit 3 via a communication signal S204, may be used. Considering variations in the L value of the transformer 201 for each substrate used in the high-voltage power supply unit 500, the duty and frequency of the drive signal S202 can be adjusted for each unit. As a method thereof, a method is known in which the output characteristics of the high-voltage power supply unit 500 are measured in advance on the production line and stored in the non-volatile memory 510. In this way, the high-voltage power supply unit 500 may include a storage unit that stores information associating the output voltage with the duty and frequency, and information on the thresholds when increasing and decreasing the output power. Further, the storage unit may store a plurality of pieces of information associating the output voltage with the duty and frequency, and threshold information, according to the current flowing through the load. In this way, by measuring in advance the output characteristics for each load current of the high-voltage power supply unit 500 and storing them in the storage unit, the output voltage can be controlled to an arbitrary value regardless of variations in the L value of the transformer 201.

[0062] As described above, according to the second embodiment, in a power supply device capable of changing the output voltage, it is possible to reduce the generation of the output voltage ripple and the power consumption.

Description of Reference Numerals

[0063] 3 Control unit 201 Transformer 203 Field-effect transistor 204 Comparator

Claims

1. A transformer having a primary winding and a secondary winding, A switching element connected in series to the primary winding and turning on or off according to an input control signal, Control means for outputting the control signal of a predetermined duty to the switching element, Stop means for stopping the input of the control signal to the switching element so that the output voltage output from the secondary side of the transformer approaches a target voltage, Comprising, The control means is a power supply device capable of changing the output voltage by switching the target voltage, The control means, When increasing the output voltage, the control signal of a first duty is output when the target voltage is lower than a first threshold value, and when the target voltage switches to a state equal to or higher than the first threshold value, the control signal of a second duty larger than the first duty is output, When decreasing the output voltage, the control signal of the second duty is output when the target voltage is equal to or higher than a second threshold value, and when the target voltage switches to a state lower than the second threshold value, the control signal of the first duty is output, The power supply device is characterized in that an absolute value of the second threshold value is larger than an absolute value of the first threshold value.

2. The first threshold value is a voltage when the switching element is turned on or off with the second duty such that the ripple of the output voltage becomes the maximum allowable ripple, according to the power supply device described in claim 1.

3. Comprising detection means for detecting a current flowing through a load to which the output voltage is supplied, The control means varies the first threshold value and / or the second threshold value based on a detection result by the detection means, according to the power supply device described in claim 1 or claim 2.

4. When decreasing the output voltage, the control means sets a threshold value for switching the duty to a third threshold value having an absolute value larger than an absolute value of the second threshold value for switching the duty when the current detected by the detection means is larger than a predetermined value, when the current detected by the detection means is equal to or less than the predetermined value, according to the power supply device described in claim 3.

5. When increasing the output voltage, the control means sets a threshold for switching the duty to a fourth threshold having an absolute value greater than that of the first threshold for switching the duty when the current detected by the detection means is greater than the predetermined value, when the current detected by the detection means is equal to or less than the predetermined value. The power supply device according to claim 3, characterized in that.

6. A storage unit that stores information associating the output voltage and the duty is provided. The power supply device according to any one of claims 3 to 5, characterized in that a plurality of pieces of the information corresponding to the current flowing through the load are stored in the storage unit.

7. The maximum value of the output voltage output according to the second duty is greater than the maximum value of the output voltage output according to the first duty. The power supply device according to any one of claims 1 to 6, characterized in that.

8. The ripple when a predetermined output voltage is output according to the second duty is greater than the ripple when an output voltage substantially the same as the predetermined output voltage is output according to the first duty. The power supply device according to any one of claims 1 to 5, characterized in that.

9. A transformer having a primary winding and a secondary winding; A switching element connected in series to the primary winding and turned on or off according to an input control signal; Control means for outputting the control signal of a predetermined frequency to the switching element; Stop means for stopping the input of the control signal to the switching element so that the output voltage output from the secondary side of the transformer approaches the target voltage; Comprising The control means is a power supply device capable of changing the output voltage by switching the target voltage. The control means When increasing the output voltage, the control signal of the first frequency is output in a state where the target voltage is lower than the first threshold, and when the target voltage switches to a state of being equal to or higher than the first threshold, the control signal of the second frequency higher than the first frequency is output. When decreasing the output voltage, the control signal of the second frequency is output in a state where the target voltage is equal to or higher than the second threshold, and when the target voltage switches to a state lower than the second threshold, the control signal of the first frequency is output. The power supply device, characterized in that the absolute value of the second threshold is greater than the absolute value of the first threshold.

10. The power supply device according to claim 9, wherein the first threshold value is a voltage when the switching element is turned on or off at the second frequency such that the ripple of the output voltage is the maximum allowable ripple.

11. Comprising detection means for detecting a current flowing through a load to which the output voltage is supplied, The control means varies the first threshold value and / or the second threshold value based on the detection result by the detection means, according to the power supply device described in claim 9 or claim 10.

12. When the control means reduces the output voltage, a threshold value for switching the frequency when the current detected by the detection means is equal to or less than a predetermined value is set to a third threshold value having an absolute value greater than that of the second threshold value for switching the frequency when the current detected by the detection means is greater than the predetermined value, according to the power supply device described in claim 11.

13. When the control means increases the output voltage, a threshold value for switching the frequency when the current detected by the detection means is equal to or less than a predetermined value is set to a fourth threshold value having an absolute value greater than that of the first threshold value for switching the frequency when the current detected by the detection means is greater than the predetermined value, according to the power supply device described in claim 11.

14. Comprising a storage unit that stores information associating the output voltage and the frequency, The power supply device according to any one of claims 11 to 13, wherein a plurality of pieces of the information corresponding to the current flowing through the load are stored in the storage unit.

15. The power supply device according to any one of claims 9 to 14, wherein the maximum value of the output voltage output according to the second frequency is greater than the maximum value of the output voltage output according to the first frequency.

16. The power supply device according to any one of claims 9 to 13, wherein the ripple when a predetermined output voltage is output according to the second frequency is greater than the ripple when substantially the same output voltage as the predetermined output voltage is output according to the first frequency.

17. Image forming means for forming an image on a recording medium, The power supply device according to any one of claims 1 to 16, An image forming apparatus, characterized by comprising.

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