Piezoelectric transformer type high-voltage power supply device and image forming apparatus

A high-voltage power supply device includes: a piezoelectric transformer; a driving unit of the piezoelectric transformer; a detection unit configured to detect an output of the piezoelectric transformer; and a control unit configured to control the output of the piezoelectric transformer by giving a driving signal to the driving unit so that the output detected by the detection unit reaches a target value, wherein the control unit changes a frequency of the driving signal without changing a duty of the driving signal so as to set the frequency of the driving signal such that the output detected by the detection unit falls within a predetermined range including the target value, and after setting the frequency for the driving signal such that the output detected by the detection unit falls within the predetermined range, changes the duty of the driving signal so that the output detected by the detection unit reaches the target value.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a piezoelectric transformer type high-voltage power supply device and an image forming apparatus and, more particularly, to a technique of controlling an output voltage or an output current.

2. Description of the Related Art

A conventionally known electrophotographic image forming apparatus uses a DC bias voltage as a voltage to be applied to the transfer member. To generate a high voltage necessary for image formation, a wire-wound electromagnetic transformer has been used conventionally. However, the output current value of the electromagnetic transformer used in the image forming apparatus of the above-described specifications is as small as several μA. Hence, the leakage current needs to be minimized in every unit. To do this, the winding of the transformer needs to be insulated by molding or the like. In addition, the transformer needs to be relatively large to its supply power. For these reasons, it is difficult to reduce the size and weight of the high-voltage power supply device.

To solve these problems, a proposal has been made to generate a high voltage using a slim and lightweight high-power piezoelectric transformer (Japanese Patent Laid-Open No. 11-206113). More specifically, using a piezoelectric transformer made of a ceramic makes it possible to generate a high voltage at an efficiency higher than that of the electromagnetic transformer and also increase the distance between the primary side electrode and the secondary side electrode. Since the special molding for insulation is unnecessary, the high-voltage generator can be made compact and lightweight.

Japanese Patent Laid-Open No. 11-206113 discloses a high-voltage power supply device which causes a voltage controlled oscillator (VCO) formed from an analog circuit to generate a driving frequency to be input to the piezoelectric transformer. As a feature of the piezoelectric transformer, its output voltage is maximized at the resonance frequency. It is therefore possible to control the output voltage by the frequency. Note that as the features of the relationship between the driving frequency and the output voltage, the output voltage is maximized at the resonance frequency and lowers as the frequency becomes higher or lower than the resonance frequency. The high-voltage power supply device described in Japanese Patent Laid-Open No. 11-206113 controls the frequency output from the VCO, thereby controlling the output voltage of the piezoelectric transformer.

Recently demanded is space saving using fewer components. As described above, the control circuit portion of the piezoelectric transformer is formed from an analog circuit in most cases and therefore includes many components. To decrease the number of components of the control circuit portion and implement a space-saving piezoelectric transformer, the control circuit portion is formed as an IC on one chip. However, since the number of pins of an IC is limited, the IC may be unusable in a small package. In addition, when the package is small, and ten-odd piezoelectric transformers are used as in a color printer, a plurality of control ICs are necessary, and a large space-saving effect is difficult to obtain. In this case, the ICs are formed on one chip together with the CPU and the ASIC of the controller of the printer engine unit. In this method, a large space-saving effect can be obtained. The control circuit can be formed from not an analog circuit as before but a digital circuit. However, when the control circuit portion of the piezoelectric transformer is digitized, driving control of the piezoelectric transformer and, more particularly, output voltage control by a variable frequency requires a high frequency accuracy. That is, for accurate frequency control, the CPU or the ASIC needs to have a very high operation clock speed.

For example, when a 10-bit counter (the MAX count is 1024) counts 602 pulses in both H and L levels (the count is 1204 in one period) for pulse generation, the output frequency value is 166.113 KHz. When 603 H pulses and 602 L pulses are counted (the count is 1205 in one period), the output frequency value is 165.975 KHz. The frequency difference Δf is 120 Hz. When the frequency changes by 100 Hz, the output voltage changes by about 20 V. To avoid any influence on an image, the voltage needs to change at an accuracy of 2.0 V or less. To change the voltage at an accuracy of 1.5 V or less, a frequency resolution of 10 Hz or less is necessary. In that case, the frequency of the operation clock of the CPU or the ASIC needs to be 2 GHz or more. That is, examples of problems posed by speeding up the internal operation clock are an increase in unwanted radiation noise, higher power consumption, and an increase in the cost caused by the semiconductor microfabrication process.

SUMMARY OF THE INVENTION

The present invention has been made in consideration of the above-described problems, and provides an inexpensive image processing apparatus which performs processing only in an image composition region while keeping image data compressed, thereby processing even high-resolution image data in a short time.

According to one aspect of the present invention, there is provided a high-voltage power supply device comprising: a piezoelectric transformer; a driving unit of the piezoelectric transformer; a detection unit configured to detect the output of the piezoelectric transformer; and a control unit configured to control the output of the piezoelectric transformer by giving a driving signal to the driving unit so that the output detected by the detection unit reaches a target value, wherein the control unit changes a frequency of the driving signal without changing a duty of the driving signal so as to set the frequency of the driving signal such that the output falls within a predetermined range including the target value, and after setting the frequency for the output within the predetermined range, changes the duty of the driving signal so that the output reaches the target value.

According to another aspect of the present invention, there is provided an image forming apparatus comprising: an image forming unit configured to form an image; and a high-voltage power supply configured to output a high voltage to the image forming unit, the high-voltage power supply comprising a piezoelectric transformer, a driving unit of the piezoelectric transformer, a detection unit configured to detect the output of the piezoelectric transformer, and a control unit configured to control the output of the piezoelectric transformer by giving a driving signal to the driving unit so that the output detected by the detection unit reaches a target value, wherein the control unit changes a frequency of the driving signal without changing a duty of the driving signal so as to set the frequency such that the output falls within a predetermined range including the target value, and after setting the frequency for the output within the predetermined range, changes the duty of the driving signal so that the output reaches the target value.

According to another aspect of the present invention, there is provided a high-voltage power supply device comprising: a piezoelectric transformer; a driving unit of the piezoelectric transformer; a detection unit configured to detect the output of the piezoelectric transformer; a control unit configured to control the output of the piezoelectric transformer by giving a driving signal to the driving unit so that the output detected by the detection unit reaches a target value; and a storage unit configured to store a frequency of the driving signal and a value of the output detected by the detection unit and corresponding to the frequency of the driving signal in association with each other, wherein the control unit causes the storage unit to store the frequency of the driving signal given to the driving unit and the value of the output corresponding to the frequency of the driving signal in association with each other upon sweeping the driving signal, supplies the frequency of the driving signal to the driving unit based on the frequency of the driving signal and the value of the output corresponding to the frequency stored in the storage unit upon frequency sweep such that the output of the piezoelectric transformer falls within a predetermined range with respect to the target value, and after that, changes a duty of the driving signal so that the value of the output reaches the target value.

According to another aspect of the present invention, there is provided a high-voltage power supply device comprising: a piezoelectric transformer; a driving unit of the piezoelectric transformer; a detection unit configured to detect the output of the piezoelectric transformer; a control unit configured to control the output of the piezoelectric transformer by giving a driving signal to the driving unit so that the output detected by the detection unit reaches a target value; and a storage unit configured to store a frequency of the driving signal and a value of the output detected by the detection unit and corresponding to the frequency of the driving signal in association with each other, wherein the control unit changes the frequency of the driving signal without changing a duty of the driving signal such that the value of the output of the piezoelectric transformer falls within a predetermined range with respect to the target value, causes the storage unit to store the frequency of the driving signal for the output within the predetermined range with respect to the target value, supplies the frequency of the driving signal for the output within the predetermined range with respect to the target value to the driving unit using the frequency of the driving signal stored in the storage unit, and after that, changes the duty of the driving signal so that the value of the output reaches the target value.

According to another aspect of the present invention, there is provided an image forming apparatus comprising: an image forming unit configured to form an image; and a high-voltage power supply configured to output a high voltage to the image forming unit, the high-voltage power supply comprising a piezoelectric transformer, a driving unit of the piezoelectric transformer, a detection unit configured to detect the output of the piezoelectric transformer, a control unit configured to control the output of the piezoelectric transformer by giving a driving signal to the driving unit so that the output detected by the detection unit reaches a target value, and a storage unit configured to store a frequency of the driving signal and a value of the output detected by the detection unit and corresponding to the frequency of the driving signal in association with each other, wherein the control unit changes the frequency of the driving signal without changing a duty of the driving signal such that the value of the output of the piezoelectric transformer falls within a predetermined range with respect to the target value, causes the storage unit to store the frequency of the driving signal for the output within the predetermined range with respect to the target value, supplies the frequency of the driving signal to the driving unit using the frequency of the driving signal stored in the storage unit so that the output falls within the predetermined range with respect to the target value, and after that, changes the duty of the driving signal so that the value of one of the output voltage and an output current reaches the target value.

According to another aspect of the present invention, there is provided an image forming apparatus comprising: an image forming unit configured to form an image; and a high-voltage power supply configured to output a high voltage to the image forming unit, the high-voltage power supply comprising a piezoelectric transformer, a driving unit of the piezoelectric transformer, a detection unit configured to detect the output of the piezoelectric transformer, a control unit configured to control the output of the piezoelectric transformer by giving a driving signal to the driving unit so that the output detected by the detection unit reaches a target value, and a storage unit configured to store a frequency of the driving signal and a value of the output detected by the detection unit and corresponding to the frequency of the driving signal in association with each other, wherein the control unit changes the frequency of the driving signal without changing a duty of the driving signal such that the value of the output of the piezoelectric transformer falls within a predetermined range with respect to the target value, causes the storage unit to store the frequency of the driving signal for the output within the predetermined range with respect to the target value, supplies the frequency of the driving signal for the output within the predetermined range with respect to the target value to the driving unit using the frequency of the driving signal stored in the storage unit, and after that, changes the duty of the driving signal so that the value of the output reaches the target value.

According to the present invention, it is possible to accurately control the output voltage without speeding up the operation clock in digital control of a piezoelectric transformer.

DESCRIPTION OF THE EMBODIMENTS

First Embodiment

The first embodiment of the present invention will now be described. However, this embodiment is merely an example, and the present invention is not limited to these arrangements. A conventional piezoelectric transformer type high-voltage power supply will briefly be described first with reference toFIG. 11. The illustrated circuit is a high-voltage power supply and includes a piezoelectric transformer (piezoelectric ceramic transformer)101of the high-voltage power supply. The output of the piezoelectric transformer101is rectified and smoothed to a positive voltage by diodes102and103and a high-voltage capacitor104and supplied to a transfer roller (not shown) serving as a load. That is, the diodes102and103and the high-voltage capacitor104function as a rectification circuit. An LC resonance circuit using an FET111as a switching element serves as a driving circuit. As described above, the DC high-voltage generation circuit includes the driving circuit, the piezoelectric transformer, and the rectification circuit.

Resistors105,106, and107divide the output voltage which is then input to the noninverting input terminal (+terminal) of an operational amplifier109via a protective resistor108. On the other hand, the inverting input terminal (−terminal) of the operational amplifier receives an analog control signal Vcont of the high-voltage power supply via a resistor114. The operational amplifier109, the resistor114, and a capacitor113that are configured as illustrated function as an integration circuit for the control signal Vcont. The control signal is smoothed by an integration time constant determined by the component values of the resistor and the capacitor, and input to the operational amplifier109. The output terminal of the operational amplifier109is connected to a voltage controlled oscillator (VCO)110. The voltage controlled oscillator (VCO)110switches the switching element111by a frequency corresponding to the output voltage from the operational amplifier109. The voltage is amplified by an inductor112and supplied to the primary side of the piezoelectric transformer.

FIG. 10is a sectional view showing the schematic arrangement of a “color laser printer” serving as an image forming apparatus according to the first embodiment to which the present invention is applicable. The color laser printer is equipped with the piezoelectric transformer type high-voltage power supply device. Referring toFIG. 10, a color laser printer401includes a deck402that stores printing paper sheets32, a paper sensor403that detects the presence/absence of the printing paper sheets32in the deck402, and a pickup roller404that extracts the printing paper sheet32from the deck402. The color laser printer401also includes a deck feed roller405that conveys the printing paper sheet32extracted by the pickup roller404, and a retard roller406that pairs off with the deck feed roller405to prevent conveyance of multiple printing paper sheets32. A registration roller pair407that synchronously conveys the printing paper sheet32and a pre-registration sensor408that detects the conveyance state of the printing paper sheet32to the registration roller pair407are disposed downstream from the deck feed roller405.

An electrostatic adsorptive feeding transfer belt (to be referred to as an ETB hereinafter)409is arranged downstream from the registration roller pair407. This image forming apparatus is a color laser printer and therefore includes exchangeable process cartridges410of a plurality of colors. Images formed by image forming units including process cartridges410Y,410M,410C, and410Bk and scanner units420Y,420M,420C, and420Bk of four colors (yellow Y, magenta M, cyan C, and black Bk), respectively, are sequentially overlaid on the ETB409by transfer rollers430Y,430M,430C, and430Bk, thereby forming a color image. The formed color image is transferred to the printing paper sheet32. The printing paper sheet32is conveyed downstream. On the downstream side is a pair of a pressurizing roller434and a fixing roller433that incorporates a heater432to thermally fix the toner image transferred to the printing paper sheet32. Also disposed are a discharge roller pair435configured to convey the printing paper sheet32from the fixing roller433and a discharge sensor436that detects the conveyance state from the fixing unit.

Each scanner unit420includes a laser unit421that emits a laser beam modulated based on an image signal output from a video controller440, a polygon mirror422and a scanner motor423configured to scan the laser beam from the laser unit421on a photosensitive drum305, and an imaging lens group424. Each process cartridge410includes the photosensitive drum305, a charging roller303, a developing roller302, and a toner container411necessary for the known electrophotography process. Each scanner unit420is detachable from the main body of the color laser printer401. Upon receiving image data output from an external device441such as a personal computer, the video controller440bitmaps the received image data to generate an image signal for image formation.

An engine controller201of the color laser printer401is formed from, for example, various input/output control circuits (not shown) and a CPU207serving as a control unit including a RAM207a, a ROM207b, a timer207c, a digital input/output port207d, and a D/A port207e. A high-voltage power supply (piezoelectric transformer type high-voltage power supply device)202includes a charging high-voltage power supply (not shown) and a developing high-voltage power supply (not shown) corresponding to the process cartridges410, and a transfer high-voltage power supply (not shown) capable of outputting high voltages corresponding to the transfer rollers430using a piezoelectric transformer. Note that the image forming apparatus has been described by exemplifying a tandem type color image forming apparatus. However, any image forming apparatus using a high bias voltage is incorporated in the scope of the present invention.

[Control and Operation Procedure of Piezoelectric Transformer Type High-Voltage Power Supply]

FIGS. 1 and 2show a block diagram and a flowchart illustrating the operation procedure of the piezoelectric transformer type high-voltage power supply according to the present invention. Note that in this embodiment, the output is described as a voltage. However, handling the output as a current is also incorporated in the scope of the present invention. The same reference numerals as in the conventional piezoelectric transformer type high-voltage power supply shown inFIG. 11denote the same parts in the piezoelectric transformer type high-voltage power supply of the present invention. The engine controller201includes a pulse generator2051that gives a driving signal to the driving circuit, and an A/D converter2052that performs A/D conversion.

FIGS. 4A to 4Dshow operation waveforms in frequency control at the start of control when an LC resonance circuit that uses the FET111as the switching element, as shown inFIG. 1, serves as the driving circuit.FIGS. 4A to 4Dshow the following waveforms by plotting time along the abscissa.FIG. 4Ashows a gate waveform Vgs of the FET111,FIG. 4Bshows a drain waveform Vds of the FET111, that is, the driving voltage of the piezoelectric transformer101,FIG. 4Cshows a current waveform Il of the inductor112, andFIG. 4Dshows a drain current waveform Id. In this case, control is performed to change the frequency while fixing the duty (frequency control). Upon frequency control at the start of control, the ON duty of the pulse generator (Pulse Width Modulation: PWM)2051that generates a driving pulse is set within such a range that allows the switching element (FET111) to do ZVS (Zero Voltage Switching), and the PWM controls the frequency. In this embodiment, an example will be explained in which the ON duty of the FET111inFIG. 4Ais 50%. In an ON time Ton (t0to t1) of the FET111, Il matches Id. This indicates that the current from the inductor112all flows into the FET111. When the FET111is turned off, Id instantaneously changes to zero, as shown inFIG. 4D. The inductor current Il that has flowed to the FET111thus far flows into a resonance capacitor116and the primary side static capacitance of the piezoelectric transformer101to charge them. The drain-source voltage Vds of the FET111begins rising. That is, as shown inFIG. 4D, immediately after the FET111has been turned off, the value of the voltage Vds largely jumps. The rising voltage waveform indicates the LC resonance phenomenon between the inductor112, the resonance capacitor116, and the primary side static capacitance of the piezoelectric transformer101. A frequency f10is approximately given by
fl0≈½π√LC(1)

In the ON time Ton (t0to t1) of the FET, an inductor current waveform Ilp1is approximately given by
Ilp1≈V/L·Ton  (2)

An energy E accumulated in the inductor112by Ilp1is lost due to the resistance component, the wiring resistance, and the like of the inductor112. If the loss is neglected, the energy E has the same value as that accumulated in the resonance capacitor116and the primary side static capacitance of the piezoelectric transformer101by a voltage amplitude Vdsp of Vds. Hence,
E≈½·LIdp2≈½·CVdsp2  (3)

approximately holds. When equation (3) is solved for Vdsp, we obtain
Vdsp≈√L/C·Idp(4)

The resonance capacitor116and the primary side static capacitance of the piezoelectric transformer101are charged by Vdsp from t1to t2. The resonance capacitor116and the primary side static capacitance of the piezoelectric transformer101are discharged from t2to t3. At this time, the accumulated charges and the removed charges are of equal value. With this LC voltage resonance, the flyback voltage waveform serving as the input voltage waveform is generated and supplied to the primary side of the piezoelectric transformer. In the drain voltage waveform Vds shown inFIG. 4B, the time from t2to t3is determined by the constants of the inductor112and the resonance capacitor116and the input side static capacitance component of the piezoelectric transformer101. In addition, Vdsp of the drain voltage waveform does not largely change in the zero voltage switching region. In this state, frequency sweep is performed in a rough frequency resolution at the start of control. The “rough frequency resolution” indicates a low frequency resolution. The “rough frequency resolution” suffices at the start because the control to a more appropriate high frequency resolution is done to approach the set voltage, as needed. Note that the specific “rough frequency resolution” to be used at the start may be obtained experimentally.

Duty Control

Upon determining that the input voltage has fallen within the range of set voltage V+α, the control is switched so as to change the duty while fixing the frequency of the pulse generator2051(duty control). The value +α (the value representing the allowable error range in which the voltage approximates the set voltage V) is defined in advance. The duty control is performed until the input voltage falls within the error range. Waveforms in duty control shown inFIGS. 5A to 5Dwill be described. The waveforms shown inFIGS. 5A to 5Dcorrespond to those inFIGS. 4A to 4D, respectively. In the duty control, the switching element (FET111) operates in the hard switching region that is not the zero voltage switching region. When the time Ton shortens up to the hard switching region, the current Ilp1of the inductor112decreases in accordance with equation (2), as shown inFIG. 5C. When the current Ilp1of the inductor112decreases, the energy accumulated in the inductor112decreases. As can be seen from equations (3) and (4), the energy accumulated in the inductor112equals that accumulated in the resonance capacitor116and the primary side static capacitance of the piezoelectric transformer101by the voltage amplitude Vdsp. Hence, when the energy of the inductor112decreases, the drain voltage Vdsp of the FET111decreases, as shown inFIG. 5B. That is, the input voltage of the piezoelectric transformer101decreases. Hence, the input voltage of the piezoelectric transformer101is changed in the hard switching region in correspondence with the time Ton that is the duty of the driving pulse, thereby controlling the output voltage of the piezoelectric transformer101.

An explanation will be done next based in the operation procedure inFIG. 2. Note that at the start of the operation procedure, frequency control is performed, as shown inFIGS. 4A to 4D. The CPU207serving as the control unit of the engine controller201sets the set voltage value (S201). To drive the piezoelectric transformer101, the CPU207sets the output frequency in the pulse generator2051(S202). The driving frequency set here is defined as the “rough frequency resolution”. The voltage detection units105,106, and107formed from resistors detect the output voltage of the piezoelectric transformer101. The A/D converter2052converts the output voltage from the analog signal into a digital signal. The CPU207compares the value converted by the A/D converter2052with the set voltage value, and determines whether the result of comparison with the set voltage value falls within a preset range (S203). If the comparison result falls outside the set range (NO in step S203), the pulse generator2051changes the output frequency serving as the driving signal from the high frequency side to the low frequency side. Thus changing the driving frequency is repeated until the voltage approximates the set voltage (S204). If the comparison result falls within the set range, the CPU207fixes the frequency set in the pulse generator2051at that time (S205).

From then on, duty control is performed, as shown inFIGS. 5A to 5D. While fixing the frequency output from the pulse generator2051, the CPU207changes the duty of the output pulse (S206). In accordance with the duty change in step S206, the driving voltage of the piezoelectric transformer101changes in correspondence with the duty (S207). In addition, the output voltage changes in correspondence with the change in the driving voltage (S208). Note that steps S207and S208are phenomena caused not by the control unit but as the result of step S206. The voltage detection units105,106, and107formed from resistors detect the changed output voltage. The A/D converter2052converts the output voltage from the analog signal into a digital signal. The CPU207compares the value converted by the A/D converter2052with the set voltage value, and determines whether the result of comparison with the set voltage value falls within a preset range (S209). If the comparison result falls outside the set range (NO in step S209), the process returns to step S206to change the duty of the driving pulse until the output voltage value reaches the target set voltage value. If the output voltage equals the set voltage (YES in step S209), the voltage will be controlled by the current duty (S210), and the processing procedure ends.

An example will be described here in which the pulse generator2051includes a digital counter circuit, and the driving frequency is 200 MHz. In step S204, the 10-bit counter (the MAX count is 1024) changes the counts of both H and L pulses, thereby changing the frequency stepwise. For example, when 602 pulses are counted in both H and L levels (the count is 1204 in one period) for pulse generation, the output frequency value is 166.113 KHz. When the frequency is fixed to this output frequency, the count is 1204 in one period. In step S206, the duty is changed by incrementing the H pulse count by one and decrementing the L pulse count by one so as to count 603 H pulses and 601 L pulses. That is, the H pulse count and the L pulse count are changed without changing the total count of H and L pulses. In the above-described example, the duty is changed by 0.08% each time. When the duty is changed by 0.1%, an output voltage change of about 2 V is obtained.

The above operation will be described with reference toFIGS. 3A and 3Bshowing the F-V characteristic of the piezoelectric transformer.FIG. 3Bis an enlarged view of part ofFIG. 3A. The solid line indicating frequency control represents the F-V characteristic until determining that the voltage falls within the range of set voltage V+α by sweeping the frequency in the rough frequency resolution from a frequency much higher than the resonance frequency of the piezoelectric transformer to a lower frequency. If the voltage falls within the range of set voltage V+α, the control is switched to duty control indicated by the broken line. The duty control is performed in a high resolution while fixing the frequency, thereby controlling the input voltage of the piezoelectric transformer in correspondence with the duty. The peak of the F-V characteristic is thus finely adjusted to control to the desired set voltage of the target value. The sweep may be done from a much lower frequency to a higher frequency.

As described above, the driving frequency of the piezoelectric transformer is changed at first in a rough frequency resolution. It is determined whether the voltage falls within the range of set voltage V+α. If the voltage falls within the set voltage range, the duty is changed to control to the desired set voltage. This allows to accurately control the output voltage without speeding up the operation clock of the CPU or the ASIC.

Second Embodiment

The second embodiment of the present invention will be described below based onFIGS. 6 and 7. In this embodiment, frequency sweep is performed, and the frequency and the output voltage (or output current) at that time are stored in association with each other. A frequency corresponding to an output voltage that approximates a set voltage is selected from the stored information and used for control.FIGS. 6 and 7show a block diagram and a flowchart illustrating the operation procedure of a piezoelectric transformer type high-voltage power supply according to the present invention. A description of the same parts as in the first embodiment will be omitted. Note thatFIG. 6is different fromFIG. 1in that an engine controller201incorporates a storage device2053.

FIGS. 6 and 7show a block diagram and a flowchart illustrating the operation procedure of the piezoelectric transformer type high-voltage power supply according to the present invention. Referring toFIG. 7, when the color printer is powered on, a CPU207in the engine controller201sweeps the output frequency of a pulse generator2051from a higher frequency to a lower frequency within a predetermined range (S301). The CPU207stores, in the storage device2053, the piezoelectric transformer output detection result obtained by the frequency sweep and the frequency set value of the pulse generator2051corresponding to the output value (S302).

FIG. 8shows the F-V characteristic of a piezoelectric transformer101in the frequency sweep. Let f0be the maximum frequency in the frequency sweep, and V0be the corresponding voltage detection value. The frequency sweep is done up to a minimum frequency fn. The frequency set values (f0, . . . , fG, . . . , fN) and the voltage detection values (V0, . . . , VG, . . . , VN) corresponding to them are stored in the storage device2053.

At the start of printing (S303), the CPU207compares the preset set voltage value with the voltage detection result stored in the storage device2053at the time of frequency sweep (S302), and determines whether the comparison result falls within a preset range (set voltage V+α) (S304). The CPU207acquires, from the storage device2053, the output frequency set value of the pulse generator2051corresponding to the voltage detection result of which the comparison result falls within the preset range, and sets the frequency in the pulse generator2051(S305). The CPU207then fixes the output frequency set in the pulse generator2051(S306). After that, the CPU207changes the duty of the pulse generator2051to control the output voltage of the piezoelectric transformer to the set voltage, as in steps S206to S210ofFIG. 2(S307to S311). If printing is to be continuously executed after the voltage setting (YES in step S312), the processing is performed (S313). When printing is all competed, the processing ends.

As described above, control information and output information of frequency sweep performed at a predetermined timing are held. The correspondence information of the output voltage and the frequency, which is necessary for comparison, in the F-V characteristic of the piezoelectric transformer101is acquired. This allows to shorten the arrival time of the set voltage at the start of control. It is also possible to obtain the same effect as in the first embodiment concerning the accuracy of the output voltage of the piezoelectric transformer.

Note that the frequency sweep performed at a predetermined timing in a predetermined frequency range can be either sweep from a frequency much higher than the resonance frequency of the piezoelectric transformer101to a lower frequency or sweep from a much lower frequency to a higher frequency.

In addition, the frequency sweep performed at a predetermined timing in a predetermined frequency range may be executed either after powering on the image forming apparatus (including a capacitor115and a resistance120) of the prior art shown inFIG. 11or upon initialization when returned from the power saving mode. To compensate for a change in the resonance frequency caused by the temperature rise of the piezoelectric transformer101and a time-rate change in the load, the frequency sweep may be executed after the image forming apparatus has printed a predetermined number of sheets so as to update the data in the storage device2053. This is also applicable to compensate for a load variation caused by exchanging process cartridge410. Upon detecting exchange of at least one of process cartridges410Y,410M,410C, and410Bk of the image forming apparatus, the CPU207may execute the frequency sweep to update the data in the storage device2053.

Third Embodiment

The third embodiment of the present invention will be described below based onFIGS. 6 and 9. In this embodiment, a frequency that approximates the output voltage to the set voltage is stored at a predetermined timing, and the output voltage (or output current) is controlled using the stored frequency any time other than the predetermined timing.FIGS. 6 and 9show a block diagram and a flowchart illustrating the operation procedure of a piezoelectric transformer type high-voltage power supply according to the present invention. A description of the same parts as in the first and second embodiments will be omitted. Consider, for example, print control executed at the start of a predetermined timing after powering on the color printer or when returned from the power saving mode. In this case, a CPU207changes the frequency from a frequency much higher than the resonance frequency of a piezoelectric transformer101to a lower frequency until determining that the output voltage falls within the preset voltage range V+α, as in steps S401to S405ofFIG. 9. Unlike the first embodiment, upon determining that the output voltage falls within the set voltage range V+α in the processing procedure ofFIG. 9, the CPU207stores the used output frequency set value of a pulse generator2051in a storage device2053(S406). After determining whether the output voltage falls within the set voltage range V+α, the CPU207fixes the frequency of the pulse generator2051and controls to the set voltage V by duty control (S407to S413).

When the color printer transits from the standby state (S414) to print control (YES in step S415), the CPU207acquires the frequency set value stored in the storage device2053in the previous print control, and sets it in the pulse generator2051(S416). Like the above-described procedure, the CPU207fixes the output frequency of the pulse generator2051to the set frequency (S406). The CPU207controls the duty of the pulse generator2051(S407) to control the output voltage of the piezoelectric transformer101to the set voltage value (S408to S412).

As described above, the output voltage of the piezoelectric transformer can be controlled in every print control without changing the frequency from a frequency much higher than the resonance frequency of the piezoelectric transformer. This allows to shorten the rising time to the set voltage.

Note that the frequency may change from a frequency much lower than the resonance frequency of the piezoelectric transformer to a higher frequency. As for the predetermined timing, to compensate for a change in the resonance frequency caused by the temperature rise of the piezoelectric transformer101and a time-rate change in the load, the setting may be executed in print control after the image forming apparatus has printed a predetermined number of sheets so as to update the data in the storage device2053. This is also applicable to compensate for a load variation caused by exchanging process cartridge410. Upon detecting exchange of at least one of process cartridges410Y,410M,410C, and410Bk of the image forming apparatus, the CPU207may execute the setting even in print control after the exchange to update the data in the storage device2053.

This application claims the benefit of Japanese Patent Application No. 2010-119730, filed May 25, 2010, which is hereby incorporated by reference herein in its entirety.