JP5744481B2 - Power supply device and image forming apparatus having power supply device - Google Patents

Description

  The present invention relates to a power supply device used in an electronic apparatus such as an image forming apparatus.

  In order to achieve the target performance in the electronic device, the ability of the power supply device that supplies electric power to the electronic device is important. For example, in an electrophotographic image forming apparatus, a toner image is formed by applying a high voltage to an image forming unit such as a charging unit, a developing unit, or a transfer unit that charges a photoreceptor when forming an image. Yes. Therefore, in the image forming apparatus, the image quality depends on the accuracy of the high voltage. That is, it is required to output a target high voltage stably. Also, in order to increase productivity by increasing the time from the start of image formation to the start of image formation until the image is formed for one sheet and the number of images formed per unit time, it is necessary to raise the target voltage. It is important how to shorten the time.

  Patent Document 1 proposes a small and lightweight high-voltage generator that employs a piezoelectric transformer instead of a wound electromagnetic transformer. In particular, the piezoelectric transformer outputs a voltage corresponding to the frequency of the signal output from the voltage controlled oscillator (VCO). The output voltage characteristic with respect to the driving frequency of the piezoelectric transformer is generally a substantially mountain-shaped characteristic. That is, the output voltage becomes maximum at the resonance frequency.

JP-A-11-206113

  In the power supply device employing the above-described piezoelectric transformer, it is necessary to perform frequency sweep until the target voltage is output. This frequency sweep is started from a certain initial frequency fi toward the resonance frequency f0. The initial frequency fi is determined by giving a sufficient margin in consideration of individual variations and circuit variations of the resonance frequency f0 of the piezoelectric transformer. For this reason, the initial frequency fi is set to be considerably higher than the drive frequency fx corresponding to the target voltage. In the method of starting the frequency sweep from the high frequency side, if the drive frequency becomes lower than the resonance frequency f0, there is a possibility that control may be impossible. In order to avoid this, it is necessary to delay the feedback time constant of the entire circuit. This is one of the factors that require a lot of time for the sweep operation. Accordingly, an object of the present invention is to shorten the time until the output voltage of the piezoelectric transformer reaches the target voltage.

The power supply device of the present invention is, for example,
A voltage controlled oscillator that generates a signal having a frequency according to the input control voltage;
A drive unit to which a signal output from the voltage controlled oscillator is input;
A piezoelectric transformer driven by the driving unit in accordance with a signal output from the voltage controlled oscillator;
A frequency control unit that starts driving the piezoelectric transformer by the drive unit at a predetermined initial frequency and sweeps the frequency through the control voltage so that an output voltage from the power supply device becomes a predetermined target voltage. When,
Analog-digital that converts information indicating the control voltage corresponding to a frequency between the frequency of the signal output from the voltage controlled oscillator and the initial frequency when the output voltage reaches the target voltage from an analog value to a digital value A converter and a digital-analog converter that converts the digital value into an analog value, the digital-analog converter latches and holds the digital value of information indicating the control voltage; and
A setting unit that sets a sweep start frequency corresponding to the information held in the holding unit in the frequency control unit instead of the initial frequency;
It is characterized by providing.
Moreover, the power supply device of the present invention is, for example,
A voltage controlled oscillator that generates a signal having a frequency according to the input control voltage;
A drive unit to which a signal output from the voltage controlled oscillator is input;
A piezoelectric transformer driven by the driving unit in accordance with a signal output from the voltage controlled oscillator;
A frequency control unit that starts driving the piezoelectric transformer by the drive unit at a predetermined initial frequency and sweeps the frequency through the control voltage so that an output voltage from the power supply device becomes a predetermined target voltage. When,
Analog-digital that converts information indicating the control voltage corresponding to a frequency between the frequency of the signal output from the voltage controlled oscillator and the initial frequency when the output voltage reaches the target voltage from an analog value to a digital value A holding unit including a converter, a memory that stores the digital value of the information indicating the control voltage, and a digital-analog converter that converts the digital value stored in the memory into an analog value;
A setting unit that sets a sweep start frequency corresponding to the information held in the holding unit in the frequency control unit instead of the initial frequency;
It is characterized by providing .

  According to the present invention, the holding unit holds information indicating the control voltage corresponding to the frequency of the signal output from the voltage controlled oscillator when the output voltage from the power supply device becomes the target voltage. further. The setting unit sets a sweep start frequency corresponding to the information held in the holding unit in the frequency control unit instead of the initial frequency. Therefore, the sweep start frequency is close to the drive frequency of the piezoelectric transformer when the output voltage reaches a predetermined target voltage, so by starting the sweep from that point, the output voltage of the piezoelectric transformer reaches the target voltage. Time can be shortened.

1 is a cross-sectional view illustrating an image forming apparatus according to an embodiment of the present invention. The circuit block diagram of the power supply device in the related technology of this invention. The figure which shows the frequency characteristic of the piezoelectric transformer in the related technology of this invention. The circuit block diagram of the power supply device in Example 1 of this invention. The circuit block diagram of the voltage controlled oscillator in the Example of this invention. The figure explaining the circuit operation | movement of the voltage controlled oscillator in the Example of this invention. The figure which shows the frequency characteristic of the piezoelectric transformer in the Example of this invention. The flowchart explaining the control of Example 1 of this invention. The circuit block diagram of the power supply device in Example 2 of this invention. The flowchart explaining control of Example 2 of this invention. The circuit block diagram of the power supply device in Example 3 of this invention. The flowchart explaining control of Example 3 of this invention.

[Example 1]
FIG. 1 is a configuration diagram of a tandem color image forming apparatus using an electrophotographic process. The image forming apparatus 100 includes an image forming unit 30 that forms a multicolor image on a medium by superposing four color toners of yellow (Y), magenta (M), cyan (C), and black (K). Yes. The controller 40 is a control unit that controls the image forming unit 30 and includes an MPU (microcomputer) 41. The MPU 41 includes a RAM 42 that functions as a work area, a ROM 43 that stores programs and data, a timer 44 that measures time, and the like. The power supply device 10 is a power supply device that is controlled by the controller 40 and applies a charging bias to the charging roller provided in the image forming unit 30, applies a developing bias to the developing roller, and applies a transfer bias to the secondary transfer roller 29. is there. In this manner, the image forming unit 30 is supplied with power from the power supply device 10 and forms an image on the recording medium.

<Basic Configuration of Power Supply Device 10>
A basic configuration of the power supply apparatus 10 will be described with reference to FIG. The piezoelectric transformer 101 is a booster that outputs an input voltage as a booster. The output of the piezoelectric transformer 101 is rectified and smoothed by a rectifying / smoothing circuit 14 and supplied from an output end 105 to a transfer roller as a load. The rectifying / smoothing circuit 14 includes diodes 102 and 103 and a high voltage capacitor 104. The rectified and smoothed output voltage is divided by a voltage dividing circuit formed by the resistors 106 and 107 to become a feedback voltage Vfb. The feedback voltage Vfb is input to the non-inverting input terminal (+ terminal) of the operational amplifier 109 via the protective resistor 108. On the other hand, the control voltage Vcont is input from the controller 40 to the inverting input terminal (− terminal) of the operational amplifier 109 via the resistor 110. The control voltage Vcont is an analog signal for controlling the power supply device 10. The operational amplifier 109, the resistor 110, the resistor 111, and the capacitor 112 form the differential amplifier circuit 11. A voltage V1 corresponding to the difference between Vcont and Vfb appears on the output line of the differential amplifier circuit 11. This voltage is input to the inverting amplifier circuit 12 at the next stage. The inverting amplifier circuit 12 is formed by an operational amplifier 120 and resistors 121, 122, 123, and 124. The resistance values of the resistors 121 and 122 are the same, and the resistance values of the resistors 123 and 124 are also the same. Therefore, the inverting amplifier circuit 12 operates as a so-called inverter having an amplification factor of 1. The voltage V2 output by inverting the voltage V1 by the inverting amplifier circuit 12 is input to the voltage controlled oscillator 130 as a control voltage.

  The voltage controlled oscillator 130 generates and outputs a signal having a frequency corresponding to the input control voltage V2. Specifically, when the control voltage V2 increases, the output frequency changes to a high frequency, and when the control voltage V2 decreases, the output frequency changes to a low frequency. That is, by sweeping the control voltage V2, the driving frequency for driving the piezoelectric transformer 101 can be swept. A feedback circuit including the differential amplifier circuit 11, the inverting amplifier circuit 12, the rectifying / smoothing circuit 14, and the like functions as a frequency control unit that controls the drive frequency. That is, the frequency control unit starts driving of the piezoelectric transformer by the driving unit at a predetermined initial frequency, and sweeps the frequency through the control voltage so that the output voltage becomes a predetermined target voltage. The output terminal of the voltage controlled oscillator 130 is connected to one end of the pull-down resistor 134 and the gate terminal of the FET 131. The FET 131, the inductor 132, and the capacitor 133 form the LC parallel resonance circuit 13. The piezoelectric transformer 101 is driven through the LC parallel resonance circuit 13 and generates a high output voltage. The LC parallel resonant circuit 13 is an example of a drive unit that receives a signal output from a voltage controlled oscillator and drives a piezoelectric transformer. The piezoelectric transformer 101 is an example of a piezoelectric element that is driven by a drive unit in accordance with a signal output from a voltage controlled oscillator.

  FIG. 3 is a diagram illustrating exemplary frequency characteristics of the piezoelectric transformer 101. The horizontal axis indicates the drive frequency, and the vertical axis indicates the output voltage. The characteristics of the piezoelectric transformer 101 generally have a characteristic that the output voltage becomes maximum at the resonance frequency f0. As shown in FIG. 2, the output voltage of the piezoelectric transformer 101 can be controlled by the drive frequency. When the power supply device 10 is activated (when high voltage output is started), the driving of the piezoelectric transformer 101 is started from the initial frequency fi, and the frequency sweep is performed from fi to the low frequency side. When the output voltage reaches the target voltage Edc when the frequency sweep reaches a certain drive frequency fx, the drive frequency sweep ends. Thereafter, the piezoelectric transformer 101 continues to output the target voltage stably at the drive frequency fx.

  In a state where the power supply device 10 stops outputting a high voltage to the image forming apparatus, both the control voltage Vcont and the feedback voltage Vfb are 0V. When the controller 40 instructs to start outputting a high voltage, the control voltage Vcont is set to a voltage corresponding to the target voltage Edc. The amplification degree of the differential amplifier circuit 11 determined by the resistor 110 and the resistor 111 is set to be very large. Therefore, V1 becomes 0V in the initial stage of high voltage output. As the feedback voltage Vfb approaches the control voltage Vcont, the voltage level of V1 increases.

  The input voltage V2 of the voltage controlled oscillator 130 is a value obtained by inverting the above-described V1 using the voltage at the non-inverting input terminal of the operational amplifier 120 as a reference voltage. As described above, since the resistance values of the resistor 123 and the resistor 124 are the same, the reference voltage is Vcc / 2. Therefore, V2 is equal to Vcc in the initial stage. As the feedback voltage Vfb approaches the control voltage Vcont, the voltage level of V2 decreases. The voltage controlled oscillator 130 outputs a signal having an initial frequency fi when the control voltage V2 is Vcc. As the control voltage V2 decreases, the output frequency of the voltage controlled oscillator 130 changes lower than fi.

<Improved Configuration of Power Supply Device 10>
As described above, when the driving of the piezoelectric transformer 101 is started from the output frequency fi, a considerable time is required until the output voltage reaches the target voltage. Therefore, the driving frequency fx corresponding to the target voltage is obtained, and the frequency sweeping is started from the vicinity thereof, so that the time is shortened in the first embodiment.

  The power supply device 10 according to the first embodiment will be described with reference to FIG. Among the constituent elements shown in FIG. 4, the same reference numerals are given to the elements common to the constituent elements shown in FIG. 2, thereby simplifying the description. Compared with FIG. 2, in FIG. 4, the output voltage from the operational amplifier 120 that is the control voltage V <b> 2 of the voltage controlled oscillator 130 is fed back via the holding circuit 20, and the non-inverting input terminal of the operational amplifier 120 of the inverting amplifier circuit 12. It differs in that it is applied to. The holding circuit 20 functions as a holding unit that holds information indicating a control voltage corresponding to the frequency of a signal output from the voltage controlled oscillator when the output voltage from the power supply device becomes the target voltage. The holding circuit 20 includes a sample hold circuit that holds the potential of the control voltage V2.

  The configuration of the holding circuit 20 will be described. The holding circuit 20 includes a sample and hold circuit that functions as a kind of holding unit or storage unit. In the holding circuit 20, operational amplifiers 201 and 206, resistors 202 and 203, a switch 204, and a capacitor 205 form a kind of sample and hold circuit, among others. The switches 204 and 298 can be realized by, for example, FETs. The first characteristic point is that the output voltage (control voltage V2) of the sampled operational amplifier 120 is divided by a voltage dividing circuit including a resistor 202 and a resistor 203, and the divided value V2 ′ is held. It is. The second characteristic point is that the output of the sample and hold circuit is connected to the non-inverting input terminal of the operational amplifier 120.

  Here, in order to make the operation and effect of this circuit easier to understand, an example of the voltage controlled oscillator 130 will be described with reference to FIGS. Although FIG. 5 illustrates a voltage controlled oscillator using a sawtooth wave generation circuit as an example, the present invention is not limited to this. This is because the present invention does not depend on the internal structure of the voltage controlled oscillator 130.

  The control voltage V2 of the voltage controlled oscillator 130 is divided by the resistors 220 and 221 and input to the non-inverting input terminal of the operational amplifier 222. The operational amplifier 222, the transistor 223, and the resistor 224 form a constant current circuit. A current i corresponding to the input voltage at the non-inverting input terminal of the operational amplifier 222 flows through the resistor 224.

  On the other hand, a capacitor 229 is connected between the collector of the transistor 223 and the constant voltage source Vcc. Due to the charging / discharging operation of the capacitor 229, the voltage Va applied to the collector becomes a sawtooth wave as shown in FIG. One terminal of the capacitor 229 is connected to the constant voltage source Vcc, and the other terminal is connected to the collector of the transistor 223. Therefore, when the current i flows, the charge charged in the capacitor 229 decreases, and the voltage Va generated at the other terminal of the capacitor 229 decreases. The voltage Va is also applied to the non-inverting input terminal of the comparator 225.

  When the voltage Va drops below the threshold voltage Vth determined by the resistors 226 and 227, the output of the comparator 225 is inverted, and as a result, the transistor 228 is turned on. Accordingly, both ends of the capacitor 229 are short-circuited by the transistor 228, and the capacitor 229 is charged again by the voltage Vcc from the voltage source. That is, the voltage Va rises again to Vcc. When the voltage Va becomes Vcc, the output of the comparator 225 is inverted again and the transistor 228 is turned off. The voltage Va decreases below Vcc as the current i flows. This operation is repeated, and the voltage Va becomes a sawtooth wave as shown in FIG.

  The voltage Va is also applied to the non-inverting input terminal of the subsequent stage comparator 230. The comparator 230 compares the voltage Va and Vduty determined by the resistors 231 and 232, and outputs a comparison result. An output voltage Vout is generated at the output terminal of the comparator 230 to which the pull-up resistor 233 is connected. As shown in FIG. 6, the output voltage Vout is a rectangular wave. Further, a three-state buffer 234 is connected to the subsequent stage of the comparator 230, and whether or not to output the output voltage from the comparator 230 to the outside is controlled by the level of the control voltage Vcont. Due to the presence of the three-state buffer 234, when the output of the high voltage is stopped, the output from the voltage controlled oscillator 130 can be stopped immediately. This output voltage Vout is input to the gate terminal of the second FET 131 shown in FIG. This is the drive frequency of the piezoelectric transformer 101.

  The repetition period T shown in FIG. 6 depends on the slope of the voltage drop of the voltage Va. This slope is determined by the current i. That is, the repetition period T can be expressed by the following equation by the control voltage V2 of the voltage controlled oscillator 130. Here, C229 is a capacitance value of the capacitor 229, and R224 is a resistance value of the resistor 224.

T = ((Vcc−Vth) / V2) × C229 × R224 (1)
Therefore, the output frequency of the voltage controlled oscillator 130 is a frequency proportional to the control voltage V2.

  Returning to the description of FIG. 4, the update method and circuit principle of the sweep start frequency in the first embodiment will be described. Here, an initial operation in which power is supplied to the power supply apparatus 10 and a high voltage is first output and an operation after that will be described separately. First, the initial operation will be described.

(Initial operation)
As can be seen from the circuit configuration of FIG. 4, the voltage applied to the non-inverting input terminal of the operational amplifier 206 is selected by the switch 204 and the switch 208. In the initial stage, the controller 40 outputs an initialization signal to the control terminal of the switch 208, so that the switch 208 is turned on. On the other hand, since the controller 40 outputs a high voltage stop signal to the control terminal of the switch 204, the switch 204 is off. In this initial state, the voltage value determined by the resistors 209 and 210 is input to the non-inverting input terminal of the operational amplifier 206. This voltage value is a voltage value corresponding to the initial frequency fi determined at the design stage of the power supply device 10. The operational amplifier 206 functions as a voltage follower like the operational amplifier 201. The output voltage of the operational amplifier 206 is input to the non-inverting input terminal of the operational amplifier 120 via the protective resistor 207. In the second embodiment, the resistance values of the resistors 209 and 210 are the same value. Therefore, the voltage at the non-inverting input terminal is Vcc / 2.

  In the first embodiment, the resistance values of the resistors 121 and 122 are set to the same value. Therefore, the inverting amplifier circuit 12 operates as a so-called inverter having an amplification factor of 1. That is, the inverting amplifier circuit 12 inverts the voltage V1 input from the previous stage using the voltage Vcc / 2 applied to the non-inverting input terminal as the reference voltage.

  As described with reference to FIG. 2, the control voltage V2 of the voltage controlled oscillator 130 is Vcc at the start of the sweep, and falls from there. That is, the frequency sweep as a power supply circuit that outputs a high voltage starts from the initial frequency, and continues to the low frequency side as the control voltage V2 decreases.

(Short startup)
Next, shortened activation will be described. The first embodiment is characterized in that a drive frequency fx ′ slightly higher than the drive frequency fx when the output voltage reached the target voltage in the previous time is held, and sweeping is started from the drive frequency fx ′. Compared with the initial frequency fi, since the drive frequency fx ′ is the same as or very close to the drive frequency fx for outputting the target voltage, it can be expected that the sweep time is significantly shortened.

  The target voltage Edc is output from the power supply device 10, and the driving frequency of the piezoelectric transformer 101 at that time is fx. Further, it is assumed that the control voltage V2 of the voltage controlled oscillator 130 at that time is Vdc.

  The controller 40 turns off the switch 208 by stopping sending the initialization signal before stopping the output of the high voltage. Subsequently, immediately before the power supply device 10 stops the high voltage output, the controller 40 outputs a pulse signal (high voltage stop signal) for stopping the high voltage output to the control terminal of the switch 204. As a result, the switch 204 is turned on for a predetermined time t. As a result, a voltage obtained by dividing Vdc by the resistors 202 and 203 is held in the capacitor 205. Here, the predetermined time t may be a time sufficient for charging the capacitor 205. The pulse signal as the high voltage stop signal may be output by the controller 40, or may be generated from the falling waveform when the control voltage Vcont falls when the high voltage output is stopped.

  Since the controller 40 reduces the control voltage Vcont to 0V, the output of the driving frequency for driving the piezoelectric transformer 101 output from the voltage controlled oscillator 130 is stopped by the function of the three-state buffer 234. As a result, the value of the high voltage output from the power supply device 10 decreases.

  The characteristic voltage division by the resistor 202 and the resistor 203 in the first embodiment is an important factor for determining how high the drive is started from the previous drive frequency fx. Here, for the sake of easy understanding, a configuration in which driving is started from the same frequency as the previous driving frequency fx will be described.

  The resistance values of the resistor 202 and the resistor 203 are the same. The voltage value held by the sample hold circuit of the holding circuit 20 is Vdc / 2, that is, the input terminals of the operational amplifier 120 are both Vdc / 2. Next, when the power supply device 10 is started up in this state, the inversion reference voltage of the inverter centering on the operational amplifier 120 is Vdc / 2. Therefore, even at the start of startup when the voltage V1 in the previous stage of the inverter is 0 V, the output voltage of the operational amplifier 120 (the control voltage V2 of the voltage controlled oscillator 130) becomes Vdc, and the power supply device 10 can be started up. That means that the drive frequency sweep for driving the piezoelectric transformer 101 is started not from the initial frequency fi but from the previous drive frequency fx.

  Now, the load on various biases actually used by the image forming apparatus 100 is not limited to the load of the transfer bias, and always varies during image formation. This is caused by uneven resistance values in the circumferential direction of the transfer roller and the charging roller. On the other hand, even if the power supply device 10 drives the piezoelectric transformer 101 at a constant frequency, the output voltage changes due to the resistance of the load connected to the output terminal 105. In order to maintain the output voltage constant, a circuit configuration is required in which the frequency is always changed following the load change.

  The frequency sweep type power supply circuit shown in FIG. 4 can only set the control region as a region located on the lower frequency side than the frequency at which the drive is started. For this reason, when driving is started at the driving frequency fx, the driving frequency can be controlled only in the lower frequency region than fx, and it is not possible to shift to the high frequency region following the load fluctuation. Therefore, instead of starting the sweep from the previous drive frequency fx, the sweep is started from a frequency higher than the previous drive frequency fx by a predetermined amount, thereby ensuring a margin for following the load fluctuation. Such a margin would not be necessary for a power supply device with sufficiently small load fluctuations. A configuration that determines a frequency that is a predetermined amount higher than the previous drive frequency fx is the voltage dividing ratio of the resistor 202 and the resistor 203. A circuit for setting a frequency higher than the previous drive frequency fx as the sweep start frequency can be realized by making the resistance value of the resistor 203 larger than the resistance value of the resistor 202. The voltage sampled and held by this voltage division ratio is (Vdc / 2) + α. When this voltage becomes the inversion reference voltage of the inverter centering on the operational amplifier 120, the initial voltage V2 can be expressed by equation (2).

V2 = Vdc + 2α (2)
That is, sweeping is started from a frequency that is higher by a frequency corresponding to + 2α. Note that the value of the sweep start frequency fi ′ can be calculated by substituting Equation (2) into Equation (1). The sweep start frequency is a frequency used when the sweep is resumed in order to resume the output of the high voltage, and thus may be called a sweep resume frequency.

  As shown in FIG. 7, the sweeping is started from the initial frequency fi in the initial stage. However, after the initial stage, the controller 40 switches on the switch 204 to set the reference voltage of the operational amplifier 120 to a voltage corresponding to the sweep start frequency fi ′ held by the sample hold circuit. As a result, the sweep is started from the sweep start frequency fi '. As apparent from FIG. 7, the sweep start frequency fi ′ is closer to the previous drive frequency fx than the initial frequency fi, so the time until the output voltage of the power supply device 10 reaches the target voltage can be shortened. . In addition, by considering the margin α, it is possible to cope with load fluctuations. The controller 40 (MPU 41) and the switch 204 function as a setting unit that sets, in the frequency control unit, the sweep start frequency corresponding to the information held in the holding unit instead of the initial frequency.

  The important point in the first embodiment is the relationship between the “resistance value ratio of the resistor 122 and the resistor 121” and the “resistance value ratio of the resistor 202 and the resistor 203”. In order to make the relationship between the resistance and the resistance value easy to understand, the resistance value of the resistance x is expressed as Rx. x indicates a reference number.

  First, the resistance value ratio between the resistor 122 and the resistor 121 is R122 / R121, which determines the amplification degree of the inverting amplifier circuit 12. In the first embodiment, it is assumed that R122 / R121 = 1. The amplification degree can be determined in accordance with the ratio between the voltage V1 input to the inverting amplifier circuit 12 and the control voltage V2 output from the inverting amplifier circuit 12. Therefore, the amplification degree is not limited to 1. The resistance value ratio R202 / R203 between the resistor 202 and the resistor 203, which is an important element of the first embodiment, is a value that determines the next inversion reference voltage.

R122 / R121> = R202 / R203 (3)
By setting the value of each resistance value so that the expression (3) always holds, sweeping from a frequency equal to or higher than the previous drive frequency fx can be started.

  Control of the secondary transfer bias when the power supply device 10 according to the first exemplary embodiment is applied to the image forming apparatus 100 will be described with reference to a flowchart of FIG. Here, when a plurality of toner images are continuously formed on an image carrier such as a photosensitive drum or an intermediate transfer member, the toner image is generated between a region where a certain toner image is formed and a region where a next toner image is formed. It is assumed that a detection pattern is formed in a non-image area (so-called paper interval). The detection pattern includes a color shift detection pattern for detecting color shift and a density detection pattern for detecting the density of the toner image for adjusting the transfer bias. It is assumed that the positive bias of the secondary transfer is switched from OFF to ON between the sheets. In this flowchart, details of control other than the positive bias between the sheets are well-known and will be omitted. In addition, the control processing according to this flowchart is executed by the MPU 41 according to a program stored in the ROM 43 in advance. Here, control processing after the print job is started will be described.

  In step S801, the MPU 41 outputs the initialization signal to the control terminal of the switch 208, thereby switching the switch 208 from off to on. Thereby, the voltage input to the non-inverting input terminal of the operational amplifier 120 can be initialized. The switch 208 and the like are initialized to initialize the frequency set in the frequency control unit in order to start driving the piezoelectric transformer by the drive unit at an initial frequency different from the frequency corresponding to the information held in the holding unit. It functions as a part. The MPU 41 keeps the high voltage stop signal off. The process of S801 may be executed before the start of the print job, for example, immediately after the main power supply of the image forming apparatus 100 is turned on. Incidentally, the state in which the switch 208 is on and the high voltage stop signal is off is a so-called standby state.

  In step S <b> 802, the MPU 41 sets and outputs a voltage for setting the output voltage to the target voltage (target equivalent voltage) as the control voltage Vcont in order to activate the power supply apparatus 10 that is responsible for supplying the transfer bias. In step S803, the power supply apparatus 10 starts sweeping the drive frequency of the piezoelectric transformer 101 from the initial frequency fi. Finally, the output voltage from the power supply device 10 rises to the target voltage Edc. Note that S803 is not the operation of the MPU 41 but the operation of the power supply apparatus 10, but is described in this flowchart for easy understanding of the first embodiment. However, in order to clearly indicate that the processing is not performed by the MPU 41, a broken line is shown.

  In step S804, the MPU 41 stands by until the trailing edge of the recording paper passes through the nip for secondary transfer. Whether the trailing edge of the recording paper has passed through the nip can be determined by the MPU 41 from the output result of a paper detection sensor (not shown). When it is confirmed that the trailing edge of the recording paper has passed through the nip, the process proceeds to S805.

  In step S805, the MPU 41 determines whether the job has ended. The MPU 41 knows how many recording sheets the image should be formed from the job data. Further, the MPU 41 counts the number of recording sheets on which images have already been formed. Therefore, the MPU 41 can determine from the count value whether the job has ended. If the job is completed, the process proceeds to S812. If the job is not completed, the process proceeds to S806.

  In step S806, the MPU 41 switches the initialization signal off in order to hold the voltage corresponding to the drive frequency fx by the sample hold circuit of the holding circuit 20. Further, in S807, the MPU 41 outputs a high voltage stop signal for one pulse. Thereby, the voltage corresponding to the drive frequency fx is held by the sample hold circuit. In S808, the MPU 41 sets the value of the control voltage Vcont to 0 V in order to stop the high voltage output from the power supply apparatus 10.

  In step S809, the MPU 41 executes the above-described inter-sheet sequence. Further, in S810, the MPU 41 sets the control voltage Vcont to the target equivalent voltage in order to cause the power supply device 10 to start outputting a high voltage again. In S811, the power supply apparatus 10 starts sweeping the drive frequency from the sweep start frequency fi 'corresponding to the voltage stored or held by the holding circuit 20. As a result, the output voltage rises again to the target voltage Edc. Note that S811 is not the operation of the MPU 41 but the operation of the power supply apparatus 10. Thereafter, the process returns to S804.

  Until the job is finished, the processes from S804 to S811 are continuously executed. When the image formation is completed for the last sheet of the job, the process proceeds to S812. In S812, the MPU 41 stops the output of the high voltage from the power supply apparatus 10 by switching the control voltage Vcont to 0V.

  As described above, according to the first embodiment, the divided value of the control voltage of the voltage controlled oscillator 130 that controls the frequency for driving the piezoelectric transformer 101 is held by the sample hold circuit. Further, in Example 1, when the driving of the piezoelectric transformer 101 is restarted next time, a control voltage corresponding to the sweep start frequency is generated with reference to the held voltage. Thereby, it becomes possible to start driving the piezoelectric transformer 101 from the vicinity of the previous drive frequency fx, and the time is shortened until the output voltage reaches the target voltage. By starting sweeping from the vicinity of the higher frequency side to the vicinity of the lower frequency side rather than from the previous drive frequency fx, it is possible to stably maintain a desired voltage output against load fluctuations. Enables frequency tracking.

[Example 2]
Example 2 will be described with reference to FIG. The second embodiment differs from the first embodiment in that an A / D converter 301 and a D / A converter 302 are used as voltage holding means corresponding to the sweep start frequency. The A / D converter 301 functions as an analog-digital converter that converts a signal output from the voltage controlled oscillator from an analog value to a digital value. The D / A converter 302 functions as a digital-analog converter that converts a digital value into an analog value, and latches and holds the digital value when the output voltage is the target voltage.

  The configuration of the second embodiment is more expensive than the configuration of the first embodiment. However, as the holding unit, the voltage at the non-inverting input terminal of the operational amplifier 120 can be maintained until the power supply is cut off. Therefore, there is an advantage that the sweep start frequency can be held across a plurality of print jobs. Note that the description of matters common to the first embodiment is omitted.

  According to FIG. 9, in addition to the A / D converter 301 and the D / A converter 302, the voltage controlled oscillator 130, its control circuits, and the D / A converter 304 for outputting the control voltage Vcont are integrated as an IC 300. IC. The MPU 41 mounted on the controller 40 transmits a communication signal and writes various data and commands in the register 303. Thereby, the MPU 41 controls the power supply device 10. That is, the register 303 stores digital data to the D / A converter 304, a bit as an initialization signal for the D / A converter 302, and a bit as a high voltage stop signal. These digital data and bits can be rewritten by the MPU 41.

  As in the first embodiment, an initialization signal is input to the D / A converter 302 so that a voltage of Vcc / 2 is output. For example, the D / A converter 302 itself is configured to output Vcc / 2 by an initialization signal (single signal). Alternatively, the initialization signal from the register 303 may be digital data, and digital data that outputs a voltage of Vcc / 2 may be sent from the register 303 to the D / A converter 302. In the second embodiment, the former configuration is adopted. The D / A converter 302 latches and outputs the digital data output from the A / D converter 301 according to the high voltage stop signal from the register 303. Specifically, when the high voltage stop signal changes from Low to High, the D / A converter 302 performs a latch operation. The configuration of the second embodiment is more complicated than the configuration of the first embodiment, but by using an IC, the unit unit price is relatively inexpensive and the occupied area on the substrate can be minimized.

  Control of the secondary transfer bias when the power supply apparatus 10 according to the second embodiment is applied to the image forming apparatus 100 will be described with reference to the flowchart of FIG. In step S <b> 1001, the MPU 41 writes a bit indicating initialization of the D / A converter 302 in the register 303. The D / A converter 302 outputs a voltage corresponding to the initial frequency fi. Note that S1001 may be executed before the start of a print job (for example, immediately after the image forming apparatus 100 is turned on). As described above, the MPU 41 and the like function as the initialization unit described above.

  In step S <b> 1002, the MPU 41 writes digital data corresponding to the target voltage to be set in the D / A converter 304 in the register 303 in order to start outputting a high voltage from the power supply device 10. The D / A converter 304 outputs the control voltage Vcont by converting the digital data written in the register 303 into analog data. In S1003, the power supply apparatus 10 starts sweeping the drive frequency from the initial frequency fi, and finally the output voltage reaches the target voltage Edc.

  In step S1004, the MPU 41 stands by until the trailing edge of the recording paper passes through the nip for secondary transfer. When it is confirmed that the trailing edge of the recording paper has passed through the nip, the process proceeds to S1005. In step S1005, the MPU 41 determines whether the job has ended. If the job is completed, the process proceeds to S1011. If the job is not completed, the process proceeds to S1006.

  In S1006, the MPU 41 writes a bit for switching the high voltage stop signal from on to off in the register 303. When this bit is written, the D / A converter 302 latches the data output from the A / D converter 301. Here, the latched data indicates a voltage value corresponding to the sweep restart frequency fi ′.

  In step S1007, the MPU 41 writes digital data for setting the control voltage Vcont to 0 V in the register 303. Thereby, since the D / A converter 304 outputs 0 V as the control voltage Vcont, the output of the high voltage from the power supply device 10 is stopped.

  In step S1008, the MPU 41 executes the above-described inter-sheet sequence. Further, in S1009, the MPU 41 sets the control voltage Vcont to the target equivalent voltage in order to cause the power supply device 10 to start outputting a high voltage again. In step S <b> 1010, the power supply apparatus 10 restarts the frequency sweep from the sweep start frequency fi ′ obtained by adding a margin to the previous drive frequency fx latched in the D / A converter 302. Thereafter, the process returns to S1004. When the job ends, the process proceeds to S1011. In step S <b> 1011, the MPU 41 writes digital data for setting the control voltage Vcont to 0 V in the register 303. Thereby, since the D / A converter 304 outputs 0 V as the control voltage Vcont, the output of the high voltage from the power supply device 10 is stopped.

  In the second embodiment, the divided voltage value of the control voltage V2 of the voltage controlled oscillator 130 that controls the frequency for driving the piezoelectric transformer 101 is held using the A / D converter 301 and the D / A converter 302. The sweep is started from the sweep start frequency fi ′ corresponding to a certain value. Therefore, the second embodiment can achieve the same effects as the first embodiment. Further, in the second exemplary embodiment, the voltage value corresponding to the sweep start frequency fi ′ can be held from when the main power supply of the image forming apparatus 100 is turned on to when it is turned off. Therefore, even when a certain print job ends and the next print job is executed, the sweep can be started from the sweep start frequency fi ′ instead of the initial frequency fi.

[Example 3]
Example 3 will be described with reference to FIG. The third embodiment is different from the first and second embodiments in that the EEPROM 401 is used as a voltage holding unit corresponding to the sweep start frequency. As shown in FIG. 11, the divided value obtained by dividing the control voltage V2 of the voltage controlled oscillator 130 by the resistors 202 and 203 is input to the A / D port included in the MPU 41. Therefore, the MPU 41 can read the value by converting the analog partial pressure value into the digital partial pressure value by the A / D converter 402. The MPU 41 holds the read value in a memory (eg, EEPROM 401), and outputs it to the D / A port connected to the non-inverting input terminal of the operational amplifier 120 via the D / A converter 403. The MPU 41 can output the control voltage Vcont via the D / A converter 404. The configuration shown in the third embodiment is more expensive than the configurations of the first and second embodiments, but the sweep start frequency fi ′ can be inherited even when the main power supply of the image forming apparatus 100 is turned off. There are advantages.

  Further, in the third embodiment, since the MPU 41 can monitor the partial pressure value, the frequency range actually used can be monitored and stored in the EEPROM. Therefore, it is possible to determine an appropriate initial frequency fi according to the resonance frequency of each mounted piezoelectric transformer 101 and to make the initial frequency fi variable.

  An appropriate initial frequency setting method according to the mounted piezoelectric transformer 101 will be described with reference to FIG. In step S1201, the MPU 41 instructs the motor drive circuit (not shown) that rotates the secondary transfer roller to start rotation. In S1202, the MPU 41 outputs the control voltage Vcont set to a value corresponding to the target voltage from the D / A converter 404. As a result, the power supply device 10 starts outputting a high voltage. However, the value corresponding to the target voltage is the lowest voltage value among the voltages used by the image forming apparatus 100. In step S1203, the piezoelectric transformer 101 starts to be driven from the highest drive frequency in the drive frequency range.

  In step S1204, the MPU 41 monitors (measures, measures, or monitors) the divided voltage value of the control voltage V2 to the voltage controlled oscillator 130 through the A / D converter 402. In step S1205, the MPU 41 stores the monitored value in the EEPROM 401. In S1206, the control voltage Vcont of 0 V is output from the D / A converter 404, and the high voltage output from the power supply apparatus 10 is stopped. In step S1207, the MPU 41 instructs the motor drive circuit (not shown) that rotates the secondary transfer roller to stop the rotation. In S1208, the MPU 41 outputs the value read from the EEPROM 401 to the D / A converter 403. As a result, a voltage corresponding to the value read from the EEPROM 401 is applied to the operational amplifier 120.

  Thereby, when the power supply device 10 outputs the next high voltage, the initial frequency of the piezoelectric transformer 101 can be changed to an appropriate initial frequency in the power supply device. The initial frequency after the change reflects the variation in resonance frequency and the variation in circuit constants. The MPU 41 may read the initial frequency determined in the design stage from the EEPROM 401 and output it to the D / A converter 403. Thereby, the MPU 41 functions as the initialization unit described above.

  Needless to say, the configuration of the third embodiment operates in the same manner as the second embodiment. In the second embodiment, the divided voltage value corresponding to the sweep start frequency is held by the D / A converter 302. However, in the third embodiment, it can be held in the EEPROM 401. The A / D converter 402 functions as an analog-digital converter that converts a signal output from the voltage controlled oscillator from an analog value to a digital value. The EEPROM 401 functions as a memory for storing a digital value when the output voltage is the target voltage. Further, the D / A converter 403 functions as a digital-analog converter that converts a digital value stored in the memory into an analog value.

  Here, the EEPROM 401 is used as an example of the memory, but another nonvolatile memory such as a flash memory may be adopted. If it is not necessary to take over the sweep start frequency fi ′ when the main power supply of the image forming apparatus 100 is turned off, the EEPROM 401 may be replaced with a volatile memory such as a DRAM.

  In the above-described embodiment, since the present invention is applied to a power supply device applied to an electrophotographic image forming apparatus, the output voltage of the power supply device 10 has been called a high voltage. Of course, the present invention may be applied to a power supply device of an electronic apparatus other than the electrophotographic image forming apparatus. In that case, the output voltage of the power supply device 10 may not be a high voltage.

  In the above-described embodiment, the frequency sweep type power supply circuit can set the control region only in the region located on the lower frequency side than the drive start frequency. However, the present invention can also be applied to a power supply circuit in which only a region on a higher frequency side than the frequency at which driving is started can be set as a control region. Furthermore, the present invention may be applied to a power supply circuit capable of sweeping the frequency on the high frequency side or the low frequency side.

Claims (4)

A power supply unit,
A voltage controlled oscillator that generates a signal having a frequency according to the input control voltage;
A drive unit to which a signal output from the voltage controlled oscillator is input;
A piezoelectric transformer driven by the driving unit in accordance with a signal output from the voltage controlled oscillator;
A frequency control unit that starts driving the piezoelectric transformer by the drive unit at a predetermined initial frequency and sweeps the frequency through the control voltage so that an output voltage from the power supply device becomes a predetermined target voltage. When,
Analog-digital that converts information indicating the control voltage corresponding to a frequency between the frequency of the signal output from the voltage controlled oscillator and the initial frequency when the output voltage reaches the target voltage from an analog value to a digital value A converter and a digital-analog converter that converts the digital value into an analog value, the digital-analog converter latches and holds the digital value of information indicating the control voltage ; and
The initial frequency instead, the power supply apparatus characterized by comprising a setting unit for setting a sweep start frequency corresponding to the information held in the holding unit to the frequency control unit. A power supply unit,
A voltage controlled oscillator that generates a signal having a frequency according to the input control voltage;
A drive unit to which a signal output from the voltage controlled oscillator is input;
A piezoelectric transformer driven by the driving unit in accordance with a signal output from the voltage controlled oscillator;
A frequency control unit that starts driving the piezoelectric transformer by the drive unit at a predetermined initial frequency and sweeps the frequency through the control voltage so that an output voltage from the power supply device becomes a predetermined target voltage. When,
Analog converting the information indicating the control voltage the output voltage corresponds to a frequency between the frequency and the initial frequency of the voltage controlled oscillator output signal when become the target voltage from analog values into digital values - a holding unit which includes an analog converter - digital converter, a memory for storing the digital value of the information indicating the control voltage, before Symbol converting the digital value stored in the memory to an analog value digital ,
The initial frequency instead, power supplies, characterized in that it comprises a <br/> a setting unit that sets the frequency control unit the sweep start frequency corresponding to the information held in the holding unit. The power supply device according to claim 2 , wherein the memory is a nonvolatile memory. An image forming apparatus,
The power supply device according to any one of claims 1 to 3 ,
An image forming apparatus comprising: an image forming unit that forms an image on a recording medium when power is supplied from the power supply device.

Images