JP2008125303A - Constant-current power supply unit and image forming device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a constant-current power supply unit capable of obtaining a circuit which can keep an output current to a load at a prescribed value with high accuracy, and can control an oscillation frequency to a piezoelectric transformer with high accuracy that is not influenced by external factors, and achieving a simple constitution and cost reduction. <P>SOLUTION: An AC voltage outputted from the piezoelectric transformer 4 is distributed by at least two resistors R13, 14, and a resistance pressure voltage dividing circuit 5 which steps down the AC voltage to be outputted is provided to return the AC voltage outputted by the resistance voltage dividing circuit 5 to an oscillation circuit 2 through a feedback circuit 9. Additionally, the oscillation circuit 2 generates an arbitrary oscillation frequency by using the AC voltage returned by the feedback circuit 9 as a control signal and outputs the frequency to the piezoelectric transformer 4. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a power supply device having a piezoelectric transformer that transforms an output voltage based on an input AC voltage, and in particular, a rectifier circuit that converts the output voltage of the piezoelectric transformer into a DC voltage, and the rectifier circuit that outputs the rectifier circuit. The present invention relates to a constant current power supply apparatus having a constant current control circuit that maintains a current value of a DC voltage at a predetermined value, and an image forming apparatus including the constant current power supply apparatus.

2. Description of the Related Art Conventionally, an electrophotographic image forming apparatus such as a copying machine, a printer apparatus, or a facsimile apparatus includes a constant current power supply device that applies a predetermined current to a charging roller of a charger or a transfer roller of a transfer apparatus. The In the image forming apparatus, it is important for the accuracy of image formation that the current applied by the constant current power supply apparatus is accurately maintained at a predetermined value.
By the way, in recent years, a small-sized piezoelectric transformer may be used for the constant current power supply device with high efficiency and less noise. This piezoelectric transformer transforms the output voltage by utilizing the resonance phenomenon of the piezoelectric vibrating body caused by the AC voltage of an arbitrary frequency input from the oscillation circuit. At this time, in order to drive the piezoelectric transformer efficiently, it is necessary to input an AC voltage at a resonance frequency unique to the piezoelectric transformer. Therefore, a method of controlling the oscillation frequency by feeding back the output voltage of the rectifier circuit that rectifies the AC voltage output from the piezoelectric transformer and outputs the DC voltage to the load to the oscillation circuit is known. However, in this method, since the DC voltage supplied to the load is branched, there is a problem that the accuracy of constant current control of the DC current supplied to the load is lowered.
Therefore, for example, Patent Document 1 proposes detecting the output voltage of the piezoelectric transformer as a feedback signal by electrostatic induction and feeding it back to the oscillation circuit. In this case, it is possible to prevent a decrease in the accuracy of constant current control of the direct current output to the load.
JP 2005-184896 A

However, in the technique of Patent Document 1, since the output of the piezoelectric transformer is detected by electrostatic induction, variations in the distance between the piezoelectric transformer and the electrode for electrostatic induction and the ambient environment (temperature) of the piezoelectric transformer. The capacitance between the piezoelectric transformer and the electrode fluctuates due to external factors such as the variation in dielectric constant due to fluctuations in the voltage and humidity, and the phase of the AC voltage fed back to the oscillation circuit varies. This causes a problem that high-precision control cannot be performed.
On the other hand, the AC voltage output from the piezoelectric transformer is branched before the rectifier circuit and directly fed back to the oscillation circuit, so that the accuracy of the oscillation frequency control due to the external factors can be prevented from being reduced. Since the output AC voltage is high voltage, it is necessary to use a high rated voltage electronic component such as a capacitor provided on the oscillation circuit and the feedback path to the oscillation circuit, resulting in a high cost. Arise.
Accordingly, the present invention has been made in view of the above circumstances, and its object is to maintain the output current to the load at a predetermined value with high accuracy and to influence the oscillation frequency to the piezoelectric transformer due to the influence of external factors. It is an object of the present invention to provide a constant current power supply device that can realize a circuit that can be controlled with high accuracy without being affected by a simple and low-cost configuration, and an image forming apparatus including the constant current power supply device.

In order to achieve the above object, the present invention transforms an output voltage based on an oscillation circuit that generates an AC voltage having an arbitrary oscillation frequency based on an input control signal and an AC voltage output from the oscillation circuit. A piezoelectric transformer; a rectifier circuit that converts an AC voltage output from the piezoelectric transformer into a DC voltage; and a rectifier that controls driving of the piezoelectric transformer based on a current value of the DC voltage output from the rectifier circuit. And a constant current control circuit that controls the current value of the DC voltage output from the circuit to be a predetermined value, the AC current output from the piezoelectric transformer. A resistance voltage dividing circuit for stepping down and outputting the AC voltage by dividing the voltage by at least two resistors, and the AC voltage output by the resistance voltage dividing circuit. And a feedback circuit that feeds back the oscillation circuit to the oscillation circuit, wherein the oscillation circuit generates an arbitrary oscillation frequency using the AC voltage fed back by the feedback circuit as the control signal. Configured as a power supply.
According to the present invention, a low AC voltage output from the piezoelectric transformer and divided by the resistance voltage dividing circuit can be fed back to the oscillation circuit. The rated voltage required for an electronic component such as a capacitor provided in the can be reduced. Therefore, it is possible to realize a circuit capable of performing control for maintaining the frequency input to the piezoelectric transformer at the resonance frequency of the piezoelectric transformer with high accuracy with a simple and low-cost configuration. Furthermore, since the AC voltage from the piezoelectric transformer is branched and fed back before the rectifier circuit, adverse effects on the constant current control circuit can be prevented, and the output current to the load can be accurately controlled. Can be kept at default value.
The present invention can also be understood as an invention of an image forming apparatus provided with the constant current power supply device. Specifically, in the image forming apparatus, it is conceivable to use the constant current power supply device in order to supply a constant current DC voltage to a charger or transfer device used for image formation. Thereby, a direct current maintained at a predetermined value with high accuracy is supplied to the charger and the transfer device, so that high image forming accuracy can be maintained.

  According to the present invention, a low AC voltage output from the piezoelectric transformer and divided by the resistance voltage dividing circuit can be fed back to the oscillation circuit. The rated voltage required for an electronic component such as a capacitor provided in the can be reduced. Therefore, it is possible to realize a circuit capable of performing control for maintaining the frequency input to the piezoelectric transformer at the resonance frequency of the piezoelectric transformer with high accuracy with a simple and low-cost configuration. Furthermore, since the AC voltage from the piezoelectric transformer is branched and fed back before the rectifier circuit, adverse effects on the constant current control circuit can be prevented, and the output current to the load can be accurately controlled. Can be kept at default value.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings for understanding of the present invention. The following embodiment is an example embodying the present invention, and does not limit the technical scope of the present invention.
FIG. 1 is a block diagram showing a schematic configuration of a constant current power supply device X according to an embodiment of the present invention, FIG. 2 is an electric circuit diagram for explaining a specific example of the constant current power supply device X, and FIG. FIG. 3 is a diagram showing voltage waveforms at points P1 to P4 in the electric circuit diagram of FIG. 2.
First, a schematic configuration of a constant current power supply device X according to an embodiment of the present invention will be described with reference to FIG.
As shown in FIG. 1, the constant current power supply device X includes a reference voltage generation circuit 1, an oscillation circuit 2, a resonance circuit 3, a piezoelectric transformer 4, a resistance voltage dividing circuit 5, a rectifier circuit 6, a DC current detection circuit 7, A current control circuit 8 and a feedback circuit 9 are provided. The constant current power supply device X boosts a DC voltage (DC24V) supplied from a DC power supply Vcc to a high voltage and outputs it from an output terminal Vout. A negative voltage (about −500 V to −1000 V) is output from the output terminal Vout.
Specifically, the constant current power supply device X applies a predetermined current to a charging roller of a charger or a transfer roller of a transfer device in an electrophotographic image forming apparatus such as a copying machine, a printer device, or a facsimile device. Used as a power source.

The reference voltage generation circuit 1 is a so-called shunt regulator, and generates a reference voltage based on a DC voltage supplied from the DC power supply Vcc.
The oscillation circuit 2 generates an AC voltage at an arbitrary oscillation frequency based on the input control signal and inputs the AC voltage to the piezoelectric transformer 4. Specifically, it is a so-called self-excited oscillation circuit in which the oscillation frequency is controlled based on the frequency of the output voltage of the piezoelectric transformer 4 fed back by the feedback circuit 9.
The resonance circuit 3 has a coil and a capacitor set so as to resonate with the internal capacitance of the piezoelectric transformer 4.

The piezoelectric transformer 4 includes a plate-like piezoelectric vibrating body made of piezoelectric ceramics such as PZT, and transforms an output voltage based on an input AC voltage using a resonance phenomenon of the piezoelectric vibrating body. It is. The AC voltage output from the piezoelectric transformer 4 is applied to the resistance voltage dividing circuit 5 and the rectifier circuit 6.
The resistance voltage dividing circuit 5 steps down and outputs the AC voltage by dividing the AC voltage output from the piezoelectric transformer 4 by at least two resistors. It is output to the feedback circuit 9.
The feedback circuit 9 is a circuit for feeding back the low-voltage AC voltage output from the resistance voltage dividing circuit 5 to the oscillation circuit 2. In the oscillation circuit 2, an AC voltage is generated at an arbitrary oscillation frequency using the frequency of the AC voltage fed back by the feedback circuit 9 as the control signal.
Here, when the output of the piezoelectric transformer 4 is detected by electrostatic induction as in the prior art (for example, see Patent Document 1), the capacitance fluctuates due to an external factor and is fed back to the oscillation circuit 2. There has been a problem that the phase of the AC voltage is varied. On the other hand, when the output of the piezoelectric transformer 4 is directly taken out, it is necessary to use a high rated voltage electronic component such as a capacitor provided on the oscillation circuit 2 or the feedback circuit 9.
However, in the constant current power supply device X, the low-voltage AC voltage obtained by stepping down the output of the piezoelectric transformer 4 by voltage division by the resistance voltage dividing circuit 5 connected to the previous stage of the rectifier circuit 6 is passed through the feedback circuit 9. Since the feedback is made to the oscillation circuit 2, the rated voltage required for the capacitor provided on the oscillation circuit 2 or the feedback circuit 9 can be lowered while preventing the fluctuation of the phase due to an external factor.

On the other hand, the rectifier circuit 6 rectifies the AC voltage output from the piezoelectric transformer 4 into a DC voltage and outputs it to an output terminal Vout connected to a load (not shown). The DC current detection circuit 7 and the constant current control circuit 8 are for maintaining the current value of the DC voltage output from the rectifier circuit 6 constant.
The DC current detection circuit 7 is for detecting a current flowing from the rectifier circuit 6 to a load connected via the output terminal Vout, and the current value detected by the DC current detection circuit 7 is , Input to the constant current control circuit 8.
Based on the current value detected by the DC current detection circuit 7, the constant current control circuit 8 controls the piezoelectric transformer 4 so that the current value output to the output terminal Vout becomes a preset default value. , That is, the voltage applied to the piezoelectric transformer 4 is controlled.
Thereby, in the constant current power supply device X, even if the load connected to the output terminal Vout fluctuates, the current value supplied to the load by the DC current detection circuit 7 and the constant current control circuit 8 is changed. Maintained constant.

Hereinafter, a specific example of the constant current power supply device X will be described with reference to FIGS. 2 and 3. FIG. 2 is an electric circuit diagram for explaining a specific example of the constant current power supply device X. FIG. 3 is a diagram showing voltage waveforms at points P1 to P4 in the electric circuit diagram of FIG. . Each circuit described here is merely an example, and other circuits that are conventionally known may be used as long as they operate in the same manner.
The reference voltage generation circuit 1 includes resistors R1 to R4 and a Zener diode ZD1, and generates a reference voltage Vref according to a DC voltage input from the DC power supply Vcc.
The oscillation circuit 2 includes resistors R5 to R12, capacitors C1 and C2, a square wave generating comparator U1, a buffer amplification comparator U2, a piezoelectric transformer driving FET Q1 and the like. In the oscillation circuit 2, the oscillation frequency of the square wave generated when the voltage from the feedback circuit 9 is not applied is the DC voltage at the points P1 and P3 and the resistors R5, R6, R9, R12 in FIG. And the capacitor C2. FIG. 3A shows the voltage waveform at the point P1 in FIG. A square wave after the square wave shown in FIG. 3A is buffered and amplified by the comparator U2 is input to the FET Q1 as a drive signal.

Here, a specific operation of the oscillation circuit 2 will be described.
In the oscillation circuit 2, when a DC voltage is applied from the power supply Vcc, the voltage at the point P1 located at the output of the comparator U1 that is an open collector rises to the reference voltage Vref. Accordingly, the output voltage of the comparator U2 also rises to the reference voltage Vref. When the capacitor C2 is charged via the resistor R12 with the reference voltage Vref, the voltage at the point P5 gradually increases and exceeds the voltage at the point P3. As a result, the voltage at the point P1 decreases to 0V. When the voltage at this point P1 becomes 0V, the output of the comparator U2 also becomes 0V, and the capacitor C2 is discharged through the resistor R12.
When the voltage at the point P5 becomes lower than the voltage at the point P3, the voltage at the point P1 rises to the reference voltage Vref and charges the capacitor C2 again. In this way, charging and discharging are repeated with the voltage at the point P3 as a reference, and the FETQ1 is turned on / off, so that oscillation is maintained at a constant period. The voltage at the point P3 is a voltage obtained by dividing the reference voltage Vref by the resistors R3 and R4. The resistors R5, R6, and R7 have a certain hysteresis width, thereby stabilizing the oscillation circuit 2 and setting the oscillation frequency to a basic frequency.

The resonance circuit 3 includes a coil L1 and a capacitor C3. The coil L1 and the capacitor C3 are set so as to resonate together with the capacitance inside the piezoelectric transformer 4.
FIG. 3B shows the voltage waveform at point P2 in FIG. 2, that is, the voltage waveform input to the piezoelectric transformer 4. As shown in FIGS. 3A and 3B, the drive voltage to the piezoelectric transformer 4 is applied to the piezoelectric transformer 4 when the FET Q1 is turned from ON to OFF.
As described above, when a driving voltage is applied to the piezoelectric transformer 4, a high-voltage AC voltage is output from the piezoelectric transformer 4 due to the resonance phenomenon of the piezoelectric vibrator. FIG. 3D shows the output voltage from the piezoelectric transformer 4, that is, the voltage waveform at the point P4 in FIG. A high-voltage AC voltage (FIG. 3D) output from the piezoelectric transformer 4 is input to the resistance voltage dividing circuit 5 and the rectifier circuit 6.

The resistance voltage dividing circuit 5 includes two resistors R13 and R14 for dividing a high-voltage AC voltage from the piezoelectric transformer 4, and the termination of the resistor R13 is in parallel with the resistor R14 and the feedback circuit 9. It is connected. Here, the resistor R13 is set to a value sufficiently larger than the resistor R14. As a result, the AC voltage output from the resistor R13 is stepped down to a sufficiently small value.
The terminal end of the resistor R14 is connected to the capacitor C7 via a point P6 in FIG. The AC voltage from the piezoelectric transformer 4 passes through the capacitor C4, R14, and then flows to the ground through the capacitor C7.
On the other hand, a capacitor C4 is provided on the feedback circuit 9, and the AC voltage from the piezoelectric transformer 4 flows to the point P3 of the resonance circuit 2 through the capacitor C4. The capacitor C4 is used to remove a DC component on the feedback circuit 9.
Thus, in the constant current power supply device X, the low-voltage AC voltage output from the resistor R13 is fed back to the point P3 of the oscillation circuit 2 through the feedback circuit 9 and the capacitor C4. Therefore, the AC voltage applied to the capacitor C4 provided on the oscillation circuit 2 or the feedback circuit 9 is sufficiently low that has been stepped down by the resistance voltage dividing circuit 5, so that the oscillation circuit 2 or A capacitor having a low rated voltage (allowable voltage) can be used as the capacitor C4, and the circuit can be simplified and the cost can be reduced. Further, since it is not necessary to step down the AC voltage with the capacitor C4, stable control can be performed by using a capacitor having a sufficiently large capacitance (that is, having a small capacitive reactance). Is possible.

  FIG. 3C shows a voltage waveform at point P3 of the oscillation circuit 2. The voltage waveform shown in FIG. 3C is obtained by superimposing the AC voltage fed back through the feedback circuit 9 (hereinafter referred to as “feedback voltage”) on the DC voltage input from the reference voltage generation circuit 1. is there. The oscillation circuit 2 generates a drive signal for the FET Q1 based on the superimposed voltage waveform.

Here, control of the drive signal of the FET Q1 based on the feedback voltage in the oscillation circuit 2 will be described.
First, when a DC voltage is applied from the power source Vcc, the voltage at the point P1 rises, the capacitor C2 is charged with time, and the voltage at the point P5 rises. On the other hand, the voltage applied to the point P3 (see FIG. 3C), that is, the threshold value of the comparator U1 varies depending on the feedback voltage fed back to the point P3. When the voltage at the point P5 increases until it exceeds the voltage at the point P3, the voltage at the point P1 becomes 0V and the capacitor C2 is discharged. That is, in the oscillation circuit 2, the timing at which the voltage at the point P1 is inverted is controlled by the feedback voltage superimposed on the voltage at the point P3.
Similarly, the timing at which the capacitor C2 is discharged is similarly a frequency dependent on the amplitude and phase of the feedback voltage because the negative voltage of the feedback voltage superimposed on the point P3 determines the threshold value of the comparator U1. Therefore, the oscillation frequency of the square wave is drawn in. Therefore, if the load connected to the output terminal Vout increases and the amplitude of the feedback voltage decreases, the phase of the feedback voltage is delayed and the oscillation frequency of the square wave decreases. Thus, the frequency of the drive voltage to the piezoelectric transformer 4 is controlled by the oscillation circuit 2 so as to be a resonance frequency specific to the piezoelectric transformer 4, so that the piezoelectric transformer 4 is always in a highly efficient state. It becomes possible to maintain.

On the other hand, the rectifier circuit 6 rectifies the AC voltage input from the piezoelectric transformer 4 into a DC voltage, and then outputs it to the output terminal Vout. And a capacitor C5. The current value of the DC voltage output from the rectifier circuit 6 varies as the load connected to the output terminal Vout varies.
However, since the constant current power supply device X is provided with the direct current detection circuit 7 and the constant current control circuit 8, the current value of the direct current voltage output from the output terminal Vout is preset. Maintained at the value. The rectifier circuit 6, the DC current detection circuit 7, and the constant current control circuit 8 are provided with resistors R20 and R21, a diode D3, and a Zener diode ZD2. A DC voltage of +500 V is always applied to the output terminal Vout from the external reverse bias power source via the diode D3.

The DC current detection circuit 7 includes a resistor R15 for detecting the current value of the DC voltage output from the rectifier circuit 6. Specifically, the current value is detected by the voltage drop generated in the resistor R15.
Here, the current flowing through the resistor R15 has the same value as the current flowing from the output terminal Vout to the load. Specifically, in the positive half cycle of the output voltage of the piezoelectric transformer 4, a current I ₁ (broken arrow in FIG. 2) flows through the diode D1, the resistor R20, the Zener diode ZD2, and the resistor R15. 4 internal capacity is charged. In the next negative half cycle, the capacitor C5 is charged with a current I ₂ (broken arrow in FIG. 2) including the charge of the internal capacitance of the piezoelectric transformer 4. The electric charge charged in the capacitor C5 flows as a current I ₃ (broken arrow in FIG. 2) to the load connected to the resistor R21 and the output terminal Vout and is discharged. In the piezoelectric transformer 4 and the rectifier circuit 6, such charging / discharging action is repeated.
At this time, since the output voltage of the piezoelectric transformer 4 is a sine wave, the current I ₁ charging the internal capacitance of the piezoelectric transformer 4, the current I ₂ charging the capacitor C5, and the capacitor C5 are discharged. Is equal to the current I ₃ (I ₁ = I ₂ = I ₃ ). Therefore, if the voltage drop across the resistor R15 due to the current flowing from the diode D1 is kept constant, the current flowing through the load connected to the output terminal Vout can be kept constant.

The constant current control circuit 8 is a circuit for keeping a voltage drop across the resistor R15 constant, and includes resistors R16 to R19, a capacitor C6, and an operational amplifier U3.
The operational amplifier U3 compares the voltage across the resistor R15 with a control voltage input from the outside, and controls the base voltage of the transistor TR1, thereby outputting the output from the rectifier circuit 6 according to the control signal. Maintain current at default. Here, the control voltage is preset as a voltage value for supplying a preset current value, for example, a current value of about 10 μA necessary for a charger or a transfer roller in the image forming apparatus to the load. Is.
At this time, as described above, in the constant current power supply device X, the AC voltage branched in the previous stage of the rectifier circuit 6 is fed back to the oscillation circuit 2 through the feedback circuit 9. The constant current control by the constant current control circuit 8 can be performed with high accuracy without affecting the detection value by the DC current detection circuit 7. Thereby, for example, by using the constant current power supply device X for a charger or a transfer device of the image forming apparatus, high image forming accuracy can be obtained in the image forming apparatus.

By the way, a DC voltage of +500 V is always applied to the output terminal Vout through the diode D3 from an external reverse bias power source. Here, when the output from the constant current control circuit 8 is OFF, that is, when the output from the piezoelectric transformer 4 is OFF, no current flows through the diode D1. Therefore, the DC voltage from the reverse bias power supply is applied to the output terminal Vout through the diode D3.
On the other hand, when the output from the constant current control circuit 8 is ON, that is, when the output from the piezoelectric transformer 4 is ON, a current flows to the point P6 via the diode D1. At this time, the value of the resistor R20 is set to a sufficiently large value so as not to cause a voltage drop across the diode D3, so that a DC voltage from the reverse bias power source flows into the resistor R15. This can be prevented and the influence of the constant current control circuit 8 on the constant current control can be prevented. The Zener diode ZD2 clamps the DC voltage + 500V of the reverse bias power source so that it does not flow, for example, about 550V. Loss is reduced.

The block diagram which shows schematic structure of the constant current power supply device X which concerns on embodiment of this invention. The electric circuit diagram for demonstrating the specific example of the constant current power supply device X which concerns on embodiment of this invention. The figure which shows the voltage waveform of each of the points P1-P4 in the electric circuit diagram of FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Reference voltage generation circuit 2 ... Oscillation circuit 3 ... Resonance circuit 4 ... Piezoelectric transformer 5 ... Resistance voltage dividing circuit 6 ... Rectification circuit 7 ... DC current detection circuit 8 ... Constant current control circuit 9 ... Feedback circuit C1-C7 ... Capacitor D1 to D3 ... diode I ₁ ~I ₃ ... current L1 ... coil Q1 ... FET
R1 to R21 ... resistor TR1 ... transistors U1 and U2 ... comparator U3 ... operational amplifiers ZD1 and ZD2 ... Zener diodes

Claims (2)

An oscillation circuit that generates an alternating voltage of an arbitrary oscillation frequency based on an input control signal, a piezoelectric transformer that transforms an output voltage based on the alternating voltage output from the oscillation circuit, and an output from the piezoelectric transformer A current value of the DC voltage output from the rectifier circuit by controlling the driving of the piezoelectric transformer based on the current value of the DC voltage output from the rectifier circuit and the rectifier circuit that converts the AC voltage into the DC voltage. A constant current power supply device comprising a constant current control circuit for controlling to a predetermined value,
A resistance voltage dividing circuit for stepping down and outputting the AC voltage by dividing the AC voltage output from the piezoelectric transformer by at least two resistors, and the AC voltage output by the resistance voltage dividing circuit as the oscillation circuit A constant current power supply device comprising: a feedback circuit that feeds back to the oscillation circuit, wherein the oscillation circuit generates an arbitrary oscillation frequency using the AC voltage fed back by the feedback circuit as the control signal.   An image forming apparatus comprising: the constant current power supply device according to claim 1; and a charger and / or a transfer device to which the DC voltage output from the constant current power supply device is applied.

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