JP6765906B2 - High-voltage generator and image forming device - Google Patents

Description

The present invention relates to a high-voltage generator that generates high voltage and an image forming apparatus that uses the high-voltage generator.

Conventionally, in an electrophotographic image forming apparatus, a high voltage is applied to a process unit such as a charging roller or a primary transfer roller, and the current flowing at that time is detected to feed back to various controls. The high voltage output and current detection are performed by the high voltage substrate in the apparatus, and the high voltage substrate is required to apply the voltage to the load and detect the current flowing through the load with high accuracy.

For example, Patent Document 1 describes a configuration in which a voltage detection unit that detects an applied voltage is not directly grounded and is connected to an operational amplifier feedback unit in a current detection circuit that uses operational amplifier feedback. A predetermined offset potential is given to the reference voltage (positive terminal voltage of the operational amplifier) in this current detection circuit. By doing so, even if there is an inflow current from another load, the operational amplifier output unit does not reach the GND level, so that the reference voltage of the voltage detection unit can be kept constant.

Japanese Unexamined Patent Publication No. 2001-169546

However, when the current detection circuit of Patent Document 1 is used to detect the current flowing by applying a negative voltage, for example, as in a charging roller, the current detection value decreases in proportion to the output current from the offset potential. .. Therefore, when a large current flows, the current detection value reaches the GND level, and the voltage of the feedback unit of the current detection circuit cannot be kept constant. At this time, since the voltage of the feedback unit is the reference voltage for voltage detection, constant voltage control cannot be performed and the output value deviates significantly from the target value.

An object of the present invention is to prevent the current detection value from dropping to the GND level even when a large current flows.

The high-voltage generator of the present invention has a transformer that boosts the input voltage, a rectifying means that rectifies the output of the transformer and supplies a DC voltage to the load, and a DC current flowing through the load to the first input terminal. It has an operational capacitor that is input and a reference value is input to the second input terminal, the output of the operational capacitor is fed back to the first input terminal of the operational object via a current detection resistor, and the direct current flowing through the load is transferred to the load. It has a current detecting means for detecting as a voltage generated in a current detecting resistor, and a voltage detecting means for detecting a DC voltage connected to the first input terminal of the operational capacitor and supplied from the rectifying means to the load. The current detecting means is a voltage dividing means that divides the output of the operational amplifier as a detection value of a direct current flowing through the load, and the voltage detecting means when the detected value of the direct current flowing through the load is lower than a predetermined value. It is characterized by having a bypass means for passing a current from a connection portion between the means and the first input terminal of the operational amplifier to the output terminal of the operational amplifier.

According to the present invention, by providing a bypass means for flowing a current from the feedback terminal of the operational amplifier to the output terminal of the operational amplifier, it is possible to prevent the output of the operational amplifier from reaching the GND level and causing an abnormality in the high voltage output. It becomes.

It is a block diagram of the image forming apparatus 1 of this Example. It is a board block diagram of this Example. It is a circuit block diagram of the charge development high voltage substrate 200a of this Example. It is explanatory drawing of the output current and the voltage of each part in the current detection circuit of this Example. It is explanatory drawing of the operation of each part at the time of the resistance value change of the charging roller 2 of this Example.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic configuration diagram of an image forming apparatus using an electrophotographic process according to an embodiment of the present invention.

In the figure, the image forming section 10 is provided with four image forming sections, which are photoconductor drums 1a to 1d, charging rollers 2a to 2d, laser scanners 3a to 3d, developing machines 4a to 4d, and an intermediate transfer belt. 5. Primary transfer rollers 6a to 6d and secondary transfer rollers 7 are arranged. The subscripts a to d of the numbers correspond to the image forming portions of yellow, magenta, cyan, and black, respectively. However, the image forming color of each image forming portion is not limited to this.

The outline of the image forming operation will be described. After the photoconductor drums 1a to 1d are uniformly charged by the charging rollers 2a to 2d, the laser scanners 3a to 3d perform exposure based on the image signal corresponding to the image to be formed, whereby the photoconductor drums 1a to 1a to An electrostatic latent image is formed on 1d. After that, the formed electrostatic latent image is developed as a toner image by the developers 4a to 4d, and each toner image is multiplex-transferred to the intermediate transfer belt 5 by the primary transfer rollers 6a to 6d. The toner image transferred to the intermediate transfer belt 5 is transferred to the recording material P conveyed from the paper cassette 9 by the secondary transfer roller 7, and fixed by the fixing device 8 to obtain a color image.

FIG. 2 is a block diagram of the charging rollers 2a to 2d, the charging developing boards 200a to 200d as a high voltage generator for generating a high voltage applied to the developing devices 4a to 4d, and the control board 100.

The charged development substrate 200a has a high voltage charging circuit 300a that generates a high voltage output (charged high voltage) applied to the charging roller 2a and a developing high voltage circuit 400a that generates a high voltage output (development high voltage) applied to the developer 4a. It is a board.

The charged development substrate 200b has a high voltage charging circuit 300b that generates a high voltage output (charged high voltage) applied to the charging roller 2b and a developing high voltage circuit 400b that generates a high voltage output (development high voltage) applied to the developer 4b. It is a board.

The charged development substrate 200c has a high voltage charging circuit 300c that generates a high voltage output (charged high voltage) applied to the charging roller 2c and a developing high voltage circuit 400c that generates a high voltage output (development high voltage) applied to the developer 4c. It is a board.

The charged development substrate 200d has a high voltage charging circuit 300d that generates a high voltage output (charged high voltage) applied to the charging roller 2d, and a developing high voltage circuit 400d that generates a high voltage output (development high voltage) applied to the developer 4d. It is a board.

The control board 100 controls the overall image forming operation by starting the operation, setting the output voltage, and acquiring the detected value for each of the charged development boards 200a to d.

Here, since the charged and developed substrates 200a to d in the present embodiment are all substrates of the same circuit, only the charged and developed substrate 200a will be described as a representative in the following description.

FIG. 3 is an explanatory diagram of a charged high voltage circuit 300a as a high voltage generator. The charged high-voltage circuit 300a is composed of a PWM smoothing unit 301, a constant voltage control unit 302, a transformer drive unit 303, a high-voltage rectifying unit 304, a voltage detection unit 305, and a current detection unit 306, and has a high voltage of negative direct current to the charging roller 2. Output HVout. A PWM signal and a transformer drive clock CLK are input from the control board 100 to the charged high-voltage circuit 300a, and 3.4V voltage and 24V voltage output from the control board 100 are further supplied. Further, the detected value (charged current detection value Isns) of the direct current (charged current Iout) flowing through the charging roller 2 is fed back to the control board 100.

<PWM smoothing part 301>
The PWM smoothing unit 301 is a low-pass filter composed of a resistor R1 and a capacitor C1 and converts an input PWM signal into a DC voltage at a predetermined cutoff frequency. The charge output (HVout) can be changed by changing the duty ratio of the input PWM signal. For example, when the duty ratio is 50%, about -800V is output, and when the duty ratio is 0%, about -1500V is output.

<Constant pressure control unit 302>
The constant pressure control unit 302 includes an operational amplifier IC1 and a capacitor C2, a transistor Q1 and an electrolytic capacitor C3. In the operational amplifier IC1, the DC voltage output from the PWM smoothing unit 301 is input to the − terminal, the voltage detection value Vsns described later is fed back to the + terminal, and the output voltage is adjusted so that the voltages of the + terminal and the − terminal match. It is an inverting amplifier circuit. Further, the capacitor C2 is an integrating element for the purpose of stabilizing the output voltage of the amplifier circuit. Further, the output of the operational amplifier IC1 is connected to the base of the transistor Q1 grounded by a collector, and the emitter of the transistor Q1 is lower than the voltage of the output terminal of the operational amplifier IC1 by the voltage between the base and emitter of the transistor Q1 (about 0.6V). Become. An electrolytic capacitor C3 for voltage stabilization is connected to the emitter of the transistor Q1.

<Transformer drive unit 303>
The transformer drive unit 303 is a circuit for driving a transformer T1 that boosts the input voltage. It is composed of a pull-down resistor R2, a damping resistor R3, and a FET Q2, and repeats ON / OFF of the FET Q2 according to the transformer drive clock CLK. In the present embodiment, the transformer drive clock CLK has a period of 50 kHz and a duty ratio of 25%. This makes it possible to control the start and stop of the operation of the transformer T1.

<High voltage rectifier 304>
The high-voltage rectifying unit 304 is composed of a high-voltage diode D1 and a high-voltage ceramic capacitor C4, and rectifies and smoothes the negative voltage of the AC voltage output from the transformer T1 to charge the negative DC voltage HVout (0V to -1500V output). Output as.

<Voltage detector 305>
The voltage detection unit 305 is composed of two resistors R4 and R5, and outputs the voltage obtained by dividing the voltage between the charged output HVout and Vref2 described later by R4 and R5 as the voltage detection value Vsns (0 to 3.0V). ..

<Current detector 306>
The current detection unit 306 is composed of an operational amplifier IC2, a plurality of resistors R7 to R12, and a diode D2, and sets the charging current Iout when the charging output HVout is applied to the charging roller as a charging current detection value Iss of 0 to 3.0V. It is a circuit to output. The Ins is detected by the value obtained by dividing the output of the operational amplifier IC2 by the resistors R8 and R11. Here, the directions of the currents (Iout and Ir7, Id2, I1, I2 described later) flowing through the circuit will be described with the direction of FIG. 3 as positive. Further, the voltage of each part in the charging current Iout when the resistance constants of the current detection unit 306 are R7: 20 kΩ, R8: 3.3 kΩ, R9: 2 kΩ, R10: 15 kΩ, R11: 1 kΩ, and R12: 1 kΩ is shown in the figure. Shown in 4.

Since the voltage Vref1 at the + terminal of the operational amplifier IC2 is a value obtained by dividing the voltage of 3.4V by the resistors R9 and R10, Vref1 [V] = 3.4 × R10 / (R9 + R10) = 3V as shown in the following equation. 1)
Will be.

The charging current Iout is input to the-terminal (first input terminal) of the operational amplifier IC2, and the voltage Vref as a reference value is input to the + terminal (second input terminal). Further, since the output terminal portion (point c) of the operational amplifier IC2 is fed back to the-terminal portion (point a) of the operational amplifier IC2 by the resistor R7 as a resistance for current detection, the voltage Vref2 at the point a is a + terminal by virtual grounding. It is equal to the voltage Vref1 of the unit and becomes a constant value.

Vref2 [V] = 3V (2)
A voltage detection unit 305 is also connected to point a, and the point a voltage Vref2 is also used as a reference voltage for voltage detection.

Since the charged output HVout is a negative high voltage output, the charged current Iout flows along the path indicated by the dotted arrow in the figure. That is, the charging current Iout passes through the transformer T1 from the charged high-voltage output terminal, and flows from the point a to the point b, R11, and GND via R7. As described above, since the point a voltage Vref2 is a constant value at 3V, the voltage at point b is a voltage that is lower than the voltage at R7 due to the charging current Iout flowing from Vref2 to R7. It can be detected as a detected value Isns. Isns is as shown in the following formula: Isns [V] = Vref2-Iout × R7 (3)
As shown by the solid arrow in FIG. 4, it is represented by a linear expression of 3V when Iout is 0 μA and 1V when Iout is 100 μA.

Further, the resistor R11 is a pull-down resistor, and the current I1 flowing through R11 is
I1 [μA] = Isns / R11 (4)
Therefore, the output current Iic2 of the operational amplifier IC2 is Iic2 [μA] = I1-Iout (5).
Will be. Therefore, the voltage Vic2 of the operational amplifier IC2 output terminal (c) is Vic2 [V] = Isns + Iic2 × R8 (6).
Will be. As shown by the broken line arrow in FIG. 4, when Iout is 0 μA, it is 12.9 V, and when Iout is 100 μA, it is 3.97 V. Therefore, the change is larger than that of Isns. Here, R8 is a protective resistor so that the output terminal of the operational amplifier is not destroyed by static electricity. In the present embodiment, R11 is arranged on the charged developing substrate 200a, but may be arranged on the control substrate 100, and the calculation formula is the same in that case as well.

On the other hand, in the conventional circuit configuration, when the charging current Iout is as large as 200 μA, Isns is calculated to be -1V from the equation (3). However, in reality, Since Isns cannot be less than 0V, Vref2 cannot maintain 3V and rises to 4V (200 μA × 20 kΩ). When Vref2 exceeds 3V, for example, 4V, the reference voltage for voltage detection deviates, so that the voltage detection value Vsns deviates from the actual value, and as a result, the charged output HVout deviates from the appropriate value.

In order to prevent this, in the present embodiment, a bypass means composed of a series connection circuit of the diode D2 and the resistor R12 is connected between the points c and a. The function of this bypass means is that when the resistance value of the charger 2 decreases and a large current flows, the charging current Iout from the connection between the voltage detecting means 305 and the first input terminal of the operational amplifier IC2 is a diode instead of the resistance R7 side. It is intended to flow to the D2 side. The resistor R12 as a bypass means and the diode D2 are connected in parallel to the resistor R7 as a current detection resistor, and the cathode of the diode D2 is arranged on the output terminal side of the operational amplifier IC2. At this time, the condition for the current to flow through the diode D2 is as follows for Vic2 obtained by using the equation (6), where Vf is the forward voltage of the diode D2.
Vref2-Vic2 ≧ Vf (7)

For example, when the charging current Iout is 50 μA, Vref2-Vic2 is −5.44 V, so no current flows through the diode D2. When the charging current Iout is 200 μA, Vref2-Vic2 becomes 7.96 V, and a current flows through the diode D2.

Further, assuming that the current at which the current starts to flow in the diode D2 is Iout', Iout' is the case where the equation (7) holds, and Iout'[μA] = (R11 / (R11 /) using the equations (2) to (7). R7 x R8 + R8 x R11 + R11 x R7))
× (Vf + (R8 / R11) × Vref2) (8)
Will be. When Vf is 0.6V, Iout'is 117.6μA. Further, the charge current detection value Isns' when the current starts to flow in the diode D2 is 0.648 V from the equation (3). This voltage of 0.648V is the upper limit of the charging current detection value Isns when the resistance value of the charger 2 is not lowered due to foreign matter contamination. Therefore, when the Ins exceeds 0.648V as a predetermined value, the current Id2 flows through the diode D2, so that the Iss becomes a constant value at 0.648V regardless of the output current Iout. Therefore, the voltage Vref2 can maintain 3V regardless of the output current Iout.

Further, as can be seen from FIG. 4, Vic2 is a signal having a large change with respect to the output current Iout. Therefore, even if the resistance value and the Vf value in the current detection unit 305 vary, the variation in Iout'is small, and it is possible to suppress it to about ± 5 μA (± 0.1 V in Isns').

FIG. 5 shows an explanation of the operation of each part when the resistance value of the charging roller 2 is 20MΩ and the resistance value of the charging roller 2 changes to 5MΩ due to the mixing of foreign matter or the like during the charging output HVout of −1000V (Iout50μA). It is a figure. Note that FIG. 3A shows a case where the diode D2 and the resistor R12 are not provided, and FIG. 3B shows a case where the diode D2 and the resistor R12 are present.

When there is no diode D2 and resistor R12 (FIG. 5 (a)), Iout (negative value) gradually rises from 50 μA at the timing Tc when the resistance value of the charging roller 2 changes, and when it reaches 150 μA, Iss is GND. Reach the level. When the Ins reaches the GND level, Vref2 cannot be maintained at 3V and Vref2 rises. When Vref2 rises, the voltage detection value VSNS also rises, so the constant voltage control unit 302 considers that the charged output HVout is lower than the target value and tries to raise the output voltage. However, since the VSNS does not match the target value, the HVout becomes the maximum output of -1500V, and the output current reaches up to 300μA. Since R7 is 20 kΩ, Vref2 at this time is 6 V or more.

On the other hand, when the diode D2 and the resistor R12 are present (FIG. 5 (b)), Iout (negative value) gradually increases from 50 μA to 117.6 μA at the timing Tc when the resistance value of the charging roller 2 changes. By the way, a current flows on the diode D2 side. At this time, Isns becomes a constant value at 3V-117.6 μA × 20 kΩ = 0.648 V from the equation (3). Due to the current flowing through the diode D2, Vref2 can maintain 3V, so that HVout is maintained at −1000V. That is, the DC current Iout (-117.6 μA) flowing in the load when the Isns reaches the predetermined value of 0.648 V is the DC current Iout (-117.6 μA) flowing in the load when the Isns reaches the GND level in the absence of the bypass means. It is lower than -150 μA).

As described above, in the present embodiment, by arranging the diode between the feedback terminal and the output terminal of the operational amplifier, even if a large current flows through the charger, the current flowing through the resistor for current detection is made constant, and the current flows. It is possible to prevent the detected value from dropping to the GND level. As a result, constant voltage control can be continued.

2 Charging roller 100 Control board 200 Charging developing board 300 Charging high-voltage circuit 306 Current detector D2 Diode IC2 Operational amplifier

Claims (7)

A transformer that boosts the input voltage and
A rectifying means that rectifies the output of the transformer and supplies a DC voltage to the load.
It has an operational amplifier in which the DC current flowing through the load is input to the first input terminal and the reference value is input to the second input terminal, and the output of the operational amplifier is the first input terminal of the operational amplifier via a current detection resistor. A current detecting means that detects the direct current flowing through the load as a voltage generated in the current detecting resistor.
A voltage detecting means connected to the first input terminal of the operational amplifier and detecting a DC voltage supplied from the rectifying means to the load, and
Have,
The current detecting means is a voltage dividing means that divides the output of the operational amplifier as a detection value of a direct current flowing through the load, and the voltage detecting means when the detected value of the direct current flowing through the load is lower than a predetermined value. A high-voltage generator, comprising: a bypass means for passing a current from a connection portion between the operational amplifier and the first input terminal of the operational amplifier to an output terminal of the operational amplifier. The resistance value of the load decreases due to the inclusion of foreign matter, and the predetermined value is lower than the detected value of the direct current in a state where the resistance value of the load does not decrease. The high-voltage generator according to claim 1. When the detected value of the DC current flowing through the load reaches the predetermined value, the DC current flowing through the load is the load when the detected value of the DC current flowing through the load reaches the GND level in the absence of the bypass means. The high-voltage generator according to claim 2, wherein the current is lower than the direct current flowing through the device. Any one of claims 1 to 3, wherein the bypass means is connected between a connection portion between the first input terminal of the operational amplifier and the current detection resistor and an output terminal of the operational amplifier. The high pressure generator described. The high-voltage generator according to claim 4, wherein the bypass means includes a resistor and a diode connected in series with the resistor and having a cathode on the output terminal side of the operational amplifier. The high-voltage generator according to claim 5, wherein the bypass means is connected in parallel with the current detection resistor. The photoconductor on which the electrostatic latent image is formed and
The charging means for charging the photoconductor and
The high-voltage generator according to any one of claims 1 to 6 is provided.
The image forming apparatus is characterized in that the high voltage generator applies a negative high voltage to the charging means, and the current detecting means detects a current flowing through the charging means.

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