JP2020157676A - Exposure device and image formation device - Google Patents

Abstract

To provide means that allows for desired current control, in a light-emitting element array in which multiple light-emitting elements are arrayed.SOLUTION: A control unit controls a current with a first current value to flow through an open drain driver in a first time and then, performs two-stage control so that a current with a second current value necessary for light emission of a light-emitting element flows through the open drain driver in a second time necessary for light exposure by the light-emitting element.SELECTED DRAWING: Figure 1

Description

The present invention relates to an exposure apparatus and an image forming apparatus in which a plurality of light emitting elements are arranged.

The conventional exposure apparatus uses a thyristor with an insulating gate as a high-speed turn-on element, and in the thyristor with an insulating gate, the pulsed current in which the electron lifetime in the n-type base layer energizes the element is 0. Set longer than the time required to increase the current value to 1 times, set the lifetime of electrons and holes shorter than the time interval until the anode voltage is applied after energizing the pulsed current, and turn on at high speed with low loss. Moreover, even if the anode voltage is raised in a short time after the current is attenuated, it is prevented from erroneously firing (see, for example, Patent Document 1).

Japanese Unexamined Patent Publication No. 5-13754

However, in the conventional technique, in high-speed pulse drive, a light emitting element having a thyristor has a small parasitic capacitance of the cathode and turns on in a short time, but a light emitting element in which a plurality of light emitting elements configured in common with the cathode are arranged. In the array, the parasitic capacitance of the cathode becomes large, the thyristor cannot be discharged in a short time, and it takes a long time to turn on.
When a large current control is applied to the open-drain cathode, the voltage changes sharply in order to turn on the thyristor. In the light emitting element array, since it takes time to turn on, the cathode voltage continues to drop during that time and undershoots, and the drain voltage for passing the current to the open drain driver becomes insufficient, which causes a problem that the intended current control cannot be performed. is there.

An object of the present invention is to solve such a problem, and an object of the present invention is to enable desired current control in a light emitting element array in which a plurality of light emitting elements are arranged.

Therefore, the present invention is an exposure apparatus in which a plurality of light emitting elements are arranged, and a thyristor provided corresponding to each of the light emitting elements and having a light emitting layer formed on the anode side and a cathode of each of the thyristors. It has an open drain driver connected to the side, a control unit that controls a current value of a current flowing through the open drain driver, and a control unit that controls the time during which the current flows, and the control unit has a first current value. After controlling the current to flow to the open drain driver in the first time, the current of the second current value required for light emission of the light emitting element is the second time required for exposure by the light emitting element. It is characterized by performing two-step control so that it flows to the open drain driver.

As described above, the present invention has an effect that desired current control can be performed in a light emitting element array in which a plurality of light emitting elements are arranged.

Circuit diagram of a control board having a light emitting element in an embodiment Block diagram showing the control configuration of the open drain driver in the embodiment Schematic side sectional view of the image forming apparatus in the examples Timing chart in the examples Timing chart of two-stage control in the embodiment

Hereinafter, examples of the exposure apparatus and the image forming apparatus according to the present invention will be described with reference to the drawings.

FIG. 3 is a schematic side sectional view of the image forming apparatus in the embodiment.
In FIG. 3, the image forming apparatus 100 forms an image on a medium and prints the image, and is, for example, an electrophotographic printer. Although the image forming apparatus 1 will be described as a monochrome printer in this embodiment, it may be a color printer. Further, in the present embodiment, the printer will be described as a printer that prints a standard medium (pre-cut paper) cut into a predetermined size in advance, but a continuous medium (for example, a medium having a label portion on the continuous paper) It may be used as a printer for printing.

In addition to printing paper, OHP paper, envelopes, copy paper, special paper, and the like can be used as the medium.

The image forming apparatus 100 includes a paper feed cassette 10, a resist roller 11, an image forming unit 12, an LED (Light Emitting Mode) head 16, a transfer roller 17, a fixing device 21, an ejection roller 24, and a paper ejection. It has a loading unit 25.
The paper feed cassette 10 stacks and accommodates the media P on which the developer image is formed, and the media P are separated one by one by the rotation of the hopping roller 10a, and the media indicated by the arrow A in the drawing is shown. Paper is fed in the transport direction and transported to the resist roller 11.

The resist roller 11 supplies the medium P conveyed from the paper feed cassette 10 by the hopping roller 10a to the image forming unit 12 at the timing of development by the image forming unit 12.
As described above, the paper feeding mechanism arranged below the image forming unit 12 has a paper feeding cassette 10, a hopping roller 10a, and a resist roller 11.

The image forming unit 12 forms a toner image as a developer image, and has a photoconductor drum 121, a charging roller 122, a developing roller 123, a toner supply roller 124, and a developing blade 125. There is.

In the photoconductor drum 121 as an image carrier, an electrostatic latent image is formed by the LED head 16. The photosensitive drum 121 is rotatably supported by the image forming unit 12 in the direction indicated by the arrow in the drawing, and the charging roller 122, the LED head 16, and the developing roller 123 are arranged in this order from the upstream in the rotation direction.

The charging roller 122 as a charger charges the surface of the photoconductor drum 121 uniformly and uniformly.
The developing roller 123 as a developing device attaches toner as a developer to an electrostatic latent image formed on the photoconductor drum 121 by the LED head 16 to form a visible toner image.

The toner supply roller 124 as a developer supply member contacts the developing roller 123 and supplies toner to the developing roller 123. The toner supply roller 124 supplies the toner contained in the toner cartridge to the developing roller 123.
The developing blade 125 is pressed by the developing roller to thin the toner supplied from the toner supply roller 124 on the developing roller 123.

The LED head 16 as an exposure device is arranged above the photoconductor drum 121 in the image forming unit 12 at a position facing the photoconductor drum 121, and selects the surface of the photoconductor drum 121 charged by the charging roller 122. To form an electrostatic latent image on the surface of the photoconductor drum 121. The LED head 16 exposes the photoconductor drum 121 according to the image data to form an electrostatic latent image.
Further, the LED head 16 has a control board extending in the main scanning direction, and LEDs as a plurality of light emitting elements are arranged in the main scanning direction on the control board to form a light emitting element array.

The transfer roller 17 as a transfer device is arranged below the photoconductor drum 121 in the image forming unit 12 at a position facing the photoconductor drum 121, charges the medium P in the opposite polarity to the toner, and charges the photoconductor drum 121. The toner image formed above is transferred to the medium P.
The fixing device 21 has a heating roller 22 and a backup roller 23, and fixes the toner image transferred to the medium P conveyed from the transfer roller 17 by heat and pressure.

The discharge roller 24 conveys the medium P on which the toner image is fixed by the fixing device 22 to the outside of the device and discharges the medium P.
The paper discharge loading unit 25 loads the medium P discharged by the discharge roller 24.

FIG. 1 is a circuit diagram of a control board having a light emitting element in the embodiment, and is a circuit diagram of a control circuit of the control board of the LED head 16 shown in FIG.
In FIG. 1, the control circuit 160 in the control board of the LED head 16 shown in FIG. 3 has a light emitting element array in which a plurality of light emitting elements are arranged, and the light emitting elements arranged in the main scanning direction of the LED head 16. A plurality of thyristors 1 (192 in this embodiment) are arranged corresponding to the above. The control circuit 160 has an anode 2, a cathode 3, an open drain driver 4, a gate 5, a resistor 6, and an LED 7.

In this embodiment, the resolution of the LED head 16 shown in FIG. 3 is 1200 dpi (dots / inch), and a control circuit 160 having 192 thyristors 1 and LEDs 7 is substantially parallel to the main scanning direction on the control board. Although they are arranged in two rows, since each control circuit 160 has the same configuration, one of the control circuits 160 will be described.

Further, in the present embodiment, 192 thyristors 1 and LEDs 7 are described as being arranged in the control circuit 160, but the number of thyristors 1 and LEDs 7 is not limited to this, depending on the resolution of the LED head. Can be changed.

The thyristor 1 has a pnpn structure in which a p-type emitter layer is formed on one surface of a high-resistance n-type base layer and a p-type base layer and an n-type emitter layer are selectively formed on the other surface. is there.
The anode 2 is connected to each thyristor 1 in common, and the cathode 3 is connected in common.

Further, an LED layer as a light emitting layer is formed on the anode 2 side of each thyristor 1 to form an LED 7. When the thyristor 1 becomes conductive, the LED 7 emits light, and when the thyristor 1 becomes non-conductive, the LED 7 emits light. Turns off.

As described above, the thyristor 1 of this embodiment is provided corresponding to each LED 7, and a light emitting layer is formed on the anode side of the thyristor 1.

The cathode 3 is connected to the open drain driver 4 and is pulled up to 5V via the resistor (R) 6 at the power supply voltage VDD.
The gates 5 of the thyristor 1, G1 to G192, are connected to a shift register, and the gate of the thyristor 1 to be driven is selected.

The gate 5 sequentially selects one thyristor 1 from a plurality of thyristors 1 by G1 to G192.
Further, a power supply voltage VDD (5V) is supplied to the terminals of the anode 2.
The open drain driver 4 is connected to the cathode 3 side of each thyristor 1.

In this open drain driver 4, the constant current Ids flowing between the terminals is controlled by the constant current control signal CS output from the control unit and the register, which will be described later, and the thyristor 1 selected by the gate 5 conducts (on) or does not conduct. It switches (off).

A constant current Ids flows through the open drain driver 4, and the drain voltage Vds between the terminals of the open drain driver 4 drops from the high level (H), so that the thyristor 1 selected by the gate 5 becomes conductive (on). , The LED 7 connected to the thyristor 1 emits light.

On the other hand, when the current flowing through the open drain driver 4 disappears, the drain voltage Vds between the terminals of the open drain driver 4 changes from a lowered state to a high level (H), so that the thyristor 1 selected by the gate 5 is not displayed. The state of continuity (off) is reached, and the LED 7 formed on the thyristor 1 is turned off.
In this way, the open drain driver 4 controls the turning on and off of the LED 7, and the LED head 16 shown in FIG. 3 performs desired image formation.

FIG. 2 is a block diagram showing a control configuration of the open drain driver in the embodiment.
In FIG. 2, the open drain driver 4 is connected to the control unit 8 via the register 9.

The control unit 8 is a control means such as a CPU (Central Processing Unit), and controls the open drain driver 4 based on a control program or the like stored in a storage means such as a memory. The control unit 8 controls the open drain driver 4 according to the image data. Further, the control unit 8 controls the current value of the current flowing through the open drain driver 4 and the time during which the current flows.

The register 9 controls the constant current control signal CS to control the constant current Ids flowing through the open drain driver 4. The register 9 can control the constant current control signal CS by writing the current value by the control unit 8.

In this embodiment, the pre-current control signal can be output by setting the current value written to the register 9 to the first current value, and the current value written to the register 9 is set to the second current value. This makes it possible to output the main current control signal.

Therefore, the control unit 8 outputs the two-stage control signal of the pre-current control signal and the main current control signal to the open drain driver 4 by controlling the constant current control signal CS via the register 9, and the open drain The constant current Ids flowing through the driver 4 can be controlled to a pre-current and a main current.

Further, the control unit 8 can control the time during which the pre-current flows and the time during which the main current flows through the open drain driver 4.

The control unit 8 of the present embodiment controls the pre-current of the first current value to flow to the open drain driver 4 in the first time, and then the second current required for light emission of the LED 7 shown in FIG. Two-step control is performed so that the value current flows through the open drain driver 4 in the second time required for exposure by the LED 7.

FIG. 4 is a timing chart in the embodiment, and is a timing chart of a signal for driving the control circuit 160 shown in FIG.
In FIG. 4, each gate G1 to G192 is set to a high level (H) by inputting a gate control signal to each of the gates G1 to G192.

When a constant current control signal (constant current control signal CS shown in FIG. 1) is input to the open drain driver 4 with the respective gates G1 to G192 at a high level (H), the constant current Ids is transmitted to the open drain driver 4. The flow changes and the drain voltage Vds connected to the cathode 3 changes.

The constant current control signal is input to the open drain driver 4 as a two-stage control signal of a pre-current control signal and a main current control signal. When the pre-current control signal is input, the pre-current Pc (current of the first current value) flows to the open drain driver 4 as a constant current Ids, and when the main current control signal is input, the main current Mc (shown in FIG. 1). The current of the second current value required for light emission of the LED 7) flows to the open drain driver 4 as a constant current Ids.

The operation of the above-described configuration will be described.
First, the printing operation of the image forming apparatus will be described with reference to FIG.
The medium P in the paper accommodating cassette 10 of the image forming apparatus 100 is unwound by the hopping roller 10a, sent to the resist roller 11, and then conveyed from the resist roller 11 to the image forming unit 12.

In the image forming unit 12, the surface of the photoconductor drum 121 is charged by the charging roller 122 and exposed by the LED head 16 to form an electrostatic latent image. A thin layer of toner is electrostatically adhered to the electrostatic latent image formed on the photoconductor drum 121 on the developing roller 123 to form a toner image.

The toner image formed on the photoconductor drum 121 is transferred to the medium P by the transfer roller 17, and the toner image is formed on the medium P. The toner remaining on the photoconductor drum 121 after the transfer is removed by the cleaning device.

The medium P on which the toner image is formed is conveyed to the fixing device 21, and the toner image is fixed on the medium P in the fixing device 21 to form an image.
The medium P on which the toner image is fixed is sandwiched by the discharge roller 24 and discharged to the paper discharge loading unit 25.

In this way, the image forming apparatus 10 forms an image on the medium P and prints it.

Next, the operation of the control circuit of the control board of the LED head 16 shown in FIG. 3 will be described with reference to FIGS. 1, 2, 4, and 5. The open drain driver 4 of the control circuit of the LED head 16 is controlled by the control unit 8 shown in FIG.

In the control circuit 160 shown in FIG. 1, as shown in FIG. 4, in the initial state (timing T0), the gates G1 to G192 are at a low level (L), and the open drain driver 4 has a high impedance (Hi-Z). The cathode 3 is pulled up and the thyristor 1 is in an off (non-conducting) state. At this time, the LED 7 is in the off state.

A gate control signal is input from the shift register of the driver, the selected gate (G1 shown in FIG. 4) is in the high level (H) state, and the other gates are in the off (low level (L)) state.
The gate is sequentially shifted in high level (H) from G1 to G192 by the gate control signal output from the shift register.

Here, the high level (H) of the gate will be described using the timing chart of FIG. Note that FIG. 5 is an enlarged view of the waveforms of the constant current Ids and the drain voltage Vds between the high level (H) of the gate G1 from the timing T1 to the timing T2 in FIG.

As shown in FIG. 5, the high level (H) period Tw of the gate is the distance between lines (line spacing) in the sub-scanning direction depending on the printing speed and resolution, and the number of times one line is lit (192 times in this embodiment). ) Is calculated.

Further, the time obtained by subtracting the shift register stabilization time (gate stabilization time) Ts and the time Tgq required to turn off the thyristor 1 from the period Tw is the time Tc that can be used for light emission control.
That is, Tc = Tw-Ts-Tgq
Will be.

The period Tw is the time when one thyristor is selected by the gate, and the time Tc is the control time that the control unit 8 can use to control the open drain driver 4.
After the stable time Ts of the shift register elapses after the gate reaches the high level (H), the pre-current control signal Pc is input to the gate.

The pre-current Ip is a discharge current of the charge Q of the parasitic capacitance of the thyristor, and is a small constant current such that the drain voltage Vds connected to the cathode 3 does not cause undershoot. The charge Q varies depending on the number of thyristors, which is the number of arrays, and the characteristics of the thyristors.

By driving the open drain driver 4 with the pre-current Ip as a small current value, the drain voltage Vds gradually decreases as shown in the waveform W1 shown in FIG.
Further, as the drain voltage Vds decreases, a voltage is applied between the anode 2 and the cathode 3 to turn on (waveform W2 shown in FIG. 5), and a current flows.

The current value of the pre-current Ip and the pre-current time Tip through which the pre-current Ip flows are adjustable, and the smaller the current value of the pre-current Ip, the longer the time until turn-on. On the other hand, the larger the current value of the pre-current Ip, the shorter the time until the turn-on is performed, but the drain voltage Vds drops sharply. In this embodiment, the current value of the pre-current Ip is set to be smaller than the current value of the main current Im.

If the current value of the constant current Ids is too large, the drain voltage Vds will continue to drop sharply until it turns on, and the minimum voltage Vmin or less will cause undershoot, the constant current Ids will not flow, and the intended control will be possible. It will disappear.

Therefore, in this embodiment, the drain voltage Vds does not undershoot, and the current value of the pre-current Ip and the pre-current time Tip through which the pre-current Ip flows are controlled until the thyristor 1 is turned on or just before the turn-on.

The main current Im is a drive current for causing the photoconductor drum 121 shown in FIG. 3 to emit light with the energy required for exposure, and the light emission time Tim as the second time is the light emission time required for exposure by the LED 7. Yes, it is controlled within the time Tc that can be used for light emission control minus the pre-current time Tip.

Therefore, "light emission time Tim <time Tc-pre-current time Tip that can be used for light emission control".

The exposure amount (J), which is the energy required for exposure, is obtained by the product of the light amount (W) and the emission time Tim. The amount of light is determined by the lens that adjusts the drive current and focal position of the light emitting element.
Further, the control unit 8 controls the main current Im to flow to the open drain driver 4 from the state in which the thyristor 1 is turned on or the state immediately before the thyristor 1 is turned on.

In the state where the thyristor 1 is turned on or immediately before the turn-on, a steep fluctuation of the drain voltage Vds is suppressed even when the pre-current Ip is changed to the main current Im, and the drain voltage Vds for driving the open drain driver 4 is set. Can be secured.
After the main current Im is applied at the emission time Tim required for exposure, the open drain driver 4 becomes high impedance, the cathode voltage is pulled up, and the thyristor 1 is turned off.

After that, the gate of the next thyristor is selected by the gate control signal, the above-mentioned control is performed, and the gates G1 to G192 are similarly controlled.
In this embodiment, the current value of the pre-current Ip is set to a current value smaller than the main current Im and the pre-current is set so that the time until the turn-on is not long and the drain voltage Vds does not cause undershoot. The pre-current time Tip for passing Ip was in the following range.

100ns (threshold value) ≤ pre-current time Tip <time available for light emission control Tc-light emission time Tim
The threshold value, which is the lower limit of the pre-current time Tip, is 100 ns (nanoseconds), and was measured experimentally.

Further, the "pre-current time Tip <time Tc-light emission time Tim that can be used for light emission control" is a condition based on the above-mentioned "light emission time Tim <time that can be used for light emission control Tc-pre-current time Tip".

As described above, in this embodiment, after the control unit 8 controls the current of the first current value (pre-current Ip) to flow to the open drain driver 4 in the first time (pre-current time Tip). , Two-step control is performed so that the current of the second current value (main current Im) required for light emission of the LED 7 flows to the open drain driver 4 in the second time (light emission time Tim) required for exposure by the LED 7. This makes it possible to perform desired current control in a light emitting element array in which a plurality of LEDs 7 are arranged.

In addition, the thyristor 1 can be turned on by controlling the pre-current without causing the cathode voltage to undershoot, and even when the main current is large, the cathode voltage does not undershoot and stable main current control is performed. Can be done.

Further, even when the main current is small, the turn-on is performed by controlling the pre-current, so that the main current control can be quickly shifted and a desired light emission time can be secured.

The case where the current control of the control circuit 160 of the LED head 16 described above is applied to the image forming apparatus 100 of this embodiment will be described below.

When the printing speed of the image forming apparatus 100 shown in FIG. 3 is 35 ppm (sheets / minute) on an A4 size printing medium, the period of one line in the main scanning direction (time to drive 192 thyristors arranged in one line). Is about 106 μs.
Since the light emitting element of one line in the main scanning direction emits light 192 times in the cycle of one line in the main scanning direction, when the period of one line is divided by the number of times of light emission and the time including the margin, the light emission per dot of the light emitting element The time will be 550 ns.

Excluding the shift register stabilization time of 50 ns and the thyristor 1 off time of 80 ns from the light emission time of 550 ns per dot of the light emitting element, the time that can be used for light emission is 410 ns.

The emission time Tim required for exposure is calculated from the exposure energy required for exposure and the characteristics of the light emitting element to be 150 ns, and the pre-current time Tip and the pre-current time Tip in which the drain voltage Vds does not undershoot within the time 260 ns that can be used for the pre-current time Tip and The current value of the pre-current Ip was derived, the pre-current time Tip was set to 200 ns, and the current value of the pre-current Ip was set to 2.5 mA.

The current value of the pre-current Ip was set to 2.5 mA according to the result of the experiment as a current value smaller than the main current Im and the drain voltage Vds does not undershoot.

By doing so, it becomes possible to perform desired current control in the image forming apparatus 100 having a light emitting element array in which a plurality of LEDs 7 are arranged.
As described above, in the present embodiment, it is possible to obtain an effect that desired current control can be performed in the light emitting element array in which a plurality of light emitting elements are arranged.

In addition, the thyristor can be turned on by controlling the pre-current without causing the cathode voltage to undershoot, and even when the main current is large, the cathode voltage does not undershoot and stable main current control can be performed. The effect of being able to do it is obtained.
Further, even when the main current is small, the turn-on is performed by controlling the pre-current, so that it is possible to quickly shift to the main current control and obtain an effect that a desired light emission time can be secured.
In this embodiment, the image forming apparatus has been described as a printer, but the present invention is not limited to this, and any apparatus having an exposure apparatus may be used as a copying machine, a facsimile machine, or a multifunction device (MFP).

1 thyristor 2 anode 3 cathode 4 open drain driver 5 gate 6 resistor 7 LED
8 Control unit 9 Register 10 Paper feed cassette 11 Resist roller 12 Image formation unit 16 LED head 17 Transfer roller 21 Fixing device 24 Discharge roller 25 Paper discharge loading unit 160 Control circuit

Claims (8)

An exposure device in which a plurality of light emitting elements are arranged.
A thyristor provided corresponding to each of the light emitting elements and having a light emitting layer formed on the anode side,
An open drain driver connected to the cathode side of each of the thyristors,
A control unit that controls the current value of the current flowing through the open drain driver and the time during which the current flows.
Have,
The control unit controls the current of the first current value to flow to the open drain driver in the first time, and then the current of the second current value required for light emission of the light emitting element is the light emitting element. An exposure apparatus characterized in that a two-step control is performed so as to flow to the open drain driver in a second time required for exposure by. In the exposure apparatus according to claim 2,
The exposure apparatus, wherein the first current value is smaller than the second current value. In the exposure apparatus according to claim 1 or 2.
It has a gate for sequentially selecting one thyristor from the plurality of the thyristors.
Of the time when one thyristor is selected by the gate, the control time that the control unit can use to control the open drain driver is Tc.
The second time, which is the light emission time required for exposure by the light emitting element, is Tim.
Then
The exposure apparatus, wherein the first time is equal to or greater than a threshold value and less than (Tc-Tim). In the exposure apparatus according to claim 3,
An exposure apparatus characterized in that the threshold value is 100 ns. In the exposure apparatus according to claim 3 or 4.
Let Tw be the time when one thyristor is selected by the gate, the stabilization time Ts of the gate, and the time Tgq required to turn off the thyristor.
The control time Tc is
Tc = Tw-Ts-Tgq
An exposure apparatus characterized by being. In the exposure apparatus according to any one of claims 1 to 5.
The control unit is an exposure device that controls the current of the second current value to flow to the open drain driver from a state in which the thyristor is turned on or a state immediately before the thyristor is turned on. An image forming apparatus comprising the exposure apparatus according to any one of claims 1 to 6. In the image forming apparatus according to claim 7,
When the printing speed is 35 ppm (sheets / minute) on an A4 size printing medium and the number of the thyristors is 192,
An image forming apparatus, characterized in that the first current value is 2.5 mA and the first time is 200 ns.

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