JPH11240198A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stably control a driving current of a laser diode in an image formation apparatus irrespective of a temperature change. SOLUTION: A reference voltage Vref2 is input to a negative input terminal of a variable gm amplifier 34, and an I0 is input as a reference current. A current output in accordance with a potential difference of two input voltages Verr and Vref2 becomes a driving current ILD (=k.I0) of a laser diode which determines a quantity of emitted light. The driving current ILD is input to a current limit circuit 35 and a current buffer 37. The current limit circuit 35 limits the current ILD corresponding to an output quantity of light, with outputting a current limited to a constant value almost equal to a reference current IPO when the current ILD increases to be close to the IPO. A function current of input and output currents of the current limit circuit 35 is supplied to a cathode of a laser diode 41 by current buffers 37, 38, a current subtraction circuit 39, a switching circuit 36 and a current addition circuit 40.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus, and more particularly, to exposing a photosensitive member by driving a laser light source based on a detection signal of detecting a laser beam from the laser light source to form a latent image on a photosensitive surface. The present invention relates to an image forming apparatus such as a copying machine, a printer, and a facsimile machine.

[0002]

2. Description of the Related Art A laser diode is used as a light emitting element to convert an electric pulse signal into a light pulse in a digital optical communication or electrophotographic image forming apparatus.
It is required that a desired amount of light be obtained even when the operating temperature of the element changes. However, the light emitting characteristics of the laser diode largely depend on the operating temperature, and it is necessary to control the laser diode driving current by a light emitting element driving circuit in order to obtain a desired light amount with a change in the operating temperature.

FIG. 9 shows, as a first conventional example, a configuration of a laser diode drive circuit by pulse current control of a cathode drive type laser.

In FIG. 9, reference numeral 101 denotes a comparator, 1
02 and 106 are reference voltage sources, 103 is a sample and hold circuit (S / H), and 104 is a hold capacitor (C
H), 105 is a current amplifier circuit, 108 is a reference current source (I
0) and 107 are switching circuits (SW), 111 is a laser diode (LD), 112 is a photodiode (PD), and 110 is a monitor resistor (RM).

In the conventional example shown in FIG. 9, during the period when the sample and hold circuit 103 is in the sampling state (hereinafter referred to as A
PC: (Automatic Power Contr.
ol), the switching circuit 107 is in the ON state, and the input data (DATA) is set so that the laser diode 111 emits light in the entire surface. The light amount from the laser diode 111 is monitored by the photodiode 112 so that the light emission amount of the laser diode 111 becomes a desired light amount, and the monitor current IM generated in the photodiode 112 is monitored by the monitor resistor 110.
, A monitor voltage VM is generated at the monitor resistor end 110. The current amplifying circuit 105 controls the laser diode drive current based on the reference current source 108 so that the monitor voltage VM is constant (that is, the light emission amount is constant).

While the sample and hold circuit 103 is in the hold period, the switching circuit 10 responds to input data.
7 turns on / off the laser diode drive current, thereby giving a pulse modulation signal to the laser diode 111.

However, in the configuration shown in FIG. 9, when the operating frequency of the optical pulse modulation increases, the light emission delay peculiar to the laser diode becomes a problem, and the transient characteristics of the modulated optical pulse deteriorate.

FIG. 10 shows a second conventional example which is one of the techniques for solving the problems of the first conventional example. In the second conventional example, a DC bias current is added to a laser diode driving current in order to improve a transient characteristic of an optical pulse due to a light emission delay of the laser diode. Since the basic configuration is the same as that of the first conventional example shown in FIG. 9, detailed description is omitted. Reference numeral 109 denotes a current source (I
B) and 115 are reference pulse current sources (IP0).

In the prior art shown in FIG. 10, the switching circuit 107 is in the ON state during the APC operation, and the input data is set so that the laser diode 111 emits light over the entire surface. The photodiode 1 is controlled so that the light emission amount of the laser diode 111 is constant in the entire light emitting state.
Based on the monitor voltage VM obtained by the configuration of the resistor 12 and the monitor resistor 110, the pulse current IP is controlled by the current amplifier circuit 105 from the reference pulse current IP0, and the pulse current IP and the bias current IB are superimposed to thereby drive the laser diode driving current. ILD has been determined.

During the hold period, the switching circuit 107 turns on / off the pulse current IP according to the input data.
By turning off the laser diode drive current ILD
Pul pulse modulation signal is given.

In the conventional example shown in FIG. 10, the light emission delay of the laser diode 111 cannot be effectively reduced unless the bias current IB is applied near the threshold current at which the laser diode 111 emits light.

However, in the conventional example shown in FIG. 10, the oscillation threshold current of the laser diode 111 varies depending on the operating temperature as described above, and also varies depending on individual elements. Without turning off, there is a high possibility that a sufficient extinction ratio cannot be obtained. Therefore,
In actual use, it is difficult to set a bias current at a fixed value near the threshold current.

FIG. 11 shows a third conventional example. In the third conventional example, a bias current is added to a drive current as in the second conventional example, but the target of the current to be controlled is the bias current, and the pulse current is fixed. Detailed description of the same reference numerals as those in FIG. 9 is omitted. 11
3 is a reference bias current source (IB0) for determining the bias current IB, and 114 is a pulse current source (IP) for giving a pulse current IP.

In the conventional example shown in FIG. 11, the switching circuit 107 is on during the APC operation, and the input data is set so that the laser diode 111 emits light. Laser diode 11 when emitting light
The photodiode 1 is adjusted so that the light emission amount of the photodiode 1 becomes a desired value.
Based on the error voltage between the monitor voltage VM obtained by the configuration of the reference resistor 12 and the monitor resistor 110 and the reference voltage Vref1 corresponding to the desired light quantity, the reference bias current IB0 is controlled by the current amplifier circuit 105 to control the bias current IB, It controls the diode drive current ILD. Also, during the hold period, the pulse current IP is turned on / off by the switching circuit 107 in accordance with the input data, thereby applying pulse data to the laser diode drive current ILD to perform optical pulse modulation.

However, in the conventional example shown in FIG. 11, when the laser diode drive current is small, such as when the laser diode 111 is operated at a low temperature, the bias current is unnecessary and there is a possibility that the laser diode 111 will fall into an incapable state.

Hereinafter, the case where the state becomes uncontrollable will be described in more detail.

FIG. 12 shows the relationship between the laser drive diode current and the light output due to a change in the operating temperature of a general laser diode.

As the operating temperature increases, the threshold current increases, and the laser diode drive current IDL increases.
In this case, the problem described above does not occur. Conversely, when the operating temperature decreases, the threshold current decreases, and the laser diode drive current IDL can be reduced.
The bias current IB is reduced so that the light output p from the laser diode becomes a desired value. However, where the bias current IB is unnecessary and a desired light output can be obtained with a value smaller than the set value of the pulse current IP, it is impossible to control the light amount to be constant. This is because the pulse current IP, which is a fixed value, cannot be reduced below the set value.

There is another peculiar phenomenon in the temperature characteristics of the laser diode. A peculiar phenomenon is that the slope efficiency (also called differential efficiency) in the laser oscillation region decreases. Therefore, in performing optical pulse modulation, if the laser diode drive current increases due to an increase in the operating temperature and the slope efficiency decreases, it is necessary to increase the pulse current without securing a sufficient extinction ratio of the laser diode. Can not.

FIG. 13 shows the relationship between the change in the laser drive current ILD with respect to the temperature change and the ratio of the pulse current and the bias current.

When the operating temperature Ta decreases from a high temperature to a low temperature, the laser drive current ILD decreases. Further, it is possible to control the amount of light emitted from the laser diode to be constant until the operating temperature decreases and the bias current IB becomes IB = 0, but when the bias current IB becomes unnecessary, the control becomes impossible. Would. That is, FIG.
In the middle region A, since a current for reducing the pulse current IP is required, it is not possible to perform control to obtain a desired light amount.

[0022]

As described above, in the prior art, in order to secure a high-speed light emitting operation of a laser diode, a DC current close to an oscillation threshold current is usually supplied as a bias current. The pulse current corresponding to the input data is superimposed on the bias current to supply the current to the laser diode. As a driving method of the light emitting operation of the laser diode, there are pulse current control and bias current control, each of which has advantages and disadvantages.

In the bias current control, high speed can be ensured in the optical pulse modulation, but the extinction ratio of the laser light can be sufficiently obtained by the change in the oscillation threshold current and the slope efficiency due to the change in the operating temperature of the laser diode. If the bias current is less than zero, it is impossible to control the light intensity to a desired value. On the other hand, in the pulse current control, the extinction ratio of the laser beam can be sufficiently secured by setting the bias current so as not to exceed the threshold current at any operating temperature.
When high-frequency optical pulse modulation is performed, the transient characteristics of the optical pulse deteriorate.

Accordingly, the present invention has been made in view of the above problems, and an object of the present invention is to provide an image forming apparatus which solves the above problems.

[0025]

According to a first aspect of the present invention, there is provided a light emitting device in which an output light quantity is dependent on a bias current and a temperature, and a detecting element for detecting the output light quantity. Means, a driving unit for driving the light emitting element by a modulation signal of an image signal and the bias current, and a photoreceptor that is exposed to output light of the light emitting element to form a latent image, The driving unit drives the light emitting element with a current that gives an amplitude level of the modulation signal when the temperature is lower than a predetermined temperature based on a current corresponding to the output light amount, and the temperature is higher than the predetermined temperature. In some cases, the light emitting element is driven by a current giving the amplitude level and the bias current.

The device according to the present invention described in claim 2 is:
2. The device according to claim 1, wherein the driving unit current-limits a current corresponding to the output light amount, and limits the current to a constant value substantially equal to the reference current value when the current corresponding to the output light amount increases and approaches a reference current value. Current limiting means for outputting the obtained current;
A current supply means for supplying a function current of an input current and an output current of the current limiting means to the light emitting element is provided.

The device according to the present invention described in claim 3 is:
2. The driving device according to claim 1, wherein the driving unit current-limits a current corresponding to the output light amount, and outputs a current limited so as to slightly increase from a reference current value with an increase in the current corresponding to the output light amount. The light emitting device includes a current limiting unit and a current supply unit that supplies a function current of an input current and an output current of the current limiting unit to the light emitting element.

The device according to the present invention described in claim 4 is:
4. The light emitting device according to claim 2, wherein the light emitting element is a laser diode having a cathode connected to the current supply unit, and the current supply unit subtracts the limited current from a current corresponding to the output light amount. A current subtracting means for generating a bias current, and a current adding means for adding the current obtained by switching the limited current by the image signal and the bias current to supply the current to the cathode.

The device according to the present invention described in claim 5 is:
The light emitting element according to claim 2 or 3, wherein the light emitting element is a laser diode having an anode connected to the current supply means, and the current supply means obtains the limited current from a current corresponding to the output light amount by the image signal. A current subtracting means for subtracting the switched current and supplying the current to the anode is provided.

The device according to the present invention described in claim 6 is:
In any one of claims 2 to 5, a means for arbitrarily setting the reference current value is provided.

[0031]

Embodiments of the present invention will be described below in detail with reference to the drawings.

(Embodiment 1) FIG. 1 is a side perspective view showing Embodiment 1 of an image forming apparatus to which the present invention is applied.

A copying machine 1 shown in FIG. 1 is a kind of an image forming apparatus, and originals stacked on an original feeder 2 are sequentially conveyed onto an original platen glass surface 3 one by one. When a document is conveyed on the platen glass surface, the lamp 5 in the scanner unit 4 is turned on, and the scanner unit 4 moves in the X direction in the drawing to irradiate the entire document surface.

The reflected light from the original is reflected by mirrors 6a, 6b, 6
After passing through c, an image described on a document is formed on the imaging surface of the linear image sensor 8 extending in the Z direction in the figure through the lens system 7. The linear image sensor 8 photoelectrically converts the formed image and outputs an image signal. This image signal is subjected to processing such as correlated double sampling and waveform shaping by a signal processing circuit (not shown).
The image data is temporarily stored in an image memory (not shown) composed of M and read out again. The read image signal is input to the exposure control unit 9.

The exposure control section 9 includes a semiconductor laser corresponding to a light source, and this semiconductor laser is controlled in light emission timing in accordance with an image signal read from an image memory. The laser beam from the semiconductor laser is applied to the rotating polygon mirror 1
It is irradiated to 0. The cylindrical photosensitive drum 11 rotates clockwise (CW) in the figure, and the photosensitive surface is uniformly charged by the charger 12. The reflected light from the rotating polygon mirror 10 is reflected by the mirror 6d and then applied to the photosensitive surface of the photosensitive drum 11 to remove a part of the charged charges, thereby forming an electrostatic latent image on the photosensitive surface. For example, a gas laser may be used as a light source instead of the semiconductor laser. In this case, however, an optical modulator that modulates light at a high speed is required.

FIG. 2 is a diagram showing a configuration of a main part in FIG. 2, the same components as those in FIG. 1 are denoted by the same reference numerals, and the mirror 6a is omitted.

Laser beam L from semiconductor laser 20
Is converted into substantially parallel light by the collimator lens 21 and the stop 22, and is incident on the rotary polygon mirror 10 via a predetermined beam. The rotating polygon mirror 10 is rotated in a counterclockwise direction (CC
W) at a constant angular velocity. Due to this rotation, the laser beam L incident on the rotating polygon mirror 10 is reflected as a deflection beam Ld that continuously changes the reflection angle.

The deflection beam Ld, which is an arc scanning light, is f-θ
The light is condensed by the lens 23 to focus on a straight line on the photosensitive surface of the photosensitive drum 11. At the same time, the f-.theta. Lens 23 corrects the distortion so as to guarantee the temporal linearity of the scanning, so that the deflection beam Ld scans the photosensitive surface of the photosensitive drum 11 at a constant speed in the direction of the arrow in FIG. Photosensitive drum 1
The formation of the electrostatic latent image on 1 is performed by variably controlling the light emission timing of the semiconductor laser 20 according to the image signal from the image memory.

The beam detector 24 detects the deflected beam Ld from the rotary polygon mirror 10, and sets the drive timing of the semiconductor laser 20 so as to start modulating the laser beam L based on the image signal after a certain time according to the distance d. Controlled. Thereby, the recording start position of the image on the photosensitive surface of the photosensitive drum 11 can be fixed.

Referring back to FIG. 1, the deflection beam Ld
As a result, the charge on the photosensitive surface is removed according to the image signal, and the photosensitive drum 11 is exposed, so that an electrostatic latent image is formed on the photosensitive drum 11. This electrostatic latent image is developed by applying toner by the developing device 13. The transfer target paper is conveyed from the transfer target paper stacking portion 14a or 14b to a position near the photosensitive drum 11 by the transfer mechanism at the timing when the electrostatic latent image formed on the photosensitive drum 11 is developed.

Then, the developed image is transferred to the transfer paper by passing the transfer paper between the transfer unit 15 and the photosensitive drum 11 disposed below the photosensitive drum 11. Thereafter, the image on the transfer receiving paper is heated by the fixing device 16,
It is fixed on the receiving paper. The transfer paper on which the image is fixed and the printing is completed is discharged to a predetermined tray of the multi-tray device 18 by the discharge roller 17.

FIG. 3 shows Embodiment 1 of a light emitting element drive circuit for driving a cathode drive type laser diode used in an image forming apparatus to which the present invention is applied.

In FIG. 3, reference numeral 31 denotes a comparator;
Is a sample hold circuit (S / H), 33 is a hold capacitor (CH), 34 is a variable gm amplifier, 35 is a current limiting circuit (LIM), and 36 is a switching circuit (S
W), 37 and 38 are current buffers, 39 is a current subtraction circuit, 40 is a current addition circuit, 41 is a laser diode (LD), 42 is a photodiode (PD), and 43 is a current-voltage conversion circuit (I / V). Show.

First, the definition of the variable gm amplifier 34 used in this embodiment will be described. Variable gm amplifier 34
Is a current amplifier having two voltages and one reference current Iin as inputs, and the output current has a certain function with respect to the reference current Iin. Output current Io of variable gm amplifier 34
ut is represented by the following equation, where 電位 vi is the potential difference between two input voltages. Here, for the sake of simplicity, the gain of the variable gm amplifier 34 is set to 1.

[0045]

Iout = f (△ vi) · Iin = k · Iin where k = f (△ vi) Here, k takes a value of 0 ≦ k ≦ 1, and hereinafter, the control function k
I will call it.

When the input potential difference Δvi changes between −1 and +1, the control function k and the output current Iout change as follows. In the above change range,
It is assumed that the control function k and the output current Iout change linearly.

[0047]

△ vi = −1 to 0 to +1 k = 0 to 0.5 to 1 Iout = 0 to Iin / 2 to Iin In FIG. 3, the monitor current IM of the output of the photodiode 42 is a current-voltage conversion circuit 43. And the output voltage VM of the current-voltage conversion circuit 43 is input to the negative input terminal of the comparator 31. A reference voltage Vref1 corresponding to a desired light amount is input to a positive input terminal of the comparator 31, and an output of the comparator 31 is input to a sample and hold circuit 32. A hold capacitor 33 is connected to the sample and hold circuit 32. The output voltage Verr of the sample hold circuit 32 is input to the positive input terminal of the variable gm amplifier 34.

The reference voltage Vref2 is input to the negative input terminal of the variable gm amplifier 34, and I0 is input as the reference current. The current output according to the potential difference between the two input voltages Verr and Vref2 determines the amount of light emission. The determined laser diode drive current ILD (= k · I0). The laser diode driving current ILD is supplied to the current limiting circuit 35.
Is input to the current buffer 37. Current limiting circuit 3
The reference pulse current IP0 is input to 5 as a current limit value, and the output of the current limit circuit 35 is a current that gives the output level of the laser diode 41 the amplitude level of the optical pulse modulation signal.

The output current of the current limiting circuit 35 is input to a switching circuit 36 and a current buffer 38. The output current of the current buffer 37 and the output current of the current buffer 38 are subjected to current subtraction in a current subtraction circuit 39, and the subtracted current is set to a low-level current during pulse modulation, and is referred to as a bias current IB. The output current IP of the switching circuit 36 (this is referred to as a pulse current) and the bias current I
B is added to the current by the current adding circuit 40. The added current is supplied to the cathode of the laser diode 41.

Here, the input / output characteristic of the current limiting circuit 35 is such that the maximum value of the output current IP is constant as the input laser diode drive current ILD increases as shown by the curve in FIG. It is assumed that the value is limited to a limit value (k2 · IPO = IP0).

Next, the operation for determining the laser diode drive current ILD in this embodiment will be described.

During the APC operation, the switching circuit 36 is on, and the input data (DAT) based on the image signal is
A) is assumed to be set so that the laser diode 41 emits light in the entire surface. Switching circuit 36
When the light output of the laser diode 41 is monitored by the photodiode 42 in a state where is turned on, the monitor current IM flows through the current-voltage conversion circuit 43, and a monitor voltage VM is generated at the output of the current-voltage conversion circuit 43.

The comparator 31 compares the monitor voltage VM with a reference voltage Vref1 corresponding to a desired light quantity, and compares the result with an error voltage Ver via a sample and hold circuit 32.
The signal is output to the variable gm amplifier 34 as r. The variable gm amplifier 34 determines the control function k based on the potential difference between the error voltage Verr and the reference voltage Vref2, and controls the laser diode drive current ILD.

When the control function k changes linearly from 0 to 1, the laser diode drive current ILD also changes linearly, and the pulse current IP and bias current IB at that time.
Is determined as follows. The variable k1 used below is a control function value when the current control starts working, and the variable k2 is a control function value when the current control works completely.

When the control function k is in the range of 0 ≦ k ≦ k1,
The laser diode drive current ILD is equal to the reference pulse current I.
Since it is smaller than P0, the current limiting circuit 35 does not operate, and the laser diode drive current ILD becomes the pulse current IP. That is, in the region of 0 ≦ k ≦ k1, the light emitting operation of the laser diode 41 is controlled by the pulse current control.

When the control function k is in the range of k1 ≦ k ≦ k2, the laser diode drive current ILD approaches the reference pulse current IP0, so that the current limiting circuit 35 starts to operate gradually, and the laser diode drive current ILD and the pulse current IP Differences occur. The difference between the current ILD and the current IP becomes the bias current IB, and when the control function k further increases, the bias current IB also gradually increases. That is, k1
The region of ≦ k ≦ k2 is a region where the transition from pulse current control to bias current control is performed.
The light emission operation is controlled in a state where pulse current control and bias current control are mixed.

When the control function k is in the range of k2 ≦ k ≦ 1, the laser diode drive current ILD is larger than the reference pulse current IP0, so that the current control circuit 35 operates sufficiently. , And the bias current IB increases linearly. That is, in the region of k2 ≦ k ≦ 1, the light emitting operation of the laser diode 41 is controlled by the bias current control.

FIG. 5 illustrates the above description, and the laser diode drive circuit of the present embodiment can continuously shift from pulse current control to bias current control. Further, during the transition, control can be performed in a state where both are mixed. The laser diode drive circuit of the present embodiment further includes a switch from bias current control to pulse current control.
The transition in the opposite direction to that described above can be performed continuously.

When the sample and hold circuit 32 is in the hold state, the control function k is determined by the error voltage Verr held by the hold capacitor 33, the laser diode drive current ILD is determined, the pulse current IP and the bias current IB Is also determined.

In order to perform the optical pulse modulation according to the input data, the switching circuit 36 is turned on / off according to the input data.

FIG. 6 is a diagram showing a laser diode drive current in this embodiment when the operating temperature changes, and corresponds to FIG.

The laser diode driving current ILD on the vertical axis
The ratio of the pulse current IP and the bias current IB constituting the above has the relationship shown in the drawing with respect to the operating temperature Ta on the horizontal axis. As can be seen from a comparison between FIG. 6 and FIG. 13, with the use of this embodiment, it is also possible to control a region that cannot be controlled by the conventional technique (a region corresponding to region A shown in FIG. 13). That is, when the operating temperature Ta is high, the pulse current is constant and the bias current control is performed, and when the operating temperature Ta is low, the control range can be wider than before by the pulse current control.

A further feature of the present embodiment is that the reference pulse current IP0
By changing the value of, the transition point of the control from the pulse current control to the bias current control or the transition point of the control from the bias current control to the pulse current control can be freely changed. In extreme cases, the reference pulse IP0
Is set to be equal to or greater than the maximum laser diode drive current ILD, the light emitting operation of the laser diode 41 is performed by pulse current control.

Further, since the bias current IB is determined from the difference between the laser drive current ILD and the pulse current IP,
The laser diode drive current ILD is equal to the error voltage Verr output from the comparator 31 irrespective of the control transition point.
Only determined by

As described above, in the present embodiment, the pulse current IPL is obtained by using the current limiting circuit 35 and the switching circuit 36 from the laser diode driving current IDL controlled so that a desired light amount is obtained by the variable gm amplifier 34. And the laser diode drive current ID
The difference between L and the pulse current IP is obtained by a current subtraction circuit 39 and used as a bias current IB. A superposition of the pulse current IP and the bias current IB by a current subtraction circuit 40 is supplied to the cathode of a laser diode 41.

Thus, during a low-temperature operation in which the laser diode drive current IDL is small, the laser diode drive current IDL is equal to or less than the limit value of the current limiting circuit 35, and all of the laser diode drive current IDL has the pulse current IP and the pulse current IPL. It can be controlled only by IP.
Therefore, the light emitting operation of the laser diode 41 is controlled by completely controlling the laser diode drive current IDL only by the pulse current control.

The laser diode driving current IDL approaches the limit value (reference pulse current value) of the current limiting circuit 35,
When the increase amount of the pulse current IP gradually decreases, and the laser diode drive current IDL is small and the bias current IB is becoming unnecessary, the difference between the laser diode drive current IDL and the pulse current IP becomes the bias current I.
Generated as B. That is, at this time, the light emitting operation of the laser diode 41 is controlled by controlling the laser diode drive current IDL in a state where the pulse control and the bias control are mixed.

Further, during a high-temperature operation in which a large laser diode drive current IDL is required due to a change in the operating temperature of the laser diode 41 or the like, the laser diode drive current IDL is equal to or larger than the limit value of the current limiting circuit 35, and the pulse current IP is The limit value of the limit circuit 35 is obtained, and the bias current IB is the difference between the laser diode drive current IDL and the pulse current IP. Therefore, the light emitting operation of the laser diode 41 is controlled by controlling the laser diode drive current IDL only by the bias current control.

The above-mentioned transition of the control, that is, the transition from the bias current control to the pulse current control, or the transition from the pulse current to the bias current can be performed automatically and continuously. Further, since the reference pulse current for determining the current limit value can be set arbitrarily, the transition point of the control can be freely changed.

(Embodiment 2) FIG. 7 shows, as Embodiment 2, an anode drive type laser diode drive circuit. Since the reference numerals used in FIG. 7 are the same as those in the first embodiment, the description of the components in the figure will be omitted.

In FIG. 7, the monitor current IM output from the photodiode 42 is input to the current-voltage conversion circuit 43, and the output voltage VM of the current-voltage conversion circuit 43 is input to the negative input terminal of the comparator 31. Comparator 3
1 has a reference voltage Vr corresponding to a desired light quantity.
ef1 is input, and the output of the comparator 31 is input to the sample hold circuit 32. A hold capacitor 33 is connected to the sample and hold circuit 32. The output voltage Verr of the sample and hold circuit 32 is
It is input to the positive input terminal of the variable gm amplifier 34.

The variable gm amplifier 34 has I as a reference current.
0 is input, and the reference voltage Vr is applied to its negative input terminal.
ef2 is input, and two input voltages Verr and Vref
The current output according to the potential difference of 2 is the laser drive current ILD (= k · I0) that determines the light emission amount. The laser diode drive current ILD is input to a current limiting circuit 35 and a current buffer 37, and the output of the current limiting circuit 35 is used as a current for giving the output light of the laser diode 41 the amplitude level of the optical pulse modulation signal. 35
Is input with a reference pulse current IP0 as a current limit value.

The output current of the current limiting circuit 35 is input to the switching circuit 36, and the output current of the current buffer 37 and the output current of the switching circuit 36 (this current is referred to as a pulse current IP) are subtracted by a current subtraction circuit 39. And
The subtracted current is supplied to the anode of the laser diode 41. Here, it is assumed that the characteristic of the current limiting circuit 35 has a characteristic shown by a curve in FIG. 4A with respect to a change in the control function k, as in the first embodiment.

Next, the operation of this embodiment will be described. It is assumed that the switching circuit 36 is off during the APC operation. When the output light from the laser diode 41 is detected by the photodiode 42 while the switching circuit 36 is off, the photodiode 42
, A monitor current IM flows. The monitor current IM is converted into a monitor voltage VM using a current-voltage conversion circuit 43, and the monitor voltage VM and a reference voltage Vref1 corresponding to a desired light quantity are converted.
Are compared by a comparator 31 and the result is variable as an error voltage Verr via a sample and hold circuit 32.
Output to the m amplifier 34. In the variable gm amplifier 34, the control function k is determined by the potential difference between the error voltage Verr and the reference voltage Vref2, and the laser diode driving current I
LD (= k · I0) is controlled.

When the sample and hold circuit 32 is in the hold state, the current output from the current limiting circuit 35 is a current that gives a modulation signal to the laser diode drive signal IDL during light pulse modulation, and is input to the switching circuit 36. To apply pulse modulation to the laser diode 41, the switching circuit 36 is turned on / off according to input data. The output current of the switching circuit 36 (this current is referred to as a pulse current IP) and the output current of the current buffer 37 are input to a current subtraction circuit 39, and the current obtained by subtracting the pulse current IP from the output current of the current buffer 37 is a laser diode. It is supplied to 41 anodes.

At this time, a current obtained by subtracting the pulse current IP from the output current of the current buffer 37 becomes a current that determines a low level in optical pulse modulation, and this current becomes a bias current IB.

Also in the present embodiment, by changing the value of the reference pulse current IP0, the control transition point from the pulse current control to the bias current control or the control transition point from the bias current control to the pulse current control is performed. Can be changed freely. Further, since the bias current IB is determined from the difference between the laser drive current ILD and the pulse current IP, the laser diode drive current ILD is determined only by the error voltage Verr output from the comparator 31 regardless of the control transition point. You.

As described above, according to the present embodiment, the current obtained by subtracting the pulse current IP from the laser diode driving current ILD is supplied to the anode of the laser diode 41. The same effect can be obtained by selectively controlling the pulse current control and / or the bias current control according to the value of the laser diode drive current ILD according to the above to control the light emitting operation of the laser diode.

(Embodiment 3) In general, as shown in FIG. 12, the temperature characteristic of a laser diode increases as the operating temperature increases, and the laser diode needs to have a laser light intensity to obtain a desired light quantity. The need to increase the diode drive current has been described above. However, there is another peculiar phenomenon in the temperature characteristics of the laser diode. This phenomenon is that the slope efficiency (also called differential efficiency) in the laser oscillation region decreases.

Therefore, in order to ensure a sufficient extinction ratio of the laser diode in performing the light pulse modulation,
As the slope efficiency decreases, the pulse current must be increased.

In the light emitting element drive circuit used in the image forming apparatus to which the present invention is applied, the current limiting capability of the current limiting circuit in the configurations of the first and second embodiments is limited so as to solve this problem. can do.

That is, as shown by the curve in FIG. 4B, in the bias current control region in the light emitting element drive circuit used in the image forming apparatus to which the present invention is applied, the pulse current is increased with the increase in the laser diode drive current. May be provided with a current limiting capability to slightly increase the current. It is easy to make the current limiting circuit have the characteristic shown by the curve (b), and FIG. 8 shows a configuration example of this embodiment.

The reference pulse current IP0 that gives the current limit value of the current limiting circuit 80 is supplied to the anode of the diode 81 and the positive input terminal of the operational amplifier 85, and the cathode of the diode 81 is connected to the upper end of the resistor R81. . The other end of the resistor R81 is grounded. Operational amplifier 8
The output of 5 is connected to the base of transistor Q2. The collector of transistor Q2 is connected to power supply voltage Vcc, and the emitter of transistor Q2 is connected to a resistor R
One end of the input terminal 82 is connected to the constant current source I1 and the negative input terminal of the operational amplifier 85.

On the other hand, the laser diode drive current ILD, which is the input of the current limiting circuit 80, is supplied to the anode of the diode 89 and the output of the variable gm amplifier 87.
The cathode of diode 89 is connected to the collector and base of transistor Q6, the base of transistor Q8, and the positive input terminal of operational amplifier 86. The collector of the transistor Q4 is connected to the reference current input terminal of the variable gm amplifier 87, and the output of the operational amplifier 86 is connected to the base of the transistor Q4. The emitter of the transistor Q4 is connected to the other end of the resistor R82 and the operational amplifier 86.
Is connected to the negative input terminal.

The emitter of the transistor Q6 is connected to the resistor R83.
And the emitter of the transistor Q8 is connected to the upper end of the resistor R84. The other ends of the resistors R83 and R84 are grounded. The output current IP can be taken out from the collector of the transistor Q8. Note that the resistance value of the resistor R82 is R1, the resistors R81, R83,
The resistance value of R83 is R2, the potential difference Vc that determines the control function α is input to the variable gm amplifier 87, and the current gain is 1.

Next, the operation of the current limiting circuit 80 having the above configuration will be described. First, in order to briefly explain the basic operation, the control function α of the variable gm amplifier 87 is set to α = 1.

Input current (laser diode drive current I
LD) is equal to or less than the reference pulse current IP0, the resistance R82
Does not flow through the input current ILD, and the input current ILD is output as it is as the output current IP.

When the input current ILD becomes equal to or more than the reference pulse current IP0, a potential difference is generated between both ends of the resistor R82, a current flows, and a current limiting operation is performed. The output current IP at this time is given by the following equation. Here, the emitter voltages of the transistors Q2 and Q4 are VIE and V2E, the base-emitter voltage of the transistor Q6 is VBE, and the forward voltage of the diode 81 is VF.

[0089]

IP = ILD-i (1) i = (V2E-V1E) / R1 = [{R2 · (ILD-i) + VBE} − (R2 · IP0 + VF)] / R1 where VF = VBE ,

[0090]

I = (R2 · IP−R2 · IP0) / R1 = (IP−IP0) · R2 / R1 (2) Therefore, substituting equation (2) into equation (1),

[0091]

## EQU00005 ## IP = ILD- (IP-IP0) .R2 / R1 = ILD-IP.R2 / R1 + IP0.R2 / R1 = (ILD + IP0.R2 / R1) / (1 + R2 / R1) = (ILD + a.IP0) / (1 + a) (a = R2 / R1) (3) Here, if the resistance ratio is selected such that the coefficient a has a very large value, the limiting capability of the current limiting circuit 80 having the configuration of FIG. And the output current IP is approximately IP = IP0. Conversely, if the resistance ratio is selected so that the coefficient a has a small value, the current limiting capability of the current limiting circuit 80 can be weakened, and the output current IP becomes the current value given by the equation (3).

Therefore, by selecting the resistance ratio and setting the value of the coefficient a in the equation (3) to an appropriate value corresponding to the temperature change of the slope efficiency, the light emitting element used in the image forming apparatus to which the present invention is applied. It is possible to control the increase in the pulse current in the bias control region in the drive circuit. Thus, a sufficient extinction ratio can be ensured even if the laser diode drive current increases and the slope efficiency decreases.

Further, in the above description, the control coefficient α of the variable gm amplifier 87 is set to 1. However, the control function α is changed by controlling the potential difference Vc inputted to the variable gm amplifier 87, and thereby the apparent function is changed. The coefficient a can also be varied. Therefore, once the resistors R81 to R81 are set.
Laser diodes with different temperature characteristics of slope efficiency that can control an increase in pulse current in a bias control region in a light emitting element driving circuit used in an image forming apparatus to which the present invention is applied without changing the value of R84. Can also be accommodated.

As described above, according to the present embodiment, when the laser diode drive current ILD increases above the reference pulse current in consideration of the temperature characteristics of the slope efficiency, the output current of the current limiting circuit increases by a certain function. By limiting the current limiting capability so that the pulse current gradually increases in the bias current control region, even if the laser diode drive current increases and the slope efficiency in the laser oscillation region decreases, extinction will occur. The ratio can be sufficiently secured. Also, by controlling the control function of the variable gm amplifier by varying the input potential difference, the current control capability can be controlled without changing the resistance value once set.

[0095]

As described above, according to the present invention, in driving a light emitting element in which the output light quantity depends on the bias current and the temperature, a current corresponding to the output light quantity is generated. When the temperature is lower than the temperature, the modulation signal is driven by the current giving the amplitude level, and when the temperature is higher, the current is applied by the current giving the amplitude level and the bias current. By controlling the light emission using the operation from high to high temperatures, there is an effect that a good image can be formed regardless of the temperature change.

[Brief description of the drawings]

FIG. 1 is a first embodiment of an image forming apparatus to which the present invention is applied.
FIG.

FIG. 2 is a diagram showing a configuration of a main part in FIG. 1;

FIG. 3 is a configuration diagram of a laser diode drive circuit according to the first embodiment of the present invention.

FIG. 4 is a characteristic diagram of a current limiting circuit used in the present invention.

FIG. 5 is a characteristic diagram illustrating a relationship between a pulse current and a bias current with respect to a control function k of the laser diode drive circuit according to the embodiment of the present invention.

FIG. 6 is a characteristic diagram showing a relationship between a pulse current and a bias current with respect to a change in operating temperature according to the embodiment of the present invention.

FIG. 7 is a configuration diagram of a laser diode drive circuit according to a second embodiment of the present invention.

FIG. 8 is a configuration diagram of a current limiting circuit according to a third embodiment of the present invention.

FIG. 9 is a circuit diagram of a laser diode drive circuit in a conventional image forming apparatus.

FIG. 10 is a circuit diagram of a laser diode driving circuit based on pulse current control in a conventional image forming apparatus.

FIG. 11 is a circuit diagram of a laser diode driving circuit based on bias current control in a conventional image forming apparatus.

FIG. 12 is a characteristic diagram showing a relationship between a drive current and a temperature change of a laser diode.

FIG. 13 is a characteristic diagram showing a relationship between a pulse current and a bias current with respect to a change in operating temperature of a drive circuit in a conventional image forming apparatus.

[Explanation of symbols]

 31, 101 Comparator 32, 103 Sample hold circuit 33, 104 Hold capacitor 34, 87 Variable gm amplifier 35 Current limiting circuit 36, 107 Switching circuit 37, 38 Current buffer 39 Current subtraction circuit 40 Current addition circuit 41, 111 Laser diode 42, 112 Photodiode 43 Current-voltage conversion circuit 81, 89 Diode 102, 106 Reference voltage source 105 Current amplifier circuit 108 Reference current source 109 Bias current source 110 Monitor resistor 113 Reference bias current source 114 Pulse current source 115 Reference pulse current source Q2, Q4 , Q6, Q8 Transistors R81-R84 Resistance

Claims (6)

[Claims] A light emitting element whose output light quantity depends on a bias current and a temperature; a detecting means for detecting the output light quantity; a driving means for driving the light emitting element with a modulation signal of an image signal and the bias current; In an image forming apparatus including a photoreceptor that is exposed by an output light of a light emitting element to form a latent image, the driving unit is configured to output a latent image based on a current corresponding to the output light amount.
When the temperature is lower than a predetermined temperature, the light emitting element is driven by a current that provides an amplitude level of the modulation signal, and when the temperature is higher than the predetermined temperature, the light emitting element is driven by the current that provides the amplitude level and the bias current. An image forming apparatus for driving a light emitting element. 2. The driving device according to claim 1, wherein the driving unit limits a current corresponding to the output light amount, and limits the current to a constant value substantially equal to the reference current value when the current corresponding to the output light amount increases and approaches a reference current value. 2. The image forming apparatus according to claim 1, further comprising: a current limiting unit configured to output the selected current; and a current supply unit configured to supply a function current of an input current and an output current of the current limiting unit to the light emitting element. apparatus. 3. The driving unit current-limits a current corresponding to the output light amount, and outputs a current limited so as to slightly increase from a reference current value with an increase in the current corresponding to the output light amount. The image forming apparatus according to claim 1, further comprising: a current limiting unit; and a current supply unit configured to supply a function current of an input current and an output current of the current limiting unit to the light emitting element. 4. The light emitting element is a laser diode having a cathode connected to the current supply means, wherein the current supply means subtracts the limited current from a current corresponding to the output light amount to produce a bias current. A current subtracting means for generating a current, and a current adding means for adding a current obtained by switching the limited current by the image signal and the bias current to supply the bias current to the cathode. The image forming apparatus as described in the above. 5. The light emitting element is a laser diode having an anode connected to the current supply means, and the current supply means switches the limited current from a current corresponding to the output light amount by the image signal. The image forming apparatus according to claim 2, further comprising a current subtracting unit that subtracts a current and supplies the current to the anode. 6. The image forming apparatus according to claim 2, further comprising a unit that arbitrarily sets the reference current value.