JPH065865B2 - Semiconductor laser protection circuit - Google Patents

Description

The present invention relates to a semiconductor laser protection circuit suitable for application to a semiconductor laser drive circuit used in an electrophotographic color copying machine, a laser printer or the like.

BACKGROUND OF THE INVENTION In an electrophotographic color copying machine or the like, there is one using a semiconductor laser as a means for forming an electrostatic latent image on a photosensitive image forming body by an image signal corresponding to a document.

FIG. 6 is a block diagram showing an example of such a color copying machine 10.

The figure shows an example of a simple color copying machine. This copying machine divides the color information of a color document into about three types of color information to record a color image. As the color information to be separated, in this example, 3 of black BK, red R and blue B are used.
A color is illustrated.

In the figure, reference numeral 11 denotes a drum-shaped image forming body,
A surface of a photoconductive photoconductor such as selenium is formed on the surface thereof so that an electrostatic image (electrostatic latent image) corresponding to an optical image can be formed.

On the peripheral surface of the image forming body 11, members described below are sequentially arranged in the direction of rotation thereof.

The surface of the image forming body 11 is uniformly charged by the charger 12. The surface of the charged image forming body 11 is subjected to image exposure (the light is indicated by 14) based on each color separation image.

After the image exposure, it is developed by a predetermined developing device. The developing devices are arranged by the number corresponding to the color separation image. In this example, the developing device 15 filled with the red toner developer, the developing device 16 filled with the blue toner developer, and the developing device 17 filled with the black toner developer are These are arranged in this order in the direction of rotation of the image forming body 11 so as to sequentially face the surface of the image forming body 11.

The developing devices 15 to 17 are sequentially selected in synchronization with the rotation of the image forming body 11. For example, when the developing device 17 is selected, a black color separation image (normal black and white image) is developed.

A pre-transfer charger 19 and a pre-transfer exposure lamp 20 are provided on the developing device 17 side to facilitate transfer of a color image onto the recording medium P. However, pre-transfer charger 1
9 and a pre-transfer exposure lamp are provided as needed.

The color image developed on the image forming body 11 is transferred onto the recording body P by the transfer device 21.

The transferred recording medium P is subjected to a fixing process by the fixing device 22 in the subsequent stage, and then the recording medium is ejected.

The static eliminator 23 is composed of one or both of a static eliminator lamp and a corona discharger for static elimination.

The cleaning device 24 is composed of a cleaning blade and a fur brush, and removes the residual toner adhering to the drum surface after transferring the color image of the image forming body 11 by these.

As the charger 12 described above, a scorotron corona discharger or the like can be used. This is because the influence of the previous charging is small and stable charging can be given to the image forming body 11.

As the image exposure, the image exposure obtained by the laser beam scanning device is used.

This is because, in the case of using the laser beam scanning device, a small and inexpensive semiconductor laser can be used as a light source of the image recording device, and a clear color image can be recorded.

The image exposure means shown in FIG. 7 shows an example of the laser beam scanning device 30.

The laser light scanning device 30 has a semiconductor laser 31, and the laser 31 is optically modulated based on a color separation image (for example, binary data).

The laser beam emitted from the laser 31 enters a mirror scanner 34 formed of a rotating polygon mirror (polygon) via a collimator lens 32 and a cylindrical lens 33.

The laser beam is deflected by the mirror scanner 34, and the laser beam is irradiated on the surface of the image forming body 11 through the f-θ lens 35 and the cylindrical lens 36 for image formation.

The laser beam is reflected by the mirror scanner 34 on the image forming body 1.
By scanning the surface of No. 1 at a constant speed in the predetermined direction a, image exposure corresponding to the color separation image is performed by such scanning.

Reference numeral 39 denotes a photo sensor, and by receiving the laser beam reflected by the mirror 38, an index signal indicating the start of scanning of the laser beam is obtained, and writing of image data for one line is performed based on this index signal. Will be done.

When the laser beam scanning device 30 is used, an electrostatic image can be formed while being shifted for each color separation image.
A clear color image can be recorded.

FIG. 8 is a systematic diagram of essential parts showing an example of a laser drive circuit 40 suitable for use in the color copying machine as described above.

The laser drive circuit 40 is provided with a light quantity stabilizing circuit for stabilizing the light quantity of the laser to a proper light quantity value in addition to the circuit for driving the laser by the modulation signal.

The reason why the light quantity stabilizing circuit is provided in this way is that the temperature characteristics of the laser are extremely poor. Considering use in an environment where the ambient temperature changes, a light quantity stabilizing circuit that stabilizes the laser light quantity is provided. Is necessary.

In this example, the microcomputer controls the appropriate light amount.

Now, the laser 31 is excited by the drive current (excitation current) output from the current generation circuit 48, and is generated with a light amount corresponding to the drive current. The laser beam emitted from the laser 31 is a photosensor (photodiode, etc.) 4
The light amount is detected by 1, and a current corresponding to the light amount is supplied to the current / voltage converting means 42, so that the voltage signal corresponding to the light amount is converted. Therefore, the photo sensor 41 and the current / voltage converting means 42 function as the light amount monitor circuit 60.

The voltage signal is converted into a digital signal by the A / D converter 51 and then supplied to the microcomputer 52, whereby a predetermined current control output signal corresponding to the value of the digital signal is output.

This control signal is latched by the latch circuit 53 and then D
By being supplied to the / A converter 47, it is converted into an analog control signal (current control signal) corresponding to the output signal. By supplying this current control signal to the current generation circuit 48, the excitation state of the laser 31 is controlled based on this current control signal, and the light amount thereof is changed.

By the way, the relationship between the drive current supplied to the laser 31 and the amount of emitted light is as shown in FIG.
Until then, the amount of light increases very little. However, it has a characteristic that the light amount rapidly increases when the drive current Ith is exceeded.

Therefore, when the drive current reaches from the inflection point Ith to zero, the range of change in the value of the control current supplied to the current generating circuit 48 is set to be large, and thereafter, the light amount appropriate value should be controlled in small steps. This is because it is possible to significantly reduce the time until convergence.

In consideration of this, the microcomputer 52
Then, depending on the value of the input digital signal supplied thereto, the following signal is output.

That is, as shown in FIG. 10, when the maximum value of the laser light amount required for data writing is Pmax and the minimum value is Pmin, the light amount smaller than the minimum value Pmin by a predetermined value is P1. The light amount between Pmin and Pmax becomes an appropriate light amount value.

The detected light intensity is P1 or more. The light amount between Pmin and Pmax becomes an appropriate light amount value.

When the amount of detected light is P1 or less, the D / A conversion device 47
Let A be the increment ΔV of the digital value given to. And
When P1 or more and Pmin or less, the increment ΔV is set to B, and in other cases, ΔV is set to −B (therefore, in this case, the decrement). However, the relationship between A and B is selected as A>B> 0.

Then, V + ΔV is calculated from the digital value V previously output to the D / A converter 47, and this is controlled as the new V to be output to the D / A converter 47.

Note that V is set to an initial value 0 in the initialization routine when the power is turned on.

From this, when the control routine for stabilizing the light quantity starts, the laser 31 is excited by the current I1 corresponding to the initial value ΔV, and in the next step, 2ΔV is used as a new output signal for the D / A converter. Output to the 47 side,
Accordingly, the laser 31 is excited by the current I2 corresponding to 2ΔV.

After that, the same control operation is repeated until the detected light intensity is P1 or more P
When it becomes less than min, B is used as ΔV instead of A this time.

As a result, the amount of change in current becomes smaller than before, but the detected light amount gradually increases as the control routine is repeated, and when the maximum light amount value Pmax is exceeded, the excitation current is controlled by -B this time. It will be.

Thus, the detected light amount settles to a value near Pmin to Pmax.

A and B can be set as follows.

That is, first, when the current setting step is repeated n times, A is set to a value slightly exceeding Pmin, and B is set to P when the current control step is repeated once or several times.
It is set to a value that exceeds max.

This is because when A and B are too large, they cannot be converged to Pmin to Pmax, and when they are too small, it takes time to converge.

Next, an example of the control program for stabilizing the light amount described above will be described with reference to FIG.

First, when the control routine for stabilizing the light amount is called, the presence or absence of the light amount control request is checked. The presence / absence of the light quantity control request can be determined by the presence / absence of a flag obtained from a control routine that controls the control of the apparatus body.
If there is no request, this control routine is exited (steps 61 and 62). In the light quantity control mode, the on state of the laser 31 is determined, and when it is off, the laser 31 is controlled to be turned on, and then the control routine is exited (steps 63 to 65).

When the laser 31 is in the ON state, after the A / D conversion data corresponding to the detected light amount is input, it is judged whether or not there is a proper range of the detected light amount (steps 66 and 67). In this case, when the detected light amount is within the proper range (Pmin to Pmax), the laser 31 is turned off and the light amount control request flag is reset, and then the control routine returns to the main processing control routine (step 68). , 69).

When the detected light quantity is not within the proper range, the detected light quantity level is compared with the set light quantity P1, and when it is P1 or less, the increment A is set as ΔV, and then V + ΔV is set as V and the D / A converter 47 is set. Output to (step 71 to
73).

Here, in the first control routine, V = 0 is initialized.

When it is detected that the detected light amount becomes P1 or more, it is determined whether or not it is within the proper range (Pmin to Pmax). Pmi
When it is less than or equal to n, the increment is set as ΔV,
Then, V + ΔV is set as V and output to the D / A converter 47 (steps 74 and 75).

As a result of such control, when the detected light amount becomes equal to or larger than Pmax, the routine proceeds to step 76, where the decrease amount -B is set as ΔV, and then V-ΔV is output to the D / A converter 47. Therefore, when the control routine 74 is called, the light amount of the laser 31 is controlled between Pmin and Pmax.

Therefore, the light quantity of the laser 31 is always excited with a light quantity value (appropriate light quantity value) between Pmin and Pmax.

The control routine is performed by a timer interrupt.

In this case, the light intensity control program described above is placed in this timer interrupt routine, or a flag is set when the timer is interrupted, the flag is checked in the main routine, and the control routine is executed only when the flag is set. To execute.

Here, since the output of the D / A converter 47 is prone to glitch (whisker-like noise), a low-pass filter is often used in the output stage of the D / A converter. Therefore, the response of the circuit is slow, and the conversion time of the A / D converter is required. Therefore, it is convenient to perform the light amount control process by the interrupt of the timer speed.

Although the light amount control starts from zero, the control routine may be configured to start from a predetermined value lower than the light amount P1.

That is, instead of setting the initial value to V = 0, V = V0 (0
It can also be set to <V0 <V1). Here, V1 is data to be given to the D / A converter corresponding to the light amount P1 at a certain temperature.

In this case, as shown in FIG. 11, the control time until the light amount is stabilized to the proper value can be further shortened.

[Problems to be Solved by the Invention] By the way, as described above, when the stabilization process for obtaining the appropriate value of the light amount is executed by using the microcomputer 52, if noise jumps into the microcomputer 52 from the outside, The microcomputer 52 may run out of control.

When the microcomputer 52 runs out of control, its output value cannot hold the light amount level when the light amount is stabilized.

In this case, if the output value of the microcomputer 52 becomes abnormally high due to runaway, the current generation circuit 48 controls the laser drive current so that the laser drive current becomes larger than that during normal operation.

Therefore, if this drive current is an excessive current to the laser 31, this excessive current causes the laser 3 to move.
1 may be destroyed.

Since the drive circuit 40 shown in FIG. 8 has no laser protection means due to such a runaway, the laser 31 cannot be effectively protected when the microcomputer 52 runs out of control.

Therefore, the present invention solves such a problem, and proposes a semiconductor laser protection circuit capable of reliably protecting the laser 31 even when the microcomputer 52 runs out of control. Is.

[Technical Means for Solving the Problems] In order to solve the above-mentioned problems, according to the present invention, a light amount monitor circuit for monitoring the emitted light amount of a semiconductor laser and an output voltage of the light amount monitor circuit are converted into a digital amount. D / A
Conversion means and D / for converting a digital amount into an analog signal
The A conversion means, the current generation means for supplying a predetermined drive current to the semiconductor laser according to the analog signal, and the digital amount given to the D / A conversion means so that the A / D conversion output is within a certain range are controlled. And a microcomputer that operates.

Further, according to the present invention, when it is detected that the D / A conversion output is abnormally greatly changed due to the runaway of the microcomputer, the output voltage of the forcible lighting means for lighting the semiconductor laser for a certain time and the light quantity monitor circuit is a certain value or more. It is characterized in that a lighting prohibition means for prohibiting the lighting of the semiconductor laser is provided when it is detected that the above condition has been detected.

[Operation] When the microcomputer runs out of control, the level of the current control signal supplied to the current generation circuit changes abruptly, so that this change time point is detected by the forced lighting means and modulated. The laser is forcibly turned on regardless of the presence or absence of a signal.

Further, by the forced lighting, it is possible for the lighting prohibiting means to detect that the light amount of the laser has reached a predetermined value or more. When a level equal to or higher than a predetermined value is detected, the lighting state of the laser is forcibly prohibited, thereby preventing the laser from being destroyed due to an excessive current. This prohibition circuit has a higher priority than the forced lighting circuit.

[Embodiment] Next, an example of a semiconductor laser protection circuit according to the present invention will be described in detail with reference to FIG.

Since the embodiment shown in FIG. 1 is applied to the semiconductor laser drive circuit shown in FIG. 8, the description of the same parts as those shown in FIG. 8 will be omitted.

FIG. 1 is an example of a laser drive circuit 40 including a laser protection circuit.

In FIG. 1, the current control signal Sa obtained from the D / A converter 47 is supplied to the current generation circuit 48 via the low pass filter 55.

At the input stage of the current generating circuit 48, input control means 90 for the lighting signal of the laser 31 is provided. In addition to the modulation signal corresponding to the image data and the lighting control signal used when stabilizing the light amount, the input control means 90 includes the forced lighting signal Sc and lighting from the forced lighting means 80 that operates when the microcomputer 52 runs out of control. The lighting prohibition signal Sf is supplied from the prohibition means 100, respectively.

The compulsory lighting signal Sc is used to compulsorily light the laser 31 for a certain period from the occurrence of the runaway, and the lighting prohibition signal Sf destroys the laser 31 from the laser light amount as a result of being driven by the compulsory lighting signal Sc. It is used to detect the level before being turned on and forcibly prohibit the lighting of the laser 31. The lighting prohibition signal Sf is also supplied to the microcomputer 52 as a runaway state determination signal. FIG. 9 shows a concrete example of the forced lighting means 80 and the input control means 90.

The forced lighting means 80 is composed of an edge detection circuit 81 and a pulse generation section 82. The edge detection circuit 81 is supplied with a current control signal Sa to detect a time point of a signal level change which occurs when the microcomputer 52 runs out of control. It

In a normal use state, the current control signal Sa has a constant level. However, when the microcomputer 52 runs out of control, the level of the current control signal Sa changes rapidly (FIG. 5A). This time point of level change is detected, and the edge detection signal Sb obtained at that time drives the pulse generator 82 to generate the compulsory lighting signal Sc having a constant pulse width (B and C in the same figure).

The input control means 90 includes an OR gate 91 and an AND gate 9.
In addition to the modulation signal, the lighting control signal sent from the microcomputer 52 side and the above-mentioned forced lighting signal Sc are supplied to the OR gate 91, and when any of these signals is supplied, OR output is obtained.

The OR output and the lighting prohibition signal Sf are supplied to the AND gate 92, and when the lighting prohibition signal Sf shown in FIG. 5F is at a high level, that is, in the normal operation mode, the OR output is directly used as the lighting signal in the current generating circuit 48. Will be supplied to.

On the other hand, when the runaway state of the microcomputer 52 is detected, the lighting prohibition signal Sf is inverted to the low level, so that the AND gate 92 is closed and the laser 3 is turned off.
Driving 1 is forcibly prohibited.

FIG. 3 shows an example of the low-pass filter 55. In this example, an RC time constant circuit of a resistor 56 and a capacitor 57 is used, and the time constant is set to be less than the pulse width T of the forced lighting signal Sc.

By the way, as described above, in addition to the forced lighting means 80,
The low pass filter 55 is provided for the following reason.

That is, when the modulation signal is 0, that is, the laser 3
1 is in a drive stopped state, the microcomputer 5
When 2 is out of control, when the current control signal Sa is at a level considerably higher than in the normal state as shown in FIG. 5A, the current generation circuit 48 outputs a considerably large drive current. It is controlled by the state. In this state, when the modulation signal becomes 1, the laser 31
Excessive current will flow instantly.

If such a state can be immediately detected by the lighting prohibition means 100, the lighting signal to the current generating circuit 48 becomes zero.
There is no particular problem.

However, when the circuit response speed of the lighting prohibition means 100 system is slow, there is a high possibility that the laser 31 will be destroyed by an excessive current.

Further, even when the modulation signal becomes 1 when the output Sl (FIG. 5A) of the low-pass filter 55 reaches the maximum level of the current control signal Sa, the current generation circuit 4
From 8 onward, since an excessive current is controlled to be supplied to the laser 31, there is a strong possibility that the laser 31 will be destroyed as described above.

Therefore, during a runaway, the laser 31
Is supplied with a predetermined lighting signal, and the low-pass filter 55 is used in synchronization with this to supply a current control signal.
Try to gradually raise the Sa level.

In this way, the laser 31 is forcibly turned on even during a runaway, and the driving current does not suddenly flow through the laser 31. The lighting prohibition means 100 detects this runaway state while the drive current is gradually increasing, and the laser 3
This is because it is possible to stop the driving of the laser 31 by the lighting prohibition signal Sf before an excessive current sufficient to destroy 1 is flown.

In this way, by using the low-pass filter 55 to moderate the rising of the current control signal Sa, it is sufficient to use circuit elements having a slow response speed as the lighting prohibition means 100 system and the forced lighting means 80. Be put to practical use.

Therefore, it becomes possible to use a circuit element having a slow response speed as the entire laser protection means. As a result, the entire circuit can be constructed at low cost.

Furthermore, by providing the low pass filter 55,
It becomes difficult for surge current to flow through the laser 31, and the laser 31
Can be prevented from premature deterioration.

FIG. 4 shows an example of the lighting prohibition means 100. The voltage signal Sd proportional to the amount of light is compared with the reference voltage REF of the reference voltage source 110 by the comparison circuit 101 (FIG. 5D). In the illustrated example, the comparison output Se is inverted to the high level for the first time when the voltage signal Sd rises above the reference voltage REF (E in the figure).

The reference voltage REF is set to Pmax or more and below a level sufficient to destroy the laser 31.

The comparison output Se is supplied to the clock terminal of the D flip-flop 102. Since the flip-flop 102 is cleared when the power source of the device is turned on, when the high level comparison output Se is obtained, the low level lighting prohibition signal Sf is output from the inverting output terminal Q thereof (F in the same figure).

As a result, when the runaway state of the microcomputer 52 is detected in this way, the level of the lighting prohibition signal Sf is inverted and the lighting state of the laser 31 is forcibly turned off. This causes the voltage signal to
Sd becomes zero.

In the above-described embodiment, the microcomputer 52 may be provided as a dedicated microcomputer for stabilizing the light amount, or a microcomputer that controls the apparatus body may be used.

If the microcomputer 52 itself has an A / D conversion function, the A / D conversion of the appropriate light amount value is performed.
The / D converter 51 can be omitted.

Similarly, when the microcomputer 52 itself has a rack function or the D / A converter 47 has a latch function, the rack circuit 53 can be omitted as a matter of course.

[Effects of the Invention] As described above, according to the present invention, since the forced lighting means 80 and the lighting prohibition means 100 are provided, as a result of the microcomputer 52 running away due to noise or the like, an excessive current flows to the laser 31. Even if it flows, this can be immediately detected and the drive state of the laser 31 can be forcibly prohibited.

As a result, it is possible to prevent damage to the laser 31 due to the runaway of the microcomputer 52.

Therefore, as described above, the present invention is extremely suitable when applied to a laser driving circuit used in a simple color copying machine, a laser printer, or the like.

[Brief description of drawings]

FIG. 1 is a system diagram of an essential part showing an example of a semiconductor laser drive circuit including a laser protection circuit according to the present invention, FIG. 2 is a system diagram showing an example of forced lighting means and input control means, and FIG. FIG. 4 is a connection diagram showing an example of a low-pass filter, FIG. 4 is a system diagram showing an example of lighting prohibition means, FIG. 5 is a waveform diagram used for explaining the operation at the time of microcomputer runaway, and FIG. 6 is suitable for application to the present invention. FIG. 7 is a configuration diagram of a main part showing an example of a simple simplified color copying machine, FIG. 7 is a configuration diagram showing an example of a laser beam scanning device used for explaining the present invention, and FIG. 8 is a system diagram of a semiconductor laser drive circuit. FIG. 9 is a characteristic diagram showing the relationship between the excitation current of the laser and the emitted light amount, FIGS. 10 and 11 are diagrams for explaining the operation of stabilizing adjustment of the light amount, and FIG. 12 is stabilization of the laser light amount. It is a control flowchart which shows an example of control operation. 10 ... Color copying machine 11 ... Drum as image forming body 30 ... Laser light scanning device 31 ... Semiconductor laser 40 ... Driving circuit 41 ... Photo sensor 42 ... Current / voltage conversion means 47 ... D / A Converter 51 ...... A / D converter 52 ...... Microcomputer 53 ...... Latch circuit 60 ...... Light intensity monitor circuit 80 ...... Forced lighting means 90 ...... Input control means 100 ...... Lighting prohibition means 55 ...... Low pass filter

Claims (4)

[Claims] 1. A semiconductor laser, a light amount monitor circuit for monitoring the emitted light amount of the semiconductor laser, A / D conversion means for converting an output voltage of the light amount monitor circuit into a digital amount, and the digital amount into an analog signal. D / A conversion means, a current generation means for supplying a predetermined drive current to the semiconductor laser according to the analog signal, and the D / A conversion output so that the A / D conversion output is within a certain range.
And a microcomputer for controlling the digital amount given to the A / A conversion means, and when it detects that the D / A conversion output abnormally greatly changes due to the runaway of the microcomputer, the semiconductor laser is kept constant. A semiconductor laser comprising: forced lighting means for lighting for a certain period of time; and lighting prohibition means for prohibiting lighting of the semiconductor laser when it is detected that the output voltage of the light quantity monitor circuit becomes a certain value or more. Protection circuit. 2. The forcible lighting means comprises edge detection means for detecting the rising edge of the D / A conversion output, and a pulse generation section for generating a forcible lighting signal for a fixed time from the edge detection signal. What is claimed is: Claim 1
A semiconductor laser protection circuit described in the paragraph. 3. The lighting prohibition means comprises a comparison circuit for comparing the output voltage of the light quantity monitor circuit with a reference voltage, and a pulse generation section for generating a predetermined pulse when the comparison output changes. The semiconductor laser protection circuit according to claim 1, wherein: 4. An input control means for supplying at least the forced lighting signal, a lighting prohibition signal, and a laser modulation signal corresponding to an image signal to the input stage of the current generation circuit. The semiconductor laser protection circuit according to any one of claims 1 to 3.