JP2012150338A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus that prevents deterioration of image quality due to delayed start-up of a laser source when the laser source is driven with high switching frequency.SOLUTION: When a light-emitting part 107 is turned on in response to a video signal Vo, a switching element 106 is turned on for operation, and a current control circuit 103 supplies current generated in accordance with voltage V1 output from a voltage source 101 as drive current Id to the light-emitting part 107. The current control circuit 103 then converges the drive current Id that is overshoot relative to a target current to be the target current based on a response control signal S8.

Description

  The present invention relates to an electrophotographic image forming apparatus.

  2. Description of the Related Art In recent years, a surface-emitting type semiconductor laser element is used as the laser light source for an image forming apparatus including an exposure device that exposes and scans a photosensitive drum with a laser light source in order to improve productivity or resolution. It is being considered. This surface-emitting type semiconductor laser element emits laser light from the surface of a wafer.

  The surface emitting semiconductor laser element can arrange light emitting points two-dimensionally with respect to the edge emitting semiconductor laser element that emits laser light from the end face of the wafer, and can easily cope with an increase in the number of beams. There is an advantage that you can. Due to this advantage, the surface emitting semiconductor laser element is regarded as promising as a laser light source of an exposure apparatus. On the other hand, the surface-emitting type semiconductor laser element has a rising timing of the amount of laser light emitted from the surface-emitting type semiconductor laser element and the timing at which the drive current is supplied, as compared with the edge-emitting type semiconductor laser element. It has a characteristic that the falling timing is delayed.

  In addition, since the switching frequency per pixel of the laser light source is proportional to the process speed and the square of the resolution, the switching frequency of the laser light source should be further increased in order to improve productivity or resolution. is required.

  Here, the rise and fall delays during switching of the laser light source affect the formation of the latent image of the pixel. For example, when the rise time is long, the latent image is thinned, and when the fall time is long, the latent image is thickened. If the thin and thick latent images are developed as they are, the image quality is deteriorated, for example, the image quality is deteriorated. The rise time and fall time vary greatly depending on the amount of laser light emitted by the laser light source, the ambient temperature of the laser light source, and the like.

  Therefore, in an electrophotographic image forming apparatus, when the switching frequency of a laser light source is increased in order to cope with higher resolution and higher speed, a fast response characteristic is required for the laser light source, and the rise time is increased. And it is necessary to shorten the fall time. For example, under the situation where the switching frequency of a conventional edge-emitting semiconductor laser device is several tens of MHz, a high frequency such as several hundred MHz is assumed as the switching frequency of the surface-emitting semiconductor laser device.

  Accordingly, the following driving method has been proposed as a driving method for shortening the rise time and the fall time of the surface emitting semiconductor laser element (see Patent Document 1). In this driving method, when the surface emitting semiconductor laser element is started up, the semiconductor laser element is driven by voltage driving, and is switched to current driving according to time to correct the rising time.

Japanese Patent No. 41233791

  However, in the driving of the surface emitting semiconductor laser element described above, the switching frequency of the laser element is set to a higher frequency than before in order to improve the resolution. Therefore, the driving method of the surface emitting semiconductor laser element requires switching between voltage driving and current driving at a very high speed on the order of n (nano) seconds. Therefore, in particular, in the PWM control for each pixel, it is very difficult to realize the driving method.

  Further, the rising and falling speeds of the light amount vary depending on the temperature of the laser element. That is, the rising speed of the light amount when the light is turned off for a short time after the lighting state continues for a long time and is turned on again is different from the rising speed of the light amount when the light is turned on after the lighting state continues for a long time. Further, when the time from turning on to turning off differs, the falling speed of the light amount changes.

  An object of the present invention is to provide an image forming apparatus capable of preventing deterioration in image quality due to a delay in rising of a laser light source when a laser light source is driven at a high switching frequency.

  In order to achieve the above object, the present invention provides an electrophotographic image forming apparatus, a laser light source for emitting laser light for exposure scanning of an image carrier, and corresponding switching in response to a video signal Drive means for supplying and interrupting a drive current to the laser light source to drive the laser light source at a frequency, and the drive means has a time constant of an operation in response to the video signal as the time constant of the laser light source. A time constant determined in accordance with output characteristics, and when the driving means turns on the laser light source in response to the video signal, the driving means overshoots a target current and then sets the time constant. An image forming apparatus is provided, wherein a current that converges to a target current is supplied to the laser light source as the drive current.

  According to the present invention, when a laser light source is driven at a high switching frequency, it is possible to prevent deterioration in image quality due to a delay in rising of the laser light source.

1 is a longitudinal sectional view illustrating a configuration of an image forming apparatus according to a first embodiment of the present invention. It is a schematic diagram which shows the principal part structure of the exposure units 26m-26k of FIG. FIG. 3 is a circuit diagram showing a configuration of a laser driver 209 in FIG. 2. (A) is a figure which shows each rise waveform of the drive current Id 'at the time of driving the light emission part 107 by the conventional constant current control, and the optical output of a light emission part. FIG. 4B is a diagram illustrating rising waveforms of the drive current Id and the light output of the light emitting unit when the light emitting unit 107 is driven by the drive circuit 209i of FIG. It is a flowchart which shows the procedure of the process which calculates the target current which the target current signal S4 shows, and the voltage which the voltage setting signal S5 sets. The video signal Vo, the drive current Id, the target current signal S4, the output voltage V1 of the voltage source 101, the applied voltage Vd to the light emitting unit 107, and response control when changing the voltage V1 output from the voltage source 101 by the voltage setting signal S5. It is a figure which shows each waveform of signal S8 and an optical output. FIG. 6 is a circuit diagram showing a configuration of a laser driver mounted on an image forming apparatus according to a second embodiment of the present invention. It is a circuit diagram which shows the structure of the laser driver mounted in the image forming apparatus which concerns on the 3rd Embodiment of this invention. It is a figure which respectively shows the waveform of the video signal Vo in the 3rd Embodiment of this invention, the waveform of the overshoot part of the current which the current control circuits 103 and 303 each output, the waveform of the drive current Id, and the waveform of the optical output. .

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(First embodiment)
FIG. 1 is a longitudinal sectional view showing a configuration of an image forming apparatus according to a first embodiment of the present invention. Here, an image forming apparatus that provides at least a color copy function will be described.

  As shown in FIG. 1, the image forming apparatus includes a reader 10 capable of reading a color image and a printer 20 capable of forming a color image. The reader 10 includes an automatic document feeder 11 that feeds documents one by one, and a scanner 12 that can read an image on a document fed by the automatic document feeder 11 in full color. The scanner 12 outputs image data (R, G, B color image data) read from a document to an image processing unit (not shown). The image processing unit performs predetermined image processing on the image data output from the scanner 12, and the image processed image data is converted into M (magenta), C (cyan), Y (yellow), and K (black). Are output to the printer 20.

  The printer 20 forms a color image or a monochrome image on a sheet based on image data of each color (M, C, Y, K) output from the image processing unit by electrophotography. Specifically, the printer 20 includes a plurality of exposure units 26m, 26c, 26y, and 26k and a plurality of image forming units 21m, 21c, 21y, and 21k.

  Each of the exposure units 26m to 26k modulates the laser beam based on the corresponding color (M, C, Y, K) image data, and the modulated laser beam is used to sensitize the corresponding image forming units 21m to 21k. The drum 22 is exposed and scanned.

  Each of the image forming units 21m to 21k includes a photosensitive drum (image carrier) 22, a charger 23, a developing device 24, and a cleaner 25. Since the configurations of the image forming units 21m to 21k are the same, only the components of the image forming unit 21m are denoted by reference numerals, and the symbols for the components of the other image forming units 21c to 21k are omitted.

  The surface of the photosensitive drum 22 is charged to a predetermined potential by the charger 23 and then exposed and scanned by the laser beams of the corresponding exposure units 26m, 26c, 26y, and 26. As a result, an electrostatic latent image of a color corresponding to the image data is formed on the surface of the photosensitive drum 22. The electrostatic latent image formed on the photosensitive drum 22 is developed into a toner image by the developing device 24, and the toner image is carried on the photosensitive drum 22. The cleaner 25 scrapes off and collects the toner remaining on the surface of the photosensitive drum 22 after the primary transfer.

  The toner images of the respective colors carried on the photosensitive drums 22 of the image forming units 21m to 21k are superimposed and transferred onto the intermediate transfer belt 29 by the corresponding primary transfer devices 28m to 28k (primary transfer). As a result, a full-color toner image is formed on the intermediate transfer belt 29.

  The toner image transferred to the intermediate transfer belt 29 is transferred from the paper feed cassette 30 or the hand tray 31 to the paper fed to the secondary transfer position T via the registration roller 32 (secondary transfer). The paper on which the toner image is transferred is guided to the fixing device 33, and the fixing device 33 fixes the toner image on the paper to the paper. The sheet on which the toner image is fixed is discharged onto the discharge tray 34.

  Next, each of the exposure units 26m to 26k will be described in detail with reference to FIG. FIG. 2 is a schematic diagram showing a main part configuration of the exposure units 26m to 26k in FIG. Here, since the configuration of each of the exposure units 26m to 26k is the same, the configuration of the exposure unit 26m will be described.

  As shown in FIG. 2, the exposure unit 26m includes a laser light source 200, a collimator lens 201, a polygon mirror 202, an f-θ lens 204, a beam detect sensor (hereinafter referred to as BD sensor) 205, a density sensor 206, and a laser driver 209. Have

  The laser light source 200 is composed of a surface emitting semiconductor laser element having a plurality of light emitting portions arranged at a predetermined interval in the sub-scanning direction. The laser light source 200 is driven at a high switching frequency such as several hundred MHz in a situation where the switching frequency of a conventional edge-emitting semiconductor laser element is several tens of MHz, for example.

  A plurality of laser beams emitted from the laser light source 200 (a plurality of laser beams emitted from the respective light emitting units) are converted into parallel laser beams via the collimator lens 201, respectively, and then rotated in a polygon mirror. 202 is incident. Here, the polygon mirror 202 is rotationally driven by the scanner motor 203 at a predetermined equiangular speed.

  Each laser beam incident on the polygon mirror 202 is reflected by the polygon mirror 202 and reaches the f-θ lens 204. Each laser beam passes through the f-θ lens 204 and is combined and scanned on the photosensitive drum 22 at a constant speed in the main scanning direction. By scanning (that is, scanning operation) of the plurality of laser beams, latent images of a plurality of main scanning direction lines are simultaneously formed on the surface of the photosensitive drum 22.

  The BD sensor 205 outputs a beam detect signal (hereinafter referred to as a BD signal) S1 to the controller 210 when detecting the laser beam reflected and input by the polygon mirror 202 during the forced lighting period in which the laser light source 200 is forcibly turned on. . The BD signal S1 is a signal that serves as a reference signal for image formation writing timing for each main scan.

  The density sensor (density detection means) 206 detects the density of the density detection pattern image formed on the surface of the photosensitive drum 22, and outputs a density signal S2 indicating the detection result (detected density) to the controller 210.

  The controller 210 includes a CPU, a ROM, a RAM, an input / output interface (not shown), and the like. The controller 210 controls the entire apparatus and executes various processes including processes to be described later.

  The controller 210 executes a write sequence based on the BD signal S1. As the writing sequence is executed, the video signal Vo is input to the laser driver 209.

  Further, the controller 210 performs a target light amount calculation process for calculating the target light amount based on the density signal S2 (density of the density detection pattern image) from the density sensor 206, and obtains the calculated target light amount. A target current calculation process for calculating the target current is performed. Then, the controller 210 outputs a target current signal S4 indicating the calculated target current to the laser driver 209.

  Further, the controller 210 performs a voltage calculation process for calculating a voltage to be set in the voltage source 101 based on the temperature signal S6 output from the temperature sensor (temperature detection means) 207. Then, the controller 210 outputs a voltage setting signal S5 indicating the calculated voltage to the laser driver 209. Here, the temperature sensor 207 is arranged to detect the ambient temperature of the laser light source 200, and the temperature signal S6 is a signal indicating the detection result (the detected ambient temperature of the laser light source 200).

  Further, the controller 210 generates a drive control signal S 7 for driving the scanner motor 203 and outputs it to the scanner motor 203.

  The laser driver 209 supplies a drive current Id to each light emitting unit in order to drive each light emitting unit of the laser light source 200 in response to the video signal Vo.

  Next, the configuration of the laser driver 209 will be described with reference to FIG. FIG. 3 is a circuit diagram showing a configuration of the laser driver 209 of FIG.

  As shown in FIG. 3, the laser driver 209 is provided with a drive circuit 209 i (i = 1 to n) for each light emitting unit 107 of the laser light source 200. Each drive circuit 209i supplies a drive current Id to the light emitting unit 107 in order to drive the corresponding light emitting unit 107 at a corresponding switching frequency (for example, a high frequency such as several hundred MHz) in response to the video signal Vo. And shut off. Each light emitting unit 107 of the laser light source 200 is provided so as to perform exposure scanning on the corresponding main scanning direction line. Here, only one drive circuit 209i for one light emitting unit 107 is shown.

  The drive circuit 209i includes a voltage source 101, a current detection circuit 102, a current control circuit 103, an error amplifier 104, a time constant circuit 105, a switching element 106, and an inverter 113.

  The voltage source 101 is a variable voltage source that outputs the voltage V1 set by the voltage setting signal S5. The voltage V1 is a voltage that generates a current that overshoots the target current, as will be described later.

  The current detection circuit 102 detects a current flowing in a current path from the voltage source 101 to the light emitting unit 107 via the switching element 106, and outputs a current detection signal S9 indicating the detected current.

  The current control circuit 103 controls the current generated according to the voltage V1 output from the voltage source 101 (current that has overshooted the target current) based on a response control signal S8 from the error amplifier 104 described later. Specifically, the current control circuit 103 first calculates the maximum current (current overshooted with respect to the target current) generated according to the voltage V1 output from the voltage source 101 based on the response control signal S8. The maximum current is an overshoot current with respect to the target current, and then the current control circuit 103 determines the drive current Id as a time constant based on the response control signal S8. The target current is converged with the time constant determined by the circuit 105.

  The error amplifier 104 detects the difference between the current detection signal S9 from the current detection circuit 102 and the target current signal S4. Then, a signal indicating the difference is output to the current control circuit 103 as a response control signal S8. Here, the difference is such that the current indicated by the current detection signal S9 gradually converges toward the target current indicated by the target current signal S4 at the time specified by the time constant determined by the time constant circuit 105. Change.

  The time constant circuit 105 includes a capacitor, and determines a time constant when the current control circuit 103 converges the drive current Id to the target current as a time constant of the operation of the drive circuit responding to the video signal Vo. Here, the time constant is determined so as to be the same as the rising delay time (delay amount) according to the output characteristics of the light emitting unit 107 (such as the rising characteristics according to the use environment). The capacitance of the capacitor is a value selected so as to obtain the time constant.

  Charge is accumulated in the capacitor while the switching element 106 is turned off. Then, the electric charge accumulated in the capacitor is discharged in response to the lighting start of the light emitting unit 107, that is, in response to the ON operation of the switching element 106, and the discharge is completed in a time defined by the time constant.

  Details of the action of the time constant determined by the time constant circuit 105 when starting up the light emitting unit 107 (each light emitting unit 107 of the laser light source 200) will be described later.

  The switching element 106 performs a switching operation (that is, an on / off operation) based on the output of the inverter 113 to which the video signal Vo is input. By this switching operation, the drive current Id is supplied to and cut off from the light emitting unit 107. The video signal Vo input to the inverter 113 is a video signal in the main scanning direction line corresponding to the light emitting unit 107.

  Next, features of the light emitting unit 107 (surface emitting semiconductor laser element) will be briefly described with reference to FIG. FIG. 4A is a diagram showing rising waveforms of the drive current Id ′ and the light output of the light emitting unit when the light emitting unit 107 is driven by the conventional constant current control. FIG. 4B is a diagram showing rising waveforms of the drive current Id and the light output of the light emitting unit when the light emitting unit 107 is driven by the drive circuit 209i of FIG.

  The light emitting unit 107 is required to raise the light output to the target light amount in a short time in response to the drive current Id. However, the light emitting unit 107 has a characteristic that the rise (rise until reaching the target light amount of light output) is delayed with respect to the rise of the drive current Id. Further, the amount of delay (delay time) varies greatly depending on the light amount of the light emitting unit 107, the ambient temperature (or the temperature of the light emitting unit 107), and the like.

  Conventionally, for example, as shown in FIG. 4A, the supply of the drive current Id 'is started with respect to the input of the video signal Vo. With the start of supply of the drive current Id ′, the light output of the light emitting unit 107 (laser beam waveform) immediately rises up to a certain amount of light, but the current control circuit has a period until it reaches the target amount of light thereafter. It rises gradually with a time constant. Here, the delay in the rise of the optical output with respect to the rise of the drive current Id ′ is affected by the temperature (the temperature of the light emitting unit 107 or its ambient temperature), and the delay amount is large at the low temperature (solid line), and the high temperature. At time, the amount of delay is small (dotted line).

  Such a delay in the rise of the laser light source affects the formation of the latent image of the pixel as described above. For example, when the rise time is long, the image quality is deteriorated such that the latent image is thinned.

  In the present embodiment, as shown in FIG. 4B, when the light emitting unit 107 is turned on in response to the video signal Vo, first, the maximum generated according to the voltage V1 output from the voltage source 101 is generated. Is supplied to the light emitting unit 107 as the drive current Id. This maximum current is a current that overshoots the target current. Thus, the light emitting unit 107 can be started up so that the light output reaches the target light amount in a shorter time (faster speed) than in the past. That is, it is possible to reduce the rising delay amount of the light emitting unit 107.

  Here, if the drive current Id that overshoots the target current continues to be supplied to the light emitting unit 107, the light output of the light emitting unit 107 exceeds the target light amount, and the latent image becomes thicker. Affects latent image formation. In order to avoid the thickening of the latent image, it is necessary to converge the drive current Id overshooting the target current to the target current in an appropriate time.

  Therefore, in the current control circuit 103, control is performed so that the drive current Id that overshoots the target current converges to the target current with the time constant (response control signal S8). As described above, this time constant is determined to be the same as the rise delay time according to the output characteristics of the light emitting unit 107. Thereby, the rising waveform of the light output of the light emitting unit 107 can be a stable waveform (substantially rectangular waveform) with a small delay amount.

  Specifically, in the drive circuit 209i, the voltage source 101 outputs the voltage V1 set by the voltage setting signal S5. Here, when an L (Low) level video signal Vo is input to the inverter 113, the inverter 113 outputs an H (High) level signal. The switching element 106 is turned off by an H level signal from the inverter 113.

  When the switching element 106 is off, the current detection circuit 102 does not detect current. As a result, the response control signal S8 output from the error amplifier 104 is a signal that causes the current control circuit 103 to output the maximum current corresponding to the voltage V1 of the voltage source 101 as the drive current Id.

  Next, when an H level video signal Vo is input to the inverter 113, an L level signal is output from the inverter 113, and the switching element 106 is turned on. Thereby, the current control circuit 103 supplies the maximum current corresponding to the voltage V1 output from the voltage source 101 to the light emitting unit 107 as the drive current Id. With such an operation, the light emitting unit 107 can be started up so that the light output immediately reaches the target light amount.

  At this time, the current detection circuit 102 detects a current supplied from the voltage source 101 through the switching element 106 to the light emitting unit 107 and outputs a current detection signal S9 to the error amplifier 104. The current detected by the current detection circuit 102 is an overshoot current with respect to the target current, and the capacitor of the time constant circuit 105 is discharged, so that the error amplifier 104 receives an input from the output via the time constant circuit 105. Negative feedback is applied. Then, with the passage of time defined by the time constant determined by the time constant circuit 105, the difference between the current detected by the current detection circuit 102 and the target current (value indicated by the response control signal S8) gradually decreases. Therefore, based on the response control signal S8, the current control circuit 103 converges the drive current Id overshooting the target current toward the target current with the time constant.

  The voltage set by the voltage setting signal S5, that is, the voltage V1 output from the voltage source 101 is determined based on the ambient temperature of the light emitting unit 107 (laser light source 200) (temperature signal S6 of the temperature sensor 207) and the target light amount. Voltage. That is, the voltage setting signal S5 makes it possible to control the maximum current supplied from the current control circuit 103 to the light emitting unit 107 when the light emitting unit 107 starts up. Therefore, it is possible to obtain a light output with a stable waveform (substantially rectangular waveform) with a small delay amount when the light emitting unit 107 starts up without being affected by the ambient temperature of the light emitting unit 107 (laser light source 200). .

  Here, for example, the case where the ambient temperature of the laser light source 200 (the temperature signal S6 of the temperature sensor 207) is the first temperature (low temperature) and the second temperature higher than the first temperature (high temperature). Suppose.

  In the case of the first temperature, the voltage V1 output from the voltage source 101 is increased by the voltage setting signal S5 as compared to the case of the second temperature. Thereby, the maximum current supplied by the current control circuit 103 in the case of the first temperature is larger than that in the case of the second temperature. That is, as shown in FIG. 4B, the high voltage V1 is set so that the rising waveform of the drive current Id is higher (the amount of overshoot with respect to the target current is larger) at low temperatures than at high temperatures. . As a result, the delay amount at low temperature (dotted line in FIG. 4A) larger than the delay amount at high temperature (solid line in FIG. 4A) at the rise of the optical output can be reduced. Therefore, it is possible to obtain a light output having a stable waveform with a very small delay amount at the time of rising at a low temperature.

  On the other hand, in the case of the second temperature, the voltage V1 output from the voltage source 101 by the voltage setting signal S5 is made smaller than in the case of the first temperature. That is, as shown in FIG. 4B, the low voltage V1 is set so that the rising waveform of the drive current Id is lower (the amount of overshoot with respect to the target current is smaller) at high temperatures than at low temperatures. .

  Next, processing for calculating the target current indicated by the target current signal S4 and the voltage set by the voltage setting signal S5 will be described with reference to FIG. FIG. 5 is a flowchart showing a processing procedure for calculating the target current indicated by the target current signal S4 and the voltage set by the voltage setting signal S5. The procedure shown in the flowchart of FIG. 5 is executed by the controller 210 at a predetermined timing such as when the power is turned on.

  As shown in FIG. 5, the controller 210 first forms a predetermined density detection pattern image (toner image) on the photosensitive drum 22 (step S500). The controller 210 detects the density of the density detection pattern image formed on the photosensitive drum 22 based on the density signal S1 from the density sensor 206 (step S501).

  Next, the controller 210 calculates a target light amount based on the detected density (step S502). Then, the controller 210 calculates a target current and a target voltage for obtaining the calculated target light amount (step S503).

  Next, the controller 210 detects the ambient temperature of the laser light source 200 (light emitting unit 107) based on the temperature signal S6 from the temperature sensor 207 (step S504). Then, the controller 210 refers to the table shown in Table 1 and calculates a correction coefficient for correcting the target voltage based on the calculated target light amount (light output) and the detected ambient temperature (step S505). ).

  Next, the controller 210 corrects the target voltage using the calculated correction coefficient, calculates the corrected voltage as the voltage V1 output from the voltage source 101 (step S506), and ends the process.

  When the target current and the voltage V1 output from the voltage source 101 are calculated in this way, the target current signal S4 indicating the target current is supplied to the current control circuit 103 and the voltage setting signal S5 for setting the voltage V1 is obtained. Each is output to the voltage source 101.

  Here, the table shown in Table 1 shows a correction coefficient α (%) for correcting the target voltage. When the target voltage is VA, the voltage V1 output from the voltage source 101 is (1 + α / 100) VA. For example, when the light output is 0.2 (mW), the correction coefficient α is 16 (%), so the voltage V1 output from the voltage source 101 is 1.16 VA.

  As the light emitting unit 107 is repeatedly turned on and off, the ambient temperature of the light emitting unit 107 varies. Along with this, the rising delay amount of the light emitting unit 107 changes. Therefore, in the present embodiment, the voltage V1 output from the voltage source 101 is changed by the voltage setting signal S5 in order to cope with the change in the rising delay amount of the light emitting unit 107 due to the change in the ambient temperature of the light emitting unit 107.

  A case where the voltage V1 output from the voltage source 101 is changed by the voltage setting signal S5 will be described with reference to FIG. FIG. 6 shows a video signal Vo, a drive current Id, a target current signal S4, an output voltage V1 of the voltage source 101, and an applied voltage Vd to the light emitting unit 107 when changing the voltage V1 output from the voltage source 101 by the voltage setting signal S5. FIG. 6 is a diagram illustrating respective waveforms of a response control signal S8 and an optical output.

  As shown in FIG. 6, in the initial state in which the switching element 106 is turned off, the drive current Id does not flow through the light emitting unit 107, and the current detection circuit 102 does not detect a current. At this time, the voltage source 101 outputs the voltage V1 set by the voltage setting signal S5, and the current control circuit 103 is in a state of supplying the maximum current by the response control signal S8 from the error amplifier 104. .

  Here, when the H-level video signal Vo is input to the inverter 113 at the timing T1, the switching element 106 is turned on. From the current control circuit 103, the maximum current corresponding to the voltage V1 output from the voltage source 101 is supplied to the light emitting unit 107 as the drive current Id. The current (current detection signal S9) detected by the current detection circuit 102 is larger than the target current indicated by the target current signal S4.

  The error amplifier 104 outputs a response control signal S8 indicating the difference between the current indicated by the current detection signal S9 and the target current indicated by the target current signal S4. The difference indicated by the response control signal S8 changes so as to gradually decrease at a time defined by the time constant determined by the time constant circuit 105. The current control circuit 103 converges the drive current Id to the target current for a time defined by the time constant determined by the time constant circuit 105 based on the response control signal S8 from the error amplifier 104. In accordance with this, the voltage Vd applied to the light emitting unit 107 gradually decreases from the voltage V1.

  When the video signal V0 becomes L level at the timing T2, the switching element 106 is turned off. Thereby, the current detected by the current detection circuit 102 becomes smaller than the target current (target current signal S4). Therefore, the difference indicated by the response control signal S8 of the error amplifier 104 gradually increases, and the current control circuit 103 enters a current control state in which the drive current Id is increased.

  When the video signal Vo having a duty lower than that of the video signal Vo in the period between the timings T1 and T2 is input to the inverter 113 in the period between the timings T3 and T4, the switching element 106 based on the video signal Vo Operates on and off. A period between the timings T2 and T3 is a period in which the time constant circuit 105 is sufficiently saturated, and the voltage V1 is output from the voltage source 101 as in the timing T1.

  When the switching element 106 is turned on, the difference indicated by the response control signal S8 gradually decreases. On the other hand, when the switching element 106 is turned off, the difference indicated by the response control signal S8 gradually increases. As described above, the difference indicated by the response control signal S8 is repeatedly increased and decreased in accordance with the ON / OFF operation of the switching element 106.

  In addition, the temperature of the light emitting unit 107 (laser light source 200) or the ambient temperature thereof changes so as to repeatedly increase and decrease in accordance with repetition of turning on and off of the light emitting unit 107. Thereby, the rising delay amount of the light emitting unit 107 varies.

  Here, since the lighting time of the light emitting unit 107 is shorter than the extinguishing time, the change in the temperature of the light emitting unit 107 or its ambient temperature is small, and the variation in the rising delay amount of the light emitting unit 107 is small. Therefore, it is not necessary to change the voltage V1 output from the voltage source 101 by the voltage setting signal S5, and the voltage V1 is held as it is.

  When the video signal Vo becomes L level at timing T4, the switching element 106 is turned off. As a result, the current control circuit 103 enters a current control state in which the drive current Id is increased by the response control signal S8 from the error amplifier 104.

  When the video signal Vo having a high duty is input as the video signal Vo in the period between the timings T5 and T6, the switching element 106 is turned on and off based on the video signal Vo. A period between timings T4 and T5 is a period in which the time constant circuit 105 is sufficiently saturated, and the voltage V1 is output from the voltage source 101 similarly to the timing T1.

  Here, in the period between the timings T5 and T6, since the lighting time of the light emitting unit 107 is longer than the extinguishing time, the difference indicated by the response control signal S8 is a little in accordance with the on / off operation of the switching element 106. Decrease gradually while repeating increase and decrease. As a result, the rising waveform (overshoot) of the drive current Id is lower than the next rising waveform, and the rising delay amount of the light emitting unit 107 cannot be reduced. Further, the temperature of the light emitting unit 107 or its ambient temperature rises, and the rising delay amount of the light emitting unit 107 varies in a decreasing direction.

  Therefore, since the delay amount of the rise of the light emitting unit 107 at a high temperature becomes small, even if the rise waveform (overshoot) of the drive current Id becomes low, it is corrected so as to become small. That is, a stable waveform with a small delay amount can be obtained as the output waveform of the light emitting unit 107 without changing the voltage output from the voltage source 101 by the voltage setting signal S5.

  At timing T7, for example, it is assumed that the rise delay amount of the light emitting unit 107 is increased due to a decrease in the ambient temperature (temperature signal S6) of the light emitting unit 107 detected by the temperature sensor 207, for example. This is determined by the controller 210 based on the temperature signal S6. In this case, the controller 210 increases the voltage V1 set to the voltage source 101, and outputs a voltage setting signal S5 for setting the increased voltage V1 to the voltage source 101. As a result, the voltage source 101 outputs a voltage V1 higher than the voltage V1 before the timing T7.

  When an H level video signal Vo is input in a period between timings T8 and T9, a drive current Id is supplied from the current control circuit 103 to the light emitting unit 107. The rising waveform (overshoot) of the drive current Id is higher than the rising waveform (overshoot) of the drive current Id at the timing T1. Thereby, the delay amount of the rise of the light emitting unit 107 that has become larger due to the decrease in the ambient temperature of the light emitting unit 107 is reduced, and it is possible to obtain a light output that immediately reaches the target light amount when the light emitting unit 107 rises. .

  Here, when the voltage V1 set to the voltage source 101 is changed, the table shown in Table 1 is referred to, and the correction coefficient is determined based on the current light output and the ambient temperature. Then, the voltage V1 set to the voltage source 101 is set by adding a voltage corresponding to the correction coefficient to the currently set voltage V1.

  As described above, according to the present embodiment, when the light emitting unit 107 (laser light source 200) is driven at a high switching frequency, the rising delay amount of the light emitting unit 107 can be reduced. As a result, it is possible to prevent deterioration in image quality due to a delay in the rise of the light emitting unit 107 (laser light source 200).

(Second embodiment)
Next, a second embodiment of the present invention will be described with reference to FIG. FIG. 7 is a circuit diagram showing a configuration of a laser driver mounted on the image forming apparatus according to the second embodiment of the present invention. Here, the same elements or members as those in the first embodiment are denoted by the same reference numerals, and description of the same elements or members is omitted or simplified.

  As shown in FIG. 7, the laser driver 209 of this embodiment is provided with a drive circuit 209 i (i = 1 to n) for each light emitting unit 107 of the laser light source 200. Each drive circuit 209 i supplies a drive current Id for driving the corresponding light emitting unit 107 to the corresponding light emitting unit 107.

  The drive circuit 209i includes a voltage source 101, a current detection circuit 102, a current control circuit 103, an error amplifier 104, a time constant circuit 105, a switching element 106, and an inverter 113. The drive circuit 209i is provided with a light amount control circuit 114 (target current determining means).

  The light amount control circuit 114 includes a photodiode (light amount detection means; hereinafter referred to as FD) 108 that detects the light amount of the laser light emitted from the light emitting unit 107 and outputs a signal indicating the detected light amount. The signal output from the FD 108 is amplified by the amplifier 109 and then input to the error amplifier 111. Further, the target light amount signal S10 output from the controller 210 is input to the error amplifier 111. Here, the target light amount signal S10 is a signal indicating the target light amount. The error amplifier 111 outputs a difference signal indicating a difference between the light amount indicated by the signal input from the FD 108 via the amplifier 109 and the target light amount indicated by the target light amount signal S10.

  The differential signal output from the error amplifier 111 is input to the sample hold circuit 112. The sample hold circuit 112 holds and outputs the differential signal output from the error amplifier 111 based on the sample / hold signal S11 from the controller 210. The difference signal output from the sample hold circuit 112 is input to the error amplifier 104.

  At the time of APC (automatic light quantity control), the controller 210 outputs a sample / hold signal S11 and a target light quantity signal S10 indicating the target light quantity so that the light quantity of the laser light emitted from the light emitting unit 107 matches the target light quantity. The current Id is controlled. When the light amount of the laser light emitted from the light emitting unit 107 matches the target light amount, the drive current Id at that time is determined as the target current and held in the sample / hold circuit 112.

  At the time of image formation, the controller 210 outputs a sample / hold signal S11 to the sample / hold circuit 112. Then, the target current obtained by APC (current when the light amount of the laser light emitted from the light emitting unit 107 matches the target light amount) held in the sample / hold circuit 112 is an error as the target current signal S4. It is output to the amplifier 104.

  Here, when the light emitting unit 107 is turned on in response to the video signal Vo, the drive current Id overshooting the target current is supplied to the light emitting unit 107 as in the first embodiment. The drive current Id that overshoots the target current converges to the target current with the time constant determined by the time constant circuit 105.

  In this way, as in the first embodiment, the rising delay amount of the light emitting unit 107 (laser light source 200) can be reduced.

(Third embodiment)
Next, a third embodiment of the present invention will be described with reference to FIGS. FIG. 8 is a circuit diagram showing a configuration of a laser driver mounted on an image forming apparatus according to the third embodiment of the present invention. FIG. 9 is a diagram showing the waveform of the video signal Vo, the waveform of the current output from each of the current control circuits 103 and 303, the waveform of the drive current Id, and the waveform of the optical output in the third embodiment of the present invention. Here, the same elements or members as those in the first embodiment are denoted by the same reference numerals, and description of the same elements or members is omitted or simplified.

  As shown in FIG. 8, the laser driver 209 of this embodiment is provided with a drive circuit 209 i (i = 1 to n) for each light emitting unit 107 of the laser light source 200. Each drive circuit 209 i supplies a drive current Id for driving the corresponding light emitting unit 107 to the corresponding light emitting unit 107.

  The drive circuit 209i includes two drive current supply systems, an inverter 113, and a switching element 106. One drive current supply system includes a voltage source 101 (first voltage source), a current detection circuit 102, a current control circuit 103 (first current control circuit), an error amplifier 104, and a time constant circuit 105 (first time). Constant circuit). Here, the voltage source 101 outputs the voltage V1 (first voltage) set by the voltage setting signal S15 from the controller 210. The time constant circuit 105 determines a time constant A when the current (first current) output from the current control circuit 103 is converged.

  The voltage source 101, the current detection circuit 102, the current control circuit 103, the error amplifier 104, and the time constant circuit 105 that constitute one drive current supply system perform the same operations as in the first embodiment. Here, the description is omitted.

  Similarly, the other driving current supply system includes a voltage source 301 (second voltage source), a current detection circuit 302, a current control circuit 303 (second current control circuit), an error amplifier 304, and a time constant circuit 305 (first constant). 2 time constant circuit).

  In the other drive current supply system, the voltage source 301 outputs the voltage V2 (second voltage) set by the voltage setting signal S35 from the controller 210. The current detection circuit 302 detects a current flowing in a current path from the voltage source 301 to the light emitting unit 107 via the switching element 106, and outputs a current detection signal S39 indicating the detected current.

  The time constant circuit 305 includes a capacitor, and determines a time constant B (first time constant) when the current (second current) output from the current control circuit 303 is converged. Here, the time constant B determined by the time constant circuit 305 is made shorter than the time constant A determined by the time constant circuit 105. The time constants A and B are determined so that the rising waveform of the light output of the light emitting unit 107 becomes a desired rectangular waveform when the currents output from the current control circuits 103 and 303 are superimposed. However, the relationship between the time constants A and B is not limited to the relationship of A> B, and may be the reverse relationship, for example.

  The error amplifier 304 detects the difference between the current detection signal S39 from the current detection circuit 302 and the target current signal S34. A signal indicating the difference between the current detection signal S39 and the target current signal S4 is output to the current control circuit 103 as a response control signal S38.

  Based on the response control signal S38 from the error amplifier 304, the current control circuit 303 performs control so that the current overshooted with respect to the target current converges to the target current with the time constant B determined by the time constant circuit 305. To do.

  In this embodiment, when the switching element 106 is turned on in response to the input of the video signal Vo, each of the current control circuits 103 and 303 converges with time constants A and B after overshooting with respect to the target current. Output current. Then, the current output from each of the current control circuits 103 and 303 is superimposed and supplied to the light emitting unit 107 as the drive current Id.

  Here, the height of the rise (overshoot) of the current output from each of the current control circuits 103 and 303 is determined by the voltages V1 and V2 output from the voltage sources 101 and 301.

  In the present embodiment, the voltage V1 output from the voltage source 101 is changed according to the ambient temperature of the laser light source 200, but the voltage V2 output from the voltage source 301 is not changed. Alternatively, the voltage V2 output from the voltage source 301 may be changed according to the ambient temperature of the laser light source 200, or the voltages V1 and V2 output from the voltage sources 101 and 301 may be changed. May be.

  Here, as shown in FIG. 9, the overshoot w1 of the current output from the current control circuit 103 has a time constant A and has a waveform that converges to the target current. The overshoot w2 of the current output from the current control circuit 303 has a time constant B and has a waveform that converges to the target current.

  Overshoot w1 and overshoot w2 are superimposed, and the superimposed current becomes a drive current Id that converges on the target current with a time constant obtained by combining time constants A and B after overshooting the target current. . Thereby, the delay amount of the rise of the light output can be reduced, and a stable light output with no delay can be obtained.

  In addition, when the voltage V1 output from the voltage source 101 is changed to a low voltage as the ambient temperature of the laser light source 200 changes, the overshoot w1 of the current output from the current control circuit 103 is indicated by a dotted line in the figure. It becomes a waveform like this.

  In the first to third embodiments, the voltage V1 output from the voltage source 101 is calculated based on the detection result of the temperature sensor 207 that detects the ambient temperature of the laser light source 200. Instead, a temperature sensor that detects the internal temperature of the laser light source 200 may be provided, and the voltage V1 output from the voltage source 101 may be calculated based on the detection result of the temperature sensor.

  As described above, according to the present embodiment, as in the first embodiment, the rising delay amount of the light emitting unit 107 (laser light source 200) can be reduced.

22 Photosensitive drums 101, 301 Voltage source 103, 303 Current control circuit 105, 305 Time constant circuit 106 Switching element 107 Light emitting unit 108 Photodiode 114 Light amount control circuit 200 Laser light source 206 Concentration sensor 207 Temperature sensor 210 Controller unit

Claims (8)

An electrophotographic image forming apparatus,
A laser light source for emitting laser light for exposure scanning of the image carrier;
Drive means for supplying and interrupting a drive current to the laser light source to drive the laser light source at a corresponding switching frequency in response to a video signal;
The driving means has a time constant determined according to the output characteristics of the laser light source as a time constant of an operation in response to the video signal,
When the laser light source is turned on in response to the video signal, the driving means uses, as the driving current, a current that converges on the target current with the time constant after overshooting the target current. An image forming apparatus, characterized by being supplied to a light source.   The image forming apparatus according to claim 1, wherein the driving unit includes a time constant circuit for determining the time constant.   The driving means outputs a voltage source that generates a voltage that generates an overshoot current with respect to the target current, and an overshoot current with respect to the target current generated according to the voltage output from the voltage source. 3. The image forming apparatus according to claim 1, further comprising a current control unit configured to converge a current overshooted with respect to the target current to the target current with the time constant after the output. Temperature detecting means for detecting either the internal temperature of the laser light source or the ambient temperature of the laser light source;
Voltage calculating means for calculating a voltage output from the voltage source based on a detection result of the temperature detecting means;
The image forming apparatus according to claim 3, wherein the voltage source outputs the calculated voltage. Density detecting means for detecting the density of an image formed on the image carrier by exposure scanning with laser light emitted from the laser light source;
Target light quantity calculation means for calculating a target light quantity based on the detection result of the density detection means;
The image according to any one of claims 1 to 4, further comprising target current calculation means for calculating a current for obtaining a target light quantity calculated by the target light quantity calculation means as the target current. Forming equipment. A light amount detecting means for detecting a light amount of laser light emitted from the laser light source;
A target light amount calculating means for calculating a target light amount of laser light emitted from the laser light source;
Target current determining means for determining, as the target current, a current when the light quantity of the laser light detected by the light quantity detecting means matches the target light quantity of the laser light calculated by the target light quantity calculating means. The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus. The driving means includes a first time constant circuit for determining a first time constant and a second time constant circuit for determining a second time constant;
When the laser light source is turned on in response to the video signal, the driving means includes a first current that converges on the target current with the first time constant after overshooting the target current. Superimposing a second current that converges on the target current with the second time constant after overshooting the target current, and supplying the superimposed current to the laser light source as the drive current. The image forming apparatus according to claim 1, wherein: The driving means includes a first voltage source that outputs a first voltage that generates a current that overshoots the target current, and a second voltage that generates a current that overshoots the target current. After outputting the second voltage source to be output and the current generated according to the first voltage output from the first voltage source, the current is converged to the target current with the first time constant. A first current control means and a second current constant generated after outputting the current generated according to the second voltage output from the second voltage source are converged to the target current with the second time constant. Two current control means,
The current output from the first current control means is the first current, and the current output from the second current control means is the second current. 8. The image forming apparatus according to 7.

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