JPH059950B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a driving device for controlling switching of a semiconductor laser.

Semiconductor lasers (hereinafter simply referred to as lasers) are used in laser printers, communications, and other applications. The laser printer will be explained with reference to the block diagram of FIG. 1. Reference numeral 1 denotes a laser driving device that outputs a signal for controlling on/off of a laser 2, 3 a collimator lens, and 4 a beam expander. Reference numeral 5 denotes a polygon mirror having a rotating polygon mirror attached to a polygon motor shaft, which scans the laser beam that has passed through the collimator lens 3 and the beam expander 4 at high speed. Reference numeral 6 denotes an fθ lens that projects the laser beam scanned by the polygon mirror 5 onto the rotating photosensitive drum 7. Reference numeral 8 denotes a light receiving element for generating a synchronization signal, which is provided near the end of the photoreceptor drum 7 and receives the laser beam to synchronize the rotation of the photoreceptor drum 7 with the scanning of the laser beam.
Further, 9 is a light output detection element, which is built into the laser 2 or installed near the laser 2, receives the laser beam of the laser 2, returns the signal to the laser driving device 1, and outputs the laser 2. We aim to stabilize the situation.

The operation of the laser 2 in the laser printer of FIG. 1 will be explained with reference to the waveform diagram of FIG. 2. When the laser beam starts scanning by rotating the polygon mirror 5 in the direction of the arrow, the laser beam enters the light receiving element 8 and a synchronizing signal is generated from the light receiving element 8. This synchronizing signal causes a control circuit (not shown) to be activated. The time from when the laser enable signal is turned off until the laser beam reaches the printing position on the photosensitive drum 7 is counted. When exposing the character portion on the photoreceptor (negative-positive), the laser 2 is also turned off during the period when the laser enable signal is turned off. When the time comes for the laser beam to reach the printing position, the laser enable signal turns on and the laser 2 starts operating. While the laser beam scans the image printing area, the laser driving device 1 operates the laser according to the image data. 2 on/off control.
When the scanning of the image printing area is completed, the laser enable signal is once turned off and then turned on again at a preset time, and the laser 2 is also turned on. During this period when the laser 2 is on, the laser beam scan returns to the starting position (the left end in the figure), the photodetector 8 generates a synchronization signal again, and the above printing operation is executed on the next line. . in this way,
The operation timing of the laser 2 is synchronized every time the laser beam scans one line on the photosensitive drum 7. In the case of positive-positive, the laser on-off is reversed.

Also, during the timing A in Figure 2, the laser 2
The optical output is detected by the optical output detection element 9, and the signal is fed back to the laser driving device 1 to stabilize the laser emission output.

As shown in FIG. 4, a conventional laser driving device 1 used in this type of laser printer, etc. is a differential amplifier including transistors Q ₁ , Q ₂ , etc.
A laser 2 is connected to the collector side of the transistor Q ₂ , and the laser 2 is controlled on/off by a laser on/off signal input to the base of the transistor Q ₁ . Amplify the optical output signal
A device is known that is configured to be connected to the base of transistor _Q3 via AMP to control the drive current.

In such a conventional circuit, a transistor is used to control the on/off of the laser 2 at high speed.
It is desirable to operate Q ₁ and Q ₂ in the active region,
Normally, a minute current flows through the laser 2, and the laser 2 always emits weak light (LED light emission). Therefore, when this circuit is applied to a laser printer, image fogging occurs, which is a practical problem.

For this reason, it is conceivable to use a high-speed switching element such as a high-speed transistor or a field-effect transistor (hereinafter referred to as FET) in place of the differential amplifier type switching circuit described above. The circuit is schematically shown in FIG. 3, and a new problem arises. That is, the assumed circuit is, as shown in FIG. 3, a circuit including a current control transistor 10 and a fixed resistor R _D in parallel between a series circuit of a laser 2 and a high-speed switching transistor Q. A laser on/off control circuit 11 is connected to the switching transistor Q, and a current control circuit 12 is connected to the current control transistor 10.

In this laser drive device, when the switching transistor Q is turned on, a current flows through the laser 2.
iQ flows and emits light. The current iQ is the current iF flowing through the current control transistor 10 and the fixed resistance R _D
The light emitting output of the laser 2 is maintained stably by controlling the current _iF by the current control circuit 12.

However, in such a drive device, when the switching transistor Q is turned on and off, the time required for fractional carriers to be injected into the base region of the current control transistor 10 or the time required for fractional carriers to escape from the base region is limited. Therefore, the load resistance RF also affects the on/off of laser 2.
This has the disadvantage of poor off-state characteristics.

An object of the present invention is to improve the responsiveness of laser light emission output adjustment in such a laser drive device without reducing the laser switching speed by the switching element.

A further object of the present invention is to further improve the accuracy of adjusting the current flowing through the laser.

In order to achieve the above object, in the present invention, as an element for adjusting the current flowing through the laser,
A FET is used, and a fixed resistor is provided in parallel to the FET in order to make the current adjustment more accurate.

Hereinafter, the present invention will be explained in detail using one example.

FIG. 5 is a circuit diagram showing an embodiment of the present invention, in which the parallel circuit provided between the laser 2 and the switching FET Q' is different from the conventional example shown in FIG.
This is a parallel circuit of a series circuit including 20 and a fixed resistor R _D. Switching FETQ′ has a laser-on
The off control circuit 11 is connected and the current control FET
A current control circuit 12 is connected to 20.

For example, J107 (manufactured by Siliconix) can be used as the FET 20 for current control.

In this embodiment, the current control circuit 12 includes a sample and hold circuit 21 that samples and holds the amplified output signal of the optical output detection element 9, and the magnitude of the output signal of this sample and hold circuit 21.
A switch 22 such as an analog switch that selects either this output signal or a reference voltage, and a switch 22 for current control based on the output signal of this switch 22.
A circuit for controlling the FET 20 is provided.

In this example, the FET 20 and the resistors R _F and R _D are:
It acts as a current limiting resistor for the laser 2. If FET20 is connected as shown in Figure 5, this FET2
0 can be treated as a pure resistance element. FET
If the resistance that appears when the 20 is turned on is r _f , then the combined resistance R of this parallel circuit is expressed as R = R _D · (R _F + r _f )/R _D + R _F + r _f ...(1), and the laser The current i _Q flowing through FET 2 is: i _Q = V _CC −V _F −V _Q /R = V _CC −V _F −V _Q /R _D・( _RF + r _f )/R _D + R _F + r _f It can be seen that i _Q can be controlled by controlling the resistance r _f of . Here, V _CC
is the power supply voltage, V _F is the forward voltage of laser 2, and V _Q is
It is the saturation voltage of FETQ′.

The following equation (2) is obtained by differentiating equation (1) for calculating the composite resistance R described above with respect to r _f .

R _D ² /(R _D +R _F +r _f ) ² ...(2) In the above equation (2), R _D , R _F , and r _f are all positive numbers, so R _D ² <(R _D +R _F +r _f ) ² , and the above
Equation (2) becomes smaller than 1.

On the other hand, the combined resistance when the fixed resistance R _D is not provided is expressed by the following equation (3).

R _F +r _f ...(3) When this equation (3) is differentiated with respect to r _f , the differential value becomes 1. In this way, the amount of current adjustment corresponding to the change in r _f is smaller when the fixed resistor R _D is provided than when the fixed resistor R _D is not provided. Therefore, it is possible to more finely adjust the current flowing through the laser 2, and more accurate current adjustment can be achieved.

Since the relationship between the gate voltage of the FET 20 and the current i _F is expressed, for example, as shown in FIG. 6, the reference voltage V _r2 may be set so that i _F is at the center of the controlled range.

In the embodiment shown in FIG. 5 set in this way, the laser on/off control circuit 11 turns on the switching FET Q' from the state where the laser 2 is off. At this time, since the signal from the optical output detection element 9 is zero or very small, the output signal of the sample-and-hold circuit 21 is smaller than the reference voltage _Vr3 . Connected to V _r1 side. the result
A current corresponding to the reference voltage V _r1 flows through the FET 20, and the laser 2 emits light at a preset current value. Next, the light emission output of the laser 2 reaches a predetermined value, and the output signal of the sample hold circuit 21 becomes the reference voltage.
When V _r3 is reached, the switch 22 is connected to the sample and hold circuit 21 side. When the light emission output of the laser 2 increases in this state, the absolute value of the gate voltage (negative) of the FET 20 increases via the amplifiers 24 and 25, increasing the resistance r _f of the FET 20, and the laser 2
The light emitting output of the laser 2 is decreased by decreasing the current _iQ . Conversely, when the light emission output of the laser 2 decreases, the light emission output of the laser 2 is increased by acting in the opposite manner to the above. Further, when the light emission output of the laser 2 decreases below a predetermined value, the switch 22 is switched to the reference voltage V _r1 side, and the FET 20 is controlled so that the current i _Q becomes the set value.

When the switching FETQ' is repeatedly turned on and off as in the printing operation of a laser printer, the switch 22 remains connected to the sample-and-hold circuit 21 by the sample-and-hold signal.

When the switching FETQ' was turned on and off by the laser on/off control circuit 11, the laser 2
The light emission intensity of is determined by FET as a current adjustment element.
20, the rise and fall are good in low-frequency switching, as shown by the solid line in Figure 7a, and the contrast or resolution is good in high-frequency switching, as shown by the solid line in Figure 7b. Summer. In FIGS. 7a and 7b, the dotted lines indicate the case where a bipolar transistor is used instead of the FET 20 as the current adjusting element.

In this embodiment, the current iQ of the laser 2 is kept constant by switching the switch 22 until the light emission output of the laser 2 reaches a constant _value , so when the power is turned on or If sampling of the voltage to the sample-and-hold circuit 21 is not performed due to a failure or the like, or if the voltage drops extremely, it is possible to prevent an accident in which an overcurrent flows through the FET 20 and damages the laser 2.

In addition, in the embodiment shown in FIG.
FETQ' may be the switching transistor Q shown in FIG. Also, FET2
The positions of the parallel circuit containing 0 and the switching FET Q' (or the switching transistor Q) may be interchanged.

As is clear from the above description, according to the present invention, a field effect transistor is used as an element for adjusting the current flowing through the laser, so that the responsiveness of the laser emission output adjustment to the laser switching operation by the switching element is improved. It is possible to provide a laser drive device that can quickly switch the laser emission output and has excellent switching characteristics of the laser emission output. Additionally, a fixed resistor was installed in parallel to the field effect transistor that adjusts the current flowing through the laser.
More accurate current adjustment becomes possible.

[Brief explanation of drawings]

FIG. 1 is a block circuit diagram showing a laser printer as an example to which the present invention is applied, and FIG.
FIG. 3 is a block circuit diagram for explaining disadvantages in the laser drive device; FIG. 4 is a circuit diagram showing an example of a conventional laser drive device; Figure 5 is a block circuit diagram showing one embodiment of the present invention;
FIG. 6 is a diagram showing the gate voltage versus load current characteristics of the FET, and FIGS. 7a and 7b are diagrams showing the laser emission output intensity. 1... Laser drive device, 2... Laser, 9...
Optical output detection element, 10... Current control transistor, 11... Laser on/off control circuit, 12...
...Current control circuit, 20...FET for current control, 2
1... Sample hold circuit, 22... Switch, R _D ... Fixed resistance, Q... Switching transistor, Q'... Switching FET.

Claims (1)

[Scope of Claims] 1. A switching element connected in series with a semiconductor laser; a parallel circuit connected in series with the semiconductor laser and including a field effect transistor and a fixed resistor in parallel; and a current flowing through the field effect transistor. A semiconductor laser drive device comprising: a current control circuit for adjusting; 2. The current control circuit maintains the current flowing through the field effect transistor at a predetermined value until the light emission output of the semiconductor laser reaches the predetermined value, and after the light emission output of the semiconductor laser reaches the predetermined value, 2. The semiconductor laser driving device according to claim 1, further comprising a protection circuit that changes the current flowing through the field effect transistor.