JPH03151679A - Protective circuit of laser diode - Google Patents

Abstract

PURPOSE:To securely protect a laser diode by absorbing surge voltage without causing delayed response by connecting parallelly to the laser diode an FET which becomes conductive upon application of zero voltage by pass thereto. CONSTITUTION:There is connected parallelly to a light regulated laser diode 12 a junction type FET 18 which becomes conductive with application of zero voltage bypass. Further, for a power supply switch 20 being on, the FET 18 is switched to a non-conductive state after surge is settled, and thereafter lasing is started by light regulation control. On the other hand, for the power supply being off, after the lasing is stopped, the FET 18 is returned to a conductive state and then the power supply switch 20 is turned off. Namely, there is provided FET 18 which has its source and drain connected parallelly to the laser diode 12 and becomes conductive between its source and drain upon the zero voltage bypass being applied to a gate thereof. Hereby, even though surge voltage is applied to the laser diode 12, the FET 18 absorbs the surge without causing delayed operation, and hence element damage can securely be protected even if the diode is subjected to a short-term surge pulse.

Description

[Detailed Description of the Invention] [Summary] Regarding a laser diode protection circuit that prevents element destruction due to surge voltage, the laser diode protection circuit uses dimming control to reliably absorb and protect even instantaneous surge pulses. A junction FET that is placed in a conductive state with zero voltage bias is connected in parallel with the laser diode, and when the power switch is turned on, the FET is turned on after the surge subsides.
Switch the FET to a non-conducting state, then start oscillation of laser light by dimming control, and on the other hand, when the power is turned off, stop oscillating the laser light and then return the FET to a conductive state.
After that, the power switch is configured to be turned off.

[Industrial Application Field] The present invention relates to a laser diode protection circuit that prevents element destruction due to surge voltage.

When a laser diode receives a surge voltage due to electrostatic induction caused by the operator's charge, or when the power is turned on,
There is a risk that the laser diode will be easily destroyed if it is subjected to surge voltage caused by switching when off, so it goes without saying that sufficient care should be taken when handling the laser diode. It is desirable to provide a protection circuit to prevent element destruction.

[Prior art] As a conventional laser diode protection circuit, for example,
The one shown in the figure is known (Japanese Patent Laid-Open No. 60-7166).

In FIG. 4, 12 is a laser diode, and a laser beam oscillation operation is performed by flowing a predetermined large current from a current supply circuit 32. In FIG.

Laser diode protection is laser diode 12
The diode D1 and the NPN transistor 32 are connected in parallel to each other. NPN) transistor 32 is controlled by NPN) transistor 34.

The operation of this protection circuit is such that if a surge voltage due to electrostatic induction is applied to the laser diode 12 by touching the terminal with a hand etc. while the power supply 36 is not connected, the NPN transistor 34 is turned off, so the NPN ) transistor 32 is placed in a conductive state and receives a bias due to the surge voltage, NPN) transistor 32 becomes conductive and absorbs the surge voltage. The diode D1 becomes conductive when a reverse surge voltage that cannot turn on the NPN transistor 32 is applied to the laser diode, and similarly absorbs the surge voltage.

[Problems to be Solved by the Invention] However, in such a conventional laser diode protection circuit, the protective NPN transistor 32 is not activated until a surge voltage is applied to the laser diode.
As a result, the laser diode cannot absorb the instantaneous surge pulse that is applied at a speed higher than the operating speed of the protection transistor, and the problem remains that the laser diode is damaged by the surge.

The present invention has been made in view of these conventional problems, and an object of the present invention is to provide a laser diode protection circuit that can reliably absorb and protect even short-term surge pulses.

[Means to solve the problem] FIG. 1 is a diagram explaining the principle of the present invention.

In FIG. 1, a laser diode 12 that oscillates a laser beam by current drive by a transistor 0 is provided, and the laser diode 12 is configured such that the intensity of the laser beam detected by a monitor light receiving element 14 by an automatic dimming means 16 reaches a set value. Dimming control is applied to control the transistor 10 so as to maintain the brightness.

The protection circuit of the present invention is provided with an FET 18 whose source and drain are connected in parallel to the laser diode 12 and whose source and drain are brought into conduction when the gate is biased at zero voltage.

Furthermore, a control means 28 is provided to protect against surge voltages that occur when the power is turned on and off, and when the light emission starts, the control means 28 first turns on the power switch 20 and then turns on the enable signal 22 to shut off the FET 18. state, and then control the control switch 24 (turn off) to start oscillation of laser light by the automatic dimming means 16. On the other hand, when light emission is stopped, control the control switch 24 to the original state (on) to turn off the laser light. After the oscillation of light is stopped, the FET 12 is brought into conduction by turning off the enable signal 22, and finally the power switch 20 is turned off.

Further, the circuit section 28 consisting of the laser diode 12, the monitor light receiving element 14, and the FET 18 is formed into a single replaceable module.

[Function] According to the laser diode protection circuit of the present invention having such a configuration, the FET 18, which is in a substantially conductive state with a low impedance of several ohms between the source and drain at zero voltage bias, is connected to the laser diode 12. Since they are connected in parallel, when a surge voltage is applied to the laser diode 12, the FET 18 absorbs the surge without causing any delay in operation, and it is possible to reliably protect the element from destruction even if it receives a short surge pulse.

In addition, in order to avoid surge damage when the power is turned on, the power switch 20 is first turned on, and the enable signal 22 is turned off to maintain the FET 18 in a conductive state in response to the surge voltage that occurs at this time. absorb into. After the surge voltage has subsided and the power supply is stable, the enable signal 22 is turned on to control the FET 18 to a non-conductive state, and finally the control switch 24 is controlled to drive the laser diode to oscillate by automatic dimming.

On the other hand, when the power is turned off, contrary to when the power is turned on, the control switch 24 is first controlled to its original state to stop laser oscillation, and then the enable signal 22 is turned off to return the FET 18 to the conductive state and enable the protection function. After that, the power switch 20 is turned off to ensure that the surge voltage generated at this time is absorbed.

[Embodiment] FIG. 2 is a block diagram showing an embodiment of the present invention.

In FIG. 2, 12 is a laser diode, and a resistor RO for current limiting and a transistor 10 are connected in series to the cathode side of the laser diode 12, and a large current is passed through the laser diode 12 under the control of the transistor 10.
The laser beam is oscillated by flowing it through the oscilloscope. In this embodiment, a DC voltage of 12 volts is applied as a power supply voltage to both ends of the series circuit of the laser diode 12, the resistor RO, and the transistor 10 via the power switch 20.

An automatic dimming circuit 16 is provided for the transistor 0, and performs dimming control by controlling the transistor 10 so that the intensity of the laser light from the laser diode 12 becomes a predetermined value. That is, the laser light from the laser diode 12 is detected by the monitor light receiving element 14, and the light receiving element 1
Variable resistor VR provided with the light receiving current of 4 in the automatic dimming circuit 16
A detection voltage Vx corresponding to the intensity of the laser beam is generated at both ends thereof. The detection voltage Vx generated across the variable resistor VR is applied to the inverting input terminal of the operational amplifier 40, and the resistors R4 and R5 are applied to the non-inverting input terminal of the operational amplifier 40.
A reference voltage Vr divided by 2 is input. The output of the operational amplifier 40 is applied to the base of the transistor 0 via a low pass filter consisting of a resistor R3 and a capacitor C4. A soft start capacitor C2 is connected in parallel with the resistor R5. Further, a control switch 24 is connected in parallel with the resistor R5, and when the control switch 24 is in the OFF state, the resistor R5 is short-circuited to set the reference voltage Vr to Vr=O volts and the dimming control is stopped. When the capacitor C2 is turned off, charging of the capacitor C2 is started via the resistor R4, and after a predetermined charging time, the voltage rises to a predetermined reference voltage Vr determined by the voltage division between the resistors R4 and R5, and steady dimming control begins.

42 is a regulator which converts the power supply voltage of -12 volts obtained through the power switch 20 into a power supply voltage of -5 volts for operating the automatic dimming circuit 16. Note that the capacitor C3 is provided for power backup.

The protection of the laser diode 12 against surge voltage is
Junction FET connected in parallel to laser diode 12
18. That is, the laser diode 12
A source of junction type FE718 between the cathode and anode of
The drains are connected in parallel. Enable signal 2 is sent from the controller 26 to the gate of the junction type FE718.
2 is given, and furthermore, the enable signal 22 is connected to the resistor R
It is pulled down by 0 1.

The junction type FE 718 is in a low impedance state of several ohms between the source and drain when the gate is in a zero voltage bias state, that is, in a conductive state, and the voltage between the gate and the source is V. When S is increased, the impedance between the source and drain increases, resulting in a transition to a non-conductive state. Normally, by setting v6s=-3 to -5 volts, a non-conducting state, that is, a high impedance state between the gate and the source can be achieved.

The controller 26 controls on/off of the power switch 20, turns on the control switch 24 provided in the automatic dimming circuit 16,
The off control and each of the enable signals 22 for the junction type FE 718 are controlled according to a predetermined sequence so that the protection function can be reliably performed against surge voltages accompanying power on and off at the time of starting light emission and stopping light emission.

The control by the controller 26 at the time of starting the light emission is to first turn on the power switch 20, and turn off the enable signal 22 when the power is turned on so that the junction FET 18 absorbs the surge voltage generated when the power switch 20 is turned on. That is, the junction type FET 18 is brought into conduction with zero volts. Power switch 2
When the surge voltage subsides and transitions to a stable state after turning on 0, the enable signal 22 is turned on and a predetermined enable on voltage in the range of 3 to 5 volts is applied to the junction type FET 1.
8 and is biased by this enable signal 22 so that the junction FET 18 becomes non-conductive. Subsequently, the controller 26 turns off the control switch 24, and the automatic dimming circuit 16 controls the transistor 10 to start the oscillation operation of the laser diode 12.

On the other hand, when the light emission is stopped, the controller 26 performs an operation opposite to that when the power is turned on. That is, first, the control switch 24 is turned on to stop the oscillation operation of the laser diode 12, and then the enable signal 22 is changed from the on state to the off state and set to zero volt bias, thereby turning on the junction type FE.
718 is made conductive, and then the power switch 20 is turned off.

Furthermore, in the embodiment of FIG.
, the monitor light receiving element 14, the junction FET 2 18, and the resistor R1 are formed as a single module. By forming the circuit section 28 into a single module in this manner, replacement of the laser diode 12 due to deterioration can be performed on a module-by-module basis.

Next, refer to the operation timing chart in FIG.
The operation of the illustrated embodiment will be explained.

First, when the apparatus is stopped, the power switch 20 is off as shown, and the control switch 24 is on.

Now, suppose that the power switch 20 is turned on by a command from the controller 26 at time t1 in FIG.
At the same time, a power supply voltage of -12 volts is applied to the collector of the transistor 10 from 0 to 15 volts, and at the same time, it is converted to 15 volts by the regulator 42 and the power supply voltage is supplied to the automatic dimming circuit 16. At this time, the enable signal 22 from the controller 26 is in an off state, that is, at zero volts, and therefore the junction FET 18 is in a zero voltage bias state, so conduction occurs between the source and drain with a low impedance of several ohms. state, the surge voltage generated when the power switch 20 is turned on is applied to the source of the junction FET l8,
It flows between the drains and is absorbed, and the laser diode 12
No surge voltage is applied to the

At time t2, when sufficient time has passed for the surge voltage to subside after the power switch 20 is turned on, the controller 2
The enable signal 22 from 6 is turned on and becomes a predetermined voltage in the range of 3 to 5 volts, and the source of the junction FET 18
The junction FET 18 of the laser diode 12 becomes high impedance between the drains and switches to a non-conducting state.
The short circuit caused by this is canceled and laser oscillation becomes possible.

Subsequently, at time t3, the controller 26 switches the control switch 24
Switch from on to off. When the control switch 24 is turned off, the short-circuited state of the parallel circuit of the resistor R5 and the capacitor C2 is released, and charging of the capacitor C2 is started according to a predetermined time constant determined by the resistor R4 and the capacitor C2, and the operational amplifier 4
The reference voltage Vr relative to 0 gradually increases.

The operational amplifier 40 outputs a detection voltage Vx across a variable resistor vR according to the light-receiving current from the monitor light-receiving element 14 and a reference voltage V
Since the light emission stops when the power is turned on, the detection voltage Vx is zero volts, and the operational amplifier 40 generates an output that increases according to the reference voltage Vr, and the base current of the transistor 10 is set to the reference voltage. The current flowing through the laser diode 12 also increases as Vr increases. When the driving current of the laser diode 12 by the transistor 10 exceeds a predetermined threshold current, the laser diode 12 starts oscillating, and the monitoring light receiving element 14 that receives the laser light from the laser diode 12 responds to the intensity of the laser light. The detected light receiving current is passed through the variable resistor VR, so that the detection voltage V according to the laser beam is
x is obtained. Thereafter, the operational amplifier 40 performs feedback control to control the current of the transistor 10 so that the detected voltage Vx matches the reference voltage Vr. Therefore, when the reference voltage Vr stabilizes to a constant value at time t4, feedback control is performed to maintain a constant laser beam intensity corresponding to the reference voltage Vr.

Next, when the controller 26 is instructed to stop emitting light at time t5, the controller 26 first turns the control switch 24 from OFF to ON, and changes the reference voltage Vr to Vr by short-circuiting the parallel circuit of the resistor R5 and the capacitor C2. = 0, and by feedback control by the operational amplifier 40, the drive current of the laser diode 12 is also lowered as the reference voltage Vr is lowered, and laser oscillation is stopped.

After stopping the laser oscillation, the controller 26 turns off the enable signal 22, which had been on until then, at time t6.
That is, the state is switched to zero bias voltage, and the junction FET 18, which has been in a non-conducting state, returns to a conducting state. Then, the controller 26 finally switches on the power switch 2 at time t7.
Turn off 0.

A surge voltage is generated when the power switch 20 is turned off, but since the junction FET 6 18 has already returned to the conductive state by turning off the enable signal 22, the surge voltage generated when the power switch 20 is turned off is applied to the source of the junction FET 18. , which flows between the drain and is absorbed, and the laser diode 1 due to the surge voltage when the switch is turned off.
2. Reliably prevents element destruction.

On the other hand, when assembling the device or replacing the laser diode 12 due to its service life, the circuit section 28 including the laser diode 12, monitor light receiving element 14, junction FET 18, and bias resistor R1 is handled as one module. At this time, the junction type FE718 is in a conductive state due to the zero voltage bias state, so even if it receives a surge voltage caused by electrostatic induction due to the charging of the handler, the surge voltage flows to the junction type FE718, which is in a conductive state, and is absorbed. Therefore, the laser diode 12 can be reliably protected from surge damage.

In addition, in the above embodiment, the FET 18 is a junction type, but
A MOS type may be used as long as it is a depression type.

[Effects of the Invention] As explained above, according to the present invention, the FET, which becomes conductive at zero voltage bias, is connected in parallel to the laser diode, so even an instantaneous surge pulse causes a response delay. The laser diode can be reliably protected by absorbing surge voltage without any damage.

Also, when the power is turned on, the FET is kept in a conductive state until the surge subsides, and then the FET is switched to a non-conductive state to allow the laser diode to oscillate; on the other hand, when the power is turned off, the FET is returned to a conductive state after the laser oscillation stops. Since the power switch is turned off after
By performing surge absorption by ET, the laser diode can be reliably protected against surge voltages when the power is turned on and off.

[Brief explanation of the drawing]

8 7 FIG. 1 is an explanatory diagram of the principle of the present invention; FIG. 2 is a block diagram of an embodiment of the present invention; FIG. 3 is an operation timing chart of the present invention; FIG. 4 is a block diagram of a conventional circuit. In the figure, 10: Transistor 12: Laser diode 14: Monitor light receiving element 16: Automatic light control means (circuit) 18: Junction type FET 20: Power switch 22 enable signal 24: Control switch 26: Control means (controller) 40 : Operational amplifier 42: Regulator

Claims (2)

[Claims] (1) A laser diode (12) that oscillates laser light by current drive by a transistor (10); an automatic dimming means (16) for controlling; an FET (18) whose source and drain are connected in parallel to the laser diode (12) and whose source and drain are brought into conduction when the gate is biased at zero voltage; At times, after turning on the power switch (20), the enable signal (22) is turned on to make the FET (18) non-conductive, and then the control switch (24) is controlled to turn on the automatic dimming circuit (16). The control switch (24
) is returned to its original state to stop laser oscillation, the FET (12) is returned to a conductive state by turning off the enable signal (22), and finally the control means (26) turns off the power switch (20). A laser diode protection circuit characterized by comprising: and; (2) The laser diode (12) and the FET (
2. Laser diode protection circuit according to claim 1, characterized in that the circuit part (28) consisting of (18) is formed in a single replaceable module.