Light emission device

Abstract

To provide a light emission device 100 for achieving rapid response for use in strobe light emission or the like while employing a current control system capable of accurately controlling intensity of emitted light.SOLUTION: A device includes: a current measurement circuit 2 for measuring current flowing to an LED circuit 1; a comparator circuit 3 for receiving a measurement signal indicating measurement current value measured by the current measurement circuit 2 and a target signal indicating predetermined target current value, and outputting a comparison signal indicating comparison result between the measurement current value and the target current value; a current control element 4 for receiving the comparison signal and controlling the current flowing to the LED circuit 1 so that a deviation between the measurement current value and the target current value approaches 0; and a pre-drive circuit 5 for outputting a pre-drive signal having a negative value to the comparator circuit 3 in place of the measurement signal when the target current value is set to 0.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a light emitting device used in a surface inspection device or an exposure device using an LED or a semiconductor laser as a light source.

Background Art

[0002] As this type of light emitting device, for example, as shown in Patent Document 1, there is known a device that uses an LED as a light source and adjusts the intensity of the emitted light (the amount of light emitted per unit time) by controlling the current flowing through the LED.

[0003] For example, in the field of surface inspection devices, in order to shorten the inspection lead time, there is a demand for a light emitting device that can emit stronger light for the purpose of increasing the shutter speed of the camera during workpiece imaging or imaging without stopping the conveyance of the workpiece.

[0004] Therefore, recently, a light emitting device has been developed that can emit very strong light, which is instantaneous but difficult to achieve with continuous light emission, by causing an LED to emit short-time pulsed light (stroboscopic light emission). In such stroboscopic light emission, good responsiveness is required.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] However, for example, when adopting a current control method that performs feedback control so that the current flowing through the LED becomes a desired target current in order to accurately control the intensity of the light during stroboscopic light emission, due to the fact that this feedback control is being performed, the responsiveness becomes worse compared to the case of open-loop control such as the voltage control method.

[0007] For example, the current control circuit operates in a feedback loop of current measurement → comparison by an error amplifier between the measured current and the target current → current adjustment by a MOSFET. However, at the time of the turn-off command, that is, when the value of the target current is 0, the feedback does not function, and the voltage V GS between the gate and source of the MOSFET continues to drop and becomes 0. When a predetermined value is set as the target current value to cause light emission from this state, the feedback control starts and the voltage V GS between the gate and source starts to rise. However, no current flows until this voltage V GS reaches the operating threshold voltage (gate threshold voltage), and that time becomes wasted time. And since the feedback control is performed during that time as well, the wasted time inevitably becomes long, having a great adverse effect on the lighting responsiveness of the LED.

[0008] The present invention has been made in view of such problems, and is a current control method capable of accurately controlling the intensity of the emitted light, and aims to achieve high-speed responsiveness that can also be used for stroboscopic light emission and the like.

Means for Solving the Problems

[0009] That is, the light emitting device according to the present invention is an LED circuit to which one or more LEDs are connected, a current measurement circuit that measures the current flowing through the LED circuit, a comparison circuit that receives a measurement signal indicating the value of the measured current by this current measurement circuit and a target signal indicating the value of a preset target current, and outputs a comparison signal indicating the comparison result between the measured current value and the target current value, and includes a current control element that receives the comparison signal and controls the current flowing through the LED circuit so that the deviation between the measured current value and the target current value approaches 0. When 0 is set as the target current value, it further includes a pre-drive circuit that outputs a pre-drive signal indicating a negative value to the comparison circuit instead of the measurement signal. The comparison circuit and the current control element are driven by the pre-drive signal, and the value of the current flowing through the LED circuit is set within a range where the lighting of the LED cannot be visually recognized.

Advantages of the Invention

[0010] According to the present invention configured as described above, since the current control element is driven by the pre-drive signal until immediately before the light emission command for the LED to such an extent that the lighting of the LED cannot be visually recognized, it can be started up in a shorter time compared to starting up from a state where the current control element is not driven at all, and the lighting responsiveness of the LED can be improved. In particular, since current feedback control is being performed, this reduction in time greatly contributes to the improvement of responsiveness.

Brief Description of the Drawings

[0011]

Figure 1

Embodiments for Carrying Out the Invention

[0012] An embodiment of the present invention will be described below with reference to the drawings.

[0013] 1. Configuration The optical emission device 100 according to the present embodiment is of a high-brightness single-pulse light emission type and can adjust the luminance of each pulse of light by current control. For example, it is used as a part of a surface inspection device (not shown) for inspecting the surfaces of various workpieces.

[0014] Specifically, as shown in FIG. 1, this light emitting device 100 includes an LED circuit 1, a current measurement circuit 2 that measures the current flowing through the LED circuit 1, a comparison circuit 3 that receives a measurement signal indicating the value of the measured current by the current measurement circuit 2 and a target signal indicating the value of a preset target current, and outputs a comparison signal indicating the comparison result between the measured current value and the target current value, and a MOSFET 4 that is a current control element that receives the comparison signal and controls the current flowing through the LED circuit 1 so that the deviation between the measured current value and the target current value approaches 0.

[0015] Next, each part will be described. The LED circuit 1 is configured by connecting a plurality of LEDs in series and / or in parallel. The current input terminal of this LED circuit 1 is connected to a second constant voltage source (output voltage is +V2) via a current limiting resistor element R7, and its current output terminal is connected to ground via a current control element and the current measurement circuit 2 described later.

[0016] Here, the current measurement circuit 2 includes a current measurement resistor element R1 that is connected in series with the LED circuit 1 and is configured such that all the current flowing through the LED circuit 1 flows into it.

[0017] Here, by connecting one terminal of the current measurement resistor element R1 to the current output terminal of the LED circuit 1 via the MOSFET 4 and connecting the other terminal to ground, the potential of one terminal of the current measurement resistor element R1 is configured to indicate the measured current value. In other words, the measurement signal is configured to be output from the one terminal.

[0018] The comparison circuit 3 is mainly composed of an amplifier OP1.

[0019] As is well known, this amplifier OP1 outputs from its output terminal a signal obtained by multiplying the difference between the voltage input to the positive input terminal and the voltage input to the negative input terminal by the amplification factor. Here, this amplifier OP1 is driven only by a positive constant voltage (+V1), and the voltage range of the comparison signal, which is the signal output from the output terminal, is defined within the range of 0 to the positive voltage (+V1).

[0020] One terminal of the current measurement resistance element R1 is connected to the negative input terminal of this amplifier OP1 via a resistance element R6 so that the measurement signal is input.

[0021] On the other hand, a target current signal indicating the value of the target current is input to the positive input terminal of the amplifier OP1. This target current signal is a voltage signal and is, here, a single pulse waveform.

[0022] Here, the MOSFET4 is of the N type. The output terminal of the amplifier OP1 is connected to the gate terminal, which is the control terminal of this MOSFET4, via a resistance element R8, and the comparison signal is input. Also, the output terminal of the LED circuit 1 is connected to the drain terminal, which is the input terminal of this MOSFET4, and one terminal of the current measurement resistance element R1 is connected to the source terminal, which is the output terminal of this MOSFET4.

[0023] With the above configuration, the feedback loop F1 shown in FIG. 1 is formed, and the comparison circuit 3 and the MOSFET4 operate so that the measured value of the current flowing through the LED circuit 1 (the measured current value measured by the current measurement circuit 2) approaches the target current value.

[0024] However, in this embodiment, when 0 is set as the target current value (hereinafter, this case is also referred to as "when the turn-off is commanded", etc.), a predrive circuit 5 is further provided that outputs a predrive signal indicating a negative value to the comparison circuit 3 instead of the measurement signal.

[0025] As shown in FIG. 1, this pre-drive circuit 5 includes a reference voltage setting circuit 51 and an amplifier OP2.

[0026] The reference voltage setting circuit 51 is composed of resistor elements R2 and R3 provided in series between the constant voltage (+V3) line and the ground, and the potential at the midpoint of the resistor elements R2 and R3 becomes the reference voltage v3.

[0027] Similar to the amplifier OP1, the amplifier OP2 outputs, from the output terminal, a signal obtained by multiplying the difference between the voltage input to the plus input terminal and the voltage input to the minus input terminal by the amplification factor. Note that this amplifier OP2 is driven by a positive constant voltage (+V3) and a negative constant voltage (-V4), and the voltage range of the signal output from the output terminal, that is, the pre-drive signal, extends from the negative voltage (-V4) to the positive voltage (+V3).

[0028] The reference voltage v3 is configured to be applied to the minus input terminal of this amplifier OP2 via a resistor element R4, while one terminal of the current measurement resistor element R1 is connected to the plus input terminal thereof so that the measurement signal is input.

[0029] Also, its output terminal is connected to the minus input terminal of the amplifier OP1 via a diode D1 which is a cut-off element. More specifically, this output terminal is connected to the cathode of the diode D1, and the anode of the diode D1 is connected to the minus input terminal of the amplifier OP1.

[0030] Note that the minus input terminal and the output terminal of this amplifier OP2 are connected via a resistor element R5, and the amplification factor of the amplifier OP2 is determined by the ratio of the values of the resistor elements R4 and R5.

[0031] 2. Operation Next, the operation of the pre-drive circuit 5 will be described. When the light is turned off at the initial stage and the voltage v5 of the target signal is set to 0, no current flows through the LED circuit 1, so the measured current value is 0, that is, the voltage v1 of the measured signal is 0.

[0032] This measured voltage v1 is compared with the reference voltage v3 in the amplifier OP2, and the value obtained by subtracting the reference voltage v3 from the measured voltage v1 is amplified and output as a pre-drive signal from the output terminal of the amplifier OP2. Here, since the measured voltage v1 is 0 and the reference voltage v3 is positive, the voltage of the pre-drive signal is negative.

[0033] Then, current is drawn in through the route shown by the dashed line in Fig. 1, and the measured voltage v1 becomes negative. Also, when this pre-drive signal having a negative value is input to the negative terminal of the amplifier OP1, since the voltage v5 of the target signal input to the positive terminal of the amplifier OP1 is 0, the voltage v4 of the comparison signal output from the amplifier OP1 becomes positive, and thereby the MOSFET4 operates to try to pass current through the LED circuit 1.

[0034] As a result, the measured voltage v1 is updated and becomes positive. Then, based on this updated measured voltage v1, the amplifier OP2, the amplifier OP1, and the MOSFET4 operate again, and a feedback loop is formed in which the measured voltage v1 is updated again.

[0035] Finally, the measured voltage v1 and the voltage v4 of the comparison signal are determined by such a feedback loop. In other words, the source voltage v1 and the gate voltage v4 of the MOSFET4 are determined.

[0036] However, in this embodiment, by appropriately setting the reference voltage v3, the amplification factor of the amplifier OP2, the values of the resistance element R1 and the resistance element R6, etc., the gate-source voltage V of the MOSFET4 determined by the feedback loop GS (=v4 - v1) becomes the operating threshold voltage V of the MOSFET4 THHereinafter, and such that it is 50% or more, more preferably 70% or more, and even more preferably 80% or more of the operating threshold voltage V TH is achieved.

[0037] As a result, when the target current value is 0, the MOSFET 4 is maintained in a state just before the amplification operation or the switching operation, so to speak, on the verge.

[0038] Next, when the target current value, that is, the voltage v5 of the target signal, is set to a predetermined positive value in order to emit pulsed light, as described above, the feedback loop F1 shown in FIG. 1 is formed, and the measured value of the current flowing through the LED circuit 1 (the voltage v1 of the measurement signal) approaches the target current value (the voltage v5 of the target signal), the comparison circuit 3 and the MOSFET 4 operate.

[0039] At this time, the voltage v1 of the measurement signal is also input to the amplifier OP2 of the pre-drive circuit 5 and compared with the reference voltage v3. Since the voltage v1 of the measurement signal during pulsed light emission is set to be significantly larger than the reference voltage v3, the output voltage of the amplifier OP2, that is, the voltage v2 of the pre-drive signal, also has the amplification factor of the amplifier OP2 applied and becomes the maximum positive value.

[0040] However, since the pre-drive signal that has become a positive voltage is not transmitted to the comparison circuit 3 by the diode D1 which is a blocking element, it does not affect the current control operation by the feedback loop F1 shown in FIG. 1 during LED lighting.

[0041] 3. Effects Thus, according to the above configuration, until just before the pulsed light emission command, the gate-source voltage V GS of the MOSFET 4 is maintained at a voltage close to the operating threshold voltage V TH , so the time (ramp-up time) to raise the gate-source voltage V GS from 0v where no current flows to the operating threshold voltage V TH can be made as short as possible, and the responsiveness of pulsed light emission can be improved.

[0042] In particular, in the case of the present embodiment in which current feedback control is performed, the aforementioned run-up time is extremely long compared to open loop control, so that the gate-source voltage V GS The operating threshold voltage V TH The shortening of the run-up time by maintaining the voltage close to 0 V contributes greatly to improving the response.

[0043] In addition, for example, the gate-source voltage V GS In a configuration in which the feedback loop F1 is maintained at a constant level, there is a possibility that light will be emitted due to instrumental errors of elements such as the amplifier OP1 and the MOSFET 4 that constitute the feedback loop F1. However, according to the present embodiment, such a situation can be prevented, and reliable operation can be achieved with a simple configuration.

[0044] 4. Other embodiments The present invention is not limited to the above-described embodiment. For example, in the above embodiment, the gate-source voltage V GS The operating threshold voltage V TH The gate-source voltage V GS , the operating threshold voltage V TH It is acceptable to set the value above 0.05V to operate the MOSFET within the linear region. In this case, current flows through the LED circuit, but the current value must be kept within a range where the lighting is not visible or where it does not affect applications such as surface inspection. In this way, the run-up time can be set to 0, further improving responsiveness.

[0045] The current control element is not limited to an N-type MOSFET, but may be a P-type MOSFET, or may be replaced with other FETs, bipolar transistors, or the like.

[0046] The cutoff element is not limited to a diode, but a switching element or the like may be used.

[0047] In addition, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit thereof.

Explanation of Reference Numerals

[0048] 100 ··· Light emitting device 1 ··· LED circuit 2 ··· Current measurement circuit 3 ··· Comparison circuit 4 ··· MOSFET (current control element) 5 ··· Predrive circuit D1 ··· Diode (blocking element)

Claims

1. an LED circuit to which one or more LEDs are connected; a current measurement circuit that measures a current flowing through the LED circuit; a comparison circuit that receives a measurement signal indicating a value of the measured current by the current measurement circuit and a target signal indicating a value of a preset target current, and outputs a comparison signal indicating a comparison result between the measured current value and the target current value; a current control element that receives the comparison signal and controls a current flowing through the LED circuit so that a deviation between the measured current value and the target current value approaches zero; when 0 is set as the target current value, further comprising a pre-drive circuit that outputs a pre-drive signal having a negative value to the comparison circuit instead of the measurement signal; a light emitting device, wherein the comparison circuit and the current control element are driven by the pre-drive signal, and a value of a current flowing through the LED circuit is set to a range where lighting of the LED cannot be visually recognized.

2. the current control element is a MOSFET, and is configured such that the comparison signal is input to a gate terminal thereof; the light emitting device according to claim 1, wherein when the pre-drive signal having a negative value and the target signal having a target current value of 0 are input, a voltage applied between a gate and a source of the MOSFET by the comparison signal output from the comparison circuit is configured to be equal to or lower than an operation threshold voltage of the MOSFET.

3. the light emitting device according to claim 1, further comprising a blocking element that blocks the pre-drive circuit and the comparison circuit when a positive value at which the LED becomes in a visually recognizable lighting state is set as the target current value.

4. the light emitting device according to claim 3, wherein the blocking element is a diode.

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