Light-emitting element drive circuit and laser oscillator

Abstract

A light-emitting element drive circuit (100) is provided with: a main voltage source (4); a boost voltage source (7); a MOSFET (3); a MOSFET (2); a diode (5); a linear regulator (6) that comprises a MOSFET (61) that is connected in series to an LD (1) and a drive unit (63) that drives the MOSFET (61), that forms a series circuit which includes the main voltage source (4), the MOSFET (3), the LD (1), the MOSFET (61), and wiring inductance (L1, L2), and that controls current flowing to the LD (1); a voltage monitor unit (8) that detects a third voltage and a fourth voltage for detecting drain-source voltage variation of the MOSFET (61); and a constant-current circuit unit (9) that controls injection of a first current into a gate of the MOSFET (61) on the basis of the third voltage and the fourth voltage.

Description

Light-emitting element drive circuit and laser oscillator

[0001] The present disclosure relates to a light-emitting element drive circuit and a laser oscillator that drive light-emitting elements including a laser diode (LD), a light-emitting diode (LED), and the like.

[0002] An LD or an LED is a light-emitting element that emits light with a luminance corresponding to the flowing current and is driven by a direct current. In a drive circuit that drives these light-emitting elements, a light-emitting element, a constant voltage source, and a linear regulator are arranged in series, and a configuration in which the current of the light-emitting element is controlled by the linear regulator is generally adopted. Further, in order to reduce the loss in the drive circuit, the voltage of the constant voltage source is set to a value slightly higher than the forward voltage of the light-emitting element. In a drive circuit having such a configuration, while the light-emitting element is constantly lit, no particular problem occurs. On the other hand, when controlling the light-emitting element from the off state to the on state, a current must be passed through the inductance of the wiring using the slight potential difference between the constant voltage source and the forward voltage of the light-emitting element as a potential gradient. For this reason, there is a problem that the rise of the current becomes extremely slow.

[0003] In order to solve the above problems, Patent Document 1 discloses that by switching a high voltage source with a relatively high voltage and a low voltage source with a relatively low voltage at high speed by a switch, the current is rapidly raised when the current of the light-emitting element rises, and after the current rise, the high voltage source is controlled to be off, and a constant current is passed through the light-emitting element only by the low voltage source. Further, in Patent Document 1, by providing a diode connected in a direction in which the energy accumulated in the wiring inductance on the anode side and the cathode side of the light-emitting element is regenerated to the high voltage source when a current flows through the light-emitting element, even when used in applications where the on / off of the current frequently occurs, an increase in circuit loss can be suppressed.

[0004] Japanese Patent Application Laid-Open No. 2022-161687

[0005] However, as will be described later, Patent Document 1 has a problem in that, due to circuit response delay during the transition from a current rise period in which the current is rapidly increased to a constant current control period in which a constant current is controlled to flow through the light-emitting element, large current overshoots and undershoots occur that may affect actual applications.

[0006] This disclosure has been made in view of the above, and aims to provide a light-emitting element driving circuit that can suppress an increase in circuit loss and suppress overshoot and undershoot of the current flowing to the light-emitting element, even in applications where current is frequently switched on and off.

[0007] To solve the above-mentioned problems and achieve the objective, the light-emitting element driving circuit according to this disclosure drives a light-emitting element. The light-emitting element driving circuit includes a first DC power supply that holds a first voltage and is connected to the anode of the light-emitting element so as to be able to apply the first voltage; a second DC power supply that is connected to the anode of the light-emitting element so as to be able to apply a second voltage higher than the first voltage; a first switch element connected in a direction that prevents the second voltage from being applied to the first DC power supply and switches the application of the first voltage to the anode of the light-emitting element on and off; a second switch element that switches the application of the second voltage to the anode of the light-emitting element on and off; and when current flows through the light-emitting element, the energy stored in the wiring inductance on the anode and cathode sides of the light-emitting element is regenerated to the second DC power supply. The device comprises a diode connected in the direction of the light-emitting element, a third switch element connected in series with the light-emitting element, and a drive unit for driving the third switch element. It also comprises a linear regulator that controls the current flowing through the light-emitting element, a voltage monitor unit that detects a third voltage and a fourth voltage for detecting voltage fluctuations between the drain and source of the third switch element, and a constant current circuit unit that controls the injection of a first current into the gate of the third switch element based on the third and fourth voltages.

[0008] The light-emitting element driving circuit of this disclosure has the effect of suppressing an increase in circuit loss and suppressing overshoot and undershoot of the current flowing to the light-emitting element, even in applications where current is frequently switched on and off.

[0009] Circuit diagram showing the configuration of the light-emitting element driving circuit according to Embodiment 1 Time chart showing the operation of the light-emitting element driving circuit according to Embodiment 1 Circuit diagram showing the configuration of the light-emitting element driving circuit according to Embodiment 2 Circuit diagram for explaining Patent Document 1

[0010] The light-emitting element driving circuit and laser oscillator according to the embodiment will be described in detail below with reference to the drawings.

[0011] Before describing the embodiments, the problems of Patent Document 1 will be explained using Figure 4. Figure 4 is a circuit diagram for explaining Patent Document 1. The light-emitting element driving circuit of Patent Document 1 comprises an LD1, a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) 2, a MOSFET 3, a low voltage source 4, a diode 5, a linear regulator 6, and a high voltage source 7. The linear regulator 6 comprises a MOSFET 61, a current detector 62, and a drive unit 63 for driving the MOSFET 61.

[0012] In the light-emitting element driving circuit of Figure 4, the LD1 is driven by switching between the high voltage of the high voltage source 7 and the low voltage of the low voltage source 4, while the current flowing through the LD1 is controlled by the linear regulator 6. Specifically, when the LD1 emits light, the circuit switches from the low voltage of the low voltage source 4 to the high voltage of the high voltage source 7 to raise the current flowing through the LD1 to the target current. The period during which the high voltage of the high voltage source 7 is supplied to the LD1 and the current flowing through the LD1 is raised to the target current is called the current rise period. Once the current flowing through the LD1 has risen to the target current, the circuit switches from the high voltage of the high voltage source 7 to the low voltage of the low voltage source 4, and the MOSFET 61 and the drive unit 63 maintain a constant current flowing through the LD1. The period during which the low voltage of the low voltage source 4 is supplied to the LD1 and the current flowing through the LD1 is controlled to be constant is called the constant current control period. When current flows through LD1, energy is stored in the wiring inductances L1 and L2 on the anode and cathode sides of LD1. Subsequently, during the current fall-off period, the energy stored in the wiring inductances L1 and L2 is regenerated to the high-voltage source 7 via the diode 5.

[0013] In the light-emitting element driving circuit of Figure 4, ideally, the linear regulator 6 controls the current so that the current flowing through LD1 remains constant even at the moment when the voltage at the anode of LD1 switches from low voltage to high voltage or from high voltage to low voltage. However, in reality, when transitioning from the current rise period to the constant current control period, that is, when the voltage at the anode of LD1 switches from low voltage to high voltage or from high voltage to low voltage, the response delay of the control of the linear regulator 6 causes current distortion, including overshoot and undershoot, in the current flowing through LD1.

[0014] This current distortion is influenced by the characteristics of the MOSFET 61, which is the current output element of the linear regulator 6 that controls the current of LD1, and the drive unit 63 that controls the gate voltage of the MOSFET 61. Generally, in a MOSFET, if you want to control the current to be constant, a constant current can be supplied if the gate voltage and the drain-source voltage of the MOSFET are constant. However, if the drain-source voltage increases and you want to maintain the original current, you can keep the current constant by relatively lowering the gate voltage. Also, if the drain-source voltage decreases, you can keep the current constant by relatively raising the gate voltage.

[0015] During the current rise period when the voltage at the anode of LD1 switches from the low voltage of the low voltage source 4 to the high voltage of the high voltage source 7, the drain-source voltage of MOSFET 61, which controls the current to flow at a constant level, rises sharply. In order to keep the current constant, it is necessary to lower the gate voltage of MOSFET 61, but due to the response delay of the drive unit 63 that controls the gate voltage of MOSFET 61, the operation to lower the gate voltage cannot keep up, resulting in an overshoot of the current flowing through LD1.

[0016] Furthermore, during the constant current control period when the anode voltage of LD1 switches from the high voltage of the high voltage source 7 to the low voltage of the low voltage source 4, the drain-source voltage of MOSFET 61, which controls the current to flow at a constant level, drops sharply. In order to keep the current constant, it is necessary to increase the gate voltage of MOSFET 61, but due to the response delay of the drive unit 63 that controls the gate voltage of MOSFET 61, the operation to increase the gate voltage cannot keep up, resulting in an undershoot of the current flowing through LD1.

[0017] Here, it is possible to improve the responsiveness of the drive unit 63, but the drive unit 63 is generally composed of an operational amplifier, and improving the responsiveness of the operational amplifier can cause unstable operation of the linear regulator 6, such as output oscillation, which may lead to current oscillation and other malfunctions.

[0018] Therefore, in this embodiment, it is possible to suppress the overshoot and undershoot of the current flowing to the LD1 without changing the response characteristics of the drive unit 63.

[0019] Embodiment 1. Figure 1 is a circuit diagram showing the configuration of a light-emitting element driving circuit 100 according to Embodiment 1. In Figure 1, components that achieve the same function as the light-emitting element driving circuit shown in Figure 4 are denoted by the same reference numerals. The light-emitting element driving circuit 100 includes an LD1, a metal-oxide-semiconductor field-effect transistor (MOSFET) 2, a MOSFET 3, a main voltage source (low voltage source) 4 which is a first DC power supply, a diode 5, a linear regulator 6, a boost voltage source (high voltage source) 7 which is a second DC power supply, a voltage monitor unit 8, and a constant current circuit unit 9. The linear regulator 6 includes a MOSFET 61, a current detector 62, a drive unit 63, and a resistive element 64. MOSFET 2 corresponds to a second switch element, MOSFET 3 corresponds to a first switch element, and MOSFET 61 corresponds to a third switch element.

[0020] LD1 is an example of a light-emitting element. The anode of LD1 is connected to the connection point between the source of MOSFET 2 and the drain of MOSFET 3. The cathode of LD1 is connected to the connection point between the anode of diode 5 and the drain of MOSFET 61. In Figure 1, LD1 is shown as a single element, but it is not limited to a single element. LD1 may consist of multiple elements connected in series or in series-parallel.

[0021] The source of MOSFET 3 is connected to the positive terminal of the main voltage source 4. The drain of MOSFET 2 is connected to the cathode of diode 5, and this connection point is connected to the positive terminal of boost voltage source 7. The source of MOSFET 61 is connected to the negative terminal of the main voltage source 4 via current detector 62, and this connection point is connected to the negative terminal of boost voltage source 7.

[0022] Wiring inductances L1 and L2 are shown on both sides of LD1. Wiring inductance L1 is the inductance of the electrical wiring between the positive terminal of the main voltage source 4 and the anode of LD1, and wiring inductance L2 is the inductance of the electrical wiring between the negative terminal of the main voltage source 4 and the cathode of LD1. When current flows through LD1, energy is stored in the wiring inductances L1 and L2 on the anode and cathode sides of LD1.

[0023] The linear regulator 6 is a component that controls the current flowing through the LD1, and comprises a MOSFET 61, a current detector 62, a drive unit 63 for driving the MOSFET 61, and a resistor 64. The resistor 64 is connected in series between the gate of the MOSFET 61 and the drive unit 63. The resistor 64 serves as both a resistor for suppressing the current of the MOSFET 61 and a resistor for converting the current from the constant current circuit 9 into a correction voltage for the gate voltage of the MOSFET 61.

[0024] The linear regulator 6 forms a series circuit with the main voltage source 4, MOSFET 3, LD1, and wiring inductances L1 and L2. Therefore, when driving LD1, the current flowing to LD1 flows through MOSFET 61. The current detector 62 can detect the current flowing through MOSFET 61, and consequently, the current flowing through LD1. The drive unit 63 compares the current command value to flow through LD1 with the current value detected by the current detector 62, and drives MOSFET 61 so that the difference between the current command value and the detected current value is zero.

[0025] The voltage monitoring unit 8 detects a third voltage and a fourth voltage to detect voltage fluctuations between the drain and source of the MOSFET 61. In Embodiment 1, the voltage monitoring unit 8 monitors the second voltage of the boost voltage source 7 as the third voltage and the anode voltage of the LD1 as the fourth voltage.

[0026] The inputs of the voltage monitor unit 8 are connected to the connection point between the drain of MOSFET 2, the cathode of diode 5, and the positive terminal of boost voltage source 7; the connection point between the anode of LD1, the source of MOSFET 2, and the drain of MOSFET 3; and the connection point between the negative terminal of boost voltage source 7 and the negative terminal of main voltage source 4. The two outputs of the voltage monitor unit 8 are connected to the constant current circuit unit 9. The voltage monitor unit 8 detects the voltage of the boost voltage source 7 using the potential at the connection point between the negative terminal of boost voltage source 7 and the negative terminal of main voltage source 4 as a reference. The voltage monitor unit 8 also detects the voltage of the anode of LD1 by detecting the voltage at the connection point between the anode of LD1, the source of MOSFET 2, and the drain of MOSFET 3, using the potential at the connection point between the negative terminal of boost voltage source 7 and the negative terminal of main voltage source 4 as a reference. The voltages of the boost voltage source 7 and the anode of LD1 monitored by the voltage monitor unit 8 are input to the constant current circuit unit 9.

[0027] In the voltage monitoring unit 8, the voltage of the boost voltage source 7 may be divided by a resistor and output, or it may be output directly to the constant current circuit unit 9 without voltage division. Alternatively, the voltage monitoring unit 8 may also divide the voltage of the anode of LD1 by a resistor and output, or it may be output directly to the constant current circuit unit 9. Furthermore, the voltage monitoring unit 8 may include a filter function or other method of level conversion.

[0028] The constant current circuit section 9 receives two voltages as input: the voltage from the boost voltage source 7 from the voltage monitor section 8 and the voltage of the anode of LD1. The output of the constant current circuit section 9 is connected to the connection point between the gate of the MOSFET 61 of the linear regulator 6 and the resistor 64. In the constant current circuit section 9, when connecting to the connection point between the gate of the MOSFET 61 of the linear regulator 6 and the resistor 64, the connection may be made directly, or a switching element such as a MOSFET may be interposed, and the current from the constant current circuit section 9 may be supplied at any desired timing by the switching element.

[0029] The constant current circuit section 9 controls the rise and fall of the gate voltage of the MOSFET 61 by injecting current into the gate of the MOSFET 61 based on two monitor voltages input from the voltage monitor section 8: the voltage of the boost voltage source 7 and the voltage of the anode of the LD1. The current injected into the gate of the MOSFET 61 from the constant current circuit section 9 corresponds to the first current.

[0030] Next, the light-emitting operation of the LD1 of the light-emitting element driving circuit 100 according to Embodiment 1 will be described. In this light-emitting element driving circuit 100, the pulse driving period in which the LD1 is repeatedly switched on and off at a set duty cycle includes an off period in which the LD1 is turned off, and the current rise period, constant current control period, and current fall period described above. During the off period, a first voltage is supplied from the main voltage source 4, and a simmer current flows through the LD1. During the current rise period, a second voltage is supplied to the LD1 from the boost voltage source 7. During the constant current control period and the current fall period, a first voltage is supplied to the LD1 from the main voltage source 4.

[0031] Therefore, when transitioning from the off period to the current rise period, MOSFET 3 is turned off and MOSFETs 2 and 61 are controlled to be turned on. As a result, the second voltage from the boost voltage source 7 is applied to the anode of LD1 via MOSFET 2, and current flows through LD1, MOSFET 61, and current detector 62. In this way, by applying the second voltage from the boost voltage source 7 to LD1 when the current rises, the current rise is accelerated.

[0032] When the current flowing through LD1 rises to the target current, MOSFET2 is controlled to turn off, but MOSFET61 remains on, and MOSFET3 is controlled to turn on. As a result, the first voltage from the main voltage source 4 is applied to the anode of LD1 via MOSFET3, and current flows through LD1, MOSFET61, and current detector 62. When current flows through LD1, energy is stored in the wiring inductances L1 and L2 on the anode and cathode sides of LD1. MOSFET61 and the drive unit 63 maintain a constant current flowing through LD1. This period corresponds to the constant current control period.

[0033] Subsequently, during the current fall-off period, MOSFET 61 is controlled to be off. At this time, MOSFET 3 remains on. This allows the energy stored in the wiring inductances L1 and L2 via diode 5 to be regenerated to the boost voltage source 7 during the current fall-off period.

[0034] Next, the function and characteristics of the circuit configuration of the light-emitting element driving circuit 100 according to Embodiment 1, which is connected as described above, will be explained. The main voltage source 4 holds a first voltage and is connected so that the first voltage can be applied to the anode of LD1. The boost voltage source 7 holds a second voltage that is higher than the first voltage and is connected so that the second voltage can be applied to the anode of LD1. The MOSFET 3 is connected in a direction that prevents the second voltage from being applied to the first DC power supply. In other words, the MOSFET 3 operates as a reverse current prevention element.

[0035] When MOSFET 2 is turned on, a second voltage from the boost voltage source 7 is applied to the anode of LD1 via MOSFET 2. When MOSFET 2 is turned off, the application of the second voltage to LD1 is stopped. In other words, MOSFET 2 switches the application of the second voltage to the anode of LD1 on and off.

[0036] When MOSFET 3 is turned on, the first voltage from the main voltage source 4 is applied to the anode of LD1 via MOSFET 3. When MOSFET 3 is turned off, the application of the first voltage to LD1 is stopped. In other words, MOSFET 3 switches the application of the first voltage to the anode of LD1 on and off.

[0037] MOSFET 3 is an example of a switching element equipped with an antiparallel diode. Antiparallel means that the anode of the diode is connected to the source of MOSFET 3, and the cathode of the diode is connected to the drain of MOSFET 3. The antiparallel diode can be an externally connected diode or a parasitic diode that is built into MOSFET 3. A parasitic diode is also called a body diode. Using a parasitic diode eliminates the need for individual diodes, thus reducing the number of components and lowering costs.

[0038] Diode 5 is connected in a direction that allows the energy stored when current flows through LD1 to be regenerated to the boost voltage source 7. With this configuration, the energy stored in the wiring inductances L1 and L2 is regenerated to the boost voltage source 7 via diode 5. Diode 5 also prevents the current caused by the voltage of the boost voltage source 7 from flowing through MOSFET 61.

[0039] The linear regulator 6 forms a series circuit with the main voltage source 4, MOSFET 3, LD1, and wiring inductances L1 and L2. As mentioned above, the resistor 64 serves as both a resistor for suppressing the current of MOSFET 61 and a resistor for converting the current from the constant current circuit 9 into a correction voltage for the gate voltage of MOSFET 61.

[0040] The voltage monitoring unit 8 monitors the second voltage of the boost voltage source 7 and the anode voltage of LD1.

[0041] The constant current circuit 9 controls the injection of current to the gate of the MOSFET 61 based on the second voltage of the boost voltage source 7 input from the voltage monitor unit 8 and the voltage of the anode of the LD1. The constant current circuit 9 uses the second voltage of the boost voltage source 7 as a reference from the two monitor voltages input from the voltage monitor unit 8 to determine the voltage difference between the voltage of the anode of the LD1 and the second voltage of the boost voltage source 7. If this voltage difference is relatively small and the voltage of the anode of the LD1 is at the same voltage level as the second voltage of the boost voltage source 7, the constant current circuit 9 relatively reduces the amount of current flowing to the connection point between the gate of the MOSFET 61 and the resistor 64.

[0042] Furthermore, when the voltage at the anode of LD1 approaches the same voltage as the second voltage of the boost voltage source 7, the constant current circuit 9 operates in such a way that the amount of current injected into the connection point between the gate of MOSFET 61 and the resistor element 64 decreases as the voltage difference between the two decreases. This current injection lowers the gate voltage of MOSFET 61. In other words, when the voltage at the anode of LD1 transitions sharply from a low voltage level to a high voltage, the gate voltage level of MOSFET 61 decreases sharply. Specifically, during the current rise period when the voltage at the anode of LD1 switches from a first voltage to a second voltage which is a relatively higher voltage than the first voltage, if the drain-source voltage of MOSFET 61 increases sharply, it is necessary to lower the gate voltage of MOSFET 61 in order to keep the drain-source current of MOSFET 61 constant. In this case, the operation to lower the gate voltage of MOSFET 61 is accelerated by relatively reducing the current flowing from the constant current circuit 9 to the connection point between the gate of MOSFET 61 and the resistor element 64. In this case, the drive unit 63 lowers the gate voltage of MOSFET 61 in order to keep the drain-source current of MOSFET 61 constant, but the operation to lower the gate voltage can be accelerated by current injection from the constant current circuit 9. In this way, the constant current circuit 9 assists the operation to lower the gate voltage of MOSFET 61 performed by the drive unit 63. In this way, the overshoot of the current flowing to LD1 caused by the response delay of the drive unit 63 can be suppressed.

[0043] On the other hand, when the voltage at the anode of LD1 moves away from the second voltage of the boost voltage source 7, the constant current circuit section 9 operates so that the amount of current injected into the connection point between the gate of MOSFET61 and the resistor element 64 increases as the voltage difference between the two increases. This current injection raises the gate voltage of MOSFET61. That is, when the voltage at the anode of LD1 sharply transitions from a high voltage level to a low voltage level, the gate voltage of MOSFET61 operates to sharply increase. Specifically, in a constant current control period in which the voltage at the anode of LD1 switches from the second voltage to the first voltage, which is a relatively lower voltage than the second voltage, when the voltage between the drain and source of MOSFET61 sharply decreases, in order to keep the current between the drain and source of MOSFET61 constant, it is necessary to raise the gate voltage of MOSFET61. In this case, the operation of raising the gate voltage of MOSFET61 is accelerated by relatively increasing the current flowing from the constant current circuit section 9 to the connection point between the gate of MOSFET61 and the resistor element 64. In this case, the drive section 63 is performing an operation of raising the gate voltage of MOSFET61 in order to keep the current between the drain and source of MOSFET61 constant, but the operation of raising the gate voltage by the current injection from the constant current circuit section 9 can be accelerated. Thus, the constant current circuit section 9 assists the operation of raising the gate voltage of MOSFET61 being executed by the drive section 63. In this way, it is possible to suppress the undershoot of the current flowing through LD1 caused by the response delay of the drive section 63.

[0044] Thus, when the voltage at the anode of LD1 moves away from the second voltage of the boost voltage source 7, the constant current circuit section 9 increases the injection current to the gate of MOSFET61 as the voltage difference between the two increases, controls the rise of the gate voltage of MOSFET61, and when the voltage at the anode of LD1 approaches the voltage equivalent to the second voltage of the boost voltage source 7, the constant current circuit section 9 decreases the injection current to the gate of MOSFET61 as the voltage difference between the two decreases, and controls the fall of the gate voltage of MOSFET61.

[0045] Figure 2 is a time chart showing the operation of the light-emitting element driving circuit according to Embodiment 2. The top figure of Figure 2 shows the anode voltage VLD of LD1, the middle figure of Figure 2 shows the LD current ILD flowing through LD1, and the bottom figure of Figure 2 shows the gate voltage VGS of MOSFET 61. The horizontal axis in each figure of Figure 2 represents time. In the top figure of Figure 2, Vmain represents the main power supply voltage corresponding to the first voltage of the main voltage source 4, and VBoost represents the boost voltage corresponding to the second voltage of the boost voltage source 7. In the middle and bottom figures of Figure 2, the solid line corresponds to ideal operation, the dashed line corresponds to operation in Patent Document 1, and the thick solid line corresponds to operation in Embodiment 1.

[0046] In Patent Document 1, when switching from the main power supply voltage Vmain to the boost voltage VBoost, a large overshoot of the LD current ILD occurs, causing a significant delay in the operation of lowering the gate voltage VGS of the MOSFET 61. In contrast, in Embodiment 1, when switching from the main power supply voltage Vmain to the boost voltage VBoost, the overshoot of the LD current ILD is reduced, and the responsiveness of the operation of lowering the gate voltage VGS of the MOSFET 61 can be improved.

[0047] Furthermore, in Patent Document 1, when switching from the boost voltage VBoost to the main power supply voltage Vmain, a large undershoot of the LD current ILD occurs, causing a significant delay in the operation of raising the gate voltage VGS of the MOSFET 61. In contrast, in Embodiment 1, when switching from the boost voltage VBoost to the main power supply voltage Vmain, the undershoot of the LD current ILD is reduced, and the responsiveness of the operation of raising the gate voltage VGS of the MOSFET 61 can be improved.

[0048] Thus, according to Embodiment 1, a voltage monitor unit 8 that monitors the second voltage of the boost voltage source 7 and the voltage at the anode of the LD1, and a constant current circuit unit 9 that controls the injection of a first current to the gate of the MOSFET 61 based on the second voltage of the boost voltage source 7 and the voltage at the anode of the LD1 are provided. Since a current is injected from the constant current circuit unit 9 to the MOSFET 61 to correct the gate voltage of the MOSFET 61, overshoot and undershoot of the current flowing through the LD1 can be suppressed without changing the response characteristics of the drive unit 63. Also, even in applications where the current is frequently turned on and off, an increase in circuit loss can be suppressed.

[0049] Embodiment 2. FIG. 3 is a circuit diagram showing the configuration of a light-emitting element drive circuit 100A according to Embodiment 2. In Embodiment 2, the connection destination of one of the three inputs of the voltage monitor unit 8 is changed from the anode of the LD1 to the drain of the MOSFET 61. Other configurations in FIG. 3 are the same as or equivalent to the configuration in FIG. 1. The same or equivalent components are denoted by the same reference numerals, and redundant descriptions are omitted.

[0050] In Embodiment 2, the voltage monitor unit 8 monitors the second voltage of the boost voltage source 7 as the third voltage and the drain voltage of the MOSFET 61 as the fourth voltage in order to detect voltage fluctuations between the drain and source of the MOSFET 61.

[0051] The constant current circuit unit 9 controls the injection of current to the gate of the MOSFET 61 based on the second voltage of the boost voltage source 7 and the drain voltage of the MOSFET 61 input from the voltage monitor unit 8. The operation of the constant current circuit unit 9 in Embodiment 2 is the same as the operation of the constant current circuit unit 9 in Embodiment 1. That is, in the constant current circuit unit 9, when the drain voltage of the MOSFET 61 moves away from the second voltage of the boost voltage source 7, the injection current to the gate of the MOSFET 61 is increased as the voltage difference between the two increases, to control the rise of the gate voltage of the MOSFET 61. When the drain voltage of the MOSFET 61 approaches the voltage of the second voltage of the boost voltage source 7, the injection current to the gate of the MOSFET 61 is decreased as the voltage difference between the two decreases, to control the fall of the gate voltage of the MOSFET 61.

[0052] As described above, according to Embodiment 2, a voltage monitoring unit 8 that monitors the second voltage of the boost voltage source 7 and the drain voltage of the MOSFET 61, and a constant current circuit unit 9 that controls the injection of a first current into the gate of the MOSFET 61 based on the second voltage of the boost voltage source 7 and the drain voltage of the MOSFET 61 are provided. Since the gate voltage of the MOSFET 61 is corrected by injecting current from the constant current circuit unit 9 into the MOSFET 61, overshoot and undershoot of the current flowing to the LD1 can be suppressed without changing the response characteristics of the drive unit 63. Furthermore, even in applications where current is frequently switched on and off, an increase in circuit loss can be suppressed.

[0053] The light-emitting element driving circuits 100 and 100A described in Embodiment 1 and Embodiment 2 are applicable to laser oscillators that perform pulse oscillation.

[0054] The configurations shown in the embodiments described above are merely examples of the content of this disclosure, and can be combined with other known technologies, combined with other embodiments, and some parts of the configuration can be omitted or modified without departing from the gist of this disclosure.

[0055] 1 LD, 2, 3, 61 MOSFET, 4 Main voltage source (low voltage source), 5 Diode, 6 Linear regulator, 7 Boost voltage source (high voltage source), 8 Voltage monitor section, 9 Constant current circuit section, 62 Current detector, 63 Drive section, 64 Resistor element, 100, 100A Light-emitting element drive circuit, L1, L2 Wiring inductance.

Claims

1. A light-emitting element driving circuit for driving a light-emitting element, comprising: a first DC power supply that holds a first voltage and is connected to the anode of the light-emitting element so as to be able to apply the first voltage; a second DC power supply that is connected to the anode of the light-emitting element so as to be able to apply a second voltage higher than the first voltage; a first switch element connected in a direction that prevents the second voltage from being applied to the first DC power supply and switches the application of the first voltage to the anode of the light-emitting element on and off; a second switch element that switches the application of the second voltage to the anode of the light-emitting element on and off; a diode connected in a direction that allows the energy stored in the wiring inductance on the anode and cathode sides of the light-emitting element to be regenerated to the second DC power supply when current flows through the light-emitting element; a third switch element connected in series with the light-emitting element; and a drive unit that drives the third switch element, comprising: a linear regulator that controls the current flowing through the light-emitting element, and which constitutes a series circuit including the first DC power supply, the first switch element, the light-emitting element, the third switch element, and the wiring inductance on the anode and cathode sides of the light-emitting element, A light-emitting element driving circuit comprising: a voltage monitoring unit for detecting a third voltage and a fourth voltage for detecting voltage fluctuations between the drain and source of the third switch element; and a constant current circuit unit for controlling the injection of a first current into the gate of the third switch element based on the third voltage and the fourth voltage.

2. The light-emitting element driving circuit according to claim 1, characterized in that the third voltage is the second voltage of the second DC power supply and the fourth voltage is the voltage of the anode of the light-emitting element.

3. The light-emitting element driving circuit according to claim 1, characterized in that the third voltage is the second voltage of the second DC power supply and the drain voltage of the third switch element.

4. The constant current circuit section controls the rise of the gate voltage of the third switch element by increasing the first current as the voltage difference between the fourth voltage and the third voltage increases when the fourth voltage moves away from the third voltage, and by decreasing the first current as the voltage difference between the fourth voltage and the third voltage decreases when the fourth voltage approaches the third voltage, thereby controlling the fall of the gate voltage of the third switch element.

5. The light-emitting element driving circuit according to any one of claims 1 to 4, wherein the linear regulator further comprises a resistive element connected between the gate of the third switch element and the driving unit, and the constant current circuit injects the first current at the connection point between the gate of the third switch element and the resistive element.

6. The light-emitting element driving circuit according to any one of claims 1 to 5, wherein the linear regulator further comprises a current detector for detecting the current flowing through the third switch element, and the drive unit controls the driving of the third switch element based on the difference between the current command value of the third switch element and the detected current value of the current detector.

7. A laser oscillator comprising: a light-emitting element; and a light-emitting element driving circuit according to any one of claims 1 to 6 for driving the light-emitting element.

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