Patent Publication Number: US-11020823-B2

Title: Laser diode driving power source and laser machining device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is based on PCT filing PCT/JP2017/005306, filed Feb. 14, 2017, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The present invention relates to a laser diode driving power source of a laser machining device that machines a workpiece, and a laser machining device. 
     BACKGROUND 
     A conventional power source for driving a laser diode (LD) outputs a direct-current voltage higher than a voltage required for the LD using a constant voltage source that has a large-capacity capacitor on an output side. This direct-current voltage is applied to the LD and a regulator circuit. The conventional LD-driving power source adjusts the voltage applied to the regulator to thereby adjust an electrical current flowing through the LD. The regulator is constructed of semiconductor elements such as a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) and a transistor. The conventional LD-driving power source can change a current value at a high speed by control of the regulator. However, because a current flowing through the regulator is loss, power efficiency is lowered. 
     An LD power-source control device employing a constant current system disclosed in Patent Literature 1 includes a power source that supplies power to a laser diode, a current-controlling switching device and a reactor that are inserted between the laser diode and the power source, a current detector, and a diode connected in parallel to a series circuit of the current-controlling switching device and the reactor, but does not include a large-capacity capacitor on an output side of the power source, thereby achieving a constant-current operation. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Patent No. 3456120 
     SUMMARY 
     Technical Problem 
     In laser machining, a machining head is moved with respect to a workpiece or the workpiece is moved with respect to the machining head, and such movement is a base for machining of the workpiece. In an LD-driving power source employing a constant current system disclosed in Patent Literature 1, when a current value is changed at a high speed by switching control, a current ripple becomes large in association with turning on and off of the switching device and a value of a current flowing through the LD fluctuates, causing fluctuation of an LD output value, that is, a laser intensity. The LD-driving power source disclosed in Patent Literature 1 has a problem that a machined-surface roughness required for machining cannot be achieved because of the fluctuation of the LD output value in some cases. 
     The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a laser diode driving power source that can reduce fluctuation of an LD output value. 
     Solution to Problem 
     In order to solve the problem mentioned above and achieve the object, the present invention provides a laser diode driving power source of a laser machining device that radiates laser light emitted from a laser diode to a workpiece to machine the workpiece, comprising: a power converter including a switching device, to supply a current to the laser diode; and a control unit to control a switching operation of the switching device, wherein a value smaller than a value that is n times a time constant for laser machining is set as a switching period of the switching device in the control unit, the n is larger than 1, and the time constant for laser machining is a value obtained by dividing a surface roughness required for machining of the workpiece by a moving velocity of the workpiece. 
     Advantageous Effects of Invention 
     The laser diode driving power source according to the present invention has an advantageous effect that it is possible to reduce fluctuation of an LD output value. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a configuration diagram of a laser diode driving power source according to the present embodiment. 
         FIG. 2  is a diagram illustrating a first configuration example of a power converter illustrated in  FIG. 1 . 
         FIG. 3  is an explanatory diagram of an output current waveform in a case of using a power converter illustrated in  FIG. 2 . 
         FIG. 4  is a diagram illustrating a second configuration example of the power converter illustrated in  FIG. 1 . 
         FIG. 5  is an explanatory diagram of an output current waveform in a case of using a power converter illustrated in  FIG. 4 . 
         FIG. 6  is a diagram illustrating a first configuration example of a control unit illustrated in  FIG. 1 . 
         FIG. 7  is a diagram illustrating a second configuration example of the control unit illustrated in  FIG. 1 . 
         FIG. 8  is a chart illustrating a relation among a switching period, fluctuation of an output current, an on-duration of a switching device, and an off-duration of the switching device. 
         FIG. 9  is a diagram illustrating a third configuration example of the control unit illustrated in  FIG. 1 . 
         FIG. 10  is an explanatory diagram of influences of fluctuation of an output current of a power converter on machining of a workpiece. 
         FIG. 11  is a diagram illustrating a modification of the laser diode driving power source according to the present embodiment. 
         FIG. 12  is a diagram illustrating current and voltage waveforms of an LD illustrated in  FIG. 11 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A laser diode driving power source and a laser machining device according to an embodiment of the present invention will be described in detail below with reference to the drawings. The present invention is not necessarily limited by the embodiment. 
     Embodiment 
       FIG. 1  is a configuration diagram of a laser diode driving power source according to the present embodiment. A laser machining device  200  illustrated in  FIG. 1  includes a laser diode driving power source  100 , an LD  20  that is a laser diode emitting laser light by a current supplied from the laser diode driving power source  100 , a machining head  22  for machining a workpiece  300 , and a fiber  21  that is an example of an optical coupling system and an optical amplifier, which transmits the laser light emitted from the LD  20  to the machining head  22 . 
     The laser diode driving power source  100  includes a power converter  10  and a control unit  11  that controls turning on and off of a switching device in the power converter  10 . The power converter  10  compares a current value detected by a current detector  13  with a current command value to control a switching operation of the switching device provided in the power converter  10 . The power converter  10  is a constant-current system power converter that converts alternating-current power supplied from an alternating-current power source  400  to direct-current power by turning on and off the switching device. 
     The laser light outputted from the LD  20  is transmitted to the machining head  22  through the fiber  21  and is focused onto the workpiece  300  by a lens  23  provided in the machining head  22 . In this manner, the workpiece  300  is machined to be cut. In machining of the workpiece  300 , it is necessary to move the focusing position of the laser on the workpiece  300 . For this reason, the workpiece  300  is placed on a workpiece moving mechanism (not illustrated) for moving the workpiece  300  or a head moving mechanism (not illustrated) for moving the machining head  22  is provided in the laser machining device  200 . 
       FIG. 2  is a diagram illustrating a first configuration example of the power converter illustrated in  FIG. 1 . A power converter  10 - 1  illustrated in  FIG. 2  is a non-insulated type and step-down chopper system power converter. The power converter  10 - 1  includes a rectifier circuit  1  that rectifies alternating-current power supplied from the alternating-current power source  400 , a capacitor  2  connected in parallel to the rectifier circuit  1 , a switching device  3 , a chopper diode  4 , and a reactor  5 . 
     A drain of the switching device  3  is connected to the rectifier circuit  1  and one end of the capacitor  2 . A source of the switching device  3  is connected to a cathode of the chopper diode  4  and one end of the reactor  5 . An anode of the chopper diode  4  is connected to the other end of the capacitor  2  and a cathode of the LD  20 . The other end of the reactor  5  is connected to an anode of the LD  20 . 
       FIG. 3  is a diagram for explaining an output current waveform in a case of using the power converter illustrated in  FIG. 2 .  FIG. 3  illustrates a current command that is a machining command  12 , a switching-device driving signal generated in the control unit  11  illustrated in  FIG. 1 , and an output current waveform of the power converter  10 - 1 . Further,  FIG. 3  illustrates a switching-device driving signal and an output current waveform when a reactor is large, and illustrates a switching-device driving signal and an output current waveform when the reactor is small. 
     When the current command is inputted to the control unit  11  and a current rapidly rises, the switching device  3  starts a switching operation with a specific switching period. Accordingly, the output current waveform rises as illustrated in  FIG. 3 . The current waveform is caused to fluctuate by turning on and off operations of the switching device. Such fluctuation is called “current ripple”. Although it is possible to reduce a fluctuation width of the current ripple if the reactor is high in value, rising of the output current becomes slower. If the reactor is reduced in value, the output current rises more quickly and therefore reaches a target current value in a short time, but the current ripple becomes larger as the reactor becomes smaller. 
       FIG. 4  is a diagram illustrating a second configuration example of the power converter illustrated in  FIG. 1 . A power converter  10 - 2  illustrated in  FIG. 4  is an insulated type power converter and is of a system in which power rectified by the rectifier circuit  1  is converted to alternating-current power by an inverter  6  and is then rectified again through a transformer  7 . A current outputted to a secondary side of the transformer  7  flows to the LD  20  via a diode  8  or a diode  9  and the reactor  5 . An output voltage is adjusted based on a turn ratio of the transformer  7 , and an output current is adjusted based on a ratio of an on-duration time to a switching period of a plurality of switching devices in the inverter  6 . 
       FIG. 5  is a diagram for explaining an output current waveform in a case of using the power converter illustrated in  FIG. 4 .  FIG. 5  illustrates a current command, an output voltage waveform of the inverter  6 , and an output current waveform of the power converter  10 - 2 . 
     Because the inverter  6  has a full-bridge configuration, the output voltage waveform of the inverter  6  is a square wave with positive and negative values as illustrated in  FIG. 5 . By adjusting the pulse width of this square wave, an output is adjusted. When the output voltage of the inverter  6  becomes asymmetrical for a positive side and a negative side, a direct-current component is temporarily generated in a current flowing through the transformer  7 . Generation of the direct-current component causes biased magnetization of a core of the transformer  7 , and leads to magnetic saturation if a current value becomes high, so that any accident of short-circuit of a current may occur. Therefore, it is necessary that the output waveform of the transformer  7  is symmetrical for the positive side and the negative side. A switching period of the power converter  10 - 2  is equal to a time of one set of a positive output and a negative output, that is, an inverter period illustrated in  FIG. 5 . 
     A configuration example of a control unit that controls an operation of the switching device  3  is described below. 
       FIG. 6  is a diagram illustrating a first configuration example of the control unit illustrated in  FIG. 1 . A control unit  11 - 1  illustrated in  FIG. 6  is an example of a control circuit that drives the non-insulated type step-down chopper system power converter  10 - 1  in PWM (Pulse Width Modulation) with a determined frequency. 
     The control unit  11 - 1  includes an amplifier  31  that amplifies a current detected by the current detector  13 , an error amplifying circuit  32  that compares the current amplified by the amplifier  31  with a current command value  35  to get an error and amplifies and outputs a signal corresponding to the error, a triangle-wave generator  34 , a PWM generating circuit  33 , and a driving circuit  30 . 
     The PWM generating circuit  33  compares a triangle wave outputted from the triangle-wave generator  34  and a signal outputted from the error amplifying circuit  32  with each other to output a PWM signal having a pulse width according to the output signal of the error amplifying circuit  32 . The driving circuit  30  converts the PWM signal outputted from the PWM generating circuit  33  to a driving signal for the switching device  3 . 
     The current detected by the current detector  13  is compared with the current command value  35  by the error amplifying circuit  32  to obtain an error, and a signal corresponding to the error is inputted to the PWM generating circuit  33 . In the PWM generating circuit  33 , the triangle wave and the output value of the error amplifying circuit  32  are compared with each other, and thereby the PWM signal having a pulse width corresponding to the output signal of the error amplifying circuit  32  is outputted. The PWM signal is amplified and converted to the driving signal in the driving circuit  30 . The switching device  3  is driven by the driving signal. In the circuit having this configuration, the frequency of the triangle-wave generator  34  is equal to a switching frequency of the switching device  3 . 
       FIG. 7  is a diagram illustrating a second configuration example of the control unit illustrated in  FIG. 1 . A control unit  11 - 2  illustrated in  FIG. 7  is an example of a control circuit having no reference-frequency circuit for driving for the control unit  11 - 2 , which is not of constant-frequency driving based on a triangle wave. Even in a circuit with a variable frequency as in the control unit  11 - 2 , it is possible to define a switching frequency of the switching device  3 . 
     The control unit  11 - 2  includes the driving circuit  30 , the amplifier  31 , and a comparator  36 . In the control unit  11 - 2 , a current detected by the current detector  13  is compared with the current command value  35  by the comparator  36 . The control unit  11 - 2  executes control in such a manner that the switching device  3  is turned off when the current detected by the current detector  13  is larger than the current command value  35 , but is turned on when the current detected by the current detector  13  is smaller than the current command value  35 . 
     Two resistors R 1  and R 2  are connected to the comparator  36 , and some hysteresis occurs in an input and an output of the comparator  36 . Assuming that a current command value is I and a current fluctuation width by the comparator  36  with a hysteresis is dI, a relation of the following expression (1) is established between the current command value I and the current fluctuation width dI. 
     
       
         
           
             
               
                 
                   [ 
                   
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
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                         R 
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     Assuming that an inductance of the reactor  5  is L, an input voltage of the power converter  10 - 1  is V in , an output voltage of the power converter  10 - 1 , that is, a forward voltage of the LD  20  is V out , and a forward voltage of the chopper diode  4  is V d , the current fluctuation width dI of an output current when the switching device  3  is on is represented by the following expression (2). Further, the current fluctuation width dI of the output current when the switching device  3  is off is represented by the following expression (3). 
     
       
         
           
             
               
                 
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       FIG. 8  is a chart illustrating a relation among a switching period, fluctuation of an output current, an on-duration time of a switching device, and an off-duration time of a switching device. dt on  in the above expression (2) is an on-duration time of the switching device  3 . dt off  in the above expression (3) is an off-duration time of the switching device  3 . A switching period dt is obtained by dt=dt on +dt off . The above expressions (2) and (3) assume a state where a current steadily flows with stability and ignore resistance loss of a circuit. From the above expressions (2) and (3), the switching period dt is represented by the following expression (4). 
     
       
         
           
             
               
                 
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       FIG. 9  is a diagram illustrating a third configuration example of the control unit illustrated in  FIG. 1 . A control unit  11 - 3  illustrated in  FIG. 9  is configured by a circuit such as a microcomputer or an FPGA (Field Programmable Gate Array). A current detected by the current detector  13  is converted to a digital signal by an AD (Analog to Digital) converter  43 . A difference between a value obtained by the conversion and a current command value  44  is obtained by a subtractor  45 . In accordance with this difference, a control signal for the switching device  3  is generated in a controller  46 . 
     This control signal is converted to a PWM signal by a PWM signal generator  42  on the basis of a constant-frequency signal that is generally outputted from an oscillator  41 , and the PWM signal inputted to the driving circuit  30  for the switching device  3 . It is optional whether this circuit configuration is partly configured as a digital circuit or is entirely realized inside the FPGA. 
     In such a circuit, a period of the signal output from the oscillator  41  has the same meaning as a switching frequency of the switching device  3 . The signal outputted from the oscillator  41  is generally generated inside a digital circuit. However, because the switching device  3  can be driven in PWM by this signal as a result, it is possible to identify a time constant for the laser machining if the frequency of this signal is measured. The time constant for the laser machining is a value obtained by dividing a surface roughness required for machining the workpiece  300  by a moving velocity of the workpiece  300 . Details of the time constant for the laser machining will be described later. 
     Next, influences of the current fluctuation width dI of an output current of the power converter  10 - 1  on machining of the workpiece  300  are described. 
       FIG. 10  is an explanatory diagram of influences of fluctuation of an output current of a power converter on machining of a workpiece.  FIG. 10  illustrates, from a top to a bottom thereof, fluctuation of an output current of the power converter  10 - 1 , an uneven surface when a machined-surface roughness is small, and an uneven surface when the machined-surface roughness is large. 
     In laser machining, the machining head  22  is moved with respect to the workpiece  300  keeping a stationary condition to cut the workpiece  300 , or the workpiece  300  is moved with respect to the machining head  22  keeping a stationary condition to cut the workpiece  300 . At that time, fluctuation of a value of a current flowing through the LD  20  causes fluctuation of an LD output value, that is, a laser intensity. Therefore, the fluctuation of the output current illustrated in the top in  FIG. 10  can be regarded as being equal to fluctuation of the laser intensity of the LD  20 . A period of the fluctuation of the laser intensity is dt p , which corresponds to a switching period. This period dt p  may be sometimes referred to as “switching time constant” or “switching period”. 
     The second and third rows from the top of  FIG. 10  shows a situation where a machined groove becomes broader or narrower for reason of the fluctuation of the laser intensity when the workpiece  300  and the machining head  22  are relatively moved. Adjacent large and small circles are in contact with each other. This is because a beam shape of the laser at a focal point of the lens  23  can be regarded as being circular and the width of circular laser light radiated onto the workpiece  300  fluctuates with time. 
     Assuming that a period of the fluctuation of the laser intensity is dx, a relation between the period dx and the period dt p  depends on a moving velocity v L  of the relative movement of the workpiece  300  and the machining head  22 , as represented by the following expression (5).
 
[Formula 5]
 
 dx=v   L   ×dt   p   (5)
 
     The period dx is not a so-called surface roughness, but is like a “surge” appearing in a machined surface. Meanwhile, in machining, “surface roughness” of machining is important as the finishing accuracy of a machined surface in many cases. There are various definitions for a surface roughness. A surface roughness dz can be defined as a width from a bottom of a concave portion to a top of a convex portion in a direction perpendicular to a laser-emitting direction in a top surface of the workpiece  300 , that is, a surface of the workpiece  300  opposed to the machining head  22 . The surface roughness dz corresponds to a difference between the smallest beam shape of the laser light and the largest beam shape of the laser light in the focal point, as illustrated in  FIG. 10 . 
     Because the period dx and the surface roughness dz are influenced by both fluctuation of the laser and a machining phenomenon during laser machining, a relation between the period dx and the surface roughness dz cannot be represented simply. However, in a case where the surface roughness dz becomes large as illustrated in the third row from the top of  FIG. 10 , there is a relation represented by the following expression (6) between the surface roughness dz and the period dx. 
     
       
         
           
             
               
                 
                   [ 
                   
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     6 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   dz 
                   ≤ 
                   
                     dx 
                     2 
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
     Here, when a surface roughness of dz 0  is required as a surface roughness included in some machining performances, it is necessary to make the surface roughness dz of a surface after being laser-machined smaller than the required surface roughness dz 0 . Accordingly, because the surface roughness dz can be dx/2 at the maximum, it is necessary that the period dx satisfies the following expression (7).
 
[Formula 7]
 
 dx≤ 2× dz   0   (7)
 
     From the relation between the period dx and the period dt p  represented by the above expression (5), the above expression (7) can be represented by the following expression (8).
 
[Formula 8]
 
 v   L   ×dt   p   =dx≤ 2× dz   0   (8)
 
     Here, a time constant dt m  of laser machining is defined by the following expression (9). The moving velocity v L  is the maximum moving velocity of a moving mechanism during machining. According to the expression (9), the time constant dt m  of the laser machining can be understood as being obtained by dividing “surface roughness” that is to be a target length as some machining phenomenon by a machining velocity. 
     
       
         
           
             
               
                 
                   [ 
                   
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     9 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     dt 
                     m 
                   
                   = 
                   
                     
                       dz 
                       0 
                     
                     
                       v 
                       L 
                     
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
     The time constant dt m  of laser machining defined in this manner and the switching period dt p  have a relation represented by the following expression (10). 
     
       
         
           
             
               
                 
                   [ 
                   
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     10 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     d 
                     ⁢ 
                     
                       t 
                       m 
                     
                   
                   = 
                   
                     
                       
                         dz 
                         0 
                       
                       
                         v 
                         L 
                       
                     
                     ≥ 
                     
                       
                         dt 
                         p 
                       
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
           
         
       
     
     The relation of the above expression (10) means the following matters. In a case of relatively moving the workpiece  300  and laser light with respect to each other to perform laser machining, it is necessary to make the switching period dt p  of the laser diode driving power source  100  smaller than a value that is twice the time constant dt m  for the laser machining, that is, a value obtained by dividing the required surface roughness dz 0  by the moving velocity v L  and doubling a result of the division in order to achieve the required surface roughness. 
     It has been already described that the switching period dt influences the machining quality. In addition, a time constant other than the time constant dt m  for the laser machining may influence the machining in some cases. In a case where feedback control is performed using a detected current value, since the laser diode driving power source  100  controls an output current, it is necessary to consider not only the switching period dt but also a feedback time from detection of the current value until the current value is reflected in a switching operation, that is, a time required for processing in a feedback control system included in the power converter  10 . Therefore, when an integration process is performed with a sufficiently long time constant in a control system, it is necessary to use a value obtained by adding a response time constant of the control system to the switching period dt as the time constant dt m  of laser machining. That is, the time constant dt m  for laser machining includes the response time constant of the feedback control system. In a control unit according to the present embodiment, when a current flowing through a laser diode is detected and feedback control is executed, a value smaller than a value obtained by doubling a time constant for laser machining is set as a sum of a response time constant of a feedback control system owned by a laser diode driving power source and a switching period. 
     The laser diode driving power source  100  according to the present embodiment can reduce influences of fluctuation of a current on a machined-surface roughness by making a time amount obtained by summing a feedback time and a switching period smaller than a value that is twice the time constant dt m  for the laser machining. 
     In the case of feedback control, it may be necessary to consider a delay caused by a time constant of a feedback system, and a time corresponding to several switching periods may be sometimes required until a current value becomes stable depending upon a control system, so that there is a disadvantage in respect of controlling the current value at a high speed in many cases. On the other hand, in the case of feedforward control, that is, in a case of performing switching with a switching width that is predetermined with respect to a target current value, or performing switching in a predetermined switching pattern, it is not necessary to perform feedback, so that the influences of fluctuation of a current on the machined-surface roughness can be further reduced. 
     A configuration example of a laser diode driving power source using a power converter of a constant current system has been described above. However, the laser diode driving power source according to the present embodiment may use a power converter of a constant voltage system. The following is description for a configuration example of a laser diode driving power source using a power converter of a constant voltage system. 
       FIG. 11  is a diagram illustrating a modification of the laser diode driving power source according to the present embodiment. A laser diode driving power source  100 A illustrated in  FIG. 11  is a boost chopper type switching power source of a constant voltage system. The laser diode driving power source  100 A includes a power converter  10 A and a control unit  11 A. In the control unit  11 A, a value smaller than a value that is twice a time constant for laser machining is set as a switching period of a switching device, as with the control unit  11  illustrated in  FIG. 1 . 
     The power converter  10 A includes the rectifier circuit  1 , the capacitor  2 , a reactor  5   a , a switching device  3   a , a diode  4   a , a capacitor  15 , and a voltage detector  14 . The control unit  11 A includes an LPF (Low Pass Filter)  37  in addition to the constituent elements of the control unit  11 - 1  illustrated in  FIG. 6 . 
     The voltage detector  14  detects an inter-end voltage of the capacitor  15 . The detected voltage is used for feedback control in the control unit  11 A. In a case where the capacitance of the capacitor  15  is set to a sufficiently large value, an output voltage is kept constant and no voltage ripple is generated. Therefore, it is assumed that the capacitance of the capacitor  15  is set to have a relatively small value. That is, it is assumed that the capacitor  15  does not have sufficient capacitance with respect to an output voltage of the power converter  10 A. Further, although the control unit  11 A executes feedback control on the output voltage of the power converter  10 A, the voltage fluctuates and therefore the LPF  37  is provided between the amplifier  31  and the error amplifying circuit  32 . 
       FIG. 12  is a diagram illustrating current and voltage waveforms of the LD illustrated in  FIG. 11 . As illustrated in  FIG. 12 , in the current and voltage waveforms of the LD  20 , an LD current is largely changed even when an LD voltage is slightly changed because of the current-voltage characteristics of the LD  20 . That is, this situation means that slight fluctuation occurs in the LD voltage because of insufficient capacitance of the capacitor  15  though the LD voltage is controlled to be constant by the laser diode driving power source  100 A. Therefore, because of the current-voltage characteristics of the LD  20 , the LD current largely fluctuates and a current ripple is generated therein, so that the current ripple reaches a level at which the current ripple causes influences on the machining. To reduce the influences on machining, it is necessary that a switching frequency of the laser diode driving power source  100 A is sufficiently high. 
     Although a switching period of a switching device is set to a value smaller than a value that is twice a time constant of laser machining in the present embodiment, the switching period of the switching device is not limited thereto, but may be set to have any value smaller than a value that is n times the time constant of laser machining, where n is a value larger than 1. As described above, as long as the switching period of the switching device is set to a value smaller than a value that is n times the time constant of laser machining, influences of fluctuation of a current on a machined-surface roughness can be reduced. 
     The laser machining device  200  according to the present embodiment may be provided with an input screen on which items for allowing input of the surface roughness dz and the moving velocity v L  are displayed. By providing this input screen, it is possible to input any values for the surface roughness dz and the moving velocity v L , and further even in a case where the surface roughness dz and the moving velocity v L  are changed, the influences of fluctuation of a current on the machined-surface roughness can be suppressed. 
     The configurations described in the above embodiment are only examples of the content of the present invention. The configurations can be combined with other publicly known techniques and partially omitted and/or modified without departing from the scope of the present invention. 
     REFERENCE SIGNS LIST 
       1  rectifier circuit,  2  capacitor,  3 ,  3   a  switching device,  4  chopper diode,  4   a  diode,  5 ,  5   a  reactor,  6  inverter,  7  transformer,  10 ,  10 - 1 ,  10 - 2 ,  10 A power converter,  11 ,  11 - 1 ,  11 - 2 ,  11 - 3 ,  11 A control unit, machining command,  13  current detector,  14  voltage detector,  15  capacitor,  20  LD,  21  fiber,  22  machining head,  23  lens,  30  riving circuit,  31  amplifier,  32  error amplifying circuit,  33  PWM generating circuit,  34  triangle-wave generator,  35  current command value,  36  comparator,  37  LPF,  41  oscillator,  42  PWM signal generator,  43  AD converter,  44  current command value,  46  controller,  100 ,  100 A laser diode driving power source,  200  laser machining device,  300  workpiece,  400  alternating-current power source, R 1 , R 2  resistor.