Patent Publication Number: US-2022239229-A1

Title: High-frequency power supply device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. § 119 to Japanese Patent Application No. 2021-011930 filed on Jan. 28, 2021, the entire disclosure of which is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a self-oscillation high-frequency power supply device. 
     Description of the Related Art 
     An example of a conventional self-oscillating high-frequency power source device is disclosed in Patent Document 1. The transformer of the high-frequency power supply device disclosed in Patent Document 1 is configured such that the primary coil and the secondary coil is formed in a coaxial configuration. The semiconductor of the high-frequency power supply device is driven by a voltage applied via the transformer. 
     PRIOR ART DOCUMENT 
     Patent Document 
     
         
         Patent Document 1: Japanese Unexamined Patent Application Publication No. 2016-51556 
       
    
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     In the above-described high-frequency power supply device, it is preferable to separately provide a configuration for appropriately setting the current flowing through the transformer because it is difficult to adjust the turn ratio of the primary coil to the secondary coil due to the structure of the transformer. 
     The present invention has been made in view of the above-described circumstances. An object of the present invention is to provide a high-frequency power supply device capable of causing an appropriate current to flow through a transformer while suppressing an excessive current from flowing through the transformer. 
     Means for Solving the Problem 
     According to a first aspect of the present invention, the present invention relates to a self-oscillation high-frequency power supply device equipped with a DC power supply, an LC resonant circuit, a switching circuit, and a transformer. The LC resonant circuit includes an induction coil for plasma generation and a capacitor. The switching circuit includes a semiconductor element, the switching circuit being configured to subject DC power supplied from the DC power supply to switching processing to supply high-frequency power to the LC resonant circuit. The transformer includes a primary coil included in the LC resonant circuit and a secondary coil connected to the semiconductor element to turn on/off the semiconductor element. The transformer is formed in a coaxial structure in which the primary coil and the secondary coil are coaxially provided. The LC resonant circuit includes a resistor connected in parallel to the primary coil. 
     Effects of the Invention 
     According to the first aspect of the present invention, the resistor is connected in parallel to the primary coil of the transformer. Therefore, it is possible to cause an appropriate current to flow through the transformer while suppressing an excessive current from flowing through the transformer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram showing a configuration example of a high-frequency power supply device of this embodiment. 
         FIG. 2  is a schematic diagram for explaining an inner configuration of a transformer of this embodiment. 
         FIG. 3  is an example of a partial plan view showing a configuration example of a MOSFET and therearound of this embodiment. 
         FIG. 4  is a cross-sectional view taken along the line A-A in  FIG. 3 . 
         FIG. 5  is a cross-sectional view taken along the line B-B in  FIG. 3 . 
         FIG. 6  is another example of a partial plan view showing a configuration example of a MOSFET and therearound of this embodiment. 
         FIG. 7  is an example of a circuit diagram showing a configuration example of an LC resonant circuit. 
         FIG. 8  is still another example of a partial plan view showing a configuration example of a MOSFET and therearound of this embodiment. 
         FIG. 9  is a schematic cross-sectional diagram showing an overall configuration of the high-frequency power supply device shown in  FIG. 1 . 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     1. Electrical Configuration of High-Frequency Power Supply Device 
       FIG. 1  is a circuit diagram showing a configuration example of a high-frequency power supply device according to this embodiment. This high-frequency power supply device is applicable to an analyzer, such as, e.g., an inductively coupled plasma (ICP) emission analyzer. This high-frequency power supply device is a self-oscillation high-frequency power supply device and is provided with a DC power supply  1 , a by-pass capacitor  2 , a switching circuit  3 , an impedance conversion circuit  4 , an LC resonant circuit  5 , and the like. 
     The DC power supply  1  is configured to set a DC voltage of the switching circuit  3  and determines high-frequency power to be supplied to the LC resonant circuit  5 . The by-pass capacitor  2  is provided between the DC power supply  1  and the switching circuit  3  to ensure a low-impedance and high-frequency current path. 
     The LC resonant circuit  5  includes an induction coil  51  and a capacitor  52  connected to the induction coil  51 . The induction coil  51  included in the LC resonant circuit  5  is for plasma generation. By supplying high-frequency power to the induction coil  51  from the DC power supply  1  via the switching circuit  3 , plasma can be generated at the plasma torch (not illustrated). 
     The impedance conversion circuit  4  includes two coils  41  and  42  and a capacitor  43  connected in series between these coils  41  and  42 . Between the switching circuit  3  and the impedance conversion circuit  4 , a loop including the coils  41  and  42  and the capacitor  43  of the impedance conversion circuit  4  is formed. Further, between the impedance conversion circuit  4  and the LC resonant circuit  5 , a loop including the capacitor  43  of the impedance conversion circuit  4  and the induction coil  51  and the capacitor  52  of the LC resonant circuit  5  is formed. 
     The switching circuit  3  is configured to include semiconductor elements and is connected to the DC power supply  1  via the semiconductor elements. In this example, the switching circuit  3  is configured by a bridge circuit including four MOSFETs (Metal Oxide Semiconductor Field-Effect Transistors)  31  ( 31   a ,  31   b ,  31   c ,  31   d ). The switching circuit  3  is configured by a bridge circuit of a half-bridge type or a full-bridge type. Note that the semiconductor element included in the switching circuit  3  is not limited to a MOSFET. Further note that the switching circuit  3  is not limited to a half-bridge type or a full-bridge type. 
     Between the drain electrode of the MOSFET  31   a  and the source electrode of the MOSFET  31   b , the DC power supply  1  and the by-pass capacitor  2  are connected. Further, the source electrode of the MOSFET  31   a  and the drain electrode of the MOSFET  31   b  are connected by a wiring  35 , and a coil  41  of the impedance conversion circuit  4  is connected to the middle portion of the wiring  35 . 
     Between the drain electrode of the MOSFET  31   c  and the source electrode of the MOSFET  31   d , the DC power supply  1  and the by-pass capacitor  2  are connected. Further, the source electrode of the MOSFET  31   c  and the drain electrode of the MOSFET  31   d  are connected by a wiring  36 , and the coil  42  of the impedance conversion circuit  4  is connected to the middle portion of the wiring  36 . 
     A gate drive circuit  32  ( 32   a ,  32   b ,  32   c ,  32   d ) is connected to a gate electrode of each MOSFET  31   a ,  31   b ,  31   c ,  31   d . Thus, the MOSFET  31  is switched to the ON state or the OFF state at a predetermined timing by the gate drive circuit  32 . Thus, it is possible to subject the DC power supplied from the DC power supply  1  to switching processing to provide high-frequency power to the LC resonant circuit  5 . 
     Each gate drive circuit  32   a ,  32   b ,  32   c ,  32   d  is provided with a coil  33  ( 33   a ,  33   b ,  33   c ,  33   d ) and a capacitor  34  ( 34   a ,  34   b ,  34   c ,  34   d ) connected in parallel to each other. The coil  33  provided to the gate drive circuit  32  constitutes a secondary coil of each transformer and is connected to the control terminals (the gate electrode, the source electrode) of the MOSFET  31  to turn the MOSFET  31  on/off (switching). The primary coil of each transformer is constituted by a coil  53  ( 53   a ,  53   b ,  53   c ,  53   d ) included in the LC resonant circuit  5 . 
     As described above, in this embodiment, the transformer configured by a pair of the coil  53  and the coil  33  is provided in association with each MOSFET  31 , so that a feedback voltage can be supplied to each MOSFET  31 . 
     Further, in this embodiment, the transformer is configured by the coil (primary coil)  53  and the coil (secondary coil)  33 . Therefore, when an excessive current flows through the primary coil  53 , an excessive current also flows through the secondary coil  33 , which may adversely affect the MOSFET  31 . 
     In this embodiment, as a configuration for appropriately setting the current flowing through the transformer, a resistor  54  ( 54   a ,  54   b ,  54   c ,  54   d ) is connected in parallel to each transformer, specifically each primary coil  53   a ,  53   b ,  53   c ,  53   d . Further, the coil  53  and the resistor  54  connected in parallel are connected in series to the induction coil  51  and the capacitor  52 . 
     The resistor  54  is an electric conductor having a resistance value. That is, the resistor  54  can also be said to be a current flow path having a resistance value. As the resistor  54 , a metallic plate, a resistive element, and the like can be exemplified. 
     As described above, by connecting the resistor  54  in parallel to the primary coil  53 , it is possible to cause an appropriate current to flow through the transformer while suppressing an excessive current from flowing through the transformer. 
     2. Internal Configuration of Transformer 
       FIG. 2  is a schematic diagram for explaining the inner configuration of the transformer  6  of this embodiment. The transformer  6  is configured by the primary coil  53  provided in the LC resonant circuit  5  and the secondary coil  33  provided in the gate drive circuit  32 . 
     In this embodiment, the transformer  6  is configured by a semi-rigid coaxial cable. The semi-rigid coaxial cable is coaxially provided with a linear center conductor  61 , a cylindrical insulator  62  covering the outer side of the center conductor  61 , and a cylindrical outer conductor  63  covering the outer side of the insulator  62 . The primary coil  53  of the transformer  6  is configured by the outer conductor  63 , and the secondary coil  33  is configured by the center conductor  61 . 
     With this, the transformer  6  is formed in a coaxial structure in which the primary coil  53  and the secondary coil  33  are coaxially provided, and even in a case where the transformer  6  is curbed or bent, the state in which the primary coil  53  and the secondary coil  33  are arranged in parallel to each other is maintained. Note that it may be configured such that the primary coil  53  of the transformer  6  is configured by the center conductor  61  and that the secondary coil  33  is configured by the outer conductor  63 . 
     3. Configuration Around MOSFET 
       FIG. 3  is an example of a partial plan view showing a configuration example of the MOSFET  31  and therearound of this embodiment.  FIG. 4  is a cross-sectional view taken along the line A-A in  FIG. 3 . Further,  FIG. 5  is a cross-sectional view taken along the line B-B in  FIG. 3 . 
     Further, although only the configuration of the periphery of one of MOSFETs  31  out of the MOSFET  31   a , etc., is shown in  FIG. 3  to  FIG. 5 , the same configuration is applied to the configuration of the periphery of each MOSFET  31 . 
     In this embodiment, each circuit, such as, e.g., the switching circuit  3 , the impedance conversion circuit  4 , and the LC resonant circuit  5 , is mounted on the circuit board  10 . The components included in each circuit are electrically connected to each other by pattern wirings  11  and pattern wirings  12 . The current flowing through the induction coil  51  for plasma generation is very large. Therefore, if the width of the pattern wiring  11  is too small, too large heat is generated, which is unacceptable. Therefore, it is preferable to use a wide pattern as the pattern wiring  11 . Note that the circuit board  10  may be a single sheet or may be divided into two or more sheets. 
     Each transformer  6  is formed in a U-shape. The U-shape means a shape in which it is curved or bent such that both the ends  60   a  and  60   b  of the transformer  6  are closely arranged to each other, which is a conception including other shapes formed substantially in a U-shape such as a semicircular shape. The transformer  6  configured by a semi-rigid coaxial cable does not have a protective covering on the outer side of the outer conductor  63  and is in a state in which the outer conductor  63  is exposed. The primary coil  53  of each transformer  6  is connected to the pattern wiring  11 . On the other hand, the input end (IN) and the output end (OUT) of the secondary coil  33  of the transformer  6  and the control terminals of the MOSFET  31  (the gate electrode  301 , the source electrode  302 ) are electrically connected by a pattern wiring  12  on a side opposite to the side of the pattern wiring  11  on the circuit board  10 . 
     Further, in this embodiment, the transformer  6  is formed in a U-shape. Thus, it is possible to suppress the attenuation of the feedback voltage due to the pattern inductance by shortening the pattern wiring  12  between the input end (IN) and the output end (OUT) of the secondary coil  33  of the transformer  6  and the control terminal (the gate electrode  301 , the source electrode  302 ) of the MOSFET  31  while securing the length capable of generating a sufficient feedback voltage. Further, by placing the primary coil  53  and the secondary coil  33  of the transformer  6  parallel, the degree of coupling of the primary coil  53  and the secondary coil  33  increases. 
     With these structural improvements, a sufficient feedback voltage can be supplied to the MOSFET  31  via the transformer  6 . That is, it is possible to provide a sufficient current to each MOSFET  31  via the transformer  6 . 
     As described above, the feedback voltage can be increased, and therefore, this embodiment eliminates the need to apply a DC bias voltage to the control terminal of each MOSFET  31 . That is, in a case where the semiconductor element of the switching circuit  3  is configured by the MOSFETs  31  as in this embodiment, the amplitude of the gate voltage of each MOSFET  31  can be increased, thereby eliminating the necessity to apply a DC bias voltage to the gate electrode of each MOSFET  31 . 
     Furthermore, as shown in  FIG. 1 , by connecting the secondary coil  33  and the capacitor  34  in parallel to the control terminals of the MOSFET  31 , it is possible to reduce the variations of the gate-source capacitance of each MOSFET  31  and the effects of variations thereof. Further, the voltage of the control terminal of each MOSFET  31  having a nonlinear gate charging characteristic changes more linearly, thereby suppressing the ringing of the switching waveform due to the inductance of the load, which enables to realize a high-power supply efficiency. 
     Further, in this embodiment, as shown in  FIG. 3  and  FIG. 5 , each resistor  54  is connected to the pattern wiring  11 . The transformer  6  is formed in a coaxial structure in which the primary coil  53  and the secondary coil  33  are arranged coaxially, and therefore, it is difficult to adjust the magnitude of the current flowing through the primary coil  53  and the secondary coil  33  by adjusting the winding ratio thereof. However, for example, by simply replacing the resistor  54  to another resistor  54  having a different resistance value, it is possible to easily adjust the magnitude of the current flowing through the primary coil  53  and the secondary coil  33 . 
     Further, the resistor  54  differs in the resistance value when the length of the current flow path differs even in the case of using an electric conductor of the same material. Specifically, even in the case of using an electrical conductor of the same material, an electrical conductor having a longer current flow path is larger in resistance. Therefore, in an electric conductor of a given material, the longer the current path of the resistor  54 , the greater the resistance of the resistor  54 . 
     Like this embodiment, in a case where the resistor  54  is formed in a U-shape in the same manner as in the transformer  6 , for example, even if the connecting positions of the end portions  540   a  and  540   b  of the resistor  54  are arranged closely to each other, an adequate resistance value can be secured. 
     Further, the resistor  54  differs in resistance value if the cross-sectional area of the current flow path differs even in the case of using an electric conductor of the same material. Specifically, even in the case of using an electrical conductor of the same material, an electrical conductor smaller in the cross-sectional area of the current flow path is larger in the resistance value. Therefore, in an electric conductor of a predetermined material, the smaller the cross-sectional area of the current path of the resistor  54 , the larger the resistance of the resistor  54 . 
     As will be apparent from the above, the resistance value of the resistor  54  is determined by the material and the shape. Note that in this embodiment, in a case where the resistance value of the resistor  54  is extremely smaller than the value of the parasitic resistance of the primary coil  53 , the magnitude of the current flowing through the resistor  54  increases. In other words, the current flowing through the primary coil  53  becomes extremely small. In this case, there is a possibility that the voltage (current) required to drive the MOSFET  31  is not supplied to the MOSFET  31 . 
     In this embodiment, as shown in  FIG. 3  to  FIG. 5 , the resistor  54  is formed in a U-shape similar to the transformer  6 , but the material and the shape of the resistor  54  are not particularly limited as long as a voltage equal to or greater than the minimum voltage required to drive the MOSFET  31  is supplied. 
     Further, as shown in  FIG. 3  to  FIG. 5 , in this embodiment, the transformer  6  and the resistor  54  extend in parallel to each other in a state of facing each other. The insulating heat sink  20  is located between the transformer  6  and the resistor  54  and is in contact with the transformer  6  and the resistor  54 . 
     Each of the transformer  6  and the resistor  54  is curved or bent in mutually parallel planes F 1  and F 2  and is in contact with the heat sink  20  in these planes F 1  and F 2 . 
     4. Modification of Heat Sink and LC Resonant Circuit 
     As shown in  FIG. 3  to  FIG. 5 , a single heat sink  20  may be in contact with a single pair of the transformer  6  and the resistor  54  on the circuit board  10 . Alternatively, as shown in  FIG. 6 , a single heat sink  20  may be in contact with a plurality of pairs of the transformer  6  and the resistor  54 . Note that  FIG. 6  shows another example of a partial plan view showing the configuration example of the MOSFETs  31  and therearound according this embodiment. Specifically,  FIG. 6  shows the configuration of the two MOSFET  31  and MOSFET  31  out of the MOSFET  31   a  and the like. 
     Further, as in the LC resonant circuit  5  shown in  FIG. 7 , the resistor  54  may be connected in parallel to a plurality of primary coils  53 . For example, as shown in  FIG. 7 , in a case where the resistor  54  is connected in parallel to two primary coils  53 , the periphery of the MOSFETs  31  can be configured as shown in  FIG. 8 . Note that  FIG. 7  is an example of the circuit diagram showing a configuration example of the LC resonant circuit  5 . Further,  FIG. 8  is still another example of a partial plan view showing the configuration example of the MOSFETs  31  and therearound of this embodiment. Furthermore, in  FIG. 8 , it specifically shows the configuration of the periphery of the two MOSFETs  31  out of the MOSFET  31   a  and the like. 
     As will be understood from the above, in this embodiment, as long as the heat sink  20  is provided on the circuit board  10 , it is possible to cool the respective transformers  6  and the respective resistors  54  by at least a single heat sink  20 . 
     The heat sink  20  is configured by, for example, a block made of aluminum nitride. However, the heat sink  20  may be made of a material other than aluminum nitride as long as the material has high thermal conductivity and is an insulating material. 
     In this embodiment, since the transformer  6  is configured by a coaxial cable, the outer surface of the transformer  6  is a circumferential surface. Therefore, in a case where the contact surface of the heat sink  20  which comes in contact with the transformer  6  is formed in a flat surface, the heat sink  20  comes in contact with only a small part of the coaxial cable, which may result in insufficient heat dissipation. For this reason, it is preferable to increase the contact area between the heat sink  20  and the transformer  6 , for example, by applying a thermal grease to the periphery of the contact portion between the transformer  6  and the heat sink  20 , or by making the contact surface of the heat sink  20  that comes into contact with the transformer  6  to be a concave surface corresponding to the outer surface of the transformer  6 . The same applies to the resistor  54 . 
     5. Overall Configuration of High-Frequency Power Supply Device 
       FIG. 9  is a schematic cross-sectional view illustrating the overall configuration of the high-frequency power supply device of  FIG. 1 . This high-frequency power supply device has a configuration provided with various components in a hollow housing  100 . The housing  100  is provided with an insertion hole for inserting cables but not provided with other openings, and therefore, the housing  100  is substantially in a sealed state. That is, the housing  100  is not provided with other openings, such as, e.g., an inlet for sucking air into the housing  100  and an outlet for discharging air from the housing  100 . 
     The space within the housing  100  is partitioned by one or more partition walls  101 . The circuit board  10  on which circuits, such as, e.g., the switching circuit  3 , the impedance conversion circuit  4 , and the LC resonant circuit  5 , are mounted is held by, for example, the partition walls  101 . Thus, various components, such as, e.g., the switching circuit  3 , the LC resonant circuit  5 , and the transformer  6 , are accommodated in the housing  100 . 
     Note that the induction coil  51  included in the LC resonant circuit  5  is not accommodated in the housing  100  but is disposed inside a plasma stand (not illustrated). The circuit board  10  accommodated in the housing  100  and the induction coil  51  provided outside the housing  100  are connected by an electric conductor, such as, e.g., a copper plate. The induction coil  51  and the plasma stand are partitioned by an insulator, such as, e.g., fluoropolymer. 
     The heat sink  23  provided in the housing  100  is mounted on the circuit board  10 . The heat sink  23  is, for example, a water-cooled system and is provided with a main body portion  21  through which a refrigerant flows and a plurality of heat dissipation fins  22  protruding from the main body portion  21 . The heat sink  23  mainly cools the MOSFETs  31  mounted on the circuit board  10 . The main body portion  21  may be brought into contact with, for example, the circuit board  10  to cool the circuit board  10  and the components mounted on the circuit board  10 . However, it is insufficient because the heat dissipation is performed via the insulating material of the circuit board  10 . 
     For this reason, in the housing  100 , in addition to the circuit board  10  and the heat sink  23  described above, an air cooling fan  40  is provided. The air cooling fan  40  is held by, for example, a partition wall  101  and faces a side of the circuit board  10  opposite to the heat sink  23  side. Thus, when the air cooling fan  40  is rotationally driven, the air in the housing  100  is blown from the air cooling fan  40  against the circuit board  10 . 
     The space in the housing  100  partitioned by the partition wall  101  constitutes a circulation path  102  for circulating the air in the housing  100 . That is, the air blown out of the outlet  401  of the air cooling fan  40  is exposed to the circuit board  10  and the heat sink  23  provided in the middle of the circulation path  102 , sucked by the air cooling fan  40  via the inlet  402 , and then blown out of the outlet  401  again. 
     The plurality of heat dissipation fins  22  provided to the heat sink  23  each extends along the flow direction of the air in the circulation path  102  and is arranged in parallel to each other with a space therebetween. As described above, the heat sink  23  (heat dissipation fin  22 ) is arranged in the middle of the circulation path  102 , and therefore, it is possible to circulate the air in the housing  100  by the air cooling fan  40  while guiding the air to the heat sink  23 . 
     As described above, in this embodiment, components, such as, e.g., the LC resonant circuit  5  except for the induction coil  51 , the switching circuit  3 , and the transformer  6 , are accommodated in the housing  100 , and the air in the housing  100  is circulated by the air cooling fan  40 . Therefore, the air outside the device is less likely to flow into the device. Thus, it is possible to prevent the components in the device from being contaminated by the air outside the device. 
     In addition, the components in the housing  100  are cooled by the water-cooled heat sink  23 , and the air to be circulated in the housing  100  by the air cooling fan  40  is circulated while being cooled by the water-cooled heat sink  23 . Therefore, high-efficiency cooling can be realized. For this reason, even in a configuration in which the air outside the device is less likely to flow into the device, it is possible to cool the components in the device well. 
     6. Other Modification 
     In the above-described embodiment, an example is described in which the LC resonant circuit  5  having the induction coil  51  is configured by the series resonant circuit in which the capacitor  52  is connected in series to the induction coil  51 . However, the present invention is not limited to such a configuration, and the LC resonant circuit  5  may be configured by a parallel resonant circuit in which the capacitor  52  is connected in parallel to the induction coil  51 . 
     The transformer  6  is not limited to a configuration constituted by a semi-rigid coaxial cable and may be constituted by other coaxial cables. In a case where a coaxial cable having a protective covering is used as the transformer  6 , it is preferable to use it by removing the protective covering. However, the present invention is not limited to the configuration using a coaxial cable and may be configured such that only the primary coil  53  and the secondary coil  33  of the transformer  6  are arranged coaxially. 
     Further, as the resistor  54 , a component provided to the high-frequency power supply device, specifically an electrically connectable component, i.e., a component having an electric conductivity and a resistance value may be used. 
     For example, in a case where the high-frequency power supply device is provided with a fan with a general-purpose metallic housing and, the fan with the general-purpose metallic housing is used as the resistor  54 , by arranging the fan such that the heat sink  20  is exposed to the air blown out of the fan, the cooling efficiency of the heat sink  20  can be improved. 
     Further, in the plurality of circuit boards  10 , even if various circuits are configured in the same manner, due to the differences in the qualities of the components, the value of the current flowing through the transformer  6  may differ slightly. In a case where a variable resistor is used as the resistor  54 , even if the current flowing through the transformer  6  differs from the original value, it is possible to adjust the current flowing through the transformer  6  as appropriate without replacing the resistor  54 . In other words, it is easy to maintain a certain level of quality in the production of the high-frequency power supply device. 
     The high-frequency power supply device according to the present invention is applicable not only to an ICP luminescence analyzer but also to other analyzers that perform an analysis using plasmas. Further, the high-frequency power supply device according to the present invention is applicable not only to an analyzer but also to other devices using plasma (e.g., high-frequency oscillation circuit for plasma CVD). 
     7. Aspects 
     It is understood by those skilled in the art that the embodiments described above are specific examples of the following aspects. 
     (Item 1) 
     A self-oscillation high-frequency power supply device, including: 
     a DC power supply; 
     an LC resonant circuit including an induction coil for plasma generation and a capacitor; 
     a switching circuit including a semiconductor element, the switching circuit being configured to subject DC power supplied from the DC power supply to switching processing to supply high-frequency power to the LC resonant circuit, and 
     a transformer including a primary coil included in the LC resonant circuit and a secondary coil connected to the semiconductor element to turn on/off the semiconductor element. 
     The transformer is formed in a coaxial structure in which the primary coil and the secondary coil are coaxially provided. 
     The LC resonant circuit includes a resistor connected in parallel to the primary coil. 
     According to the self-oscillation high-frequency power supply device as recited in the above-described Item 1, with the resistor connected in parallel to the primary coil of the transformer, it is possible to cause an appropriate current to flow through the transformer while suppressing an excessive current from flowing through to the transformer. 
     (Item 2) 
     In the self-oscillation high-frequency power supply device as recited in above-described Item 1, 
     the transformer and the resistor may be each formed in a U-shape. 
     According to the self-oscillation high-frequency power supply device as recited in the above-described Item 2, it is possible to suppress the attenuation of the feedback voltage due to the pattern inductance by shortening the wiring between the input end and the output end of the secondary coil of the transformer and the semiconductor while securing the length as long as possible to generate a sufficient feedback voltage. Further, it is possible to ensure a sufficient resistance value even if the connecting positions of the ends of the resistor are closely arranged. 
     (Item 3) 
     In the self-oscillation high-frequency power supply device as recited in above-described Item 1, it may be further provided with: 
     a heat sink with electrical insulation properties. 
     The heat sink may be in contact with both the transformer and the resistor. 
     According to the self-oscillation high-frequency power supply device as recited in the above-described Item 3, since the heat generation from the transformer and the resistor is dissipated through the heat sink, it is possible to achieve a high heat dissipation efficiency. 
     (Item 4) 
     In the self-oscillation high-frequency power supply device as recited in above-described Item 3, 
     the transformer and the resistor may extend in parallel to each other in a state of facing each other, and 
     the heat sink may be positioned between the transformer and the resistor. 
     According to the self-oscillation high-frequency power supply device recited in the above-described Item 4, it is possible to cool the transformer and the resistor with at least one heat sink. 
     (Item 5) 
     In the self-oscillation high-frequency power supply device as recited in any one of the above-described Items 1 to 4, 
     the resistor may be a variable resistor. 
     According to the self-oscillation high-frequency power supply device as recited in the above-described Item 5, it is possible to adjust the current flowing through the transformer without replacing the resistor.