Patent Publication Number: US-2021166863-A1

Title: Reactor structure

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present disclosure relates to a reactor structure. 
     Description of the Background Art 
     For example, an electric vehicle such as an electric car or a plug-in hybrid vehicle has a power conversion device for driving a motor using power from a high-voltage battery as motive power. In the power conversion device, a reactor is used for various purposes such as smoothing power or stepping up/down voltage. 
     In a reactor of a power conversion device for an electric vehicle that requires large power density, loss density is large and forced cooling is performed by using a filler such as a potting material. Here, the loss is loss in the reactor, and specifically, refers to loss occurring in a winding and a core forming the reactor. 
     One of conventional reactors includes: a reactor body having a core and a coil mounted onto the core; a case storing the reactor body and having an opening through which a part of the reactor body protrudes to the outside; a bus bar which is a conductive member electrically connected to the coil and covers a part of a side surface of the reactor body protruding from the opening; and a terminal block having an extending portion provided along an edge of the opening and formed by a resin material in which a part of the bus bar is embedded, the terminal block supporting a part electrically connecting the bus bar and the outside (see Patent Document 1). 
     In Patent Document 1, the following configuration is often adopted. That is, the core and the coil are mounted onto the case or the like having a dug part for preventing the filler from flowing, and the filler is poured and solidified. Then, the case is attached to a cooler of the power conversion device, and thus the coil and the core which are heat generating bodies are cooled by the cooler via the filler and the case. 
     Patent Document 1: Japanese Laid-Open Patent Publication No. 2019-121665 
     The size of the reactor is restricted by factors due to heat dissipation property and loss. In order to downsize the reactor, it is necessary to improve heat dissipation property and reduce loss. Factors influencing loss include ripple current. In order to reduce loss, it is necessary to increase the inductance value and thus reduce ripple current. However, in general, it is necessary to enlarge the size of the reactor in order to increase the inductance value. Meanwhile, in order to improve the heat dissipation property, it is conceivable that the case is made of metal and the coil and the core which are heat generating bodies are located as close to the case as possible so that the thermal resistance is reduced. However, the metal member blocks a leakage magnetic flux of the reactor, so that the inductance value is reduced, and loss is increased. 
     SUMMARY OF THE INVENTION 
     The present disclosure has been made to solve the above problem, and an object of the present disclosure is to provide a reactor structure that increases an inductance value without blocking a leakage magnetic flux and improves cooling performance by directly cooling a coil and a core by a cooler via a cooling member. 
     A reactor structure according to the present disclosure has a core wound by a coil, a winding cooling portion for cooling the coil is in contact with a cooler via a coil cooling member formed by non-fluid material, a core cooling portion for cooling the core is in contact with the cooler via a core cooling member formed by non-fluid material, and a resin mold member covering the coil and the core retains the coil and the core and fixes the coil and the core to the cooler. 
     The reactor structure according to the present disclosure can increase the inductance value without blocking the leakage magnetic flux and improve cooling performance by directly cooling the coil and the core by the cooler via the cooling member. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram showing the configuration of a power conversion device according to embodiment 1; 
         FIG. 2  is an exploded perspective view showing a reactor structure according to embodiment 1; 
         FIG. 3  is a perspective view showing the structure of a reactor body according to embodiment 1; 
         FIG. 4  is a perspective view showing the structure of the reactor body according to embodiment 1; 
         FIG. 5  is a sectional view showing a reactor according to embodiment 1; 
         FIG. 6  is a perspective view showing the structure of a core of the reactor according to embodiment 1; 
         FIG. 7  is a perspective view showing the structure of a comparative reactor; 
         FIG. 8  is a sectional view of the reactor according to embodiment 1, along a plane perpendicular to an X-axis direction; 
         FIG. 9  is a sectional view showing a comparative reactor structure, along a plane perpendicular to the X-axis direction; 
         FIG. 10  is a graph showing the relationship between the inductance value and the frequency; and 
         FIG. 11  is a side view showing the case in which a magnetically coupled reactor is used as a reactor according to embodiment 2. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiment 1 
     Hereinafter, a power conversion device according to the present embodiment will be described with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference characters, and will not be repeatedly described. The present embodiment aims to achieve size reduction and cost reduction of a reactor used in a power conversion device.  FIG. 1  is a schematic diagram showing the configuration of the power conversion device according to embodiment 1. In  FIG. 1 , the power conversion device  2  is a single-switch-type boost DC/DC converter which boosts the voltage of DC power from a DC input power supply  1  and supplies the power to a load  3 . 
     The power conversion device  2  includes a boost reactor  4 , semiconductor switching elements  5   a ,  5   b , an input power smoothing capacitor  6 , and an output power smoothing capacitor  7 . The semiconductor switching elements  5   a ,  5   b  are connected in series to each other, and a connection point (neutral point) N therebetween is connected to one terminal of a winding of the boost reactor  4 . Another terminal of the winding of the boost reactor  4  on the side that is not connected to the connection point N between the semiconductor switching elements  5   a ,  5   b  is connected to a positive terminal of the input power smoothing capacitor  6 . A terminal of the semiconductor switching element  5   a  on the side that is not connected to the neutral point N is connected to a positive terminal of the output power smoothing capacitor  7 . A terminal of the semiconductor switching element  5   b  on the side that is not connected to the neutral point N is connected to a cathode terminal of the output power smoothing capacitor  7  and a cathode terminal of the input power smoothing capacitor  6 . 
     By switching operations of the semiconductor switching elements  5   a ,  5   b , the boost reactor  4  repeatedly stores/discharges electric energy as magnetic energy, whereby boosting operation is performed. Here, the operation principle of the boost DC/DC converter is commonly well-known, and therefore the description thereof is omitted. 
       FIG. 2  is an exploded perspective view showing the structure of the boost reactor  4 . In  FIG. 2 , the boost reactor  4  includes a boost reactor body  200 , a cooler  210 , core cooling members  220   a ,  220   b , and coil cooling members  230   a ,  230   b . The boost reactor body  200  includes a thermistor  101 , a coil  102 , a resin mold member  201 , screws  202 , and screw holes  203 . 
       FIG. 3  and  FIG. 4  are perspective views showing the structure of the boost reactor body.  FIG. 3  is a perspective view seen from below side, and  FIG. 4  is a perspective view seen from above side. In  FIG. 2 , the arrow direction of Z axis is defined as upper side, and the side opposite to the arrow direction is defined as lower side. X axis and Y axis are axes extending in directions perpendicular to Z axis.  FIG. 4  shows the state in which the resin mold member  201  is removed from the boost reactor body  200 . In the drawing, the boost reactor body  200  is formed by the resin mold member  201  covering the thermistor  101 , the coil  102 , and the core  105 . 
     Two windings  103   a ,  103   b  forming the coil  102  have ends connected to each other at the outside, and other ends serving as terminals of the boost reactor  4 . The windings  103   a ,  103   b  are wound around the core  105 , and the turns ratio thereof is one to one. The windings  103   a ,  103   b  are wound such that magnetic fluxes generated from the respective windings  103   a ,  103   b  are directed in the same direction inside the core  105  (cumulative connection). 
     The resin mold member  201  serves to retain the thermistor  101 , the coil  102 , and the core  105  and also to fix the boost reactor body  200  to the cooler  210 . As shown in  FIG. 3 , winding cooling portions  104   a ,  104   b  for cooling the coil  102  and core cooling portions  107   a ,  107   b  for cooling the core  105  are provided, and these portions are not covered by a resin mold. Other part that is not covered by a resin mold may be provided, unless the function is lost. Although the winding cooling portions  104   a ,  104   b  and the core cooling portions  107   a ,  107   b  are located on the lower side of the reactor, their locations are not limited thereto. For example, as shown in  FIG. 5 , they may be provided at an upper part U or side surfaces S 1 , S 2  of the reactor. Cooling portions may be provided at appropriate parts in accordance with the shapes of the reactor and the cooler  210 , whereby cooling performance can be enhanced. 
     The winding cooling portions  104   a ,  104   b  and the core cooling portions  107   a ,  107   b  are in contact with the cooler  210  via the core cooling members  220   a ,  220   b  and the coil cooling members  230   a ,  230   b , respectively. The core cooling members  220   a ,  220   b  and the coil cooling members  230   a ,  230   b  are formed as members separate from each other. However, without limitation thereto, they may be integrated into one cooling member. As shown in  FIG. 2 , the cooler  210  is provided with bases  211   a ,  211   b  for mounting the core cooling members  220   a ,  220   b  thereon. 
     The material of the cooling members forming the core cooling members  220   a ,  220   b  and the coil cooling members  230   a ,  230   b  is a non-fluid material such as a semisolid or a solid. Examples thereof include a silicone-type heat dissipation sheet, a curable silicone-type gap filler, and heat dissipation grease. By using such a non-fluid material, it becomes unnecessary to provide a dug structure for preventing a cooling member from flowing, which would be needed in the case of using a fluid material (potting). Since the dug structure is eliminated, metal members for covering side surfaces of the reactor are not provided, and thus the inductance increases, whereby size reduction of the reactor can be achieved. 
       FIG. 6  is a perspective view showing the structure of the core  105  of the boost reactor  4 . The core  105  is formed by two core members  106   a ,  106   b , and their respective ends are in contact with each other at core member end abutting portions  108   a ,  108   b . In this state, the resin mold member  201  fixes the core  105 . Here, an example in which the core  105  is formed by two core members has been shown, but the structure thereof is not limited thereto. 
     Hereinafter, a problem in a boost reactor having a comparative structure will be described. Induced voltage is generated in accordance with change in current in the reactor, and the ratio of the change in current and the induced voltage is self-inductance L. In the power conversion device  2 , in the boost reactor  4  during boost operation, induced voltage is determined by input voltages Vin, Vout for each operation mode, and thus ripple current depending on the self-inductance L occurs. 
     Increase in ripple current leads to increase in winding loss of the boost reactor  4 . And increase in ripple current leads to increase in loss in the input power smoothing capacitor  6 , the output power smoothing capacitor  7 , and the semiconductor switching elements  5   a ,  5   b.    
     That is, regarding the relationship between ripple current and winding loss, loss occurring in the winding is separated into DC loss due to a DC current component and AC loss due to a ripple component. Where the AC loss is Wcoil_ac [W], the winding resistance is Rcoil [Ω], and the ripple current value is Irip [Arms], the AC loss Wcoil_ac [W] is represented by the following Expression (1). 
       Wcoil_ac=Irip 2 ×Rcoil . . .   (1)
 
     Thus, the AC loss is proportional to the square of the ripple current value and therefore increase in the ripple current leads to increase in loss. 
     Meanwhile, regarding the input power smoothing capacitor  6  and the output power smoothing capacitor  7 , where loss occurring in the capacitor is Wco [W], a resistance component of the capacitor is ESRco [Ω], and current flowing through the capacitor is Ico [Arms], the capacitor loss is represented by the following Expression (2). 
       Wco=Ico 2 ×ESRco . . .   (2)
 
     In both of the input power smoothing capacitor  6  and the output power smoothing capacitor  7 , current Ico flowing through the capacitor increases corresponding to increase in the ripple current of the reactor. Therefore, if the ripple current increases, loss in each capacitor increases. 
     Also regarding the semiconductor switching element, similarly to the above, if the ripple current of the reactor increases, ripple of current flowing through the semiconductor switching element increases, so that loss in a member forming the semiconductor switching element increases. 
     As understood from the above description, from the viewpoint of loss and heat generation, it is desirable that the self-inductance L is increased and the ripple current is decreased. 
     The inductance value L of the reactor is represented by the following Expression (3). 
         L=N   2 ×(μ r·μ   0   ·s )/ lc . . .    (3)
 
     Here, lc is a core magnetic path length, μr is relative permeability of the core, and μ 0  is vacuum permeability. 
     In order to increase the inductance L, a method of increasing the number N of turns of the coil or increasing a core sectional area S is commonly adopted. 
     Main factors that restrict the size of the reactor are heat dissipation property and loss. In order to reduce the size of the reactor, it is desirable that the amount of loss is reduced while the inductance value is increased. However, when increasing the inductance value by the above method, there is a problem that the size of the reactor is enlarged and thus size reduction is limited. 
       FIG. 7  is a perspective view showing the structure of a comparative boost reactor. In  FIG. 7 , a coil and a core of a boost reactor body  300  are the same as those of the boost reactor body  200  shown in  FIG. 2 . In  FIG. 7 , the boost reactor body  300  includes the thermistor  101 , the coil  102 , a case  301 , a filler  302 , and a core mold member  303 . 
     The core mold member  303  covers the core and serves to protect the core surface and position the coil  102 . The filler  302  is, for example, formed by a silicone-type potting material, and serves to cool the coil  102  and the core and fix the core. The case  301  serves to prevent the filler  302  from flowing out. 
     In order to improve heat dissipation property, the case  301  is made of a metal member such as aluminum and is provided to be close to the coil  102  and the core which are heat generating bodies. When a metal member is present close to the reactor, the metal member blocks a leakage magnetic flux generated from the reactor. Here, the leakage magnetic flux is a magnetic flux emitted directly to a space from the core or the coil of the reactor. The leakage magnetic flux also contributes to the inductance value, and when the leakage magnetic flux decreases, the self-inductance value decreases. Therefore, while heat dissipation performance is improved owing to the case  301 , there is a problem that the amount of loss in the reactor increases. 
     The present embodiment has been made to solve such a problem, and in the boost reactor  4  of the power conversion device  2  according to the present embodiment, a resin member is used for a mechanism for retaining the reactor while high heat dissipation property is maintained. Thus, a large amount of leakage magnetic flux can be utilized. As a result, it is possible to increase the inductance value without changing the structures of the coil and the core. Further, loss in the reactor is reduced, size reduction thereof is achieved, and production thereof can be performed at low cost. 
     Hereinafter, the effects of the boost reactor  4  of the power conversion device according to the present embodiment will be described.  FIG. 8  is a sectional view of the boost reactor according to the present embodiment, along a plane perpendicular to the X-axis direction.  FIG. 9  is a sectional view showing a comparative boost reactor structure, along a plane perpendicular to the X-axis direction. 
     In  FIG. 9 , in the comparative boost reactor, since the filler  302  serves to fix the coil and the core, the reactor needs to be covered, including side surfaces thereof, by the metal case  301 . Therefore, with regard to the leakage magnetic flux  9  generated from the coil and the core, a leakage magnetic flux in the Y-axis direction is blocked by the case  301 . 
     On the other hand, in  FIG. 8 , in the boost reactor according to the present embodiment, the boost reactor body  200  is fixed by the resin mold member  201 , whereby a function for fixing to the cooling members can be eliminated and thus cooling surfaces can be localized. That is, in the present embodiment, as shown in  FIG. 8 , cooling surfaces are only three surfaces of the core cooling members  220   a ,  220   b  and the coil cooling member  230   b , and thus the cooling surfaces can be localized. To the contrary, in  FIG. 9 , the entire surface of the filler  302  is a cooling surface. Therefore, the cooling surface includes not only the bottom surfaces of the coil  102  and the core  106  but also side surfaces of the coil  102  and the core  106 , so that the cooling surface cannot be localized. Accordingly, in the present embodiment, a metal case for covering side surfaces of the reactor is not needed. Therefore, the leakage magnetic flux  8  generated from the coil and the core can spread also in the Y-axis direction and thus the magnetic flux amount thereof is larger than the amount of the leakage magnetic flux  9  shown in  FIG. 9 , so that the inductance value increases. In the present embodiment, the core, the coil, and the like are fixed to the cooler  210  by a fixation portion that the resin mold member  201  has. 
     In the comparative boost reactor, the coil and the core are cooled by a cooler  310  via the filler  302 , the case  301 , and heat generation grease  320 . On the other hand, the coil  102  and the core  105  of the boost reactor  4  according to the present embodiment are directly cooled by the cooler  210  via the coil cooling members  230   a ,  230   b  and the core cooling members  220   a ,  220   b , respectively. Thus, the thermal resistance to reach the cooler  210  can be reduced, so that cooling performance is improved. 
     Further, since a metal case is not needed, the reactor body can be downsized and production can be performed at low cost. 
     In the power conversion device  2 , when a housing or a housing-like large metal member such as a bus bar covering one surface of the reactor is located close to the boost reactor  4 , the effects of the present embodiment are influenced. In order to sufficiently obtain the effects of the present embodiment, it is necessary to ensure an area in which a leakage magnetic flux can be generated. It is preferable to, except for the surface having the cooling portion, ensure a space of at least 10 mm from ends of the core and the coil which are magnetic flux generation sources, i.e., separate the large metal member from ends of the core and the coil by at least 10 mm. It is noted that influence of a small metal member such as a screw for tightening the terminal block or the like can be neglected. 
     As shown in  FIG. 7 , the core of the comparative boost reactor is fixed by the filler  302 . Since the hardness of the filler is small, it is impossible to fix the core member end abutting portions  108   a ,  108   b  of the core members  106   a ,  106   b  while being in contact with each other by the filler  302  alone. Therefore, it is necessary to fix the core member end abutting portions  108   a ,  108   b  by an adhesive agent. 
     On the other hand, in the boost reactor  4  according to the present embodiment, molding is performed by the resin mold member  201  in a state in which the ends of the core members  106   a ,  106   b  abut on each other. Thus, stress due to thermal compression generated during molding can be continued to be applied, whereby the core member end abutting portions  108   a ,  108   b  can be fixed in a state in which they are abutting on each other. In the comparative boost reactor, since an adhesive agent is used, there is a risk that, when the temperature increases, the adhesive agent is disabled, so that the reactor is disabled. However, in the boost reactor  4  according to the present embodiment, such a risk is eliminated. Accordingly, the reactor can be operated even at a higher temperature, and size reduction in the reactor can be achieved. 
     As the core of the boost reactor according to the present embodiment, a dust core may be used. The dust core exhibits a great saturation magnetic flux density and is suitable for large power application, but has comparatively small permeability. Therefore, the ratio of an inductance value due to a leakage magnetic flux increases relative to an inductance value generated by the core, whereby a great inductance increase effect is obtained. In particular, when using Sendust which is a dust core having small relative permeability, a significant effect can be obtained. However, application of the present embodiment is not limited thereto. A material such as an electromagnetic steel sheet or a ferrite having high relative permeability may be used as the core. This provides the same effects as those described above. 
       FIG. 10  is a graph showing the relationship between the inductance value and the frequency, and shows comparison between the inductance values of the boost reactor of the present embodiment and the comparative boost reactor. In  FIG. 10 , the horizontal axis indicates the frequency, the vertical axis indicates the ratio of the inductance relative to the inductance of the boost reactor of the present embodiment at 100 Hz, a dotted line represents the boost reactor of the present embodiment, and a solid line represents the comparative boost reactor. Block of a leakage magnetic flux by the metal member is due to eddy current occurring in a metal housing, and greatly varies with accordance to the frequency (magnetic flux change amount). That is, as the frequency becomes higher, the magnetic flux block effect increases. As shown in  FIG. 10 , in particular, when the drive frequency for the semiconductor switching elements  5   a ,  5   b  of the power conversion device  2  is 1 kHz or higher, reduction in the inductance in the present embodiment is small, but the reduction rate in the comparative boost reactor is large. Therefore, the present embodiment provides particularly significant effects when the drive frequency of the power conversion device is 1 kHz or higher. 
     As described above, in the present embodiment, a structure that allows a large amount of leakage magnetic flux to be utilized while keeping high heat dissipation property, is used. Whereby it is possible to increase the inductance value so as to reduce loss, without changing the material and the structure of the coil and the core. That is, in the reactor structure according to the present embodiment, the resin member is used for the mechanism for retaining the reactor. Whereby it is possible to increase the inductance value without blocking the leakage magnetic flux. In addition, the coil and the core can be directly cooled by the cooler via the cooling members, whereby cooling performance can be improved. Further, size reduction of the reactor structure can be achieved and production can be performed at low cost. 
     Embodiment 2 
     In the above description, the boost reactor body  200  of the power conversion device is configured such that the two windings  103   a ,  103   b  are cumulatively connected to form one coil. The cumulative connection is based on the premise that a magnetic path is formed inside the core. On the other hand, in the case of being applied to the power conversion device and the reactor configuration based on the premise that a magnetic path is formed outside the core and the leakage magnetic flux is utilized as inductance, higher effects are provided. That is, the inductance value can be further increased. In the reactor based on the premise that the leakage magnetic flux is utilized as inductance, the absolute amount of the leakage magnetic flux is large, and the ratio of the inductance value based thereon with respect to the inductance value generated by the core becomes large. Therefore, the leakage magnetic flux increase effect obtained in the present embodiment is relatively large. 
     An example of such a power conversion device is a multiphase boost converter formed by a boost reactor having a plurality of windings. Further, an example of such a boost reactor is a magnetically coupled reactor in which magnetic fluxes generated from the respective windings are canceled out (differential connection).  FIG. 11  is a side view showing the case in which the magnetically coupled reactor is used as the boost reactor. In  FIG. 11 , a coil  1101  is wound around a core  1102 , and a magnetic flux M is generated. It is noted that the positional relationship between: the cooling member, the cooler, and the resin mold member; and the core  1102  and the coil  1101 , is the same as that shown in embodiment 1. 
     In the above embodiments, a boost DC/DC converter has been shown as the circuit configuration of the power conversion device. However, this is merely an example, and the power conversion device may be configured by other circuit such as an AC/DC converter circuit or an insulation-type step-down DC/DC converter circuit. Also in this case, the same effects as described above are obtained. 
     In addition, the number, dimensions, materials, and the like of the components described above may be changed appropriately. 
     Although the disclosure is described above in terms of various exemplary embodiments and implementations, it should be understood that the various features, aspects, and functionality described in one or more of the individual embodiments are not limited in their applicability to the particular embodiment with which they are described, but they can be applied, alone or in various combinations to one or more of the embodiments of the disclosure. 
     It is therefore understood that numerous modifications which have not been exemplified can be devised without departing from the scope of the present disclosure. For example, at least one of the constituent components may be modified, added, or eliminated. At least one of the constituent components mentioned in at least one of the preferred embodiments may be selected and combined with the constituent components mentioned in another preferred embodiment.