Patent Publication Number: US-10326379-B2

Title: Power conversion device

Description:
TECHNICAL FIELD 
     The present invention relates to a DC-DC converter, and particularly to a DC-DC converter which is used in an electric vehicle and a plug-in hybrid vehicle. 
     BACKGROUND ART 
     An electric vehicle and a plug-in hybrid vehicle include an inverter device which drives a motor by a high voltage battery for a driving force and a low voltage battery which is used to operate auxiliaries such as lights, a radio, and the like of the vehicle. In such a vehicle, there is mounted a DC-DC converter which converts power from the high voltage battery to the low voltage battery or from the low voltage battery to the high voltage battery (for example, see PTL 1). In a housing of the DC-DC converter, a coolant channel is provided in a converter housing to cool down heated components therein, and a coolant such as a long-life coolant is supplied. 
     By the way, an output current required for the DC-DC converter for a vehicle reaches even 200 A. Therefore, heat amounts of the secondary winding and the secondary circuit of a transformer are large, and thus it is difficult to achieve a miniaturization and a low cost due to a countermeasure against the temperature rise. For example, as disclosed in PTL 2, there may be employed a configuration of a parallel DC-DC converter in which the secondary circuit is manufactured in a module type and provided in parallel to share the output current and to disperse the heat amount. 
     CITATION LIST 
     Patent Literature 
     PTL 1: JP 2005-143215 A 
     PTL 2: JP 2001-223491 A 
     SUMMARY OF INVENTION 
     Technical Problem 
     In the configuration disclosed in PTL 2, the secondary circuit modules are only provided in parallel, but the heat amount of the transformer itself is not able to be reduced. In addition, since there is generated a difference in wiring inductance from each of the secondary circuit modules to the transformer, the current is not able to be equally shared. Therefore, there is a need to design a sufficient margin for heat of the components used in the secondary circuit module, and there is a concern that an increase in size and the costs up are caused. 
     A primary object of the invention is to provide a power conversion device which suppresses a current unbalance of a plurality of secondary circuit modules provided in parallel, and is manufactured in a compact size and at a low cost. 
     Solution to Problem 
     A power conversion device according to the invention boosts downs and outputs an input voltage, and includes an input circuit module which includes a switching element, and an output circuit module which includes a transformer and a rectifier element. A plurality of the output circuit modules are provided to have almost the same structure. The plurality of output circuit modules are electrically connected to the input circuit module. 
     Advantageous Effects of Invention 
     According to the invention, it is possible to achieve a miniaturization and a low cost of a power conversion device. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram illustrating a circuit configuration of a DC-DC converter of the invention. 
         FIG. 2  is a diagram illustrating a circuit configuration of the DC-DC converter of the invention. 
         FIG. 3A  is a perspective exterior view of a module which is used in the DC-DC converter of the invention. 
         FIG. 3B  is a bird&#39;s eye view of the module which is used in the DC-DC converter of the invention. 
         FIG. 4  is a diagram illustrating a mounting configuration of the DC-DC converter of the invention. 
         FIG. 5  is a diagram illustrating a circuit configuration of a conventional DC-DC converter. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of a power conversion device according to the invention will be described with reference to the drawings. In the following, the description will be given about a DC-DC converter which includes a high-voltage primary circuit which converts a high DC voltage into an AC voltage, a transformer which converts a high AC voltage into a low AC voltage, and a low-voltage secondary circuit which converts a low AC voltage into a DC voltage. Further, the same components in the respective drawings will be denoted with the same symbols, and the redundant description will be omitted. 
     A typical main circuit configuration of the DC-DC converter is illustrated in  FIG. 5 . The DC-DC converter includes high voltage terminals  103   a  and  103   b  and a low voltage terminal  112 . As a high-voltage primary circuit  101 , there is illustrated a circuit configuration in which four MOSFETs  105   a  to  105   d  are connected in an H bridge shape and a smoothing capacitor  104  is connected in the input side. A primary winding of a transformer  107  is connected to the output line. A center tap type transformer is employed as the transformer  107  in which a center point of the secondary winding is led out. A smoothing circuit made of a chock coil  108  and a capacitor  110  is connected to a rectifying circuit which is configured using diodes  109   a  and  109   b  or a MOSFET as a low voltage circuit. 
     An output current obtained by the DC-DC converter for a vehicle reaches even 200 A. Therefore, heat amounts of the secondary winding and the secondary circuit of the transformer are large, and thus it is difficult to achieve a miniaturization and a low cost due to a countermeasure against the temperature rise. Therefore, there is a conventional technique employing a configuration of a parallel DC-DC converter in which the secondary circuits are manufactured in a module type, and provided in parallel to share the output current and make the heat amount dispersed. 
     However, there are two problems in the configuration as follows. First, the transformer rises in temperature. The heat amount of the transformer itself is not changed only by arranging secondary circuit modules in parallel. Therefore, there is a need to increase a diameter of the winding to reduce a copper loss or to increase a cross-sectional area of a core to reduce an iron loss, which causes an increase in size of the transformer and the costs up. 
     Second, if there is a difference in inductance between the transformer and the wiring connecting the respective secondary circuit modules, the current may be not equally shared to the respective secondary circuit modules. If the inductance of the wirings connecting the transformer and the respective secondary circuit modules can be made equally, such a problem does not occur. However, the design may be difficult in many cases due to magnetic interference of each wiring and a layout restriction of the housing inside. In a case where the respective secondary circuit modules are unbalanced in current, there is a need to design a sufficient margin for heat of the components used inside the secondary circuit module, which causes an increase in size of the component and the costs up. Therefore, a conventional parallel configuration fails in miniaturization of the DC-DC converter and the costs down. 
       FIG. 1  is a diagram illustrating a circuit configuration of the DC-DC converter in this embodiment. As a high-voltage primary circuit  101 , there is illustrated a circuit configuration in which four MOSFETs  105   a  to  105   d  are connected in an H bridge shape and the smoothing capacitor  104  is connected in the input side. The primary windings of a plurality of transformer  201  to  204  are connected to the output line of the primary circuit  101  in parallel. While not illustrated in the drawing, the transformers  202  to  204  mean the same transformer as the transformer  201 . These transformers each are provided with a core around which the winding is wound, and the transformers are magnetically connected. Therefore, these transformers can be freely divided and disposed in separate places. In addition, since the plurality of transformers having the same configuration are connected, the current is divided into the respective transformers, and the power is transferred and shared onto the secondary side. Similarly, losses such as a copper loss caused by a winding resistance and an iron loss of the core may also be shared onto the respective transformers. 
     A principle of suppressing the temperature rise using a plurality of transformers connected in parallel and the reason of a miniaturization and the low cost will be described below. 
     A volume of a single type of transformer in the related art is represented by “V”, and the heat amount is represented by “Q”. In addition, assuming that the transformer is a cube shape, the length of one side is L(1) and a cross-sectional area of the transformer facing a converter housing is S(1). There is provided a water channel through which a coolant flows to the converter housing. A thermal resistance from the transformer to the coolant is set to Rt-w(1). A thermal resistance Rt-w(1) from the center of the transformer to the coolant is proportional to a height L(1) of the transformer, and inversely proportional to a cross-sectional area S(1). When a thermal conductivity of the transformer is set to “A”, the following Expression (1) is obtained. 
     
       
         
           
             
               
                 
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     On the other hand, in a case where the number of transformers provided in parallel is set to “N” in a multiple parallel scheme, and the area of the transformer per each one is assumed as V/N, the length L(N) of one side of the transformer is represented by the following Expression (2).
 
[Expression 2]
 
 L ( N )= L (1)/ N   1/3   (2)
 
     In addition, the cross-sectional area S(N) of the transformer facing the housing is represented by Expression (3).
 
[Expression 3]
 
 S ( N )= S (1)/ N   2/3   (3)
 
     In a case where the thermal conductivity A of the transformer is constant regardless of the volume of the transformer, a thermal resistance Rt-w(N) from N transformers provided in parallel to the coolant becomes the following Expression (4). 
     
       
         
           
             
               
                 
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     Therefore, a temperature rise ΔT(N) from the water channel to the center of the transformer is represented by the following Expression (5) on the basis of the heat amount Q/N and the above expression, so that it can be seen that the temperature rise is suppressed by increasing the number (N) provided in parallel. 
     
       
         
           
             
               
                 
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     As described above, since the temperature rise of the transformer can be suppressed by increasing the number of transformers provided in parallel. Therefore, a heat dissipation member such as a potting resin, a heat dissipation sheet, and a thermal grease can be reduced so as to lower the cost. In addition, the number of windings or the diameter of the winding of the transformer may be designed small in order to leave a thermal margin in each transformer. In some cases, a general compact transformer or a compact core may be utilized, so that the cost can be lowered further more. 
     Next,  FIG. 2  illustrates a circuit configuration in which the transformer and the low-voltage secondary circuit are provided in parallel. The high-voltage primary circuit  101  of the circuit configuration illustrated in  FIG. 2  has the similar configuration to the high-voltage circuit  101  illustrated in  FIG. 1 . In the embodiment illustrated in  FIG. 2 , output circuit blocks  301  to  304  are connected to the output line of the primary circuit  101  in parallel. Herein, the circuit blocks  302  to  304  are configured by the same component as that of the circuit block  301  while not denoted in the drawing. 
     The output circuit block  301  is configured by a transformer  311 , rectifier diodes  312   a  and  312   b , a chock coil  313 , and a smoothing capacitor  314 . The inputs and the outputs of the output circuit blocks  301  to  304  are connected to the other circuit blocks in parallel. 
     In a conventional configuration of a parallel DC-DC converter, for example, in a configuration in which a single transformer is provided and only the secondary circuits are connected in parallel, there is a need to match the wiring inductance from the transformer to a rectifier element in the secondary circuits in order to remove the current unbalance between the secondary circuits. However, it is difficult to make the wiring length equal due to a restriction on the layout in many cases. In addition, it is difficult to design the inductance to be exactly equal due to magnetic interference with respect to the other wirings. 
     On the other hand, the DC-DC converter illustrated in the embodiment of  FIG. 2  can improve the flexibility in design of wirings between the element of the secondary circuit and the transformer and easily remove the inductance difference of the wirings by providing the transformer in each secondary circuit. Therefore, the current unbalance of the respective secondary circuits can be removed, which contributes to a miniaturization and a low cost of the DC-DC converter. 
       FIG. 3  is an example of a module mounting structure for realizing the circuit block  301  illustrated in  FIG. 2 .  FIG. 3( a )  is a perspective exterior view of a module  401 , and  FIG. 3( b )  is an exploded perspective view of the module  401 . 
     A transformer  411  and a chock coil  413 , which are magnetic components among the components of the circuit block  301 , are stored in a module case  416  which is designed for the purpose of component support and heat radiation. While not illustrated in the drawing, the case may be filled with a potting resin or the like in order to increase a radiation performance of the transformer  411  and the chock coil  413 . 
     Rectifier diodes  412   a  and  412   b  are attached to the side wall surface of the module case  416 . While not illustrated in the drawing, a heat dissipation sheet or the like may be interposed between the rectifier diodes  412   a  and  412   b  and the side wall surface of the module case  416  for the insulation and the heat radiation. 
     Respective connection terminals of the transformer  411 , the chock coil  413 , and the rectifier diodes  412   a  and  412   b  are connected to a circuit board  415  provided on the module case  416  using solder to sure electrical connection. In addition, some of components of the circuit block  301  which are small and can be mounted on the surface, for example, a capacitor  414 , are mounted on the circuit board  415 . As described above, the respective components are disposed, and the components are electrically connected by a wiring pattern provided in the circuit board  415 , so that the circuit configuration of the circuit block  301  can be realized. 
     If the mounting structure of the module  401  is applied to the entire circuit block as a mounting structure to realize the circuit blocks  301  to  304  illustrated in  FIG. 2 , most the current unbalance between the respective modules disappears. 
     There is a need to design a variation in the wiring inductance between the transformer and the rectifier diode to be in a several nH order or less to suppress the current unbalance between the respective circuit blocks. The connection between the transformer and the rectifier diode in the module  401  is formed by the wiring pattern on the circuit board  415 . Therefore, the wiring pattern may be similarly applied to the respective circuit blocks. In other words, if the mounting structure of the module  401  is commonly used in the respective circuit blocks, the wiring pattern is also automatically shared. Therefore, most the current unbalance disappears. 
     As described above, if the current unbalance between the respective modules is removed, there is no need to take the current unbalance into consideration for the margin of the thermal design. Therefore, it is possible to use smaller components as the components used in the module. 
       FIG. 4  illustrates an example of the mounting structure in a case where a plurality of modules  401  described using  FIG. 3  is used to form the DC-DC converter. Modules  501  to  504  are those which employ the similar mounting structure to the module  401  in  FIG. 3 . 
     Each module is electrically connected to a primary circuit module  505 , and forms the circuit illustrated in FIG.  2 . The module case of each module is attached to a DC-DC converter housing  506  formed with a coolant channel using screws so as to secure a cooling performance. 
     In the structure illustrated in  FIG. 4 , a variation in the wiring inductance which causes a difference of a wiring length easily occurs in a wiring for connecting the respective modules  501  to  504  and the primary circuit module  505 . However, in a case where a winding ratio N 1 /N 2  of a primary winding N 1  and the secondary winding N 2  of the transformer is large, the variation in the wiring inductance of the primary winding does not cause an actual problem. The reason is that, in a case where the winding ratio N 1 /N 2  is large, a leakage inductance of the transformer is larger than the wiring inductance in the primary side of the transformer, and the influence of the wiring becomes small. In the case of the DC-DC converter for a vehicle, the winding ratio N 1 /N 2  is normally about “10”, and the primary leakage inductance of the transformer  1  is about several uH. On the contrary, the wiring inductance is in a several nH order, and is sufficiently smaller than the primary leakage inductance of the transformer. Therefore, the DC-DC converter of the invention can be said as a configuration which is applied to a vehicle. 
     In addition, the above-described modules  501  to  504  are designed to have almost the same structure, so that the shapes and the dimensions of the respective components are almost the same. As described above, the connection wiring between the transformer and the rectifier diode is formed to be almost the same shape with respect to the respective modules. In addition, besides the connection wiring between the transformer and the rectifier diode, the connection wiring between the rectifier diode and the capacitor element and the connection wiring between the rectifier diode and the chock coil are also formed to have almost the same shape with respect to the respective modules. 
     Further, the expression “almost the same shape” described in this embodiment means a shape which is formed to have the same shape in terms of design idea, but does not intend a difference in shape caused by a variation in a dimensional tolerance or a manufacturing process. In other words, the invention is intended to easily equalize a variation in the wiring inductance caused by a layout of the components between modules, but it is not considered to make the wiring inductance between the modules exactly same. 
     REFERENCE SIGNS LIST 
     
         
           101  high-voltage primary circuit 
           102  low-voltage secondary circuit 
           103   a ,  103   b  high-voltage input terminal 
           104  smoothing capacitor 
           105   a ,  105   b ,  105   c ,  105   d  MOSFET 
           107  transformer 
           108  chock coil 
           109   a ,  109   b  rectifier diode 
           110  smoothing capacitor 
           112  low-voltage output terminal 
           201 ,  202 ,  203 ,  204  transformer 
           301  circuit block 
           311  transformer 
           312   a ,  312   b  rectifier diode (rectifier element) 
           313  chock coil (coil element) 
           314  capacitor (capacitor element) 
           401  module (output circuit module) 
           411  transformer 
           412   a ,  412   b  rectifier diode (rectifier element) 
           413  chock coil (coil element) 
           414  capacitor (capacitor element) 
           415  circuit board 
           416  module case 
           501 ,  502 ,  503 ,  504  module (output circuit module) 
           505  high-voltage primary circuit 
           506  housing