Abstract:
A power switching module is provided having at least one power switch placed above at least one other power switch, each power switch in turn including an upper wall and a lower wall, each of which is cooled through thermal conduction by a cooling medium that circulates in channels and voids that are provided along the walls for this purpose.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to a power switching module and a multiphase inverter equipped with the module.  
           [0002]    In particular, the present invention relates to a power switching module comprising:  
           [0003]    at least one power switch, i.e., at least one top power switch, placed above at least one other power switch, i.e., above at least one bottom power switch, and with each such power switch comprising upper and lower walls that are each adapted to be cooled through thermal conduction by a cooling medium, and  
           [0004]    lower closed channels and upper closed channels configured to circulate a cooling fluid along such lower and upper walls, respectively, of such bottom and top power switches.  
           [0005]    Power switching modules of the above kind are used, for example, in three-phase power inverters supplying power to motors of electrical machines, such as the traction motors of a train.  
           [0006]    Such power inverters conventionally comprise two interrupters per phase.  
           [0007]    The currents to be switched being relatively high, for example on the order of a few thousand amperes, each interrupter is commonly formed of a plurality of power switches, each of which is in turn formed of a multitude of individual interrupters, for example insulated gate bipolar transistors (IGBT). Because of the presence of this multitude of transistors, such power interrupters are bulky and must be cooled to prevent overheating.  
           [0008]    To this end, it is known in the art to form a power switching module in which the two interrupters of the same phase are disposed one above the other, so as to thereby reduce the overall size of the module. In such a prior art embodiment, a cooling circuit to evacuate heat generated by the transistors is formed only on the exterior surfaces of the switches of the module. Although this kind of module comprising two interrupters one above the other is less bulky than those used until now, it still has a large overall size.  
           [0009]    Thus the invention aims to reduce even further the overall size of a power switching module comprising at least two interrupters placed one above the other.  
           [0010]    The patent document GB 1 416 561 discloses a modular arrangement of power semiconductor components in which semiconductor elements are disposed in disc-shaped packages. A cooling liquid may circulate in a space where spring means are located between two stacked packages. This liquid cools the lower wall of the top package and the upper wall of the bottom package. Furthermore, the external walls of this modular arrangement are also cooled by a cooling liquid that circulates lengthwise of the module. Thus it is known in the art to cool a power switching module by a two-fold circulation of cooling liquid, firstly along external walls of the module and secondly inside the module. However, in the modular arrangement described in the above patent document, the spring means in the space between the two stacked packages impedes the flow of cooling liquid along the faces of the internal packages of the module and this limits thermal exchange efficiency. The overall size of the module is still large in relation to the amount of current that the module is able to switch. A relatively large quantity of cooling liquid is needed to dissipate heat via the interior faces of the components.  
           [0011]    Thus the teaching of the above document cannot, starting with a prior art switching module in which a cooling circuit is formed exclusively on the exterior surfaces of each of the switches of the module, lead to modifying the module to reduce further its overall size, which is the aim of the invention.  
         SUMMARY OF THE INVENTION  
         [0012]    An object of the invention, therefore, is to provide a power switching module comprising:  
           [0013]    at least one top power switch placed above at least one bottom power switch, and with each such power switch comprising upper and lower walls that are each adapted to be cooled through thermal conduction by a cooling medium, and  
           [0014]    lower closed channels and upper closed channels configured to circulate a cooling fluid along such lower and upper walls, respectively, of such bottom and top power switches,  
           [0015]    and wherein said power switching module further includes a lower void, i.e., passage, in said power switching module located along and above the upper wall of each bottom power switch as well as an upper void, i.e., passage, in said power switching module located along and below the lower wall of each top power switch to cool said walls by circulating a cooling fluid in the lower void and in the upper void.  
           [0016]    In the power switching module of the present invention, the switches of each interrupter are cooled both from above and from below. Thus, the heat created by the switching losses of the transistors is removed through two walls. The cooling fluid circulates without impediment in each of the aforementioned lower and upper voids. Accordingly, the transistors, therefore, are cooled more effectively than in the prior art. In general, the amount of current that a transistor is able to switch is limited primarily by the transistor&#39;s capacity to dissipate heat. In particular, it has been found that a transistor, which is cooled, is able to switch currents up to five times the nominal current for the transistor. Thus, for the same electrical specifications, the power interrupter formed by the power switches used in the power switching module of the present invention includes fewer transistors than a prior art power interrupter cooled by means of only one wall. As such, because the number of transistors in each power switch of the power interrupter is reduced, the overall size of each such power interrupter is also reduced. Moreover, it has been found that this reduction in the size of each such power interrupter more than adequately counterbalances the increase in the size of the power switching module of the present invention, which results from the presence of additional walls and a second cooling fluid circulation circuit. Consequently, the result is that a power switching module constructed in accordance with the present invention, which is equipped with two cooling circuits, has an overall size that is smaller than that of a prior art power switching module, which has only one cooling circuit.  
           [0017]    According to particular embodiments, the power switching module of the present invention may include one or more of the following features:  
           [0018]    wherein the lower void and the upper void have two ends interconnected to form a single cooling fluid circulation circuit;  
           [0019]    wherein each power switch includes a single heatsink that is thermally connected to one of the lower or upper walls, the heatsink being equipped with fins that are adapted to be in direct contact with the cooling liquid, which is circulating in the channels or the voids;  
           [0020]    wherein only the upper wall of each top power switch includes a heatsink and only the lower wall of each bottom power switch includes a heatsink;  
           [0021]    wherein the fins of the heatsinks are parallel to the main cooling fluid circulation direction;  
           [0022]    wherein the lower channels and the upper channels are connected to each other to form a single cooling fluid circulation circuit along the upper wall of each top power switch and along the lower wall of each bottom power switch;  
           [0023]    wherein each cooling fluid circulation circuit is connected to the same pump to circulate the cooling fluid in each circulation circuit;  
           [0024]    wherein at least the ends of a void and a channel are connected to a common cooling fluid inlet or outlet connector;  
           [0025]    wherein a spacer of substantially parallelepiped shape is housed between the top power switch or switches and the bottom power switch or switches so as to maintain a predetermined distance between them and the spacer includes a housing for each power switch;  
           [0026]    wherein the shape of the spacer is selected so as to be capable of sealing hermetically the open side of the lower channels and the upper channels;  
           [0027]    wherein each power switch includes a plurality of transistors and electrical tracks to which the emitter and/or the gate of each transistor is welded, the tracks being formed on an inside surface of the wall cooled by cooling fluid that circulates in the void;  
           [0028]    wherein each power switch further includes electrical tracks to which the collector of each transistor is welded, the tracks being formed on an inside surface of the wall cooled by cooling fluid that circulates in a channel; and  
           [0029]    wherein the transistors are electrically connected to the electrical tracks by molten weld cylinders.  
           [0030]    A further object of the present invention is also to provide a multiphase inverter in which each phase includes two interrupters, each of which is comprised of at least one switch, and wherein the two interrupters of the same phase are created utilizing a single power switching module, which is constructed in accordance with the present invention.  
           [0031]    The invention will be better understood on reading the following description, which is given solely by way of example, and with reference to the figures of the drawing. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0032]    [0032]FIG. 1 is a block diagram of a power inverter that includes a plurality of interrupters;  
         [0033]    [0033]FIG. 2 is a partial block diagram of one of the interrupters of the power inverter of FIG. 1;  
         [0034]    [0034]FIG. 3 is an exploded view of a power switch employed in one of the interrupters of the power inverter of FIG. 1;  
         [0035]    [0035]FIGS. 4 and 5 are cross-sectional views taken along the line IV-IV in FIG. 5 and the line V-V in FIG. 4, respectively, of an interrupter of the power inverter of FIG. 1; and  
         [0036]    [0036]FIGS. 6 and 7 are cross-sectional views taken along the line VI-VI in FIG. 7 and the line VII-VII in FIG. 6, respectively, of a power switching module constructed in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0037]    [0037]FIG. 1 shows a three-phase power inverter  2  adapted to supply power to a rotating electrical machine  4  such as a motor under the control of an electronic processor  6 .  
         [0038]    The motor  4  is, for example, one of the traction motors of a train.  
         [0039]    The processor  6  and methods of controlling a three-phase converter are known in the art. For this reason they will not be described in detail here.  
         [0040]    The inverter  2  is a conventional circuit and comprises three identical electrical phases  10 ,  12  and  14  each consisting of two identical power interrupters  16  and  18 . In this example each power interrupter is operable to switch currents of several thousand amperes and is also able to withstand potential differences of several thousand volts.  
         [0041]    To this end, each interrupter is comprised of a plurality of electrical switches that are connected in parallel.  
         [0042]    By way of exemplification and not limitation, FIG. 2 shows an interrupter that consists of four identical switches  20  to  23 . Since the switches  20  to  23  are identical, only the switch  20  is shown and described herein in detail.  
         [0043]    In accordance with this embodiment, each switch consists of four insulated gate bipolar transistors (IGBT)  26  to  29  connected in parallel and two diodes  32 ,  33  connected in non-parallel relation to the terminals of the transistors  26  to  29 .  
         [0044]    Each transistor  26  to  29 , by way of exemplification and not limitation, can withstand a voltage of 3 000 volts and a maximum nominal current of 150 amperes. Connecting the transistors  26  to  29  in parallel thus produces a switch  20  that is able to switch a current much greater than that which each of the transistors  26  to  29  thereof is able to withstand.  
         [0045]    The gate of each of the transistors  26  to  29  is connected via a respective resistor  36  to  39  to a gate control electrode that is not shown.  
         [0046]    [0046]FIG. 3 is an exploded perspective view showing the structure of the switch  20 . The switch  20  includes a substrate or bottom wall  50 , which is made from an electrically insulative and thermally conductive material, such as a dielectric material. The substrate  50  is substantially rectangular and is disposed horizontally.  
         [0047]    Etched electrical tracks  52  located on an upward facing interior surface of the substrate  50  connect the collectors of the transistors  26  to  29  to collector electrodes  54 . The collector electrodes  54  are brazed to the tracks  52  and project to the outside on the rear shorter side of the substrate  50 .  
         [0048]    Each transistor  26  to  29  has two opposite plane faces. In FIG. 3, the lower face of each transistor  26  to  29  is shown as being carried by the collector electrodes of the respective transistor, and the upper face of each transistor  26  to  29  is shown as being carried by the gate and emitter electrodes of the same transistor.  
         [0049]    The collector of each of the transistors  26  to  29  and the cathode of the diodes  32  and  33  are welded to the tracks  52 . In addition to providing the electrical connection, welding assures the thermal connection of the collector of the transistors  26  to  29  to the substrate  50 . To ensure a good transfer of heat between the collector of each transistor  26  to  29  and the substrate  50 , the welding area is made as large as possible.  
         [0050]    The switch  20  also includes a rectangular substrate or upper wall  60  made from an electrically insulative and thermally conductive material. Etched electrical tracks  62 ,  64  are located on the interior face of the substrate  60  so as to face the substrate  50 . The track  62  connects the gates of the transistors  26  to  29  to a gate electrode  66  via resistors  36  to  39 . The track  64  connects the emitters of the transistors  26  to  29  to emitter electrodes  68 .  
         [0051]    In accordance with the embodiment of the present invention depicted in FIG. 3, the gate and emitter electrodes  66 ,  68  are fixed to the substrate  50 .  
         [0052]    Finally, the switch  20  also includes a temperature sensor  70  and a heatsink  72 .  
         [0053]    The temperature sensor  70  is fixed to the inside surface of the substrate  50 .  
         [0054]    The heatsink  72  is a copper heatsink comprised of a multitude of parallel fins  74 . The space between two consecutive fins  74  forms a narrow channel of substantially rectangular cross-section having, for example, a depth of 3 mm and a width of 1 mm. The heatsink  72  is welded to the outside surface of the substrate  50  so that the fins  74  are parallel to the shorter side of the substrate  50 .  
         [0055]    The substrate  60  is fixed to the substrate  50 , so that the substrate  60  lies parallel to and above the substrate  50 , preferably by means of a welding method known as the “bump” welding process, using weld cylinders. To this end, weld cylinders  80  are disposed on the surfaces to be electrically and thermally connected to the tracks  62  and  64  of the substrate  60 . In accordance with the embodiment of the present invention depicted in FIG. 3, the cylinders  80  are disposed in particular on the anode surface of the diodes  32  and  33 , and on the surfaces of the emitters and the gates of the transistors  26  to  29 . Other cylinders  80  are also disposed so as to be operable to connect the gate electrodes  66  and the emitter electrodes  68  to the tracks  62  and  64 .  
         [0056]    To assemble the substrate  60  to the substrate  50 , the cylinders  80  are melted to connect the substrate  60  to the substrate  50  electrically, thermally and mechanically. Moreover, thanks to the use of the cylinders  80 , the contact area between the tracks  62 ,  64  and the corresponding surfaces of the emitters and the gates of the transistors  26  to  29  is large thereby ensuring good heat transfer between the welded surfaces.  
         [0057]    Because it has been found that the quantity of heat that may be removed via the substrate  50  to which the collectors are welded is much higher than that which can be removed via the substrate  60  to which the emitters and the gates are connected, only one heatsink  72  is fixed to the collector side of the transistors  26  to  29 .  
         [0058]    Once the substrate  60  has been welded to the substrate  50 , the space between these two substrates  50 ,  60  is filled with a dielectric gel to isolate the electrical components from the external environment.  
         [0059]    [0059]FIGS. 4 and 5 illustrate the structure of the interrupter  18  diagrammatically. The interrupter  18  includes a horizontal lower support  90  made from an electrically insulative material such as a plastic material.  
         [0060]    By way of exemplification and not limitation, the support  90  may comprise a horizontal plate  92  embodying the shape of a parallelepiped that is supported by four vertical feet  94  at the corners thereof.  
         [0061]    There is an open channel  96  of rectangular shape that extends parallel to the longer side of the plate  92  on the upper surface thereof. The channel  96  is operative to receive a cooling liquid and the heatsinks  72  of the switches  20  to  23 . To this end, the width and the depth of the open channel  96  correspond to the width and the height of the heatsink  72 . In particular, the depth is chosen so that the free edge at the bottom of each fin  74  is in contact with the bottom of the channel  96  so as to define between the fins  74  a cooling fluid circulation mini-channel. This kind of configuration makes the fins  74  more efficient.  
         [0062]    The four switches  20  to  23  forming the interrupter  18  are nested in the support  90  so that their respective heatsinks  72  are accommodated in the channel  96 . In accordance with the embodiment of the present invention depicted in FIGS. 4 and 5, the switches  20  to  23  are disposed so that the fins  74  of the heatsinks  72  are parallel to the longer side of the support  90 . This configuration facilitates circulation of cooling liquid in the channel  96 . Further, in accordance with the embodiment of the present invention depicted in FIGS. 4 and 5, the substrates  50  of the switches  20  to  23  are lined up one behind the other in a common plane that is parallel to the upper surface of the plate  92 . The lower surface of the substrates  50  is preferably glued and sealed to the edges of the channel  96 .  
         [0063]    [0063]FIGS. 6 and 7 illustrate diagrammatically the structure of an assembled power switching module  100 . The power switching module  100  includes the interrupters  16  and  18  that are disposed, by way of exemplification and not limitation, one above the other in a mirror image configuration.  
         [0064]    The structure of the interrupter  16  is identical to that of the interrupter  18 , which has been described hereinbefore with reference to FIGS. 4 and 5. In particular, the interrupter  16  comprises four switches  110  to  113 , respectively, that are identical to the switches  20  to  23  and that are fixed to a support  120 , which is identical to the support  90 . In accordance with the embodiment of the present invention depicted in FIGS. 6 and 7, the reference numbers, which have been used hereinbefore in connection with the description and illustration of the switch  20 , are also being used for purposes of identifying identical parts of the switches  110  to  113 . The switches  110  to  113  are accommodated in an open channel  116 .  
         [0065]    When assembled, the respective substrates  60  of the switches  16  and  18  face each other.  
         [0066]    A horizontal spacer  130  having the shape of a parallelepiped is housed between the respective substrates  60  of the interrupters  16  and  18  so as to thereby hold the switches  110  to  113  at a predetermined distance from the switches  20  to  23 .  
         [0067]    The spacer  130  includes a housing for each of the switches  20  to  23  and for each of the switches  110  to  113 . Moreover, the shape of the spacer  130  is selected so as to be operable to provide a hermetic seal on the open side of the channels  116  and  96 .  
         [0068]    Inside the spacer  130  is a single first circuit  132  for circulation of a cooling liquid for cooling all of the transistors  26  to  29  of the module  100  via the substrates  60 . To this end, two horizontal rectilinear voids, i.e., passages,  133  and  134  of rectangular cross-section extend inside the spacer  130  parallel to the longer side of the spacer  130  and along the outside surface of each of the substrates  60 . As viewed with reference to FIG. 7, the left-hand end of these voids  133  and  134  is open and their right-hand ends are fluidically connected via a first semicircular void  136 . The thickness of the voids  133  and  134  is preferably less than or equal to 1 mm to maximize heat transfer efficiency relative to the volume of cooling liquid employed.  
         [0069]    The spacer  130  also has at its right-hand end a second semicircular void  138  that is fluidically connected to the right-hand ends of the channels  96  and  116 . This produces a second circuit  140  for circulation of cooling liquid for cooling all of the transistors  26  to  29  of the module  100  via the substrates  50 .  
         [0070]    The circuits  132  and  140  each have a U-shaped longitudinal section.  
         [0071]    As viewed also with reference to FIG. 7, the right-hand end of the spacer  130 , which contains the voids  136  and  138 , may by way of exemplification and not limitation be either an integral portion of the spacer  130  or an integral portion of one of the supports  90  or  120 .  
         [0072]    Inside the module  100 , the first and second circuits  132  and  140  are fluidically independent of each other.  
         [0073]    The left-hand ends, as viewed with reference to FIG. 7, of the channels  96  and of the voids  133  are fluidically connected via a Y-shaped connector  141  to the outlet of a common pump  142 .  
         [0074]    Similarly, the left-hand ends, as viewed with reference to FIG. 7, of the channel  116  and of the void  134  are fluidically connected by a Y-shaped connector  144  to the inlet of the same pump  142 . Thus a single pump is used to circulate the cooling liquid in both circuits  132  and  140 .  
         [0075]    In accordance with the embodiment of the present invention depicted in FIGS. 6 and 7, the outlet of the pump  142  is operable to discharge cooling liquid into the circuits  132  and  140 , and the inlet of the pump  142  is operable to receive cooling liquid present in the circuits  132  and  140 . The direction of circulation of the cooling liquid within the module  100  is shown by the arrows that are depicted in FIG. 7.  
         [0076]    The operation of the module  100  will next be described.  
         [0077]    When the interrupters  16  and  18  operate, each transistor  26  to  29  produces heat because of switching losses and also because the transistor is conducting electricity. The greater portion of the heat so produced is transferred to the heatsink  72  via the collector of each transistor  26  to  29  and the substrate  50 . The presence of the narrow channels formed by the spaces between the fins  74  of the heatsink  72  increases the area of contact with the cooling liquid circulating in the circuit  140 . Heat is, therefore, transferred more efficiently between the heatsink  72  and the cooling liquid.  
         [0078]    The remainder of the heat produced by the transistors  26  to  29  is transferred via the substrates  60  directly to the cooling liquid circulating in the circuit  132 .  
         [0079]    Thus, heating of each transistor  26  to  29  is limited considerably since the transistors  26  to  29  are cooled via their upper and lower faces. It is then possible to use the transistors  26  to  29  to switch currents much higher than the nominal current for which they are designed. Therefore, it is possible, thanks to the presence of the two cooling circuits  140  and  132 , respectively, for the upper and lower faces of the transistors  26  to  29 , either to fabricate a switching module  100  capable of switching much higher currents with the same number of transistors  26  to  29 , or to reduce the number of transistors  26  to  29  and thereby the size of the module  100  in order to be able to switch exactly the same current.  
         [0080]    In accordance with the present invention the cooling liquid is preferably water. However, other cooling liquids could also be utilized instead. In addition, it is equally possible to replace the cooling liquid with a gas, such as an inert gas.  
         [0081]    In accordance with another embodiment of the present invention, the circuits  132  and  140  may be completely independent from one amother, i.e., they may have no fluidic connectors  141  and  144  at the left-hand ends, as viewed with reference to FIG. 7, of the channels  96  and  116  and of the voids  133  and  134 , respectively. Even though an embodiment of this kind has no particular advantage a priori, using the pump  142  to circulate a cooling liquid in the circuit  140  and another pump to circulate an inert cooling gas in the circuit  132  may be envisaged.  
         [0082]    The module  100  of the preferred embodiment of the present invention described here includes only one first circuit  132  for cooling the substrates  60  and only one second circuit for cooling the substrates  50 , with both circuits  132  and  140  being fed by the same pump  142 . However, in accordance with yet another embodiment of the present invention, the power module may include a plurality of first cooling circuits for the substrates  60  and a plurality of second cooling circuits for the substrates  50 . To this end, the channels  96  and  116  and the voids  133  and  134  are open at each of their right-hand and left-hand ends, as viewed with reference to FIG. 7. Such left-hand open ends are connected to a first pump in a similar manner to that described and illustrated with reference to FIG. 7 and such right-hand open ends in a similar manner are connected to a second pump. In such an embodiment of the present invention, the channels  96  and  116  form two independent first circuits for circulation of a fluid for cooling the substrates  50  of the interrupters  16  and  18 , respectively. While, the voids  133  and  134  form two independent second circuits for circulating a fluid for cooling the substrates  60  of the interrupters  16  and  18 , respectively.  
         [0083]    The module  100  has been described in a specific situation in which the transistors are insulated gate bipolar transistors. However, in a different embodiment of the present invention, the transistors could equally well without departing from the essence of the present invention be bipolar transistors or metal oxide silicon field-effect transistors (MOSFET), or could be replaced by other electronic components that need to be cooled efficiently.  
         [0084]    The interrupters  16  and  18  are described here in the particular situation in which they comprise four switches  20  to  23 . However, in a different embodiment, a power interrupter  16  and  18  may equally well without departing from the essence of the present invention include more than four switches or may include fewer than four switches, such as, for example, only one switch.  
         [0085]    While the invention has been illustrated and described as embodied in specific embodiments, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.