Abstract:
A stack assembly comprising a housing into which an alternating sequence of elements such as heat sinks and printed circuit boards (PCBs) carrying press-packaged semiconductor devices may be mounted and placed under pressure by a clamping device for use in a variety of power system applications. The PCBs are mounted to the heat sinks by a bracket which axially aligns the press-packaged devices with a longitudinal axis defined by the clamping device. Heat sink compartments in the housing are sized slightly larger than the heat sinks to allow the heat sinks some horizontal play within the compartment when the clamping device is loosened. This is further achieved by using floating anchors to connect the heat sinks to the housing and a flexible connector to connect the heat sinks to a power source. The heat sinks may thus be easily shifted to remove a malfunctioning PCB whilst the bulk of the stack remains in the assembled state, thereby facilitating the rapid re-assembly of the stack.

Description:
FIELD OF INVENTION 
     The invention generally relates to a housing device for mounting an assembly of press-pack semiconductor devices and associated heat sinks, and to a stacked an assembly of assembly of such devices. 
     BACKGROUND OF INVENTION 
     Medium to high voltage semiconductor devices such as gate turnoff thyristors (GTOs), silicon controlled rectifiers (SCRs) and insulated gate bipolar transistors (IGBTs) are used in a variety of power system applications. For example, IGBTs may be used as the switching elements in a power inverter bridge controlling a 1200 horsepower motor. These mediums to high power semiconductor devices are characterized by current and voltage ratings of approximately 100 to 300 amps and 1.2 to 10 kV. 
     Due to the relatively high power capacities of such semiconductor devices they have to be packaged so as to contend with a number of issues, including heat dissipation, electrical contact characteristics and arcing. One common form of packaging used by the manufacturers of such devices is the “press pack”. In this packaging structure the semiconductor material is enclosed in a typically cylindrically-shaped casing. The tubular body of the casing is constructed out of an electrically non-conductive material; ceramic is often used for its durability at high temperatures. The tubular body is capped with electrically conductive metallic plates which function as some (or all) of the terminals of the semiconductor device, such as the anode and cathode of a GTO or thyristor. These terminal end-faces present relatively broad planar surfaces for enabling good electrical and thermal contact with other power circuit components such as electrically conductive heat sinks. To further ensure good electrical contact and meet other operating requirements, press pack devices require a pre-specified amount of pressure to be applied thereto, typically in the range of 2-20 kN, although much higher forces are also possible. 
     The pressure or mounting force applied to the press-pack devices is provided by some sort of clamping mechanism. A typical clamping mechanism comprises two threaded rods fitted with plates for applying pressure provided by clamping nuts. Vice-like clamping mechanisms can also be used. These clamping mechanism are also often used to stack multiple numbers of press-pack devices and heat sinks together in abutting relationship. The resultant assembly, or “stack”, can be used in a variety of power circuits such as the leg of an inverter and minimizes the number of clamping mechanisms required, which are extraneous elements of the power circuit. 
     In assembling the stack the conventional practice is to axially align all of the elements thereof in order to ensure uniform application of the mounting force. The way this was accomplished in the prior art is through the use of small guide pins inserted into locating holes formed on the abutting faces of the press-pack devices and heat sinks. Many press-pack devices are manufactured with small holes situated in the centre of the terminal end-faces thereof for this purpose. However, a significant problem arises with this system when it is necessary to replace one of the press-pack devices in the field. More specifically, the heat sinks of the stack are typically quasi-rigidly mounted to a supporting structure such as a housing or cabinet and therefore capable of moving apart only a few thousands of an inch to allow for thermal expansion. This distance is considerably less than the length of the guide pin as disposed in the locating hole. So, to replace one press-pack device in a large stack often meant the whole stack had to be removed from the cabinet, disassembled to replace the press-pack device, then re-assembled and re-installed. This task could require well over an hour. Alternatively, field personnel would attempt to bypass the disassembly procedure altogether by trying to pry out a press-pack device from the stack through the use of sheer force. This usually resulted in a significant scarring or gouging of the terminal end-faces of the press-pack device caused by dragging it over the embedded locating pins, and the gouges were often significant enough so as to render the press-pack devices inoperative because of a change in the thermal transfer characteristics. 
     A further limitation of conventional stack assemblies is that they do not readily accommodate the installation and removal of a press-pack semiconductor device which is mounted onto a printed circuit board. 
     SUMMARY OF INVENTION 
     The invention seeks to overcome various limitations of the prior art by providing a housing have compartments for mounting a predetermined arrangement of heat sinks and printed circuit boards (“PCBs”) carrying press-pack semiconductor devices. The housing also includes a compartment for accommodating a force application member which provides the necessary mounting force required by the press-pack devices. The housing receives and distributes this force amongst the foregoing elements. 
     According to one aspect of the invention, a stack assembly is provided which includes the following components: one or more heat sinks, one or more PCBs, each having a press-packaged semiconductor device mounted therein, a plate having a force applying member, and a housing having compartments for accommodating the aforementioned component in a predetermined abutting arrangement. The compartments accommodating the heat sinks and PCBs are sized slightly larger than the heat sinks and the PCBs to allow each such component a predetermined amount of horizontal play in its corresponding compartment. Each PCB includes a bracket for mounting the PCB onto a corresponding heat sink. The mounting bracket and the size of the heat sink compartments are configured to substantially axially align the press-pack devices with a longitudinal axis defined by the force applying member. 
     In the preferred embodiment, the compartments for accommodating the heat sinks are provided with locating nubs for positioning the heat sinks. The width between locating nubs is slightly larger than the width of a corresponding heat sink, thereby providing the heat sink with a predetermined amount of horizontal play in its compartment. 
     In the preferred embodiment, the rear wall of each heat sink compartment is formed with at least one longitudinal receiving slot for receiving a heat sink anchor. The heat sink anchor features a body, which is fitted in the slot and has a breadth or width smaller than the length of the slot so as to be able to slide therein. Two flanges of the anchor respectively abut opposite sides of the rear wall so as to retain the anchor to the housing. The anchor also features a bore for enabling the heat sink to be fastened thereto. In this manner the anchor is floatingly mounted to the housing and does not interfere with the horizontal play afforded to the heat sink by the excess width between the locating nubs. 
     In the preferred embodiment, a flexible connector is used to connect a heat sink to a power source. The flexible connector includes a power lead and a bus bar. The power lead is connected at one end to a heat sink and at the other to the bus bar. The bus bar is rigidly connected to a terminal. To permit play in the heat sink in at least the horizontal direction, the bus bar is curved. 
     In an alternative embodiment, the flexible connector includes: a power bar with a transverse non-threaded bore, a bus bar with a slot, and at least one spacer with a longitudinal bore. The spacer is inserted through the non-threaded bore in the power lead and through the slot in the bus bar. A bolt is passed through the bore in the spacer and secured with a nut. The spacer is longer than the combined thickness of the power lead and bus bar thereby permitting play in the power lead and heat sink connected thereto. A wire connected at one end to the power lead and at the other to the bus bar ensures good electrical contact between the power lead and bus bar. 
     In the preferred embodiment, a recess in the heat sink compartment furthest from the force applying member accommodates a reaction plate which bears against the housing and distributes the clamping force indirectly applied to it by the force applying member through the sequence of axially aligned stack elements. 
     In the preferred embodiment each heat sink compartment has a rear wall having an opening of approximately the same shape and area of the rear face of a heat sink. A gasket is installed between the heat sink and the rear wall to form an airtight seal there between. 
     The housing also features compartments for accommodating resistor networks. Each resistor network compartment is provided with registering slots for engaging the edges of horizontal locator plates that are used to hold a resistor network in place in the compartment. 
     In the preferred embodiment, one form of heat sink anchors provide a means to connect the stack assembly to a power supply lead. These anchors have a flange for abutting the housing and a shaft connected at one end to the flange. The shaft has a diameter sized smaller than the length of the longitudinal slot in the rear wall so as to slide therein. A clip fits into a circumferential groove of the shaft so as to permit the anchor to be floatingly mounted to the housing through the slot. The anchor also features a smooth longitudinal bore which mediates the passage of a bolt between a heat sink and a power lead having a threaded bore, thus tightly securing the heat sink to the power lead via this anchor. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The foregoing and other aspects of the invention will become more apparent from the following description of specific embodiments thereof and the accompanying drawings illustrating these embodiments. In the drawings: 
     FIG. 1A is a front top left perspective view of a six unit stack assembly including a housing therefor; 
     FIG. 1B is a front top right and partially exploded perspective view of the stack assembly of FIG. 1A showing a PCB in isolation; 
     FIG. 2A is a front right bottom perspective view of a housing, taken in isolation, for a two unit stack assembly; 
     FIG. 2B is a front left top perspective view of the housing unit of FIG. 2A; 
     FIG. 3 is an exploded view of a first embodiment of an anchor used to secure a heat sink to the housing; 
     FIG. 4 is a perspective view of a second embodiment of an anchor used to secure a heat sink and a power lug to the housing; 
     FIG. 5 is an isolated perspective view of a heat sink; 
     FIG. 6 is a rear left bottom and partially exploded perspective view of the stack assembly of FIGS. 1A and 1B and showing a power resistor network in isolation; 
     FIG. 7A is a rear right top perspective view of the housing shown in FIGS. 2A and 2B; 
     FIG. 7B is a rear left bottom perspective view of the housing shown in FIGS. 2A and 2B; 
     FIG. 8 is an isolated perspective view of a power resistor network; 
     FIG. 9 is an isolated perspective view of a locator plate used to mount the resistor network to the housing. 
     FIG. 10A is an isolated perspective view of the bus assembly; 
     FIG. 10B is an isolated side view of the bus assembly; 
     FIG. 11A is an exploded perspective view of an alternative bus assembly; 
     FIG. 11B is an assembled top view of an alternative bus assembly; 
     FIG. 12 is a cut out view of the second embodiment of an anchor used to secure a heat sink to the housing and to connect the heat sink to a power lead; and 
     FIG. 13 is a rear left bottom perspective view of the stack assembly of FIGS. 1A and 1B showing power lugs and bus bars attached thereto. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to FIGS. 1A,  1 B,  2 A and  2 B a stack assembly  10  is shown which includes a housing  12 . The housing  12  is preferably formed from moulded epoxy but may also be made from some other strong, electrically non-conductive material. The housing  12  features a plurality of compartments  14  and  16  that are designed to respectively accommodate heat sinks  18  and PCBs  20 . Each PCB  20  has a press-pack semiconductor device  22  mounted thereon (as shown best in the exploded view of FIG.  1 B). PCB  20  also features a bracket  24  for mounting the PCB  20  onto a corresponding heat sink  18 , as described in greater detail below. Within each heat sink compartment  14  a set of locating nubs  26  (as seen best in FIGS. 2A and 2B) are provided for positioning one of the heat sinks  18  therein. The width W between locating nubs  26  (within a compartment) is slightly larger than the width of a corresponding heat sink  18  in order to allow a predetermined amount of horizontal play while the heat sink is installed in its compartment  14 . Note that the housing  12  may be configured to hold a different number of heat sinks, as required. 
     The housing  12  additionally includes a compartment  28  for accommodating a clamping mechanism. In the illustrated embodiment the clamping mechanism comprises a plate  32  which has a force-applying member  34  depending therefrom via a threaded shaft  36 . The mounting force provided by the clamping mechanism is maintained by a preset washer spring  37  which is set by adjusting a tensioning nut  38 . A recess  40  in the heat sink compartment  14  furthest from the clamping plate  32  accommodates a reaction plate  42 . The force-applying member  34  and its associated shaft  36  define a longitudinal axis L of the stack assembly  10 . 
     The horizontal play afforded to the heat sinks occurs in the direction of this axis L. A flange  44  having holes  46  traverses a perimeter of the housing  12  in a plane parallel to a rear wall  50  (as seen best in FIGS. 2A and 2B) of the heat sink compartments  14 . The flange  44  and holes  46  may be used to install the stack assembly  10  to a cabinet wall or other support structure (not shown). 
     Referring specifically to FIG. 1B, the stack assembly  10  is illustrated in a partially exploded view in which one PCB  20  is shown in isolation. As noted above, PCB  20  features mounting bracket  24  for mounting the PCB  20  directly onto one of the heat sinks  18 . Each mounting bracket  24  is provided with one or more holes  52  through which screws  54  pass in order to seat into screw holes  56  located in an exterior surface of the corresponding heat sink  18 . The mounting bracket  24 , in conjunction with the depth of the corresponding heat sink compartment  14 , is configured to substantially axially align the centre C of the conductive faces  58  of a press pack device  22  with the longitudinal axis L of the stack assembly  10 . The press pack devices  22  and heat sinks  18  are thus arranged in abutting, axially aligned relationship such that current may flow through the heat sinks and into the press-pack semiconductor device, and heat may flow from the press pack devices  22  to the heat sinks  18 . 
     When all of the elements of the stack are inserted into their respective compartments in the housing  12 , the tensioning nut  38  may be tightened. This causes the force applying member  34  of the clamping mechanism to press against the adjacent first heat sink  18 , and hence to apply a mounting force to the other elements of the stack assembly  10 . The plates  32  and  42  of the clamping mechanism bear against the housing  12  which functions to distribute the clamping or mounting force amongst the elements of the stack. The housing  12  thus eliminates the need for the clamping rods of a prior art stack assembly. 
     To remove a PCB  20  from the stack assembly  10 , it is only necessary to loosen the tensioning nut  38  and loosen the screws  54  affixing the selected PCB  20  to its corresponding heat sink  18 . Given the horizontal play provided to the heat sinks  18  in their compartments  14 , it then becomes a simple matter to remove the selected PCB  20  whilst the bulk of the stack assembly remains in an assembled and axially aligned state. The removal of the PCB  20 , for example for servicing, and the subsequent reassembly of the stack will thus only take a few minutes to accomplish. 
     Referring additionally to FIGS. 3,  4  and  5 , a means for floatingly mounting the heat sinks  18  to the housing unit  12  is shown and described. As shown best in FIGS. 2A and 2B longitudinal slots  60  are molded or cut into a rear wall  50  of each heat sink compartment  18 . Anchors  62  (FIG. 3) and  72  (FIG. 4) are slidingly fitted into these slots. More particularly anchor  62  (FIG. 3) is used exclusively to mount heat sink  18 . This anchor features a cylindrical portion  64  having a diameter smaller than the longitudinal dimension of slot  60 . The cylindrical portion  64  is fitted within the slot  60  and may slide therealong. A flange  67  which is sized larger than the slot abuts one side of the rear wall  50  of compartment  14 . Once the anchor  62  is inserted into slot  60 , a washer  63  is placed over the distal end of the anchor  62  and a clip  66  is fitted into a circumferential grove  68  of the anchor  62 . The washer  63  is held in place by the clip  66  and abuts the opposite side of the rear wall  50  so as to retain the anchor  62  to the rear wall  50  of the housing unit  12 . The distance between the flange  67  and the washer  63  is slightly greater than the thickness of the rear wall  50 . The cylindrical portion  64  of the anchor  62  features a threaded bore  69 . The heat sink  18  features apertured wings or ears  70  (FIG. 5) so that the heat sink  18  can be fastened to the threaded bore  69  via a bolt (not shown). In this manner anchor  62  may be floatingly mounted to the housing unit  12  in such a way as to not interfere with the horizontal play permitted to the heat sink  18  when disposed in its compartment  14 . 
     Anchor  72  (FIG. 4) is used to fasten the heat sink  18  (which conducts electricity to and from the press-pack semiconductor device) to the housing  12  as well as to a power lead or lug  74 . This is shown in FIGS. 12 and 13. As with anchor  62 , anchor  72  (FIG. 4) features a cylindrical portion  76  having a diameter smaller than the longitudinal dimension of slot  60 . The cylindrical portion  76  is fitted within slot  60  and may slide therealong. This anchor also features a flange  78  which is sized larger than slot  60  such that the flange will abut one side of the rear wall  50  of compartment  14  when the anchor  72  is inserted in slot  60 . A washer  63  is fit over the cylindrical portion  76  and is held in place by a clip  80 , which is fitted into a circumferential grove  82  of the anchor  72 . The washer  63 , held in place by the clip  80 , will abut the other side of the rear wall  50  and thereby retain the anchor  72  to the housing  12 . The distance between the flange  78  and the washer  63  is slightly greater than the thickness of the rear wall  50 . Anchor  72 , however, features a non-threaded bore  84  as well as a flared opening  86 . The power lug  74  features a conical front end  88  which may be seated into the opening  86 . The front end  88  of the lug features a threaded inner bore  90  (shown in phantom) to which one of the apertured ears  70  of heat sink  18  may be fastened via a bolt (not shown). In this manner the anchor  72  may be floatingly mounted to the housing  12  in such a way so as not to interfere with the horizontal play permitted between the heat sink  18  and the locating nubs  26  of its corresponding compartment  14 . It will thus also be seen that anchor  72  features a means for simultaneously mounting a power lug to the housing  12 . 
     Referring additionally to FIGS. 10A and 10B the power lug  74  features a rectangular fin  73  (FIG. 4) at its rear end which is used to attach the power lug  74  to a bus bar  77 . The bus bar  77  is rigidly connected at its opposite end to a terminal  83 . The rectangular fin  73  is provided with two non-threaded bores  75  therethrough. The bus bar  77  is constructed of a flat generally rectangular piece of metal and is provided with two non-threaded bores  79  passing through the thickness of the bus bar  77 . The bores  79  correspond in size and location to the bores  75  in the power lug  74 . The power lug  74  is attached to the bus bar  77  via two bolts (not shown), each passing through corresponding bores  75  and  79 , respectively. Each bolt is secured with a nut (not shown). The bus bar  77  features a U-shaped bend  81  in the same direction as the axis L. The U-shaped bend  81  provides the bus bar  77  with some flexibility, permitting the power lug  74 , as well as the rigidly connected heat sink  18 , some horizontal play. The bus bar  77  may alternatively be provided with some flexibility by constructing the bus bar  77  using three thin, flat, generally rectangular pieces of metal laminated to one another (not shown). This arrangement eliminates the need for a U-shaped bend  81 . 
     In an alternative embodiment, horizontal play between the power lug  74  and the bus bar  77  is achieved using the arrangement shown in FIGS. 11A and 11B. The bores  75  through the rectangular fin  73  are at least the diameter of cylindrical spacers  85 . A straight bus bar  77 ′ features a longitudinal slot  91  that is at least as wide as the diameter of the cylindrical spacers  85 . To connect the power lug  74  to the bus bar  77 ′, the cylindrical spacers  85  are inserted through the bores  75  and through the slot  91 . The cylindrical spacers  85  are provided with a smooth longitudinal bore  87  for receiving a bolt  89 . The bolt  89  secures the bus bar  77 ′ to the power lug  74  via the cylindrical spacers  85  and is secured with a nut  93 . The cylindrical spacers  85  must be longer than the total thickness of the bus bar  77 ′ and the rectangular fin  73  so that there is some horizontal play in the power lug  84  and the heat sink  18  connected thereto. 
     Good electrical contact between the power lug  74  and bus bar  77 ′ is achieved by securing a wire  95 , such as a braided wire, at one end to the bus bar  77 ′ and at the other end to the power lug  74 . The flexible connector may be secured to the bus bar  77 ′ by a bolt  97  inserted through a non-threaded bore  101  and secured by a nut  103 . The flexible connector  95  is secured to the power lug  74  using a bolt  97  screwed into a threaded bore  99  (shown in phantom) in the shaft of the power lug  74 . 
     As shown in FIGS. 2A and 2B, each heat sink  18  is mounted over a rectangular opening  92  located in the rear wall  50  of heat sink compartment  14 . The opening  92  is substantially the same shape and area of, but slightly smaller than, the rear face of heat sink  18 . A cooling airflow may be drawn via a fan (not shown) through fissures  94  (FIG. 5) of the heat sinks  18 . In order to ensure that the cooling air flow flows only through the heat sinks, gaskets  96  are disposed between the rear face of the heat sink and the rear wall  50  of the housing unit in order to provide a solid seal therebetween. 
     Referring additionally to FIGS. 6,  7 A,  7 B,  8  and  9 , an opposite side of the housing  12  has compartments  100  that are designed to accommodate a resister network  102 . Each resistor network  102  comprises a plurality of power resisters  104  (see best in FIG. 8) which are mounted on a frame  106  which in turn is mounted to support plates  108  (one of the support plates is shown in stippled lines in FIG.  8 ). The support plates  108  feature edges  110  for engaging grooves  114  of a locator plate  112  (FIG.  9 ). The locator plate  112  also features tapered edges  116  which are designed to engage registering slots  118  molded into the resistor network compartment  100  (see FIGS. 7 a,    7   b ). When a resistor network  102  is installed into compartment  100  (FIG.  6 ), the locator plates  112 , placed at both ends of the resistor network  102 , guide the resistor network  102  into the compartment  100  via the registration slots  118 . The resistor networks  102  are further retained by a retaining rod  120  that runs longitudinally along the centre line of the rear face of the housing  12  and passes through mounting holes  122  (see best in FIG. 7 a ) located in walls  124  of the housing  12 . 
     Cooling air is drawn transversely through the stack assembly  10  by a fan (not shown). Cooling air first passes through the heat sinks  18 , thus cooling the press-pack semiconductor devices  22 . The air, now slightly warmer, then passes through the resister networks  102 . This design optimizes the effect of the cooling air flow because the resisters can withstand higher temperatures than the semiconductor devices. 
     Capacitors  126  may also be attached to the rear of the housing unit  12  at regular intervals corresponding with the location of a dividing wall  128  associated with each compartment  100 . In this manner the housing  12  can mount all of the typical elements, i.e., semiconductor devices, heat sinks, and resistors and capacitors for snubbing networks, associated with medium voltage power circuitry. Those skilled in the art will understand that numerous variations modifications may be made to the embodiment described herein without departing from the spirit and scope of the invention.