Abstract:
A power resistor may be formed as a stacked arrangement of first and second terminal plates positioned on either side of a resistor plate and insulated therefrom by interposing first and second insulator plates. Preferably, the insulator plates are metallic plates with non-conductive surfaces. As an example, anodized aluminum plates may be used. The metallic insulator plates provide good thermal conduction paths between the resistor plate and the opposing terminal plates, allowing efficient heat transfer from the power resistor. Further, with metallic insulators, each layer in the stack may be made of metal with attendant structural advantages. For example, the stacked resistor may be subjected to significant compressive force in mounting without need for special precautions or load distribution measures, as might be required with ceramic insulating layers. Preferably, the stack includes interlayer features allowing it to be frictionally fitted together, thus simplifying assembly.

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to electrical resistors, and particularly relates to power resistor applications. 
     Power resistors find broad application across a variety of system types and applications. In many electric motor applications, power resistors are used as braking loads for the motors. In other applications, such as in frequency converters, power resistors are used in snubber circuits that protect high-power switching devices by suppressing voltage and current spikes arising from line switching actions. 
     While these and other applications differ significantly in terms of end purpose, many of the design and operating challenges imposed on power resistors are common across the range of uses. Power resistors must generally provide reliable, safe, long term operation under rated power conditions, which often entails extended operation at high power levels. Because the dissipation of electrical energy through resistance involves converting electrical energy to thermal energy, power resistor reliability depends on good thermal management. 
     Often, power resistors are required to dissipate so much electrical power that the resultant heat would be damaging absent some form of heat sinking. Heat sinking entails placing a heat-generating object in thermal contact with a heat-dissipating object. In practical terms, for power resistors, this often entails placing the power resistor in good thermal contact with a larger heat radiator, or even in contact with a liquid cooled heat sink. 
     In whatever configuration successful thermal management depends on the efficient conduction of heat away from the power resistor and into the heat sink. Without good thermal conduction, operating temperature of the power resistor may rise to dangerous and damaging levels, which is unacceptable in any practical system. 
     Many techniques exist for enhancing the thermal performance of power resistors. First, the resistors themselves may be made to have good intrinsic heat dissipation characteristics. Imbuing such characteristics conventionally entails making the resistor larger, which has obvious disadvantages in size-constrained systems. This is exacerbated by the tendency to use banks of power resistors, rather than just one or two such devices in a given system. 
     Other approaches focus on establishing a thermal conduction path between each power resistor and the heat sink. Maintaining good heat flow requires minimizing thermal resistance at the junction between the power resistor and the heat sink. Techniques for accomplishing this minimization include the use of thermal bonding compounds, along with mating the power resistor to the heat sink under relatively high contact pressure. Inherent in these approaches is the notion of providing generally smooth, flat mating surfaces between resistor and heat sink. 
     Arguably, too many tradeoffs arise from the above considerations. Installation considerations limit the size and therefore intrinsic heat dissipation capability of power resistors, which imposes the requirement to efficiently conduct heat out of the power resistor into a heat sink. Thus, power resistor package must comprise materials that provide good thermal conduction, yet the need to minimize thermal contact resistance requires relatively high contact pressure requirements. The resultant mechanical stresses suggest the need for mechanically robust power resistor packages, but this must be balanced against the thermal properties of the materials used. 
     Thus, a power resistor that embodies good thermal conduction, mechanical robustness, and small size is needed. Preferably, this power resistor would be relatively simple to manufacture, and would accommodate various mounting arrangements. The present invention addresses these and other needs, as will be made evident later herein. 
     BRIEF SUMMARY OF THE INVENTION 
     A power resistor includes features that enhance its performance in high-power electrical systems, and may be formed in a stacked arrangement with opposing terminal plates. Generally, the power resistor includes two terminals for contacting with an external system, and a resistor element providing the desired electrical resistance between the two terminals. Preferably, an electrical insulator is positioned between each terminal and the resistor element to prevent electrical shorting between the two terminals across the resistor element. By using electrical insulators with favorable thermal conduction characteristics, the insulators provide efficient thermal conduction paths from the resistor element into the two terminals, one or both of which may be in contact with an external heat sink. 
     In some embodiments, the insulators are made from aluminum or other metal to capitalize on the good thermal conduction and mechanical strength of metal. A surface treatment, such as an anodization process for aluminum, is used to render the metallic insulator&#39;s surface nonconductive. The use of treated metal as electrical insulation within a power resistor structure provides the power resistor the ability to withstand compressive mounting forces without need for special precautions, as well as providing good thermal conduction between the resistor element and the terminals. 
     When implemented in a stacked arrangement, the component pieces comprising the power resistor are preferably joined by mechanically pressing them together. An exemplary stack includes top and bottom terminals with a resistor element positioned between them, and with an insulator positioned between each terminal and the resistor element. The different elements within the stack include mechanical features that establish the desired electrical contact points between the two terminals and the resistor element, and that further provide the inter-element contacts that join the stack when mechanically pressed together. 
     In an exemplary stack arrangement, the terminals, insulators, and the resistor are all substantially flat plates or discs that stack together. In this arrangement, the top or first terminal has a raised projection on its inner surface that is preferably centrally located. The insulator disposed between this first terminal and the resistor element has a central cutout or opening that exposes the resistor element. The terminal&#39;s projection passes through the opening, making mechanical and electrical contact with the resistor element. The resistor element may include a opening corresponding to the shape of the terminal&#39;s projection and sized to allow the resistor element to be pressed onto the projection, thus fixing the resistor element to the top terminal, with the intervening insulator sandwiched between them. 
     Similarly, the bottom or second terminal has a perimeter lip formed on its inner surface, with the lip defining an inset area or recess. The second insulator is sized to fit into this recess and is positioned therein. In turn, the resistor element is sized such that its outer perimeter conforms to the terminal&#39;s perimeter lip, and just matches or is slightly larger in size than the inset area. Thus, the resistor element may be joined to the second terminal by pressing it into the inset area. In this manner, only the outer perimeter of resistor element is in electrical contact with the second terminal. 
     When implemented in the above stacking arrangement, the power resistor comprises a small, mechanically robust package that is well suited for high power applications. The power resistor is well suited for continuous power dissipation, and for operation subject to high power level transient voltage spikes. Its use of thermally efficient electrical insulators to draw heat from the resistor element into the two terminals allows the power resistor to dissipate very high levels of electrical power, provided proper heat sinking measures are taken. The preferably flat and outwardly smooth terminals complement heat-sinking arrangements by providing relatively large, low thermal-resistance surface areas for contacting the power resistor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded view of an exemplary power resistor in accordance with the present invention. 
     FIG. 2 is a cross-sectional view of the power resistor detailed in FIG.  1 . 
     FIG. 3A is a diagram of an exemplary configuration for the resistor element of FIG.  1 . 
     FIG. 3B is a diagram of an exemplary alternate configuration for the resistor element of FIG.  1 . 
     FIG. 4A is a diagram of an exemplary circuit application for the power resistor of FIG.  1 . 
     FIG. 4B is a diagram of a mechanical arrangement for heat sinking the circuit of FIG.  4 A. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates an exemplary configuration for the power resistor of the present invention. In this embodiment, a power resistor  20  comprises top and bottom terminals  22 A and  22 B, top and bottom electrical insulating elements  24 A and  24 B, and a resistor element  26 . Order of stacking comprises terminal-insulator-resistor-insulator-terminal. When stacked together, the elements comprising the power resistor  20  define an electrical path from the terminal  22 A to the terminal  22 B through the resistor  26 . 
     As will be detailed later, the power resistor stack preferably uses mechanical interference between the top and bottom terminals  22 A and  22 B and the resistor  26  for fastening. That is, the stack elements are aligned and then mechanically pressed together. After such pressing, an optional high current pulse may be used to effectively weld the resistor  26  to the respective terminals  22 A and  22 B. 
     Terminals  22 A and  22 B preferably comprise metallic conductors formed as plates or discs with smooth, flat outer surfaces. Copper represents an exemplary terminal material because of its low electrical and thermal resistances. As with any thermal conduction application, maximizing the surface area at heat transfer interfaces reduces thermal resistance. Thus, the flat outer surfaces of the terminals  22 A and  22 B provide essentially ideal contact surfaces for external mounting arrangements and heat sinks. However, in some applications, advantage may be gained by forming or attaching some type of mounting or fastening feature on the exterior of one or both terminals  22 A and  22 B. Such variations are reasonably understood to apply to any device that may be used in a broad range of applications. 
     The resistor element  26  defines the electrical resistance seen by current flowing through the power resistor  20 . As with the terminals  22 A and  22 B, the resistor  26  is preferably formed as a metal plate or disc, with its specific geometry normally chosen to complement that of the terminals. A range of metals or alloys thereof may be selected for forming the resistor  26 . Generally, a balance between desired operating temperature, thermal expansion characteristics, and specific resistance determines material selection. Later discussion provides more particular structural details for the resistor  26 , and makes clear how a desired current path is established between the terminals  22 A and  22 B through the resistor  26 . 
     By internally separating the terminals  22 A and  22 B from the resistor  26 , the insulators  24 A and  24 B also help establish the desired electrical path. Simply, the insulators  24 A and  24 B are non-conductive elements interposed between the resistor  26  and respective terminals  22 A and  22 B. Metal represents an ideal material selection for the insulators but for its electrical conductivity. That is, metal generally provides good thermal conduction, and responds well to mechanical strain. This last characteristic is particularly beneficial as power resistors are often mounted to heat sinks under large compressive force to maximize heat transfer from the power resistor. Such forces can typically range as high as 60,000 Newtons (N). 
     Rendering the metallic material used for the insulators  24 A and  24 B non-conductive may involve one or more approaches. Certain surface coatings or jackets might be applied to the base metal to render its surface non-conductive. However, any approach adopted should not seriously degrade heat resistance, thermal conduction, and mechanical suitability. With aluminum as the base metal, the insulators  24 A and  24 B may be anodized, in which a hard, non-conductive and continuous layer of aluminum oxide is formed on the aluminum&#39;s outer surface. 
     Anodization may rely on the ELOXAL process, which is well documented in the art. With ELOXAL treatment, a microcrystalline layer of Aluminum Oxide (e.g., Al 2 O 3 ) forms on the surface of the aluminum work piece. As is known, with proper control of the ELOXAL process parameters, the aluminum oxide may be formed in a continuous, uniform layer over the work piece&#39;s surface at a thickness in excess of 25 um. At the expense of additional process time, the layer thickness may be increased to 40 um or more, although such thickness is typically not necessary. 
     Anodized aluminum satisfies the requirement of being electrically non-conductive while still having excellent thermal conduction properties. For reference, base aluminum has a characteristic thermal conductivity in the range of 195 Watts per Kelvin•meter (W/Km). Typical electrically insulative plastic materials have thermal conductivities ranging from 0.6 to 3.5 W/Km. In contrast, Al 2 O 3  at roughly 96% purity has a thermal conductivity in the range of 26 W/Km, which is significantly better than typical plastic insulators. Ceramic (e.g., Aluminum Nitride or AIN) offers excellent thermal conduction, having a thermal conductivity in the range of 110 to 180 W/Km. However, ceramic is relatively expensive, and its fragility leaves it ill suited for the high mechanical stresses power resistors are often subjected to without special precautions in mounting or in resistor construction. 
     Anodized aluminum also has a surface hardness that complements its use in the power resistor stacking arrangement. As the various stack elements likely comprise different materials (e.g., different metals), different layers of the stack might be expected to have differing thermal expansion characteristics. Thus, the resistor  26 , terminals  22 A and  22 B, and insulators  24 A and  24 B may all expand or contract to a greater or lesser extent relative to each other as the power resistor  20  heats and cools. Differences in expansion may cause some movement between stack layers, so providing the insulators  24 A and  24 B with a hard surface layer helps maintain the integrity of the nonconductive insulator surfaces over the operating life of the power resistor  20 . 
     As noted above, the interposition of insulators  24 A and  24 B restricts electrical contact between the resistor  26  and respective terminals  22 A and  22 B to desired contact points or areas. The restriction of electrical contact with the terminals combined with the design of the resistor  26  defines the electrical path through the power resistor  20 . 
     To establish this path, a preferably cylindrical projection  30  on the inner surface of terminal  22 A projects downward through a central opening  32  of the insulator  24 A to make contact with the resistor  26 . Preferably, the resistor  26  includes a central opening  34  that allows it to be pressed onto the projection  30 . Thus, the projection  30  projects into and engages with the opening  34 . 
     This engagement between the projection  30  and the inner surface of the opening  34  provides electrical and mechanical contact between the terminal  22 A and the resistor  26 . Preferably, the opening  32  in the insulator  24 A is slightly larger than the diameter of the projection  30 , whereas the diameter of the opening  34  in resistor  26  is slightly smaller than the diameter of the projection  30 . As the height of the projection  30  at least preferably equals the combined thickness of insulator  24 A and resistor  26 , this sizing of the openings allows the resistor  26  to be seated onto the projection  30  by a mechanical press, with the insulator  24 A positioned between it and the terminal  22 A. 
     With electrical contact between the terminal  22 A and the resistor  26  thus made, allowing the resistor  26  to contact the bottom terminal  22 B along a perimeter contact area  36  completes the electrical path through the power resistor  20 . The terminal  22 B has a perimeter lip  38 , here a circumferential lip or ridge, which defines an interior inset region  40 . The depth of the inset region  40  is preferably sufficient to receive the combined height of the insulator  22 B stacked together with the resistor  26 . As such, the outer diameter of the insulator  24 B is made slightly smaller than the inner diameter of the perimeter lip  38 , such that the insulator  24 B drops into the inset area  40 . 
     In contrast, the outer diameter of the resistor  26  is generally made equal to or slightly larger than the inner diameter of the perimeter lip  38 , such that there is a defined amount of mechanical interference between the resistor  26  and the terminal  22 B as the resistor  26  is pressed into the inset area  40  of the terminal  22 B. This allows the resistor  26  to be securely joined with the terminal  22 B by mechanically pressing it into the inset area  40 . 
     With the designed-in mechanical interference between the terminals  22 A and  22 B and the resistor  26 , the component parts of the stack may be frictionally fitted together by mechanical press. One might place the insulator  24 B into the inset area  40  of terminal  22 B, and then press the resistor  26  into place. This subassembly might then be fitted onto the central projection  30  of the terminal  22 A and pressed into place, with the insulator  24 A placed on the terminal  22 A before attaching the subassembly. Preferably, however, the elements comprising the power resistor  20  are aligned in their proper stack order, and pressed together in one operation. 
     FIG. 2 is a cross-sectional diagram of the power resistor stack  20 . The use of insulators  24 A and  24 B in restricting electrical contact between the resistor  26  and the top and bottom terminals  22 A and  22 B is more clearly shown in this cross-sectional view. Note that the height of the projection  30  may vary, although it should terminate before extending through the plane of the bottom terminal  22 B. 
     Current may flow through the power resistor  20  in either direction, but for purposes of discussion current is assumed to enter the top of the power resistor  20 . Electrical current flows into the top terminal  22 A and into the resistor  26  via contact between the projection  30  and inner surface of opening  34  in the resistor  26 . Current then flows outward through the resistor  26  in a path defined by the cut pattern of the resistor  26 . This cut pattern is discussed more clearly later herein. Contact between the outer circumference of the resistor  26  and the inner wall of the lip  38  formed in the bottom terminal  22 B allows the current to flow into the terminal  22 B and on into exterior devices or systems. 
     One or more elements within the stack comprising the power resistor  20  may take on other geometries. For example, the stack may comprise rectangular plates. This configuration may have advantages for arrays of power resistors  20 . As the overall geometry of the stack elements may change, so too may the geometry of the interior features of the stack that permit mechanical joining. Thus, the terminal  22 A may have one or more non-cylindrical projections  30  for contacting and fastening to the resistor  26 . Similarly, the perimeter lip  38  of the terminal  22 B may be changed or altered as needed to conform to the overall geometry of the terminal. 
     FIG. 3A illustrates an exemplary embodiment for the resistor  26 . Preferably the resistor is a metallic disc or plate having the inner and outer contact areas or points  34  and  36  earlier discussed. 
     One or more cut lines  50  determine the electrical path between contact areas  34  and  36  of the resistor  26 . These cut lines  50  may be etched, machined, laser cut, or formed by any other suitable process. In the illustration, cut lines  50  comprise a single continuous spiral cut made from the inner region of the resistor  26  continuing on in a spiral pattern to its outer area. This defines a conduction path of a desired length. This length, along with the specific resistance of the material from which the resistor is formed determines the electrical resistance of the power resistor  20 , ignoring any contact resistances. 
     FIG. 3B depicts an alternate exemplary configuration for the resistor  26 . In some applications, the electrical system in which the power resistor  20  is used may be sensitive to inductance. In such instances, it may be desirable to configure the resistor  26  to have as low an inductance as possible. Thus, the cut lines  50  may be varied or altered to minimize or eliminate inductance in the current path between the contact areas  34  and  36 . 
     A power resistor  20  formed in accordance with the above exemplary details provides axial current and heat flow, which may simplify mounting within an electrical system, and complements compressive mounting against a heat sink. Here, axial heat flow denotes a general heat flow direction that is normal to the plane of the insulators  24 A and  24 B. 
     FIG. 4A is a simplified diagram of a typical circuit in which the power resistor  20  might be used. The circuit comprises the power resistor  20  electrically connected in series with a high-power semiconductor device  60 , which may, for example, be a diode. This type of arrangement finds common application in a variety of circuits, such as in some types of charging and discharging applications. For example, the power resistor  20  might serve to limit inrush current into a capacitor bank (not shown) during charging, while the semiconductor  60  may act to block reverse current from the capacitor bank, or serve some switching function. In operation, then, the power resistor  20  and the semiconductor  60  may generate significant heat, depending upon the magnitude and frequency of the current pulses passing through them. 
     An exemplary mechanical for heat sinking the circuit of FIG. 4A is illustrated in FIG.  4 B. It should be noted that the illustrated arrangement is simply one of many possible physical arrangements for using the power resistor  20  in practical applications. 
     As shown, the power resistor  20  and the semiconductor  60  are pressed against opposing sides of a water-cooled heat sink  62 . Cooling fluid, which may or may not be water circulates through the heat sink  62  and serves to conduct heat away from the power resistor  20  and semiconductor  60 . Pressure plates  64  interface the semiconductor  60  and the power resistor  20  to opposing screw clamps  66 , which may be tightened to achieve the desired compression for efficient heat sinking. 
     In exemplary embodiments, the material selection and structure of the power resistor  20  allows it to achieve heat dissipation performance better than 500 Watts per square inch (500 W/in 2 ), and voltage withstand capabilities greater than one kilo-volt (1 KV). As such, the exemplary power resistor  20  provides a comparatively small package capable of operating under high voltages and demanding thermal conditions. 
     Variations of the present invention may be practiced without departing from its scope and intent. Details in the above discussion and accompanying illustrations are exemplary and should not be construed as limiting. Indeed, the present invention is limited only by the following claims and their reasonable equivalents.