Abstract:
A lug with relatively small resistance is provided that allows current to be determined between a source and a load by measuring the voltage drop across the lug. The voltage drop is sufficient to be above the electronic noise, and yet the resistance of the lug is low enough so that the heat produced by current flow is small compared with the heat generated by the source. The material comprising the lug is of a type and strength to be readily manufactured, and its resistivity varies by a relatively small amount with temperature.

Description:
FIELD OF THE INVENTION 
     This invention relates to a current-sense resistor, and more particularly to a current-sense resistive lug used as a connector in a hybrid power module. 
     BACKGROUND OF THE INVENTION 
     Electrical and electronic applications frequently use modules for many purposes. A module comprises several electronic components such as transistors connected in a standard arrangement in one package. Monolithic modules, also known as integrated circuits, consist of one piece of semiconductor material, typically silicon. Hybrid, or multi-chip, modules consist of two or more pieces of semiconductor material connected to one substrate. 
     Modules have the principal advantage of saving cost and speeding manufacturing compared with hand-assembled components. In addition, all the heat from a module is dissipated in one surface. This may aid in the design of the device that incorporates the module. 
     One use of modules is in control circuits for electric motors. These modules may be used as inverters to convert DC to AC to power a motor. It is often useful to measure the current between one or more transistor switches within the module and a resistive or inductive load. The load may be a motor winding connected to the transistor switches. The modules may be used instead to convert AC to DC, that is, as synchronous rectifiers. In that application, it may be useful to measure the current at the input to the transistor switches. 
     A current measurement may be used to protect the switch or the load from damage. Secondly, the measurement may be used to control the torque of the motor. Thirdly, the measurement may be used to control the positional state of the motor, for example, the angle of rotation. For these and other reasons, it is desirable to measure the current. The measured current may be DC or AC, and the AC current may be sinusoidal, square wave, or other waveforms. 
     In several prior approaches, devices external to the module, or additional devices within the module, were used to measure current. The laws of physics allow current to be measured in only a few ways. For example, Hall effect transducers are known in the art. Secondly, current transformers are well known in the art. Thirdly, additional current-sense resistor shunts within or external to the module are well known. Finally, it is also known to use transistors having Kelvin connections. Shunt resistors or Kelvin connections carry a fraction of the current to be measured. 
     In an application in which power transistors are mounted on a substrate to form a switch in a module, a sense resistor might be made part of the substrate. The resistor might also be mounted at right angles to the substrate. However, all the designs described herein before require space, add weight, generate heat, and increase the expense or difficulty of manufacturing the hardware. 
     It is known to use a conducting lug, for example, one made of tinned copper, to connect cables or bus bars to transistor switches, or to other electronic components. A lug is simply a projection on a metal part that is used as a connection. These prior lugs could not be used to sense current because the voltage drop across them would be near zero, and would be lost in the noise. 
     DISCLOSURE OF THE INVENTION 
     Accordingly, an object of the present invention is to provide a current-sense apparatus that will fit within a power module. 
     Another object of the invention is to provide a current-sense lug with relatively small resistance that allows current to be determined by measuring the voltage drop across the lug. 
     Still another object of the invention is that the voltage drop across the lug is sufficient to be above the electronic noise, and yet the resistance of the lug is low enough so that the heat produced is small compared with the heat generated by the transistor switches. 
     Yet another object of the present invention is that the resistance of the lug varies by only a relatively small amount with temperature. 
     A further object of the present invention is that the material comprising the lug is of a type and strength to be readily manufactured. 
     Another object of the invention is that the lug has sufficient strength to help fasten a cable end to transistor switches. The term “cable” as used herein should be understood to mean a cable, a bus bar, or other similar devices for conducting electricity. 
     Yet another object of the invention is that the lug carries the entire current to be measured. 
     Still another object of the invention is that the lug is used not only to secure the cable, but also to sense current, so that the current-sense function takes up no additional space in the module. 
     A major step in the invention is the recognition that the existing means to connect a cable to a transistor switch can also be used to sense current if the resistance value of the connector lug is of a certain, predetermined value. 
     According to the invention, a power hybrid module has at least one transistor connected as a switch, a cable connecting the output point of the switch to an inductive or resistive load such as a motor winding, and a lug that fastens and electrically connects the cable to the output point of the switch. The lug has a predetermined, known resistance value that allows the current in the cable to be determined by measuring the voltage drop across the lug, the lug resistance varies minimally with temperature, and the lug material has sufficient strength to be manufactured and to help fasten the cable. 
     The invention has utility because it provides a miniature current-sense apparatus that fits within a power module. No additional parts are used, no additional space is needed, no weight is added, and relatively little heat is dissipated. The material comprising the lug costs little compared with the remainder of the module. The material is of a type and strength to be readily manufactured. Current is determined by measuring the voltage drop across the lug, which electrically connects and physically attaches the cable to the transistor switch. Unlike prior shunt resistors or Kelvin connections, the lug carries the entire current. The lug is used, therefore, not only to attach the cable, but also to sense current; hence, the current-sense function takes up no additional space in the module. The voltage drop across the lug is sufficient to be above the electronic noise. The heat produced by the lug, however, is small compared with the heat generated by the transistor switches. The current-sense process is nearly linear because the resistance of the lug varies by only a relatively small amount with temperature. 
     The above and other objects, features, and advantages of this invention will become apparent when the following description is read in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an electrical schematic diagram of a hybrid power module driving an inductive load through a lug; 
     FIG. 2 is a cutaway side view, partially exploded, of the hybrid power module of FIG. 1; and 
     FIG. 3 is an isometric view of the lug in the hybrid power module of FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As illustrated schematically in FIG. 1, a hybrid power module  100  consists of two bipolar transistors  110  and  120  connected as a switch. The emitter  122  of the first transistor  110  is connected to the collector  124  of the second transistor  120 . The collector  126  of the first transistor  110  is connected externally to a positive DC power source V+, while the emitter  128  of the second transistor  120  is connected to a negative DC power source V−, which may be ground. The emitter of the first transistor  110  and collector  124  of the second transistor  120  are connected together at a pad  130 . A first flyback diode  131  is connected between the collector and emitter of first transistor  110  for control. A second flyback diode  132  is connected between the collector and emitter of second transistor  120 . The bases  132 ,  134  of the two transistors form the inputs to the switch. 
     The signals at the switch inputs are relatively large in magnitude so that the transistors are either “on” or “off” (i.e., the transistors  110 ,  120  are each operated as a switch). When the second transistor  120  is on (or active mode), and the first transistor  110  is off, the voltage between the collector and emitter, Vce, for the second transistor  120  is relatively small (e.g., approximately 0.2 V for a silicon bipolar transistor). The negative DC power source V− (or ground) then acts as though it were attached to the output pad  130 ; therefore, the output is relatively small or negative. When the second transistor  120  is off, the Vce of the second transistor  120  acts as an open circuit (Vce has relatively high resistance). When the first transistor  110  is on, Vce is relatively small, so V+ is seen by the output  130 . Thus, the output  130  of the module alternates between V+ and V−. 
     A lug  140  connects the pad  130  to a first end of a cable  150 . The term “cable” as used herein should be understood to mean a cable, a bus bar, or other similar devices for conducting electricity. The second end of the cable is connected to a load  160 . This load may be inductive, as illustrated in FIG. 1, resistive, or a mixture of the two. The current in the load is greater than the current at the input; the module  100  acts as a gate that switches a larger current controlled by a smaller current. 
     In FIG. 2, transistors  110  and  120  are mounted on a substrate  170 . The transistors are connected at the pad  130 . The lug  140  has lower and upper ends. The lower end is rigidly attached to the pad  130 . The upper end of the lug is connected to the cable  150  by means of a fastener  200  and a screw  210 . Wires  180  and  190  are connected near the first and second ends of the lug. 
     Referring to FIG. 3, the lug  140  shown in isometric view in the preferred embodiment can be made of various materials. The lug can be mass-produced by forming or stamping it from sheets of the material and bending as required. The preferred material will be discussed in more detail herein below. The lug  140  has a predetermined width w and thickness t. The height h is the distance between the junctions of the connecting wires  180  and  190  with the lug. 
     Equation 1 can be used to compute the electrical resistance R of the lug between the connecting wires  180  and  190 : 
     
       
         R=ρh/wt  Eq.1 
       
     
     where ρ is the resistivity, also called specific resistance, of the lug material. Resistivity of a material is the resistance between two parallel faces that a cubic centimeter offers to electricity flowing perpendicular to the two faces. Resistivity is usually expressed in ohm-meters, that is, resistance times distance. 
     It is often desirable to measure a current I at the input or output of the module  100 . A current measurement may be used to protect the switch or the load from damage. The measurement may be used to control the torque or the angle of rotation of the motor. For these and other reasons, it is desirable to measure the current, which may be DC or AC. Because all the current passes through the lug  140 , and the resistance R of the lug is known, one skilled in the art will realize that Eq. 2 (Ohm&#39;s law) can be used to measure I: 
     
       
         I=V/R  Eq.2 
       
     
     Where V is the voltage drop measured between wires  180  and  190 . The voltage can be measured with a difference amplifier or other circuitry. 
     When the hybrid power module  100  is used to drive a motor winding  160 , including possibly a three-phase motor winding, typical currents between a few amperes and many hundred amperes may pass through the lug  140 . In order to measure the current with accuracy, the lug is designed to produce a voltage drop of about 50 to 500 mV. This is sufficient to be above the electrical noise, but small enough so that negligible heat, measured by I times I times R, is produced by the required resistance of the lug. Depending upon the current for the application involved, the predetermined resistance value of the lug  140  may be between 0.5 milliohm and 250 milliohms. Because the size of the lug may vary, the resistivity of the lug material may between 500 and 2000 nano-ohm-meters. To achieve linear current sense capability, the resistance value of the lug must vary with temperature by no greater than ±30 parts per million per degree Celsius. 
     Typical dimensions for the lug  140  to fit within a hybrid power module  100  are a width w and a height h of a relatively small number of centimeters, and a thickness t of a relatively few tenths of a centimeter. Using Eq. 1, a resistivity of a few hundred to a few thousand nano-ohm-meters (nΩ-m), and preferably between 300 and 3000 nΩ-m, is necessary for the lug material. 
     In the preferred embodiment, the lug  140  is made of a resistance alloy that has the following properties: 
     a) uniform resistivity throughout; 
     b) stable resistance with time; 
     c) relatively high ductility and mechanical strength for ease of manufacture; 
     d) ability to be soldered or welded; 
     e) relatively small temperature coefficient, that is, change of resistance with temperature; 
     f) relatively small thermoelectric potential with copper; 
     g) relatively good corrosion resistance; and 
     h) low cost compared with the module. 
     Nickel-base alloys have properties a) through h). Thus, these alloys are used in the preferred embodiment. For example, an alloy of 76 nickel-17 chromium-4 silicon-3 manganese has a resistivity of 1330 nΩ-m. The temperature coefficient of resistance for this alloy is ±20 parts per million per ° C. Its thermoelectric potential is −1 μV per ° C. versus copper, and its tensile strength is 900-1380 Mpa. 
     Thus, the current-sense lug  140  of the invention provides a highly reliable, compact means to measure current in a module without overheating the module  100  or adding size, complexity, cost, or weight. 
     This invention of a current-sense lug has been illustrated and described with respect to a specific embodiment thereof. It should be understood by those skilled in the art that this is not an exclusive embodiment. Although this invention is described in detail for use in a hybrid power module with bipolar transistors, those skilled in the art will realize that other types of transistors or other electronic components may be connected to the current-sense lug. In particular, CMOS or MOSFET switches, insulated gate bipolar transistors (IGBTs), or other semiconductors may be used instead of bipolar transistors. 
     The current-sense lug can be made in any size or shape. Wires  180  and  190  can be replaced by connections already in place to the two ends of the lug, although some accuracy may be sacrificed. Those skilled in the art will also realize that the present invention can be used anywhere that current needs to be measured in a reliable, compact manner. The lug need not be inside a module. 
     In fact, the invention need not be in the form of a lug; the invention can be a component of relatively small resistance used in any electrical application. Obvious size, shape, and material modifications to the current-sense device can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention should be determined not by the embodiments in the foregoing specification, but by the appended claims and their legal equivalents. 
     All the foregoing variations are irrelevant. It suffices for the invention that a power hybrid module has at least one transistor connected as a switch, a cable connecting the output point of the switch to an inductive or resistive load, and a lug that fastens and electrically connects the cable to the output point of the switch. The lug has known resistance that allows the current in the cable to be determined by measuring the voltage drop across the lug, the lug resistance varies minimally with temperature, and the lug material has sufficient strength to be manufactured and to help fasten the cable.