Patent Publication Number: US-8531264-B2

Title: Current sensing resistor and method for manufacturing the same

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a resistor, and more particularly, to a current sensing resistor. 
     2. Description of the Prior Art 
     Current sensing resistors have been used in the electronic industry for many years, and are formed on the basis of the Kelvin theory or the 4-wire theory. The current sensing resistor is mainly used for application of low resistance, and has the advantages of low temperature coefficient and high heat dissipation performance when compared with general resistors. A conventional current sensing resistor (such as the U.S. Pat. No. 5,999,085) adopts a structure where a metal plate with fixed resistance is a middle portion and each of the two opposite terminals of the plate is fixedly connected to a side portion with high electrical conductivity. Each of the pair of side portions has a slot, dividing each of the pair of side portions into a current terminal and a sensing terminal. The length of the slot may be used for deciding the stability of resistance of the current sensing resistor. 
     The conventional current sensing resistor is formed through the fixed connection of different materials of metal or alloy, which is time-consuming during manufacturing and is also difficult to control the material characteristics of the metal or alloy. Moreover, other methods such as soldering or adhering are inevitably used during the fixed connection process, and the use of extra materials renders that the conventional current sensing resistor is incapable of fully demonstrating the material characteristics of a resistor substrate. As a result, the stability of resistance of the current sensing resistor is affected. 
     Therefore, a current sensing resistor made through an integral molding method is required in the market, allowing such current sensing resistor to be formed by only one material of metal or alloy. Therefore, the characteristics of the metal or alloy may be fully demonstrated, and it will also be easier to select the corresponding metal or alloy according to the required resistor characteristics. In this manner, manufacturing is more convenient, and the stability of resistance of the current sensing resistor is further improved. 
     SUMMARY OF THE INVENTION 
     In order to achieve the above objectives and efficacies, the present invention adopts an innovative technical means and an innovative method. 
     An embodiment of the present invention provides a current sensing resistor, which is made by a highly electrically conductive metal plate, and the metal plate includes: a middle portion; a first portion, located at one side of the middle portion, having a first slot; and a second portion, located at the other side of the middle portion opposite to the first portion, having a second slot; where each of the first portion and the second portion is divided into a current terminal and a sensing terminal by the first slot and the second slot respectively, and the current terminals of the first portion and the second portion have a length that is greater than that of the sensing terminals of the first portion and the second portion; characterized in that the middle portion has a middle slot and the length of the middle slot can be used for controlling the stability of resistance for the current sensing resistor. 
     Another embodiment of the present invention provides a method for manufacturing a current sensing resistor, which includes: forming at least one resistor substrates on a highly electrically conductive metal plate through stamping, where the resistor substrate has a middle slot at a middle portion and has a slot at each of the two side portions of the middle portion; forming a passivation layer at the middle portion of the resistor substrate; and forming a conductive layer at the two side portions of the resistor substrate. 
     In order to make the aforementioned objectives, features and advantages of the present invention more comprehensible, exemplary embodiments with accompanying drawings are described in detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows the structure of a current sensing resistor according to an embodiment of the present invention; 
         FIG. 2  is an equivalent diagram of a current sensing resistor of  FIG. 1 ; 
         FIG. 3   a  is a diagram of a relationship between the magnitude of current flowing through a current sensing resistor and the magnitude of resistance of the current sensing resistor according to an embodiment of the present invention; 
         FIG. 3   b  is a diagram of a relationship between the magnitude of current flowing through a conventional current sensing resistor and the magnitude of resistance of the conventional current sensing resistor; 
         FIG. 3   c  is a diagram of a relationship between the temperature and the magnitude of resistance of a current sensing resistor according to an embodiment of the present invention; and 
         FIG. 4  shows a method for manufacturing a current sensing resistor according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows an embodiment of the present invention, which is a current sensing resistor  100  made by a highly electrically conductive metal plate, and the current sensing resistor  100  may be divided into two portions, namely a middle portion  102  and a pair of side portions  104 , where the pair of side portions  104  are respectively located at two opposite sides of the middle portion  102 . In an embodiment of the present invention, the side portions may be a first portion and a second portion, which are generally referred to as the side portions  104  herein. Each of the side portions  104  has a slot  112 , and each of the side portions  104  may be divided into a current terminal  106  and a sensing terminal  108  by the slot  112 . The middle portion of the current sensing resistor  100  includes a middle slot  110 , and the depth of the middle slot  110  is used for deciding the stability of resistance of the current sensing resistor  100 . 
     Current flowing through the current sensing resistor  100  mainly passes through the current terminal  106 . Therefore, the length of the current terminal  106  should be greater than that of the sensing terminal  108 , and the length of the current terminal  106  is selected according to the magnitude of the current. 
     In an embodiment, the current terminal  106  and the sensing terminal  108  of the pair of side portions  104  may include a conductive layer (not shown), so that four terminals of the current sensing resistor  100  may be connected to an external circuit. In a preferable embodiment, the material of the conductive layer may include Cu, Ni or Sn. 
     In an embodiment, the material of the metal plate may have a low resistance coefficient and a low resistance-temperature coefficient. The material of the metal plate may be selected according to the characteristics (such as the resistance coefficient or the resistance-temperature coefficient) of the desired current sensing resistor  100 . In a preferable embodiment, the material of the metal plate may include Cu—Mn alloy, Ni—Cu alloy or Mn—Cu—Sn alloy. 
     In another embodiment, the middle portion  102  may be covered with a passivation layer (not shown), for protecting a resistor body portion of the current sensing resistor  100 . In a preferable embodiment, materials such as either resin or a high polymer material may be used for the passivation layer. As shown in  FIG. 1 , in a preferable embodiment, the length (or depth) of the middle slot  110  is greater than or equal to the length of the slot  112  plus the length of the sensing terminal  108 . 
       FIG. 2  is an equivalent diagram of the current sensing resistor  100 . As shown in  FIG. 2 , when the resistance of the current sensing resistor  100  is measured, the current terminal  106  needs to be connected to an ammeter  122 , and the sensing terminal  108  needs to be connected to a voltmeter  120 . The voltage of the voltmeter  120  is divided by the current of the ammeter  122  according to the Ohm&#39;s law, to obtain the resistance of the current sensing resistor  100 . 
       FIG. 3   a  is a measurement result according to an embodiment of the present invention, and a relationship between the resistance of the current sensing resistor  100  and the current passing through the current sensing resistor  100  is measured. An abscissa represents the current, and a unit thereof is ampere; an ordinate represents the magnitude of resistance of the current sensing resistor  100 , and a unit thereof is milliohm. In the present invention, when the current passing through the current sensing resistor  100  is increased from 1 ampere to 30 amperes, the resistance of the current sensing resistor  100  is changed only by 0.004 milliohm.  FIG. 3   b  is a measurement result of a conventional current sensing resistor. When the current passing through the conventional current sensing resistor is increased from 1 ampere to 30 amperes, the resistance of the conventional current sensing resistor is changed by 0.6 milliohm. Therefore, it can be known that, with the same amount of current change (30 amperes), the resistance change of the current sensing resistor  100  of the present invention is much smaller than that of the conventional current sensing resistor. 
     In addition,  FIG. 3   c  is another measurement result according to an embodiment of the present invention, which shows a relationship between the temperature and the magnitude of resistance of the current sensing resistor  100  under a fixed current (30 amperes in this embodiment). An abscissa represents the temperature, and a unit thereof is degree Celsius; an ordinate represents the magnitude of resistance of the current sensing resistor  100 , and a unit thereof is milliohm. In addition to showing the measurement result of an embodiment of the present invention,  FIG. 3   c  also includes a measurement result of the conventional current sensing resistor for comparison. It can be known from  FIG. 3   c  that, when an operating temperature of the conventional current sensing resistor is increased from 20 degrees Celsius to 100 degrees Celsius, the resistance thereof is increased by 0.06 milliohm. When the operating temperature of the current sensing resistor  100  of the present invention is increased from 20 degrees Celsius to 100 degrees Celsius, the resistance thereof is decreased by 0.025 milliohm. 
     Referring to  FIGS. 3   a  to  3   c , when compared with the conventional current sensing resistor, the current sensing resistor  100  of the present invention has a smaller resistance change when the current changes. In addition, the current sensing resistor  100  of the present invention also has a lower temperature coefficient. The lower temperature coefficient may resist a resistance measurement offset caused by a temperature rise due to a high-voltage pulse or high-temperature environment. Therefore, the current sensing resistor  100  of the present invention is more stable. 
       FIG. 4  shows a method for manufacturing a current sensing resistor according to the present invention. In Step S 41 , the material of a highly electrically conductive metal plate  402  is selected according to the desired resistance characteristics (such as the resistance coefficient or resistance-temperature coefficient) of a resistor. In Step S 42 , at least one resistor substrate is formed on the highly electrically conductive metal plate  402  through stamping or cutting. In Step S 43 , a passivation layer  404  is formed at a middle portion of the resistor substrate, where materials such as either resin or a high polymer material may be used for the passivation layer. In Step S 45 , the resistor substrates are divided into separate resistors through punching or cutting. In Step S 46 , a conductive layer  405  is respectively formed at the two side portions of each resistor substrate. 
     In another embodiment, electrodes of the resistor may be connected to an external conductive element  406  in Step S 46  of the method, such that the resistance of the current sensing resistor may be measured and/or the stability of resistance may be adjusted by controlling the length of a middle slot. 
     According to an embodiment of the present invention, the material of the metal plate  402  may include Cu—Mn alloy, Ni—Cu alloy or Mn—Cu—Sn alloy, and the conductive layer may be formed by plating Cu, Ni or Sn. 
     In another embodiment, in Step S 44  of the method, a trademark, a resistance or a related pattern is marked on the passivation layer. 
     In another embodiment, Step S 45  and Step S 46  of the method may be interchanged, if required, and the above steps merely demonstrate one of the embodiments. 
     The technical content and features of the present invention have been described; however, persons of ordinary skill in the technical field of the present invention can still make variations and modifications without departing from the teachings and disclosure of the present invention. Therefore, the disclosed embodiments are not intended to limit the present invention. Modifications and variations made without departing from the present invention shall fall within the scope of the present invention as specified in the following claims. 
     LIST OF REFERENCE NUMERALS 
     
         
           100  Current sensing resistor 
           102  Middle portion 
           104  Side portion 
           106  Current terminal 
           108  Sensing terminal 
           110  Middle slot 
           112  Slot 
           120  Voltmeter 
           122  Ammeter 
           402  Metal plate 
           404  Passivation layer 
           405  Conductive layer 
           406  Conductive element