Abstract:
A method and circuit for implementing high current capability Kelvin connections and measuring the resistance of the contacts and connections to verify that the conducting path is capable of carrying the high current without damage or degraded performance. Included as well is the means and circuit for efficiently dividing a high current test stimulus current into 2 paths with low losses and voltage drops.

Description:
RELATED APPLICATION DATA 
     This application is a continuation-in-part of U.S. application Ser. No. 13/090,771 filed on Apr. 20, 2011, which claims the benefit of U.S. Application No. 61/355,804 filed on Jun. 17, 2010, each of which is hereby incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates generally to testing of integrated circuits and power semiconductor devices, and more particularly, to a method and apparatus for implementing Kelvin connections with verification and contact resistance testing capability that is especially useful for high current applications. In addition, these techniques are scalable to more than two Kelvin connections. 
     DESCRIPTION OF THE RELATED ART 
     Precision measurement circuits often make use of Kelvin connections (see, e.g.,  FIG. 1 ) to provide accurate measurements by removing the effect of contact resistance in series with the measurement stimuli. A force lead connection  10  is typically used to supply the stimuli, and a sense lead  12 , which is simply a connection that carries no current, is used to make the measurement. Because the sense lead  12  carries no, or negligible current, the effect of voltage drop in the contacts and leads is eliminated. In a typical test application, dual contacts are made to each lead of the device under test. For integrated circuit chip packages and power semiconductor devices this is often done with a socket configuration having two separate spring contacts to each lead of the device. The force lead is typically connected to one contact and the sense lead to the other. 
     Testing to verify that the Kelvin connection is properly made confirms both leads are making proper contact. In other types of high current testing, both leads may be used as parallel paths for lower resistance and higher current carrying capacity. In this case, testing the “Kelvin connection” verifies that both leads  10 ,  12  are properly connected to the test point so both leads can pass high currents and share the load. 
     In testing of high current devices, a means is used to determine the integrity of the test connections with the device terminals to insure the leads actually are making contact and that the quality of the contact is satisfactory. Connections that have high resistance may cause inaccurate tests, or may damage test contacts. In some situations, high currents are passed to the device under test, while simultaneously, high voltages may be present. The circuitry for determining proper connection integrity must then also be able to withstand the presence of these high voltages during the course of the test. Typically, measurement circuits that can withstand high voltages are high impedance. However, testing for low values of contact resistance generally requires substantial measurement currents, which requires low impedance circuits to avoid dissipating unacceptable amounts of power which would affect measurements. 
     In some high current testing applications a Force/Sense Kelvin measurement is not required but a modified dual contact system is used to provide parallel high current paths. In this case, there are two Force contacts and the contact resistance of both connections of the contact pair is important because both are passing high current. Splitting the current into two parallel paths has other beneficial effects such as lowering the overall path inductance and resistance. In these cases the current handling capability of individual contacts, such as socket contacts or handler contacts, may not be sufficient to handle the full current required for testing. Adding parallel contacts provides additional current handling capability and reduces stress on the test fixtures. The problem with this approach is that it is difficult to efficiently divide the high current test stimulus in two different paths. Additionally, existing test circuits for verifying the contact integrity are not optimal. The requirement for very low contact resistance in high current paths makes it difficult to verify using existing methods. 
     Existing test circuits for verification of Kelvin contacts typically use active circuitry which floats with the measurement leads to verify the contact resistance. An example of this type circuitry using optical couplers  20  is shown in  FIGS. 2 a  and 2 b   . These circuits use a significant number of components and have the disadvantage of requiring a floating power circuit  22 , which may have significant capacitance to ground. The circuit is also hard programmed to detect a certain level of contact resistance to determine a failure. 
     Another example of a circuit designed to verify the quality of the Kelvin contact connections is incorporated in U.S. Pat. No. 5,999,002, Fasnacht et al, which is a continuation in part of U.S. Pat. No. 5,886,530. This circuit attempts to use a transformer to isolate the Kelvin contact resistance measurement from the test stimuli. The technique taught in U.S. Pat. No. 5,999,002 employs a simple single voltage pulse applied to the primary of the transformer that is affected by the secondary resistance of the transformer including the contact resistance between the force and sense leads. The secondary of the transformer is capacitively coupled to the force and sense contacts to isolate the circuit from the measurement stimuli. Although difficult to follow, the techniques disclosed in U.S. Pat. No. 5,999,002 may have theoretically useful properties but this reference basically provides an indication of continuity and not a measurement of resistance in ohms. 
     As best understood, the technique taught in U.S. Pat. No. 5,999,002, from a practical standpoint, has several problems. First, the secondary of the transformer must be capacitively coupled to the Kelvin contacts to isolate the secondary from the measurement circuit. This creates capacitive loading on the measurement signals, which could degrade the quality of any dynamic measurements. Second, the resistance error threshold of any practical version of the circuit is quite high in comparison to the desired low resistance path of a high current Kelvin connection. In addition, the error threshold is fixed by component values and cannot be programmed or otherwise readily changed to different levels. 
     The nature of the circuit used in U.S. Pat. No. 5,999,002 seriously limits the sensitivity of the resistance test, so it is difficult, if not impossible, to set tight limits on the contact resistance. In high current testing this can be a very significant problem. Since the circuit uses only a logic level threshold that is compared to the transformer&#39;s primary voltage to determine if the contact is good or bad, it does not provide any quantitative measurement of the actual level of the contact resistance. The contact resistance measurement may be extremely important and the actual requirements may vary with different levels of current required for testing different types of devices due to heating and over the lifetime of the contacts. In addition, the measurement current used to determine the contact resistance is not well understood, or predictable, since all that is used is the primary voltage level and current is not a factor. Lastly, as pointed out in U.S. Pat. No. 5,999,002 there are issues with noise pickup and voltage spikes that must be filtered out or ignored in some manner. 
     Accordingly, there is a need in the art for a fast, accurate method of verifying the contact resistance between the Force and Sense, or two Force leads of a Kelvin connection. The method should be sensitive and accurate enough to discriminate very low resistance values to ensure the connection can pass very high currents, including the case where two parallel forcing leads are used to increase current capability. The method should also be able to provide measurement capability to determine the actual resistance value, not just a comparison to limit in order to accommodate the wide range of possible requirements in a single test station. Measurements should be performed as quickly as possible, especially if there are more than two contacts without the interference of noise or voltages that must be filtered out. The measurement current used should be well defined and predictable. Finally, the method should provide the best means to manage a high current test stimulus that must be divided into two separate Force paths with minimal losses and yet be able to verify the contact resistance at the Kelvin connection. 
     SUMMARY 
     Accordingly, a method and system in accordance with the present invention enables integrity verification of Kelvin connections to an integrated circuit, power semiconductor device or other electronic assembly with speed and repeatability. Additionally, a method and system in accordance with the present invention enable measurement of the actual contact resistance of a Kelvin connection with a predetermined measurement current. Further, a method and system in accordance with the present invention enable integrity verification of Kelvin connections in the presence of high voltages. Also, a method and system in accordance with the present invention provide a simple method of utilizing the same connections to Kelvin connected leads that may be used to provide two parallel high current paths to a test point. The integrity verification of such connection can be scaled and expanded to a plurality of connections. In addition, little or no power is dissipated until the resistance measurement is performed. Finally, it is important to note this method and system utilize a predetermined measurement current while performing the actual resistance measurement of the Kelvin connection. 
     According to one aspect of the invention, a device for measuring impedance of a Kelvin connection to a circuit of interest and for injecting a test stimulus into the circuit of interest includes: a first transformer having a primary winding and a secondary winding; a first input device for receiving a Kelvin test input waveform, the first input device electrically coupled to the primary winding; first and second test leads for connection to the circuit of interest, the first and second test leads electrically connected to first and second ends, respectively, of the secondary winding; a sensing device electrically coupled to the primary winding, the sensing device operative to provide a measurement corresponding to a contact impedance across at least one of the first and second test leads based on impedance reflected from the secondary winding to the primary winding in response to application of the Kelvin test input waveform to the first input device. 
     According to one aspect of the invention, the device includes a second input device for receiving the test stimulus, the second input device electrically coupled to the secondary winding. 
     According to one aspect of the invention, the secondary winding comprises a first winding part and a second winding part, one end of the first winding part connected to one end of the second winding part via a center tap, and wherein the second input device is electrically connected to the center tap. 
     According to one aspect of the invention, a turns ratio of the first winding part is equal to a turns ratio of the second winding part. 
     According to one aspect of the invention, a turns ratio of the first winding part is different from a turns ratio of the second winding part. 
     According to one aspect of the invention, the sensing device comprises a resistor connected to the primary winding. 
     According to one aspect of the invention, the first input device comprises a driver amplifier electrically connected to one end of the primary winding. 
     According to one aspect of the invention, the first input device comprises a switching device electrically connected to one end of the primary winding. 
     According to one aspect of the invention, the first input device comprises a switching device configured to selectively couple a first and second end of the primary winding to the Kelvin test input waveform. 
     According to one aspect of the invention, the device includes a waveform generator for generating the Kelvin test input waveform, the waveform generator operatively coupled to the first input device. 
     According to one aspect of the invention, the waveform generator is configured to generate two alternating waveforms out of phase from one another, and the first input device provides one of the two alternating waveforms to a first end of the primary winding, and the other of the two alternating waveforms to a second end of the primary winding. 
     According to one aspect of the invention, the device includes a resistor electrically connected in series between the secondary winding and one of the first or second test leads. 
     According to one aspect of the invention, the device includes a switching device having a switch portion and an enable portion for controlling a state of the switch portion, the switch portion being electrically connected in series between the secondary winding and one of the first or second test leads to selectively couple the secondary winding to the one of the first or second test leads, and the enable portion being electrically connected to an input of the first input device to control the state of the switch portion. 
     According to one aspect of the invention, the device includes a second transformer and a third transformer, the second and third transformers each including a secondary winding with a center tap, wherein the center tap of the second transformer is electrically connected to one end of secondary winding of the first transformer and the center tap of the third transformer is electrically connected to the other end of the secondary winding of the first transformer. 
     According to one aspect of the invention, the device includes: a third test lead and a fourth test lead for connection to the circuit of interest; and a second transformer and a third transformer, the first, second and third transformers each including a center tap in the secondary winding, wherein the center tap of the first transformer is electrically connected to one end of the secondary winding of the second transformer, the center tap of the third transformer is electrically connected to the other end of the secondary winding of the second transformer, a primary winding of the first transformer is electrically connected to a primary winding of the third transformer, and the third and fourth test leads are electrically connected to first and second ends, respectively, of the secondary winding of the third transformer. 
     According to one aspect of the invention, the device includes a comparator operatively coupled to the sensing device, the comparator configured generate a signal indicative of the measured impedance being at least one of above or below a predetermined threshold. 
     According to one aspect of the invention, a method for measuring contact impedance of a Kelvin connection using at least one transformer having a primary winding and a secondary winding is provided. The method includes: connecting each end of the secondary winding to a respective contact of a contact pair for which the impedance is to be measured; applying a waveform to the primary winding; sensing an impedance reflected from the secondary winding to the primary winding in response to the applied waveform; and correlating the reflected impedance to the contact impedance. 
     According to one aspect of the invention, sensing an impedance includes sensing flow of current in the primary winding in response to the applied waveform. 
     According to one aspect of the invention, the method includes: comparing the sensed current flow to a predetermined value; and concluding the resistance of the contact pair is acceptable or unacceptable based on the comparison. 
     According to one aspect of the invention, sensing the reflected impedance includes sensing current flowing from a center tap of the primary winding in response to the applied waveform. 
     According to one aspect of the invention, applying a waveform includes applying a first alternating waveform to a first end of the primary winding, and a second alternating waveform to a second end of the primary winding. 
     According to one aspect of the invention, applying a waveform includes applying a single square-wave pulse to a first end of the primary winding, and sensing the reflected impedance includes sensing current flowing from a second end of the primary winding. 
     According to one aspect of the invention, the second waveform is out of phase from first waveform. 
     According to one aspect of the invention, a method of detecting a resistance imbalance between two connections to a circuit of interest is provided. The method includes: coupling a secondary winding of a transformer to the circuit of interest; injecting a test stimulus current at a center tap of the secondary winding; concluding there is a resistance imbalance between the two connections when a voltage across a primary winding of the transformer exceeds a predetermined voltage. 
     These and further features of the present invention will be apparent with reference to the following description and attached drawings. In the description and drawings, particular embodiments of the invention have been disclosed in detail as being indicative of some of the ways in which the principles of the invention may be employed, but it is understood that the invention is not limited correspondingly in scope. Rather, the invention includes all changes, modifications and equivalents coming within the scope of the claims appended hereto. 
     Features that are described and/or illustrated with respect to one embodiment may be used in the same way or in a similar way in one or more other embodiments and/or in combination with or instead of the features of the other embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a conventional Kelvin connected measurement circuit. 
         FIG. 2 a    is a schematic diagram of a typical conventional Kelvin verification circuit with separate Force and Sense Kelvin connections. 
         FIG. 2 b    is a schematic diagram of a conventional Kelvin verification circuit with two parallel high current, or Force, Kelvin connections from a single high current test stimulus. Therefore, two Kelvin diodes are utilized to isolate the Force connections for Kelvin verification. 
         FIG. 3 a    is a schematic diagram of an exemplary high current Kelvin verification circuit in accordance with the present invention and showing a Test Stimulus being divided into two parallel force paths without any Kelvin diodes utilizing a single transformer. 
         FIG. 3 b    is a schematic diagram of showing how the circuitry in  FIG. 3 a    can be expanded to a plurality of contacts which are driven in parallel. 
         FIG. 3 c    is a graph illustrating an exemplary representation of R kelvin  vs V 1 . 
         FIG. 4  is a schematic diagram on another exemplary high current Kelvin verification circuit showing a simplified drive configuration which utilizes a single pulse to verify the Kelvin connection of two high current, and balanced, paths in accordance with the present invention. 
         FIG. 5  is a schematic diagram of yet another exemplary Kevin verification circuit between Force and Sense contacts in accordance with the present invention. Different turns ratios on the secondaries of the transformer divide the Test Stimulus Current whereby essentially all current flows in the Force contact but still provides a Sense contact for voltage measurement. 
         FIG. 6  is a schematic diagram of yet another exemplary Kelvin verification circuit between Force and Sense contacts in accordance with the present invention. In this case, the transformer has the same low number of turns but one secondary which is connected to the Sense contact has a resistor in series with the current path to help divert essentially all the Test Stimulus Current into the Force contact. 
         FIG. 7  is a schematic diagram of yet another exemplary Kelvin verification circuit between Force and Sense contacts utilizing an optically isolated switch in accordance with the present invention. The optical switch is closed to perform the Kelvin verification and then opened just before the Test Stimulus Current is applied. Therefore all of the Test Stimulus Current flows through the Force connection which allows the Sense connection to measure voltages accurately 
         FIG. 8  is a schematic diagram of yet another exemplary Kelvin verification circuit which can be scaled to a plurality Force Kelvin connections utilizing a input single transformer in accordance with the present invention. This results in only one Kelvin verification measurement to verify all Kelvin connections using the same current path as the Test Stimulus Current. 
         FIG. 9  is a schematic diagram of yet another exemplary Kelvin verification circuit which can be scaled to a plurality Force Kelvin connections utilizing transformers driven in series in accordance with the present invention. This enables a number of Kelvin contacts to be verified at the same time. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Embodiments of the present invention will now be described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. It will be understood that the figures are not necessarily to scale. 
     A circuit has been developed that provides several advantages over previously employed circuitry. The advantages can include: 
     1. relative simplicity 
     2. high voltage isolation 
     3. high current capability 
     4. maximal utilization of test contacts 
     5. variable threshold 
     6. ruggedness 
     7. low voltage and power losses 
     8. high speed measurements 
     9. adaptable to a plurality of contacts 
     10. ability to maintain current sharing in contacts 
     11. low noise measurements 
     An embodiment of a circuit  30  for high current capability is shown in  FIG. 3 a   . The circuit is advantageous in that it combines dividing high current into two current paths with Kelvin contact resistance measurement utilizing only one transformer. 
     Test contact points  32 ,  34  are contacts that will be connecting to the device under test and the contact resistance of these connections will be determined via the circuit shown. Note that Kelvin diodes are not needed since the same transformer, T 1 , is also used to divide the Test Stimulus current. 
     A test stimulus  36  can be injected into the circuit via line  36   a , which is electrically connected to a center tap  38  on a secondary winding  40  of transformer T 1 . One end of the secondary winding  40  is electrically connected to first contact point  32  and another end of the secondary winding  40  is electrically connected to second contact point  34 . 
     Input drivers, such as transistors Q 1  and Q 2 , provide signals to respective ends of the primary winding  44  based on a signal  48  applied to gates of the respective transistors. Additionally, the primary winding  44  of transformer T 1  includes a center tap  46  connected to signal common through resistor R 1 . A buffer amplifier  50  is electrically connected to resistor R 1  to obtain a voltage drop across the resistor R 1 , and an output of buffer amplifier  50  is connected to a positive input of comparator  52  via an RC network formed from resistor R 2  and capacitor C 1 . A negative input to comparator  52  is connected to a Kelvin Test Limit voltage. 
     During operation, test stimulus  36 , such as current stimulus having a predetermined magnitude, can be provided on line  36   a  from the high current source that will be used in testing of the device. As can be seen in  FIG. 3 a   , the test stimulus  36  is applied to a center tap  38  on a secondary winding  40  of a transformer T 1 . This winding is preferably constructed with heavy gauge wire so that it can pass the high test currents. The applied test stimulus  36  will then accurately divide (one-half of the current passing though each contact points) with low losses and pass on to each of the test contact points  32 ,  34 . Accordingly, both contact points  32 ,  34  may be used for passing currents resulting from the test stimulus  36 , which allows application of approximately twice the test current as compared to schemes that use conventional Force and Sense connections for forcing the test stimulus on one line and sensing of voltage on the other. 
     The primary winding  44  of the transformer T 1  is fed with alternating pulses of opposite phase via transistors Q 1  and Q 2  (e.g., switching devices). The return path for the current through either half of the primary winding  44  is via the center tap  46  and a current sensing device, such as resistor R 1 . While a resistor is shown as a current sensing device, any current to voltage conversion technique may be utilized without departing from the scope of the invention. 
     During the contact measurement operation, driver transistors (not shown) generate alternating pulsed gate waveforms  48  shown in  FIG. 3 a   , which are applied to the gates of Q 1  and Q 2 , respectively. As Q 1  and Q 2  alternately conduct V+(e.g., 5 VDC) to the corresponding ends of the primary winding  44 , the resulting alternating voltage applied to the primary winding appears across the secondary winding  40  in relation to the turns ratio of the windings. Since higher currents are typically applied to measure low values of contact resistance, it is then advantageous to utilize a higher turns count on the primary side and a lower turns count on the secondary side. In one embodiment, the turns ratio is on the order of 3:1, primary to secondary. This ratio is optimized to suit the expected Kelvin contact resistance and desired measurement current. 
     Note that each winding  40  and  44  is symmetrical about the center tap (which insures that, during testing, the high current test stimulus will divide evenly, plus the mutual inductance will cancel the inductance in each of the two secondary high current paths). The secondary voltage is then applied across the Kelvin test contacts  32  and  34  and a current will flow in relation to the applied primary voltage turns ratio (which is chosen to be compatible with the expected Kelvin contact resistance range and desired measurement current) and the actual contact resistance. Due to the coupling between the primary and secondary windings, a current will flow in the primary winding  44  of the transformer T 1  due to the reflected secondary impedance. This impedance is the contact resistance that is reflected back to the primary winding in relation to the turns ratio of the transformer T 1 , and this impedance can be determined by the voltage appearing across R 1  due to the resulting primary current. The voltage across R 1  can then be used to calculate the contact resistance of the testing contacts. 
     Due to the arrangement of the circuitry, gate pulses, and windings, the voltage appearing at R 1  is essentially DC, referenced to ground, and requires very little additional detection circuitry or filtering. If the turns ratio, applied voltage, and value of R 1  are properly chosen so as not to saturate the transformer core, a signal of sufficient amplitude will be developed and additional amplification of the voltage across R 1  may not be required. The turns ratio, applied voltage, and value of R 1  can be chosen such that the transformer core will not saturate if the applied primary voltages do not exceed the Volt-Seconds rating of the transformer core. 
     The current through R 1  increases as the Kelvin resistance (R Kelvin ) decreases and thus the voltage across R 1  is indicative of the Kelvin resistance. The voltage across R 1  (or other sensing device) can be provided to a buffer amplifier  50  and then to comparator  52 , which can be configured to provide an output indicative of whether the measured impedance is acceptable or unacceptable (e.g., above a predetermined threshold, below a predetermined threshold, etc.). 
     It is further shown that any variations in the output of the buffer amplifier  50  can be filtered with R 2  and C 1 . This filter will add a slight time delay but results in a precise signal level that can be used by a comparator to determine if the Kelvin resistance is acceptable. It is noted that this signal level could also be input to an Analog-to-Digital converter. The actual Kelvin resistance can then be measured from the digital output of the Analog-to-Digital conversion. As the Kelvin resistance decreases, the voltage V 1  increases and, after a time delay, V 2  rises above a Kelvin test limit voltage setpoint provided to the comparator  52  and a Kelvin PASS signal is output. 
     At this point the advantages of using a current technique to measure Kelvin resistance through the transformer T 1  will be discussed. Namely, the current in the transformer starts at zero (0) amps and builds up to a maximum value based on the applied voltage, the value of R 1 , turns ratio and contact resistance, which is maintained until the transformer saturates. The Kelvin resistance can be read after a known time delay as the current rises to its final value. The current rise time is primarily due to leakage inductance in the transformer T 1 . There is also very little noise since the leakage inductance acts a filter to keep the current signal from picking up high frequency noise. The maximum or value of secondary current used for the resistance measurement is based on the primary applied voltage divided by the resistor R 1  and then multiplied by the turns ratio. This is expressed as:
 
 I   sec   =V   +   /R 1*( N   pri   /N   sec ), in amps
 
Since higher measurement current enables accurate reading of low resistances, using the turns ratio of the transformer is a good method to achieve high measurement currents with low power dissipation. The basic transformer relationships are:
 
 Z   pri   /Z   sec =( N   pri   /N   sec ) 2  or in terms of resistances,
 
 R   pri   /R   sec =( N   pri   /N   sec ) 2  or in terms of  R   kelvin  and  R   reflected  
 
 R   reflected   /R   kelvin =( N   pri   /N   sec ) 2  
 
     i.e., the impedance ratio is equal to the square of the turns ratio
 
 V   r1 =( R 1)*( V   + )/( R   reflected   +R 1)
 
Therefore:
 
 V   r1 =( R 1)*( V   + )/( R 1+(( N   pri   /N   sec ) 2   *R   kelvin ))
 
     Solving for the unknown impedance across the contact points gives: 
     
       
         
           
             
               Z 
               sec 
             
             = 
             
               
                 
                   ( 
                   
                     
                       N 
                       pri 
                     
                     / 
                     
                       N 
                       sec 
                     
                   
                   ) 
                 
                 2 
               
               
                 
                   R 
                   1 
                 
                 ⁡ 
                 
                   ( 
                   
                     
                       
                         V 
                         + 
                       
                       
                         V 
                         
                           R 
                           1 
                         
                       
                     
                     - 
                     1 
                   
                   ) 
                 
               
             
           
         
       
     
     And the final result is the expression to determine R kelvin :
 
 R   kelvin =( N   sec   /N   pri ) 2   *R 1*(( V   +   /V   r1 )−1)
 
Thus by specifying a suitable measurement current for the Kelvin contacts  32  and  34 , the associated components then can be determined such that R kelvin  can be measured.
 
     During the testing of the device, the pulsing of transistors Q 1  and Q 2  may be stopped, and the test stimulus current may be applied to the center tap  38  of the secondary winding  40 . Since the current flows in opposing directions in each half of the winding  40 , no net flux is produced and no currents will appear on the primary winding  44 . This also cancels out the inductance of the secondary winding  40  so there is essentially no net effect of transformer T 1  and the test stimulus current is efficiently divided into two separate current paths with low losses. In the situation that the currents do not evenly balance due to unequal contact resistance, currents will be induced in the primary winding  44 . Due to the choice of a relatively small magnetic core for the transformer T 1 , the energy that is coupled through the transformer before the core saturates is small, and can be easily absorbed by Q 1 , Q 2 , small transient suppressor diodes or Zeners (not shown). 
     The magnetic core selected for transformer T 1 , along with the number of turns on the primary winding are selected such that saturation of the core does not occur at the switching frequency chosen for the drive signal to Q 1  and Q 2  when the secondary is open circuited. Further, the turns ratio of the primary to secondary can be optimized to provide maximal sensitivity to low contact resistances, while minimizing the power that must be applied to the primary. 
     In graphical terms, a typical representation of R kelvin  vs V 1  is illustrated in  FIG. 3 c   . Note the response is most sensitive at lower values of R kelvin , which is most important for high current testing since any resistance can cause poor test results or damage the Device Under Test. Also note the beneficial effect of increasing the Primary-to-Secondary turns ratio of the transformer T 1  results in improving the sensitivity of the method at low values of R kelvin . Low values of R kelvin  can be easily, quickly and accurately resolved. 
     As shown in  FIG. 3 b    this basic technique can be scaled up to handle more than just one pair of Kelvin contacts while at the same time utilizing the same transformer and current paths used by the Test Stimulus Current. More specifically,  FIG. 3 b    includes the transformer T 1  as described with respect to  FIG. 3 a    and associated circuitry (not shown), as well as two additional transformers T 2  and T 3 . Instead of the secondary windings of transformer T 1  being coupled to the Kelvin contacts, a first end of secondary winding  40  is connected to a center tap  38   a  of transformer T 2 , and a second end of secondary winding  40  is connected to a center tap  38   b  of transformer T 3 . A first end of transformer T 2 &#39;s secondary winding  40   a  is connected to first contact  32   a , and a second end of transformer T 2 &#39;s secondary winding  40   a  is connected to a second contact  34   a . Similarly, a first end of transformer T 3 &#39;s secondary winding  40   b  is connected to first contact  32   b , and a second end of transformer T 3 &#39;s secondary winding  40   b  is connected to a second contact  34   b . Primary windings of transformers T 2  and T 3  also include a respective center tap  46   a  and  46   b  connected to signal common through resistors R 1  and R 2 , respectively. Transistors Q 1  and Q 2  provide driving signals on respective ends of the transformer primary windings  44   a  and  44   b . The primary winding  44  of transformer T 1  is connected as shown in  FIG. 3 a   . Further, and although not shown, a buffer and/or comparator may be coupled across resistors R 1   a  and R 1   b . In the circuit of  FIG. 3 b   , the current in each leg of windings  40   a  and  40   b  is halved such that now one quarter of the current flows through each test Kelvin contact  32   a ,  34   a ,  32   b  and  34   b.    
     Referring now to  FIG. 4 , a circuit is shown that combines division of the test stimulus current  36  into two current paths with Kelvin contact resistance measurement using only a single transformer. More specifically, the basic Kelvin verification method has been further simplified to use a single winding  44 ′ on the primary of the transformer T 10 , and to utilize just one pulse  48 ′. Like before, the transformer T 10  is chosen to be compatible with the expected Kelvin contact resistance range and desired measurement current. The single pulse may be applied to the primary winding  44 ′ via driver U 1  (which may be any commercially available Driver U 1 ) to enable measurement of R kelvin . No filtering of any kind is needed. The resulting current in the primary of the transformer T 10  is based on the reflected Kelvin resistance, and is converted to voltage V R1  by resistor R 1 , which can be sampled after a short delay. 
     If the voltage V R1  rises above a Kelvin Test Limit Voltage provided to the comparator (not shown in  FIG. 4 ), then it can be concluded that R kelvin  is less than the maximum resistance limit. The only delay in the circuit of  FIG. 4  is due to the small amount of leakage inductance of T 10 . Further, there is very little noise or error voltages. A resistance measurement can be obtained based on the value of V R1 . For example, V R1  may be sampled by an Analog-to-Digital Converter, which can produce a digital value corresponding to the a resistance measurement in ohms. The circuit of  FIG. 4  is advantageous in that a simple, lower-cost circuit can be used to provide accurate Kelvin measurements. 
       FIG. 5  illustrates another embodiment of a circuit in accordance with the present invention. The embodiment of  FIG. 5  is similar to the embodiment of  FIG. 4 . However, the basic Kelvin verification method has been modified for a Force and Sense Kelvin connections  32  and  34  using a different number of turns on the two secondary windings of transformer T 20 . The desired effect is to direct 99% (or other predetermined percentage) of the Test Stimulus Current  36  into the Force Kelvin contact  32 , which then allows the Sense Kelvin contact  34  (which carries very little current) to accurately measure voltages. Therefore, the Sense Kelvin connection can be connected to the Sensing Circuitry. Kelvin resistance can be measured with only a single pulse, and the resulting current in the primary of transformer T 20  is based on the reflected Kelvin resistance. The primary current is converted to voltage V R1  by resistor R 1 . 
     The primary winding of T 20  has been increased to keep the ratio of the Primary-to-Secondary at 3:1. Increasing the primary windings is relatively easy since the currents are low and the primary is low voltage. 
     Referring now to  FIG. 6 , the basic Kelvin verification method has been modified for a Force and Sense Kelvin connections. The circuit of  FIG. 6  combines division of the test stimulus current into two unequal current paths with total Kelvin contact resistance measurement (R 3 +R Kelvin ) using only a single transformer. The circuit of  FIG. 6  is similar to the circuit of  FIG. 4 , except that resistor R 3  has been added in the secondary of T 20 . The resistor R 3  causes an imbalance in the transformer secondary winding  40  which results in a rise in flux in the transformer T 20 . As a result, transformer T 20 , which preferably has a relatively small core with a low volt-seconds product (thus enabling saturation at low currents), will saturate when the Test Stimulus is first applied to the center tap  38 . When the transformer T 20  saturates, due to the low resistance through the transformer to the force contact  32  essentially all of the test current flows through the Force Kelvin connection  32 . The Sense Kelvin connection  34  to the device under test will carry little or no current and thus accurately sense voltage. 
     The point of saturation is determined by the volt-seconds rating of the transformer T 20 , which is usually on the order of 50 volt-microseconds. Therefore with a 5 volt drop across the resistor R 3 , the other side of the transformer T 20  will need to also generate a 5 volt offset. After just 10 microseconds the transformer T 20  would saturate and essentially all the current would pass through the Force Kelvin contact  32 . The Sense Kelvin connection  34 , which now carries little or no current (most current passes through the Force Kelvin connection  32  due to it having a significantly lower resistance) will then sense the voltage accurately. Note the resistor R 3  will be added to the total Kelvin Resistance measurement. As a result, if R 3  is a precision resistor, R kelvin  can be determined based on the difference between the measured resistance and the value of R 3 . Finally R 3  can be used to identify test fixtures or setups that are subject to being exchanged for different testing purposes such as with probe cards. The voltage V R1  across R 1  due to a single gate pulse is a factor of the Kelvin resistance and can be sampled after a short delay. 
     Referring to  FIG. 7 , another embodiment of a circuit in accordance with the present invention is illustrated. The circuit of  FIG. 7  is similar to the circuit of  FIG. 4 , except that the transformer T 30  has a single-primary and single-secondary winding, and is only used from measuring the Kelvin resistance by sensing the resulting current that flows in the primary winding and through R 1 . Additionally, transformer T 30  does not include a center tap on the secondary winding  40 ′, and an opto switch  20  is connected in one leg of the secondary winding  40 ′. Since the secondary winding does not have a center tap, the test stimulus current  36  is input directly to the Force Kelvin contact  32 . In  FIG. 7  the basic Kelvin verification method has been modified to isolate the Force and Sense Kelvin connections  32  and  34  using the opto-isolator  20 . In this case, the Test Stimulus Current  36  can only flow through the Force Kelvin contact  32  when the opto-isolator  20  is turned OFF. The opto-isolator  20  can be turned ON just before a Kelvin Resistance measurement is performed. The opto-isolator  20  then can be turned OFF after the Kelvin test and before the test stimulus is applied to the Force Kelvin connection  32 . Note the Test Stimulus current  36  does not flow through the transformer T 30  at any time. Again, the voltage V R1  across R 1  due to a single gate pulse is a factor of the Kelvin resistance and can be sampled after a short delay. 
     Referring to  FIG. 8 , the basic Kelvin verification method as shown in  FIG. 4  is applied to a plurality of Kelvin connections  32   a - 32   d  (similar to the configuration shown in  FIG. 3 b   ). While the configuration of the primary windings of transformers T 2  and T 3  is not shown in  FIG. 8 , it is understood that the primary windings may be configured as shown in  FIG. 3 b    or as shown in  FIG. 9  (discussed below). 
     In  FIG. 8 , multiple Kelvin connections are grouped together and sampled using a single Kelvin test. In this regard, the Kelvin connections are individually checked for connection to the common bus. 
     In operation the Kelvin resistance measurement current is divided equally by the secondary of each transformer T 2  and T 3 . As a result, the test stimulus current is applied to all four Kelvin connections such that each connection carries one-quarter of the test stimulus current. 
     The voltage V R1  is a measurement of the combination of series-parallel Kelvin resistance. The circuit of  FIG. 8  is advantageous in that one Kelvin test can quickly determine a) if any one or more Kelvin Force connections are high resistance and b) verify that the Test Stimulus Current path is complete. 
     Moving now to  FIG. 9 , the basic Kelvin verification method is applied to a plurality of Kelvin connections whereby a single Kelvin test can be used to sense if any pair of Kelvin Force Connections are high resistance. The circuit of  FIG. 9  includes the transformer T 1  as described with respect to  FIG. 3 a    and associated circuitry coupled to the primary winding (not shown), as well as two additional transformers T 2  and T 3 . Instead of the secondary winding  40  of transformer T 1  being coupled to the Kelvin contacts, a first end of secondary winding  40  is connected to a center tap  38   a  of transformer T 2 , and a second end of secondary winding  40  is connected to a center tap  38   b  of transformer T 3 . A first end of transformer T 2 &#39;s secondary winding  40   a  is connected to first Kelvin contact  32   a , and a second end of transformer T 2 &#39;s secondary winding  40   a  is connected to a second Kelvin contact  34   a . Similarly, a first end of transformer T 3 &#39;s secondary winding  40   b  is connected to third Kelvin contact  32   b , and a second end of transformer T 3 &#39;s secondary winding  40   b  is connected to a fourth Kelvin contact  34   b . A driver U 1  is coupled to one end of primary winding  44   a  of transformer T 2 , and the other end or primary winding  44   a  is connected to one end of primary winding  44   b  of transformer T 3 . The other end of primary winding  44   b  is connected to signal common via resistor R 1 . 
     The voltage developed across resistor R 1  the result of the sum of the two Kelvin resistors R kelvin1  and R kelvin2 . 
     The circuit of  FIG. 9  is advantageous in that it enables multiple Kelvin connections to be simultaneously tested during a single test. In this regard, one quarter of the stimulus current applied to the center tap  38  of transformer T 1  passes through each Kelvin contact  32   a ,  34   a ,  32   b , and  34   b.    
     Although the invention has been shown and described with respect to a certain preferred embodiment or embodiments, it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described elements (components, assemblies, devices, compositions, etc.), the terms (including a reference to a “means”) used to describe such elements are intended to correspond, unless otherwise indicated, to any element which performs the specified function of the described element (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary embodiment or embodiments of the invention. In addition, while a particular feature of the invention may have been described above with respect to only one or more of several illustrated embodiments, such feature may be combined with one or more other features of the other embodiments, as may be desired and advantageous for any given or particular application.