Abstract:
A method for diagnosing faults using a load. In the method IGBTs are controlled such that certain currents are expected. If the currents are not as expected, a fault may be diagnosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims the benefit of U.S. Provisional Patent Application No. 60/383,822, entitled “Method and Apparatus for Insulated Gate Bipolar Transistor Converter Circuit Fault Diagnostics,” filed May 28, 2002, which is hereby incorporated herein by reference in its entirety. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates generally to the field of electronics, and more specifically to a method and an associated apparatus for measuring and diagnosing faulted insulated gate bipolar transistors (“IGBTs”).  
           [0004]    2. Description of the Related Art  
           [0005]    The bipolar junction transistor (“BJT”), including its extension, the Darlington device, and the metal oxide semiconductor field effect transistor (“MOSFET”) are commercially-available advanced electronics devices. Each device has characteristics that complement the other in some respects. Relative to MOSFETS, BJTs have lower conduction losses in the ON-state and larger blocking voltages, but also have lower switching speeds. In contrast, MOSFETs switch relatively faster, but have relatively larger conduction losses in the ON-state. In order to overcome these performance limitations of the BJT and the MOSFET, the IGBT was designed. This device has significantly superior characteristics for low and medium-frequency applications in comparison to the BJT and the MOSFET Specifically, the IGBT is a voltage control device that can turn ON and OFF at a very high speed, and can deliver very high current compared to conventional bipolar transistors. Furthermore, its power rating can be improved by increasing both current and voltage. For this reason, IGBTs are preferred in some applications over both BJTs and MOSFETs.  
           [0006]    IGBTs presently serve a number of traditional markets, including motor drives and welding applications. However, with the emergence of new market segments, it is expected that IGBTs will continue to be a growing part of other industries, such as the semiconductor industry. In particular, the automotive and power supply markets, including uninterruptible power supplies (“UPSs”) and switch mode power supplies (“SPSs”), are expected to drive near term growth.  
           [0007]    Due to the cost reduction and performance enhancement of the microprocessor, three-phase AC motor drives are becoming increasingly popular and may eventually replace conventional DC motor drives as the dominant motor drive. Presently, in the electric vehicle (“EV”) field, almost all EVs, including hybrid electric vehicles (“HEVs”) and fuel cell vehicles, use AC motor drives. One of the most important functions of the AC motor controller is to convert DC power to three-phase AC power. IGBTs are typically utilized to perform this conversion.  
           [0008]    Referring to FIG. 1, there is illustrated the structure of a typical IGBT  1 . This structure is very similar to that of a vertically-diffused MOSFET, featuring a double diffusion of a p-type region and an n-type region. An inversion layer may be formed under a gate  2  of the IGBT  1  by applying the correct voltage to the gate contact  3 , much like a MOSFET. The main difference between the MOSFET and the IGBT is the use of a p +  substrate layer in the IGBT for a drain. Because of this, the IGBT  1  becomes a bipolar device as the p-type region injects holes into the n-type region.  
           [0009]    The gate voltage, V G , controls the ON/OFF state of the IGBT  1 . If the voltage applied to the gate contact  3  with respect to the emitter  4  is less than a threshold voltage, V Th , then no MOSFET inversion layer is created and the device is turned OFF. In this instance, any applied forward voltage will fall across a reversed bias junction, J 2 . The only current to flow will be a small leakage current.  
           [0010]    To turn ON the IGBT  1 , the gate voltage V G  is increased to a point where it is greater than the threshold voltage V Th . This results in an inversion layer forming under the gate  2 , thereby providing a channel linking the source to the drift region of the IGBT  1 . Electrons are then injected from the source into the drift region, while at the same time junction J 3 , which is forward biased, injects holes into the n −  doped drift region. Some of the injected holes will recombine in the drift region, while others will cross the drift region via diffusion and will reach the junction J 3  with the p-type region where they will be collected.  
           [0011]    The p-type region exhibits a type of lateral resistance. If current flowing through this resistance is high enough, it will produce a voltage drop that will forward bias the junction with the n +  region, turning ON a parasitic transistor that forms part of a parasitic thyristor. Once this happens, there is a high injection of electrons from the n +  region into the p-type region, resulting in loss of all gate control. This is known as latch up, and usually leads to device destruction.  
           [0012]    [0012]FIG. 2 is a circuit diagram illustrating a test structure  10  for performing diagnostics on IGBTs in a manufacturing facility. Six IGBTs, individually referenced as A+, A−, B+, B−, C+ and C−, are provided electrically coupled and drivable as a three-phase AC inverter  12 , which is to be tested. The test structure  10  includes a voltage source V dc , a tester  14 , an inverter drive  16  including a microprocessor (not shown in FIG. 2), a controlled-area network (“CAN”)  18 , and a test circuit  20 . The tester  14 , located at the end of the manufacturing line, is coupled to the microprocessor of the inverter drive  16  via the CAN  18  for passing commands and data between the tester  14  and the microprocessor. The test circuit  20  includes five relays, individually referenced as Ra 1 , Ra 2 , Ra 3 , Ra 4  and Ra 5 , a current sensor  22  and a current limiter  24 . The tester  14  controls the relays Ra 1 , Ra 2 , Ra 3 , Ra 4  and Ra 5  in the test circuit  20 , monitors the voltage of each of the IGBTs A+, A−, B+, B−, C+, C−, and makes decisions based upon production tests. The inverter driver  16  provides drive signals to the gates of the IGBTs A+, A−, B+, B−, C+, C−, which are synchronized with the states of the relays Ra 1 , Ra 2 , Ra 3 , Ra 4  and Ra 5  by the tester  14  to selectively turn each individual IGBT ON and OFF, one at a time, thereby controlling the current going through the IGBTs during testing. Two current sensing signals I a  and I b  provide the phase current to the microprocessor of the inverter drive  16 . The collector-emitter voltage V ce  across the collector c and emitter e of each of the IGBTs, is measured during the testing of the IGBTs and provided to the tester  14 .  
           [0013]    Testing of the IGBT switching circuits, for example, A+, requires control relays Ra 1  and Ra 5  to be closed. After the microprocessor of the inverter drive  16  commands IGBT A+ ON, current will travel through its collector and emitter terminals, c and e respectively, relay Ra 1 , current limiter  24  and relay Ra 5 . If IGBT A+ is not faulty, the collector-emitter voltage V ce  across the IGBT A+ will be close to zero volts and the current feedback I a  will be equal to a predetermined value. The microprocessor of the inverter drive  16  reads the phase current I a  and sends that value to the tester  14  via the CAN  18 .  
           [0014]    Further reference to FIG. 2 shows the absence of a current feedback sensor for the C phase, and hence, no current information available for the C phase. This is due to hardware limitations and cost. Accordingly, the two C phase IGBTs C+, C− can only be tested with a measurement of the respective collector-emitter voltages V ce . In order to test all six IGBTs A+, A−, B+, B−, C+, C−, at least six measurements are required. Among the six measurements, four require both current and voltage measurements, while two require only voltage measurements.  
           [0015]    As illustrated above, the IGBTs have rather complicated gate drive circuits and can be easily damaged resulting in undiscovered errors. This makes the manufacture of IGBT based power circuits and drive circuits difficult and complex. Further, in the case of a faulted IGBT  1  in the field, diagnostics that pinpoint the exact faulted transistor are also difficult and challenging. Accordingly, there is a need for an improved method of detecting faulted IGBTs.  
         BRIEF SUMMARY OF THE INVENTION  
         [0016]    By providing an improved testing method, both manufacturing test procedures and equipment may be simplified, and production time and cost may be reduced. Additionally, such a detection method may provide a diagnostic routine and equipment capable of locating a faulted IGBT in the field.  
           [0017]    In the disclosed embodiments, the present invention may alleviate the drawbacks described above with respect to diagnosing faulted IGBT circuits. The present invention provides a method and apparatus for measuring the current across various IGBT circuits by connecting the output of an inverter to a three-phase resistor load having a common resistance value.  
           [0018]    It should be understood that the present invention is not limited to uses related to EVs, or even AC induction drives, but is applicable to any inverter applications, including distributed power, such as fuel cells, micro-turbines and windmills, static/dynamic power quality converters, and so forth.  
           [0019]    The beneficial effects described above apply generally to each of the exemplary descriptions and characterizations of the devices and mechanisms disclosed herein. The specific structures through which these benefits are delivered will be described in detail herein below.  
           [0020]    In one aspect, a method for measuring fault diagnostics for an IGBT power converter circuit includes selectively turning ON a first, a second and a third IGBT, wherein each of the first, the second and the third IGBT may be either upper (A+, B+ or C+) or lower (A−, B− or C−) ones of IGBT pairs (i.e., first IGBT pair A+ and A−, second IGBT pair B+ and B−, and third IGBT pair C+ and C−). These upper and lower IGBTs are illustrated in FIG. 3. One skilled in the art will readily recognize that the use of the terms “upper” and “lower” are for convenience in referring the relative electrical positions of the IGBTs in the electrically coupled pairs, and these terms do not imply any specific spatial orientation within a power converter, vehicle, other device, or with respect to any other spatial reference frame.  
           [0021]    The method also includes measuring a current feedback of the first and the second IGBT and comparing the current feedback of the first and the second IGBT to a current value of the third IGBT, wherein the current value is determined by a resistor value. The method further includes determining a fault-state for the IGBTs and concluding that the IGBTs are either normal, open or shorted based upon the results of the comparison. When all the IGBTs are normal, no voltage measurements are required. The output of an inverter is connected to a multi-phase resistor load and when one faulty IGBT exists, a gate drive fault is generated at the faulty IGBT where the fault is an open fault. Where the fault in an IGBT is a short, a diagnostics circuit shuts down to protect the remainder of the IGBTs.  
           [0022]    In another aspect, an apparatus for measuring fault diagnostics for an IGBT power converter circuit includes a plurality of IGBTs which may be selectively turned ON, wherein the plurality of IGBTs may be grouped in pairs and identified as either upper IGBTs (A+, B+ or C+) or lower IGBTs (A−, B− or C−) of each pair. The apparatus also includes a resistor load coupled to the plurality of IGBTs, a current sensor coupled to two of the three phase outputs and an inverter drive, wherein the inverter drive is operable for receiving current feedback from the plurality of IGBTs. The apparatus further includes a tester in communication with the inverter drive and a voltage source coupled to the plurality of IGBTs.  
           [0023]    In another aspect, a fault determination method for assessing a condition of a power converter circuit, the power converter circuit having a number N of pairs of insulated gate bipolar transistors (“IGBTs”), each pair of IGBTs having an upper IGBT coupled to a first polarity of a DC power source and a lower IGBT coupled to a second polarity of the DC power source includes but is not limited to: selectively placing in a conducting state at least one upper IGBT during a first time and selectively placing in a conducting state at least one lower IGBT during the first time; and determining a set of IGBT operational states in response to at least one of a magnitude and a direction of a current through a load between the at least one upper and the at least one lower IGBT during the first time.  
           [0024]    In another aspect, a fault determination method for assessing a condition of a power converter circuit, the power converter circuit having a number N of pairs of insulated gate bipolar transistors (“IGBTs”), each pair of IGBTs having an upper IGBT coupled to a first polarity of a DC power source and a lower IGBT coupled to a second polarity of the DC power source includes but is not limited to: during a first time interval, controlling at least one upper IGBT and at least one lower IGBT such that at least one first-time-interval expected current will flow through a part of a resistive network if the at least one upper IGBT and the at least one lower IGBT are normal, sensing at least one of a magnitude and a direction of at least one first-time-interval current through the part of the resistive network, comparing the at least one first-time-interval sensed current with the at least one first-time-interval expected current; and concluding a state of at least one IGBT in response to the comparing.  
           [0025]    In another aspect, a fault determination method for assessing a condition of a power converter circuit, the power converter circuit having a number N of pairs of insulated gate bipolar transistors (“IGBTs”), each pair of IGBTs having an upper IGBT coupled to a first polarity of a DC power source and a lower IGBT coupled to a second polarity of the DC power source includes but is not limited to: during a first time interval and in response to a motor indicating a fault, sequentially controlling at least one upper IGBT and at least one lower IGBT such that at least one first-time-interval expected current will flow through a part of motor windings if the at least one upper IGBT and the at least one lower IGBT are normal, sensing at least one of a magnitude and a direction of at least one first-time-interval current through the part of the motor windings; comparing the at least one first-time-interval sensed current with the at least one first-time-interval expected current and concluding a state of at least one IGBT in response to the comparing.  
           [0026]    In one or more various embodiments, related systems include but are not limited to circuitry and/or programming for effecting the foregoing-referenced method embodiments; the circuitry and/or programming can be virtually any combination of hardware, software, and/or firmware configured to effect the foregoing referenced method embodiments depending upon the design choices of the system designer.  
           [0027]    The foregoing is a summary and thus contains, by necessity, simplifications, generalizations and omissions of detail; consequently, those skilled in the art will appreciate that the summary is illustrative only and is NOT intended to be in any way limiting. Other aspects, inventive features, and advantages of the devices and/or processes described herein, as defined by the claims, will become apparent in the detailed description set forth herein. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)  
       [0028]    [0028]FIG. 1 is an illustration of the structure of a typical IGBT as known in the art.  
         [0029]    [0029]FIG. 2 is a circuit diagram of a test structure including a test circuit for testing IGBTs in a three-phase AC inverter.  
         [0030]    [0030]FIG. 3 is a circuit diagram of a test structure for testing IGBTs in a three-phase AC inverter according to one illustrated embodiment of the present invention.  
         [0031]    [0031]FIG. 4 is a functional block diagram of a portion of the test structure of FIG. 3, illustrating the testing of one phase of the three-phase AC inverter according to one illustrated embodiment of the invention.  
         [0032]    [0032]FIG. 5 is a circuit diagram of a test structure for in field testing of IGBTs in a three-phase AC inverter, such as for testing in a three-phase AC inverter mounted in a power conversion module of a vehicle, according to another illustrated embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0033]    As required, detailed embodiments of the present invention are disclosed herein. However, it is to be understood that the disclosed embodiments are merely exemplary of the present invention that may be embodied in various and alternative forms. The figures are not necessarily to scale, e.g., some features may be exaggerated or minimized to show the details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention. Unless the context requires otherwise, throughout this specification and claims which follow, the word “comprise” and variations thereof, such as, “comprises” and “comprising” are to be construed in an open, inclusive sense, that is as “including, but limited to.” 
         [0034]    [0034]FIG. 3 shows a test structure  30  including a voltage source V dc , a tester  32 , an inverter drive  34  having a microprocessor  36  (FIG. 4), a controlled area network (“CAN”)  38 , and a test circuit  40  for testing an inverter  12 , according to one illustrated embodiment of the present invention. The inverter  12  may, or may not, be part of a power converter, for example for use in a vehicle or stationary power application. The test structure  30  may include an optional printer or display  41  for reviewing test results.  
         [0035]    In contrast to the test circuit  20  of FIG. 2, the output of the inverter  12  is connected to a three-phase resistor load  42 , of the test circuit  40 , formed by three resistors R 1 , R 2 , R 3  all having at least approximately the same value of resistance R. Two current feedbacks, I a  and I b , are returned to the microprocessor  36  (FIG. 4) of the inverter drive  34 , while a set of gate drive circuitries on a gate drive board  44  (FIG. 4) monitor the voltages V ce  across each IGBT A+, A−, B+, B−, C+, C−. This test structure  30  is simpler, and makes fuller use of the capabilities of the microprocessor  36  (FIG. 4) than the test structure  10  of FIG. 2, and in certain embodiments may eliminate the need for the tester  32 .  
         [0036]    [0036]FIG. 4 is a functional block diagram of a portion of the inverter drive  34 , driving a single pair of the IGBTs A+, A− for testing a first phase A of the inverter  12 . The microprocessor  36  handles all the input signals and manages and controls the outputs based on control algorithms. Among the outputs, the microprocessor  36  provides three Pulse Width Modulation (PWM) output signals  46 A,  46 B,  46 C to a Logic Cell Array (LCA)  48 . The LCA  48  may take the form of an appropriately programmed Field Programmable Gate Array (“FPGA”). The LCA  48  is a programmable digital circuit that generates six IGBT gate control signals  50 A+,  50 A− (only two shown) based on the three PWM output signals  46 A,  46 B,  46 C from the microprocessor  36 . The LCA  48  also handles all of the fault signals collectively referenced as  52  coming from the circuitries on the gate drive board  44 . It should be noted that FIG. 4 only shows one of the three phases of the gate drive circuitry. It should also be noted that the phases are referred to as first, second and third phases for convenience only and such reference should not be interpreted as an enumeration or ordering the operation of the corresponding IGBTs A+, A−, B+, B−, C+, C−.  
         [0037]    For every IGBT A+, A−, B+, B−, C+, C− there is an isolated control signal V ge  for the gate control, and the forward voltage (or conducting or collector-emitter voltage) V ce  is measured to detect a De-Saturation (DESAT) fault. When an IGBT is forward conducting, if there is a large current going passing through the IGBT, the collector-emitter voltage V ce  will increase as the conducting current increases. As soon as the collector-emitter voltage V ce  reaches a certain voltage level (corresponding to a certain current level), the corresponding drive circuit on the gate drive board  44  will generate a fault  52 , called a DESAT fault, and shut down the gate control signals V ga .  
         [0038]    The following are examples of six different situations, each illustrating the acts taken to test and diagnose all six IGBTs A+, A−, B+, B−, C+, C−.  
         [0039]    All IGBTs Normal (i.e., Not Faulty)  
         [0040]    Only two test measurements are needed to determine if all IGBTs A+, A−, B+, B−, C+, C− are normal. For example, the microprocessor  36  commands IGBTs C−, A+ and B+ ON at a same time. If all six IGBTs A+, A−, B+, B−, C+, C− are in a normal, non-faulty condition, the current I a  flowing through the first phase upper IGBT A+ and the current I b -flowing through the second phase upper IGBT B+ should be equal to the same predetermined value, I, the normal current flowing through when no fault exists, which is half of the value of the current flowing the through the third phase lower IGBT C−. Current value is determined by the resistance R. Likewise, commanding the third phase upper IGBT C+, the first phase lower IGBT A− and the second phase lower IGBT B− ON at a same time, current feedback I a  and I b  will have the same predetermined value I, but with opposite current direction to the first test. This is summarized in Table 1.  
                           TABLE 1                       Command ON   Current Feedback   Fault   Conclusion                   A+, B+, C−   I a  = I b  = I   None   A+, B+ and C− are OK       A−, B−, C+   I a  = I b  = −I   None   A−, B− and C+ are OK                  
 
         [0041]    In the above measurements, even though no current feedback information is available for the C phase, the IGBTs C+, C− in the C phase can be viewed as normal if the correct current feedback values are observed in phase A and B. Furthermore, since the circuits of the gate drive board  44  monitor the voltage V ce  between the collector c and the emitter e of the IGBT that is being turned ON, no voltage measurements by the tester  32  are required. Advantageously, measurement and testing time may be greatly reduced.  
         [0042]    One Faulty IGBT in any Single Phase  
         [0043]    At least three steps are required to determine the faulty IGBT in this scenario. Consider for example, the case of the first phase upper IGBT A+ being open or the first phase lower IGBT A− being shorted. When the upper IGBTs A+, B+ and Care commanded ON, if the first phase upper IGBT A+ is faulty, i.e., A+ is open, the collector-emitter voltage V ce  across the first phase upper IGBT A+ will be high, generating a gate drive fault  52   A+  sent to the microprocessor  36 . However, if the first phase lower IGBT A− is the faulty transistor, i.e., the first phase lower IGBT A− is shorted but first phase upper IGBT A+ is normal, then when first phase upper IGBT A+ is commanded ON a large current will pass through the first phase pair of IGBTs A+, A− due to the shorted first phase lower IGBT A−. The DESAT circuit will shut down the inverter  12  to protect the IGBTs and generate a fault at  52   A+ . Accordingly, the same fault signal will be generated for two different faults at two different places.  
         [0044]    In order to be able to determine whether first phase upper IGBT A+ is open or first phase lower IGBT A− is shorted, and thereby determine where the fault occurred, only the third phase upper IGBT C+ needs to be turned ON. If the current feedback I a  is equal to approximately −1.5 times the predetermined current I, then the first phase lower IGBT A− is shorted. If no current feedback is observed (i.e., I a  is equal to I b  is equal to zero), then the first phase upper IGBT A+ is open, and the first phase lower IGBT A− is not faulty.  
         [0045]    To check and to determine if the second phase upper IGBT B+, the second phase lower IGBT B−, the third phase upper IGBT C+ and the third phase lower IGBT C− are normal with respect to predetermined current feed back I a  and I b , the second phase upper IGBT B+ and third phase lower IGBT C− should be turned ON, followed by the turning ON of the second phase lower IGBT B− and third phase upper IGBT C+. In the case of the second phase upper IGBT B+ and third phase lower IGBT C− being turned ON, the current feedback I b  should be the negative of 1.5 times the predetermined current I.  
         [0046]    In a like manner, a single opened or shorted IGBT in phase B or C can be identified. Table 2 summarizes detection of one fault as described above.  
                           TABLE 2                           Current               Command ON   Feedback   Fault   Conclusion                   A+, B+, C−   None   A+   Either A+ open or A− shorted       A−, B−, C+   I a  = I b  = −I   None   A−, B− and C+ OK; or A− shorted       C+   I a  = −1.51 or   None   A− is shorted and A+ may be OK           I a  = I b  = 0       A+ is open and A− is OK       B+, C−   I b  = −1.51   None   B+ and C− OK       B−, C+   I b  = 1.51   None   B− and C+ OK                  
 
         [0047]    One Or Two Faulty IGBTs in Phase C  
         [0048]    Consider the example of the fault(s) of C+ open and/or C− shorted. To perform the test, the first phase upper IGBT A+, second phase upper IGBT B+ and third phase lower IGBT C− are commanded ON by the microprocessor  36 . Similar to the previous example, if the third phase upper IGBT C+ is open, its current-emitter voltage V ce  will be high, causing a corresponding gate drive fault signal (i.e., fault C+) to be sent out by the microprocessor  36 . However, if the third phase lower IGBT C− is shorted and the third phase upper IGBT C+ may or may not be open, then when the first phase lower IGBT A−, the second phase lower IGBT B− and the third phase upper IGBT C+ are commanded ON in the next step of testing, a large current will pass through the IGBTs due to the shorted C−. The DESAT circuit shuts down protecting the inverter  12  and generates a fault  50  (i.e., fault C+). Again, the same fault signal is generated for two different faults at two different places.  
         [0049]    The next step in the testing is to determine whether just one IGBT in a single phase is faulty, or both IGBTs in that phase are faulty. Here the microprocessor  36  commands ON both the first phase upper and the second phase lower IGBTs, A+, B−, respectively. If the results of the current feedback are I a  is equal to 1.5 times the predetermined current I and I b  is equal to −1.5 times the predetermined current I, then it may be concluded that the third phase upper IGBT C+ is open. If the results of the current feedback for both I a  and I b  is negative I, then it may be concluded that the third phase lower IGBT C− is shorted, and that the first phase upper and second phase lower IGBTs A+, B−, respectively, are normal. Further testing is still required to determine whether the first phase upper IGBT A− and the second phase upper IGBT B+ are normal.  
         [0050]    Determination of whether all the IGBTs have been checked can be made by commanding the first phase lower IGBT A− and the second phase upper IGBT B+ ON. If the results of the current feedback are I a  is equal to −1.5 times the predetermined current I and I b  is equal to 1.5 times the predetermined current I, then it may be concluded that the third phase upper IGBT C+ is open, and that the first phase lower IGBT A− and the second phase upper IGBT B+ are normal. If the results of the current feedback are I a  is equal to −I and I b  is equal to I, then it may be concluded that the third phase lower IGBT C− is shorted.  
         [0051]    Table 3 summarizes detection of one or two faults in one phase as described above.  
                           TABLE 3                       Command ON   Current Feedback   Fault   Conclusion                   A−, B−, C+   None   C+   Either C+ open and/or C−                   shorted       A+, B+, C−   I a  = I b  = −I   None   A− and B− OK; C− may be                   shorted       A+, B−   I a  = −I b  = −1.51   None   C+ open; A+ and B− may           or       be OK           I a  = −I b  = −I       C− shorted; A+ and B− may                   be OK       A−, B+   I b  = −I a  = I   None   C− shorted; A− and B+ OK           or       C+ open; A− and B+ OK           I b  = −I a  = 1.51                  
 
         [0052]    One or Two Faulty Transistors in any Two Phases  
         [0053]    Consider the example of the first phase upper IGBT A+ open and/or the first phase lower IGBT A− shorted, and the second phase upper IGBT B+ open and/or the second phase lower IGBT B− shorted. The same logic as shown previously applies, the microprocessor  36  commanding the first phase upper, second phase upper and third phase lower IGBTs A+, B+ and C−, respectively, ON. In this situation where there is one or two faulty IGBTs in any two phases, no current feedback will be detected, and a fault will be indicated in both the A and B phases. Accordingly, all that is known at this point of the testing is that first phase upper IGBT A+ is open and/or the first phase lower IGBT A− is shorted, and the second phase upper IGBT B+ is open and/or the second phase lower IGBT B− is shorted.  
         [0054]    As before, the microprocessor  36  next commands ON the first phase lower, the second phase lower, and the third phase upper IGBTs A−, B− and C+, respectively. If the current feedback shows both I a  and I b  to be the opposite of the predetermined current I, then it may be concluded that either the first phase lower IGBT A−, the second phase lower IGBT B− and the third phase upper IGBT C+ are normal, or that the first phase lower IGBT A− and the second phase lower IGBT B− are both shorted. It is still unknown at this point whether the first and second phase upper IGBTs A+, B+, respectively, are open.  
         [0055]    The next step indicates where the faults occurred in the A and B phases. To do so, the microprocessor  36  turns ON only the third phase upper IGBT C+. If the current feedback for both I a  and I b  is the opposite of the predetermined current I, then it is determined that at least the first and second phase lower IGBTs A−, B−, respectively, are faulty. The first and second phase upper IGBTs A+, B+, respectively, may or may not be faulty. If the current feedback for I a  is both the opposite and 150% of the amount of the predetermined current I and no current feedback is given by I b , then it is determined that at least first phase lower IGBT A− and the second phase upper IGBT B+ are faulty. The first phase upper IGBT A+ may or may not be faulty. If the current feedback for I b  is both the opposite and 150% of the amount of the predetermined current I and no current feedback is given by I a , then it is determined that at least the second phase lower IGBT B− and the first phase upper IGBT A+ are both faulty. The second phase upper IGBT B+ may or may not be faulty. If no current feedbacks are both zero, then it may be concluded that both the first and the second upper IGBTs A+, B+, respectively, are open.  
         [0056]    Table 4 summarizes detection of one or two faults in two phases, but not in phase C, as described above.  
                           TABLE 4                       Command ON   Current Feedback   Fault   Conclusion                   A+, B+, C−   None   A+   A+ open and/or A− shorted               and/or   and/or               B+   B+ open and/or B− shorted       A−, B−, C+   I a  = −I and/or   None   A−, B−, C+ OK; or           I b  = −I       A− and/or B− shorted       C+   I a  = −I and   None   A− shorted; A+ may be           I b  = −I or       open or OK and           I a  = −1.51 and       B− shorted; B+ may be           I b  = 0 or       open or OK           I b  = −1.51 and       A− shorted; A+ may be           I a  = 0 or       open or OK and B+ open           I a  = I b  = 0       B− shorted; B+ may be                   open or OK and A+ open                   A+ and B+ are open                  
 
         [0057]    It should be noted that every IGBT has been tested through the current feedback signal except C−. However, since no fault signal is generated in the first test, it is understood that the correct voltage level has occurred at C−.  
         [0058]    Table 5 illustrates the test procedure for the case of faulty transistors in two phases where one of the faults occurs in the C phase. Table 5 illustrates the steps followed and the possible results for the situation of third phase upper IGBT C+ open and/or third phase lower IGBT C− shorted, and first phase upper IGBT A+ open and/or first phase lower IGBT A− shorted.  
                           TABLE 5                       Command ON   Current Feedback   Fault   Conclusion                   A+, B+, C−   None   A+   A+ open and/or A− shorted;                   and B+ and C− OK; or                   B+ open and/or C− shorted       A−, B−, C+   None   C+   C+ open and/or C− shorted;                   and A− and B− OK; or                   A− and/or B− shorted       B+   I a  = −I and   None   A−, C− shorted; B+ OK           I b  = 21 or       A− shorted; C+ open           I a  = −1.51 = −I b         C− shorted; A+ open           or       C+ and A+ are open           I b  = −1.51,           I a  = 0 or           I b  = I a  = 0                  
 
         [0059]    It should be noted that every IGBT has been tested through the current feedback signal except the second phase lower IGBT B−. However, since no fault signal is generated in the first test, it is understood that the correct voltage level has occurred at the second phase lower IGBT B−. Furthermore, the test cases with faulty IGBTs in phase B and C are similar to the above case.  
         [0060]    Faulty IGBTs in all Three Phases  
         [0061]    Consider the situation of the first phase upper IGBT A+ open and/or the first phase lower IGBT A− shorted, the second phase upper IGBT B+ open and/or the second phase lower IGBT B− shorted, and the third phase upper IGBT C+ open and/or the third phase lower IGBT C− shorted. When the first phase upper IGBT A+, the second phase upper IGBT B+ and third phase lower IGBT C− are commanded ON, followed by commanding ON the first phase lower IGBT A−, the second phase lower IGBT B− and the third phase upper IGBT C+, faults will occur in every phase (A, B and C). In this situation, there is no need to continue testing as all power modules will be seen as faulty devices.  
         [0062]    As seen from the above six situations, if there is no faulty IGBT in the inverter circuit  12 , as is the instance in the majority of cases, only two tests are required to diagnose the same. Should any IGBT failure exist, up to five tests are required in order to locate the failed IGBT(s). Other than a three-phase resistor load, no extra hardware circuit is required. Additionally, this method and apparatus for testing makes full use of the intelligent microprocessor  36  and extra information from the circuits of the gate drive board  44 .  
         [0063]    IGBT Fault Detection With an AC Motor Connected  
         [0064]    In the field, after a failure occurs on the inverter  12 , the microprocessor  36  generates a gate drive fault indicating that one of the IGBTs failed without the above described logic. However, due to the complexity of the gate drive circuit and the inaccessibility of each individual IGBT, the above described system and method is very difficult to determine which, if any, IGBT failed without the above described logic. With very minor changes to the above, the above described system and method may be used as an AC inverter field diagnostic tool. Referring to FIG. 5, instead of utilizing the three-phase resistor load described above with reference to FIG. 3, the stator windings  60   a ,  60   b ,  60   c  of a three-phase motor  62  are used as the load. Such a diagnostics method and tool avoids the difficulty and problem of removing parts only to find that no IGBT has failed. The inverter  12  and test circuitry  30  may be part of a power module  64  for installation in a vehicle  66 .  
         [0065]    AC motors  62  have very low impedance at stand still. Thus, in order to avoid huge current passing through the motor&#39;s windings, the IGBTs&#39; ON time should be very short, i.e., pulsed. In other words, the IGBTs&#39; ON time should be less than the switching period of the inverter  12 . Also, the turn ON signal for upper and lower IGBTs A+, B+, C+ and A−, B−, C−, respectively, of each phase should not overlap each other. The output may be configured as either an output compare or a pulse width modulation (“PWM”) function so that the duty cycle is less than the switching period.  
         [0066]    For example, when an ON signal for the first phase upper IGBT A+ is generated, normally it is a PWM signal. When testing occurs in a manufacturing facility, as illustrated in FIG. 3, a resistor is generally available for limiting the current, with the current level determined by the resistance R and the DC bus voltage level. Typically, in a manufacturing facility environment, a power supply will be available for adjusting the DC voltage level. Accordingly, through the use of the resistor R 1 , R 2 , R 3  the current can be turned ON and kept ON without the concern of current overload.  
         [0067]    However, in the case of an AC motor  62  in the field, for example in a vehicle  66 , the current cannot always be ON because it would be too high, overloading the IGBTs. Since there is no power supply available that can adjust the DC bus voltage level, a PWM signal is used that limits the ON time, or duty cycle, to a very short time. By adjusting the duty cycle, the current can be measured within a reasonable range.  
         [0068]    The V ge  shown in FIG. 4 is the gate control signal for controlling the IGBT and provided via the gate driving circuitry of the gate drive board  44 . The control voltage V ce  signal is also connected with the gate drive board. This V ce  signal, or DESAT signal, is measured. When there is a large current passing through, the voltage V ce  will increase causing a fault signal to be generated shutting down the IGBT.  
         [0069]    Due to hardware constraints, only one output signal is available for controlling both IGBTs in a single phase. The gate drive circuitry of the gate drive board  44  constructs the two control signals based upon the output control signal from the microprocessor  36 . Therefore, if a short transistor ON time is required, both IGBTs of a phase will be turned ON within the one switching period. Typically, when an inverter  12  fails or generates a false fault signal (i.e., when there is no real fault), only one or, at most, two IGBTs have failed. Accordingly, the same principle as taught above can be used for detecting the failed IGBT(s). Following are three situations that further exemplify this.  
         [0070]    Fault Signal Occurs at One Phase  
         [0071]    Consider the example of a fault at the first phase upper IGBT A+. The motor  62  has indicated a fault in the A phase, but it is unknown whether the fault is with the first phase upper IGBT A+ transistor or with the first phase lower IGBT A−. Accordingly, the microprocessor  36  executes the following steps to determine where the fault occurred.  
         [0072]    The second phase IGBTs B+, B− are first turned ON by the microprocessor  36 , the upper IGBT B+ followed by the lower IGBT B−. No fault should be indicated in the B phase. The microprocessor  36  confirms that the duty cycle is small enough that no high current is flowing through the IGBTs. If the phase A current is equal to the input current I m  and the negative of the phase B current, then it may be concluded that first phase lower IGBT A− has shorted. If the phase A current is equal to the phase B current and both are equal to zero, then it may be concluded that there is no short, i.e., first phase lower IGBT A− is not faulty, and either the first phase upper IGBT A+ is open or not faulty.  
         [0073]    In order to determine whether first phase upper IGBT A+ is open or not faulty, the microprocessor  36  turns ON first phase upper IGBT A+ and second phase lower IGBT B−, followed by first phase lower IGBT A− and second phase upper IGBT B+. If the phase A current is equal to the input current I m  and the negative of the phase B current, then no fault will be generated and it may be concluded that both first phase IGBTs A+, A− are not faulty. However, if the phase A current is equal to the phase B current and both are equal to zero, then a fault will be generated for the A phase, indicating that first phase upper IGBT A+ is open. Table 6 summarizes the above steps.  
                           TABLE 6                       Command ON   Current Feedback   Fault   Conclusion                   B+, then B−   I a  = −I b  = I m  or   None   A− shorted or           I a  = I b  = 0       A− OK; either A+ open or                   A+ OK       A+, B−; then   I a  = −I b  = I m  or   None   A+ and A− are OK       A−, B+   I a  = I b  = 0   A+   or A+ is open                  
 
         [0074]    Fault Signal Occurs at Two Phases  
         [0075]    Consider the example of a fault at both the first and third phase upper IGBTs A+, C+, respectively. In this example, the motor  62  has indicated a fault in both the A and C phases, but it is unknown where the fault has occurred in each phase. In order to determined which IGBT within each phase is faulty, the microprocessor  36  executes the following.  
         [0076]    Initially the microprocessor  36  turns ON the B phase IGBTs B+, B−, the upper IGBT B+ followed by the lower IGBT B−, making certain that the duty cycle is small enough so that no high current is flowing through the IGBTs. With fault signals from both the A and C phases, one of three results should occur upon turning ON the B phase. If the phase A current is equal to the input current I m  and the negative of the phase B current, then no fault will be shown indicating that no fault has occurred within the B phase. It may be concluded that first phase lower IGBT A− has shorted, and that the third phase lower IGBT C− may not be faulty.  
         [0077]    If the phase B current is equal to twice the negative of the phase A current, then no fault has occurred within the B phase and it may be concluded that the first and third phase lower IGBTs A−, C−, respectively, have shorted. If the phase A current is equal to the phase B current and both are equal to zero, then no fault has occurred within the B phase and it may be concluded that either the first and the second upper IGBTs A+, C+, respectively, are open or are not faulty.  
         [0078]    In the instance that no current feedback is provided, i.e., the phase A current is equal to the phase B current and both are equal to zero, the microprocessor  36  performs two further steps to determine whether the first and/or the second upper IGBTs A+, C+, respectively, are open or whether a false fault signal occurred. The microprocessor  36  first turns ON the first phase upper IGBT A+ and the second phase lower IGBT B−, followed by the first phase lower IGBT and the second phase upper IGBT in order to determine whether the first phase upper IGBT A+ is open or not. If the phase A current is equal to the feedback current I m  and is the same as the negative of the phase B current, and no fault has occurred within the A phase, then it may be concluded that the first phase IGBTs A+, A− are not faulty. If the phase A current is equal to the phase B current and both are equal to zero, then a fault has occurred within the A phase and it may be concluded that first phase upper IGBT A+ is open.  
         [0079]    In order to determine whether the third phase upper IGBT C+ is open or not, the microprocessor  36  turns ON the third phase upper IGBT C+ and the second phase lower IGBT B−, followed by the third phase lower IGBT C− and the second phase upper IGBT B+. If the phase B current is equal to the input current I m , then no fault has occurred within the C phase and it may be concluded that third phase IGBTs C+ and Care not faulty. If the phase A current is equal to the phase B current and both are equal to zero, then a fault has occurred within the C phase and it may be concluded that third phase upper IGBT C+ is open.  
         [0080]    Table 7 summarizes the above, given the condition that the first and second phase upper IGBTs A+, C+, respectively, are identified as faulted in the first test.  
                           TABLE 7                       Command ON   Current Feedback   Fault   Conclusion                   B+, then B−   I b  = −I a  = I m  or   None   A− shorted; C− may be OK           I b  = −21 a  Or   None   Both A− and C− are shorted           I b  = I m , I a  = 0 or   None   C− shorted, A+ open, A−           I a  = I b  = 0   None   may be OK                   A+ and C+ open; or A+ and                   C+ OK       A+, B−; then   I a  = −I b  = I m or     None   A+ and A− are OK       A−, B+   I a  = I b  = 0   A+   A+ is open       C+, B−; then   I b  = I m  or   None   C+ and C− are OK       C−, B+   I a  = I b  = 0   C+   C+ is open                  
 
         [0081]    Fault Signal Occurs at all Three Phases  
         [0082]    In such a situation, no test is needed as all power modules will be seen as faulty devices.  
         [0083]    While there has been disclosed effective and efficient embodiments of the invention using specific terms, it should be well understood that the invention is not limited to such embodiments as there might be changes made in the arrangement, disposition, and form of the parts without departing from the principle of the present invention as comprehended within the scope of the accompanying claims.  
         [0084]    All of the above U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet, are incorporated herein by reference, in their entirety.  
         [0085]    From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.