Abstract:
The present invention relates to a method for determining an actual junction temperature (T j ) and/or an actual collector current (I C ) of an IGBT device, wherein the IGBT device has a main emitter (E M ) and an auxiliary emitter (E A ), comprising the steps of;
       measuring the characteristics of an emitter voltage drop (V EE′ ) as a difference between a main emitter voltage (V E ) at the main emitter (E M ) and an auxiliary emitter voltage (V E′ ) at the auxiliary emitter (E A ) during a switching operation of the IGBT device; and   determining the junction temperature and/or the collector current (I C ) based on the characteristics of the emitter voltage drop (V EE′ ).

Description:
TECHNICAL FIELD 
     The present invention relates to methods for determining an actual junction temperature of an IGBT device. 
     RELATED ART 
     A junction temperature of an IGBT semiconductor device in power applications is an important parameter for estimating the remaining lifetime of the IGBT device since the junction temperature provides essential information about the aging of such a device. Hence, the accurate estimation of the junction temperature becomes significant in estimating the remaining lifetime of an IGBT device and of an IGBT module including one or more IGBT devices. 
     As the junction temperature depends on operating conditions of the IGBT device and becomes highly dependent on the switching current, a number of approaches for measuring the load current of the IGBT device during operation, such as a Rogowski coil, current transformers and the like, have been made. However, these types of sensors are bulky and do not provide a practical approach to obtain an indication about the load current of the IGBT device. 
     Other techniques for measuring the junction temperature of the IGBT device may include mounting a thermocouple inside the packaged IGBT device or close thereto or measuring the internal gate resistance on the IGBT device. These techniques either involve direct access to the device or require specially designed IGBT chips or devices. 
     Hence, the above techniques are not well-suited for online measurement of the junction temperature of IGBT devices, particularly if there is a need for a timely resolution of the measurement to be in the range of several hundreds of Hz or several kHz. These measurement frequency correspond to the level of a PWM frequency (PWM: pulse width modulation) with which IGBT devices in many applications are operated. 
     As techniques requiring a manipulation of the IGBT device itself or additional components to be closely arranged at the IGBT device are costly and complex to implement, it is an object to provide a method and an apparatus for estimating a junction temperature from a direct measurement of electrical characteristics of the IGBT device. Furthermore, it is an object to provide a method for simultaneously estimating an IGBT current and a junction temperature for the same time instance. 
     U.S. Pat. No. 6,097,582A1 discloses a protection circuit for a power semiconductor switching device with a gate terminal controlling the current flow between power terminals of the device. The current through the device is estimated by integrating the voltage across an inductance in the current flow path of the device and providing a fault signal if the estimated current exceeds a selected value. The gate terminal is limited to a selected control level intermediate the full-on and full-off current levels of the switching device when the fault signal is provided. The current through the IGBT is measured based on the voltage drop between the power emitter terminal and the Kelvin emitter terminal of the device. 
     WO2013045960A2 discloses a method for measuring a silicon temperature of a power insulated gate bipolar transistors which is performed by measuring threshold voltage at turning on, which linearly decreases as the transistor silicon temperature increases, wherein the moment, at which the voltage between the gate and the control emitter corresponds to the threshold voltage, is determined by voltage detection between the control and power connections of the emitter. 
     SUMMARY OF THE INVENTION 
     These objects have been achieved by the method for determining an actual junction temperature of an IGBT device according to claim  1  and by the apparatus according to the further independent claim. 
     Further embodiments are indicated in the depending subclaims. 
     According to a first aspect, a method for determining an actual junction temperature of an IGBT device is provided, wherein the IGBT device has a main emitter and an auxiliary emitter. The method comprises the steps of:
         measuring the characteristics of an emitter voltage drop as a difference between a main emitter voltage at the main emitter and an auxiliary emitter voltage at the auxiliary emitter during a switching operation of the IGBT device; and   determining the junction temperature based on the characteristics of the emitter voltage drop,   wherein the junction temperature may be determined by the steps of:   assigning a reference peak voltage of the emitter voltage drop during the switching operation to a provided collector emitter voltage of the IGBT device and a determined collector current, wherein the reference peak voltage corresponds to a peak voltage of the emitter voltage drop during a switching operation at a reference temperature;   determining a peak voltage of the emitter voltage drop during the switching operation; and   determining the actual junction temperature depending on the determined peak voltage and the reference peak voltage.       

     One basic idea of the above method is to estimate a junction temperature depending on an emitter voltage drop between a main emitter voltage and an auxiliary emitter voltage of the IGBT device. Normally, the emitter terminals of the IGBT device are split into the power emitter of the main emitter that is connected to the power circuit and the auxiliary emitter (or Kelvin emitter) that is connected to the gate drive unit. Due to the bonding wires and the terminal fittings, the internal connections of the IGBT device cause parasitic inductances between the main emitter terminal and the auxiliary emitter terminal of the IGBT device. During a switching operation of the IGBT device, there is an emitter voltage drop across the parasitic inductance between the emitter terminals which can be used for estimating both the junction temperature. In particular, the emitter voltage drop between the main and auxiliary emitter terminals during a switching operation provides voltage characteristics having a peak voltage which depends on the junction temperature and an integral of the characteristics of the emitter voltage drop during switching which depends on a collector current. 
     This method allows for an estimation of the junction temperature by means of a direct measurement of electrical characteristics of the IGBT device. This allows for calculating the remaining lifetime of the IGBT device by detecting the power cycles of the semiconductor. Furthermore, a system health check can be performed during factory testing and during operation, so that preventive maintenance can be carried out. 
     Also, the actual junction temperature can be used to implement temperature protection for the IGBT device. Furthermore, when using the IGBT devices in converter design, it is possible to reduce the margins which were necessary when measuring the junction temperature with a delay or with measurement tolerance. 
     Moreover, the collector current may be determined based on an integration of the emitter voltage drop over time during the switching operation. 
     According to an embodiment, assigning the reference peak voltage may be performed by a provided lookup function determining a relation between peak voltages at reference temperatures and collector currents. 
     The peak voltage of the emitter voltage drop may be determined during the switching operation by loading a memory capacitance with a load current proportional to the emitter voltage drop and providing the memory capacitance voltage as an indication of the peak voltage. 
     The actual junction temperature may be determined depending on a peak voltage difference between the determined peak voltage and the reference peak voltage and a sensitivity of the characteristics of the peak voltage over junction temperature of the IGBT device. 
     According to a further aspect, an apparatus for determining an actual junction temperature and/or an actual collector current of an IGBT device is provided, wherein the IGBT device has a main emitter and an auxiliary emitter, wherein:
         means are provided for measuring the characteristics of an emitter voltage drop as a difference between a main emitter voltage at the main emitter and an auxiliary emitter voltage at the auxiliary emitter during a switching operation of the IGBT device; and   means are provided for determining the junction temperature and/or the collector current based on the characteristics of the emitter voltage drop. The apparatus may further comprise:   a first lookup function block for assigning a reference peak voltage of the emitter voltage drop during the switching operation to a provided collector emitter voltage of the IGBT device and a determined collector current, wherein the reference peak voltage corresponds to a peak voltage of the emitter voltage drop during a switching operation at a reference temperature; and   a second lookup function block for determining the actual junction temperature depending on the determined peak voltage and the reference peak voltage.       

     A peak voltage detector may be provided for determining a peak voltage of the emitter voltage drop during the switching operation. 
     An integrator may be provided which is configured to determine the collector current based on an integration of the emitter voltage drop over time during the switching operation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments are described in more detail in conjunction with the accompanying drawings, in which: 
         FIG. 1  shows a schematic view of an IGBT module having two parallel connected IGBT devices; 
         FIG. 2  shows a voltage-time diagram of the emitter voltage drop between a main emitter and an auxiliary emitter of an IGBT device during a switching operation at 25° C.; 
         FIG. 3  shows a voltage-time diagram of the emitter voltage drop between a main emitter and an auxiliary emitter of an IGBT device during a switching operation for different junction temperatures; 
         FIG. 4  shows a diagram indicating the characteristics of the peak voltage of the emitter voltage drop over junction temperature for different collector currents; 
         FIG. 5  shows the variation of a junction temperature sensitivity of the peak voltage as a function of the switching current; 
         FIG. 6  shows a diagram indicating the characteristics of the peak voltage over the collector current for different junction temperatures; 
         FIG. 7  shows a variation of the peak voltage of the emitter voltage drop over a collector emitter voltage for two different junction temperatures of 25° C. and 100° C. and two currents of 200 A and 400 A; 
         FIG. 8  shows a block diagram illustrating the function of an apparatus for estimating the junction temperature of an IGBT device; and 
         FIG. 9  shows a block diagram for an exemplary circuit implementation of a peak detector as used in the apparatus of  FIG. 8 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
       FIG. 1  shows a schematic of an IGBT module  1  for use in power applications. The IGBT module  1  comprises one or more IGBT devices  2  which are connected in parallel. Each of the IGBT devices  2  is integrally formed as a semiconductor chip and comprises an IGBT semiconductor component as known in the art. 
     The terminals of the IGBT module  1  are: an insulated gate G, a main collector C, a sense collector C S , a main emitter E M  and an auxiliary emitter E A , each of which is connected to the respective internal terminal of the IGBT device  2 , so that the terminals of the module behave like common terminals for all of the included IGBT devices  2 . Although, substantially, the IGBT is a three-terminal power semiconductor device in many applications, the auxiliary emitter E A  may be fed out to a separate external terminal. While the main emitter E M  is connected to the power circuit and is part of the power path, the auxiliary emitter E A  may be connected to a gate drive unit (not shown). 
     Internal connections in the IGBT module  1  result in parasitic inductances L 1  to L 5 , L B , wherein the sense collector C S  is provided with the parasitic inductance L 1 , the power path of the main collector C is provided with the parasitic inductance L 2 , the power path of the main emitter E M  is provided with the parasitic inductance L 3 , the auxiliary emitter E A  is provided with the parasitic inductance L 4 , the gate is provided with the parasitic inductance L 5 , and the internal terminals of the IGBT chip are provided with bonding parasitic inductances L B . Hence, there are parasitic inductances L 3  and L 4  between the main emitter E M  and the auxiliary emitter E A  which are caused by terminal configurations. Over the main collector C and the main emitter E M , a DC link voltage V DC  may be applied. 
     Between the auxiliary emitter E A  and the main emitter E M , an emitter voltage drop V EE′  can be observed which occurs during a switching operation of the IGBT device  2 . This emitter voltage drop V EE′  shows characteristics which depend on the junction temperature, the collector current I C  and the collector emitter voltage V DC . Therefore, the parasitic inductance between the auxiliary emitter E A  and the main emitter E M  can be used as a sensor to extract the current flowing through the IGBT device  2  and its junction temperature. The derivative of the collector current I C  flowing through this parasitic inductance generates a voltage drop given by 
                 V     EE   ′       =       L   σ     ⁢       ⅆ     I   C         ⅆ   t           ,         
where V EE′  corresponds to the emitter voltage drop measured between the auxiliary emitter E A  and the main emitter E M , L σ  corresponds to the parasitic inductance across which the voltage drop is measured (e. g. L 3 , as dl C /dt occurs only through L 3 , and L 4  sees only gate current), and I C  corresponds to the collector current.
 
       FIG. 2  illustrates the characteristics of the voltage drop V EE′  over time during a switching operation while the collector current I C  can be obtained by: 
                 I   C     =       1     L   σ       ⁢     ∫       V     EE     ′   ⁢                 ⁢     ⅆ   t             ,         
which is indicated by the dotted line triangle.
 
     The voltage difference between the auxiliary emitter voltage V E′  and the main emitter voltage V E , when measured at different temperatures, results in different peak voltages during the switching operations. The difference of the peak voltages at a particular collector current I C  is used as an indicator for the junction temperature of the IGBT device  2 . 
     In  FIG. 3 , it can be seen that the peak voltages of the emitter voltage drop V EE′  decrease over increasing junction temperatures. However, the area of the curve at each measurement temperature remains constant as the different curves get wider with increasing temperatures. Hence, the switching collector current I C  should remain approximately the same at all junction temperatures in this measurement condition. Since the measured parasitic inductance is constant over temperature, the change of the peak voltages of the emitter voltage drop V EE′  at different temperatures should be due to the change of dl C /dt over temperature. The term dl C /dT decreases with temperature as the device mobility is decreasing at higher temperatures and the IGBT device  2  gets slower in switching. 
     The sensitivity of the peak voltage of the emitter voltage drop V EE′  is substantially linear over the junction temperature, so that a peak voltage measured at an unknown junction temperature can be used to determine the actual junction temperature T j  of the IGBT device  2  by comparing the peak voltage at the unknown junction temperature with a peak voltage at a reference junction temperature, such as at 25° C. Hence the unknown temperature can be estimated by comparing the value of the peak voltage of the emitter voltage drop V EE′  for different currents and voltages at the reference junction temperature and the peak voltage obtained at a similar current and voltage for an unknown temperature. 
     In  FIG. 4 , the characteristics of the peak voltage of the emitter voltage drop V EE′  over temperature are illustrated for different collector currents I C . It can be seen that the variation of the peak voltage decreases over temperature and increases with the switching current at all measurement temperatures. 
     As further shown in  FIG. 5 , the variation of the temperature sensitivity of the peak voltage as a function of the switching current increases with the collector current I C  for a particular device. Hence, the collector current I C  has to be known in order to extract the actual junction temperature T j  using the peak voltage. 
     Furthermore, it can be seen from  FIG. 6  that the peak voltage of the emitter voltage drop V EE′  increases with the switching current at any given temperature. Also, the peak voltage is higher for the lowest temperature at any given collector current I C . 
     A variation of the peak voltage of the emitter voltage drop V EE′  over a collector emitter voltage V DC  for two different junction temperatures of 25° C. and 100° C. and two currents of 200 A and 400 A is shown in  FIG. 7 . It can be seen that at any given collector current I C  and junction temperature, the peak voltage increases up to a certain voltage and then either remains constant or decreases slightly thereafter, depending on measurement temperature and switching current. Generally, it can be seen that the sensitivity of the peak voltage over the collector emitter voltage V DC  is higher at higher switching currents for any given temperature. Hence, both collector current I C  and the collector emitter voltage as the DC link voltage V DC  have has to be known in order to extract the actual junction temperature T j  using the peak voltage. 
     In  FIG. 8 , a block diagram for the simultaneous estimation of the collector current I C  and the junction temperature from the emitter voltage drop V EE′  across the parasitic inductance is shown.  FIG. 8  illustrates the measurement circuit  10 , wherein the emitter voltage drop V EE′  between the main emitter voltage V E  and the auxiliary emitter voltage V E′  is tapped and a corresponding emitter voltage drop V EE′  is simultaneously supplied to an integrator  11  and a peak detector  12 . From the integrator  11 , a collector current I C  is obtained according to the above formula, an indication of which is supplied to a first lookup function in a first lookup function block  13 . In the first lookup function block  13 , a reference peak voltage is determined using the thus obtained collector current I C  and the collector emitter voltage which may substantially correspond to a DC link voltage V DC . The reference peak voltage corresponds to a peak voltage that would be obtained at a reference junction temperature, which may be 25° C., at the respective collector current I C  and the respective collector emitter voltage V DC . 
     The peak detector  12  receives the emitter voltage drop V EE′  across the parasitic inductance and detects a peak amplitude during a switching process in the IGBT device  2 . The indication of the peak amplitude is provided to a comparator  14  together with the result of the reference peak voltage from the first lookup function block  13 . The comparator  14  provides a voltage peak difference which is supplied to a second lookup function block  15 . The second lookup function block  15  provides a second lookup function which associates the actual junction temperature T j  of the IGBT device  2  with the collector current I C , the collector emitter voltage V DC  and the voltage peak difference with respect to the reference peak voltage. 
     The first lookup function can provide information regarding the peak voltage measured at a reference junction temperature and a peak voltage sensitivity (mV/° C.) at a particular collector current I C  and collector emitter voltage V DC  for various switching conditions. The difference in the peak voltage given by the comparator  14  can be used to estimate the junction temperature using the peak voltage sensitivity mV/° C. at a particular current and collector emitter voltage V DC . 
       FIG. 9  schematically shows a peak detector  12 . The block diagram shows an active diode block  21 , a purpose of which is to transmit the emitter voltage drop V EE′  to a memory capacitance block  22  through a diode with a theoretical zero-voltage drop. The active diode block  21  also provides the loading current to the memory capacitance MC of the memory capacitance block  22 . Due to the presence of the diode in the active diode block  21 , the memory capacitance MC cannot discharge and retains the higher voltage. The active diode block  21  is configured to be fast enough in order to capture a small spike. After reading the memory capacitance MC with an output buffer unit  23 , the memory capacitance MC is reset to an initial state by means of a reset unit  24 . 
     The output of the output buffer unit  23  is fed to an analog-digital converter  25  to convert the buffer output into digital data. The digital data is processed in a processing unit  26  to control the reset unit  24  for resetting the memory capacitance MC. 
     The above apparatus allows for estimating the actual junction temperature T j  of an IGBT device  2  by measuring electrical characteristics. The emitter voltage drop V EE′  between the auxiliary emitter voltage V E′  and the main emitter voltage V E  during a turn-off of the IGBT device  2  is characterized with respect to its junction temperature T j , its collector emitter voltage V DC  and its collector current I C , so that the voltage shift information of the peak voltage of the emitter voltage drop V EE′  can be used together with a lookup function to obtain the actual junction temperature T j . Furthermore, as the collector current I C  is also required for estimating the junction temperature T j , the values for the actual collector current I C  and the junction temperature derived therefrom correspond to the same time instance, which allows for a more accurate estimation of the remaining lifetime of the IGBT device  2 . 
     REFERENCE LIST 
       1  IGBT module 
       2  IGBT device 
       10  measurement circuit 
       11  integrator 
       12  peak detector 
       13  first lookup function block 
       14  comparator 
       15  second lookup function block 
       21  active diode block 
       22  memory capacitance block 
       23  output buffer unit 
       24  reset unit 
       25  analog-digital converter 
       26  processing unit