Patent Publication Number: US-2023146986-A1

Title: Short circuit detection and protection for an insulated gate component by monitoring and checking the gate voltage

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to foreign French patent application No. FR 2111832, filed on Nov. 8, 2021, the disclosure of which is incorporated by reference in its entirety. 
     FIELD OF THE INVENTION 
     The field of the invention is that of the power electronic module controls used to drive the operation of power electronic components. As is known, the power modules represent electronic components handling high powers and, thereby, demanding particular attention in their design and their tracking throughout their life and use. 
     BACKGROUND 
     More specifically, the invention deals with the protection of the insulated gate power components such as insulated gate power transistors IGBT (Insulated Gate Bipolar Transistor) or even insulated gate field effect transistors MOSFET (Metal Oxide Semiconductor Field Effect Transistor). 
     These power components are, as is known, the subject of particular attention as to their state of operation in order to know, at each instant, if this component might be impacted by any degradation. Generally, checking the good state of this type of power component is done via current flowing through it between the two terminals of these three-pole components, namely the collector and the emitter for an IGBT transistor or even the drain and the source for a transistor of MOS type. Indeed, if these two terminals are passed through by an abnormal current or a short-circuit current, then it is probable that the operation of the transistor, even the very integrity of the component, is degraded. 
     Currently, the checking of the current flowing through the two terminals mentioned above is done via a tracking of the voltage or of the current directly between the two terminals mentioned, that is to say at the conduction channel of a transistor. 
     Usually, this operation of monitoring the integrity of the power component, directly on the current passing through the two terminals of the three-pole component, is done according to the following means: Addition of an insulated current sensor at one of the two terminals of the power transistor. As an indication, a transformer positioned at the corrector (or the drain) makes it possible to directly evaluate the current passing through that terminal. Nevertheless, the use of a sensor causes the volume of the transistor to be increased. 
     The measurement of the current can also be done via a resistor positioned in series (that is to say a shunt) with respect to one of the two terminals passed through by the current that has to be measured. However, the use of a resistor for this purpose induces a high consumption of energy that the resistor dissipates. 
     The measurement of the current passing through the collector of the power transistor can also be done via a current mirror incorporated in the power component. The current mirror is an electrical circuit which makes it possible to copy the current passing through the collector and thus to have easy access to a portion of the value of the current passing through the latter. Nevertheless, this type of electrical circuit is very specific to particular types of electronic component. This architecture then necessitates large investment in terms of adaptation. 
     It is also possible to envisage studying the voltage at the collector via a diode. Nevertheless, this option increases the overall bulk in the power transistor and presents the drawback of being blind to the detection of a short-circuit for a time that corresponds to the off/on transition of the power transistor. 
     SUMMARY OF THE INVENTION 
     The invention aims to mitigate all or part of the problems cited above by proposing a detection device for detecting a short-circuit current between the collector (or the drain) and the emitter (or the source) of the power transistor that is simple, compact and that has a relatively low production cost. 
     To this end, the subject of the invention is a power stage comprising a power transistor controlled via a driver, the power transistor comprising three terminals, including a collector c, an emitter e and a gate g linked to the driver, the power stage also comprising a detection device for detecting a short-circuit current cc between the collector c and the emitter e, the detection device comprising a voltage sensor capable of detecting a voltage Vge at the gate g of the power transistor outside of a predefined voltage range. 
     According to one aspect of the invention, the detection device comprises an energy storage device configured to deliver an additional voltage to the gate g if the equivalent capacitance of the gate Cge is discharged. 
     According to one aspect of the invention, the energy storage device is an inductor positioned between the driver and the gate g of the power transistor. 
     According to one aspect of the invention, the power stage comprises a voltage reducer capable of reducing the voltage Vge at the gate g to a threshold voltage. 
     According to one aspect of the invention, the power stage comprises a recorder capable of recording a power transistor operation fault. 
     The invention also deals with a method for detecting a short-circuit current cc between a collector c and an emitter e of a power transistor comprising a gate g, the method comprising the following steps:
     Detecting a voltage Vge at the gate g of the power transistor outside of a predefined voltage range,   Reducing the voltage at the gate g of the power transistor to a predefined reduction voltage,   Switching off the power transistor.   

     The invention offers the advantage of allowing the analysis of the current between the collector and the emitter of a power transistor by monitoring the gate voltage of the power transistor via a detection device that is compact and of simple design. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood and other advantages will become apparent on reading the detailed description of an embodiment given as an example, the description being illustrated by the attached drawing in which: 
         FIG.  1    represents a power stage comprising a power transistor and a detection device for detecting a short-circuit current according to the invention, 
         FIG.  2    represents a method for detecting a short-circuit current between a drain and a source of a power transistor according to the invention. 
         FIG.  3    represents the method for detecting a short-circuit current via a timing diagram. 
     
    
    
     For clarity, the same elements will bear the same references in the various figures. 
     DETAILED DESCRIPTION 
       FIG.  1    represents a power stage  2  comprising a power transistor  4  controlled via a driver  6 . The power transistor  4  comprises three terminals, including a collector c, an emitter e and a gate g linked to the driver  6 . The power transistor  4  is an insulated gate bipolar transistor (IGBT). As a variant, the power transistor  4  is a field-effect transistor (metal oxide semiconductor field-effect transistor, or MOSFET) and therefore comprises a drain, a source and a gate linked to the driver  6 . Consequently, for a field-effect transistor, the drain can be likened to the collector c, the source can be likened to the emitter e. 
     In this context, the invention consists in detecting and protecting the insulated gate power component, namely the power transistor  4 , when the latter is passed through by an abnormal current. This abnormal current, called “short-circuit current”, appears when a low impedance, called “short-circuit”, is present at the terminals of an electrical source and has an intensity greater than a predefined intensity for so-called “normal” operation of the power transistor  4 . This short-circuit current can be of two types: 
     A type 1 short-circuit which is a short-circuit applied during conduction of the power transistor  4 , that is to say when the abnormal current is superimposed on a normal current. Consequently, in a type 1 short-circuit, a voltage measured between the gate g and the emitter e of the power transistor  4 , called gate voltage Vge, of the power transistor  4  increases rapidly and in the same proportion as the increase in the short-circuit current. 
     A type 2 short-circuit which is a short-circuit established before the power transistor  4  is made to conduct. Consequently, when the power transistor  4  starts conducting, the abnormal current or short-circuit current immediately passes through the power transistor  4 . In a type 2 short-circuit, the equivalent capacitance Cge seen from the gate g of the power transistor  4  remains constant and of low value compared to an equivalent capacitance measured at the gate of the power transistor  4  in so-called “normal” operation. 
     In other words, the type 1 short-circuit and the type 2 short-circuit are both linked to a circuit external to the power transistor  4 , forming a closed electrical loop and comprising an electrical load, connected to the power transistor  4  via the collector c and the emitter e. Thus, a type 1 short-circuit is a short-circuit appearing in this external circuit while the power transistor  4  is already conducting whereas a type 2 short-circuit is a short-circuit already present in the external circuit before the conduction of the power transistor  4  is established. 
     Thus, the power stage  2  comprises a detection device  8  for detecting a short-circuit current cc between the collector c and the emitter e. The detection device  8  comprises a voltage sensor  82  capable of detecting and of measuring the voltage between the gate g and the emitter e called Vge at the gate g of the power transistor  4 . More specifically, the voltage sensor  82  is configured to compare this measurement of gate voltage Vge to a defined voltage range, and signal if the measurement of gate voltage Vge is outside of the predefined voltage range. Indeed, the presence of a short-circuit in the power transistor  4 , and in particular between the collector c and the emitter e, can be reflected by a significant increase in the intensity of the current between these two terminals of the power transistor  4  and causes degradation of the power stage  2 . Now, this overvoltage induced by the increase in the current which passes through the power transistor  4  is expressed according to the following formula: 
     
       
         
           
             Δ 
             V 
             g 
             = 
             
               
                 Δ 
                 I 
                 d 
                 s 
               
               
                 G 
                 m 
               
             
           
         
       
     
     In which ΔVg represents the variation of voltage Vge at the gate, ΔIds represents the variation of the current between the collector c and the emitter e and Gm represents the transconductance of the power transistor  4 . As an indicative example, the transconductance Gm is equal to 300 Siemens for example. 
     Consequently, it is possible to observe a proportional relationship, for the same transconductance of the power transistor  4 , between the voltage Vge at the gate and the current between the collector c and the emitter e. More specifically, upon a significant increase in the intensity of the current between the collector c and the emitter e, synonymous with short-circuit current cc, the voltage Vge at the gate is also subjected to an increase in the voltage measurement performed by means of a voltage sensor  82 . The voltage sensor  82 , which is linked directly to the gate g, therefore makes it possible to measure the voltage Vge at the gate g so as to be able to detect a short-circuit current cc between the collector c and the emitter e of the power transistor  4 . 
     As an example, the predefined voltage range within which the voltage Vge of the gate g lies is a voltage range lying between 0 volts and +20 volts. As a variant, this predefined voltage range is a voltage range lying between 2 volts and 20 volts in the case of use of an insulated gate field-effect transistor MOSFET. According to another variant, for the case of use of an insulated gate power transistor IGBT, the predefined voltage range is a voltage range lying between 5 volts and 20 volts. 
     Furthermore, the short-circuit can also be present before the power transistor  4  is made to conduct in the case of a type 2 short-circuit. 
     When the power transistor starts conducting, an abnormal current, or to put it another way, a short-circuit current, passes immediately through the power transistor. Also, the voltage value at the gate that is measurable does not increase as rapidly and it is not as significant as for a type 1 short-circuit. Consequently, the simple measurement of the voltage at the gate Vge is no longer sufficient since this measurement can be done later. Now, upon a short-circuit, the equivalent capacitance seen by the gate g of the power transistor remains constant and of low value, i.e. approximately ten times lower than when a “normal” current passes through the power transistor. Conversely, in normal operation, that is to say when the power transistor is saturated, the value of the capacitance, seen by the gate g, is increased, notably because of the Miller effect, when the voltage measured at the collector c is low. This equivalent capacitance measured at the gate g is of high value, and generally greater than a microfarad, at the switching instants. And, in a short-circuit, the switching instant does not then affect the value of the equivalent gate capacitance. The value of the equivalent gate capacitance remains constant and of low value, that is to say lower than a hundred nanofarads. This change can be understood as a change of charges at the gate. 
     Consequently, the variation of the voltage ΔVg at the gate g is not significant enough in order to reveal a short-circuit current cc present in this transition phase. In order to reveal this type of short-circuit, the detection device  8  comprises a capacitance measurement sensor  83  capable of detecting and of measuring an equivalent capacitance value Cge of the gate g at the gate g of the power transistor  4 . More specifically, the capacitance sensor  83  is configured to compare this gate equivalent capacitance measurement Cge to a predefined capacitance value, and signal if the gate equivalent capacitance measurement Cge is lower than the predefined capacitance value. In other words, when the equivalent capacitance of the gate Cge between the gate g and the conduction channel is discharged, the capacitance measurement sensor  83  is capable of detecting this equivalent capacitance value Cge, measuring this equivalent capacitance value Cge, comparing this equivalent capacitance value Cge with the normally measurable predefined capacitance value and signal the malfunction. Indeed, in nominal operation of the power transistor  4 , that is to say at the switching instants, the equivalent capacitance of the gate Cge is temporarily of high value, that is to say for example greater than 1 µF. Conversely, when the conduction of the power transistor  4  is established, the equivalent capacitance of the gate Cge is of low value, that is to say for example approximately 100 nF. Consequently, when the measurement of the equivalent capacitance Cge at the gate g of the power transistor is lower than this low equivalent capacitance value for when the power transistor  4  is operating with conduction, a type 2 short-circuit is detected. As an indicative example, the predefined capacitance value can be a capacitance value of 100 nF. 
     As an indicative example, the capacitance measurement sensor  83  may be a Wheatstone bridge. 
     As a variant, It is also possible to envisage measuring this equivalent capacitance value of the gate Cge according to the following formula: 
     
       
         
           
             
               C 
               
                 g 
                 e 
               
             
             = 
             
               
                 
                   Q 
                   
                     g 
                     e 
                   
                 
               
               
                 
                   V 
                   
                     g 
                     e 
                   
                 
               
             
           
         
       
     
     In which Q ge  represents the gate charge and V ge  represents the voltage between the gate and the emitter. The charge Qge can be defined as integrating, with respect to time, the intensity of the current passing through the gate g. Furthermore, the value of the voltage at the gate Vge is already known through the voltage sensor  82 . Thus, the capacitance measurement sensor  83  can also be a device for measuring the intensity of the current at the gate g configured to determine, according to the above formula, the equivalent capacitance value of the gate Cge. 
     The detection device  8  also comprises an energy storage device  84  configured to deliver an additional voltage to the gate g when the equivalent capacitance value measured at the gate Cge by the capacitance measurement sensor  83  is lower than the predefined capacitance value at the gate g. In other words, when the capacitance measurement sensor  83  detects that the equivalent capacitance value measured at the gate Cge is lower than the predefined capacitance value at the gate g, the capacitance measurement sensor  83  drives the energy storage device  84  such that the energy storage device  84  releases an additional voltage to the gate g. Now, the releasing of this additional voltage to the gate g then makes it possible to generate an overvoltage at the gate g. Consequently, the voltage Vge between the gate g and the emitter e increases in proportion to the additional voltage delivered by the storage device  84  until the predefined voltage range is exceeded. Consequently, the voltage sensor  82  detects that the voltage Vge at the gate g of the power transistor  4  is outside of the predefined voltage range and therefore signals a short-circuit in the power transistor  4  that makes it possible to switch off the power transistor  4  before its components are degraded. The energy storage device  84  is positioned between the driver  6  and the gate g of the power transistor  4  so as to be able to deliver the additional voltage to the gate g. As an indicative example, this additional voltage is the voltage necessary for the sum of the voltage measured at the gate g and of the additional voltage to be greater than the maximum voltage value of the predefined voltage range. 
     As an indicative example, the energy storage device  84  is an inductor, with an inductance value of approximately 100 nH, positioned between the driver  6  and the gate g of the power transistor  4 . The inductor has the advantage of being a passive component that makes it possible to directly respond to a pulsed signal supplied by the driver  6  and allow the storage and restoration of sufficient energy to induce an overvoltage, when the equivalent capacitance seen from the gate of the power transistor  4  is low and discharged. 
     As a variant, the storage device  84  is an active generator that makes it possible to generate an additional voltage at the gate and that is driven by information from the driver  6 . 
     Hereinafter in the description, the storage device  84  is likened to an inductor  84 . Thus, as stated previously, the inductor  84  is positioned in series between the driver  6  and the gate g so as to produce an RLC resonant circuit between the driver  6 , via a resistor included in the driver  6 , the inductor  84  and the capacitance of the gate g and therefore ascertain the damping of the system. Now, the following relationship makes it possible to reveal an under-damping of the RLC resonant circuit that allows the inductor  84  to inject the additional voltage to the gate g according to the condition: 
     
       
         
           
             
               
                 R 
                 g 
               
               2 
             
             
               
                 
                   
                     C 
                     g 
                     e 
                     _ 
                     c 
                     c 
                   
                   L 
                 
               
             
             &lt; 
             1 
           
         
       
     
     In which Rg represents the resistance of the driver  6  in the RLC resonant circuit, Cge_cc represents the equivalent capacitance of the gate g when the power transistor  4  is subjected to a short-circuit and L represents the inductance value of the inductor  84 . 
     Consequently, the inductor  84  has the advantage of allowing additional voltage to be added at the gate g of the power transistor so as to reveal the fact that the value of the voltage Vge of the gate g is not within the voltage range and therefore the presence of a short-circuit current cc during this transition phase of the power transistor  4 . 
     Furthermore, the power stage  2  comprises a voltage reducer  86  capable of reducing the voltage Vge at the gate g to a threshold voltage, when a short-circuit current cc is detected between the collector c and the emitter e via the voltage sensor  82 . More specifically, when the voltage Vge at the gate g detected by the voltage sensor  82  is not within the predefined voltage range, the voltage reducer  86  makes it possible to reduce this voltage Vge at the gate g in order to lie within the voltage range. As an example, the threshold voltage can be a voltage lower than 20 volts and/or greater than the voltage at which the power transistor starts to conduct, called Vgsth, and is preferentially a voltage of 10 volts. The voltage reducer  86  offers the advantage of being able to reduce the short-circuit current according to the relationship ΔVg =  
     
       
         
           
             
               
                 Δ 
                 I 
                 d 
                 s 
               
               
                 G 
                 m 
               
             
             , 
           
         
       
     
     before switching off the power transistor  4 . 
     The power stage  2  also comprises a recorder  88  capable of recording the short-circuit of the power transistor  4 . As an example, the recorder  88  is a capacitor which is discharged following the short-circuit in order to reveal a power transistor  4  operating fault. 
     Also proposed is a detection method  100 , represented in  FIG.  2   , for detecting a short-circuit current cc between a collector c and an emitter e of a power transistor  4 , comprising a gate g. The detection method  100  comprises the following steps: 
     Detecting (step  120 ) the voltage Vge at the gate g of the power transistor  4  outside of the predefined voltage range. As an example, the predefined voltage range within which the voltage Vge of the gate g lies is a voltage range lying between 0 volts and +20 volts. The detection  120  of the voltage Vge is performed via the voltage sensor  84 . 
     Reducing the voltage (step  130 ) at the gate g of the power transistor  4  to a predefined voltage. As a preferential example, the predefined reduction voltage can be 10 volts. 
     Switching off (step  140 ) the power transistor  4 . 
     The reduction of the voltage (step  130 ) at the gate g of the power transistor  4  can cause the power transistor  4  to be automatically switched off (step  140 ). As a variant, this switching off (step  140 ) can be controlled. Nevertheless, the control of the switching off  140  of the power transistor  4  must be prompt. 
     The detection method  100  can also comprise a step of restoration (step  110 ) of additional energy upstream of the detection step  120  if the equivalent capacitance of the gate Cge is discharged and lower than the predefined capacitance, that is to say, for example, a capacitance of approximately 100 nF. 
     The step of restoration  110  of the stored energy to the gate g makes it possible to add the additional energy that reveals a short-circuit when the power transistor  4  is switched. 
     The detection method  100  can also comprise, following the detection step  120 , a step of recording of the fault (step  128 ) of the power transistor  4 . 
     The detection method  100  can also be interpreted according to the timing diagram represented in  FIG.  3   . Indeed, a short-circuit current at the power transistor  4  can be interpreted as a strong increase in the current Ip passing through the power transistor  4 . Now, in the case of a type 1 short-circuit, this increase in the current is accompanied by an increase in the voltage Vge at the gate g of the power transistor  4 . Consequently, the detection  120  is made when the voltage Vge is greater than 20 volts for example. This is then followed, in order not to damage the power transistor  4 , by reduction of the voltage Vge (step  130 ) at the gate g of the power transistor  4  to the predefined voltage, such as 10 volts for example, and the switching off  140  of the power transistor  4 . In the case of a type 2 short-circuit, the restoration  110  of sufficient energy to induce the overvoltage, if the equivalent capacitance of the gate Cge is discharged and lower than the predefined capacitance, that is to say, for example, a capacitance of approximately 100 nF, makes it possible to generate the detection  120  of this overvoltage at the gate. Furthermore, the step of recording of the fault  128  makes it possible, upon the detection  120  of the power transistor  4  operating fault, to store the fault in real time. 
     The invention therefore proposes incorporating a voltage sensor at the gate of the power transistor in order to detect an abnormal voltage, that is to say a voltage outside of a predefined voltage range for the gate g and incorporate a means that makes it possible to store and restore sufficient energy to induce a detection of an abnormal voltage, when the equivalent capacitance Cge seen from the gate of the power transistor  4  is discharged for the case of a type 2 short-circuit. The invention therefore offers the ability to detect a short-circuit in the power transistor based on the measurement of the voltage at the gate of the power transistor.