Short circuit protection circuit and method for insulated gate bipolar transistor

An assembly includes an insulated gate bipolar transistor (IGBT), a gate driver and a short-circuit protection circuit. The gate driver is adapted to supply voltage to a gate terminal of the IGBT. The short-circuit protection circuit includes an IGBT short-circuit detector for determining whether the IGBT is short-circuited, and a supply voltage regulator for regulating the supply voltage in response to the IGBT short-circuit detector determining that the IGBT is short-circuited.

BACKGROUND

Embodiments of the invention relate generally to a circuit and method for protecting insulated gate bipolar transistors (IGBTs) against short circuit conditions.

An IGBT is a switching transistor used to permit power flow in when it is on and to stop power flow when it is off. It is a solid-state device and has no physical moving parts. Instead of opening and closing a physical connection, the IGBT is operated by applying voltage to a semiconductor component, which changes its properties to create or block an electrical path. IGBTs are commonly used as switches, e.g., in chopper and frequency converter applications to control and convert electrical power by switching electrical devices on and off at predetermined instances.

IGBTs usually are designed to reliably handle circuit currents under normal as well as estimated overload conditions. Generally, to a certain extent, IGBTs are capable of withstanding fault conditions caused by anomalous operations. However, under a fault or short-circuit condition, an IGBT may be subjected to a very high surge current. The short-circuit current may be four times its rated current, resulting in both high voltage and high current simultaneously being applied to the IGBT. The IGBT under the fault or short-circuit condition may be subjected to a power loss with consequent increased thermal stress, which may damage the IGBT. Therefore, protecting IGBTs against short circuit conditions is important.

Existed methods manage to reduce a peak current at the beginning of the short circuit fault by dynamically reducing resistance of a turn-on resistor in a gate driver for the IGBT. However, the short-circuit fault current at a steady state is still not reduced, resulting in the considerable thermal loss during the fault.

As another method to protect against short circuit conditions, U.S. Pat. No. 6,104,149 describes using a shunt resistor in series with the emitter of the IGBT to reduce the overall short-circuit fault current. However, the power loss of the IGBT cannot be significantly reduced through the described method. Instead, adding such a series-connected shunt resistor will increase the power loss, resulting in additional junction temperature increase of the IGBT die if the shunt resistor is embedded into the IGBT die. Moreover, the actual gate voltage cannot be accurately controlled in the described method.

For these and other reasons, there is a need for embodiments of the present invention.

BRIEF DESCRIPTION

In accordance with an embodiment, an assembly including an insulated gate bipolar transistor (IGBT), a gate driver and a short-circuit protection circuit is provided. The gate driver is adapted to supply voltage to a gate terminal of the IGBT. The short-circuit protection circuit includes an IGBT short-circuit detector for determining whether the IGBT is short-circuited, and a supply voltage regulator for regulating the supply voltage in response to the IGBT short-circuit detector determining that the IGBT is short-circuited.

In accordance with another embodiment disclosed herein, a method for protecting an insulated gate bipolar transistor (IGBT) against short circuit conditions is provided. The method includes determining whether the IGBT is short-circuited; and reducing a supply voltage applied to a gate terminal of the IGBT in response to a determination that the IGBT is short-circuited.

In accordance with yet another embodiment disclosed herein, a method for protecting at least two series connected insulated gate bipolar transistors (IGBTs) against short circuit conditions is provided. The method includes determining whether any one of the IGBTs is short-circuited; and reducing supply voltages applied to gate terminals of all the IGBTs in response to a determination that any one of the IGBTs is short-circuited.

DETAILED DESCRIPTION

Embodiments of the invention include an assembly including an insulated gate bipolar transistor (IGBT) and a short-circuit protection circuit designed to protect the IGBT against short-circuit failure. When a short circuit occurs, the short circuit level is proportional to a gate voltage applied on the IGBT. The smaller the gate voltage is, the smaller the short circuit current it will be. Since the gate voltage is directly supplied by a supply voltage (drive voltage), a reduction of the supply voltage can lead to a reduction of the gate voltage. In the proposed assembly, the short-circuit protection circuit is configured to reduce the short-circuit current by reducing the supply voltage and thereby reducing the gate voltage applied on the IGBT during a short-circuit fault transient. As such, the power loss of the IGBT during the short-circuit fault transient can be significantly reduced. Thus, both ruggedness and reliability of the IGBT can be guaranteed under the short-circuit condition, and the durability of the IGBT can be significantly enhanced.

Unless defined otherwise, technical and scientific terms used herein have the same meaning as is commonly understood by one of ordinary skill in the art to which this invention belongs. The terms “first”, “second”, and the like, as used herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. Also, the terms “a” and “an” do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced items. Moreover, the terms “coupled” and “connected” are not intended to distinguish between a direct or indirect coupling/connection between two components. Rather, such components may be directly or indirectly coupled/connected unless otherwise indicated.

All voltages used herein are measured with respect to a common reference point (or ground, as indicated by ground symbols inFIG. 9), which is coupled to the emitter of the IGBT.

Referring toFIG. 1, a typical IGBT has three terminals: gate (G), emitter (E) and collector (C), and is characterized by its collector current (IC) decreasing with the decrease of its gate-emitter voltage (VGE). The curve inFIG. 2shows how ICvaries with VGEand the collector-emitter voltage (VCE) in an exemplary IGBT. As illustrated, for each specific VGEvalue, ICincreases with VCEand then reaches a plateau. The level of the short-circuit current is determined by the gate-emitter voltage VGEof the IGBT. There is a decrease in overall ICwhen the value of VGEdecreases. Thus, the short-circuit current can be reduced by reducing a voltage applied at a gate terminal of the IGBT during a short-circuit fault transient.

Referring toFIG. 3, an assembly100having an IGBT and a short-circuit protection circuit is provided. A gate driver102is used to apply a supply voltage (VCC) to the gate terminal of the IGBT, which controls the IGBT to be turned on and turned off. The short-circuit protection circuit includes an IGBT short-circuit detector104for determining whether the IGBT is short circuited, and a VCCregulator106for regulating VCCin response to the detector104determining that the IGBT is short-circuited and then turning off the IGBT. Specifically, once a short-circuit fault is sensed, short-circuit protection is activated by driving the VCCregulator106to reduce the supply voltage VCCapplied to the IGBT and then turn off the IGBT.

In one embodiment, the IGBT short-circuit detector104comprises a circuit capable of determining whether a short circuit is occurring by comparing a detected voltage that is proportional to the collector-emitter voltage of the IGBT with a reference voltage. Once the detected voltage is above the reference voltage, a signal is given to indicate that the IGBT is short-circuited. The short-circuit detector104may include a reference voltage source for setting a reference voltage, a voltage-steadied circuit coupled at the collector terminal of the IGBT for outputting a detected voltage that is proportional to (but typically much lower than) the collector-emitter voltage of the IGBT, and a comparator for comparing the detected voltage and the reference voltage for supplying a comparison signal. In one example implementation, the short-circuit detector104may include at least one diode, at least one resistor, at least one capacitor and at least one comparator (e.g., operational amplifier) (not shown inFIG. 3).

The VCCregulator106may be a circuit capable of regulating VCCbased on a constant current provided by a power source, and in one example may be implemented by a circuit including at least one capacitor, at least one resistor, at least one controllable electronic switch, at least one transistor and at least one voltage reference generator (not shown inFIG. 3). In some implementations, the VCCregulator106includes a voltage divider which is capable of obtaining more than one voltage from a single power source. The voltage divider may include one or more resistors connected in series, with two terminal contacts and one or more middle contacts. As current flows through the one or more resistors, different voltages can be obtained between different contacts. Details about the voltage divider will be illustrated hereinafter in conjunction withFIG. 9. In some implementations, the VCCregulator106includes at least two voltage reference generators, which provide relatively higher and lower voltage reference values for generating relatively higher and lower supply voltages, respectively, details of which will be illustrated hereinafter in conjunction withFIG. 10.

In some circumstances when high levels of electrical power may flow through the IGBT that serves as a switch, two or more IGBTs may be connected in series as a single switch to reduce the voltage value on each of the series connected IGBTs and therefore, avoid damaging the IGBTs because of the high levels of electrical power. Referring toFIG. 4, in an exemplary assembly210, IGBT-1, IGBT-2. . . IGBT-n are connected in series, and each of the IGBTs is associated with a gate driver212and a short-circuit protection circuit, including an IGBT short-circuit detector214and a VCCregulator216, which are similar to these as described above.

A possible issue which may occur with multiple series-connected IGBTs during the short circuit fault is that the voltage imbalance among the IGBTs may result in different triggering times of short circuit protection among the IGBTs. In order to prevent any single IGBT exceeding its breakdown voltage due to the unsynchronized triggering times of the short-circuit protection, in one embodiment, the protection circuits are used to provide the short-circuit protection for all the series-connected IGBTs at the same time. In this embodiment, all the IGBTs are turned off at the same time during the short-circuit protection. An example of such an embodiment is illustrated inFIG. 5. Compared with the assembly210ofFIG. 4, an assembly220illustrated inFIG. 5further includes a distributor218, which is provided to achieve the synchronization of the turn-off gating voltages during the short-circuit fault. The distributor218receives the signals from the short-circuit detectors214and uses those to drive VCCregulators216for all the series-connected IGBTs to synchronously regulate VCCin response to any one of the IGBT short circuit detectors214determining that an IGBT is short circuited. That is to say, once a short circuit occurs to any one of the series-connected IGBTs, the distributor218activates the VCCregulators216for all the IGBTs to synchronously reduce their VCCvalues and enables all the IGBTs to be synchronously turned off. The distributor may comprise, for example, a central controller such as a complex programmable logic device (CPID), field-programmable gate array (FPGA), or a single chip microcomputer (SCM).

Reduction of the supply voltage VCCresults in reduction of the short circuit current. As to a circuit including one or more series-connected IGBTs, the allowable energy and thermal losses sustained by the IGBTs under short circuit fault is proportional to the product of the voltage stress and the value of short circuit current. Reduction of the short circuit current can reduce thermal stresses applied on the IGBTs and thereby improve the durability and reliability of the IGBTs. Moreover, the reduction of short-circuit current decreases the di/dt during the turn-off transients when the protection is activated, significantly reducing the voltage stress applied on the IGBTs during the turn-off transients. As used herein, di/dt is the instantaneous rate of change of current with respect to time at a point, for example, at the point t=0. Thus, the IGBTs can be operated within a safe operating area (SOA) with sufficient margins.

In some cases where the short circuit fault is detected by comparing a detected voltage proportional to the collector-emitter voltage of the IGBT with a reference voltage, turning off of the IGBT may falsely trigger short-circuit protection because the IGBT in a turnoff status also has a relatively high collector-emitter voltage. In order to prevent false triggering of short-circuit protection due to the turning off of the IGBT, an IGBT on/off status detector may be added to the assembly or circuit, as described above, to determine whether the IGBT is turned on. In such embodiments, only when the IGBT is turned on will the VCCregulator reduce the VCCin response to the signal output by the IGBT short-circuit detector.

For example, an IGBT on/off status detector may be added to an assembly similar to the one shown inFIG. 3to provide an assembly300as illustrated inFIG. 6. In the assembly300, a gate driver302is used to apply a supply voltage VCCto the gate terminal of the IGBT, an IGBT short-circuit detector304is used to determine whether a short-circuit fault occurs to the IGBT, an IGBT on/off status detector306is used to determine whether the IGBT is turned on, and a VCCregulator308is used to regulate VCCin response to the IGBT on/off status detector306determining that the IGBT is turned on and the IGBT short-circuit detector304determining that the IGBT is short-circuited.

The IGBT on/off status detector306may be a circuit capable of determining whether the IGBT is turned on by comparing a sensed voltage at a node coupled to the gate terminal of the IGBT with a reference voltage. Once the sensed voltage is below the reference voltage, a signal is given to indicate that the IGBT is turned on.

An assembly410having two or more IGBTs connected in series, and each of which is associated with a short-circuit protection circuit including an IGBT on/off status detector, is illustrated inFIG. 7. In the assembly410, IGBT-1, IGBT-2. . . IGBT-n are connected in series, and each of the IGBTs is associated with a gate driver412, and a short-circuit protection circuit including an IGBT short-circuit detector414, an IGBT on/off status detector416and a VCCregulator418, which are similar to these as described above.

In one embodiment, the protection circuits are used to provide the short-circuit protection for all the series-connected IGBTs ofFIG. 7at the same time. An example of such an embodiment is illustrated inFIG. 8. Compared with the assembly410ofFIG. 7, an assembly420ofFIG. 8further includes a distributor419, which is provided to achieve the synchronization of the turn-off gating voltages for all the IGBTs during the short-circuit fault. The distributor419is coupled to the IGBT short-circuit detectors414, IGBT on/off status detectors416, and VCCregulators418for all the IGBTs. The distributor419is able to drive the VCCregulators418for all the IGBTs to synchronously regulate VCCin response to a determination that any one of the IGBTs is turned on and short circuited. Once a short-circuit fault occurs to any IGBT in a turn-on status, the distributor419activates the VCCregulators418for all the IGBTs to synchronously regulate their VCCvalues. Therefore, it can be guaranteed that the short-circuit protection for all the series-connected IGBTs is triggered at the same time and the series-connected IGBTs are thus turned off at the same time during the short-circuit protection.

FIG. 9illustrates a detailed schematic view of an implementation of the assembly ofFIG. 8in accordance with one embodiment of the present disclosure. As shown inFIG. 9, an assembly500including a plurality of IGBT assemblies501is provided. Each of the IGBT assemblies501includes an IGBT, a gate driver503and a short-circuit protection circuit505. The plurality of IGBT assemblies are commonly coupled to a distributor507adapted to synchronize short-circuit protection and turn-off for the different IGBTs. For clear illustration and ease of explanation, only one IGBT assembly501is shown inFIG. 9, and the other IGBT assemblies similar to the one shown are omitted.

In the illustrated IGBT assembly501, the gate driver503is coupled to the gate terminal of the IGBT and applies a supply voltage to the gate terminal of the IGBT to turn on and off the IGBT. The gate driver503shown inFIG. 9includes a turn-on resistor509, a P-channel metal-oxide-semiconductor field-effect transistor (MOSFET)511, and an N-channel MOSFET513. During operation, when a gating on logic signal arrives, the P-channel MOSFET511conducts so that a supply voltage VCCdrives through the turn-on resistor509to drive the IGBT. The emitter terminal of the IGBT is coupled to a reference ground. The collector terminal of the IGBT is coupled with a short-circuit detector515, which detects whether the IGBT has been short circuited.

The short-circuit detector515includes a comparator517. A non-inverting input of the comparator517is connected to the collector terminal of the IGBT through, for example, four series connected diodes D1, D2, D3and D4, whereas an inverting input of the comparator517is connected to the reference ground through a reference voltage source519. A pull-up resister521is connected between the supply voltage and the non-inverting input of the comparator517. A capacitor523is connected between the non-inverting input end of the comparator517and the reference ground. An output of the comparator517is supplied to the distributor507. The comparator517compares a detected voltage VDand a reference voltage generated by the reference voltage source519. When the IGBT is not short-circuited, VDis about zero and is below the reference voltage, such that the comparator517outputs a negative signal indicating that the IGBT is not short-circuited. When the IGBT is short-circuited, the collector-to-emitter voltage of the IGBT increases to a level approaching the DC-link voltage. The diodes D1, D2, D3and D4are reverse biased. The capacitor523is charged and VDwill exponentially increase to VCC. When VDincreases to a comparative threshold value that equals to the reference voltage, the comparator517outputs a positive signal indicating that the IGBT is short-circuited.

An on/off status detector525is coupled to a node at a drain terminal of the N-channel MOSFET513(with a voltage of VX). The on/off status detector525includes a comparator527. A non-inverting input of the comparator527is connected to a resistor531, whereas an inverting input of the comparator527is connected to the reference ground through a reference voltage source529. The reference voltage source529generates a reference voltage for comparison. A capacitor533is connected between the non-inverting input of the comparator527and the reference ground. An output of the comparator527is supplied to the distributor507. When a gating-on voltage is applied, the P-channel MOSFET511conducts to turn on the IGBT. The supply voltage VCCis coupled to the gate end of the N-channel MOSFET513resulting in the turn-on of the N-channel MOSFET513. Therefore VXis about zero and is below the reference voltage generated by the reference voltage source529, such that the comparator527outputs a positive signal indicating the IGBT is turned on. When a gating-off voltage is applied and thereby the IGBT is turned off, VXsubstantially equals the supply voltage VCCand is above the reference voltage generated by the reference voltage source529, such that the comparator527outputs a negative signal indicating the IGBT is turned off.

The distributor507has an output coupled to a VCCregulator535. The VCCregulator535includes a DC current source537, a capacitor538, a controllable electronic switch539, a switch driver541, a voltage reference generator543, resistors R1, R2and R3, and a transistor545. The voltage reference generator543generates a voltage reference value VK. In the illustrated embodiment, the switch driver541is coupled to the output of distributor507, which receives a signal from the output of distributor507and drives the switch539that is in parallel with the resistor R1to switch between an on-status and an off-status. When the switch539is off, the supply voltage VCCcan be calculated as:
VCC=VK*(R1+R2+R3)/R3

When the switch539is on, the supply voltage VCCcan be calculated as:
VCC=VK*(R2+R3)/R3

Since the voltage reference value VKis kept unchanged, the change of the voltage divider ratio ((R1+R2+R3)/R3or (R2+R3)/R3) eventually changes the supply voltage VCCfor driving turn-on gate voltage for IGBT. By choosing appropriate values for R1, R2and/or R3, the regulated output supply voltage VCCcan be changed from the linear regulated power supply. Thus, the VCCregulator is able to regulate the supply voltage VCCbetween a relatively higher value and a relatively smaller value, and the VCCcan be reduced in response to a determination that a short circuit is occurring in a turned on IGBT. Since short-circuit current level of the IGBT is lower when the gate voltage VCCis reduced according to the IGBT characteristics, the short circuit current of the IGBT can be significantly reduced by reducing the VCC.

The other IGBT assemblies (not shown) are coupled to the distributor507in a similar way. Once comparators of both a short-circuit detector and an on/off status detector of any one of the IGBT assemblies output positive signals, indicating that the IGBT of that IGBT assembly is turned on and short-circuited, the distributor507drives VCCregulators of all the IGBT assemblies to synchronously reduce VCCto protect the IGBTs against a short-circuit failure.

As to the VCCregulator, there may be various implementations. For example, the VCCregulator535in the assembly500may be replaced with another VCCregulator565as shown inFIG. 10. The VCCregulator565includes a DC current source567, a capacitor568, a controllable electronic switch569, a switch driver571, and at least two voltage reference generators573and575, and a transistor577. The switch driver571drives the switch569to switch between the voltage reference generators573and575, which provide relatively higher and lower voltage reference values VK1and VK2, respectively. A higher voltage reference value generates a relatively higher supply voltage VCC. Therefore, by designing the voltage reference generators573and575to provide appropriate higher and lower voltage reference points, the supply voltage VCCcan be regulated between a higher value and a lower value, which are suitable for IGBT under a normal operation and a short circuit condition, respectively. Once a short circuit is sensed, the VCCregulator565can be operated to choose the lower VCC.

In another aspect, methods for operating the IGBT assemblies described above to protect an IGBT against short circuit conditions are also provided.FIG. 11illustrates a flowchart of a method710for operating the IGBT assembly100shown inFIG. 1to protect the IGBT against short circuit conditions in accordance with one embodiment of the present disclosure. In the method710, an assembly including an IGBT is operated in step711. A parameter of the IGBT is sensed to determine whether the IGBT is short-circuited in step712. A supply voltage applied to a gate terminal of the IGBT is reduced in step713if it is found that the IGBT is short-circuited based on the result of step712.

FIG. 12illustrates a flowchart of a method720for operating the IGBT assembly300shown inFIG. 6to protect the IGBT against short circuit conditions in accordance with one embodiment of the present disclosure. In the method720, an assembly including an IGBT is operated in step721. A parameter of the IGBT is sensed to determine whether the IGBT is turned on in step722. It is determined whether the IGBT is short-circuited in step723. A supply voltage applied to a gate terminal of the IGBT is reduced in step724if it is found that the IGBT is turned on and short-circuited based on the results of steps722and723.

FIG. 13illustrates a flowchart of a method730for operating the assembly220shown inFIG. 5to protect the IGBTs of the assembly against short circuit conditions in accordance with one embodiment of the present disclosure. In the method730, the assembly including a plurality of series-connected IGBTs is operated in step731. Each of the IGBTs is evaluated to determine whether any one of the IGBTs is short-circuited in step732. Supply voltages applied to all the IGBT are synchronously reduced in step733if it is found that any one of the IGBTs is short-circuited based on the result of step732.

FIG. 14illustrates a flowchart of a method740for operating the assembly420shown inFIG. 8to protect the IGBTs of the assembly against short circuit conditions in accordance with one embodiment of the present disclosure. In the method740, the assembly including a plurality of series-connected IGBTs is operated in step741. Each of the IGBTs is evaluated to determine whether any one of the IGBTs is turned on and short-circuited in step742. Supply voltages applied to all the IGBT are synchronously reduced in step743if it is found that any one of the IGBTs is turned on and short-circuited based on the result of step742.

To demonstrate the effectiveness of the proposed short circuit protection scheme, experiments were conducted to compare the short circuit behavior of an IGBT without a short circuit protection circuit and the short circuit behavior of an IGBT with a short circuit protection circuit as described above.

FIG. 15shows the short circuit behavior of an Infineon 1700V/450 A IGBT without short circuit protection. It can be seen that the voltage across the IGBT (VCE) is approximately 900V and while the short circuit current is around 1750 A during the short circuit fault. After about 7.3 us, the IGBT is turned off. The voltage across the IGBT is up to about 1545V due to the large di/dt during turn-off transients.FIG. 16shows the short circuit behavior of an Infineon 1700V/450 A IGBT with a proposed short circuit protection circuit. Due to the reduction of the gate voltage, the short circuit current is decreased to 1050 A at the steady state, indicating that there would be about a 40% reduction of thermal losses for the IGBT during such a short-circuit fault condition. After the gating on voltage lasts for around 7 us, the IGBT is turned off. Since the short circuit current is reduced to a lower level, the voltage spike applied across the IGBT is reduced to 1428V. Therefore, the IGBT is protected within its SOA with enough margins.

It is to be understood that not necessarily all such objects or advantages described above may be achieved in accordance with any particular embodiment. Thus, for example, those skilled in the art will recognize that the systems and techniques described herein may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein.