| IGBT protection Chen Yihuai, Hu Weihua, Wang Yan (Xi'an Electronic Engineering Research Institute, Xi'an, Shaanxi 710100) Abstract: Through the analysis of the damage mechanism of IGBT, according to the cause of its damage, corresponding measures are taken to protect it in order to ensure its safe and reliable operation. Keywords: IGBT; MOSFET; drive; overvoltage; surge; buffer; overcurrent; overheating; protection 0 Introduction The insulated gate bipolar transistor IGBT is a device composed of MOSFET and bipolar transistor. Its input is MOSFET and its output is PNP transistor. Therefore, it can be regarded as a Darlington transistor with MOS input. It combines the advantages of these two devices, with the advantages of simple and fast driving of MOSFET devices and the advantages of large capacity of bipolar devices. Therefore, it has been more and more widely used in modern power electronic technology. In medium and large power switching power supply devices, IGBT has gradually replaced thyristor or GTO due to its simple control drive circuit, high operating frequency and large capacity. However, in the switching power supply device, it is easy to be damaged because it works under the conditions of high frequency, high voltage and high current. In addition, the power supply as the front stage of the system is subject to greater stress due to the influence of power grid fluctuations, lightning strikes and other reasons. Therefore, the reliability of IGBT is directly related to the reliability of the power supply. Therefore, in addition to considering derating when selecting IGBT, the protection design of IGBT is also a key consideration in the design of power supply. 1 Working principle of IGBT The equivalent circuit of IGBT is shown in Figure 1. As shown in Figure 1, if a positive driving voltage is applied between the gate and emitter of the IGBT, the MOSFET is turned on, so that the collector and base of the PNP transistor are in a low resistance state, making the transistor turned on; if the voltage between the gate and emitter of the IGBT is 0V, the MOSFET is turned off, cutting off the supply of base current of the PNP transistor, making the transistor turned off. 
Figure 1 IGBT equivalent circuit It can be seen that the safety and reliability of IGBT is mainly determined by the following factors: ——The voltage between the gate and emitter of the IGBT; ——The voltage between the collector and emitter of the IGBT; ——The current flowing through the collector-emitter of the IGBT; ——IGBT junction temperature. If the voltage between the gate and emitter of the IGBT, that is, the driving voltage, is too low, the IGBT cannot work stably and normally. If it is too high and exceeds the withstand voltage between the gate and the emitter, the IGBT may be permanently damaged. Similarly, if the allowable voltage applied to the collector and emitter of the IGBT exceeds the withstand voltage between the collector and the emitter, the current flowing through the collector and the emitter of the IGBT exceeds the maximum allowable current of the collector and the emitter, and the junction temperature of the IGBT exceeds the allowable value of its junction temperature, the IGBT may be permanently damaged. 2. Protection measures When designing the circuit, appropriate protection measures should be taken according to the factors that affect the reliability of IGBT. 2.1 IGBT gate protection The guaranteed value of the gate-emitter drive voltage VGE of the IGBT is ±20V. If a voltage exceeding the guaranteed value is applied between its gate and emitter, the IGBT may be damaged. Therefore, a gate voltage limiting circuit should be set in the IGBT drive circuit. In addition, if the gate and emitter of the IGBT are open, and a voltage is applied between its collector and emitter, as the collector potential changes, the gate potential increases due to the presence of parasitic capacitance between the gate and the collector and emitter, and current flows through the collector-emitter. At this time, if the collector and emitter are in a high-voltage state, the IGBT may heat up or even be damaged. If the gate circuit of the equipment is disconnected during transportation or vibration, and a voltage is applied to the main circuit without being noticed, the IGBT may be damaged. To prevent this from happening, a resistor of several tens of kΩ should be connected between the gate and emitter of the IGBT, and this resistor should be as close to the gate and emitter as possible. As shown in Figure 2. 
Figure 2 Gate protection circuit Since IGBT is a composite of power MOSFET and PNP bipolar transistor, especially its gate is MOS structure, in addition to the above protection, like other MOS structure devices, IGBT is also very sensitive to static voltage, so the following matters must be paid attention to when assembling and welding IGBT: ——Before touching the IGBT with your hands, you should discharge the static electricity on your body before operating it, and try not to touch the drive terminal part of the module. If you must touch it, make sure that all the static electricity on your body has been discharged; ——During welding operations, in order to prevent static electricity from damaging the IGBT, the welding machine must be reliably grounded. 2.2 Overvoltage protection between collector and emitter There are two main situations in which overvoltage is generated. One is that the DC voltage applied to the collector-emitter of the IGBT is too high, and the other is that the surge voltage on the collector-emitter is too high. 2.2.1 DC overvoltage The reason for DC overvoltage is due to abnormal input AC power or the previous input of IGBT. The solution is to perform derating design when selecting IGBT; in addition, the input of IGBT can be disconnected when this overvoltage is detected to ensure the safety of IGBT. 2.2.2 Surge voltage protection Due to the existence of distributed inductance in the circuit and the high switching speed of the IGBT, when the IGBT is turned off and the reverse recovery diode connected in parallel with it recovers in reverse, a large surge voltage Ld i / d t will be generated , threatening the safety of the IGBT. Usually the surge voltage waveform of IGBT is shown in Figure 3. 
Figure 3 IGBT surge voltage waveform In the figure: v CE is the voltage waveform between the collector and emitter of the IGBT; i c is the collector current of IGBT; U d is the DC voltage of the input IGBT; V CESP = U d＋L d i c /d t , is the peak value of surge voltage. If V CESP exceeds the IGBT's collector-emitter withstand voltage V CES , the IGBT may be damaged. The main solutions are: ——Consider the design margin when selecting IGBT; ——Adjust the R g of the IGBT drive circuit during circuit design to make di / dt as small as possible; ——Install the electrolytic capacitor as close to the IGBT as possible to reduce the distributed inductance; ——Install a buffer protection circuit as needed to bypass high-frequency surge voltage. Since the buffer protection circuit plays an important role in the safe operation of IGBT, the types and characteristics of the buffer protection circuit are introduced here. —— C snubber circuit As shown in Figure 4(a), a thin film capacitor is used and installed close to the IGBT. Its characteristic is that the circuit is simple. Its disadvantage is that the LC resonant circuit composed of distributed inductance and snubber capacitor is prone to voltage oscillation, and the collector current is large when the IGBT is turned on. —— RC snubber circuit As shown in Figure 4(b), it is suitable for chopper circuits. However, when using large-capacity IGBTs, the snubber resistance value must be increased. Otherwise, the collector current will be too large when turned on, which will limit the IGBT function to a certain extent. —— RC D snubber circuit As shown in Figure 4(c), compared with the RC snubber circuit, its characteristic is that the snubber diode is added to increase the snubber resistance, avoiding the problem of IGBT function being blocked when turned on. 
(a) C snubber circuit (b) RC snubber circuit 
(c) RC D snubber circuit (d) Discharge prevention snubber circuit Figure 4 Buffer protection circuit The loss caused by the snubber resistor in the snubber circuit is P =  LI 2 f＋ CU d 2 f Where: L is the distributed inductance in the main circuit; I is the collector current when the IGBT is turned off; f is the switching frequency of the IGBT; C is the buffer capacitor; Ud is the DC voltage value. ——The discharge prevention snubber circuit is shown in Figure 4(d). Compared with the RC D snubber circuit, it has the characteristics of small loss and is suitable for high-frequency switching. The loss generated on the snubber resistor in this snubber circuit is P =  LI 2 f Select appropriate buffer protection circuit according to actual situation to suppress shutdown surge voltage. When assembling, try to reduce the distributed inductance of main circuit and buffer circuit as much as possible. The shorter and thicker the wiring, the better. 2.3 Collector current overcurrent protection There are three main methods for overcurrent protection of IGBT. 2.3.1 Using resistors or current transformers to detect overcurrent for protection As shown in Figure 5 (a) and Figure 5 (b), a resistor or current transformer can be connected in series with the IGBT to detect the current flowing through the collector of the IGBT. When an overcurrent occurs, the control actuator disconnects the input of the IGBT to protect the IGBT. 2.3.2 IGBT V CE(sat) detects overcurrent for protection As shown in Figure 5 (c), since V CE(sat) = I c R CE(sat) , when I c increases, V CE(sat) also increases. If the gate voltage is high and V CE is high, an overcurrent condition occurs. At this time, the AND gate outputs a high level and outputs an overcurrent signal, controlling the actuator to disconnect the input of the IGBT and protect the IGBT. 2.3.3 Detecting load current for protection This method is basically the same as the detection method in Figure 5 (a), but Figure 5 (a) is a direct method, while this is an indirect method, as shown in Figure 5 (d). If the load is short-circuited or the load current increases, the collector current of the previous IGBT may also increase, causing damage to the IGBT. After the abnormality is detected at the load (or the next circuit of the IGBT), the control actuator cuts off the input of the IGBT to achieve the purpose of protection. 
(a) Detecting overcurrent using a resistor (b) Detecting overcurrent using a current transformer 
(c) Overcurrent detection by V CE(sat) (d) Overcurrent detection by load current Figure 5 Collector overcurrent protection circuit 2.4 Overheat protection Generally, the current flowing through the IGBT is large and the switching frequency is high, so the device loss is also relatively large. If the heat cannot be dissipated in time, causing the junction temperature Tj of the device to exceed Tjmax , the IGBT may be damaged. The power consumption of IGBT includes steady-state power consumption and dynamic power consumption, and its dynamic power consumption includes turn-on power consumption and turn-off power consumption. When performing thermal design, it is necessary not only to ensure that it can fully dissipate heat during normal operation, but also to ensure that the junction temperature of the IGBT does not exceed T jmax when a short-term overload occurs . Of course, due to the limitations of the size and weight of the equipment and considerations of cost performance, the cooling system cannot be expanded indefinitely. A temperature relay can be installed near the IGBT to detect the operating temperature of the IGBT. The control actuator cuts off the input of the IGBT in the event of an abnormality to protect its safety. In addition, the following matters should be noted when installing and fixing the IGBT on the heat sink: ——Since the thermal resistance varies with the installation position of the IGBT, if only one IGBT is installed on the heat sink, it should be installed in the middle to minimize the thermal resistance; when several IGBTs are to be installed, appropriate space should be left according to the heat generation of each IGBT; ——When using a heat sink with texture, the wider direction of the IGBT should be along the texture of the heat sink to reduce the deformation of the heat sink; ——The surface finish of the heat sink should be ≤10μm. If the surface of the heat sink is uneven, the contact thermal resistance between the heat sink and the device will be greatly increased, and even a large tension will be generated on the substrate between the IGBT die and the tube shell, damaging the insulation layer of the IGBT. ——In order to reduce the contact thermal resistance, it is best to Apply thermal grease between IGBT modules. 3 Conclusion When applying IGBT, appropriate protection measures should be taken according to the actual situation. As long as effective protection measures are taken in overvoltage, overcurrent, overheating and other aspects, good results can be achieved in actual applications to ensure the safe and reliable operation of IGBT. | 
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