An Insulated Gate Bipolar Transistor (IGBT) is a device composed of a MOSFET and a bipolar transistor. Its input electrode is a MOSFET, and its output electrode is a PNP transistor; therefore, it can be considered a Darlington transistor with a MOSFET input. It combines the advantages of both devices, possessing the simplicity and speed of MOSFET driving and the large capacitance of bipolar transistors. Consequently, it has found increasingly widespread application in modern power electronics technology. In medium-to-high power switching power supplies, IGBTs have gradually replaced thyristors or GTOs due to their simple control and drive circuits, high operating frequency, and large capacitance. However, in switching power supplies, the high frequency, high voltage, and high current conditions make them susceptible to damage. Furthermore, as the front-end of the system, the power supply is subjected to greater stress due to grid fluctuations, lightning strikes, and other factors. Therefore, the reliability of the IGBT directly affects the reliability of the power supply. Thus, in addition to derating considerations, IGBT protection design is a crucial aspect of power supply design when selecting IGBTs. 1. IGBT Working Principle If a positive driving voltage is applied between the gate and emitter of the IGBT, the MOSFET turns on, creating a low-resistance state between the collector and base of the PNP transistor, thus turning the transistor on. If the voltage between the gate and emitter of the IGBT is 0V, the MOSFET turns off, cutting off the base current supply to the PNP transistor, causing the transistor to turn off. Therefore, the safety and reliability of the IGBT are 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 junction of the IGBT; —The junction temperature of the IGBT. If the voltage between the gate and emitter of the IGBT, i.e., the drive voltage, is too low, the IGBT cannot operate stably and normally. If it is too high, exceeding the gate-emitter withstand voltage, the IGBT may be permanently damaged. Similarly, if the allowable voltage applied to the collector and emitter of the IGBT exceeds the collector-emitter withstand voltage, the current flowing through the collector-emitter exceeds the maximum allowable current, or the junction temperature exceeds its allowable value, the IGBT may be permanently damaged. 2. Protection Measures When designing the circuit, appropriate protection measures should be taken in response to factors affecting the reliability of the IGBT. 2.1 Protection of the IGBT Gate 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. Furthermore, if the gate and emitter of an IGBT are open-circuited, and a voltage is applied between its collector and emitter, the gate potential will rise due to the parasitic capacitance between the gate and the collector and emitter, causing current to flow through the collector-emitter junction. If the collector and emitter are under high voltage, the IGBT may overheat or even be damaged. If the gate circuit is broken during transport or vibration, and voltage is applied to the main circuit unnoticed, the IGBT may be damaged. To prevent this, a resistor of several tens of kΩ should be connected in parallel between the gate and emitter of the IGBT, and this resistor should be as close to the gate and emitter as possible. Because IGBTs are a composite of power MOSFETs and PNP bipolar transistors, especially since their gates are MOS structures, in addition to the protections mentioned above, like other MOS structure devices, IGBTs are also very sensitive to static voltage. Therefore, the following precautions must be taken when assembling and soldering IGBTs: —Before touching the IGBT, discharge any static electricity from your body first, and avoid touching the drive terminals of the module as much as possible. If contact is necessary, ensure that all static electricity on your body has been discharged. —During soldering, to prevent static electricity from potentially damaging the IGBT, the soldering machine must be reliably grounded. 2.2 Overvoltage Protection Between Collector and Emitter Overvoltage mainly occurs in two situations: one is an excessively high DC voltage applied between the IGBT collector and emitter, and the other is an excessively high surge voltage on the collector and emitter. 2.2.1 DC Overvoltage DC overvoltage is caused by an abnormality in the input AC power supply or the input stage preceding the IGBT. The solution is to implement derating design when selecting IGBTs; additionally, the IGBT input can be disconnected upon detection of this overvoltage to ensure IGBT safety. 2.2.2 Surge Voltage Protection Due to the presence of distributed inductance in the circuit, coupled with the high switching speed of the IGBT, a large surge voltage Ldi/dt will be generated when the IGBT is turned off and when the parallel reverse recovery diode recovers in reverse, threatening the IGBT's safety. If VCESP exceeds the IGBT's collector-emitter withstand voltage VCES, it may damage the IGBT. The main solutions are: —Consider design margin when selecting IGBTs; —Adjust Rg of the IGBT drive circuit during circuit design to minimize di/dt; —Mount the electrolytic capacitor as close to the IGBT as possible to reduce distributed inductance; —Add a buffer protection circuit as needed to bypass high-frequency surge voltages. Since the buffer protection circuit plays a crucial role in the safe operation of the IGBT, the types and characteristics of buffer protection circuits will be introduced here. —C-type snubber circuit, using a thin-film capacitor and mounted close to the IGBT, is characterized by its simple circuitry. However, its disadvantage is that the distributed inductance and snubber capacitor form an LC resonant circuit, which is prone to voltage oscillations, and the collector current is relatively large when the IGBT is turned on. —RC-type snubber circuit is suitable for chopper circuits, but when using large-capacity IGBTs, the snubber resistor value must be increased; otherwise, the excessive collector current during turn-on will limit the IGBT's function. —RCD-type snubber circuit, compared to the RC snubber circuit, adds a snubber diode, thereby increasing the snubber resistor and avoiding the problem of IGBT function being blocked during turn-on. The loss generated by the snubber resistor in this snubber circuit is P = L1²f + Ud²f, where: L is the distributed inductance in the main circuit; I is the collector current when the IGBT is turned off; f is the IGBT switching frequency; C is the snubber capacitor; and Ud is the DC voltage value. —Discharge-blocking snubber circuit, compared to the RCD snubber circuit, generates less loss and is suitable for high-frequency switching. The loss generated on the buffer resistor in this buffer circuit is: P = 1/2LI²f + 1/2CUf. Select an appropriate buffer protection circuit based on the actual situation to suppress the surge voltage during shutdown. During assembly, minimize the distributed inductance of the main circuit and buffer circuit; shorter and thicker wiring is better. 2.3 Collector Current Overcurrent Protection There are three main methods for overcurrent protection of IGBTs. 2.3.1 Overcurrent Detection Protection Using Resistors or Current Transformers A resistor or current transformer can be connected in series with the IGBT to detect the current flowing through the IGBT collector. When an overcurrent occurs, the control actuator disconnects the IGBT input, thus protecting the IGBT. 2.3.2 Overcurrent protection via IGBT VCE(sat) detection: Since VCE(sat) = IcRCE(sat), as Ic increases, VCE(sat) also increases. If the gate voltage is high and VCE is high, an overcurrent condition occurs. The AND gate outputs a high level, sending an overcurrent signal to control the actuator to disconnect the IGBT input, protecting the IGBT. 2.3.3 Load current detection protection: If the load is short-circuited or the load current increases, the collector current of the preceding IGBT may increase, leading to IGBT damage. An abnormality detected at the load (or the subsequent stage of the IGBT circuit) controls the actuator to disconnect the IGBT input, achieving protection. 2.4 Overheat protection: Generally, the current flowing through the IGBT is large, and the switching frequency is high, resulting in significant device losses. If heat cannot be dissipated in time, causing the junction temperature Tj to exceed Tjmax, the IGBT may be damaged. IGBT power consumption includes steady-state power consumption and dynamic power consumption, with dynamic power consumption further divided into turn-on power consumption and turn-off power consumption. In thermal design, it's crucial to ensure adequate heat dissipation not only during normal operation but also that the IGBT junction temperature does not exceed Tjmax during short-term overloads. Of course, limitations imposed by equipment size and weight, as well as cost-effectiveness considerations, mean the cooling system cannot be expanded indefinitely. A temperature relay can be installed near the IGBT to monitor its operating temperature. The control actuator can then cut off the IGBT's input in case of an anomaly, protecting its safety. In addition to the above, the following points should be noted when mounting IGBTs onto a heatsink: —Since thermal resistance varies depending on the IGBT's mounting position, if only one IGBT is mounted on the heatsink, it should be installed in the center to minimize thermal resistance; when mounting several IGBTs, sufficient space should be provided according to the heat generation of each IGBT; —When using a textured heatsink, the wider direction of the IGBT should follow the heatsink's texture to reduce heatsink deformation; —The surface finish of the heatsink mounting surface should be ≤10μm. Uneven heatsink surfaces will significantly increase the contact thermal resistance between the heatsink and the device, and may even generate significant tension on the substrate between the IGBT die and the case, damaging the IGBT's insulation layer; —To reduce contact thermal resistance, it is best to apply thermal grease between the heatsink and the IGBT module. 3 Conclusion When using IGBTs, appropriate protective measures should be taken according to the actual situation. As long as effective protection measures are taken in terms of overvoltage, overcurrent, and overheating, good results can be achieved in practical applications, ensuring the safe and reliable operation of IGBTs.