What causes the failure of IGBT and protection methods
How to use IGBT
IGBT insulated gate bipolar transistor is a typical bipolar MOS composite power device. It combines the power MOSFET process technology to integrate the power MOSFET and the power tube GTR in the same chip. The device has the characteristics of high switching frequency, large input impedance, good thermal stability, simple drive circuit, low saturation voltage and large current. It is widely used as a power device in industrial control, power electronic systems and other fields (for example: servo motor Speed regulation, variable frequency power supply). In order to make the system we designed can work more safely and reliably, the protection of IGBT is particularly important.
At present, in the process of using and designing IGBTs, the extensive design mode is basically adopted-the required margin is large, the system is huge, but it is still unable to resist external interference and various failure problems caused by its own system . Shunlei Electronics uses its production and design advantages in the semiconductor field, combined with the characteristics of transient suppression diodes, and breaks through the design bottleneck by integrating the internal and external systems on the basis of studying the failure mechanism of IGBTs. This article will break through the traditional protection methods and discuss the solution of IGBT system circuit protection design.
IGBT failure occasions: from within the system, such as the distributed stray inductance of the power system, motor induced electromotive force, and load mutations will cause overvoltage and overcurrent; from outside the system, such as grid fluctuations, power line induction, surges, etc. In the final analysis, IGBT failure is mainly caused by the overvoltage/overcurrent of the collector and emitter and the overvoltage/overcurrent of the gate.
IGBT failure mechanism: IGBT is short-circuited due to the above reasons, and a large transient current will be generated-the current change rate di/dt is too large when it is turned off. The existence of leakage inductance and lead inductance will cause overvoltage of the IGBT collector, which will generate a holding effect inside the device, which will make the IGBT lock failure. At the same time, a higher overvoltage will cause the IGBT to break down. The IGBT enters the amplifying area due to the above reasons, which increases the switching loss of the tube.
IGBT traditional failure prevention mechanism: minimize the wiring inductance and capacitance of the main circuit to reduce the turn-off overvoltage; place a freewheeling diode between the collector and the emitter, and connect the RC circuit and RCD circuit, etc. ; At the gate, select the series impedance reasonably according to the circuit capacity, and connect the Zener diode in parallel to prevent gate overvoltage.
Causes of IGBT failure
1. Overheating can easily damage the collector. The instantaneous overheating caused by excessive current and the main reason is that continuous overheating caused by poor heat dissipation will damage the IGBT. If the device continues to be short-circuited, the power consumption generated by the large current will cause a temperature rise. Due to the small heat capacity of the chip, its temperature rises rapidly. If the chip temperature exceeds the silicon intrinsic temperature, the device will lose its blocking ability and the gate control cannot protect it. This causes the IGBT to fail. In practical applications, the maximum allowable working temperature is generally around 125°C.
2. Exceeding the shut-off safe working area will cause damage due to the holding effect. The holding effect is divided into static holding effect and dynamic holding effect. The IGBT has a PNPN 4-layer structure. Because there is a parasitic thyristor in the body, when the collector current increases to a certain level, the parasitic thyristor can be turned on, and the gate loses control and forms a self-locking phenomenon. This is the so-called static engine. Live effect. After the holding effect of the IGBT, the collector current increases, resulting in excessive power consumption, which leads to device failure. The dynamic holding effect is mainly because the current drops too fast when the device is turned off at a high speed, and the dvCE/dt is very large, which causes a large displacement current and can also cause the parasitic thyristor to self-lock.
3. In addition to short-circuit, through-through and other faults, transient overcurrent IGBTs withstand large-scale overcurrent during operation also include the reverse recovery current of the freewheeling diode, the discharge current of the snubber capacitor and the peak current caused by noise interference. Although this transient overcurrent lasts for a short time, if no measures are taken, it will increase the burden on the IGBT and may also cause the IGBT to fail.
4. Overvoltage causes breakdown of the collector and emitter or breakdown of the gate and emitter.
IGBT protection method
When an overcurrent condition occurs, the IGBT must be maintained in the short-circuit safe operating area. The time that the IGBT withstands a short circuit is closely related to the power supply voltage, the gate drive voltage and the junction temperature. In order to prevent damage to the IGBT due to short-circuit faults, there must be a complete detection and protection link. General detection methods are divided into current sensors and IGBT under-saturation protection.
1. Turn off the drive signal immediately
When the load of the inverter power supply is too large or the output is short-circuited, the current sensor on the input DC bus of the inverter bridge is used for detection. When the detected current value exceeds the set threshold, the protection action blocks the drive signals of all bridge arms. This kind of protection method is the most straightforward, but the absorption circuit and clamp circuit must be specially designed to make it suitable for short-circuit conditions. The disadvantage of this method is that it will cause the IGBT to withstand excessive stress when it is turned off, especially when turning off the inductive ultra-large current, you must pay attention to the lifting effect.
2. First reduce the gate voltage and then turn off the drive signal
The short-circuit current of the IGBT is closely related to the gate voltage. The higher the gate voltage, the greater the current during short-circuit. In the case of short circuit or transient overcurrent, if vGS can be reduced step by step or ramped down instantaneously, the short circuit current will be reduced, and the allowable overcurrent time will be longer. When the IGBT is turned off, di/dt is also reduced. Limit the amplitude of overcurrent.