Analyzing IGBT Failures and Protection Methods: A Guide for Beginners

IGBTs, or Insulated Gate Bipolar Transistors, are a type of bipolar MOS composite power device. These components combine the high switching frequency and large input impedance of power MOSFETs with the high current capabilities and low saturation voltage of GTRs (Gate Turn-Off Thyristors). This integration makes IGBTs highly efficient for switching applications. However, even with robust designs, IGBTs can fail due to internal and external factors. Here, we’ll explore the common causes of IGBT failures and methods for protecting them.

Common Causes of IGBT Failures

1. Overheating Overheating is one of the primary causes of IGBT failure. Instantaneous overheating due to excessive current or sustained overheating due to poor heat dissipation can damage the IGBT’s collector. When a device experiences continuous short-circuiting, the resulting high current generates significant power dissipation, leading to a rapid temperature rise. If the chip temperature exceeds the intrinsic temperature of silicon, the device loses its blocking capability, causing the gate control to fail and leading to IGBT failure. Typically, the maximum allowable operating temperature is around 125°C.
2. Exceeding the Safe Operating Area The latch-up effect can cause IGBT failure when the device operates outside its safe operating area. Latch-up occurs in both static and dynamic forms. Static latch-up happens when the parasitic thyristor within the IGBT turns on due to increased collector current, causing a self-sustaining high current flow that the gate cannot control. Dynamic latch-up occurs during rapid turn-off when the current drops too quickly, leading to high dvCE/dt and triggering the parasitic thyristor. Both forms result in excessive power dissipation and device failure.
3. Transient Overcurrent IGBTs often encounter large transient overcurrents during operation, such as from short circuits, shoot-through faults, reverse recovery current of freewheeling diodes, discharge currents from snubber capacitors, and noise-induced spike currents. Although these overcurrents are brief, they can increase the stress on the IGBT and potentially lead to failure if not properly managed.
4. Overvoltage Overvoltage can cause breakdown between the collector and emitter or between the gate and emitter. This results in catastrophic failure of the IGBT.

Methods for Protecting IGBTs

To ensure the longevity and reliability of IGBTs, it is crucial to implement effective protection methods. Here are some strategies:

1. Immediate Shutdown of Drive Signals In cases of excessive load or output short circuits in an inverter, current sensors on the DC bus detect the current. When the current exceeds a preset threshold, the protection system immediately shuts down all drive signals to the bridge arms. This method is straightforward but requires specially designed snubber and clamping circuits to handle short-circuit conditions. The downside is the significant stress imposed on the IGBT during shutdown, particularly when dealing with large inductive currents, which can exacerbate the latch-up effect.
2. Gradual Reduction of Gate Voltage Before Shutdown The short-circuit current of an IGBT is closely related to the gate voltage (VGS). Higher gate voltages result in larger short-circuit currents. In the event of a short circuit or transient overcurrent, gradually reducing the VGS can decrease the short-circuit current and extend the allowable overcurrent duration. This method also reduces di/dt during shutdown, limiting the magnitude of overcurrent.
3. Use of Snubber and Clamping Circuits Implementing RC snubber circuits, RCD circuits, and freewheeling diodes between the collector and emitter can mitigate overvoltage during switching. These components help absorb and dissipate transient energy, protecting the IGBT from voltage spikes.
4. Gate Protection To prevent gate overvoltage, use appropriate series resistances and Zener diodes. Properly selecting these components based on the circuit’s capacity ensures that the gate voltage remains within safe limits, thereby protecting the gate from damage.

Conclusion

Understanding the common causes of IGBT failures and the methods to protect them is crucial for anyone working with these components. Overheating, exceeding the safe operating area, transient overcurrent, and overvoltage are the main culprits behind IGBT failures. By implementing immediate drive signal shutdowns, gradual gate voltage reduction, snubber and clamping circuits, and gate protection, you can significantly enhance the reliability and lifespan of IGBTs. This knowledge is essential for designing robust systems and ensuring their safe and stable operation in various applications.