IGBT (insulated gate bipolar transistor) is a power semiconductor device widely used in the field of power electronics. During the switching process of IGBT, the turn-off spike phenomenon of the upper and lower bridge arms is an important factor affecting the performance and reliability of the device. The reasons for the different turn-off spikes of the upper and lower bridges are relatively complex, involving circuit design, device characteristics, drive parameters and other aspects. The following is a detailed analysis of this issue.

1. Circuit design differences
When IGBT modules are used in parallel, the turn-off spikes of the upper and lower bridge arms may be inconsistent due to differences in circuit design. For example, the parasitic inductance and capacitance parameters of the upper and lower bridge arms are inconsistent, or the design of the drive circuit is different, which may cause the difference in turn-off spikes. The asymmetry in the circuit design will lead to uneven current and voltage distribution, resulting in different spike voltages.

2. IGBT device characteristics
The characteristic differences of the IGBT device itself will also affect the turn-off spike. For example, different batches of IGBT devices may have parameter fluctuations, such as differences in threshold voltage, carrier lifetime and other parameters. These differences will be reflected in the switching process, resulting in inconsistent turn-off spikes of the upper and lower bridge arms.

3. Drive parameter settings
The setting of drive parameters has a significant impact on the switching characteristics of the IGBT. Different settings of parameters such as gate resistance and gate capacitance will affect the switching speed of the IGBT and the current change rate during the turn-off process. The increase in gate resistance will slow down the turn-off speed and increase the turn-off loss, but reduce the voltage spike. The selection of gate capacitance affects the device turn-off process, and a suitable value needs to be selected to avoid LC oscillation problems.

4. System loop inductance
The size of the system loop inductance directly affects the voltage spike at turn-off. A larger loop inductance will generate a higher back electromotive force when the IGBT is turned off, thus forming a spike. If the loop inductance of the upper and lower bridge arms is inconsistent, the size of the turn-off spike will be different.

5. Temperature influence
Temperature has an important influence on the switching characteristics of the IGBT. At high temperatures, the carrier mobility of the IGBT will decrease, affecting the switching speed and, in turn, the size of the turn-off spike. If the operating temperature of the upper and lower bridge arms is inconsistent, it may cause a difference in the turn-off spike.

6. Device aging and stress
Long-term operation and repeated switching will cause IGBT devices to age and their characteristics will change. Aged devices may exhibit different switching characteristics from new devices, which may also cause inconsistent turn-off spikes between the upper and lower bridge arms.

7. Electromagnetic interference
Electromagnetic interference (EMI) may also affect the switching process of the IGBT. Electromagnetic interference may cause gate voltage fluctuations, affecting the accurate switching of the IGBT, resulting in different turn-off spikes.

8. Gate drive circuit design
The design of the gate drive circuit is critical to the switching characteristics of the IGBT. A well-designed drive circuit can provide a delay-free drive pulse, reduce switching losses, and suppress current spikes. If the gate drive circuit design of the upper and lower bridge arms is inconsistent, it may cause a difference in the turn-off spike.

Conclusion
There are many reasons for the inconsistency of the turn-off spikes between the upper and lower bridges, and multiple factors such as circuit design, device characteristics, and drive parameters need to be considered comprehensively. In practical applications, reasonable circuit design, accurate device selection, optimized drive parameter settings, and effective electromagnetic compatibility design should be used to reduce the difference in turn-off spikes between the upper and lower bridges and improve system stability and reliability. At the same time, regular maintenance and monitoring can also help to promptly discover and resolve problems that may cause inconsistent spikes.