Mechanism of surge voltage generated by power devices
Generation mechanism of inrush voltage of inverter power switch
As mentioned above, the laminated busbar can avoid the breakdown of the switching device due to the surge voltage. The following briefly uses IGBT to explain the serious impact of the stray inductance on the power equipment.
As a semiconductor switching device, IGBT has four operating conditions: off-state, on-state, on-state, and off-state during operation. The switching voltage spike mainly occurs during the switching state of IGBT.
figure 2. Typical main circuit structure of inverter power supply
Figure 2 shows the typical main circuit structure of a phase of the inverter power supply, which is mainly composed of the DC side supporting capacitor, DC bus bar, switching device and load. In practical applications, due to the inevitable existence of distributed parameters in the line, there is a line distributed inductance Ls between the DC-side supporting capacitor and the switching device. The line distribution inductance Ls includes the parasitic inductance of the DC-side capacitor, the stray inductance of the bolt pin, the stray inductance of the main circuit bus and the parasitic inductance of the switching device IGBT .
At the moment when T1 is turned off, the inductance Ls will prevent the current from decreasing rapidly, and a transient spike voltage ΔU will be induced across Ls (the direction is shown as left, negative, and positive in Figure 2). The magnitude is determined by the following formula:
ΔU=Ls*di/dt (Formula 1)
Therefore, the voltage that T1 bears when it is turned off is Vd+Ls*di/dt, that is, the voltage and current stress when the IGBT switches operate and the distributed inductance in the main circuit have a very important relationship.