# IGBT Overvoltage Protection Based on Dynamic Voltage Feedback and Active Clamping
## 2 Analysis of IGBT Turn-Off Voltage Spike  
According to the working principle of an IGBT, the commutation circuit of an IGBT with an inductive load can be simplified to the application circuit shown in Figure 1, where \(Q\) is the IGBT to be tested, \(R_{g}\) is the gate resistance, \(D\) is the antiparallel diode, L is the load inductance, and \(V_{DC}\) is the \(DC\) voltage source. When testing the IGBT through the circuit in Figure 1, according to the measured waveform of the IGBT tube, the turn-off process of the IGBT can be divided into five stages, as shown in Figure 2, where \(V_{th}\) is the threshold voltage of the IGBT, \(V_{m}\) is the Miller plateau voltage of the IGBT, \(V_{H}\) is the positive terminal voltage, and \(V_{L}\) is the negative terminal voltage. Assuming that the IGBT is turned on in the initial state, at \(t_{0}\), the control signal is reversed from high to low, and the IGBT is controlled to turn off and the IGBT enters stage I (\(t_{0}\)\(\sim\)\(t_{1}\)), the IGBT current is the load inductor current \(i_{L}\) and remains unchanged, and the gate voltage \(V_{ge}\) begins to drop to the Miller plateau. Because of the higher Miller voltage \(V_{m}\), the IGBT works as usual, and the IGBT turn-on voltage drop and turn-on current remain unchanged. In stage II (\(t_{1}\)\(\sim\)\(t_{2}\)), the collector-emitter voltage \(V_{ce}\) begins to rise, the total current \(I_{T}\) remains unchanged, and the IGBT begins to exit the saturation region and charges the Miller capacitor to maintain this state, thereby forming a Miller platform. The Miller platform ends and the turn-off process enters stage III (\(t_{2}\)\(\sim\)\(t_{3}\)), the IGBT begins to exit the saturation region, the gate voltage \(V_{ge}\) continues to drop to the threshold voltage \(V_{th}\), \(V_{ce}\) rises to the DC voltage value, and the IGBT current \(I_{T}\) decreases slightly. At this time, the descending speed depends on the ascending speed of \(V_{ge}\). From time \(t_{3}\), \(V_{ce}\) rises to the stable value of the DC voltage, the freewheeling diode starts to conducting, and \(I_{T}\) starts to drop rapidly. Because of the existence of parasitic inductance in the circuit, resonance occurs, making the voltage at both ends of the IGBT higher than the voltage of the DC bus, thus generating a voltage spike. In stage V, \(V_{ge}\) continues to drop, \(I_{T}\) enters the current tailing process, and the IGBT exits the saturation zone and enters the cut-off zone. At time \(t_{5}\), \(V_{ge}\) drops to the turn-off voltage (a normally negative value ensures reliable turn-off), the tailing current ends, the IGBT completely enters the cut-off zone, and the whole turn-off process ends. Figure 1: IGBT typical application circuit. According to the IGBT turn-off process, the turn-off overvoltage spike of the IGBT is inevitable, which is due to the switching action of the IGBT, and the current value flowing through the parasitic inductance changes rapidly. Therefore, the turn-off current change rate and parasitic inductance \(L_{q}\) determine the turn-off overvoltage peak value \(\Delta U\) of the IGBT, and the relationship is as follows:  
\[\Delta U=L_{q}\frac{\mathrm{d}i_{c}}{\mathrm{d}t} \tag{1}\]  
\[i_{c}=i_{L}+C_{gc}\frac{\mathrm{d}V_{cr}}{\mathrm{d}t} \tag{2}\]  
Formula (1) shows that reducing the rate of change of the off current and the parasitic inductance can reduce the turn-off overvoltage spike. The main influence of the parasitic inductance comes from the connection inductance between the IGBT module and the DC capacitor. In practice, the parasitic inductance can be reduced by reducing the length of the DC loop or reducing the length of the positive and negative DC bus bars while increasing their width. Because of the limitation of the process level, when the parasitic inductance is limited to 200 nH, greatly reducing it further is difficult. In Formula (2), \(i_{L}\) is the load current, and the current value of the IGBT is related to its voltage rise rate.