# IGBT Overvoltage Protection Based on Dynamic Voltage Feedback and Active Clamping
## 4 Parameter Influence of the Dynamic Voltage Feedback Circuit
### \(C_{1}\) Impact Mechanism Analysis  
As seen from Table 1, \(C_{1}\), \(R_{4}\) and \(U_{Z51}\) have the most important influence on the performance of the dynamic voltage feedback circuit. To deeply study the influence of \(C_{1}\), \(R_{4}\) and \(U_{Z51}\) on the dynamic voltage feedback circuit, we provide a basis for the parameter selection of the dynamic voltage feedback circuit, analyze the role of the dynamic voltage feedback circuit in each stage of the IGBT turn-off process, and list the corresponding expressions. Table 1 shows that in stage III, \(C_{1}\), \(R_{4}\) and \(U_{Z51}\) influence the turn-off voltage rise rate. When the IGBT is turned off in stage III, its collector-emitter voltage can be expressed as follows:  
\[V_{cc}(t)=V_{gc}(t)+V_{gc}(t) \tag{7}\]  
Since \(V_{cc}\) continues to rise at this time, its value is related to \(V_{gc}\) and \(V_{gc}\), and the values of \(V_{gc}\) and \(V_{gc}\) in stage III can be based on the following formulas:  
\[ \begin{cases}&V_{gc}(t)=V_{m}-V_{ce-on}+\frac{1}{C_{gc}}\int\limits_{i1}^{t}i _{gc}(t)\\ &V_{gc}(t)=V_{L}-I_{g}(t)\bullet R_{g}\end{cases}  \tag{8}\]  
In Formula (8), \(V_{m}\) is the Miller voltage, and \(V_{ce-on}\) is the turn-on voltage of the IGBT. Since both voltages are fixed values, which are determined by the IGBT manufacturing process, the Miller capacitance \(C_{gc}\) becomes an important factor at this time. After the \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Time**} & \multirow{2}{*}{**Regular Constant**} & \multicolumn{2}{c}{**Dynamic Voltage**} & \multirow{2}{*}{**Result**} \\  & & **Feedback Parameters** & \\ \hline \(t_{0}\)\(\sim\)\(t_{1}\) & \(R_{g}\) & None & Turn-off delay \\ \(t_{1}\)\(\sim\)\(t_{2}\) & \(R_{g}\) & None & d\(U_{ce}\)/dt \\ \(t_{2}\)\(\sim\)\(t_{3}\) & \(R_{g}\) d\(U_{cc}\)/dt & \(C_{1}\)\(R_{4}\)\(U_{Z51}\) & d\(U_{cc}\)/dt \\ \(t_{3}\)\(\sim\)\(t_{4}\) & \(R_{g}\) d\(U_{cc}\)/dt & \(C_{1}\)\(R_{4}\) & d\(U_{cc}\) d\(I_{c}\)/dt \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Influence of parameters on turn-off characteristics at various stages. dynamic voltage feedback circuit is added, after entering stage III, the transient suppression diode \(Z_{s1}\) is reversely broken down, and capacitor \(C_{1}\) begins to charge. At this time, the dynamic feedback circuit is turned on, and \(C_{1}\) is equivalent to being connected in parallel with both ends of \(C_{gc}\). The value is much larger than \(C_{gc}\), raising the Miller capacitance value of the IGBT and thereby reducing \(V_{gc}\). Since the dynamic voltage feedback circuit feeds back the gate current of the IGBT as \(i_{\text{C1}}\), \(I_{g}(t)=i_{\text{C1}}+i_{g}\) increases accordingly, \(V_{ge}\) decreases, and finally \(V_{ce}\) decreases, suppressing the voltage spike. When setting the value of capacitor \(C_{1}\) constant, the current discharged through \(C_{1}\) can be obtained from Formula (6), which acts on the push-pull circuit of the control pole, according to the output characteristic curve of the triode. When the triode works in the amplification area, the base current of the triode increases, the emitter current also increases, and finally, the IGBT gate current increases. According to Formula (8), \(V_{ge}\) decreases, which reduces \(V_{ce}\). From the changes in \(V_{gc}(t)\) and \(V_{ge}(t)\) in Formula (8), it can be concluded that when \(R_{4}\) and \(U_{Z51}\) remain unchanged and \(C_{1}\) increases, the values of \(V_{ge}\) and \(V_{gc}\) will decrease, resulting in a decrease in \(V_{ce}\). The simulation platform shown in Figure 3 is built in SABER, and the simulation diagrams in Figures 4 and 5 represent the \(V_{ge}\) and \(V_{ce}\) turn-off waveforms corresponding to different \(C_{1}\) values, respectively. The change in \(C_{1}\) has a substantial impact on the turn-off voltage spike.