# IGBT Overvoltage Protection Based on Dynamic Voltage Feedback and Active Clamping
## 3 Overvoltage Suppression Circuit Combining an Active Clamp and Dynamic Feedback
### Turn Off Voltage Spike Suppression Process  
Before the IGBT is turned off, the whole circuit is in the on state, and the collector potential of the IGBT (Q4) is 0. Because of the action of the transient suppression diode, the active clamp and the dynamic voltage feedback circuit do not work. Figure 2 shows that at time \(t_{0}\), the IGBT control signal is inverted, and the circuit operation process is as follows: The switching waveform does not change from \(t_{0}\) to \(t_{1}\), and the overvoltage suppression circuit stops working. The voltage and current of the IGBT at this stage meet the following requirements:  
\[I_{T}(t)=i_{L} \tag{3}\]  
\[V_{cc}(t)=V_{on} \tag{4}\]  
where \(V_{on}\) is the on-voltage drop of the IGBT in stage I.  
From time \(t_{1}\), \(V_{cc}\) starts to rise, and when \(V_{cc}\) rises to the breakdown voltage \(U_{Z54}\) of the transient suppression diode \(Z_{s4}\), which is lower than the breakdown voltage of \(Z_{s2}\), \(Z_{s3}\) and \(Z_{s4}\), \(Z_{s4}\) is broken down, and capacitor \(C_{2}\) begins to charge. Since \(C_{2}\) is parallel to the Miller capacitance and much larger than the Miller capacitance, the Miller capacitance can be ignored, and the voltage rise speed of \(V_{cc}\) can be slowed down. The voltage rise speed of \(V_{cc}\) at this time can be expressed as follows:  
\[\frac{\mathrm{d}V_{cc}}{\mathrm{d}t}=\frac{I_{g}}{C_{2}} \tag{5}\]  
When \(V_{cc}\) rises to the breakdown voltage \(U_{Z51}\) of \(Z_{s1}\), \(C_{1}\) starts to charge. The current \(i_{C1}\) flowing through \(C_{1}\) can be expressed as follows:  
\[i_{C1}=C_{1}\frac{\mathrm{d}U_{cc}}{\mathrm{d}t} \tag{6}\]  
Figure 3: Overvoltage suppression schematic.  
\(I_{C1}\) passes through resistor \(R_{4}\). Because of the presence of the diode, \(i_{C1}\) is fed back to the gate push-pull circuit composed of \(T_{1}\) and \(T_{2}\) to increase its current driving ability, reduce the falling rate of the gate voltage, and compensate for the delay of the entire active gate-driving circuit. At time \(t_{3}\), \(I_{T}\) starts to drop. The voltage drop rate of the gate at this time directly reflects the change rate of \(I_{T}\), and the function of \(i_{C1}\) is equivalent to increasing the value of \(I_{T}\), reducing the discharge speed of the gate, lessening the change rate of \(I_{T}\), and reducing the voltage spike; at the same time, since \(V_{cc}\) exceeds the clamping voltage set by \(Z_{s2}\), \(Z_{s3}\) and \(Z_{s4}\) at time \(t_{3}\), the voltage spike appears, the transient suppression diodes \(Z_{s2}\), \(Z_{s3}\) and \(Z_{s4}\) of the active clamping circuit are simultaneously broken down, and the active clamping circuit starts to work. The voltage spike is absorbed by the active clamping circuit and fed back to the IGBT gate, thereby raising the gate voltage, stopping \(V_{cc}\) from rising, and finally falling and stabilizing near the DC bus voltage of the circuit until the entire off process is completed.  
From the characteristic curve of the IGBT observed during the whole turning-off process of the IGBT, the main function of the dynamic voltage feedback circuit is to prolong the voltage rise time of \(V_{cc}\). The main function of the active clamp circuit is to quickly react and compensate the IGBT gate after \(V_{cc}\) reaches the voltage peak to pull down \(V_{cc}\), avoid an excessive voltage peak and protect the circuit.