In addition, due to changes in the gate input and feedback capacitances, the new generation of high-voltage IGBTs exhibit different input characteristics. This must be taken into account when designing the gate drive. Since the gate uses an RC loop (resistance-capacitance loop), the amount of current and voltage variation (dI/dt and dv/dt) per unit time can be independently adjusted to achieve switching in safe operating areas of IGBTs and diodes. Loss is minimized.
Reliable short-circuit tolerance
Short-circuit tolerance is one of the most important properties of IGBTs. The short-circuit current is limited to 8 to 10 times of the rated current, resulting in a large increase in dissipated power. For example, a 2KV 12KA IGBT will have a loss of 24 MW. Therefore, for high-voltage IGBTs, it is necessary to reduce losses by reducing the short-circuit current (Isc). For the 3300V IGBT, the DC voltage of its application circuit is typically around 1500V~2000V, which is twice that of the 1600V IGBT. Therefore, in order to obtain the same loss as the 1600V IGBT, the current must be reduced. This can be achieved by adopting The optimized high-voltage cell design achieves a reduction of the short-circuit current to 5 times its rated current.
Dynamic transmission characteristics
The IGBT cell design has considered the influence of the input and feedback capacitances because they have a significant influence on the dynamic transmission characteristics of the device. This shows that under the same driving conditions, the high-voltage IGBT is very different from the 1200V and 1600V.
IGBT turn-on
The open process of IGBT can be divided into four processes as shown in Fig. 1 and Table 1 according to time, as follows:
First, the gate firing voltage VGE is less than the threshold voltage VTh. The time constants of its gate resistor RG and gate firing capacitor CGEI determine this process. When the collector current IC and the collector voltage VCE of the device remain unchanged, CGEI is the only factor that affects the turn-on delay time tdon.
Second, when the gate-emitter voltage VGE reaches its threshold voltage ,, the open process enters the second stage, and the IGBT starts to conduct. Its current rate of rise dI/dt is the same as the gate-emitter voltage VGE and the device's transconductance gfs. Relationship: dIc/dt = gfs(Ic)*dVGE/dt
Among them, dVGE/dt is determined by the gate resistance RG and gate firing capacitance CGEI of the device (for high voltage IGBTs, the gate capacitance CGC is negligible).
The third and third stages start when the collector current reaches the maximum value ICmax (inverse peak current IRM of FWD plus load current IL) and overcome the reverse voltage VR to make the diode cut off. At this time, the IGBT's collector voltage VCE begins to drop. As the VCE drops, the field capacitance reactance CGC between the voltage-controlled gate sets increases by nearly a factor of ten. When the gate-fired driving voltage is kept constant, all the gate currents are put into the discharge of the growing CGC. Therefore, the conduction at this stage is affected by the time constant of the gate resistance and the field capacitance. This time constant determines the device's voltage rate of change, dVCE/dt, and has a significant effect on the device's conduction loss.
Fourth, after the turn-on, the device enters a steady conduction state.
The control of dIC/dt and dVCE/dt increases the field capacitance and decreases the gate emission capacitance. Such an IGBT using a generic "R"-gate drive will result in an increase in dI/dt and a decrease in dV/dt. An increase in dI/dt causes the device to withstand higher pressures during FWD reverse recovery and higher negative dI/dt values ​​due to diode recovery, resulting in device overvoltage due to stray inductance. The low dV/dt value causes high switching losses. Therefore, only by changing the size of the gate resistor RG to balance can resolve the dI / dt and dV / dt size conflict. The value of RG must ensure that the adjustment of dIc/dt is always in the safe working area of ​​the device, but the value of dV/dt will be so low that the turn-on loss will not be acceptable. Therefore, the solution is to use the "RC" gate drive, which is to connect the additional capacitor CGE between the gates of the IGBT. The capacitance is used to adjust the rise of the gate firing voltage and the current change rate dIc/dt in the above-mentioned second opening process. However, the CGE has no effect on the opening of the third process because no change of dVGE/dt is caused. An increase in dVCE/dt reduces the turn-on losses of the device and the gate resistance controls the change in dV/dt on the FWD to not exceed its critical value. After the gate resistor RG is determined, the appropriate dIc/dt value can be set by adjusting the external CGE.
The results of using "RC"-gate drive are shown in Fig. 3a, b, and c. The dIc/dt setting is about 5kA/μs. Different dVCE/dt values ​​are determined by different RC values. Properly selecting the RC value can significantly reduce the turn-on losses of the device by more than 50%.
IGBT driving conditions
High-voltage IGBTs and diodes have their limitations in switching speed. When dIF/dt is the limiting value of the freewheeling diode FWD, the dVCE/dt value of the IGBT at turn-off is the maximum value. Of course, the changes in these two limits can be adjusted by changing the gate drive conditions of the IGBT. The FWD cut-off is controlled by the driving conditions of the IGBT turn-on. Always turn off the IGBT in its safe working area. In order to independently control the dV/dt, dI/dt, and dV/dt at turn-off, three passive components must be used, as shown in Fig.4. When a standard ±15V gate driver is used, it can be Turn on the gate resistor Ron (adjust dVon/dt), turn off the gate resistor Roff (adjust dVoff/dt), and turn on the emitter capacitance CGE (adjust dIon/dt) to adjust the change slope of the IGBT/FWD limit. The capacitance CGE has little effect on dI/dt when the IGBT is turned off, as shown in Fig.5.
Summary Limited by the safe operating area of ​​high-voltage IGBTs and high-voltage FWDs, RC-gate drivers with three passive components (Ron, Roff, CGE) are used to control the voltage and change the slope of the off current.
The change in capacitive reactance between the gate and gate set caused by different input and transmission characteristics can be compensated by the RC-gate drive scheme.