The most critical moments in the gate drive are the turn-on and turn-off of the IGBT. The goal is to perform this function quickly, but with minimal noise and ringing when the IGBT is turned on. A rise/fall time that is too fast can cause unwanted ringing and bad EMI, while a rise/fall time that is too slow increases the switching losses of the IGBT.
Figure 1.0 Gate Drive components for DGD0216
The above example shows the gate driver components of the DGD0216 from Diodes. The rise and fall times of the gate drive to the IGBT can be selectively controlled through careful selection of RG and RRG. When turned on, all current will flow from the IC through RG and charge the IGBT gate capacitance, so increasing or decreasing RG will increase or decrease the rise time in the application. With the addition of DRG, the fall time can be controlled independently because the turn-off current flows from the IGBT gate capacitance through RRG and DRG to the driver in the IC to GND. Therefore, increasing or decreasing the RRG will increase or decrease the fall time. Sometimes this level of fine control is not needed, in which case you may just use RG.
Increasing turn-on and turn-off has the effect of limiting ringing and noise caused by parasitic inductance, so in noisy environments, gate resistance may need to be increased. Gate component selection is a compromise between faster rise time with more ringing and worse EMI but higher efficiency on one end, versus slower rise time and better EMI and noise performance contrasted with worse efficiency on the other end. The exact value depends on the application’s parameters and system requirements. The RG value is usually between 5Ω and 50Ω, and the optimum value is determined by the IGBT gate capacitance and the drive current of the gate driver. The RRG value is usually between 3Ω and 20Ω, and the optimum value is determined by the IGBT gate capacitance and the drive current of the gate driver.
The gate-to-source capacitor CG is also used to minimize ringing and noise and provide overall stability if the gate driver and IGBT are not optimally matched. CG is not required for most systems (increasing gate resistance reduces ringing and provides system stability), but if needed, CG = 1nF is a good typical value.