Mitigation Methods for Parasitic Turn-on Effect Due to Miller Capacitor - White Paper

One of the common problems faced when switching an IGBT is parasitic turn-on due to Miller capacitor. This effect is noticeable in single supply gate driver (0 to +15V). Due to this gate-collector coupling, a high dV/ dt transient created during IGBT turn-off can induce parasitic turn-on (Gate voltage, VGE) which is potentially dangerous (Figure 1). There are three classical solutions to the above problem; the first being to vary the gate resistor (Figure 2), second to add a capacitor between gate and emitter (Figure 3) and third to use negative gate drive (Figure 4). A fourth simple and effective solution is the Active Clamp technique (Figure 5). The on-gate resistor, RGON influences the voltage and current change during IGBT turn-on. Increasing this resistor reduces the voltage and current changes but increases switching losses. Parasitic turn-on can be prevented by reducing the offgate resistor, RGOFF. The smaller RGOFF will also reduce switching loss during IGBT turn-off. However, the trade off is higher over-shoot Vce and oscillation during turnoff due to stray inductances.