What Is SiC MOSFET?

Application Characteristics

SiC MOSFETs offer superior switching performance and higher thermal conductivity, enabling more compact and efficient power systems in EVs, solar inverters, and industrial drives.

In actual driving conditions, the peak values of the source and sink current are often limited by the impedance of the gate drive loop. This includes not only the internal pull-up and pull-down resistances (R_OL, R_OH) of the driver IC, but also the external gate resistance (R_{gate_ext}) and the internal gate resistance of the MOSFET (R_{gate_in}).

Taking the UCC21530 as an example, the actual maximum turn-on and turn-off drive currents of the chip are shown in Figure 5.The values of R_OL, R_OH, and R_NMOS are generally provided in the datasheets of the driver ICs.

C. Miller Clamping

Another common method to prevent false turn-on of SiC MOSFETs is to implement a Miller clamping circuit or use a chip with a Miller clamping function, as shown in Figure 6. The Miller clamping function works by sampling the gate voltage and comparing it with the threshold voltage. When the gate voltage drops below the threshold voltage, the comparator flips, causing the built-in Miller clamping MOSFET to turn on. This creates a low impedance path, which effectively clamps the gate of the SiC MOSFET and prevents false turn-on The position of the Miller clamp greatly affects its effectiveness.

The purpose of clamping is to provide a low impedance path for the Miller current to flow towards the ground. Using an internal clamp reduces the components needed for circuit construction, and the clamp should be placed as close as possible to the power switch. The Miller current path may include parasitic resistance (R_p) and inductance (L_p), as shown in Figure 7. If the current is large enough, the Miller clamp will not significantly affect the performance of the driver. An external Miller clamp is controlled by the driver and is placed externally, as shown in Figure 8. This configuration allows the clamp to be positioned very close to the power switch to minimize any impedance in the current path..

E. Overcurrent Detection

DESAT is a common short-circuit protection method for IGBTs. However, due to differences in I-V characteristics, it is not always suitable for SiC MOSFETs. The transition of a SiC MOSFET from the linear region to the saturation region is not clearly defined, so using a single voltage threshold for DESAT detection may not be very accurate. A more applicable method is overcurrent detection, which measures the current flowing through a precision shunt resistor (R_shunt). Although R_shunt is typically in the milliohm range and carries relatively large currents—resulting in higher power losses—it offers more accurate monitoring with fewer required circuit components. Fewer components mean faster response time, which is critical for SiC MOSFETs due to their shorter short-circuit withstand time (SCWT) compared to IGBTs.

To address the power loss issue, some power modules integrate current scaling functions to reduce the current flowing through the shunt resistor (see Figure 10). The built-in shunt circuit within the module reduces the power dissipated across the shunt resistor. The corresponding scaling ratio is provided by the power device manufacturer. This approach results in lower power consumption than traditional shunt-based measurements and enables more accurate current sensing