Blog Rewrite Version

As power electronic systems evolve toward higher efficiency, higher power density, and higher switching frequencies, the limitations of traditional Si MOSFETs and IGBTs in high-voltage scenarios have become increasingly apparent. The value of SiC MOSFETs lies in achieving a better balance between high voltage rating, low conduction loss, and high-speed switching.

The following sections outline the key characteristics of SiC MOSFETs compared to traditional silicon devices, covering device structure, on-resistance, temperature characteristics, and gate drive.

I. Why Are SiC MOSFETs More Advantageous in High-Voltage Applications?

For traditional Si devices, as the voltage rating increases, the drift layer must be made thicker and more lightly doped, which significantly increases the specific on-resistance. Consequently, IGBTs were often preferred for applications above 600 V in the past.

IGBTs reduce conduction loss through conductivity modulation, but this also introduces minority carrier storage effects. The tail current formed during turn-off increases switching loss, limiting high-frequency applications.

The advantage of SiC is that its drift layer can operate under higher electric fields, eliminating the need for conductivity modulation while still achieving high voltage ratings and low on-resistance. As a majority carrier device, the SiC MOSFET does not exhibit the significant tail current seen in IGBTs during turn-off, making it more suitable for high-frequency switching.

Comparison of voltage ratings and application ranges for Si, IGBT, SJ-MOSFET, and SiC MOSFET

II. How Does SiC Achieve High Voltage Rating with Low On-Resistance?

The original text notes that the critical breakdown field of SiC is approximately 10 times that of Si. This means that for the same voltage rating, SiC can use a thinner, lower-impedance drift layer, reducing the specific on-resistance from a device structure perspective.

Taking 900 V as an example, the comparison provided in the original text states that to achieve the same on-resistance, the chip area required for a SiC MOSFET is approximately 1/35 that of a standard Si MOSFET and 1/10 that of an SJ-MOSFET. Reducing the chip area also helps lower gate charge Qg and junction capacitance.

Trend of specific on-resistance with increasing voltage rating for different devices

Additionally, SiC can relatively easily scale to 1700 V or even higher voltage ratings while maintaining low on-resistance, which is one of its core advantages in high-voltage power conversion.

III. Why Is the SiC MOSFET More Suitable for High Frequencies Than the IGBT?

Although the bipolar conduction mechanism of IGBTs helps reduce conduction loss, it produces carrier storage effects, thereby limiting turn-off speed. The SiC MOSFET does not rely on minority carrier injection, so no significant tail current occurs during turn-off.

Therefore, replacing IGBTs with SiC MOSFETs can significantly reduce switching loss and further increase the switching frequency. Higher switching frequencies also allow for the reduction of passive components such as inductors and capacitors, thereby driving up the overall power density of the system.

IV. From the VD-ID Characteristic, SiC Is More Linear Over a Wide Current Range

The original text points out that, unlike IGBTs, SiC MOSFETs do not have a distinct turn-on voltage, allowing them to maintain low conduction loss from low currents to higher currents.

This differs from the characteristics of IGBTs. IGBTs exhibit a distinct voltage drop threshold, whereas the conduction behavior of MOSFETs is closer to a resistive characteristic, making it easier to achieve high efficiency over a wider load range.

Comparison of VD-ID characteristics for different devices at 25°C and 150°C

V. At High Temperatures, the On-Resistance of SiC Increases More Gradually

Rising temperatures cause the on-resistance of MOSFETs to increase. The original text mentions that for Si MOSFETs, the on-resistance at 150°C can rise to more than twice the value at room temperature, whereas the increase rate for SiC MOSFETs is relatively lower.

This means that in high-temperature environments or high-power-density designs, the variation in conduction loss for SiC is easier to control, which is beneficial for thermal design and efficiency budgeting.

VI. The Gate Drive Voltage for SiC MOSFETs Cannot Simply Follow That of Standard Si MOSFETs

The drift layer impedance of SiC MOSFETs is very low, but the mobility of the MOS channel is relatively low, so the resistance contribution of the channel portion cannot be ignored. Increasing the gate drive voltage can further reduce the on-resistance.

The original text points out that if the 10 V to 15 V gate drive commonly used for traditional IGBTs or Si MOSFETs is applied, the low on-resistance advantage of SiC may not be fully realized. To achieve a lower RDS(on), the original text recommends using a gate drive of approximately 18 V.

Trend of SiC MOSFET on-resistance with respect to VGS and temperature

7. What to Focus on When Designing with SiC MOSFET ?

· Do not directly reuse the gate drive voltage of Si MOSFETs or IGBTs.

· Verify the RDS(on) at the target VGS by referring to the datasheet.

· Leverage the advantages of low Qg, low junction capacitance, and low reverse recovery loss to increase the switching frequency.

· After increasing the frequency, the drive loop and PCB parasitic parameters become more sensitive.

· In high-temperature applications, consider the variation of on-resistance with temperature.

8. Suitable Application Scenarios

The original text mentions that SiC MOSFETs are mainly used in scenarios such as power supplies for industrial equipment, inverters, and converters in high-efficiency power regulators. Their advantages are particularly suitable for systems requiring high voltage withstand capability, high frequency, high efficiency, and reduced sizes of heat sinks and passive components.

Summary

The core advantages of SiC MOSFETs can be summarized as: maintaining a relatively low on-resistance under high voltage conditions, no significant IGBT-like tail current during turn-off, the ability to support higher switching frequencies, and relatively mild growth in on-resistance at high temperatures. It is not simply a "more advanced version" that replaces traditional MOSFETs, but a class of power devices that requires re-evaluation of gate drive and high-frequency layout. Only by designing the device characteristics, drive voltage, and system frequency together can the advantages of SiC be fully realized.