What are the advantages of SiC MOSFETs compared with equivalent silicon devices?

Talking about the design of power converters, wide bandgap (WBG) technologies such as silicon carbide (SiC) are realistic considerations in device selection today. The introduction of 650VSiCMOSFETs makes them more attractive for some applications that have never been considered before. These devices show very good durability in high-efficiency hard-switching topologies, and thus are the realization of power factor correction (PFC) for kilowatt-level power supply solutions. ) Ideal for applications. Moreover, because they also support higher switching frequencies, smaller magnetic components can be selected, thereby reducing the size of many designs.

No free lunch

Although wide bandgap devices have many benefits, these advantages cannot be realized simply by replacing the space left by silicon-based devices with SiCMOSFETs. Engineers need to spend some time to understand the characteristics of wide bandgap devices to take full advantage of the full benefits of new devices, while also understanding their respective limitations and failure modes. The forward voltage of the body diode in the CoolSiC device is four times higher than that of the silicon MOSFET, so the efficiency of the LLC converter may drop by 0.5% under light load. The high efficiency of the PFC topology can also be achieved by using the channel for boosting instead of the body diode.

On-resistance is equivalent to silicon in the operating temperature range

A key comparison parameter is the on-resistance RDS(on). The surface parameters of silicon MOSFET seem to be better than SiC, but due to its lower multiplication factor (κ), at 100°C, an 84mΩ CoolSiC device has the same RDS(on) as a 57mΩ CoolMOS device (see Figure 1). ). Compared with silicon MOSFETs, CoolSiC also has a higher breakdown voltage V (BR) DSS, making it very beneficial in applications that require startup in a low temperature environment.

What are the advantages of SiC MOSFETs compared with equivalent silicon devices?

Figure 1: CoolSiC device temperature has a smaller effect on RDS(on) than CoolMOS, so the on-resistance does not change much in the typical operating temperature range.

The EiceDRIVER series is still an ideal synergistic device for CoolSiCMOSFET. However, in order to obtain the lower RDS(on) defined in the data sheet, a gate voltage (VGS) of 18V is required instead of the typical 12V of silicon MOSFETs. If you choose a new gate driver, you need to choose a driver with a 13V undervoltage lockout function to ensure that the target application operates safely under abnormal conditions. Another benefit of SiC is that the temperature between 25°C and 150°C has a limited effect on the transmission characteristics (see Figure 2).

What are the advantages of SiC MOSFETs compared with equivalent silicon devices?

Figure 2: The transfer characteristics at 25°C (left) and 150°C (right) show that SiC devices are much less affected than silicon MOSFETs.

Avoid negative gate voltage

Negative gate voltage can cause long-term degradation of the SiCMOSFET, leading to potential failures. Therefore, the design engineer should absolutely guarantee that VGS will not run below -2V for more than 15ns. If this happens, the drift of the gate threshold voltage (VGS(th)) may cause the RDS(on) to increase during the entire application life, which will eventually lead to a hard-won system efficiency drop, which is positive in many cases. SiC was chosen because of its high efficiency.

For silicon MOSFETs, it is usually necessary to use a high-value resistor to avoid negative VGS, thereby slowing down di/dt and dv/dt. However, for SiC devices, the preferred method is to insert a diode voltage clamp between the gate and source. If the negative voltage is purely an inductance issue, it is strongly recommended to choose a CoolSiC device with a Kelvinsource, which may result in EON loss three times lower than a device without it (see Figure 3).

What are the advantages of SiC MOSFETs compared with equivalent silicon devices?

Figure 3: In order to avoid the negative gate voltage of the SiCMOSFET, diode clamps, independent common terminals, and Kelvin sources should be considered.

Achieve higher than 99% efficiency

Another advantage of CoolSiCMOSFETs is that when the drain-source voltage VDS is higher than 50V, they have a higher output capacitance COSS, which can reduce the overshoot level without the need for gate resistance. The QOSS characteristic of SiC technology also facilitates the adoption of hard switching and resonant switching topologies because fewer discharges are required, which will affect the Eon loss in the continuous conduction mode (CCM) totem pole PFC. Using 48mΩ devices, the efficiency can reach more than 99% for 3.3kWCCM totem pole PFC (see Figure 4), and the highest efficiency peak value possible with CoolMOS in a dual boost (DualBoost) PFC design is 98.85%. Moreover, despite the higher cost of SiCMOSFETs, SiC-based designs are generally more cost-competitive.

What are the advantages of SiC MOSFETs compared with equivalent silicon devices?

Figure 4: Even the 107mΩ CoolSiCCCM totem pole PFC has an efficiency close to 99%, and most of them exceed the best CoolMOS dual boost PFC method.

in conclusion

SiC MOSFETs have a series of advantages over equivalent silicon devices, coupled with their durability in hard-switching applications, making them worthy of consideration in most high-efficiency power conversion applications. The introduction of the 650VCoolSiC series makes SiCMOSFET technology more economical and feasible for applications that need to push the power conversion efficiency to the limit.