How to design in SiC MOSFETs to improve EV traction inverter efficiency
Another key advantage of SiC is its thermal conductivity, which is about three times higher than Si. Higher thermal conductivity results in a smaller junction temperature (T j ) rise for a given power dissipation. SiC MOSFETs can also tolerate a higher maximum junction temperature (T j(max) ) than Si. A typical T j(max) value for a Si MOSFET is 150˚C; SiC devices can withstand a T j(max) of up 600˚C, although commercial devices are typically rated at 175 to 200˚C. Table 2 provides a comparison of properties between Si and
4H-SiC (the crystalline form of SiC commonly used to manufacture MOSFETs). The high breakdown voltage, low R DS(ON) , high thermal conductivity, and high T j(max) allow a SiC MOSFET to handle much higher
is looking to maximize energy density, is a restriction on the maximum operating frequency due to their “tailing current” and relatively slow turn-off. In contrast, a SiC MOSFET is able to handle high-frequency switching on par with a Si MOSFET, but with the voltage and current handling capability of an IGBT. Wider availability of SiC MOSFETs Until recently, the relatively high price of SiC MOSFETs has seen their use limited to traction inverters for luxury EVs, but falling prices have seen SiC MOSFETs become an option for a wider variety. Two examples of this new generation of SiC power MOSFETS come from ON Semiconductor : the NTBG020N090SC1 and the NTBG020N120SC1 . The major difference between the devices is that the former has a maximum drain-to-source breakdown voltage (V (BR)DSS ) of 900 volts, with a gate- to-source voltage (V GS ) of 0 volts and a continuous drain current (I D ) of 1 milliamp (mA), while the latter has a maximum V (BR)DSS of 1200 volts (under the same conditions). The maximum T j for both devices is 175˚C. Both devices are single N-channel MOSFETs in a D2PAK- 7L package (Figure 2).
current and voltage than a similarly-sized Si MOSFET.
IGBTs are also capable of handling high voltages and currents and tend to be less expensive than SiC MOSFETs – a key reason for them finding favor in traction inverter designs. The downside of IGBTs, particularly when the developer
Table 2: A SiC MOSFET's breakdown field, thermal conductivity, and maximum junction temperature make it a better choice than Si for high-current and high- voltage switching applications. (Image source: ON Semiconductor)
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