How to design in SiC MOSFETs to improve EV traction inverter efficiency
However, the key gate drive challenge arises from the fact that a large V GS (up to 20 volts) must be applied to ensure a low R DS(ON) . Operating a SiC MOSFET at a V GS that is too low can result in thermal stress or even failure due to power dissipation (Figure 3). Moreover, because a SiC MOSFET is a low-gain device, the designer must take into account the impact this has on several other important dynamic characteristics when designing a gate drive circuit. These characteristics include the gate charge Miller plateau and the requirement for overcurrent protection. These design complications demand a specialized gate driver with the following attributes: ■ An ability to provide a VGS drive of -5 to 20 volts to take full advantage of the SiC MOSFET performance benefits. To provide adequate overhead to meet this requirement, the gate drive circuit should be able to withstand VDD = 25 volts and VEE = −10 volts. ■ VGS must have fast rise and fall edges, of the order of a few nanoseconds (ns). ■ The gate drive must be able to source high peak gate current on the order of several amperes, across the entire MOSFET Miller plateau region.
■ The sink current rating should exceed that which would be required to just discharge the input capacitance of the SiC MOSFET. A minimum peak sink current rating on the order of 10 A should be considered for high- performance, half−bridge power topologies. ■ Low parasitic inductance for high−speed switching. ■ Small driver package able to be located as close as possible to the SiC MOSFET and to boost energy density. ■ A desaturation (DESAT) function capable of detection, fault reporting, and protection for long- term reliable operation.
■ A VDD undervoltage lockout (UVLO) level that is matched to the requirement that VGS > 16 volts before switching begins. ■ VEE UVLO monitoring capability to assure the negative voltage rail is within an acceptable range. ON Semiconductor has introduced a gate driver designed to meet these requirements in traction inverter designs. The NCP51705MNTXG SiC MOSFET gate driver features a high level of Figure 4: The NCP51705MNTXG’s DESAT function measures V DS for anomalous behavior during periods of maximum I D and implements overcurrent protection. (Image source: ON Semiconductor)
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