The fundamental advantage of SiC and other wide bandgap devices stems from their band gap, the energy difference between the top of the valence band and the bottom of the conduction band. Electrons move from the low-energy valence band to the high-energy conduction band, making the material conductive. Moving an electron from the valence band to the conduction band requires 1.1 eV. SiC, on the other hand, has a band gap of 3.2 eV, so moving electrons to the SiC conduction band requires significantly more energy. For a given chip size, this translates to a higher breakdown voltage than silicon devices. In practice, the advantages of SiC chips are more akin to being tailored for electric vehicles, such as smaller size, lower on-resistance (RDSON), and faster switching speeds.
The three main limitations of electric vehicles are charging time, driving range, and cost. Boosting the high-voltage section of the inverter circuit (known as the DC link) to 800 V or even 1,000 V reduces current, allowing for lighter cables and magnetic components. Higher voltages require switching devices with higher breakdown voltages, typically up to 1200 V. For standard silicon MOSFETs, scaling the breakdown voltage to that level while maintaining high current is impractical because the required die size becomes much larger. Bipolar silicon devices (primarily insulated bipolar gate transistors (IGBTs)) can address this issue, but at the expense of switching speed and limited power conversion efficiency. The wide bandgap of SiC allows unipolar FET devices (with significantly smaller die sizes) to exhibit the same breakdown voltage and rated current as conventional IGBTs. This characteristic brings several improvements to power conversion systems while allowing for higher DC bus voltages and reduced vehicle weight.
To improve the driving range of electric vehicles, either battery capacity must be increased or vehicle efficiency must be improved. Generally, increasing battery capacity increases cost, size, and weight, so designers focus on improving the efficiency of the vehicle's power conversion system. By using the right switching devices, designers can increase the power switching frequency to improve efficiency, while simultaneously reducing the size of magnetic components, thus lowering cost and weight. Furthermore, efficient converters require less heat dissipation and cooling systems.
SiC FETs are naturally suited to these high switching frequencies because they consume very little energy per charge/discharge cycle. Furthermore, the material properties of SiC, combined with its smaller die size, allow it to operate at higher temperatures while exhibiting lower losses than IGBTs.
How does the RDSON of the Cree Wolfspeed E3M0065090D automotive SiC FET change with temperature?
Unlike IGBTs, SiC FETs have an RDSON specification, and the rated RDSON changes very little with temperature. This concept is crucial for high-power electric vehicle applications, where switching devices handle kilowatts of power and often reach high temperatures. Furthermore, IGBTs are typically optimized for maximum current. Below the maximum load, their conduction losses increase dramatically. However, SiC FETs maintain their efficiency under low loads. This behavior is particularly useful in automobiles, where systems such as traction inverters operate under varying loads for extended periods.
All these improvements in SiC FETs collectively lead to higher efficiency, smaller cells, and lower costs, enabling the design of more powerful electric vehicles. However, adopting SiC technology requires designers to learn new technologies, and some of the most important technologies are concentrated on the gate driver.
SiC FETs, with their smaller die size and higher switching frequencies, require slightly different gate drive techniques. The smaller die size makes SiC FETs more susceptible to damage, while the higher frequencies necessitate gate drivers with higher performance. Finally, SiC FETs typically require higher gate drive signals and negative gate voltages in the off-state. The latest isolated gate drivers integrate the functionality needed to meet all these requirements.
Many high-voltage automotive systems use isolation devices, such as isolated gate drivers, to separate the low-voltage controller from the high-voltage portion of the system. The high switching frequencies used in most SiC FET designs expose isolated gate drivers to rapid transients. Gate drivers with a common-mode transient immunity (CMTI) of at least 100 kV/µsec can withstand these transients. Furthermore, the propagation delay and inter-channel skew of the driver must typically be below 10 ns to keep the design stable at such high speeds. As automotive systems increase DC link voltages, isolated gate drivers must also have sufficient maximum isolation operating voltage (VIORM). Thanks to technological advancements, designers can easily select isolated gate drivers that meet the requirements of SiC FET systems.
Many new isolated gate drivers, such as the Silicon Labs Si828x, also include integrated Miller clamping and desaturation detection to protect SiC devices. In half-bridge or full-bridge configurations, the voltage across the drain of the switching device in the lower half of the bridge changes rapidly when the upper device is turned on. This change induces a current in the gate to deplete parasitic capacitance that would otherwise discharge through the gate and turn on the lower device. This "Miller parasitic turn-on" can lead to breakdown, which can rapidly damage SiC devices.
Integrated Miller clamp on the Silicon Labs Si828x isolated gate driver.
When the integrated Miller clamp reaches a preset threshold, it creates a parasitic capacitance from the gate to the drain. Furthermore, abnormal load conditions can cause the switching device to saturate and become damaged. However, the Silicon Labs Si828x gate driver integrates a desaturation circuit. If the voltage on the switching device rises above the configured threshold, the gate driver responds quickly and shuts it down normally. It uses a soft-shutdown circuit to limit the induced shutdown voltage on the switching device.
For SiC FETs, protection circuitry must react quickly (typically less than 1.8 microseconds) to take effect. By integrating these three functions into the gate driver, designing robust and reliable SiC power converters becomes simpler.
Integrated desaturation circuitry on the Silicon Labs Si828x isolated gate driver.
The final aspect of driving a SiC FET is the use of a negative voltage when the FET is turned off. This negative voltage works in conjunction with Miller clamping to ensure the FET is in the off state, a crucial aspect of controlling the shoot-through current in high-frequency power converters. Methods for generating the necessary negative voltage rails are beyond the scope of this article. However, selecting a gate driver with an integrated DC/DC converter often simplifies the design.
In summary, SiC switches offer unprecedentedly faster switching speeds, higher efficiency, and higher power density. Furthermore, high breakdown voltage and thermal characteristics are fundamental requirements for electric vehicle powertrains. These advantages, coupled with improved functionality of isolated gate drivers, make them a core technology in the electrification revolution.
