Application of GaN and SiC in electric vehicles

Nearly every component of vehicle design, including chassis, powertrain, infotainment, connectivity and driver assistance systems (ADAS), is undergoing rapid development and innovation in the automotive sector.

Designers are seeking advancements to take the next step in innovation by moving from silicon-based solutions to power semiconductor technologies using wide bandgap (WBG) materials such as silicon carbide (SiC) and gallium nitride (GaN). They seek higher power density and more efficient circuits for electric vehicles (EVs).

Silicon carbide and gallium nitride are often used in energy conversion systems

In addition to the high-voltage battery (400V to 800V) and the associated battery management system (BMS), electric vehicles also include at least four types of energy conversion systems.

On-board Charger (OBC): Converts external power to electricity suitable for the EV battery while managing charging speed, keeping battery temperature within a safe range, providing charging information and ensuring high charging efficiency.

DC-DC Converter: Usually from high voltage to 12V, used to power low voltage electronic devices.

DC-AC traction inverter: used to drive an electric motor (usually a three-phase AC motor).

AC-DC converter: used to charge the vehicle battery during brake energy recuperation and from a standard residential or high-power charging station.

Advantages of Silicon Carbide

SiC has been a technology accelerator for electric vehicles. With a wider bandgap, stronger breakdown electric field, and higher thermal conductivity, SiC is becoming increasingly popular in power electronics as silicon approaches its theoretical limit. Silicon carbide-based MOSFETs are more efficient than silicon-based MOSFETs in terms of losses, switching frequency, and power density.

The concept of using SiC in electric vehicles emerged as people try to improve the efficiency and range of electric vehicles while reducing their weight and price to increase the power density of control electronics.

Because SiC devices have several desirable qualities compared to commonly used silicon, they are increasingly used in high-voltage power converters with stringent size, weight, and efficiency requirements. Since SiC has a thermal conductivity nearly three times higher than silicon, components can dissipate heat more quickly.

由于SiC器件，通态电阻和开关损耗也显著降低。这一点意义重大，因为碳化硅比传统硅更有效地散热，随着硅基器件尺寸越来越小，从电气转换过程中提取热量变得更具挑战性，散热问题也重新进入到要专业解决方案的状态，但这带来的是数倍的功率密度，这对电动汽车来说是十分值得的。

就电动汽车而言，牵引逆变器可以节省大部分电力，其中SiC FET 可以取代绝缘栅双极晶体管 (IGBT)，从而显著提高效率。由于电机是磁性组件，并且其尺寸不会随着逆变器开关频率的升高而直接减小，因此开关频率保持在较低水平（通常为8kHz）。

The circuit diagram of a typical traction inverter is shown in the figure and consists of three half-bridge elements (high-side and low-side switches - one for each motor phase - and gate drivers to control the low-side switching of each transistor. This topology has long been based on discrete or power module IGBTs and freewheeling diodes.

Today, six parallel low RDS(ON) SiC FETs with efficiencies greater than 99% at 200kW output could replace IGBTs and their parallel diodes, reducing power losses by a factor of 3. At lighter loads and high frequency use, the improvement is even better, with losses 5 to 6 times lower than IGBT technology, and the advantages of much lower gate drive power and no "knee" voltage, allowing for better control at light loads. Lower losses mean smaller, lighter, cheaper heat sinks and greater range for vehicles operating at low loads and high frequencies, making them ideal for urban driving scenarios.

Due to SiC's higher defect density and substrate (wafer) manufacturing methods, it is still much more expensive than silicon. However, chipmakers have been able to reduce overall production costs by using large quantities of substrates and reducing fault density, and cost control and mass production of SiC are no longer insurmountable defects.

Advantages of GaN

另一种比硅大近3倍的WBG材料是GaN。氮化镓不能用于超低压应用，但它具有允许更高击穿电压和更高热稳定性的优点。氮化镓可显著提高功率转换级的效率，使其成为制造肖特基二极管、功率 MOSFET 和高效电压转换器的理想硅替代品。与硅相比，宽带隙材料还具有显著优势，包括更高的能源效率、更小的尺寸、更轻的重量和更低的总体成本。

While SiC can compete with IGBT transistors in high power and ultra-high voltage (over 650V) applications, GaN can compete with current MOSFETs and super junction (SJ) MOSFETs in power applications up to 650V, and GaN FETs can switch voltages >100V/ns. GaN reverse recovery is zero, so their switching power losses are very low, and for applications that require switching frequencies in megahertz, GaN may be the best choice. OBCs and high-voltage to low-voltage DC-DC converters rated up to 25kW are well suited for GaN.

The switching frequencies in the traction inverters in current electric vehicles are as high as 20kHz and the voltages are as high as 1,000V. This is very close to the operating limits of silicon-based MOSFETs and IGBTs. Without considerable technological advances, silicon-based MOSFETs and IGBTs will have difficulty meeting the more stringent operating specifications of the next generation of electric vehicles. These limitations are caused by the physical limitations of silicon semiconductors and the design of the devices themselves. Large IGBTs and MOSFETs have difficulty switching at high frequencies and endure switching losses due to the gradual transition from the on state to the off state.

Although inverters are more efficient at higher operating frequencies, these improvements are quickly offset by the switching losses inherent in the device. In addition, there is a limit to the inverter’s operating frequency beyond which operation is not possible due to the device’s longer switching cycles.

GaN and SiC technologies complement each other and will continue to do so. GaN devices perform well in applications from tens of volts to hundreds of volts, while SiC is better suited for supply voltages from about one volt to thousands of volts. They currently cover different voltage ranges. For medium and low voltage applications (below 1200V), GaN has at least 3 times lower switching losses than SiC at 650V. SiC can be used in some 650V products, but is usually manufactured for 1200V or greater.

Silicon remains competitive up to 650 V. However, at higher voltages, SiC and GaN enable efficient high-frequency and high-current operation. All devices are suitable for 400-V EV bus voltage, and around 650 V is where the main clash between Si, SiC, and GaN occurs. Although GaN is not as developed as SiC, many experts agree that it also has great prospects in the automotive industry.