WBG Semiconductors Revolutionize Automotive Design

Using silicon carbide and gallium nitride to meet electric vehicle design requirements is now becoming the standard for next-generation automotive design that promotes sustainability. Aerodynamic lines or lighter materials are not enough to ensure the efficiency of electric vehicles. To meet efficiency and power density requirements, power electronics designers must look to new technologies.

Advanced wide bandgap (WBG) semiconductor materials, especially GaN and SiC, represent an improvement over existing semiconductor technologies such as MOSFETs and IGBTs. Basically, the bandgap corresponds to the energy required to excite an electron from the valence band of a material to the conduction band. In this sense, the bandgap of WBG materials is much higher than that of silicon. WBG semiconductors allow devices to operate at much higher voltages, frequencies, and temperatures than silicon, and with significantly lower switching and conduction losses. The conduction and switching characteristics of WBG materials are also about 10 times better than conventional silicon. These properties make WBG technology a natural application for power electronics, especially for EV applications, as SiC components and GaN can simultaneously have smaller size, higher speed, and higher efficiency.

However, the advantages of WBG devices must also be evaluated against the complexity and higher costs of mass production. While WBG components may initially be more expensive, their costs continue to fall and ultimately save costs in the entire system in the future. For example, using SiC devices in electric vehicles may increase initial costs by several hundred euros, but the result is an overall savings because the battery costs less, less space is required, and the cooling solution is simpler at the construction level - for example, using a smaller heat sink.

Technical Considerations for SiC and GaN Devices in Automotive Design

WBG power technology is key to the success of electric and hybrid vehicles, helping to accelerate the adoption of electric vehicles around the world by overcoming some of the inherent limitations of electric vehicles. To meet the increasing efficiency and power density requirements of electric vehicle systems (such as inverters and integrated chargers), automotive power electronics designers can take advantage of state-of-the-art WBG semiconductors such as SiC and GaN. As mentioned above, these products offer a range of features compared to traditional silicon devices, including lower losses, higher switching frequencies, higher operating temperatures, stability in harsh environments, and higher breakdown voltage. SiC is configured as a key technology designed for multiple electric vehicle applications, such as traction inverters, on-board chargers (OBCs), and DC/DC power converters.

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GaN and SiC can operate at higher temperatures with similar life expectancies, or they can operate at similar temperatures to silicon devices but with longer lifespans. Today, power electronics system designers have a number of design options to choose from, depending on the requirements of their specific application. Overall, using WBG materials allows you to choose different design strategies and paths depending on the goals of your end project. For example, we can decide to use the same switching frequency and increase the output power, or we can use the same switching frequency and reduce the amount of heat sinking required in the system, saving on the overall cost of the components. Otherwise, designers can choose to increase the switching frequency while keeping the power losses in the switch the same. As you can see, there are many customizable options.

Figure 1: Relationship between WBG technology, operating frequency and system power (Source: STMicroelectronics)

Figure 2: SiC applications in electric vehicle systems (Source: STMicroelectronics)

Inverter

The inverter controls the electric traction motor in electric vehicles. This is a key component in the electric propulsion system that can benefit from WBG devices. The main function of the inverter is to convert the DC voltage into a three-phase AC waveform to drive the car engine, and then convert the AC voltage generated by regenerative braking into a DC voltage to charge the battery. To drive the electric motor, the inverter converts the energy stored in the battery pack into AC power, so the lower the losses in the conversion stage, the more efficient the system. SiC devices have higher conductivity and higher switching frequency compared to silicon devices. Therefore, SiC reduces power losses because less energy is dissipated as heat. Therefore, the higher the efficiency of SiC-based inverters, the longer the range of electric vehicles.

Today, many electric vehicle manufacturers are integrating SiC power modules into the main inverter. Using SiC to build electric vehicle inverters can reduce their size by about 5 times, reduce their weight by about 3 times, and reduce power losses by half compared to their silicon counterparts. For example, OBC and DC/DC converter designs can be integrated into smaller, lighter, and more efficient packages than similar designs built with silicon devices.

Marginal blood cells

Electric vehicle charging systems (also known as OBCs) require the conversion of electrical energy from AC (usually from the distribution network) to DC. With WBG devices, new circuits for charging electric vehicles can be realized. With a band gap 2 to 3 times larger than silicon, WBG devices can withstand larger voltages and electric fields because electrons require more energy to move from the valence band to the conduction band. The breakdown voltage of WBG semiconductors is much higher, while the on-resistance is very small. This simplifies the design and improves the efficiency of the charging circuit. The low value of R DS(on) also reduces switching and power losses, thereby reducing the circuit size.

Another advantage of WBG devices is that they generate lower temperatures than silicon-based devices under the same operating conditions. In power circuits, SiC devices can withstand junction temperatures even higher than 200°C, while silicon devices can reach a maximum of around 150°C. Therefore, using WBG devices in electric vehicle chargers can achieve higher switching speeds and better energy efficiency, resulting in more compact and easier to cool modules.

The OBC is installed at the factory. In a pure electric vehicle or plug-in hybrid vehicle, the OBC provides the means to charge the battery from the home AC grid or a private or public charging station socket. The OBC uses an AC/DC converter to convert the 50/60 Hz AC voltage (100 to 240 V) to a DC voltage to charge the high-voltage vehicle battery (typically around 400 V DC). It also adjusts the DC current level according to the battery requirements, provides galvanic isolation, and includes AC/DC power factor correction.

In a typical electric vehicle OBC, SiC diodes are often used. OBCs require the highest efficiency and reliability to ensure fast charging times, but must also meet design specifications for application space and weight. OBC designs using GaN technology can simplify cooling systems and reduce charging time and energy losses. Bidirectional OBCs are a key development for electric vehicle adoption in future modern sustainable smart grid infrastructure. Bidirectional OBCs allow electric vehicles to act as energy storage or other uses to help manage supply and demand changes and help stabilize loads within the grid. GaN and SiC-based devices enable advanced bidirectional topologies and can optimize power converter configurations.

Although GaN power devices seem to be slightly behind SiC at a commercial level, they are rapidly gaining market share due to their excellent efficiency performance. Similar to SiC, GaN has lower switching losses, higher switching speeds and higher power density, and allows the size of the entire system to be reduced, which is related to weight and total cost. While typical silicon MOSFETs have lower switching speeds, GaN devices switch at higher speeds to achieve the lowest possible losses. Based on this level of operation, the system layout can also make an important contribution to the performance. Several manufacturers have developed automotive-grade SiC devices for OBC applications in electric and hybrid vehicles to reduce energy losses and achieve better electrical performance under load conditions.

DC/DC Power Converters

DC/DC converters power a variety of loads throughout the vehicle. When designing DC/DC converters for automotive applications, GaN devices can save power and significantly reduce circuit size and weight compared to ordinary silicon MOSFETs, while also achieving better thermal management performance and reliability. In the field of high-voltage and high-power applications, these devices bring advantages to the automotive field, enabling smaller and lighter modules, thereby saving space and improving energy efficiency. In addition, GaN ICs combined with 650-/700-V power transistors and optimized gate control can provide a solution to meet power efficiency needs. Lower energy losses at high speeds can improve the range, thereby increasing the switching frequency of the conversion circuit operating at 300 to 800 kHz, allowing the use of smaller passive components to maximize power density within a compact module size.

Figure 3: FF08MR12W1MA1_B11A EasyPACK CoolSiC automotive MOSFET 1,200-V half-bridge module (Source: Infineon Technologies)

Figure 3 shows an 8 mΩ half-bridge module using Infineon’s new 1,200 V automotive CoolSiC MOSFETs. With full automotive qualification, the application range of CoolSiC has now been extended to high-voltage applications with high efficiency and high switching frequency requirements, such as HV/HV DC/DC boost converters, multiphase inverters and fast-switching auxiliary drives such as fuel cell compressors.