Wide bandgap (WBG) semiconductors silicon carbide (SiC) and gallium nitride (GaN) are used for a variety of applications in EVs. Weight and size are even more important in EVs than in conventional internal combustion engine (ICE) vehicles. SiC and GaN can switch at higher frequencies compared with silicon (Si) power devices. That enables smaller and lighter power converters with better electromagnetic interference (EMI) performance compared with Si designs, and they are more efficient. Those attributes directly contribute to improvements in EV range and reduce the range anxiety of EV users. This FAQ reviews the applications for WBGs in battery electric vehicles and hybrid electric vehicles.
While there’s no strict dividing line between the uses of SiC and GaN in EVs, it’s been generally true that SiC has been used for higher voltage and wattage applications like the traction drive inverter in a conventional EV and the bidirectional dc/dc converter where power levels can range from 60 to 150 kW. Traction inverters in light EVs are often made with GaN. DC fast chargers that are external to the EV also tend to use SiC. Onboard battery chargers (OBCs) and air conditioning (A/C) systems are considered medium power applications, up to about 40 kW for the OBC and 5 kW for the A/C. Depending on the design goals, OBCs and A/Cs can employ either SiC or GaN. GaN tends to be used in applications up to several hundred volts while SiC dominates at 1 kV and above (Figure 1).
Figure 1. GaN is often found in applications up to several hundred volts while SiC is commonly found at 1 kV and higher (Image: SMicroelectronics).
Additionally, GaN has about the same thermal conductivity as Si, while SiC is over twice as good at conducting heat. SiC also has a relatively low thermal coefficient of expansion, which makes it less prone to damage from high temperatures and thermal cycling. In many applications like OBCs, the choice between GaN and SiC is not always clear-cut.
Shifting OBC designs
There are two classes of OBCs: 7.2 to 11 kW single-phase, and 22 kW three-phase. One of the challenges related to designing OBCs is that EV battery pack voltages are rising. With a 400 V battery pack voltage, GaN is often selected for OBCs, and for the emerging 800 V EV battery packs, SiC can be preferred. That picture, however, is changing, and GaN is challenging SiC for higher voltage designs.
A recently introduced 11 kW, 800 V OBC reference design using GaN switches delivers higher power density compared with a SiC solution. The GaN OBC uses a three-level topology that delivers improved switching performance and cuts the voltage stress on the transistor in half. In addition, the design uses an insulated metal substrate (IMS) interface for improved thermal performance.
48 Vdc for HEVs and more
The continued advancement of 48 Vdc mild hybrid EVs (HEVs) is also driving GaN adoption. The use of GaN enables power converters to switch at 250 kHz compared to 125 kHz for Si-MOSFET-based converters. Auxiliary functions like variable speed A/C, electric steering, and suspension use 48 Vdc power. In those applications, GaN offers increased efficiency, smaller size, and lower cost compared with Si-based solutions.
Figure 2. 48 Vdc mild HEV power bus architecture (Image: Efficient Power Conversion).
Summary
WBGs are key technologies in EVs. They enable smaller and lighter power converters and higher conversion efficiencies that combine to improve EV range. While there are places where SiC and GaN compete, there are also clear use cases for both technologies; for example, SiC is used in 1+ kV applications, and GaN fits well into many 48 Vdc designs.
References
- GaN-Based 800V On-Board Charger (OBC) Reference Design, GaN Systems
- How Silicon Carbide Technology Changes Automotive On Board Charging, onsemi
- Hybrid and Electric Powertrain 48 V, Efficient Power Conversion
- SiC power modules for your EV design, STMicroelectronics
- Wide Band Gap Devices and Their Application in Power Electronics, MDPI energies
Filed Under: FAQs, Featured, Onboard Charging, Power Electronics, Transistors