Automotive Technology

SiC MOSFETs – The Perfect Fit For Modern Electrical Vehicles

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The strive for increased energy efficiency, lighter and smaller applications as well as optimised system cost has been determining developments in power electronics for years. Optimisation of silicon based solutions like MOSFETs and IGBTs in the high voltage range (above 600V) is close to reach its limits. Due to the increasing demand for HEV/EVs as well as compact solutions for solar inverters, industrial drives and high frequency power supplies semiconductor manufacturers need to find new ways to achieve the goal of better efficiency.

Wide band gap (WBG) technologies like SiC and GaN can offer significant advantages over silicon-based (Si) power transistors. Thus those materials are the basis for a variety of new products like STMicroelectronics’ SiC MOSFET range.

WBG Characteristics & Advantages

The band gap of WBG materials is about three times the one of Si materials. This is the reason for the capability of withstanding higher breakdown voltages as well as the reduction of the temperature dependency of those innovative technologies. Furthermore, WBG materials show higher electron saturation velocity parameters which results in higher maximum switching frequency and smaller switching losses of the final component. It might be worth noting that this parameter is even higher for GaN materials than for SiC which is the reason that GaN is widely used in high switching frequency applications like audio amplifiers.

SiC allows for higher breakdown voltages without compromising on-resistance compared to GaN and Si due to the optimal critical electrical field parameter Ec (4H-SiC = 3*106; GaN = 2.2*106; Si = 3*105). Therefore SiC is well suited for applications like HEV/EV inverters, smart power grid or rail traction. 

Good thermal conductivity is another advantage of WBG power switches as higher temperatures can be reached in a safe manner compared to Si semiconductors.

In a nutshell WBG devices offer improved on-resistance/area and lower leakage current compared to Si devices. SiC is capable of handling higher breakdown voltages and GaN materials allow for higher switching frequencies thus both technologies offer individual advantages in different market segments and applications.

SiC in HEV/EVs

The features of SiC MOSFETs are highly relevant for the automotive segment and especially for EVs. Car manufacturers are pressured to develop electrical drive trains which offer the complex combination of long range, high power and efficiency. Reducing the environmental impact is a challenge other high power segments share, however the automotive segment is getting the most attention due to governmental and environmental demands.

Using SiC MOSFETs is a feasible method for designers in order to realise modern EV applications meeting the new market requirements. Comparing an ST 1200 V, 80 mΩ  SCT30N120 SiC MOSFET with a trench field-stop IGBT of the same voltage rating and equivalent on-resistance exposes the significantly reduced switching losses, even at high temperatures of the WBG components (see graphic below).

Switching loss comparison SiC MOSFET vs. IGBT (image: ST)

The high operating temperature as well as the low power losses of the SiC MOSFET help designers to reduce the heatsink size. In addition the improved switching losses enable operation at very high switching frequencies. This in turn helps to reduce the size of passive components. Furthermore, the very fast and robust intrinsic body diode of ST’s SiC MOSFETs eliminates the need for an external freewheeling diode.

SiC – Cost Saving Potential & Outlook

The features described above result in increased power density, higher efficiency, smaller size and reduced weight of the system. In addition to efficiency the costs of EVs are an important success factor. Therefore it is worth noting that SiC technology offers the potential to decrease system and product costs.
Possibilities to cut expenses vary from design to design. In general, costs can be reduced in terms of passive components and cooling system of the application. Due to the higher degree of efficiency as well as the weight reduction other components of EVs can be reduced in size and price as well (e.g. shrinking battery size). 
Looking at the expenses related to EVs in a more comprehensive way there is additional potential for savings. Lighter and smaller components and applications allow to cut expenses within the supply chain (e.g. warehouse and transportation costs).

Numerous market segments face similar challenges of improving energy efficiency and implementation of sustainable energy sources. Thus there is a growing demand for high voltage switching applications. There is no doubt that WBG devices will play a significant role in those applications in the future.

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cover image: Tesla/Alexis Georgeson