Aly Mashaly, Rohm Semiconductor
As the automotive industry moves towards zero-emissions transportation, auto manufacturers are swiftly ramping up their electrification programmes. Most OEMs plan to be ready to supply large volumes of battery electric vehicles (BEVs) as well as hybrid electric vehicles (HEVs) worldwide by 2025.
To meet buyers’ expectations in regard to range, charging time and performance, these electric vehicles require power electronic systems that enable efficient and effective operation at increased coolant temperatures. Because of its physical properties, silicon carbide (SiC) – a semiconductor material – has great potential to meet the demands associated with these market trends. As a result, SiC power semiconductors are now being more widely developed. Preparations for anticipated large volumes of SiC components are in full swing. As market leader in the SiC field, back in early 2018 Rohm announced it was increasing production capacity for SiC components to sixteen times the existing capacity by 2025.
Since 2010, various aspects of the available silicon carbide products have undergone extensive investigation in numerous research and development projects. Today, the value of SiC is undisputed among power electronic experts. Despite price differences between SiC and Si semiconductor components, the integration of SiC at system level pays off. The cost benefits have been demonstrated in a wide range of comparisons. In addition, silicon carbide has reached a degree of maturity that has led to SiC being treated as a potential solution for power electronic systems in applications with high reliability requirements – such as in the automotive sector. Interest in SiC is therefore growing very rapidly at the present time.
One of the potential applications for SiC is the inverter for the main drive system in electric vehicles. To analyse the advantages of silicon carbide in the powertrain, in 2016 Rohm launched a partnership with Monégasque motor racing team Venturi for the Formula E championship series. The drive power is specified in advance by the International Automobile Federation (FIA). In seasons 3 and 4, up to 200 kW (268 bhp) of power was available. From season 5 onwards, the cars will have a power output of up to 250 kW (335 bhp).
Advantages of SiC over Si
By using SiC power components in the powertrain, it is possible to increase the overall efficiency of the powertrain (see fig. 1). The Rohm BSM600D12P3G001 G-type module was used in the SiC-based inverter. This half-bridge module is based on SiC trench gate MOSFETs and SiC Schottky barrier diodes (SBDs). At a junction temperature of 150 °C and rated current of 600 A, the total switching losses of the G-type module with SiC components are around 65% lower than with IGBT modules. Because of the lower switching losses, the SiC module can operate at high currents with a much higher switching frequency.
Figure 2 shows two drive inverters. The inverter on the left has a rated output of 200 kW and uses power modules based on Si IGBTs and Si fast recovery diodes (FRDs). The inverter on the right was built using the newly developed SiC modules (G-type), and has a rated power output of 220 kW. Thanks to the SiC MOSFETs and SiC SBDs, a better concept for the motor control strategy, an efficient cooling system, a low-inductance busbar design and a compact DC link capacitor were successfully implemented. Both inverters are water-cooled and both types can be used with battery systems up to 800 V.
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Figure 2 – Comparison of two drive inverters: left 200 kW “IGBT-based”, right 220 kW “SiC-based”
As the table shows, the SiC-based inverter is more efficient and has a higher power density. It also achieves a power difference of around 20 kW compared to the IGBT-based drive inverter, which means more traction for the vehicle while driving.
Higher efficiency instead of a bigger battery
Among the various vehicle manufacturers, there is an increasing trend for higher battery capacity. This results from the attempt to solve the problem of battery range in electric vehicles.
In the drive system, both the inverter and the motor consume a substantial proportion of the energy stored in the battery. So any increase in the efficiency of the powertrain would bring benefits. Figure 3 shows the economic benefits of SiC depending on battery capacity in 2025.
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Figure 3 – Cost advantages of SiC for the powertrain in 2025
Based on the inverter efficiency mentioned above and a standardised driving cycle for passenger cars – the Worldwide Harmonised Light Vehicle Test Procedure (WLTP) – the possible capacity improvement is up to 5%. The 400 Arms inverter only offers an economic benefit with batteries larger than 32 kWh. Since the market for SiC is growing rapidly at the moment, it is expected that prices for SiC MOSFETs will soon move closer to prices for Si IGBTs, bringing cost advantages. It is highly probable that SiC components will displace Si IGBTs, not in all but in many applications.
It is therefore anticipated that with the same battery capacity, using SiC in the powertrain will achieve a longer range and/or a considerable reduction in battery size and hence battery weight as well as cell costs for the same range. In this way, efficiency and weight improvements will bring a cost benefit to car buyers.
Trends in brief:
In electromobility applications, power semiconductors based on silicon carbide offer a series of practical advantages over silicon-based components: higher efficiency enables more power and/or smaller batteries or an increase in range, and all with a more compact overall design.