“Silicon carbide is driving further development of electric vehicles, enabling them to have lower costs, longer ranges, more spacious designs, and higher power densities. Compared with standard internal combustion engines, electric vehicles do not require a fuel tank and engine, so more differentiated designs can be developed to make more efficient use of interior space and a more comfortable ride. However, the market share of BEVs in global new vehicle sales remains low due to model limitations, long charging times, insufficient fast-charging infrastructure, and high prices.
New product introduction
In 2021, Infineon introduces a new HybridPACK™ Drive CoolSiC™ power module. This is a full-bridge module with 1200V blocking voltage optimized for traction inverters in electric vehicles. This product enables higher efficiency for inverters with longer range and lower battery costs, especially for vehicles with 800V battery systems and larger battery capacities. Click to read the original text at the end of the article to learn more about new products.
In the current market, SiC and IGBT still have their own characteristics. This article analyzes their technical differences in detail, as well as the advantages brought by using SiC in main inverters, OBCs and DC-DC converters.
Silicon carbide is driving further development of electric vehicles, enabling them to have lower costs, longer ranges, more spacious designs, and higher power densities. Compared with standard internal combustion engines, electric vehicles do not require a fuel tank and engine, so more differentiated designs can be developed to make more efficient use of interior space and a more comfortable ride. However, the market share of BEVs in global new vehicle sales remains low due to model limitations, long charging times, insufficient fast-charging infrastructure, and high prices.
The high cost of materials and components is the main reason for the high price of pure electric vehicles. If an electric powertrain is defined as consisting of a battery, an electric motor and an inverter, the powertrain accounts for about 50% of the total cost of a pure electric vehicle. In terms of powertrain, the battery cost accounts for more than 60%. That is to say, the cost of the battery accounts for more than 35% of the cost of the whole vehicle.
Increasing the power density of electric powertrains is one way to reduce costs. The US Department of Energy has set a goal of increasing the power density of high-voltage power electronics by a factor of seven by 2025. However, due to the limited installation space, especially for high-performance vehicles, high power density is even more necessary. Because increased power density can reduce the size of powertrain components, which further optimizes vehicle interior space.
Currently, power modules using silicon IGBT technology dominate EV applications. However, after decades of development, silicon-based power devices are approaching the material limit. Therefore, it is very difficult to further increase its power density.
Therefore, the semiconductor industry has been developing wide-bandgap power devices such as silicon carbide MOSFETs. The power density targets set by the US Department of Energy are based on the utilization of wide-bandgap power devices.
Wide-bandgap power devices are more expensive than silicon devices, but can reduce overall powertrain costs due to reduced size and weight of power components, especially battery capacity savings within the same range.
Difference analysis of SiC and Si technical characteristics
Silicon carbide has become an alternative to silicon in power devices. Wide band gap, higher breakdown electric field, improved thermal conductivity, and higher operating temperature are 4 key advantages of SiC:
The band gap of silicon carbide is 3 times larger than that of silicon, which translates into a 10 times higher breakdown electric field. When designing unipolar devices with high voltages (usually 1200V or higher), such as MOSFETs, silicon carbide can benefit greatly.
The thermal conductivity of silicon carbide is 3 times that of silicon and similar to that of copper. Therefore, the heat generated by the power loss can be conducted away from the silicon carbide with a small temperature change.
Due to the higher melting temperature, silicon carbide devices can theoretically operate well above 200°C. Since cooling requirements are significantly reduced, the cost of cooling systems can be significantly reduced.
Silicon carbide devices have thinner drift layers or higher doping concentrations due to higher breakdown electric fields. Therefore, they have lower resistance compared to silicon devices of the same breakdown voltage.
Silicon carbide can be used to design unipolar devices, such as high voltage MOSFETs, that theoretically do not generate tail currents. Therefore, compared to silicon IGBTs, silicon carbide MOSFETs have lower switching losses and higher performance body diodes, enabling faster switching frequencies.
Silicon carbide devices can operate at higher temperatures, up to 200°C or higher. However, packaging technology limits the maximum operating temperature. To make SiC operate at high temperatures, many new packaging technologies are being developed.
Silicon carbide devices have a smaller die area and generate less gate charge and capacitance, enabling higher switching speeds and lower switching losses.
Silicon carbide MOSFETs can operate at high switching frequencies, enabling smaller magnetics and lower power losses. Low power losses combined with high operating temperature and high thermal conductivity reduce cooling requirements, resulting in smaller cooling systems. In power converter applications, high switching frequencies can also reduce output capacitors.
The use of SiC MOSFETs in high voltage applications (eg above 600V) allows a simplified topology due to the high breakdown voltage, whereas the topology chosen for silicon IGBTs is not as high due to their breakdown voltage typically in the range of 650V to 750V. all the same. The simplified topology requires fewer components, i.e. fewer power switches and gate drivers, and less design effort in terms of control algorithms.
Individual SiC power devices cost more than their silicon equivalents, but using SiC devices can save system cost because fewer components are required, smaller passive component size, smaller cooling system, and the same mileage range. Smaller battery capacity and less design development effort.
Application of SiC in main inverter, OBC, DC-DC
As mentioned above, SiC power devices offer significant system advantages in terms of power density, efficiency and cooling operation due to their lower losses compared to silicon IGBTs. Silicon carbide’s benefits are particularly evident in applications such as main inverters, on-board chargers (OBCs), and DC-DC converters.
The main inverter not only drives the electric motor, but is also used for regenerative braking and feeding energy back to the battery. This means that the main inverter ensures bidirectional energy transfer between the battery and the motor. Car chargers are AC to DC power converters used to charge the battery. DC-DC converters transfer energy from one voltage level to another.
The diagram below shows a DC-DC converter (high voltage to low voltage) that converts energy from a high voltage battery to low voltage energy, charges the low voltage battery and powers a 12V Electronic system. In other electric vehicles, such as fuel cell based vehicles, there are other types of DC-DC converters.
Silicon carbide brings higher inverter efficiency, smaller system size, lower system cost and longer driving range for main inverter applications. Both on-board chargers and DC-DC converters are power applications for which SiC offers higher switching frequency FSW, higher efficiency, bidirectional operation, smaller passive components, smaller system size and lower power consumption. system cost.
Daimler and Infineon have collaborated on a study on the advantages of silicon carbide in main inverter systems. The study used the package form of Infineon’s automotive-grade power module HybridPACK™ Drive, one based on 750V EDT2 IGBT technology, and the other using 1200V CoolSiC™ silicon carbide MOSFET technology.
Energy consumption comparisons are conducted using the WLTP cycle on 400V and 800V 240kW electric SUVs.
The study shows that, under the same driving conditions and mileage, in a 400V system equipped with a 1200V SIC MOSFET, the energy consumption of the inverter is reduced by 63%, resulting in 6.9% energy saving in the WLTP drive cycle.
In an 800 V system equipped with 1200V SIC MOSFETs, the inverter energy consumption is reduced by 69% and the vehicle energy consumption is reduced by 7.6%. The reduction in vehicle energy consumption by SiC is still underestimated because the impact of weight reduction in the battery system is not considered.
So, what are the benefits of using silicon carbide in terms of cost? Silicon carbide inverters are more expensive than their silicon equivalents. However, according to the aforementioned reduction in energy consumption, the efficiency of the vehicle system is increased, thus requiring less battery capacity. System cost savings of up to 6% can be achieved as battery cost savings outweigh the added cost of SiC.
On-board chargers typically have two units: an AC-DC boost topology “power factor correction” (PFC) unit, followed by an isolated DC-DC unit. PFC can be implemented with various topologies such as classic boost and totem pole. Compared to the classic boost topology, the totem-pole PFC exhibits higher power density and efficiency because it has a bridgeless PFC that greatly reduces the number of diodes.
Infineon has also studied the benefits of SiC in PFC cells. The devices studied are a silicon-based 650V TRENCHSTOPTM F5 IGBT and a silicon carbide-based 1200V CoolSiC™ MOSFET.
Power loss comparisons were made on a 3.3 kW totem-pole PFC at 400V output. Using 1200V SiC MOSFETs reduces power consumption by 52%. However, the power consumption of 1200V SiC devices is still underestimated, as a fair comparison should be made with 650V SiC devices. The 650V SiC device has lower resistance and lower conduction losses than the 1200V equivalent. In any case, silicon carbide MOSFETs can achieve higher efficiency.
High-voltage-to-low-voltage DC-DC converters in pure electric vehicles typically convert up to 3 kW of power and require high efficiency. There must be isolation between the high voltage battery and the low voltage system. Due to its high efficiency, isolated resonant converters are a good application.
DC-DC converters operate at partial load most of the time. Such as 10% to 20% load, which makes the efficiency of part load critical.
Infineon’s high-performance silicon-based CoolMOS™ CFD7 superjunction MOSFETs achieve good efficiency. The use of next-generation CoolSiC™ technology can further improve efficiency, especially when operating at part loads.
Fuel cell electric vehicles powered by hydrogen are another vehicle with huge market potential. There are two types of high voltage DC-DC converter applications in fuel cell electric vehicles.
In a typical fuel cell system, there is a DC-DC boost converter that boosts the voltage of the fuel cell stack to power the inverter system. Another DC-DC bidirectional converter feeds the battery energy to the inverter system and also uses the regenerated energy from the motor to charge the battery. In addition, fuel cell vehicles have inverter systems of similar specifications to other electric vehicles.
The power density and efficiency of DC-DC converters and inverter systems can be improved by using silicon carbide power devices. The end customer will benefit from less hydrogen consumption, as the price of hydrogen is still high, or, with the same amount of hydrogen, the car can achieve a longer range.
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