Using SiC to Improve Electric Powertrain Efficiency
The range autonomy of an electric vehicle (EV) depends on a multitude of parameters, particularly battery capacity. A 10% improvement in powertrain efficiency amounts to a similar increase in battery capacity without any added weight and with a much lower extra cost. The development of silicon carbide (SiC) semiconductors that can be integrated into power electronic systems instead of silicon (Si) semiconductors makes this possible. Powerful electronic control units (ECUs) such as those in self-driving vehicles will also benefit from their advantages. SiC semiconductors are already being used by a some high-end makers and their widespread use is not far off. However, the main challenge today is industrial, not technical.
Inverter using silicon carbide (SiC) semiconductors
The range autonomy of an electric vehicle (EV) depends on a multitude of parameters, particularly battery capacity. A 10% improvement in powertrain efficiency amounts to a similar increase in battery capacity without any added weight and with a much lower extra cost. The development of silicon carbide (SiC) semiconductors that can be integrated into power electronic systems instead of silicon (Si) semiconductors makes this possible. Powerful electronic control units (ECUs) such as those in self-driving vehicles will also benefit from their advantages. SiC semiconductors are already being used by a some high-end makers and their widespread use is not far off. However, the main challenge today is industrial, not technical.
The efficiency of an electric powertrain depends on the efficiency of its battery, motor, and inverter. It generally varies between 90 and 97% depending on the current and its frequency. Power electronics account for around 25% of all electrical losses, of which 20% are believed to be directly related to power semiconductors alone.
Used in diodes and transistors, semiconductors control electric current by cutting it off at a very high frequency. This requires a high voltage differential, which causes energy to be lost and dissipated as heat.
Replacing silicon (Si) chips by silicon carbide (SiC) chips reduces the cut-off voltage differential, even at high frequencies. An identical result would be achieved with a drop in circuit resistance.
A sharp rise in value
According to ZVEI, an EV contains about €410 worth of semiconductor chips. Experts estimate that with the deployment of autonomous driving, this figure will increase by about €900.
Eagerly awaited SiCs
STMicroelectronics estimates that average switching function losses are halved when Si chips are replaced by SiCs, and that this benefit increases with the decrease in through-current. For example, the efficiency of an inverter built with SiC semiconductors instead of Si semiconductors is about 2% higher under high-load conditions and up to 10% higher under low-load conditions. If SiC MOSFETs are used, these gains can be as much as 12%.
Delphi Technologies calculates that the average cut-off function losses are reduced by 70%.
In addition, the lower heat release obviates the need for a liquid cooling system, as passive cooling may be sufficient. Another upshot is that the footprints of electronic systems can be reduced by as much as 80% in the most extreme cases; their weights can be also be reduced, albeit to a lesser degree. In addition, the maximum frequency of high current cut-offs is higher — around 200 kHz vs just 20 kHz — which helps to improve efficiency.
For example, Oak Ridge National Laboratory reports that the specific power of a voltage converter is 2.9 kW/kg with SiC vs 1.6 kW/kg with Si, and that its power density is 6.7 kW/L with SiC vs 3.9 kW/L with Si. The maximum operating temperature is also higher: 200 °C with SiC compared to 170 °C with Si.
Comparison of inverter losses between SiC and Si semiconductors (STMicroelectronics)
World firsts
When the first SiC diode was patented, in 1906, by the Englishman Henry Harrison Chase Dunwoody, its use was limited to low currents. It was not until the advent of new manufacturing techniques that SiC diodes could be used at high currents: Cree released the first commercial SiC wafers, in 1991; Wolfspeed released the first commercial SiC diodes, in 2002; and Roma released the first commercial SiC transistors, in 2010.
Toyota also claims that SiC semiconductors can reduce the footprint of power electronics by 80% compared to Si semiconductors, particularly thanks to a reduction in the size of coils and capacitors, which account for about 40% of the size of power electronic systems. The carmaker estimates that the fuel consumption of its hybrid EVs, tested under the Japanese JC08 driving cycle, can be reduced by 10%.
The experts at Infineon claim a 5–10% increase in range autonomy.
In the case of an EV, this higher efficiency represents savings of 1–2 kWh for every 100 km, or an extended range of up to about 35 km for a mid-range saloon equipped with a 50 kWh battery.
Technical challenges
SiC MOSFET by Delphi Technologies. Left: top view; middle: SiC cutaway; right: wafer.
The current technical challenge is to achieve the high reliability rate typically seen in the automotive industry.
The cost is also higher. Semiconductor chips are made from round silicon or silicon carbide wafers. Their manufacturing entails a number of chemical and physical processes to give their surfaces very fine structures that will ultimately form chips measuring a few millimetres thick. In all, this can take up to 14 weeks.
SiC semiconductors are made from 150-mm-diameter wafers and Toyota estimates that one wafer can provide enough chips for six EVs. Future wafers, with a diameter of 200 mm, will lower production costs.
Bosch wafer
An industrial revolution
Another challenge is that the explosion in demand, particularly in the mobile-phone sector, has created a shortage of chips and memory chips that is impacting the automotive industry.
To complicate matters further, companies related to SiC-chip technology are being acquired right and left because SiC chips are becoming integral to today's electric motors. 'The semiconductor market has undergone a drastic change', says Jérome Boudonnet, Director of Automotive Sales at Siemens Digital Industries Software. 'It used to be that there were specialists for each layer. For example, TSMC sold its semiconductors to Qualcomm, which produced chips and then sold them to Samsung. Nowadays, Samsung and Apple are making their own chips'.
The trend among end-product suppliers is for them to develop their own chips. But they're not alone; carmakers are also tapping into this trend. 'Tesla was the disruptor that changed the game', says Jérome Boudonnet. 'It develops its own chips and subcontracts their manufacture to other firms, such as the French company STMicroelectronics. Tesla is following Apple's lead. Faced with the need to integrate and control every layer — semiconductors, electronics, embedded software, RF chips — around its iPhone, Apple has had to make acquisitions and develop its products itself. This control allows it to control the vertical value chain from top to bottom'.
And Tesla is not alone. 'Mazda is currently looking into doing the same and General Motors is developing more and more components', Boudonnet says. 'And it's conceivable that a maker like Tesla could buy a chip fabrication plant'.
The first automotive application of SiC chips was in 2014, when they were added to an onboard charger. In 2018, Tesla added an inverter using SiC MOSFETs to its Model 3, the first carmaker ever to do so. Several carmakers have already announced that SiC MOSFETs will be used in their mass-produced vehicles that will be rolled out in 2020 or 2021.
Various semiconductor applications in power electronics (Infineon)
The next step will be to use gallium nitride (GaN) semiconductors; GaN is a material that is used to produce LEDs. These chips offer an even higher cut-off frequency, but are as yet limited in power. A voltage converter made with GaN chips would offer the advantage of being 40% smaller and 70% lighter for an equivalent power. However, its disadvantage would be a lower thermal conductivity.
Written by: Yvonnick Gazeau
Sources: Delphi Technologies, Infineon, Oak Ridge National Laboratory, Siemens Digital Industries Software, STMicroelectronics, Toyota
