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SiC switches in electric vehicles – will they dominate the drivetrain?

Wide bandgap (WBG) semiconductors are finding applications in all types of power conversion including in electric vehicles. With their promise of higher efficiency and faster switching speeds yielding potential savings in cost, size and energy, WBG devices are commonly used in chargers and auxiliary converters but have yet to displace IGBTs in traction inverters in significant volume. This article explains how the latest-generation SiC FETs are ideally suited to new inverter designs with lower losses than IGBTs and proven robustness against short circuits, even at high temperatures and under repetitive stress.

38% of US cars in the year 1900 were electric vehicles

Yes, you read that right and it’s true1 .. Of all US automobiles in 1900, 38% (33,842) were powered by electricity, 40% by steam and just 22% by gasoline. However, when Henry Ford mass-produced cheap gas-powered cars, the percentage diminished dramatically. Today the percentage of EVs on the roads is less than 1%, but it is predicted that 65%-75% of light-duty vehicles in the US will be electrically powered by 20502 .

Modern electric vehicles (EVs) have improved dramatically since the Toyota Prius hit the Japanese streets in 1997. Sophisticated battery and motor technologies now offer a range of 300 miles and more. The uptake of EVs predicted for 2050 does however rely on certain assumptions: purchase affordability; continuing high oil prices; stricter health and environmental regulations and further technological advances for better range and quicker charging.

An EV has a conversion efficiency of 59%-62% from battery energy to power at the wheels, seemingly giving some scope for improvement. Electrical engineers might roll their eyes and point out that the modern internal combustion engine is struggling to achieve 21%, but at least there is a possible roadmap for better performance from EVs with new semiconductor switches available to use in the drivetrain.

Key to better range is the efficiency of power conversion. This is not just in the motor drive electronics – significant energy is used in auxiliary functions such as lighting, air conditioning and even infotainment systems. Therefore, much effort has gone into reducing the draw from these areas by various measures, such as using LEDs for running lights. The various power converters that step down the main battery voltage, typically from 400 V to 12 V or 24 V for these functions can now include the latest topologies and exotic semiconductors to achieve best efficiency with the risks inherent in new technology acceptable with non-safety-critical applications (Figure 1 ).

Figure 1

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Power conversion components in an electric vehicle (Image source: US Department of Energy)

Power conversion components in an electric vehicle (Image source: US Department of Energy)

For the drivetrain, the motor control electronics is seen as life-critical, so designers are forced to ‘play safe’ and stick to tried-and-tested technologies. In practice this has meant using IGBT switches which have proven their robustness over more than 30 years. For example, beneath the high-tech exterior of a Tesla model S are 66 IGBTs in TO-247 packages controlling the traction motors. The very same IGBTs would have been quite commonplace in a 1980s industrial process controller. Newer models have just started to use SiC FETs.

Wide bandgap semiconductors are now contenders in motor control

IGBTs have been replaced in many modern applications with newer technologies such as silicon MOSFETs and now wide bandgap (WBG) semiconductors fabricated in silicon carbide (SiC) and gallium nitride (GaN) materials. The headline advantages are faster switching meaning smaller external components such as a magnetics and capacitors. The combination gives higher efficiency, smaller size and weight and consequential lower overall costs. WBG devices also operate at high temperatures, typically 200o C for SiC with peak temperatures allowed over 600o C, depending on the device.

SiC FET primers and how they score

A particular type of WBG device is the SiC FET, a composite or ‘cascode’ of a SiC JFET and a Si MOSFET, which is normally-OFF with no bias and can switch in nanoseconds. Compared with SiC MOSFETs and GaN devices, it is very easy to drive and its figure of merit, RDSA, the normalised ON-resistance with die area is excellent (Figure 2 ). The device, because of its vertical construction, has extremely low internal capacitances, making switching transitions extremely low-loss. The SiC FET has a very fast body diode which reduces losses in applications such as motor drives, and does not require the use of external SiC Schottky Diodes.

Figure 2

SiC FET (cascode) RDSA - normalized ON-resistance with die area comparison

SiC FET (cascode) RDSA – normalized ON-resistance with die area comparison

SiC FETs in electric vehicle drives

So why have these miracle devices not made inroads into EV motor control when there is a drive for higher performance solutions? Apart from the natural conservatism of car system designers, there are some practical reasons: a WBG device is seen as expensive compared with an IGBT of similar ratings; motor inductance does not scale down as in DC-DC converters making higher switching frequencies less attractive; and the high switching speed means high dV/dt rates that can stress the insulation of motor windings. Also, there is a nagging doubt about the reliability of WBG devices generally under the harsh conditions of the motor drive with its potential short-circuits, back-EMFs and general high-temperature environment.

The real temptation is the possibility of improving efficiency. This translates to more available energy and better range. Heatsinks can be smaller, reducing costs and weight which again in turn helps extend range. Efficiency is particularly improved under typical operating conditions compared with IGBTs which have a ‘knee’ voltage, giving effectively a minimum power loss which is present under all driving conditions. This is shown in Figure 3 below, where we compare 200A, 1200V IGBT modules using two 1cmX1cm IGBT die vs a 200A, 1200V SiC FET module with two 0.6 X 0.6cm SiC stack cascode die.

Figure 3

Conduction loss for 1200V SiC FETs using 36% of the IGBT chip area. In this 200A, 1200V module, on-state voltage drop with SiC FETs is much lower than the IGBT drop for all currents below 200A both at room and elevated temperatures.

Conduction loss for 1200V SiC FETs using 36% of the IGBT chip area. In this 200A, 1200V module, on-state voltage drop with SiC FETs is much lower than the IGBT drop for all currents below 200A both at room and elevated temperatures.

SiC FETs are uniquely able to provide the lowest conduction losses in a given module footprint. For sure, in a ground-up design, a WBG motor drive could be switched at a higher frequency than IGBTs with sufficient EMI control designed-in, giving all the WBG benefits. Even cost should not be an issue going forward. The die of a SiC FET is much smaller than an IGBT or SiC MOSFET of equivalent ratings for example, meaning more yield per wafer and if the cost savings of smaller heatsinks and filters are factored in, it all starts to make sound economic and practical sense.

SiC FETs have proven reliability

We’re now left with those concerns about reliability – which for some WBG devices are very valid. For example, SiC MOSFETs and GaN devices are extremely sensitive to gate voltages with absolute maximum values very close to recommended operating conditions. SiC FETs on the other hand are tolerant of a wide range of gate voltages with wide margins to absolute maximums.

Short-circuit rating is perhaps the major concern in EV motor drives with IGBTs the benchmark for robustness. Certainly, GaN devices are poor performers here but once again the SiC FET scores. There is a natural ‘pinch-off’ mechanism in the vertical channel of the built-in JFET device that limits current and makes the short circuit gate drive voltage independent, unlike with SiC MOSFETs or IGBTs. The high peak temperature allowed with SiC JFET also allows extended short circuit durations. In automotive applications, there is an expectation that a short circuit should be withstood for 5μs before protection mechanisms kick in. Tests with 650 V SiC FETs from UnitedSiC show at least 8 μs withstand with a 400 V DC bus (Figure 4 ) with no degradation of ON-resistance or gate threshold after 100 short circuit events and at elevated temperature.

Figure 4

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Short circuit performance of SiC FET

Short circuit performance of SiC FET

The other stress that occurs in motor drive applications is back EMF from the motor. Again, GaN is not immune but SiC FETs have very good avalanche ratings with the internal JFET turning on to clamp the voltage as its gate drain junction breaks over. More tests by UnitedSiC show no failures with SiC FET parts in avalanche for 1000 hours at 150o 3 with 100% production tests of avalanche capability as a backstop.

The compelling case

Modern wide bandgap devices such as SiC FETs from UnitedSiC are real contenders for the next generation of EV motor drives answering the demand for better performance, overall cost savings and proven, robust operation in this demanding environment. As a result, SiC is expected to dominate the drivetrain in the coming decade.

References

[1] The history of the electric car

[2] Energy Innovation.org

[3] Robustness of SiC JFETs and Cascodes

[4] United SIC.com

Author Bio

Dr. Anup Bhalla oversees all product development at UnitedSiC and became an investor in the company when he joined in 2012. Prior to joining UnitedSiC, Anup held various product development and marketing positions at Alpha and Omega Semiconductor where he was a co-founder of the Company. He is the author or co-author of nearly 100 patents through his career at Harris, Vishay Siliconix, AOS, and UnitedSiC. He received his bachelors’ degree from the Indian Institute of Technology, Delhi, and his Ph.D. from Rensselaer Polytechnic Institute, both in Electrical Engineering.

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