Electric vehicles: Considering silicon carbide over silicon
Until recently, silicon carbide ( name="_Hlk39588110">SiC) has not been extensively tapped for use as a semiconductor, compared to nearly ubiquitous silicon (Si) and gallium arsenide (GaAs) semiconductors. Optimized to harness or limit stray inductance, SiC semiconductors offer several advantages in power electronics. The adoption of SiC-based power electronics could be a major differentiator in next generation electrified vehicles (EVs). It might be said that power electronics make or break EVs and are just as important as battery chemistry and electric motor-generator (MG) construction relating to overall energy efficiency, available power and range per cycle.
Silicon vs. silicon carbide transistors
Silicon semiconductor insulated-gate bipolar transistors (IGBT) have long been paired with flyback diodes in industrial traction drives, voltage inverters and power transmission devices. For electric mobility companies, implementing silicon power electronics was the logical choice when engineering traction systems for low- and high-power EVs.
Wide bandgap (WBG) SiC metal-oxide-semiconductor field-effect transistors (MOSFET), though, offer several advantages over silicon semiconductors. For the same capacity, SiC MOSFETs, at just one-tenth the layer thickness, can operate at higher temperatures. Silicon carbide exhibits linear conductance losses across the power band and low switching losses allow for more consistent high-frequency operation.
Still, uptake of SiC MOSFETs has been slow. Despite its advantages, complex SiC wafer production elevates pricing. With technological advances in electric mobility and increasing demand for high-efficiency power electronics, SiC MOSFET costs are expected to fall.
The advantage of silicon carbide in EV applications
Engine or motor power and efficiency are always at odds, whether internal combustion (IC) or MG. More power is needed for heavy acceleration, so vehicles are almost universally overpowered. Conversely, the more powerful the IC or MG, the less efficient it is while cruising or under light acceleration. For this reason, hybrid EVs (HEVs) demonstrate a good balance, improving both low-power MG efficiency and high-power IC efficiency. For HEV and battery EV (BEV) traction drives, SiC power electronics demonstrate even higher efficiency potential.
SiC clearly has the advantage at higher switching frequencies, exhibiting fewer conductance losses during certain operation modes. SiC MOSFET losses increase linearly, while Si insulated-gate bipolar transistor (IGBT) losses increase logarithmically (Figure 1). Up to about 50% power, SiC MOSFET power savings are dramatic, up to 75% more efficient than comparable Si IGBTs.
At low switching frequencies, around 10 kHz, SiC MOSFET inverters are exceptionally efficient. Compared to an Si IGBT, the SiC MOSFET runs up to 75% more efficiently. With inverter losses tacked on to other system losses (Figure 2), one can see a clear advantage of the SiC MOSFET power inverter. Assuming energy consumption of 20 kWh/100 km, this translates to about 5% energy savings.
Considering the efficiency gains over an Si IGBT power converter, equipping an EV with a SiC MOSFET power converter could require a battery pack that is 5% smaller and lighter or with a 5% increased range per cycle.
From chip manufacture to automobile manufacture, 3% to 11% energy savings have been noted. As SiC production costs continue to fall, its efficiency advantages will be easier to implement. Especially in high power categories, as seen in Tesla Model 3 sedans and Formula E racers, which run upwards of 600 V DC (volts direct current).
Method to limit or harness stray inductances
Increasing switching frequency in a SiC MOSFET converter results in further advantages, but construction is critical. Take, for example, a two-level main propulsion converter containing six SiC MOSFETs (Figure 3). Stray inductance, due to the proximity of individual SiC MOSFETs, results in lower efficiency and decreased performance. Additionally, this stray inductance can cause overvoltage stress and premature module failure. Typical remedies increase weight and costs, mitigating any possible benefit.
Constructing the load current paths parallel on the DC side is one way to address stray inductance. Using wide, flat conductors in parallel results in strong magnetic coupling and lower stray inductance — the entire magnetic field from one conductor saturates the opposite conductor in the load path. Consequently, semiconductor switching DC+ to DC- and vice-versa results in little change to the magnetic field. Harnessing these stray inductances improves efficiency.
Further, individual SiC MOSFETs should be physically arranged to allow for parallel conductor layout, as shown in Figure 4. If flat copper bar conductors are laid out in parallel, it becomes easier to isolate DC+ and DC- connections. Because automotive batteries are limited to 1,200 V DC, only a thin layer of insulation is necessary and is easily implementable. For EV modules, stray inductance values below 10 nH (nano Henrys) can be achieved.
Aside from the layout of the conductors in the converter module, parasitic inductances impact current in individual MOSFETs, limiting module performance. Just as parallel conductors reduce stray inductance and improve power slew rates, laying out SiC MOSFETs and their connections in parallel results in similar stray induction drops within the converter module. Preliminary testing suggests internal stray inductance can be depressed lower than 6 nH per switching path. Optimized layout and construction of the SiC MOSFET converter results in total stray inductances under 20 nH, allowing engineers to take full advantage of SiC’s benefits.
Further analysis implementing SiC MOSFET converters
As SiC MOSFET efficiency is pushed higher, further research on the AC MG side of the circuit should reveal more performance and efficiency gains. Preliminary studies suggest shorter cable paths and higher switching frequencies can reduce stray inductance and eliminate the need for additional low-pass filters on the MG. Developing the ideal arrangement will ultimately result in lighter and more economical HEVs and BEVs, with better range and power potential.
About ABB Semiconductors
ABB Semiconductors has been manufacturing power electronics for over a century. At plants in Lenzburg, Switzerland, and Prague, Czech Republic, ABB covers design, sourcing and manufacture of the highest quality, most reliable semiconductor packages used in rail, industrial and energy sectors. At the PCIM (Power Conversion and Intelligent Motion) conference, October 2019, ABB Semiconductors demonstrated RoadPak, a conversion of high-efficiency rail-application SiC MOSFET power electronics modules to smaller EV applications.
For more information on ABB Semiconductor, visit the ABB Semiconductor website.