Car manufacturers are working on fast-charging systems aimed at facilitating the use of electric cars. Wide-bandgap semiconductor SiC is central to those efforts...
There is an opportunity for remarkable growth in the market for hybrid and electric vehicles (H/EVs), but we must innovate our way past some technological barriers on the sustainable-mobility landscape if we hope to see electrically powered vehicles become commonplace on European roads. Much of the public still needs to be persuaded of the efficiency of electromobility. Toward that end, several car manufacturers are working on fast-charging systems aimed at facilitating the use of electric cars. Wide-bandgap semiconductor silicon carbide (SiC) is central to those efforts.
DC fast-charging stations are an interesting field of application for SiC modules. To achieve the ambitious goals on power density and system efficiency that are being set by industries and governments, SiC transistors and diodes are needed. But developers must ensure a correct approach to the fast-charging system, with sufficient insulation and appropriate modularity.
Battery charging is a mostly constant-current application with a low demand for dynamic power. The main trend here is attaining the highest possible efficiency throughout the battery charge cycle. Today, 15- to 20-kW units use discrete components in 19-inch × 3U × 800-mm modules with forced-air cooling. New infrastructure is targeting DC chargers exceeding 350 kW, leading to the use of liquid cooling to enable a power growth increase per sub-unit to 60 to 75 kW in even smaller form factors.
The power supply blocks of the charger consist of an AC/DC front end followed by a DC/DC converter to provide the charging voltage to the battery. The AC/DC section converts the power supply from the distribution network to a useful DC voltage, avoiding ripple fluctuations. The DC/DC converter provides electrical isolation from the vehicle chassis for safety reasons while providing the necessary DC-charging voltage to the vehicle.
By replacing silicon-based designs using IGBTs or MOSFETs in the AC/DC block of the charger with SiC devices, the circuit design is simplified while the power density and, hence, the efficiency are significantly increased, enabling reductions in parts count and in system size, weight, and cost.
With a simple change in the control software, the SiC block can also enable the bidirectionality needed to allow the vehicle battery to become part of a smart grid. Enabling such bidirectionality with a silicon solution would require the use of more hardware in a far more complex circuit design.
Because the TO-247 and TO-220 formats can be used for packaging, SiC devices also enable rapid replacement of silicon IGBTs and MOSFETs with the new SiC alternatives. In contrast, devices built with another wide-bandgap semiconductor, gallium nitride (GaN), have better results with surface-mount–device (SMD) formats. While SMDs are lighter and smaller, they cannot serve as swap-in replacements, relegating their use to new projects.
Infineon offers a pair of power modules that can be used in combination for 50-/60-kW EV charging solutions. The Easy 1B (F4-23MR12W1M1_B11) integrates a four-pack topology for the DC/DC stage of the charging station. The Easy 2B (F3L15MR12W2M1_B69) has a three-stage configuration that is well suited for the Vienna Rectifier, which is common for the power-factor correction (PFC) stage in this application.
The modules use Infineon’s CoolSiC diodes, rugged and efficient devices that were designed to meet requirements for use in hybrid and electric vehicles. An improvement on Infineon’s last-generation Schottky diodes, the CoolSiC diodes have better figures of merit, minimizing power losses.
The use of SiC in the drivetrain also ensures greater efficiency and, by extension, vehicle autonomy. AC Propulsion took advantage of high-performance SiC FETs to hit all the system power targets for an EV traction inverter design. The company designed in UnitedSiC’s UF3SC120009K4S, a 1,200-V, 9-mΩ SiC FET delivering improved efficiency over competing SiC devices in three-phase AC traction inverters for EVs. The devices returned >99% efficiency in AC Propulsion’s design, even when switching at frequencies of >20 kHz and at 2× the frequency of IGBTs.
The UF3SC120009K4S is packaged in the TO-247 format, making it a cost-effective drop-in replacement for silicon equivalents. Its efficiency allows the use of a self-contained heat sink. The SiC projects and devices discussed here demonstrate the progress being made on increased efficiency. Over time, such advances will gain consumer confidence for improving EV technology.