How SiC turbocharges EV infrastructure building blocks

Designers of automotive subsystems constantly strive to develop innovative solutions to extend the range and reduce the charging time of electric vehicles (EVs). In the pursuit of these goals, they have pushed silicon-based technologies close to their physical limits in terms of size, weight, and power efficiency and are transitioning to silicon carbide (SiC) solutions to address these challenges. In comparison to silicon, SiC devices offer lower on-resistance, faster switching speeds, and the ability to withstand larger voltages and currents at higher junction temperatures.

The trend toward higher voltages like 800 V within EVs is also driving new designs for traction-inverters, DC-DC converters, onboard chargers, and compressors for heat-pumps and fuel-cells. Here, high-voltage SiC MOSFET’s and diode’s rugged performance are well-suited for EVs, especially in commercial and off-road applications where availability is key.

At the same time, the existing network of 400-V charging infrastructure for the mainstream vehicles will also need to accommodate the newer 800-V vehicle designs. As a result, the increasing need for high voltages is driving the development of booster DC-DC modules in the car to bring the voltage rails together.

SiC technology can also act as the switching element in a solid-state circuit breaker, or E-Fuse, to protect electric components in the vehicle and diagnose fault events before becoming a hard failure. Downtime for repairs and cost can be saved by improved diagnosis and configuration options compared to mechanical solutions.

Next, there is an increasing demand for fast DC charging infrastructure to charge a vehicle quickly. This is particularly important for commercial applications—ranging from trucks and buses to mining and construction equipment—that must work for as long as possible.

Below is a sneak peek at three EV design areas where SiC power semiconductors offer higher levels of power conversion efficiency, power density, and reliability.

1. Solid-state circuit breakers

Using SiC for a solid-state circuit breaker brings several advantages compared to traditional circuit protection solutions. The technology can switch fast using a software configurable trip profile, for instance, via a LIN interface, to interrupt a circuit in microseconds. That’s 100–500 times faster than traditional mechanical approaches because of its high-voltage solid-state design.

The E-Fuse is resettable to avoid the need to replace physical fuses, which provides a reliable, long-term solution if a circuit is regularly interrupted. The potential risks of electric arcs when switching high voltage DC currents with mechanical contacts are eliminated when using a solid-state E-Fuse solution.

Figure 1 The E-Fuse demonstrator comprises 700-V and 1,200-V MOSFET switches alongside current sensing, amplifiers, LIN interface and an 8-bit PIC microcontroller featuring core independent peripherals. Source: Microchip

2. Fast charging

EVs, commercial, and off-road vehicles require fast charging capability. While a car can sit on the driveway overnight to charge, transport busses or construction equipment need to operate effectively throughout the day or night. So, they are moving to battery packs at 800 V or even 1,000 V to provide the power levels necessary for larger vehicles with heavy hauls.

These onboard charger designs mandate higher levels of power, and here, SiC technology can provide an optimal solution. Devices rated at voltages of 1,200 V and even 1,700 V provide developers with higher design margin. This can translate into higher peak performance for the vehicle, less redundancy, and easier manufacturing of elements. The higher efficiency of SiC compared to silicon IGBTs also means smaller heatsinks are needed, reducing the weight of the vehicle.

A technology demonstrator of an isolated 30 kW DC-DC charger, shown in Figure 2, is based on avalanche-rated 1,200-V MOSFETs and 1,200V diodes. The design features >98% peak efficiency, 650–750 V input voltage and 150–600 V output voltage at 50–60 A maximum at 140 kHz switching frequency. The PCB layout is optimized for safety, current, mechanical stress, and noise immunity.

Figure 2 The 30-kW DC-DC converter employs SiC MOSFETs and diodes. Source: Microchip

In addition, power factor correction (PFC) devices are generally required to do the AC-to-DC conversion and to keep the AC input current phase shift within well-defined limits against the AC input voltage, ensuring a near-unity power factor and low total harmonic distortion (THD).

Moreover, in the future, powering energy from the vehicle battery back into the grid will be a required option. This capability of bidirectional charging can be demonstrated by an 11 kW SiC-based PFC design in a Totem-Pole scheme.

3. 150-kW infrastructure charger

Silicon carbide is also key for the charging infrastructure. The same advantages of higher voltages and currents coupled with higher efficiency for smaller cooling elements lead to smaller designs of chargers. While the size of the charger is not as critical for commercial and off-road vehicles that are stored in a depot overnight, it’s relevant for domestic bidirectional DC chargers, which are gaining popularity.

Similarly, public Level 3 DC fast chargers bypass the onboard charger (OBC) of the vehicle to directly charge the battery via the EV’s battery management system (BMS). Bypassing the OBC enables significantly higher charge rates, with charger output power ranging from 50 kW to 350 kW.

Using a modular design approach means a PFC front-end is used for the AC-to-DC conversion, often from higher AC voltages such as 480 V, with a series of isolated DC-DC converter modules in parallel to provide the power to the vehicle.

Figure 3 SiC power semiconductors are becoming critical in EV charging infrastructure. Source: Microchip

This design approach allows a range of chargers to be developed from the basic modules to meet the different requirements of a vehicle operator. As the needs of the vehicles evolve, requiring higher power for faster charging, the charging infrastructure can be varied using SiC devices. This approach is being used for fast charging systems up to 150 kW and for even higher performance systems.

Using digital power management and a combination of SiC MOSFETs and diodes enables designs that offer high system efficiency and integration, high-power density, and advanced digital control loops and increased flexibility in various power topologies for DC fast charger applications. These can be coupled with analog, power management, wireless and wired connectivity, energy metering, memory, security, and human machine interface (HMI) devices to complete a Level 3 DC fast charging design.

Andreas von Hofen is marketing manager at Microchip Technology’s Automotive Products Group.

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