Benefits and Design Challenges in Shifting EV Bus to 800V

Article By : Stefano Lovati

Although most EVs today use 400-V batteries, there is a marked transition to the 800-V architecture.

The electrification of the world’s vehicle fleet is under way, challenging vehicle designers to maximize range and minimize charging times. Several technical innovations have recently been introduced or are being tested on electric vehicles, ranging from new materials and chemical processes for battery manufacture to new systems for ultra-fast charging.

One solution likely to have a major impact on the way EVs are designed relates to the EV battery voltage. Although most EVs today use 400V batteries, there is a marked transition to the 800-V architecture, affecting an ever-increasing number of newly manufactured vehicles.

400V to 800V: the benefits

The first advantage of the 800-V bus is that it allows speeds and charging times that were previously unattainable. In theory, a 50% reduction in recharging time and a recharging power of up to 350–360 kW are possible. That’s why the automotive sector is progressively migrating to this high-voltage architecture, now only available in some high-end EVs.

There are two possible routes to increasing the power sent to the battery during charging: Increase the voltage or increase the current. The second option is the least advantageous, as it would require larger electrical cables to support a higher flow of current. It should be noted that some fast-charging stations today use liquid cooling systems to avoid damage to the cables or to the charging connectors.

A better solution to increase the power transferred to the battery is to increase the voltage. By virtue of the lower resistance that the flow of electricity encounters as it passes through the conductor, this approach reduces the required cable size. And because there is less heat to dissipate, thermal management is improved.

Charging at high currents becomes particularly insidious if the heat cannot be removed quickly enough. In this case, dendrite crystals can develop on the surface of the anode, with consequent degradation of battery performance and potential risk of failure.

From a physical point of view, the rationale that justifies the advantage of switching to the 800-V bus is linked to the formula:

Shifting the EV Bus to 800 V: Benefits and Design Challenges

where E is the electric energy in joules, V the voltage in volts, I the current in amperes, and t the time in seconds. From Equation I, we can compute time t as follows:

Shifting the EV Bus to 800 V: Benefits and Design Challenges

From Equation II, it follows that with the same current I, the charging time t can be reduced by increasing V.

The challenges

Implementation of the 800-V architecture, however, has significant impacts both on the currently deployed charging infrastructure and on EV design costs.

The vehicles currently produced by Tesla, for example, use a 400-V architecture, on which both the on-board electrical devices and the proprietary charging stations are sized. According to Tesla, the advantages of a possible migration to the 800-V bus on production vehicles would be canceled out by the increased costs required to redesign other on-board devices and to update the more than 33,000 charging stations of the Supercharger network.

Conversely, Tesla is evaluating the possibility of introducing the 800-V solution on EVs currently under development, namely the Semi truck (Figure 1) and the Cybertruck pickup.

Tesla Semi, an all-electric semitruck
Figure 1: Tesla Semi, an all-electric semitruck (Source: Tesla Motors)

Another challenge is the need to review the design of different parts of the EV, starting at the initial stages of the design project. Increasing the battery voltage from 400 V to 800 V requires significant changes to the motor, inverter, conductors, insulation systems, and more.

Among the possible technical limitations, the high-voltage conductors used at 800 V require greater insulation than is required for voltages up to 400 V.

Deploying SiC to ease the transition

To support the increasing number of EVs, a charging infrastructure is needed that reduces charging times to less than 15 minutes for 80% of capacity. Upgrading to an 800-V architecture offers significant benefits, such as higher charge power delivery, lower charge current, reduced cable power losses, lower battery overheating, reduced overall vehicle weight, and, ultimately, reduced costs.

Silicon carbide technology is key to implementing the transition to the 800-V architecture. Compared with traditional silicon devices, SiC power devices offer several advantages, including:

• A 10× higher electric field, which allows for higher blocking voltages in a smaller die area than silicon. This allows SiC MOSFETs to operate with breakdown voltages even higher than 3 kV, while a silicon MOSFET is typically limited to less than 1 kV
• Lower on-resistance (RDS(on)) and lower off-state leakage currents than silicon
• Very low or no reverse-recovery current, combined with switching frequencies up to 5× higher than silicon, increasing efficiency and allowing reductions in the size and weight of capacitors and magnetic components
• Increased thermal conductivity, which gives SiC devices high strength and the ability to withstand high temperatures, thus reducing or eliminating the need for cooling systems

Figure 2 shows on-resistance as a function of voltage for different types of transistors. In theory, SiC-DMOS devices should offer very low RDS(on) values even at very high voltages. In the figure, the real SiC-DMOS devices fall within the area marked with an ellipse. It can be seen that at the same voltage, the DMOS devices exhibit significantly lower RDS(on) values than transistors made with other technologies.

On-resistance vs. voltage for different transistors
Figure 2: On-resistance vs. voltage for different transistors (Source: Rohm Semiconductor)

The properties listed above make SiC devices with 1.2-kV breakdown voltage the ideal solution, from both a performance and a cost perspective, for the implementation of the 800-V architecture on EVs.

SiC applications in 800-V architecture

The block diagram of a typical EV fast-charging station is shown in Figure 3. The AC power is first filtered to suppress spurious components or spikes and is then converted to DC by the inverter (AC/DC converter). This active front-end block gets single-phase or three-phase power from the grid and outputs to DC intermediate voltages.

Block diagram of a fast-charging station
Figure 3: Block diagram of a fast-charging station (Source: Wolfspeed)

The following stage is the isolated DC/DC, which includes a DC/AC and an AC/DC converter. This stage provides the required high-voltage level to the battery and, of course, needs to be properly isolated.

Charging stations usually include 15- to 30-kW modules stacked to reach 150 kW today but are potentially able to provide 350 kW of power. The latest-generation SiC devices, combined with proper packages and circuit topologies, will soon allow deployment of a reduced number of 60-kW blocks.

The block diagram of a generic vehicle in Figure 4 shows how different types of on-board devices can be properly designed (or adapted) to implement an 800-V architecture by replacing traditional silicon-based IGBTs or silicon MOSFETs with SiC power devices rated for 1,200 V or higher. These devices include a traction inverter, main DC/DC converter, on-board charger, and auxiliary DC/DC converter.

How SiC can replace Si in EVs
Figure 4: How SiC can replace Si in EVs (Source: STMicroelectronics)

The future

Increasing the voltage from 400 V to 800 V reduces the charging time of EVs from 40 minutes to less than 15 minutes and makes it possible to improve a vehicle’s efficiency, reduce its weight, and, consequently, reduce its final price. The latest-generation SiC devices are proposed as the ideal solutions for implementing 800-V architectures, thanks to their superior electrical characteristics to silicon in high-voltage applications and to the high reliability and maturity the devices have achieved. However, the increase in the voltage level has significant impacts both on the charging infrastructure and on the EV design, requiring an upgrade of insulation measures.

As matters stand, most car manufacturers appear to agree that future EVs will be based on 800-V architectures. One alternative currently being explored is the practice of “battery swapping” to reduce the time that a driver must spend ensuring that a vehicle is adequately charged. But if charging times of just over 10 minutes are achieved soon with fast-charging technology enabled by SiC-based 800-V solutions, such alternative approaches may prove unnecessary.

 

This article was originally published on EE Times Europe.

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