Innovating the Charging Infrastructure for EV

Article By : M. Di Paolo Emilio

Leveraging SiC, EV charging will switch into high gear when stored power can be returned to the network as needed and once cars can be charged wirelessly or while en route.

Electric cars are becoming popular, with their environmentally-friendly characteristics in terms of qualities, functional simplicity, and above all, energy efficiency. The functional thrust is driven by the electric motor whose construction is simple if compared to the combustion engine. Concerning energy efficiency, the comparison between combustion cars and electric cars is emblematic: The combustion car has an energy efficiency of 16% while the electric one has an energy efficiency of 85%. The electrical nature of propulsion has an advantage over the one based on combustion — the regeneration of energy.

Electricity offers a lot of flexibility, including the use of various forms of energy harvesting that help recharge the battery and therefore prolong the operation of the vehicle itself. Energy harvesting technologies are, therefore, in the foreground of the electric car research and development scenario.

The autonomy of an electric car directly reflects the efficiency of its powertrain and its energy management system. Furthermore, the necessary infrastructure, such as the powerful fast-charging systems that now reach the power of several hundred kilowatts, are likewise required to comply with strictly pre-established size and efficiency limits. By its specific physical properties, silicon carbide (SiC) represents a valid response to these new market requirements.

In hybrid and electric vehicles, the leading electronic power systems are the DC/DC boost converters and the DC/AC inverters. The electronic systems developed for e-mobility range from temperature, current, and voltage sensors to semiconductors based on SiC and gallium nitride (GaN).

SiC Powerful
Today, autonomy and long charging times are significant obstacles to the spread of electric vehicles. For a quick recharge, more power is necessary to make recharging possible in a shorter time. Since the volume of space available inside the vehicle is limited, the battery charging system must offer high power density; only in this way, is it possible to integrate these systems into the vehicle.

At the center of any electric vehicle (EV) or plug-in hybrid (HEV) we can find a high-voltage battery (200 to 450 VDC) and its charging system. The onboard charger (OBC) provides the means to recharge the battery from the AC mains at home or outlets found in public or private charging stations. From a three-phase high-power converter from 3.6 kW to 22 kW single-phase, today’s OBCs must have the highest possible efficiency and reliability to ensure fast charging times and meet limited space and weight requirements.

The need to set up charging stations with a compact and efficient design is common to all fast charging systems, and the current SiC power modules allow the creation of systems with the required power density and efficiency. In order to achieve ambitious goals regarding power density and system efficiency, it is necessary to use SiC transistors and diodes.

The superior electric field strength of the high hardness SiC substrate allows the use of thinner base structures. This makes it possible to reach a tenth of the thickness compared to the silicon epitaxial layers. The trend for batteries is to increase their capacity, and this feature is associated with shorter charging times. In turn, this requires an OBC characterized by high power and efficiency, for example, 11 kW and 22 kW.

With the introduction of the SCT3xHR series, ROHM now offers the most extensive line in the field of AEC-Q101 qualified SiC MOSFETs, which guarantees the high reliability required for the onboard charger and DC/DC converters for automotive applications (Figure 1). STMicroelectronics also has a wide range of AEC-Q101 qualified MOSFETs, silicon and silicon carbide (SiC) diodes, as well as 32-bit SPC5 automotive microcontrollers to enable scalable, cost-effective, and energy-efficient solutions for implementing these demanding converters (Figure 2).

Figure 1: Thermal characteristics of SCT3xHR. (Source: ROHM)

Figure 2: Block diagram of an electric system for an EV. (Source: STMicroelectronics)

Vehicle to Grid
The expected arrival of millions of battery-powered electric vehicles on the road over the next ten years presents a significant challenge for the electricity grid. The need to balance the network is constantly increasing together with the spread of production from nonprogrammable renewable sources.

The intelligent management of car batteries becomes extremely attractive when they are connected to the network through domestic recharging wall boxes, or corporate or public recharging stations. The car battery can be used to supply power to the network as well as to take it, depending on the immediate needs of absorbed power.

The system provides for the return of the energy accumulated in the vehicle, or the withdrawal by the network (toward the batteries) using a remote control. The key technology for the realization of the system is a bidirectional power inverter that is coupled on the auto side directly to the high voltage battery (300 to 500 Volts) and on the low voltage network side (Figure 3).

Figure 3: Vehicle-to-grid (V2G) technology.

Vehicle-to-grid (V2G) technology has the potential to allow a more balanced and efficient electricity grid. It will be key to balance supply and demand that should accompany the increase in electricity demand.

Wireless Charging
An incredibly exciting area is that of wireless charging of electric vehicles, thanks to recharging points located in garages or public parking lots. The charging points do not necessarily have to be precisely aligned with the receiver under the car. In the long-term, an attempt will be made to develop a micro-loading version, which could make it possible to integrate long loading plates and public roads, so as to load EV/HEV vehicles even while they are in motion, but this will depend on the number of difficulties encountered both on the national and local administrative levels.

In order for V2G technology to operate without interruption, to offer the advantages of network stabilization, and to allow vehicles to act as generators and data sources, wireless charging technology must be incorporated not only into the vehicles themselves but also into domestic and urban infrastructures where the vehicles are recharged. This would allow vehicles to be highly available if needed.

Wireless charging based on magnetic resonance technology allows electric vehicles, regardless of type or size, to charge automatically and safely by placing flexible coil on a source pad, using materials such as concrete and asphalt. Wireless power will allow vehicles to recharge and implement V2G technology automatically, constantly allowing incentives and attenuations without the need for human intervention (Figure 4).

Figure 4: Block diagram of wireless charging for EV.

Wideband semiconductor technologies and fast charging stations, supported by digital network capabilities, will help speed the spread of electric cars. As global demand for electric vehicles increases, so will the need to support the charging infrastructure. The innovative charging technologies of electric vehicles can be a catalyst for change, helping to promote the adoption of e-mobility and do a lot to achieve the goal of reducing carbon emissions.

The power electronics for EVs are enriched with SiC power devices that satisfy the need to improve: the energy efficiency of the systems; the strength and power density in electric vehicles; and the high-power applications where high voltage and high power are required — thus making an essential contribution to system performance and long-term reliability. SiC MOSFETs and SiC Schottky Barrier Diodes (SBDs) ensure maximum switching efficiency at high frequencies.

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