Bench Talk

Charging an electric vehicle (EV) can be a lengthy process, usually occurring as an overnight process from a home-mounted AC supply. However, Level 3 “fast” DC charging techniques promise much faster-charging capabilities that reduce charging times to minutes rather than hours. In this blog, we will explore how the efficiency of conversion relies on high-speed power conversion and how new wide-bandgap technologies are well suited for this task.

EV adoption is growing and has the potential to accelerate even faster as potential EV adopters start to do their research. The US Energy Information Administration (EIA) forecasts growth from 2018 to 2050 of 29% for the combined categories of 160.9km, 321.8km, and 482.8km range passenger EVs. Thanks to federal and state initiatives, as well as incentives promoting the viability of owning an electric vehicle, consumers considering buying a new car are far more likely to include an EV option on their shortlist. Whether would-be consumers simply consider it the “right choice for Planet Earth,” or their research takes a more detailed and informed approach, the chances are that range is one of the key selection criteria. Those who drill down a little further then ask the question of how long it takes to recharge the vehicle. The vast majority of vehicle owners equate the recharge process to refueling the vehicle with gas, something that typically takes no more than 10 to 15 minutes. Today, however, most vehicles rely on an onboard AC charging approach that can take overnight or at least many hours (Table 1). Most deployed EV charging infrastructure across the nation is currently Level 1—typically from a home supply—or Level 2—three-phase parking lots and retail locations. The Society of Automotive Engineers (SAE) defined these different levels of charging. SAE standard J1772 sets the charging plug and socket arrangement for Level 1 and Level 2. For Level 2 and Level 3, SAE stipulates a combination plug and socket format.

Table 1: The Types of EV Charging Stations table outlines the levels of EV charging, charging times, and power requirements. (Source: ON Semiconductor)

As Table 1 illustrates, in order to recharge an EV in a duration similar to filling the tank with gas, you need a Level 4 charger and a vehicle capable of being DC-charged. The amount of power involved in Level 4 charging is very high and moves the design emphasis away from the vehicle’s onboard AC-DC charger to a high-power, high-efficiency DC charging infrastructure. Today, Level 4 charging infrastructure is technically possible, but would place significant demands on the local power grid distribution network, which is why Level 3 chargers are a promising solution with their balance of charge time, cost, and grid load.

Level 3 stations, also known as “fast” charging stations, can supply up to a maximum of 500A and require an efficient three-phase power conversion topology for which the Vienna rectifier-based power factor correction (PFC) with a DC-DC converter approach is often used (Figure 1). This method of AC-DC conversion uses three different voltage levels from the grid three-phase supply and is an efficient, high-density, low bill of materials (BOM) method of achieving the desired output power.

Figure 1: The illustration depicts an EV Level 3 charger using a Vienna PFC converter topology. (Source: ON Semiconductor)

Despite the benefits of using the Vienna topology, the need for higher-power conversion switching frequencies and the resultant switching losses, coupled with the need to manage heat generated by conversion losses, can add up. These, together with the space-constraints imposed by charging locations, mean that power-supply design engineers have been searching for semiconductor process technologies that go beyond the current characteristics and properties of silicon-based diodes and MOSFETs.

Wide-bandgap semiconductor process technologies, such as silicon carbide (SiC), offer fast switching speeds compared to traditional silicon counterparts, which in turn, allows for smaller inductors and capacitors, lowering BOM cost and the amount of board space required (Figure 2). SiC MOSFETs also exhibit much lower RDS_(ON), and hence lower switching loss characteristics, typically a factor of 100 times less than a silicon MOSFET. Overall, SiC devices, thanks to their wider conduction bandgap, have a higher breakdown voltage, typically a factor of 10 times the dielectric field strength of silicon. SiC also has a higher-temperature conductivity, allowing devices to run hotter. Together, all the benefits of using SiC diodes and MOSFETs for a Level 3 charger yield a more compact, higher-efficiency, and higher-performance charging station. The charger circuitry is not only lighter, but the components are likely to cost less too.

Figure 2: The image provides a comparison of material properties and application advantages of SiC devices. (Source: ON Semiconductor)

ON Semiconductor is a leading supplier of SiC-based wide-bandgap diodes and MOSFETs suitable for use in Level 3 chargers. Diodes include 650V and 1200V, available in a wide range of package formats including Decawatt Package (DPAK), TO-220, Direct Bonded Copper (DBC) and baseplate-mounted modules. An example is the FFSH50120A, a 50A, 1200V reverse voltage Schottky SiC diode fabricated in a TO-247-2 package and capable of operating up to +175°C and dissipating up to 730W.

The SiC MOSFET range includes the 1200V automotive grade AEC-Q101 certified N-channel NVHL080N120SC1 through-hole mounting device that can continuously deliver up to 44A and has a maximum RDS_(ON) of 110mΩ.

Silicon carbide-based wide-bandgap diodes and MOSFETs exhibit the perfect performance characteristics for use in Level 3 charging stations. Their high-speed switching credentials, compact dimensions, and robust attributes make them the ideal choice for designing high-power, energy-efficient and compact chargers.