Feeding power-hungry EV charging stations


As the electric vehicle (EV) market grows, the challenge for governments and original equipment manufacturers (OEMs) is to promote the adoption of EVs by deploying a comprehensive charging infrastructure. This means deploying an infrastructure that not only supports today’s use cases of mostly short local trips, but also supports faster charging compared to home-based chargers, thus easing concerns about charge times when drivers have to go on longer trips.

 

Currently, the Society of Automotive Engineers (SAE) defines three different levels of charging stations, also known as electric vehicle supply equipment (EVSE). Level 3 EVSE differs from Levels 1 and 2 in that the AC-to-DC power conversion takes place in the charging station, so it’s possible to supply a high-voltage DC line to the battery to shorten the charging time. As a result, the cost and complexity of a Level 3 station is significantly greater. These stations can supply anywhere from 300V up to ~920V at a maximum of ~500A. The approximate charging time will be around 10 to 30 minutes, depending on the energy level in the battery. Unlike Levels 1 and 2, which are more typical of residential installations where EVs recharge overnight, the more expensive Level 3 DC fast charging stations are usually found in public settings and will likely resemble gas stations one day. See Table 1 below.

 

Table 1: Classification of EV chargers

 

Using a Vienna rectifier for Level 3 charging

To support the high power levels of fast chargers, AC-to-DC rectifier designs will need a three-phase AC input power factor correction (PFC) stage. Two popular power topologies for implementing three-phase PFC are a three-phase totem-pole PFC converter or a Vienna rectifier-based PFC converter. Among the two topologies, Vienna rectifier-based converters (Figure 1) are becoming more popular because of their three-level switching implementation, higher efficiency, reduced voltage stress on components and higher power density. This type of rectifier is a unidirectional, three-phase pulse-width modulation (PWM) rectifier.

When compared to a boost-type PWM rectifier, the Vienna topology uses multilevel switching (three levels), which reduces both the inductance value requirement and the voltage stress on the switches by half. This improves efficiency and power density.

 

Figure 1: Vienna rectifier topology for EV charging stations

 

C2000™ real-time MCUs for Vienna rectifiers

C2000 real-time MCUs are controllers designed specifically for power electronic applications and have features that are a good fit for complex control topologies like with a Vienna rectifier. These features include:

  • An optimized central processing unit (CPU) that enables fast execution of the control loop. The on-chip trigonometric math unit (TMU) accelerates trigonometric operations, which imparts additional speed in control-loop execution and reduces the overall million-instructions-per-second (MIPS) requirement.
  • The control law accelerator (CLA) is a secondary core that is available to offload control-type tasks from the main CPU (C28x), thus freeing up bandwidth on the C28x MCU for other operations. Additionally, the CLA can act as a parallel processing unit to run the control loop faster, therefore enabling higher switching frequency control of the Vienna rectifier.
  • The integrated comparator subsystem (CMPSS) integrates protection for overcurrent and overvoltage without the use of external devices, thus making the board lower cost and with increased power density.

Many more features within the C2000 MCU’s real-time control architecture and peripherals can help you solve design challenges related to high power conversion. For more details, see the white paper, “Maximizing power for Level 3 EV charging stations.”