Efficiency in electric vehicle charger design

EBV Elektronik’s director segment city and infrastructure, Andreji Orel

EBV Elektronik’s director segment city and infrastructure, Andreji Orel, explores the trends, options and trade-offs in semiconductors for EV charger design

Electric vehicle (EV) DC chargers rely on semiconductor power switches for efficiency, low cost and small size. DC EV chargers serve as AC-DC power supplies with unique characteristics. AC charging incurs losses in the vehicle’s onboard charger (OBC), designed by car manufacturers to protect the battery and impose charging limitations. In contrast, DC charging bypasses the OBC, enabling alternative charging protection mechanisms and faster charging. Energy efficiency is crucial for optimal performance, necessitating high efficiency in DC EV chargers. Affordable, reliable and efficient equipment usage is imperative to minimise operational expenses throughout the charger’s lifetime.

Designing DC EV chargers presents challenges. Chargers require high, variable voltage outputs to accommodate different batteries and must follow timed, sequential, constant current and constant voltage charging regimes. Power levels can be extremely high, with ultra-fast chargers rated in fractions of a megawatt. Bi-directional energy flow is also becoming an important consideration.

Silicon MOSFETs, an older technology, offer high switching frequency and can function as switches or synchronous rectifiers for bi-directional energy flow. However, at higher power levels, MOSFETs have resistive properties leading to increased dissipation. Parallel operation can mitigate this but adds complexity and cost.

Wide band-gap devices (WBG) like silicon carbide (SiC) and gallium nitride (GaN) are considered the future of power switches. They exhibit lower dynamic and static losses compared to silicon and can operate at higher temperatures. However, SiC and GaN devices have specific gate drive requirements and limitations. For instance, SiC MOSFETs have a high forward drop in their body diode, while GaN HEMT cells show a high reverse voltage drop during dead time. GaN parts are also rated at relatively low voltages due to the absence of a protective avalanche effect.

Optimal designs of DC EV chargers often incorporate a combination of Si-MOSFETs, SiC-MOSFETs and SiC diodes. The specific power rating of the charger stage determines the choice of components and configuration.

For charger powertrains rated less than 50kW, a Vienna rectifier stage is commonly used. This stage serves for mains rectification and power factor correction. It benefits from lower-voltage Si-MOSFETs due to their cost-effectiveness and advantages in terms of on-resistance. The switches in the Vienna rectifier experience half-voltage stress. Chargers exceeding 50kW employ an active front end, while higher power levels (eg 100kW) may use a diode rectifier configuration.

AC-DC conversion in ACDC stages varies: lower power levels use unidirectional power factor correction (PFC), and higher power levels use bidirectional active rectifier configuration. An isolated multiphase transformer rectifier setup may be necessary for specific charger requirements.

DC-DC conversion considers isolation, energy flow and can be either isolated or non-isolated based on requirements. It can also be unidirectional or bi-directional to accommodate different charging and energy flow scenarios, ensuring efficient and safe charger operation.

Modularity is a normal approach due to maintenance and scalability in high-power DC EV charger design. Chargers use modular sub-units rated 25 to 50kW, that can be stacked or paralleled for higher power. This approach reduces semiconductor stresses and can minimise electromagnetic interference. Modularity also provides flexibility, ensuring optimal overall efficiency, as subunits can be switched in as needed. Additionally, a single sub-unit failure only reduces maximum power output without disabling the entire charger.