Electric vehicle chargers switch to bidirectional designs

Oct 7, 2022
Schematic representation of a blue electric car with charging cable
As part of a global effort to reduce carbon footprint, automotive manufacturers are replacing gasoline with cleaner fuel-less alternatives. As a result, the electric vehicle (EV) transportation segment is experiencing rapid growth. As EVs operated purely on electric power continue to increase market share, the installed battery capacity per vehicle is also increasing.

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Consumers are also demanding faster charging times for larger capacity batteries. This demand is also spurring an increase in battery operating voltage from 400V to 800V, beginning with high-performance vehicles.

The power levels required for EV chargers can vary widely. Low-power applications for electric scooters or mini EVs, can demand less than 2kW from a single-phase supply. Medium power applications for vehicles such as Tesla’s Model 3 or the Chevy Volt can require up to 6.6kW from a single-phase supply. High power applications for sports EVs are beginning to call for 3-phase designs capable of up to 22kW.

As operating voltages and battery capacities continue to increase, designers are also switching from unidirectional charging systems—where power flows from the grid to the charger to the EV battery pack—to bidirectional systems— where power can flow in either direction. Figure 1 shows in purple the areas where bidirectional designs are starting to appear.

Electrical circuit diagrams with AC/DC stages, sensors, relays and converters
Fig. 1: The EV charging infrastructure is switching from unidirectional to bidirectional topologies (Source: RECOM)

Why the move to bidirectional OBC? An EV equipped with sufficient battery capacity is potentially capable of acting as an energy storage system (ESS), enabling a variety of vehicle-to-other use cases. Together these can be grouped under the category V2X (Vehicle to Everything), and include:

  • V2L (Vehicle to Load) – DC/DC or DC/AC to charge e-bikes and scooters, power camping equipment.
  • V2G (Vehicle to Grid) – AC/DC/AC for grid balancing/energy shaving, on-board chargers.
  • V2H (Vehicle to Home) – AC/DC/DC for smart home (self-sufficiency: mains + solar + battery + EV).
  • V2V (Vehicle to Vehicle) – DC/DC for fleet battery balancing/conditioning (cascadable)

The transfer of power can occur in either direction in these use cases. As a result, the EV charging infrastructure is migrating from unidirectional to bidirectional designs for both the EV on-board charger (OBC) and the fixed-base EV charging station. To make battery-powered vehicles cost-effective, both types of chargers must be as efficient and economical as possible.

Bidirectional EV charging system blocks

A design for a bidirectional EV charger requires a bidirectional AC/DC supply; this typically consists of a bidirectional power factor correction (PFC) circuit followed by an isolated bidirectional DC/DC converter. For OBC use, added emphasis is placed on high power density and maximized efficiency to make the most of the available space and minimize the weight. The designer cannot use the tried-and-true standard building blocks for a bidirectional charger. For the PFC, the traditional boost topology will not suffice as it is not bidirectional. Switching to a totem pole topology solves the problem.

For high efficiency and low noise, the totem pole PFC must operate in continuous conduction mode (CCM). This requires the use of silicon carbide (SiC) power devices. SiC devices exhibit very low on-state resistance (RDS(on), allowing for low switching loss and low conduction loss. Thus, SiC is ideal for high-current switching applications. What about the DC/DC converter? An LLC resonant converter has long been a popular choice for the DC/DC converter stage in a high-power, high-efficiency charger, but this is also a unidirectional topology. A bidirectional CLLC resonant converter is preferred for the DC/DC stage, as it combines high efficiency with a wide output voltage range in both charging and discharging modes.
Circuit diagram of a 3-phase bidirectional AC/DC charging output stage
Fig. 2: A 3-phase bidirectional AC/DC charger power stage (source: RECOM)
The CLLC employs soft switching for highest efficiency: zero voltage switching (ZVS) on the primary side, and ZVS combined with zero current switching (ZCS) on the secondary side. Again, switching to SiC technology yields efficiency improvements. Figure 2 shows a representative bidirectional power stage for a three-phase charging application. This design requires 14 power transistors. It is an isolated topology, so the power transistors are paired with isolated gate drivers and isolated DC/DC power supplies.

RECOM provides a comprehensive range of isolated DC/DC converters for high-power switching applications, no matter whether silicon IGBTs, SiC MOSFETS, or an emerging technology such as GaN HEMTs are used.

Modules for battery charging applications: The rest of the story

Of course, an EV charging system includes much more than the PFC and the CLLC blocks discussed above. While power conversion stages at the multi kW level are improving with the latest topologies and semiconductor technologies, all other supplies must also match the EV charger requirements in terms of efficiency and value. As Figure 1 illustrates, a complete design contains a variety of other power blocks: isolated and non-isolated DC/DC converters to accommodate microcontrollers, HMI functions, and a wireless block; an auxiliary power supply; and an AC/DC supply for relays and contactors.

There are other considerations charging wall boxes and charging stations are often installed in overvoltage category three (OVC III) environments with the potential for significant dips, power surges, and transients from lightning strikes, which the power supplies must also withstand. Additionally, temperature variations can be extreme, and the AC supply voltage available may be three-phase 480VAC or 277VAC. All AC/DC and DC/DC converters, and other blocks must operate reliably in this environment.

Whether the design calls for high or low power levels, RECOM provides a range of low-power AC/DC modules, DC/DC converters and switching regulators that match the battery charging application requirements in everything from EV chargers to low-power charging in portable products. RECOM can supply high-reliability custom battery chargers, conditioners and bidirectional inverters based on proven platform designs from three-phase AC supplies with power ratings of up to 30kW or even higher with paralleled units.

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