What is bidirectional power transfer?

Bidirectional power transfer (BPT) is a key technology enabling efficient and cost-effective electric vehicles (EVs). Simply put, BPT allows electrical energy to flow in both directions through a power converter. Energy recycling is the primary function of BPT. EVs use both fully electronic BPT and electromechanically based BPT.

The uses of BPT begin with battery testing and formation before the EV is built and extends into multiple EV operating modes. This FAQ reviews the basic principles of BPT, beginning with dc/dc converters, extending to onboard battery chargers (OBCs), and closes by looking at BPT using traction motors for regenerative braking.

The basic building block of BPT is the dc/dc converter. Bidirectional dc/dc converters can be isolated or non-isolated. They can boost (step-up) or buck (step down) the dc voltage from one side of the converter to the other. Bidirectional dc/dc converters have a range of applications in EVs and a corresponding range of voltage pairs. It begins with battery formation, when the materials in the battery become electrochemically active and where energy consumption can account for 30% of manufacturing cost. The use of BPT enables that energy to be recycled and used for the formation of subsequent cells, for other manufacturing processes, or sent back to the grid.

The power demands of hybrid electric vehicles (HEVs) often exceed the 3-kW limit of conventional 12 V automotive power systems. Those vehicles use 12V to 48V bidirectional dc/dc converters and a 48 V battery to power the higher wattage components (Figure 1). In a battery EV (BEV) with a high voltage (400 to 800 V) battery pack, bidirectional dc/dc converters are used to provide power to conventional 12 V power buses.

Figure 1. Bidirectional 48-to-12 Vdc conversion is important in many HEVs (Image: Efficient Power Conversion).

Charging
Bidirectional OBCs add a bidirectional power factor correction (PFC) front end to an isolated bidirectional dc/dc converter. That requires a new approach to PFC. The conventional boost topology is not bidirectional and therefore not suitable. More complex PFC and dc/dc topologies are needed that are enabled with a combination of MCU-based digital control architectures and silicon carbide (SiC) power transistors.

For example, a three-phase totem pole PFC topology with SiC transistors can be operated in continuous conduction mode (CCM) for high efficiency and low electromagnetic interference (EMI). And a capacitor-inductor-inductor-capacitor (CLLC) resonant bidirectional dc/dc converter can provide high efficiency in both buck and boost modes. The CLLC uses SiC power transistors with zero voltage switching (ZVS) on the primary side, and ZVS combined with zero current switching (ZCS) on the secondary side. The combination of the totem pole PFC and CLCC dc/dc supports high-efficiency BPT (Figure 2).

Figure 2. This bidirectional OBC combines a totem pole PFC with a CLCC dc/dc converter (Image: Recom).

Stopping
The motor drive traction inverter is also bidirectional and can be used to drive the motor and for regenerative (regen) braking. When regen braking is implemented, the motor acts as a generator and converts the vehicle’s motion into electricity. The electricity goes backward through the drive inverter and is stored in supercapacitors or in the main battery pack depending on the powertrain architecture. Regen braking improves vehicle efficiency but is not sufficient to completely stop the vehicle. Regen must be combined with conventional friction braking for a complete braking system.

Summary
BPT is an important technology for EVs. It’s used to improve the efficiency of battery manufacturing and EV operation. Bidirectional dc/dc converters form the core of many BPT systems and can be combined with a bidirectional PFC to form a bidirectional OBC. Bidirectional traction inverters work together with the traction motor to implement regen braking in an electromechanical-electronic BPT system.

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