Bidirectional power transfer (BPT) is a critical 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 fully electronic and electromechanical-based BPT.
The use of BPT begins with battery testing and formation before an EV is built and extends into multiple EV operating modes. This article reviews the basic principles of bidirectional power transfer, beginning with dc/dc converters, extending to onboard battery chargers, and concluding with using BPT and 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) dc voltage from one side of a converter to the other.
Bidirectional dc/dc converters offer a range of applications in EVs and a corresponding number of voltage pairs. The process begins with battery formation when the materials in the battery become electrochemically active — with energy consumption accounting for about 30% of manufacturing costs. BPT enables that energy to be recycled and used for forming 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 the conventional 12-V automotive power systems. These vehicles use 12 to 48-V 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 critical in many HEVs (Image: Efficient Power Conversion).
Charging
Bidirectional onboard battery chargers (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, unsuitable. More complex PFC and dc/dc topologies are needed, 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). Additionally, a capacitor-inductor-inductor-capacitor (CLLC) resonant bidirectional dc/dc converter can provide high efficiency in 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 totem pole PFC and CLCC dc/dc combination 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 the main battery pack, depending on the powertrain architecture. Regen braking improves vehicle efficiency but is insufficient to stop the vehicle completely. Regen must be combined with conventional friction braking for a complete braking system.
Summary
BPT is an essential technology for EVs. It improves 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 with the traction motor to implement regen braking in an electromechanical-electronic BPT system.
References
- Bidirectional Charging and Electric Vehicles for Mobile Storage, US Department of Energy
- Bidirectional dc/dc converter topology comparison and design, Texas Instruments
- Electric vehicle chargers switch to bidirectional designs, Recom
- Hybrid and Electric Powertrain 48 V, Efficient Power Conversion
- What Is Bidirectional Charging?, EV Connect
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