Electric vehicle (EV) onboard chargers (OBCs) convert alternating current (ac) into direct current (dc) for lithium-ion (Li-ion) battery traction packs. This article explains why Li-ion batteries require dc and highlights how OBCs safely and efficiently transform electric currents. It also reviews the two primary OBC categories — single-phase and three-phase — exploring new bidirectional OBCs, which are poised to facilitate vehicle-to-everything (V2X) and vehicle-to-grid (V2G) charging.
The power requirements of EV charging
Most homes and businesses in North America are wired to deliver 110 to 120 Vac of power — and 220 to 240 Vac in Europe, the Middle East, and Asia. Because electrochemical reactions such as lithium intercalation and redox require a consistent, direct flow of energy, EV Li-ion batteries can only safely accept a dc charge.
Figure 1. A cross-sectional illustration of internal EV systems, highlighting the OBC, traction battery pack, charge port, power electronics controller, and other key components. (Image: Recurrent Auto)
OBCs (Figure 1) convert ac to dc and work in tandem with EV chargers and battery management systems (BMS), dynamically regulating voltage and current for Level 1 (120 Vac) and Level 2 charging (240 Vac).
Specifically, Level 1 outputs 1.8 kW, while Level 2 supports up to 19.2 kW in North America and 22 kW in Europe. Commercial Level 3 dc fast chargers (operating at 400 to 900 Vdc) bypass OBCs, delivering 50 to 350 kW directly to EV battery packs.
EV OBCs convert ac to dc and regulate the resulting current with:
- Rectification: Converts ac into pulsating dc power with diodes, thyristors, and transistors.
- Power factor correction (PFC): Aligns the charger’s input current with the input voltage waveform, boosting efficiency, reducing harmonics, and minimizing power losses.
- Dc-dc conversion: Dynamically adjusts voltage and current levels with pulse width modulation (PWM) and resonant converters — stabilizing pulsating Vdc power into consistent Vdc output.
Single-phase versus three-phase OBCs
OBC power capacity, which is divided into single-phase and three-phase types, directly impacts EV charging speeds. Single-phase OBCs, for example, typically support maximum dc charging rates of 7.2 to 11 kW, with three-phase OBCs scaling up to 22 kW for faster charging.
Compact EVs such as the Nissan LEAF, Hyundai Ioniq, and Kia Soul offer dc charging rates of 6.6 to 7.2 kW. Premium EV models like the Jaguar I-Pace, Porsche Taycan, and Polestar 4 (Long Range) provide higher dc charging rates from 11 to 22 kW. Notably, OBCs in Tesla’s Model 3, Model S, and Model Y feature 11-kW dc charging capabilities.
Figure 2. A new EV OBC before installation. (Image: PushEVs)
To ensure safe and efficient interoperability with electric vehicle supply equipment (EVSE), OBCs (Figure 2) comply with key industry protocols such as the Society of Automotive Engineers (SAE) J1772 and International Electrotechnical Commission (IEC) 61851-1. For example, the galvanic isolation of key circuits prevents current spikes from causing electrical failures — and automatic shutoff mechanisms mitigate the effects of dangerously excessive power loads.
Additionally, OBCs regulate internal temperature with advanced thermal management systems that typically include heat sinks and fans.
A bidirectional future
Most OBCs are designed for a single purpose — recharging EV Li-ion battery traction packs by converting ac to dc. Pending widespread adoption, bidirectional OBCs that seamlessly transform dc to ac will one day enable EVs to supply electrical grids during peak hours.
Bidirectional OBCs comprise a two-way ac-dc converter, typically a PFC or active front end (AFE) circuit, followed by an isolated bidirectional dc-dc converter. Notably, new PFC totem-pole designs use two high-frequency silicon carbide (SiC) or gallium nitride (GaN) switches to facilitate two-way energy flows efficiently. PFC totem-pole designs also reduce switching and conduction losses — effectively boosting power density and thermal performance.
Many companies are developing bidirectional OBCs. Wolfspeed, for example, has debuted a 6.6-kW OBC demo board comprising a bidirectional totem-pole PFC ac-dc stage and an isolated bidirectional dc-dc stage based on CLLC topology with variable dc link voltage. The OBC demo board accepts 90 to 265 Vac, providing 250 to 450 Vdc at the output with more than 96% efficiency in both charging and inversion modes.
Meanwhile, BorgWarner recently confirmed plans to supply bidirectional 800-V OBCs to a major North American OEM. Leveraging SiC power switches, the BorgWarner bidirectional OBC incorporates a vehicle-to-load (V2L) operating mode to support the safe charging of various standalone applications.
Summary
Most homes and businesses in North America are wired to deliver 110 to 120 Vac of power — and 220 to 240 Vac in Europe, the Middle East, and Asia. EV Li-ion batteries, however, require a dc charge. Single-phase or three-phase OBCs convert ac to dc, operating with EV chargers and the BMS to dynamically regulate voltage and current for Level 1 and Level 2 charging. Pending widespread adoption, bidirectional EV OBCs will one day facilitate the safe and efficient energy transfer from Li-ion batteries back to the grid.
References
- What is an Electric Car Onboard Charger?, WePowerYourCar
- What Really Limits EV Charging Speed? Not What You Think!, Recurrent Auto
- What is an Onboard Charger?, Aptiv
- On-board Charger, EVExpert
- Onboard Chargers, E-Mobility Engineering
- Thermal Management of On-board Chargers in E-Vehicles, Electronics Cooling
- An Onboard Charger (OBC) is an Essential Component of an EV, LinkedIn Pulse
Filed Under: Charging, FAQs, Onboard Charging