Onboard Charger Assembly

Overview

The onboard charger (OBC) is the power-electronics box that lets the car charge from ordinary AC supplies — a home socket, a wallbox, or a public AC station. The grid delivers alternating current, but the HV Battery Pack can only be charged with direct current at its own voltage, so the OBC rectifies the AC, corrects its power factor, isolates it galvanically, and converts it to a controlled DC current matched to the pack. It handles 7.4 kW from a single phase up to 11–22 kW from three phases, weighs only a few kilograms, and lives on the high-voltage side of the car alongside the DC-DC Converter and the Electric Drive Unit inverters.

How it's built / Construction

The OBC is a two-stage converter in a sealed aluminium housing. The first stage is a power-factor-correction (PFC) rectifier that turns the incoming AC into a smooth high-voltage DC link while drawing a clean, sinusoidal current from the grid so the car looks like a well-behaved resistive load. The second stage is an isolated DC-DC converter: it switches the DC-link at high frequency through a transformer, which both provides the legally required galvanic isolation between the grid and the vehicle's high-voltage bus and lets the output voltage be set to whatever the HV Battery Pack currently needs.

The switches are increasingly wide-bandgap devices — silicon-carbide (SiC) or gallium-nitride (GaN) MOSFETs — which switch fast enough to shrink the magnetic components and reach high efficiency. A control board runs the converter, talks to the Pack BMS (Master) over the vehicle network, and manages the charging handshake. Because the OBC dissipates a few hundred watts of loss, it is liquid-cooled, sharing a coolant loop in the Thermal System with the other power electronics.

Key specifications explained

Power (7.4 / 11–22 kW) sets how fast the car charges on AC. A single-phase 7.4 kW OBC adds roughly 40 km of range per hour; an 11 kW three-phase unit adds about 60 km/h, and 22 kW double that — useful where three-phase supply is common. This is the ceiling for AC charging; DC fast charging bypasses the OBC entirely, feeding the pack directly from an off-board charger through the Charge Port (CCS), which is why DC can reach 150–250 kW while the OBC tops out in the tens of kilowatts.

Galvanic isolation is a safety requirement: the high-frequency transformer ensures no direct electrical connection exists between the grid and the car's HV bus, so a fault cannot energise the chassis. Efficiency (~94–96%) turns directly into wasted energy and heat; the remaining few percent is what the liquid cooling must remove. Topology (PFC plus isolated DC-DC) is shared by nearly all OBCs; the differences are in the device technology and the number of phases handled.

Manufacturing & assembly

The OBC is built as a surface-mount power-electronics assembly: PCBs are populated and reflow-soldered, the magnetic components (PFC inductor and isolation transformer) are wound and installed, and power devices are mounted to a cold-plate or the housing for cooling. The unit is potted or sealed for vibration and moisture resistance and given connectors for AC input, HV DC output, coolant, and the low-voltage control network.

Every unit is tested end-of-line across its input range — different voltages and phase counts — verifying output regulation, isolation resistance (a hipot test confirms grid-to-vehicle separation), power-factor and efficiency targets, and thermal behaviour under load. In the car the OBC bolts near the Charge Port (CCS) and the high-voltage junction, its coolant ports join the Thermal System, and its control lines join the Low-Voltage Electronics network.

Role in the vehicle / where it fits

During AC charging the OBC is the gatekeeper. The Charge Port (CCS) mates with the cable and signals the available current; the OBC and the Pack BMS (Master) negotiate a charging current; the OBC converts grid AC into the right DC and ramps it into the HV Battery Pack while the Thermal System keeps both the pack and the OBC cool. It works closely with the DC-DC Converter, which is often packaged with it, and reports to the Low-Voltage Electronics throughout. During DC fast charging the OBC steps aside and the Charge Port (CCS) connects the external charger straight to the pack.

Charging standards and the handshake

AC charging is governed by connector and signalling standards — the J1772 inlet in North America, the Type 2 inlet in Europe, both usually combined with DC pins into a CCS Charge Port (CCS). When a cable is plugged in, a low-voltage control-pilot signal lets the station and the car negotiate before any power flows: the station advertises the maximum current the circuit can supply, the car confirms it is ready and latches the connector, and only then does the OBC begin to draw current, ramping up smoothly to avoid disturbing the grid. A proximity signal tells the car a plug is connected and prevents it from driving off while tethered.

Smart-charging features layer on top of this handshake. The car can schedule charging for off-peak hours or cheap tariff windows, limit current to stay within a home's service, and — on bidirectional units — reverse the flow for vehicle-to-home or vehicle-to-grid, coordinating with the Pack BMS (Master) and the utility's signals. Throughout, the Low-Voltage Electronics monitor connector temperature and isolation, ready to stop charging if a fault appears. These protocols are why a car from one maker charges safely at a station from another: the handshake, not just the plug shape, is standardised.

Variants & alternatives

OBCs vary mainly by power and phase support: a basic single-phase 7.4 kW unit for markets and homes with single-phase supply, versus an 11 kW or 22 kW three-phase unit elsewhere. Bidirectional OBCs add the ability to push power back out — vehicle-to-load, vehicle-to-home, or vehicle-to-grid — turning the car into a mobile battery, at the cost of a more complex converter.

A growing trend is integration: combining the OBC, the DC-DC Converter, and sometimes the Traction Inverter into a single multi-function power unit that shares magnetics, cooling, and control to save cost and mass. Some cars even reuse the Traction Motor (PMSM) windings and inverter as part of the charger. Whatever the form, the OBC's role is unchanged: take whatever AC the grid offers and deliver clean, isolated, controlled DC to the battery.