System Role & Power Classes of the On-Board Charger

The on-board charger (OBC) is the AC–DC power module that converts grid or wall-outlet AC into a high-voltage DC bus for the traction battery. It sits between the vehicle inlet and the high-voltage battery system, coordinating charging limits with the BMS and the vehicle controller.

An OBC is not a roadside DC fast charger: DC fast chargers generate high-voltage DC outside the vehicle, whereas the OBC performs the AC–DC conversion on the vehicle. It is also different from a low-voltage DC–DC converter, which steps the high-voltage bus down to 12 V or 48 V for body and chassis loads.

From a system perspective, the OBC must deliver the required charging power at high efficiency, meet grid power-quality limits for power factor and harmonic distortion, and maintain safe coordination with the battery, contactors and vehicle networks.

Typical OBC Power Classes and Use Cases

In this page we focus on the AC–DC on-board charger module itself. Battery management, cell balancing and state-of-health algorithms are covered in the Battery Management System (BMS) topic, while external DC charging protocols and inlet hardware are covered in the DC Fast-Charge Interface topic. Low-voltage power rails for 12 V and 48 V loads belong to the Low-Voltage DC–DC Converter topic.

Modern OBC designs are under strong pressure to deliver higher power density, peak efficiency above 95 %, robust EMI margins and automotive-grade safety levels. This drives the adoption of SiC-based power stages, digital PFC control, integrated gate drivers and strong connectivity into the vehicle networks.

Power Stage Topologies & System Variants

From the vehicle inlet to the high-voltage bus, the on-board charger is built as a chain of power stages. A typical path is: AC inlet and protection, EMI filter, rectifier, power-factor-correction (PFC) stage, DC link capacitors, isolated DC–DC (often LLC) and finally the high-voltage DC bus that connects to the battery contactors and the BMS.

At system level the topology choice aims to meet power and efficiency targets while achieving high power factor and low harmonic distortion, managing thermal stress and making EMI compliance realistic across vehicle and grid variants.

PFC Stage Options in OBC Designs

Conventional boost PFC uses a rectifier bridge followed by a boost stage. It is mature, well understood and supported by many controllers, making it a common choice around the 3.3–3.7 kW range where cost and design simplicity are strong drivers. Efficiency is good but the bridge and boost switch conduction losses limit performance at higher power.

Bridgeless PFC reduces losses by eliminating or partially bypassing the input bridge. It improves efficiency in mid-power OBCs, but introduces more complex current paths and a tighter EMI and layout challenge. This makes it attractive for 7–11 kW tiers where incremental efficiency gains justify the added design effort.

Totem-pole PFC combines a high-frequency leg (often using SiC MOSFETs) with a low-frequency leg to emulate a rectifier and PFC stage with very low conduction loss. It is a leading choice for high-power, high-efficiency OBCs in the 11–22 kW range but demands fast gate drivers, high CMTI and carefully planned current sensing and layout, which are covered in dedicated gate-driver and current sensing topics.

Isolated DC–DC Stage Options

The isolated DC–DC stage converts the regulated DC link into a battery-compatible high-voltage output and provides galvanic isolation. LLC resonant converters are widely used in OBCs because they offer high efficiency and soft switching over a wide input range. The trade-offs in magnetic design, resonant-tank tuning and control implementation are treated in separate LLC architecture topics.

Phase-shift full-bridge (PSFB) and related hard-switched variants remain relevant in some legacy or cost-sensitive platforms. They can be simpler to control but typically place more stress on switches and magnetics. In some bidirectional OBC architectures, specialised DC–DC topologies are used to support power flow from the battery back to the grid or home installation.

Unidirectional vs. Bidirectional OBC Architectures

Unidirectional OBCs only support power flow from the grid to the battery. Their PFC and DC–DC stages, gate drivers and measurement paths are optimised for this direction, with protection focusing on over-voltage, over-current, over-temperature and insulation faults during charging.

Bidirectional OBCs enable vehicle-to-grid (V2G) or vehicle-to-home (V2H) modes. They require topologies that can reverse power flow, extended measurement and protection coverage for both directions and tighter coordination with the BMS and grid interface. Detailed bi-directional control strategies and standards are covered in dedicated V2G and DC fast-charge interface topics.

While this page focuses on the OBC power path on the high-voltage side, low-voltage DC–DC converters feeding 12 V and 48 V rails are described in the LV DC–DC converter module topic, and external DC charging hardware and protocols are handled in the DC fast-charge interface topic. This separation keeps the OBC discussion centred on the AC–DC conversion block inside the vehicle.

Control, Sensing & Isolation Overview

Beyond the power stages, an on-board charger relies on several interacting signal chains: the PFC control loop that shapes the input current, the isolated DC–DC control loop that regulates battery charging, a network of measurement and protection sensors and isolated communication links to the battery and vehicle controllers.

PFC Control Loop

The PFC control loop keeps the DC link voltage within its target window while shaping the input current to track the AC voltage waveform and maintain a high power factor. This can be implemented using a dedicated PFC controller IC or firmware running on a digital controller. In both cases the loop closes around AC or inductor current and DC link voltage feedback.

Typical hardware includes current sensing on the AC input or PFC inductor, DC link voltage measurement, temperature monitoring of key devices and PWM outputs to the gate drivers. Detailed current-loop and voltage-loop design, compensation and stability analysis are handled in dedicated PFC controller topics rather than on this OBC page.

LLC / DC–DC Control Loop

The isolated DC–DC stage control loop regulates the high-voltage output that feeds the traction battery across a wide operating range. Depending on topology, the controller adjusts switching frequency, phase shift or both to achieve soft switching, protect the transformer and magnetic components and keep losses low across operating points.

This loop relies on measurements of primary or secondary currents, output voltage and several temperature sensors on transformer cores and power devices. Instantaneous peak currents are used to enforce hard limits, while averaged values control charging profiles requested by the BMS. The underlying resonant and full-bridge control schemes are described in LLC and DC–DC controller architecture topics.

Measurement & Protection Sensing

Around these main loops, the OBC includes a measurement layer that feeds both control and protection. Key points include high-voltage bus voltage, AC input or PFC inductor current, DC link capacitor current or temperature and the temperatures of power semiconductors, magnetics and heatsinks. These signals support efficiency optimisation, thermal management and protection sequencing.

A subset of these measurements is classified as safety-related. High-voltage bus over-voltage, over-current on the battery side, critical device temperatures and insulation-monitor outputs feed safety monitors and may be acquired by redundant channels or hardware comparators. The detailed design of shunt, isolated amplifier and sigma-delta measurement circuits is covered in the current sensing and power measurement domain.

Isolation & Communication Links

Signal chains must cross isolation barriers between the high-voltage power domain and the low-voltage control domain. Isolated gate drivers carry PWM signals to the PFC and DC–DC switches, while isolated amplifiers or sigma-delta modulators bring current and voltage information back to the controller. Digital isolators or isolated transceivers link the OBC controller to CAN, LIN or Ethernet networks that sit on the vehicle side of the barrier.

Through these communication links the OBC exchanges charging limits, status and diagnostics with the BMS and vehicle control unit. Details of network protocols, update mechanisms and cyber security belong to in-vehicle networking and gateway topics, while isolation device selection and CMTI performance are covered in dedicated isolation and gate-driver pages.

Isolation Strategy, ASIL & Diagnostics

The on-board charger is part of the high-voltage traction system and must meet automotive functional safety expectations while remaining safely isolated from the vehicle body and low-voltage domains. This section looks at safety integrity levels, isolation planning and the diagnostic signals that feed protection and system monitoring.

Functional Safety Levels in OBC Applications

Many OBC implementations carry functions allocated around ASIL B or ASIL C, while the overall traction system safety goal is closed together with the BMS and vehicle controller. Safety-relevant behaviour includes preventing over-charging, managing over-current and over-temperature events and placing the system into a defined safe state when critical faults are detected.

In practice this means that selected sensing channels, protection thresholds and shut-down paths must be designed with diagnostic coverage, failure detection and safe reaction in mind. The detailed safety concept, including decomposition and safety cases, is typically handled at the vehicle platform level and is referenced by the OBC design.

Functions Requiring Redundancy or Self-Checking

Several OBC functions are strong candidates for redundancy or self-tests. High-voltage bus over-voltage and battery-side over-current protection often use both sampled measurements and hardware comparators to enforce hard limits. Critical device temperature sensors can be checked against plausibility windows or backed up by secondary sensing points.

Insulation monitoring between the high-voltage system and chassis typically reports a fault flag that must be acted on by the OBC, either by preventing charging or by forcing a controlled shut-down. Loss of communication with the BMS or vehicle controller also requires a defined safe reaction, such as reducing power or terminating charging after a timeout.

Isolation Strategy & Safety Standards

The OBC isolation strategy separates the high-voltage power domain from low-voltage control electronics and the vehicle body. Isolation is achieved through the main power transformer in the DC–DC stage, isolated gate drivers, isolated current and voltage feedback paths, isolation monitoring hardware and PCB creepage and clearance consistent with automotive insulation classes.

Standards such as IEC 61851 for conductive charging systems and general insulation and spacing rules from IEC 62368 and IEC 60664 influence the choice of insulation level, test voltages and allowable creepage and clearance distances. This page only references the role of these standards; detailed creepage tables, insulation coordination and packaging trade-offs are addressed in dedicated isolation and safety-packaging topics.

Diagnostic & Monitoring Signals

Safety and diagnostic functions rely on a structured set of signals. These include over-current detection on the battery and power-stage paths, over-voltage detection on the HV bus and DC link, over-temperature sensing on power semiconductors, magnetics and capacitors and the output of the insulation monitoring device. Many of these are exposed both as ADC channels and as discrete PG or FAULT pins.

Fault flags and status registers from gate drivers, supervisors, power monitors and communication transceivers feed the safety layer and are forwarded to the BMS and vehicle controller. General safety architectures for power monitoring, redundant channels and system-level fault handling are described in safety architecture and power monitoring topics; the OBC page focuses on which signals must exist and how they relate to charging behaviour.

IC Categories & 7-Brand Mapping for the OBC

This section groups on-board charger ICs by function and maps them across seven major automotive suppliers. The goal is to help engineering and procurement teams quickly identify relevant MCU, power, sensing, isolation and communication families that are commonly used in OBC designs, together with examples of part numbers and the reasons they fit this application.

Control & Digital

Control and digital devices run the PFC and DC–DC algorithms, interface to vehicle networks and host safety and diagnostic software. Typical choices are automotive MCUs or DSPs with fast ADCs and PWM units, dedicated PFC and LLC controllers and safety PMICs and supervisors that support functional safety goals.

In OBC projects, MCU and DSP families such as TI C2000, ST SPC58, NXP S32K and Infineon AURIX are attractive because they combine fast ADCs, flexible PWM units and rich networking with automotive-grade safety documentation. Dedicated PFC and LLC controllers from multiple vendors simplify interleaved PFC and resonant stage design, while safety PMIC and supervisor devices provide the monitored power rails, watchdogs and reset sequencing needed to meet ASIL targets.

Power & Gate Drive

Gate drivers and power-interface ICs connect the control loops to SiC and MOSFET power stages. They must withstand high dv/dt, provide fast and symmetric drive strengths, integrate fault detection such as DESAT and support safe turn-off during short-circuit and over-current events.

OBC power stages increasingly rely on isolated gate drivers capable of handling SiC-level dv/dt while still integrating DESAT, soft turn-off and Miller clamp functions. Families such as TI UCC21xx/UCC217xx, ST STGAP, onsemi NCP51xx and Infineon EiceDRIVER devices are widely used. Half-bridge and bridge drivers with built-in protections reduce external component count, while discrete helpers and clamps still matter for fine-tuning switching performance and robustness.

Measurement & Isolation

Measurement and isolation ICs capture high-voltage currents, bus voltages and temperatures and bring them safely into the control domain. Shunt amplifiers and sigma-delta modulators provide precise current feedback, while high-voltage monitors, isolated amplifiers and digital isolators maintain galvanic separation between the HV stages and low-voltage logic.

For OBC applications, low-offset shunt amplifiers and isolated sigma-delta modulators are key to obtaining accurate current and voltage feedback across temperature and lifetime, especially on SiC-based stages. Digital isolators and isolated CAN transceivers from TI, ST, NXP, Microchip and ADI are widely used to bridge the isolation barrier while maintaining EMC performance and robust communications.

Communication & Interface

Communication and interface ICs connect the on-board charger to the battery management system, vehicle control unit and in some platforms to Ethernet-based gateways or V2G infrastructure. CAN FD, LIN and automotive Ethernet PHYs are the primary building blocks, with optional PLC and V2G front-ends in advanced systems.

In many OBCs, a simple CAN FD link to the BMS and a second CAN or Ethernet link to the vehicle gateway are sufficient. NXP TJA10xx, TI TCAN10xx and Microchip MCP2562 families are frequently chosen because they offer robust EMC, low-power modes and automotive qualification. Ethernet PHY choices depend on whether the OBC is a simple node on the backbone or part of a more integrated powertrain domain controller.

Bias & Auxiliary Power

Bias and auxiliary power ICs generate the low-voltage rails for controllers, gate drivers, sensors and communication transceivers. Typical building blocks are off-line or high-voltage flyback controllers for housekeeping supplies, DC–DC buck or buck-boost converters for local rails and voltage references and supervisors for monitored start-up and reset.

Auxiliary supplies in an OBC must tolerate wide input ranges, hot environments and strong EMI while remaining small and efficient. Flyback controllers, buck converters and PMICs from TI, ST, Renesas, onsemi and others cover most of these needs, while robust LDOs, references and supervisors protect sensitive control and sensing circuits from undervoltage, overvoltage and unstable power conditions.

Layout, Isolation Barrier & EMI Considerations

This section turns the on-board charger topology and safety strategy into layout-level guidance. It is written so that a layout or hardware lead can turn each bullet into a checklist item during design reviews. The focus is on physical partitioning, isolation barrier routing, EMI and grounding strategy and thermal placement rather than detailed formulas.

Physical Partitioning & Power Zones

Partitioning the PCB and mechanics into clear functional zones helps control current loops, EMI and safety distances. The goal is to keep high-energy switching areas compact and predictable while giving low-voltage control and sensing circuits a quiet environment.

• AC Input & EMI Zone – Place fuses, surge arresters, X/Y capacitors, common-mode chokes and line filters near the AC inlet and mains connector. Keep the DM filter loop between line, neutral and bridge input as tight as possible and provide a clean path for Y-capacitor currents to chassis or vehicle ground.
• PFC Zone – Group the PFC inductor, PFC switches, bridge rectifier and current-sense components into a compact island. Minimise the high-frequency loop formed by the PFC switch, PFC diode/bridge and DC link capacitor. Avoid routing this loop underneath control or sensing circuits.
• LLC / DC–DC Zone – Place the resonant tank (transformer and resonant capacitors), main bridge and secondary rectifiers close together. Keep primary switching loops short and symmetric and align the secondary current path to the DC bus and battery interface. Do not snake HV outputs through low-voltage areas.
• HV DC Bus & HV Battery Interface Zone – Position DC link capacitors, busbars, contactors and the HV connector as a coherent block. The DC link capacitor bank should sit physically between PFC and DC–DC zones to minimise stray inductance. Reserve sufficient creepage and clearance around busbars, contactor terminals and the battery connector.
• Control Board / Low-Voltage Logic Zone – Locate MCU/DSP, gate-driver control sides, current/voltage front-ends, PMICs and communication PHYs on the low-voltage side, clearly separated from HV copper. Keep sensitive analogue sensing near the low-voltage side of isolation devices and give digital domains (MCU cores, Ethernet PHYs, clocks) their own well-routed areas.

Package-level decisions, power module footprints and mechanical details such as busbars and brackets are handled in the dedicated power-stage modules and packaging topics. Here the emphasis is on defining clean zones that naturally support those choices.

Isolation Barrier Routing & Creepage Awareness

The isolation barrier should be visible on the PCB as a deliberate strip that separates high-voltage power and sensing from low-voltage control and communication. Both signal crossings and isolated power supplies must respect this physical boundary.

• Define a clear isolation “band” – Reserve a continuous region where no copper or components cross between the HV domain and the LV domain except through isolation devices. Group all isolated gate drivers, isolated amplifiers and digital isolators along this band.
• Bundle signal crossings – Route PWM pairs, sigma-delta data, SPI/I²C links and CAN/Ethernet signals that cross the barrier as tight bundles with controlled spacing. Avoid single “stray” crossings that are easy to miss in review.
• Separate isolated power rails – Keep HV-side driver supplies and LV-side controller supplies clearly separated. Use dedicated isolated converters (fly-buck, flyback or transformer-based modules) and avoid returning any auxiliary supply ground across the barrier without a deliberate isolation element.
• Creepage and clearance awareness – Around the isolation band, maximise clearance by pulling back copper, enlarging pad spacing and using slots or cut-outs where required. Detailed creepage/clearance tables and insulation coordination are defined in isolation and safety-packaging topics; this page focuses on making the barrier visible and reviewable in the layout.

EMI & Grounding Strategy (Common-Mode vs Differential)

EMI behaviour in an OBC is dominated by differential-mode switching loops and common-mode coupling to chassis, vehicle ground and heatsinks. The aim is to keep switching loops compact, give common-mode currents a controlled path and avoid random return paths through sensitive circuits.

• Differential-mode loops – Keep the high-frequency loops of the PFC and LLC stages (switch–diode–capacitor/inductor) as small and planar as possible. Use solid reference planes under these loops and avoid routing low-level signals perpendicular to them.
• Common-mode paths – Identify high dv/dt nodes and their capacitive coupling to heatsinks, shields, enclosure and vehicle ground. Provide intentional common-mode paths through Y capacitors and filters so that common-mode currents do not search for unintended return routes.
• Ground partitioning – Separate HV power ground, analogue sensing ground and digital/communication ground. Define where these regions meet (for example at a star point near the controller) and avoid large power currents flowing through the analogue ground plane.
• OBC-specific EMI concerns – Consider mains-side conducted emissions, coupling to vehicle ground networks and to large heatsinks or cold plates. Ensure heatsinks attached to high dv/dt switches are either tied to a controlled potential or shielded to prevent radiated emissions from coupling into harnesses and the vehicle body.

Detailed filter design, emission limits and test setups are handled in the EMC / EMI subsystem topics. Here the focus is on the placement and routing decisions that give those filters a fair chance to work.

Thermal Layout & Package Placement

Thermal layout ensures that PFC and DC–DC switches, magnetics and DC link capacitors operate within their limits over lifetime. Mechanical and cooling concepts differ by platform, but certain placement rules are common across OBC designs.

• Locate high-loss devices on defined cooling paths – Place PFC and DC–DC MOSFETs or SiC modules, high-current rectifiers and main magnetics where they can couple efficiently to heatsinks, cold plates or airflow. Avoid stacking several major heat sources into a corner that has poor cooling access.
• Protect lifetime-critical components – Position electrolytic capacitors, film capacitors and current-sense shunts away from the hottest areas or provide shielding copper and airflow to keep their case temperatures under control.
• Place temperature sensors at meaningful points – Attach sensors where they track real stress: on the body or terminals of electrolytics, close to hot windings or cores of inductors and transformers and near the hottest positions on MOSFET or SiC module copper. Ensure the sensing points align with the safety and derating strategy.
• Coordinate with power-module and mechanical design – Use OBC layout reviews to check that module footprints, mounting hardware and thermal interfaces match the cooling concept. Detailed package comparisons and module-level thermal guidance are covered in the power stage modules and packaging topics.

BOM & Procurement Notes for the OBC

This section helps procurement teams and small to mid-volume integrators express the OBC requirements as clear BOM and RFQ fields. Instead of listing only part numbers, the aim is to describe the AC input, power levels, safety goals, communication interfaces and environment so that suppliers can propose suitable IC families, power modules and reference designs.

Core Required BOM Fields

The following fields should be filled in for every OBC project. They can be placed in the header of the BOM, in an RFQ template or in the design requirements document that accompanies the part list.

• AC input voltage range – e.g. “85–265 VAC single-phase” or “3× 380–480 VAC three-phase”.
• AC input phases and frequency – single / three-phase, 50/60 Hz or specified frequency range.
• Target output power and efficiency – e.g. “7.4 kW, ≥96% at 230 VAC and nominal battery voltage”.
• Power factor and THD limits – e.g. “PF ≥ 0.99, THD < 5% at nominal load”.
• OBC directionality and V2G/V2H support – unidirectional or bidirectional; specify if ISO 15118 / V2G capabilities are required.
• HV battery voltage range – e.g. “250–450 V” or “400–800 V”.
• Maximum charge current and profiles – e.g. “up to 32 A continuous, support for constant-current/constant-voltage and derating at high temperature”.
• Functional safety target – target ASIL level for OBC functions (e.g. ASIL B / C contribution) and whether safety MCU/PMIC support is required (safety manual, FMEDA, certified libraries).
• Communication interfaces – internal (CAN FD, LIN, 100BASE-T1 / 1000BASE-T1 Ethernet) and external interfaces (PLC for ISO 15118, diagnostic UART, service port).
• Diagnostic and logging requirements – whether the OBC must expose detailed fault codes, counters and lifetime data over CAN or Ethernet.
• Environmental conditions – ambient or housing temperature range (e.g. “-40…+105 °C”), IP rating (e.g. IP54, IP67), expected humidity and condensation conditions.
• Vibration and mechanical requirements – reference to the applicable automotive vibration standard (e.g. ISO 16750) or OEM internal specifications, including mounting constraints and connector orientation.

Optional Selection Fields

The next set of fields help align expectations between engineering and suppliers. They are not always mandatory but give a clearer picture of preferred architectures and IC types.

• PFC controller type – analog / digital (MCU-based) / combo controller. This indicates whether the PFC will be governed by a dedicated analog IC, a digital control platform or a combined PFC + DC–DC controller.
• PFC topology – e.g. “single-phase boost”, “interleaved boost” or “totem-pole PFC (Si / SiC)”. This strongly influences gate-driver choice and EMI behaviour.
• DC–DC topology – e.g. “LLC resonant”, “phase-shift full bridge” or “dual-active-bridge” for bidirectional systems.
• Power switch technology – Si MOSFET, SiC MOSFET or mixed. This guides the selection of gate drivers, isolation, layout rules and cooling approach.
• Gate driver isolation strength – basic / reinforced / reinforced with DESAT and soft turn-off / specifically rated for SiC. This indicates how much integrated protection is expected in the driver stage.
• Current and voltage measurement implementation – shunt-based with simple amplifiers, isolated amplifiers, sigma-delta modulators or integrated power monitors. This influences both accuracy and safety architecture.
• Safety architecture hint – single-channel monitored, dual-channel monitored, or external safety MCU supervising the OBC controller and power stages.
• Preferred IC ecosystem – if the platform prefers a dominant supplier for MCU, gate drivers and PMIC (e.g. “TI C2000-based”, “AURIX-based”), this field helps align suggested IC families with corporate strategy.

Example BOM Requirement Combinations

The table below shows how the fields above can be combined into short, descriptive “solution lines”. They are written as combinations of topology, control and interface choices, without tying the design to specific part numbers.

These examples are starting points. In practice, project teams can copy the field names into their own BOM templates and adjust the values to match vehicle architecture, OEM standards and preferred semiconductor ecosystems. Detailed IC family choices and part-number-level recommendations remain in the IC categories and seven-brand mapping section, keeping the BOM fields stable even when device selections evolve.

FAQs about On-Board Chargers (OBC)

These twelve questions highlight the main decisions behind an on-board charger design: power rating, topology, SiC adoption, sensing, isolation, safety levels, interfaces and BOM planning.