The on-board charger (OBC) is one of the most critical power electronic subsystems in an electric vehicle. It converts AC grid power into the DC voltage required to charge the vehicle’s traction battery, and it must do so efficiently, safely, and within stringent size and weight constraints. At EVSELab, we have developed deep expertise in OBC design through our Ladybug platform, a 3.3 kW on-board charger module that showcases our approach to high-efficiency, high-density power conversion.
OBC Architecture Overview
A typical on-board charger consists of two main power conversion stages connected in series:
Stage 1: AC/DC Conversion with Power Factor Correction (PFC)
The PFC stage converts the AC input from the grid into a regulated intermediate DC bus voltage. Its primary functions include:
- Power factor correction: Shaping the input current to be sinusoidal and in phase with the input voltage, achieving a power factor close to unity (typically > 0.99)
- Total harmonic distortion (THD) reduction: Ensuring the input current THD meets regulatory limits (typically < 5% per IEC 61000-3-2)
- DC bus voltage regulation: Maintaining a stable intermediate DC bus voltage (typically 380–420 V for single-phase systems)
Stage 2: Isolated DC/DC Conversion
The DC/DC stage converts the intermediate DC bus voltage to the battery charging voltage. Key requirements include:
- Galvanic isolation: Providing safety-critical isolation between the AC grid and the vehicle’s high-voltage battery system
- Output voltage regulation: Supporting the full battery voltage range (typically 250–450 V for 400 V battery systems, or 500–850 V for 800 V systems)
- Current regulation: Implementing constant current / constant voltage (CC/CV) charging profiles as required by the battery management system (BMS)
The two-stage architecture provides independent control of power factor and output regulation, enabling optimization of each stage for its specific function.
PFC Topology Selection
The choice of PFC topology has a significant impact on efficiency, cost, and complexity. Here are the most common options for OBC applications:
Conventional Boost PFC
The single-phase boost PFC is the simplest and most widely used topology:
- Advantages: Simple control, well-understood, low component count
- Disadvantages: Diode bridge rectifier causes ~1% efficiency loss; limited to continuous conduction mode (CCM) or boundary conduction mode (BCM)
- Typical efficiency: 96–97%
- Best suited for: Cost-sensitive applications where maximum efficiency is not the primary goal
Bridgeless Totem-Pole PFC
The totem-pole bridgeless PFC eliminates the input diode bridge rectifier by using active switches for rectification:
- Advantages: Eliminates diode bridge losses, achieves highest PFC efficiency, lower component count
- Disadvantages: Requires WBG devices (SiC or GaN) for high-frequency operation; more complex control during AC zero-crossing
- Typical efficiency: 98.5–99.5%
- Best suited for: Premium OBC designs targeting maximum efficiency and power density
Interleaved PFC
Interleaving multiple PFC phases provides several benefits:
- Reduced input current ripple and smaller input EMI filter
- Reduced inductor size per phase
- Improved transient response
- Better thermal distribution
At EVSELab, our Ladybug platform employs a totem-pole bridgeless PFC with SiC MOSFETs, achieving PFC stage efficiency above 99% at nominal operating conditions.
DC/DC Topology Selection
Phase-Shifted Full-Bridge (PSFB)
The PSFB topology uses four switches in an H-bridge configuration with phase-shifted PWM control:
- Advantages: Zero-voltage switching (ZVS) achievable over a wide load range; well-suited for high power
- Disadvantages: Circulating current during freewheeling interval; duty cycle loss at light loads; output rectifier voltage stress
- Typical applications: OBC designs above 3.3 kW; industrial power supplies
LLC Resonant Converter
The LLC resonant converter has become the preferred topology for modern OBC designs:
- Advantages: Zero-voltage switching (ZVS) for primary switches and zero-current switching (ZCS) for secondary rectifiers; no reverse recovery losses; low EMI; high efficiency across wide load range
- Disadvantages: More complex design process; frequency modulation control; transformer design requires careful attention to magnetizing inductance and leakage inductance
- Typical efficiency: 97–98.5%
- Typical applications: Premium OBC designs, server power supplies, telecom rectifiers
CLLC Resonant Converter (Bidirectional)
For vehicle-to-grid (V2G) and vehicle-to-home (V2H) applications, the CLLC topology extends the LLC converter to bidirectional operation:
- Symmetric resonant tank enables efficient power transfer in both directions
- Same ZVS/ZCS benefits as LLC
- Enables the vehicle to export power back to the grid or home during peak demand
EVSELab’s Topology Choice
The Ladybug platform uses an LLC resonant converter for the DC/DC stage, leveraging SiC MOSFETs on the primary side and SiC Schottky diodes on the secondary side. This combination achieves DC/DC stage efficiency of 97.5% at full load.
The overall Ladybug system efficiency (PFC + DC/DC) reaches 96.5% peak, which is competitive with the best-in-class OBC modules available globally.
Thermal Design Considerations
Thermal management is arguably the most challenging aspect of OBC design due to the severe constraints:
Operating Environment
- Ambient temperature: Automotive OBCs must operate from -40 degrees C to +85 degrees C ambient
- Coolant temperature: Liquid-cooled OBCs typically interface with the vehicle’s glycol cooling loop at 65 degrees C max coolant temperature
- Altitude: Reduced air density at high altitudes degrades cooling performance for air-cooled designs
Heat Generation Sources
The primary heat sources in an OBC include:
- Power semiconductors: SiC/GaN MOSFETs and diodes (conduction + switching losses)
- Magnetic components: Inductor and transformer core losses (hysteresis + eddy current) and winding losses (DC resistance + AC proximity effect)
- Capacitors: ESR losses in electrolytic and film capacitors
- PCB traces: I2R losses in high-current copper traces
Cooling Approaches
Air-cooled designs (typical for aftermarket and lower-power OBCs):
- Aluminum heatsink with forced air convection
- Thermal interface material (TIM) between components and heatsink
- Fan selection based on required airflow at acceptable noise levels
- Typical power density: 1.5–2.5 kW/L
Liquid-cooled designs (typical for OEM automotive applications):
- Cold plate with internal fluid channels
- Direct mounting of power semiconductors to cold plate
- Thermal interface material optimization for minimum thermal resistance
- Typical power density: 3–5 kW/L
Thermal Simulation Workflow
At EVSELab, our thermal design process follows a rigorous simulation-driven approach:
- Loss budget calculation: Detailed calculation of losses for every component at worst-case operating conditions
- CFD simulation: Computational fluid dynamics modeling to predict temperature distribution
- Thermal prototype validation: Infrared thermal imaging and thermocouple measurements on prototype hardware
- Design iteration: Refinement of heatsink geometry, TIM selection, and component placement based on measured data
EMC Design and Compliance
Electromagnetic compatibility (EMC) is a critical design consideration that must be addressed from the earliest stages of OBC development.
Conducted Emissions
OBCs must meet conducted emission limits per CISPR 11 (industrial) or CISPR 25 (automotive). Key design techniques include:
- Input EMI filter: Typically a two-stage LC filter with common-mode and differential-mode filtering
- PCB layout: Minimizing high-frequency loop areas, proper ground plane design, and strategic placement of decoupling capacitors
- Switching behavior optimization: Controlling dV/dt and dI/dt through gate drive resistor selection and active gate drive techniques
Radiated Emissions
Radiated EMI becomes increasingly important at higher switching frequencies. Mitigation strategies include:
- Shielded enclosure design with proper gasket sealing
- Cable filtering and ferrite placement on input and output harnesses
- Minimizing parasitic antenna structures in the PCB layout
Immunity Requirements
Automotive OBCs must withstand various electromagnetic disturbances:
- ESD: IEC 61000-4-2 (contact and air discharge up to 8 kV / 15 kV)
- Radiated immunity: ISO 11452-2 (bulk current injection) and ISO 11452-4 (radiated RF)
- Conducted immunity: ISO 7637-2 (transient pulses on power lines)
- Bulk current injection: ISO 11452-4
EVSELab’s EMC Approach
We integrate EMC considerations throughout the design process rather than treating compliance as a final-stage test:
- EMC requirements are captured in Phase 1 (Plan and Define) of our APQP process
- Pre-compliance simulation is performed during Phase 2 (Product Design)
- PCB layout is reviewed against our EMC design guidelines before manufacturing
- Pre-compliance measurements are taken on first prototypes before formal testing
Control System Design
Digital Control Platform
Modern OBCs use digital control implemented on microcontrollers (MCU) or digital signal processors (DSP). EVSELab’s standard control platform is based on:
- TI C2000 series DSP (TMS320F28xxx) for its dedicated PWM peripherals and ADC performance
- Alternatively, ST STM32G4 series for cost-optimized designs with sufficient performance
Control Loop Architecture
A typical OBC control structure includes:
- PFC voltage loop: Regulates the DC bus voltage with a bandwidth of 10–30 Hz
- PFC current loop: Shapes the input current with a bandwidth of 1–5 kHz
- DC/DC output voltage loop: Regulates the battery voltage during CV mode
- DC/DC output current loop: Regulates the charging current during CC mode
- Communication interface: CAN bus or LIN interface to the vehicle’s BMS and VCU
Safety-Critical Functions
The OBC firmware must implement several safety functions:
- Over-voltage protection (input and output)
- Over-current protection
- Over-temperature protection (semiconductor junction, ambient, coolant)
- Ground fault detection
- Isolation monitoring
- Watchdog timer and self-diagnostic routines
The Ladybug Platform: A Case Study
EVSELab’s Ladybug platform demonstrates our OBC design capabilities:
Key Specifications
- Rated power: 3.3 kW
- Input voltage: 85–265 VAC, 47–63 Hz
- Output voltage: 200–450 VDC (configurable for 400 V or 800 V battery systems)
- Peak efficiency: 96.5%
- Power factor: > 0.99 at rated load
- Dimensions: 260 x 180 x 65 mm
- Weight: < 2.5 kg
- Cooling: Forced air (liquid-cooled variant available)
- Protection rating: IP67 (with enclosure)
Design Highlights
- Totem-pole bridgeless PFC with SiC MOSFETs
- LLC resonant DC/DC with SiC primary switches and SiC Schottky secondary rectifiers
- TI C2000 digital control with CAN bus communication
- Designed for IEC 61851 and IEC 62368 compliance
- APQP-validated development process from concept to production
Conclusion
Designing a high-efficiency on-board charger requires a multidisciplinary approach combining power electronics expertise, thermal engineering, EMC design, control systems engineering, and rigorous quality processes. At EVSELab, we bring all of these disciplines together under our APQP framework to deliver OBC solutions that meet the demanding requirements of the electric vehicle industry.
Whether you need a standard OBC module or a custom design tailored to your vehicle platform, our engineering team in Ho Chi Minh City is ready to help.
Interested in our Ladybug OBC platform or custom OBC design services? Contact EVSELab to start a conversation.
