OBC Topology and Component Requirements
Modern OBC systems typically employ a two-stage approach:
- PFC Stage: Boost converter to achieve unity power factor and regulate intermediate bus voltage
- DC-DC Isolated Stage: LLC resonant converter for efficient power transfer and galvanic isolation
For a typical 3.3kW OBC system:
PFC Inductor Selection
Calculate based on ripple requirements and operating conditions:
L = V_in(avg) × D_avg / (f_s × ΔI_L)
For 3.3kW system with 230VAC input (nominal 330VDC), 70kHz switching frequency:
- Recommended inductance: 330µH at 16A saturation current
- DC resistance: <10mΩ for efficiency
- Recommended series: B78105N2102M (1mH, 10A)
- For OBC: Consider 2-3× current derating to account for harsh environment
LLC Transformer Design
The LLC transformer design requires careful attention to magnetizing inductance and leakage inductance for optimal ZVS operation:
- Turns ratio: Optimized for input/output voltage range
- Magnetizing inductance: Determines ZVS range and power transfer capability
- Leakage inductance: Combined with external inductor for resonant frequency
- Insulation: Reinforced for EV safety requirements
Recommended: EPCOS automotive-grade transformers with specialized materials for high-temperature operation and enhanced reliability.
EMI Filter Design for Automotive Compliance
Automotive OBC systems must comply with CISPR 25 Class 5 conducted emissions limits. The EMI filter should be designed to provide:
- Differential mode attenuation in 150kHz-30MHz range
- Common mode attenuation in 30MHz-1000MHz range
- Bi-directional filtering (charging and discharging)
- Safety compliance for high-voltage DC systems
Multi-stage filtering approach using:
- Common mode chokes for common mode noise suppression
- Safety capacitors (X/Y types) for differential and common mode filtering
- Additional high-frequency filtering for GHz range emissions
Thermal Management Considerations
Underhood installation subjects magnetic components to elevated temperatures. Considerations include:
- Component derating for elevated temperature operation
- Adequate airflow around magnetic components
- Thermal modeling to verify component temperatures
- Enhanced cooling for high-power applications
- Thermal management impact on component life
For magnetic components in OBC applications, target component temperatures <85°C for optimal reliability in automotive environment.
