📋 Case Study

Automotive Tier-1 Battery Management System (BMS) Radiated Emissions Failure

Failed CISPR 25 Class 5 radiated emissions at 120–180 MHz due to DC-DC converter switching noise coupling into CAN bus traces

🏗️ Project Overview

High-voltage 800V BMS for next-gen EV platform

🎯 Challenge

Failed CISPR 25 Class 5 radiated emissions at 120–180 MHz due to DC-DC converter switching noise coupling into CAN bus traces

🔧 Design Approach

Redesigned PCB stackup with dedicated ground plane under high-speed traces; added common-mode chokes on CAN lines; relocated ferrite beads to filter converter output ripple

📐 Design Diagram

_sw = 2 MHzCAN Bus TracesCM ChokeGround Plane (Solid)Z_gnd ↓Coupling Pathk ≈ 0.018Z = 42 Ω @ 150 MHzNoise Coupling → CANMitigation StrategyFerrite BeadRelocated to filter rippleTest ResultPASS CISPR 25 Class 5ΔL = 12 dB↓ @ 150 MHz

AI-generated project design illustration

📐 Key Calculations

Coupling Path Impedance

Z = 2πf × L + R

Result: 42 Ω @ 150 MHz

Determined choke inductance needed for >30 dB attenuation

Near-Field Coupling Coefficient

k ≈ (h/d)²

Result: 0.018

Validated spacing improvement reduced coupling by 72%

📊 Results

Passed CISPR 25 Class 5 with 8.2 dB margin at worst frequency; no CAN communication errors observed across thermal cycling (-40°C to +105°C)

💡 Lessons Learned

•PCB layer ordering dominates high-frequency EMI more than component selection alone
•CAN common-mode noise must be addressed at the transceiver pin—not just at connectors

✅ Key Takeaways

1PCB layer ordering dominates high-frequency EMI more than component selection alone
2CAN common-mode noise must be addressed at the transceiver pin—not just at connectors