Electromagnetic Compatibility for Electric Vehicles

Compliance to CISPR-25 Conducted Emissions Voltage

When comes to achieving EMC compliance for automotive specs a 2-layer PCB works only for very simple designs. Modules having CAN/LIN, USB, E-Net, LVDS are normally using 4-layer PCBs. If adding CPU and memory a 4-layer PCB may not be good enough.

Ford (FMC) mentioned 20 years ago in their EMC design guide for 2-layer PCB to use the “Ground Grid Technique” for top and bottom side of PCB. They also recommended the use of "Faraday Cage" by installing ground vias around the perimeter of the PCB every say 15mm that are connected together on both PCB sides by 0.4 mm thick traces.

The problem with CE-V is that even using a “ground grid” or “faraday cage” won’t prevent DUT’s noise to be coupled into Supply Lines. Assuming that in your case CE-V are higher on GND line I would:

  1. determine potential sources of noise with harmonics in FM band
  2. determine the noise coupling method (e.g. common mode current, common return path, ground loops)
  3. Clamp ferrite corres on the entire test harness to lower CE-V noise below limit. Clamp the same ferrite separately on VBATT or GND line to determine which one is more affected. 

RF filters are effective for emissions exceeding the limit with 2-3 dB. For conducted emissions exceeding the limit with 10 dB you better try to fix the layout first.

Christian Rosu Feb 8, 2020.