The problem becomes greater with the high currents involved in electric vehicles. Use of the metal chassis as a return is cheaper and lighter than having a dedicated cable, but almost certainly will not deliver the same level of EMI performance. However, edge rates of switching power converters can be controlled to minimise the spectrum of emissions, and topologies can be chosen that naturally have lower emissions without an efficiency penalty. For example, resonant conversion techniques are increasingly common, but at the expense of complexity. But taking a holistic approach, any extra expense can be offset by savings in filtering elsewhere.
Filters to slow data edges can be employed along with balanced-pair data cabling for minimum radiation and pick-up. Although screens can effectively protect signals, they can also provide unwanted return paths for power or other signals without careful termination.
Differential and common-mode chokes in signal and power lines can be helpful, but they will also require capacitors to shunt away interference. If EMI is just blocked with the high impedance of an inductive filter, it generates a voltage which can then be coupled elsewhere as interference. Filters can also interact to produce unwanted resonances and voltage peaking, so these need to be designed carefully with appropriate damping and controlled input/output impedances.
Transient limiters are also useful in the form of varistors, clamp zeners or simply diodes on inputs to clamp the signals to the rail voltages as a maximum. However, varistors are low cost and have a wear-out mechanism with stress, so cannot deliver reliable performance over long periods.
At the subsystem level, modules will have been evaluated for their EMC performance on an individual basis, but they will produce quite different performances with real-life I/O, which is why holistic testing is vital. Overall, there are an almost infinite number of permutations of interference types, levels and effects, so FMEA (Failure Mode and Effect Analysis) will be necessary to mitigate the effects with a hierarchy of fail-safe mechanisms.
Components intended for automotive will be rated for harsh temperatures and for thermal shock and high vibration. Employed in the Murata GCJ, GCG and GCB series of multilayer capacitors, for example, techniques are used to ‘proof’ component terminations against mechanical stress such as ‘soft’ electrodes or even conductive glue. Additionally, high-reliability types may offer a special construction, so that a stress-induced internal electrode short circuit means reduced capacitance rather than causing a total end-to-end short.