Noise suppression technologies/case study introduction (Automotive)

CAN FD Noise Suppression-1 (1/2)

24/12/2021

INDEX

1. What is CAN FD?

CAN FD is a type of interior LAN that connects and communicates between the ECU and computer in an automobile. CAN FD can communicate at a faster speed than the conventional CAN standard.

2. Noise problems

CAN acceleration and noise problems

CAN FD accelerates the bit rate beyond the conventional CAN standard.
As a result, the emission noise problem which occurs during CAN communication increases.

Noise problem in CAN-FD

In CAN FD and other interfaces, a transmission system called "differential transmission" is used.
Because the signal is transmitted through the potential difference between the two signal lines in differential transmission, it has the characteristic of being resistant to radiated noise from the outside. Moreover, the radio waves occurring in both lines due to the signal cancel each other out, so it is unlikely to be externally emitted as noise.

For external noise

For noise emissions

However, on a circuit board a common mode current will be generated by various factors. When this current flows into the cable of the differential transmission line, common mode noise is emitted from the cable.

3. Noise problem suppression

Using a common mode choke coil

A common mode choke coil is suited as a measure to address common mode noise.
A common mode choke coil is a coil that magnetically couples both lines and can remove the common mode noise without affecting the differential signal or other differential mode signals.

The reverse phase (differential mode) signal is canceled by the magnetic field -> passes through
The in phase (common mode) noise is superimposed by the magnetic field -> reflected

Common mode choke coil quality and mode conversion

In principle, the common mode choke coil has the effect of removing the common mode noise, but when the balance between the coils of both lines is lost, a mode conversion occurs and part of the differential mode signal may be converted to common mode and become noise. Therefore, it is important to use a balanced common mode choke coil with little mode conversion.