Common-Mode Chokes in CAN Networks: Source of Unexpected Transients
Common-mode chokes are frequently used in automotive CAN networks to increase system reliability with respect to electromagnetic compatibility (EMC). Electromagnetic interference emitted from an Electronic Control Module (ECU) through the CAN transceiver can be filtered, thus limiting unwanted high-frequency noise on the communication bus. Another reason for using a common-mode choke is attempting to improve the susceptibility (immunity) of the transceiver to electromagnetic disturbances on the bus.
While the above mentioned effects of the common-mode choke are beneficial, unexpected results can occur under certain conditions. EMC susceptibility can be degraded in some frequency ranges, bus signal integrity worsened, and extremely high transient voltages under bus-failure conditions can be generated, which, in the worst case, can lead to damage in the CAN transceiver and other network components.
Care should be taken in the choice of common-mode choke (winding type, core type, and inductance value), along with the termination and protection scheme of the node and bus to prevent damage to the CAN transceiver or other network components. This application report addresses some of the system-level considerations to take into account during network and node design.