In automotive CAN networks, common-mode chokes are often used to increase system reliability with respect to electromagnetic compatibility (EMC). Electromagnetic interference emitted from an electronic control module (ECU) through the CAN high-speed transceiver can be filtered, thus limiting unwanted high-frequency noise on the bus lines. 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.
Ole-Kristian Skroppa and Scott Monroe from Texas Instruments discuss the use of common mode chokes in detail in the SLLA271 application report (Common-mode chokes in CAN networks: Source of unexpected transients). They recommend taking care 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. The referenced application report addresses some of the system-level considerations to take into account during network and node design.
Jim Lepkowski from On Semiconductor gives in the AND8169/D application note (EMI/ESD protection solutions for the CAN bus) some guidance how to use common-mode chokes in CAN high-speed networks.
Common-mode choke circuits
Common-mode chokes are an effective tool for attenuating the noise that is common to both of the transceiver bus lines. Chokes function by providing high-impedance for common mode signals and a low-impedance for differential signals, which increases the common mode rejection ratio (CMRR) of the transceiver. Chokes are an effective device to implement filtering without adding a large amount of distortion on high-speed data lines. The common mode choke functions limits the magnitude of an over-voltage surge on the bus lines by functioning as a filter; thus, it is recommended that TVS devices be added to the circuit to provide clamping protection.
Fig. 1: High-Speed CAN protection circuits combining common-mode choke, capacitors and TVS diodes (Source: On Semiconductor)
But there are several disadvantages of chokes. One issue with chokes is that their inductance and the capacitance of the board and transceiver can form a resonant tank circuit that will oscillate. Oscillations on the CAN bus lines will result in false bit detections at the transceiver. Another issue with choke filters is that any mismatch in the inductance of the two coils will cause distortion in the signal waveforms. Therefore, Jim Lepkowski recommends using a combination of a common mode choke, capacitors and TVS diodes to solve the most stringent EMI emission and immunity requirements.
One of the most severe unexpected consequences from common-mode choke use is the extremely high-transient voltages that may be generated by the inductive fly-back during a short circuit of a CAN bus line to a DC voltage. As the transceiver drives the bus level from dominant-to-recessive or recessive-to-dominant during this short-circuit condition, the change in current through the common-mode choke may fly-back in excess of 65 V.