In CAN node design, we often add various devices to the CAN interface to make bus communication more reliable. However, not all applications require this; excessive protection not only increases costs, but the parasitic parameters of the devices inevitably affect signal quality. This article will briefly introduce the role of common-mode inductors in bus design.
In practical applications, we see many CAN products using common-mode inductors, but in routine testing, they don't show any significant improvement in any performance metrics; in fact, they can negatively impact waveform quality. To ensure reliability, many engineers add comprehensive peripheral circuitry to the CAN bus as a precaution. However, CAN chips already have good ESD and transient voltage protection capabilities, and some transceivers themselves have excellent EMC performance. In applications, we can add protection and filtering peripherals one by one according to design requirements. Whether or not to add a common-mode inductor to the CAN bus is primarily a matter of electromagnetic compatibility (EMC).
1. Common mode inductor
First, we introduce common-mode interference. Figures 1 and 2 show differential-mode and common-mode interference and their transmission paths, respectively. The driver and receiver in the figures use differential signal transmission, similar to a CAN bus. Differential-mode interference is generated between the two transmission lines, while common-mode interference is generated simultaneously in both lines, with its potential referenced to ground.
Figure 1 Differential mode interference and transmission path
Figure 2 Common-mode interference and transmission path
A common-mode inductor consists of coils wound with the same number of turns but in opposite directions on the upper and lower halves of a magnetic ring. Since common-mode interference is uniform, the magnetic field lines formed within the ring superimpose, and the high inductance attenuates the interference. For differential-mode signals, the magnetic field lines cancel each other out, offering no suppression effect; only the coil resistance and very small leakage inductance slightly affect the differential-mode signal. Essentially, a common-mode inductor is a bidirectional filter, filtering out common-mode interference on the signal lines while preventing the signal lines themselves from emitting electromagnetic interference. The interference signal in Figure 2 is effectively suppressed by the common-mode inductor, while the differential signal is almost unaffected.
2. CAN bus characteristics
The CAN transceiver internally uses open-source and open-drain outputs for CANH and CANL, respectively, and the driving circuit is shown in Figure 3. This method allows the bus to easily achieve dominant level driving, while recessive level is achieved through discharge of the terminating resistor.
Figure 3 CAN transceiver driver circuit
The inherent differential transmission mode of the CAN bus gives it excellent common-mode interference suppression capabilities, as shown in Figure 4. Subtracting CANH and CANL effectively eliminates external common-mode interference. However, CANH and CANL are not ideally symmetrical, and their rapidly rising edges can cause EMC problems. The bus waveform appears perfect on an oscilloscope, and tests for electrostatic discharge (ESD), EFT, surge, and conducted emissions immunity show no abnormalities. However, conducted emissions tests fail to meet the limits; the seemingly normal bus is actually transmitting conducted interference.
Figure 4 CAN transmission waveform
3. Why add a common-mode inductor?
Regarding EMC issues with the CAN interface, besides selecting a CAN transceiver chip with better performance and compliance requirements, another simple method is to add external components to the CAN interface. A common-mode inductor is a good choice. The existing automotive electronics CISPR25 standard has very strict requirements for conducted interference limits. Many CAN transceivers exceed these limits. Figure 5 shows the conducted interference of the CAN interface with and without a common-mode inductor, tested according to automotive-grade limits. The common-mode inductor value is 51μH. It can be seen that the noise improvement is significant across all frequency bands, and the test results still have a large margin.
Figure 5 Conducted disturbance test
Common-mode inductors are effective in reducing conducted interference and can help us quickly meet testing requirements and existing automotive requirements. However, adding common-mode inductors to the bus also introduces two problems: resonance and transient voltage. Common-mode inductors inevitably have parasitic inductance and DC resistance. Considering factors such as the number of bus nodes and communication distance, resonance can occur, affecting the bus signal quality. As shown in Figure 6, the green waveform represents the bus waveform with added common-mode inductor, and there is already obvious resonance at the falling edge of the signal. In addition, common-mode inductors have a large inductance and are directly connected to the transceiver interface. In practical applications, short circuits, hot-plugging, and other conditions can cause transient high voltages across the common-mode inductor, which can directly damage the transceiver in severe cases.
Figure 6 shows the CAN waveform with added common-mode inductor.
4. Summary
The advantages and disadvantages of using common-mode inductors for buses are quite obvious. They can filter out common-mode electromagnetic interference from signal lines, attenuate the high-frequency part of differential signals, suppress electromagnetic interference emitted by the CAN interface itself, and have a good effect on improving conducted emissions. However, the resonance and transient voltage they bring still need to be considered in applications. These are detrimental to the bus signal quality in long-distance, multi-node communication. For general industrial applications, there are no strict requirements for conducted emissions, so common-mode inductors are not necessary.
ZLG Zhiyuan Electronics, based on years of experience in bus protection design, has launched the CTM1051(A)HP series of high-protection isolation modules. This series complies with the international ISO11898-2 standard, offering electrostatic discharge protection up to ±8kV for contact, ±15kV for air discharge, and ±4kV for surge protection. It provides an isolated CAN solution, as shown in Figure 7, suitable for various harsh industrial environments. It is easy to use, plug and play, as illustrated in Figure 8.
Figure 7 EMC performance of CTM1051(A)HP
Figure 8 Application Principle Diagram
