In the design of CAN node, we usually add various devices for the can interface in order to make the bus communication more reliable, but not all applications need it in practice. Excessive protection not only increases the cost, but also the parasitic parameters of devices will inevitably affect the signal quality. This paper will briefly introduce the role of common mode inductance in bus.
In practical application, we see that many can products will use common mode inductors, but we can’t see that it can significantly improve any index in routine test, but affect the waveform quality. Many engineers will add a comprehensive peripheral circuit to can just in case to ensure reliability. Can chip has good anti-static and transient voltage capability, and some transceivers also have good EMC performance. We can add protection, filtering and other peripherals one by one according to the design requirements in application. Whether to add common mode inductance to can bus is mainly considered from the aspect of electromagnetic compatibility.
1. Common mode inductance
First, common mode interference is introduced. Figure 1 and Figure 2 show differential mode interference, common mode interference and their transmission paths respectively. The driver and receiver in the figure are differential signal transmission, similar to can bus. Differential mode interference is generated between two transmission lines, while common mode interference is generated in both lines at the same time, and its potential is based on the ground.
Figure 1 differential mode interference and transmission path
Figure 2 common mode interference and transmission path
Common mode inductors are coils with the same number of turns but opposite direction wound on the upper and lower half rings of a magnetic ring. The common mode interference is the same, so the magnetic lines of force formed in the magnetic ring are superimposed with each other, and the inductance impedance is large, so it plays the role of attenuation interference. For the differential mode signal, the magnetic lines of force formed in the magnetic ring cancel each other and have no inhibitory effect. Only the coil resistance and small leakage inductance have a slight impact on the differential mode signal. The common mode inductor is essentially a bidirectional filter. On the one hand, it filters out the common mode signal interference on the signal line, and on the other hand, it suppresses the electromagnetic interference from the signal line itself. The interference signal in Figure 2 can be well suppressed by the common mode inductance, while the differential signal has little effect.
2. Can bus characteristics
Canh and canl in the can transceiver are open source and open drain output respectively. The driving circuit is shown in Figure 3. In this way, the bus can easily realize the driving of explicit level, while the implicit level is realized by terminal resistance discharge.
Figure 3 can transceiver drive circuit
The inherent differential transmission form of the bus makes can have a good ability to suppress common mode interference. As shown in Figure 4, the common mode interference from the outside can be well eliminated by subtracting canh and canl, but canh and canl are not ideal symmetrical and fast rising jump edges, which will bring EMC problems. We can see that the bus waveform is perfect through the oscilloscope, and there is no abnormality in static electricity, EFT, surge, conduction disturbance and immunity. However, when testing conducted emission, it can not meet the limit requirements. The bus that looks very normal is actually sending conducted interference to the outside.
Figure 4 can transmission waveform
3. Why add a common mode inductor?
For the EMC problem of can interface, in addition to selecting can transceiver chips with better performance and symbol requirements, another simple method is to add peripherals to the can interface. Common mode inductance is a good choice. In the existing CISPR 25 standard for automotive electronics, there are very strict requirements for the limit of conducted disturbance. Many can transceivers will exceed the limit. As shown in Figure 5, the conducted disturbance of can interface with and without common mode inductance is tested according to the limit of vehicle regulations, and the common mode inductance value is 51 μ H. It can be seen that the noise is obviously improved in each frequency band, and there is still a large margin in the test results.
Figure 5 conducted disturbance test
Common mode inductance plays an obvious role in reducing conduction disturbance, which can help us quickly pass the test requirements and meet the existing automotive requirements. However, adding common mode inductance to the bus will also bring two problems: resonance and transient voltage. The common mode inductance will inevitably have parasitic inductance and DC resistance. Considering the number of bus nodes, communication distance and other factors, it will cause resonance and affect the bus signal quality. As shown in Figure 6, the green waveform is the bus waveform to increase the common mode inductance, and there is obvious resonance on the falling edge of the signal. In addition, the common mode inductance has large inductance and is directly connected to the transceiver interface. In practical application, short circuit, hot plug and other states will produce transient high voltage at both ends of the common mode inductance, which will directly damage the transceiver in serious cases.
Fig. 6 can waveform of adding common mode inductance
The advantages and disadvantages of common mode inductor used in bus are obvious. It can filter the common mode electromagnetic interference of signal line, attenuate the high-frequency part of differential signal, suppress the electromagnetic interference emitted by can interface itself, and improve the conducted disturbance. However, its application still needs to consider the resonance and transient voltage, which are in long distance, In multi node communication, the bus signal quality is unfavorable. For general industrial applications, there are no strict requirements for conduction and emission, so the common mode inductance can not be increased.
Based on years of bus protection design accumulation, ZLG Zhiyuan electronics has launched ctm1051 (a) HP series isolation module with high protection level. This series complies with the international iso11898-2 standard. The electrostatic protection grade can reach contact ± 8Kv, air discharge ± 15kV and surge protection can reach ± 4KV. The isolation can solution is shown in Figure 7 below. It can be applied to various harsh industrial site environments. The application is simple, plug and play, and the application schematic diagram is shown in Figure 8 below.
Figure 7 ctm1051 (a) EMC performance of HP
Fig. 8 application schematic diagram