What is magnetic field coupling in electromagnetic compatibility

1、 What is magnetic field coupling

Magnetic field coupling in electromagnetic compatibility is caused by the electro-magnetic effect of conduction current. We need the signal to flow through the design path, but the parasitic inductance caused by the external environment provides a path with a lower impedance to the current than the original path.

2、 Generation of magnetic field coupling

Figure 1 shows two vias on a PCB that pass through the upper heat dissipation fins (mentioned for electric field coupling). Now, the wiring is buried in different intermediate layers of the 4-layer board (as shown in Figure 2) to improve the electric field coupling effect discussed earlier. Generally, some locations of wiring require changing the wiring layer to avoid other wiring or components, so that the computer's routing is shown in Figure 2.

Figure 1. Through holes on a PCB

Figure 2. Through holes (inside) on a PCB

If the second via is connected to an unshielded cable, the magnetic flux line caused by the current in the first via is captured by the second via (as shown in Figure 3). At this time, the second via is conductive to the cable due to the current induced by the magnetic flux line. Therefore, it is easier to conduct current and cause potential radiation paths.

Figure 3. PCB through-hole magnetic field lines

3、 Effect of magnetic field coupling

Assuming that there is also a large enough parasitic capacitance between this cable and the high-frequency signal via, some current will flow along this path (as shown in Figure 4). The combination of parasitic mutual inductance and parasitic capacitance is more likely to cause high-frequency harmonic currents to flow through this unshielded cable, and the radiation source exceeds the limit due to this circuit.

图4.寄生回路电流路径

In the example (Figure 5), there is a high-frequency through-hole near the flat wire, which, according to the model listed this time, can easily lead to radiation interference in the magnetic field coupling. Later, in the rectification process, shielding or magnetic rings are needed to solve the problem, which not only increases project time but also increases material costs. The final effect may not necessarily meet expectations, so early resolution at the source is the best way.

图5

4、 Summary

All excessive radiation is caused by unreasonable circuits. Based on this principle, we should consider the coupling of electric and magnetic fields in high-frequency signals in actual project wiring to avoid the occurrence of other abnormal circuits besides the designed circuits. If the influence of electrical and magnetic coupling can be considered in advance to avoid radiation interference, the radiation problems encountered in the later stage will be much less.