8 tricks to solve EMI conducted interference

The interference signals generated by electronic devices in electromagnetic interference EMI are transmitted through wires or public power lines, and the interference generated by each other is called conducted interference. Conducted interference has confused many electronic engineers. How to solve conducted interference? Find the right method, and you will find that conducted interference is actually very easy to solve. Just increase the number of EMC filter sections in the power input circuit and adjust the parameters of each filter section appropriately, and basically meet the requirements. The organizer of the 7th Circuit Protection and Electromagnetic Compatibility Seminar summarized eight countermeasures to solve the problem of conducting interference.

Countermeasure 1: Minimize the effective area of ​​each loop

Figure 1

Conducted interference is divided into differential mode interference DI and common mode interference CI. Let's first look at how conducted interference is generated. As shown in Figure 1, loop current generates conducted interference. There are several loop currents here. We can regard each loop as an induction coil, or the primary and secondary of a transformer coil. When current flows through a loop, an induced electromotive force will be generated in another loop, thereby generating interference. The most effective way to reduce interference is to minimize the effective area of ​​each loop.

Countermeasure 2: Shield, reduce the area of ​​each current loop and the area and length of the live conductor

As shown in Figure 2, e1, e2, e3, and e4 are differential mode interference signals generated by the magnetic field induction on the loop; e5, e6, e7, and e8 are common mode interference signals generated by the magnetic field induction on the ground loop. One end of the common mode signal is the entire circuit board, and the other end is the earth. The common end in the circuit board cannot be considered as grounding. Do not connect the common end to the casing unless the casing is connected to the earth. Otherwise, connecting the common end to the casing will increase the effective area of ​​the radiating antenna, and the common mode radiation interference will be more serious. There are two ways to reduce radiation interference: one is shielding, and the other is to reduce the area of ​​each current loop (magnetic field interference) and the area and length of the charged conductor (electric field interference).

Countermeasure 3: Magnetic shielding of transformers to minimize the effective area of ​​each current loop

As shown in Figure 3, among all electromagnetic induction interference, the interference caused by transformer leakage inductance is the most serious. If the leakage inductance of the transformer is regarded as the primary of the transformer induction coil, then other loops can be regarded as the secondary of the transformer. Therefore, interference signals will be induced in the loops around the transformer. The method to reduce interference is, on the one hand, to magnetically shield the transformer, and on the other hand, to minimize the effective area of ​​each current loop.

Countermeasure 4: Shield the transformer with copper foil

As shown in Figure 4, the purpose of transformer shielding is to reduce the electromagnetic induction interference caused by the transformer leakage flux to the surrounding circuits, as well as the electromagnetic radiation interference caused to the outside. In principle, non-magnetic materials cannot directly shield the leakage flux, but copper foil is a good conductor. When the alternating leakage flux passes through the copper foil, eddy currents will be generated. The direction of the magnetic field generated by the eddy currents is just opposite to the direction of the leakage flux, so part of the leakage flux can be offset. Therefore, copper foil can also play a good shielding role for the flux.

Countermeasure 5: Use two-wire transmission and impedance matching

Figure 5

As shown in Figure 5, if the currents of two adjacent wires are equal in magnitude and opposite in direction, the magnetic lines of force they generate can cancel each other out. For circuits with severe interference or those that are easily interfered with, try to use two-wire transmission signals instead of using a common ground to transmit signals. The smaller the common ground current, the smaller the interference. When the length of the wire is equal to or greater than a quarter wavelength, impedance matching must be considered for the line that transmits the signal. Unmatched transmission lines will generate standing waves and produce strong radiation interference to surrounding circuits.

Countermeasure 6: Reduce the area of ​​the current loop

Figure 6

As shown in Figure 6, the magnetic field radiation interference is mainly caused by the magnetic flux generated by the high-frequency current loop flowing into the receiving loop. Therefore, the area of ​​the high-frequency current loop and the area of ​​the receiving loop should be minimized. Where: e1, Φ1, S1, B1 are the electromotive force, magnetic flux, area, and magnetic flux density generated in the radiation current loop; e2, Φ2, S2, B2 are the electromotive force, magnetic flux, area, and magnetic flux density generated in the radiation current loop.

Figure 7

The following is a detailed explanation of the current loop radiation using Figure 7. As shown in the figure, S1 is the rectifier output filter loop, C1 is the energy storage filter capacitor, and i1 is the loop high-frequency current. This current is the largest among all current loops, and the magnetic field interference it generates is also the most serious. The area of ​​S1 should be minimized.

In the S2 loop, there is basically no high-frequency loop current. ∆I2 is mainly the power supply ripple current, and the high-frequency component is relatively small, so the area size of S2 basically does not need to be considered. C2 is an energy storage filter capacitor, which is specifically used to provide energy for the load R1. R1 and R2 are not simple load resistors, but high-frequency circuit loads. The high-frequency current i3 is basically provided by C2. The position of C2 is relatively important. Its connection position should be considered to minimize the area of ​​S3. There is also a ∆I3 in S3, which is mainly the power supply ripple current and also has a small amount of high-frequency current component. In the S4 loop, there is basically no high-frequency loop current. ∆I4 is mainly the power supply ripple current, and the high-frequency component is relatively small, so the area size of S4 basically does not need to be considered. The situation of the S5 loop is basically the same as that of the S3 loop, and the current loop area of ​​i5 should also be as small as possible.

Countermeasure 7: Do not use multiple circuits in series to supply power

The current loops in Figure 7 are connected in series to supply power, which can easily cause current common-mode interference, especially in high-frequency amplifier circuits, which can generate high-frequency noise. The reason for current common-mode interference is: ∆I2 = ∆I3+ ∆I4+ ∆I5

Figure 8

In FIG8 , the current loops are separated from each other and powered in parallel. Each current loop is independent and will not generate current common-mode interference.

Countermeasure 8: Avoid interference signals from resonating in the circuit

Fig. 9

As shown in Figure 9, one pole of the common mode antenna is the entire circuit board, and the other pole is the ground wire in the connecting cable. The most effective way to reduce radiation interference is to shield the entire circuit board and ground the shell. The cause of electric field radiation interference is that the high-frequency signal charges the conductor or lead, and the length and surface area of ​​the conductor should be minimized. The cause of magnetic field interference is that high-frequency current flows through the conductor or loop, and the length and area of ​​the current loop in the circuit board should be minimized. The higher the frequency, the more serious the electromagnetic radiation interference; when the length of the carrier can be compared with the wavelength of the signal, the interference signal radiation will be enhanced.

When the length of the current carrier is exactly equal to an integer multiple of one-quarter wavelength of the interference signal, the interference signal will resonate in the circuit. At this time, the radiation interference is the strongest. This situation should be avoided as much as possible.

Do you think that by following these eight steps, conducted interference is under control? Finally, the spectrum of various interference pulse waveforms is attached for your reference (as shown in Figure 10). Any non-sinusoidal wave can be regarded as a superposition of many signals with different rising and falling rates (or sine waves of different frequencies), and the intensity of electromagnetic radiation is proportional to the rate of change of voltage or current.

Spectra of various interference pulse waveforms: