These methods that failed the EMC test

First of all, it is necessary to diagnose the product according to the actual situation, analyze the source of the interference and the ways and methods of mutual interference. Then according to the analysis results, targeted rectification. In general, the main rectification methods are as follows.

1. Reducing the interference source On the basis of finding the interference source, the interference source can be reduced within the allowable range. The methods of reducing the source are generally as follows:

a Add a decoupling capacitor between the VCC and GND of the IC. The capacity of this capacitor is between 0.01μF-0.1μF. Pay attention to the lead of the capacitor when installing, so that it is as short as possible. b Add an attenuator while ensuring sensitivity and signal-to-noise ratio. For example, the crystal oscillator in VCD and DVD players has a serious impact on electromagnetic compatibility. Reducing the amplitude is one of the feasible methods, but it is not the only solution. c Another indirect method is to keep signal lines away from interference sources.

2. Classification and sorting of wires and cables In electronic equipment, inter-line coupling is an important way and an important cause of interference. Because of the frequency factor, it can be roughly divided into high-frequency coupling and low-frequency coupling. Due to the different coupling methods, the rectification methods are also different, discussed separately below:

(1) Low frequency coupling Low frequency coupling is the case where the length of the guide line is equal to or less than 1/16 wavelength. Low frequency coupling can be divided into electric field and magnetic field coupling. The physical model of electric field coupling is capacitive coupling, so the main purpose of rectification is to reduce the distribution For coupling capacitance or reducing the coupling amount, the following methods can be used:

a Increasing the circuit spacing is the most effective method to reduce the distributed capacitance. b Add high conductivity shield and ground the shield at one point to effectively suppress low frequency electric field interference. c Additional filters can reduce the amount of coupling between the two circuits. d Reduce the input impedance. For example, the input impedance of CMOS circuits is very high, and it is extremely sensitive to electric field interference. A capacitor or a resistor with a lower resistance can be connected in parallel at the input end within the allowable range. The physical model of magnetic field coupling is inductive coupling, and its coupling is mainly coupled through the distributed mutual inductance between the lines. Therefore, the main method of rectification is to destroy or reduce its coupling amount. Generally, the following methods can be used:

a Add filter, pay attention to the input and output impedance of the filter and its frequency response when adding the filter.

b Reduce the loop area of the sensitive circuit and the source circuit, that is, try to make the signal line or current carrying line close to or twisted together with its return line. c Increase the distance between the two circuits to reduce the mutual inductance between the lines and reduce the amount of coupling. d If possible, try to make the sensitive loop and source loop plane orthogonal or close to orthogonal to reduce the amount of coupling between the two circuits. e Using high permeability materials to wrap sensitive wires can effectively solve the problem of magnetic field interference. It is worth noting that a closed magnetic circuit should be formed, and efforts to reduce the magnetic resistance of the magnetic circuit will be more effective.

(2) High-frequency coupling High-frequency coupling refers to traces longer than 1/4 wavelength. Due to the presence of voltage and current standing waves in the circuit, the amount of coupling will be enhanced, which can be solved by the following methods:

a Shorten the ground wire as much as possible, and use the surface contact method as far as possible to connect with the ground of the shell. b Rearrange the input and output lines of the filter to prevent coupling between the input and output lines and ensure that the filtering effect of the filter does not deteriorate. c The shielding layer of the shielded cable is grounded at multiple points. d Connect the floating pin of the connector to ground potential to prevent its antenna effect.

3. Improving the ground wire system The ideal ground wire is a zero-impedance, zero-potential physical entity. It is not only a reference point for signals, but also no voltage drop when current flows. In specific electrical and electronic equipment, such an ideal ground wire does not exist, and a voltage drop will inevitably occur when current flows through the ground wire. According to this, the formation mechanism of interference in the ground wire can be summarized as the following two points:

First, reduce low impedance and power feeder impedance. Second, correctly select the grounding method and block the ground loop. According to the grounding method, there are floating ground, single-point grounding, multi-point grounding, and mixed grounding. If the interference of the sensitive line mainly comes from the external space or the system casing, it can be solved by the method of floating ground, but the floating ground equipment is prone to static electricity accumulation. When the charge reaches a certain level, electrostatic discharge will occur, so the floating ground is not suitable for use. For general electronic equipment. Single-point grounding is suitable for low-frequency circuits. In order to prevent power frequency currents and other stray currents from generating ground potential differences between points on the signal ground, the signal ground is isolated from the power supply and the safety ground. Dot connection. Single-point grounding is mainly applicable to frequencies below 3MHz. Multi-point grounding is the only practical grounding method for high-frequency signals. It will show transmission line characteristics at radio frequency. To make multi-point grounding effective, when the length of the grounding conductor exceeds the maximum frequency of 1/8 wavelength, multi-point grounding requires an equipotential Ground plane. Multi-point grounding is suitable for 300KHz or more. Hybrid grounding is suitable for electronic circuits that have high and low frequencies.

4. Shielding is one of the important measures to improve the electromagnetic compatibility performance of electronic systems and electronic equipment. It can effectively suppress various electromagnetic interferences propagating through space. According to the mechanism, shielding can be divided into magnetic field shielding, electric field shielding and electromagnetic shielding. Electric field shielding should pay attention to the following points: a. Select materials with high conductivity and good grounding. b Correctly select the grounding point and reasonable shape, it is best to directly shield the shield. Magnetic field shielding usually only refers to the shielding of DC or very low frequency magnetic fields, and its shielding effectiveness is far less than that of electric field shielding and electromagnetic shielding. Magnetic shielding is often the focus of engineering. When magnetic shielding:

a Use ferromagnetic materials. b Keep the magnetic shield away from magnetic components to prevent magnetic short circuit. c Double-layer shielding or even three-layer shielding can be used. d The opening of the upper side of the shielding body should pay attention to the direction of the opening. The long side of the slit should be parallel to the direction of magnetic flux as much as possible to minimize the length of the magnetic circuit. Generally speaking, the magnetic shield does not need to be grounded, but to prevent electric field induction, it is better to be grounded. The electromagnetic field will be attenuated to a certain extent when passing through a metal or a barrier that has an attenuation effect on the electromagnetic field, that is, a shielding effect on the electromagnetic field is generated. In the actual rectification process, depending on the specific needs, choose which kind of shield and the shape, size and grounding method of the shield.

5. Change the wiring structure of the circuit board. Some frequency points are determined by the distribution parameters of the wiring on the circuit board. The above method is not very useful. Such rectification is changed by adding small inductance, capacitance, and magnetic beads to the wiring. The circuit parameter structure makes it move to the frequency point where the limit value is higher. For this type of interference, if you want to fundamentally solve its impact, you must re-wire.

Summary: In general, the previous methods are good for improving electromagnetic compatibility, but the most widely used method is to change the structure of the ground wire and the classification of wire and cable. These methods not only save costs, but also are the most effective rectification methods. Although shielding will increase the cost, its shielding effectiveness is sometimes unmatched by other methods. Therefore, in the actual rectification, the methods of changing the structure of the ground wire, the sorting and shielding of the wires and cables, and the shielding should be the main method, and the other methods should be supplemented.