This paper discusses the problems commonly used in EMC rectification and tries to discuss it.
First, the product should be diagnosed according to the actual situation, and the ways and means of interference sources and their mutual interference should be analyzed. According to the analysis results, the targeted rectification is carried out.
In general, the main methods of rectification are as follows:1 Attenuating the interference source On the basis of finding the interference source, the interference source can be weakened within the allowable range. The method of weakening the source generally has the following methods:
a Add a decoupling capacitor between Vcc and GND of the IC. The capacitance of this capacitor is between 0.01μF and 0.1μF. Pay attention to the lead of the capacitor when installing, so that the shorter it is, the better.
b Add an attenuator with sensitivity and signal-to-noise ratio guaranteed. For example, the crystal oscillator in VCD and DVD disc players has a serious impact on electromagnetic compatibility. It is one of the feasible methods to reduce the amplitude, but it is not the only solution.
c Another indirect method is to keep the signal line away from the source of interference.
2 Classification of wire and cable In electronic equipment, line-to-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 method of rectification is different, and the following are discussed separately:
(1) Low-frequency coupling Low-frequency coupling is the case where the guiding line length is equal to or less than 1/16 wavelength. The 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. To couple the capacitor or reduce the amount of coupling, the following method can be used:
a Increasing the circuit spacing is the most efficient way to reduce the distributed capacitance.
b Add a highly conductive shield and make the shield a single point ground to effectively suppress low frequency electric field interference.
c An additional filter reduces the amount of coupling between the two circuits.
d Reduce the input impedance, such as the high input impedance of CMOS circuits, which is extremely sensitive to electric field interference.
Connect a capacitor or a lower resistance to the input terminal within the allowable range. The physical model of magnetic field coupling is inductive coupling, and its coupling is mainly coupled by the mutual inductance between the lines. Therefore, the main method of rectification is to destroy or reduce the coupling amount. Generally, the following methods can be used:
[1] Add a filter and pay attention to the input and output impedance of the filter and its frequency response when adding the filter.
[2] Reduce the loop area between the sensitive loop and the source loop, that is, try to make the signal line or the current-carrying line close or twist together with its return line.
[3] Increase the spacing between the two circuits to reduce the mutual inductance between the lines to reduce the coupling amount.
[4] If possible, try to reduce the coupling between the two circuits by making the sensitive loop and the source loop plane orthogonal or close to each other.
[5] Using high-magnetic materials to wrap sensitive lines can effectively solve the problem of magnetic field interference. It is worth noting that it is necessary to form a closed and magnetic circuit, and it is more effective to reduce the magnetic resistance of the magnetic circuit.
(2) High-frequency coupling High-frequency coupling refers to a trace longer than 1/4 wavelength. Due to the standing wave of voltage and current appearing in the circuit, the coupling amount is enhanced and can be solved by the following method:
a Try to shorten the grounding wire and use the surface contact with the outer casing as much as possible.
b Rearrange the input and output lines of the filter to prevent coupling between the input and output lines to ensure that the filtering effect of the filter does not change.
c Shielded cable shield is grounded at multiple points.
d Connect the flying pin of the connector to ground potential to prevent its antenna effect.
3 Improving the ground system The ideal ground wire is a zero-impedance, zero-potential physical entity that is not only the reference point of the signal, but also does not generate a voltage drop when current flows. In a specific electrical and electronic device, this ideal ground line does not exist, and a voltage drop is inevitably generated when current flows through the ground. According to this, according to the mechanism of interference formation in the ground line, it can be summarized as the following two points:
First, reduce the low impedance and power supply line impedance.
Second, the grounding method and the ground loop are correctly selected. According to the grounding method, there are suspended 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 means of suspension. However, the suspended device is prone to static electricity accumulation. When the charge reaches a certain level, electrostatic discharge will occur, so the suspension should not be used. For general electronic equipment. Single-point grounding is suitable for low-frequency circuits. To prevent power frequency current and other stray currents from generating ground potential difference between the various points on the signal ground, the signal ground is isolated from the power supply and the safety ground. Point to connect. Single-point grounding is mainly suitable for frequencies below 3MHz. Multi-point grounding is the only practical grounding method for high-frequency signals. The characteristics of transmission lines are presented in RF. For the effectiveness of multi-point grounding, 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 above 300KHz. Hybrid grounding is suitable for use in electronic circuits where there are high frequencies and low frequencies.
4 Shielding shielding is one of the important measures to improve the electromagnetic compatibility of electronic systems and electronic equipment. It can effectively suppress various electromagnetic interferences transmitted through space. Shielding can be divided into magnetic field shielding and electric field shielding and electromagnetic shielding according to the mechanism. Electric field shielding should pay attention to the following points:
a Choose a material with high conductivity and a good grounding.
b Correctly select the grounding point and a reasonable shape. It is best to ground the shield directly. Magnetic field shielding usually refers only to shielding of DC or low frequency magnetic fields. Its shielding effectiveness is far less than electric field shielding and electromagnetic shielding. Magnetic shielding is often the focus of engineering. When magnetic shielding:
[1] Use ferromagnetic materials.
[2] Keep the magnetic shield away from magnetic components to prevent magnetic short circuits.
[3] Double or even three layers of shielding can be used.
[4] The opening on the upper side of the shield should pay attention to the direction of the opening. As far as possible, the long side of the slit is parallel to the flow direction of the magnetic flux, so that the length of the magnetic circuit is minimized. In general, the magnetic shield does not need to be grounded, but to prevent electric field induction, it is better to ground. When an electromagnetic field passes through a metal or a barrier that attenuates an electromagnetic field, it is subject to a certain degree of attenuation, that is, a shielding effect on the electromagnetic field. In the actual rectification process, depending on the specific needs, which shield and shield shape, size, and grounding method are selected.
5 Change the wiring structure of the circuit board. Some frequency points are determined by the distribution parameters of the traces on the circuit board. It is not very useful by the above methods. Such rectification changes the circuit by adding small inductance, capacitance and magnetic beads to the trace. The parameter structure is moved to a frequency point where the limit is required. For such interference, in order to fundamentally solve its impact, it is necessary to re-route.
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