This article aims at the problems commonly used in EMC rectification, discusses and tries to discuss the issues.
First of all, it is necessary to diagnose the product according to the actual situation, and analyze the sources and the ways and ways of their interference. According to the results of the analysis, targeted corrections will be made.
In general, the main rectification methods are as follows.
1. To weaken the interference source, and to find the interference source, we can reduce the interference source within the permissible range. Generally, the method of weakening the source is as follows:
a. Add a decoupling capacitor between Vcc and GND of IC. The capacitance of this capacitor is between 0. 01μF and 0.1μF. Please pay attention to the lead of the capacitor when installing, making it as short as possible.
b. Add an attenuator to ensure the sensitivity and signal-to-noise ratio. For example, crystal oscillators in VCD and DVD disc players have a serious impact on electromagnetic compatibility. Reducing their amplitude is one of the feasible methods, but it is not the only solution.
c. An indirect method is to keep the signal line away from the interference source.
2, wire and cable sorting, in the electronic equipment, coupling between the lines is an important way, but also an important cause of interference, because the frequency factor, can be roughly divided into high-frequency coupling and low-frequency coupling. Due to the different coupling methods, the rectification methods are also different. The following are discussed separately:
(1) Low-frequency coupling, low-frequency coupling is the case where the length of the guideline is equal to or less than 1/16 wavelength, and low-frequency coupling can be further divided into electric field and magnetic field coupling. The physical model of the electric field coupling is capacitive coupling, so the main purpose of rectification is to reduce To distribute the coupling capacitance or reduce the coupling amount, use the following method:
a. Increasing the circuit spacing is the most effective way to reduce the distributed capacitance.
b. Adding a high-conductivity shield and making the shield single-point grounding can 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, such as CMOS circuit input impedance is high, very sensitive to the electric field interference, can be connected to the input side of a capacitor or lower resistance in the allowable range.
The physical model of magnetic field coupling is inductive coupling. The coupling is mainly coupled through the mutual inductance between lines. Therefore, the main method of rectification is to destroy or reduce the amount of coupling. Generally, the following methods can be used:
a. Add filters. Pay attention to the input/output impedance of the filter and its frequency response when adding filters.
b. Reduce the loop area between the sensitive loop and the source loop, that is, try to make the signal line or current-carrying line close to or twisted with the loop.
c. Increase the spacing between the two circuits so as to reduce the mutual inductance between the lines to reduce the coupling amount.
d. If possible, minimize the amount of coupling between the two circuits by making the sensitive loop and the source loop plane orthogonal or nearly orthogonal.
e. Using high permeability materials to wrap sensitive lines can effectively solve the magnetic field interference problem. It is worth noting that it is necessary to construct a closed magnetic circuit, and efforts to reduce the magnetic resistance of the magnetic circuit will be more effective.
(2) High-frequency coupling and high-frequency coupling means that the length of the 1/4-wavelength trace increases the amount of coupling due to the standing wave of voltage and current in the circuit. The following method can be used to solve the problem:
a. Minimize the grounding wire and use surface contact with the grounding of the housing 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 deteriorate.
c. Shielded cable shields are grounded at multiple points.
d. Connect the connector's dangling pin to ground potential to prevent its antenna effect.
3, to improve the ground system, the ideal ground is a zero-impedance, zero-potential physical entity, it is not only the reference point of the signal, and current will not produce a voltage drop. In a specific electrical and electronic device, this ideal ground line does not exist, and when the current flows through the ground, it will inevitably produce a voltage drop. According to this, the formation mechanism of interference in the ground can be summarized as the following two points:
First, reduce the low impedance and power supply line impedance.
Secondly, the correct choice of grounding method and the ground loop to block, according to the grounding method to have a suspended ground, single-point grounding, multi-point grounding, mixed grounding. If the interference from the sensitive line mainly comes from the external space or the system enclosure, the suspended ground can be used to solve this problem. However, the suspended device is prone to static electricity accumulation. When the charge reaches a certain level, electrostatic discharge will occur, so the floating ground should not be used. In general electronic equipment.
Single-point grounding is suitable for low-frequency circuits. In order to prevent the generation of ground potential difference between power-frequency currents and other stray currents at various points on the signal ground, the signal ground is isolated from the power supply and the safety ground, and the power line is connected to the earth at a single location. Point connection. Single point grounding is primarily suitable for frequencies below 3MHz. Multi-point grounding is the only practical grounding method for high-frequency signals. It exhibits transmission line characteristics in radio frequency. To make multi-point grounding more effective, when the length of grounding conductor exceeds the maximum frequency of 1/8 wavelength, multipoint grounding requires an equipotential Ground plane. Multi-point grounding is suitable for more than 300KHz. Hybrid grounding is suitable for electronic circuits that have high frequencies and low frequencies.
4. Shielding and shielding is one of the important measures to improve the electromagnetic compatibility of electronic systems and electronic devices. It can effectively suppress various electromagnetic interferences that propagate through space. According to the mechanism of shielding can be divided into magnetic shielding and electric field shielding and electromagnetic shielding. Electric field shielding should pay attention to the following points:
a. Select materials with high electrical conductivity and have good grounding.
b. The correct choice of grounding point and reasonable shape, it is best to shield directly grounded.
Magnetic shielding usually refers to the shielding of DC or even low-frequency magnetic fields. Its shielding performance is far inferior to electric field shielding and electromagnetic shielding. Magnetic shielding is often the focus of engineering. When magnetic shielding is:
a. Use ferromagnetic materials.
b, magnetic shielding body away from magnetic components, to prevent magnetic short circuit.
c. Double or even three layers can be used.
d, the upper hole of the shield body should pay attention to the direction of the hole, as long as possible so that the long side of the seam is parallel to the magnetic flux, so that the magnetic circuit length increases minimally.
In general, magnetic shielding does not require grounding, but it is better to prevent electric field induction or grounding. The electromagnetic field will be attenuated to a certain extent when it passes through a metal or a barrier that attenuates the electromagnetic field, that is, it produces a shielding effect on the electromagnetic field. In the actual rectification process, depending on the needs of the specific choice of shielding and shielding body shape, size, grounding methods.
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. It is not useful by the aforementioned method. Such rectification will increase the inductance, capacitance, and magnetic beads in the wiring. Change the circuit parameter structure to move it to the frequency point where the limit value is higher. For this kind of interference, we must re-wiring if we want to fundamentally solve its impact.
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