Product electromagnetic compatibility design (serial)

Electromagnetic interference that causes equipment performance degradation or failure must have three elements, namely, there is an electromagnetic disturbance source; there is a device that is sensitive to interference; and there is a propagation path of electromagnetic interference. In order to solve the electromagnetic compatibility problem of equipment, satisfactory results can be achieved whether by suppressing the escape of disturbance sources, cutting off the propagation path of electromagnetic interference, or improving the anti-interference ability of the equipment itself. To this end, this lecture intends to discuss measures to improve the electromagnetic compatibility of equipment from three different aspects, including methods to suppress product harassment; methods to improve product immunity; and key points in the design of electromagnetic compatibility within the product.

First, we discuss how to suppress product harassment.

There are three basic methods to suppress product harassment, namely grounding, shielding and filtering. Each method has a unique role in the design of circuits and systems, but they are interrelated in use. For example, good grounding can reduce the equipment's requirements for shielding and filtering; and good shielding can also reduce the requirements for filtering.

1 ground

"Grounding" includes the signal grounding inside the equipment and the equipment grounding. The two have different concepts and different purposes.

1.1 Signal grounding of equipment

The signal ground of the device may be a certain point in the device or a thin metal plate as the ground reference point of the signal. It provides a common reference potential for all signals in the device.

There are three basic signal grounding methods in practice, namely floating ground, single-point grounding and multi-point grounding.

(1) Floating land

The purpose of a floating ground is to isolate a circuit or equipment from a common ground system or common conductors that may cause circulating currents. Floating ground also makes it easier to match circuits with different potentials.

Methods to achieve floating circuits or equipment include transformer isolation and photoelectric isolation.

The biggest advantage of floating ground is its good anti-interference performance.

The disadvantage of floating ground is that because the equipment is not connected to the public ground, it is easy to cause static electricity accumulation between the two. When the charge accumulates to a certain extent, the potential difference between the equipment ground and the public ground may cause severe electrostatic discharge, which becomes A very destructive source of harassment.

A compromise is to place a large value bleeder resistor across the floating ground and common ground to discharge the accumulated charge. Pay attention to controlling the impedance of the bleed resistor. A resistance value that is too low will affect the qualification of the leakage current of the equipment.

(2) Single point grounding

Single-point grounding means that only one physical point in a circuit or equipment is defined as the ground reference point, and all points in the circuit or equipment that need to be grounded are connected to this point.

For a system, if single-point grounding is used, each device in the system has its own single-point grounding point, and then the "ground" of each device is connected to the only designated reference grounding point in the system.

The disadvantage of single-point grounding is that when the operating frequency of the system is very high, so that the wavelength of the signal is comparable to the length of the grounding wire (such as reaching 1/4 wavelength), the grounding wire cannot be considered as an ordinary connecting wire, and it will appear Some kind of reactance effect makes the grounding effect unsatisfactory. At this time, the concept of multi-point grounding must be introduced.

(3) Multi-point grounding

Multi-point grounding means that all points in the equipment that need to be grounded are directly connected to the grounding plane closest to it, so as to keep the grounding wire as short as possible. The ground plane mentioned here can be the base plate of the equipment, a dedicated ground bus, or even the frame of the equipment.

The advantage of multi-point grounding is that it is simple. All points that need to be grounded can be grounded nearby, thus greatly reducing the phenomenon of high standing waves on the grounding line. Therefore, multi-point grounding performs well when used at high frequencies.

Multi-point grounding has higher maintenance requirements for grounding points. Any reasons such as rust or looseness can cause the grounding effect to deteriorate and make the equipment unreliable.

(4) Hybrid grounding

The respective advantages and disadvantages of single-point and multi-point grounding have prompted people to think of hybrid grounding. Only the points that need to be grounded nearby are directly connected to the ground plane (or points that need high-frequency grounding are connected to the ground plane through bypass capacitors), and all other points are single-point grounded.

The boundary between single-point grounding and multi-point grounding is usually 0.05 times the wavelength λ of the circulating signal. When the length of the single-point grounding wire reaches more than 0.05λ, multi-point grounding should be used.

(5) Processing of signal ground wires

Signal grounding is the establishment of a conductive path between two designated points (one of which is a reference point called "ground") in order to achieve a connection between a circuit and the chassis, or between a circuit and a designated ground plate. The most critical thing is to emphasize good connections and establish low-impedance paths, which is especially important for the flow of RF current in ground loops.

1.2 Grounding of equipment

(1) Grounding of equipment

In practice, in addition to carefully considering the signal grounding inside the equipment, it is usually necessary to connect the equipment's signal ground, chassis and earth together, and use the earth as the grounding reference point of the equipment. There are three purposes for connecting equipment to the earth:

① The safety grounding of the equipment ensures the safety protection of the equipment operators.

② Discharge the charge accumulated on the chassis to avoid increasing the potential of the chassis due to charge accumulation and causing unstable circuit operation.

③ Prevent the equipment from changing its potential to the earth under the action of the external electromagnetic environment, causing instability in the operation of the equipment.

It can be seen that in addition to considering the safety of personnel and equipment, connecting equipment to the ground is also an important means to suppress interference. In practice, if grounding can be used in conjunction with shielding, filtering and other technologies, it will play a more effective role in improving the electromagnetic compatibility of the equipment.

(2) Grounding method and grounding resistance

An important indicator of the effectiveness of grounding is grounding resistance. Ground resistance is not only related to the way the ground electrode is made, but also related to the properties of the earth itself.

People are accustomed to using underground metal pipes as ground electrodes because they have a larger contact area with the earth and can achieve smaller ground resistance. However, if the practice is not standardized, fault current and stray current flowing into the pipeline can easily cause harm to pipeline maintenance personnel. In addition, any non-metallic duct components can undermine the effectiveness of the grounding.

The correct way to connect to the earth is to use a copper rod with a diameter of 1cm to 2cm (2m to 4m long) and drive it into the ground to a depth of more than 2m. The grounding resistance of a copper rod is about 25Ω, which is sufficient for some low-power electrical equipment. If you want a smaller ground resistance, you can increase the salt and moisture in the area near the copper rods, and if necessary, several copper rods can be interconnected into a network. After weighing the equipment investment and the requirements for lightning protection, power failure protection and electromagnetic pulse protection, it is reasonable to set the grounding resistance to 10Ω as the design target.

2 shield

Shielding can effectively suppress electromagnetic interference propagating through space. There are two purposes of shielding: one is to limit the radiated electromagnetic energy inside the equipment from leaving a certain area; the other is to prevent external radiated electromagnetic energy from entering a certain area.

According to the role of shielding, there are three types: electric field shielding, magnetic field shielding and electromagnetic field shielding.

2.1 Electric field shielding

The mutual induction between objects with different potentials in the equipment (including wires) can be regarded as the voltage distribution between distributed capacitors (see Figure 1.1). In Figure 1.1, the voltage relationship between interference source A and induced object B is

    UB=(C1/C1+C2)UA. In order to weaken the induction of A to B, the methods that can be used are:

(1) Increase the distance between A and B to reduce the distributed capacitance C1.

(2) Keep B as close to the ground plate as possible to increase the capacitance C2 of B to ground.

(3) Insert a metal shielding plate between A and B (see Figure 1.2).

The function of the shielding plate is:

①The existence of the shielding plate increases the distance between A and B, resulting in a reduction in the distributed capacitance C1′ between A and B.

② When the shielding plate is close to the protected B, the capacitance C4 of B to the ground increases. The function and status of C4 are the same as C2.

In Figure 1.2, there are no special requirements for the thickness of the shielding plate, but it is required to be a good conductor, have sufficient strength, and be well grounded.

2.2 Magnetic field shielding

Magnetic field shielding usually refers to the shielding of DC or low-frequency magnetic fields, and its effect is much worse than the shielding of electric fields and electromagnetic fields.

The main mechanism of magnetic field shielding is to use the high magnetic permeability and low magnetic resistance characteristics of the shield to act as a magnetic shunt on the magnetic flux, thereby greatly weakening the magnetic field inside the shield (Figure 1.3).

The design points of magnetic field shielding are:

(1) Use materials with high magnetic permeability to reduce the magnetic resistance of the shield.

(2) Increasing the wall thickness of the shield also reduces the magnetic resistance of the shield.

(3) The shielded object should be placed in the center of the shield, and the magnetic flux should not pass through the shielded object as much as possible to avoid reducing the shielding effect.

(4) Pay attention to the structure of the shielding body. All gaps, ventilation holes, etc. should be distributed along the direction of the magnetic field and try not to block the passage of magnetic flux.

(5) A double-layer shielding structure can be used for strong magnetic fields. When shielding a strong external magnetic field, the outer shield is required to be made of materials that are not prone to magnetic saturation, such as silicon steel, while the inner layer is made of highly magnetically permeable materials that are easy to saturate, such as permalloy. On the contrary, the order of materials used in the shield is also reversed. When installing the two-layer shield, attention should be paid to the magnetic circuit insulation between each other. If the shield does not require grounding, it can be supported by insulating materials. If grounding is required, a metal non-ferromagnetic material can be used as a support. Starting from the fact that the shield can have both electrical and magnetic shielding functions, it is usually grounded.

2.3 Electromagnetic field shielding

The purpose of electromagnetic field shielding is to prevent electromagnetic fields from propagating in space.

Different from the mechanism of electric field and magnetic field shielding, the mechanism of electromagnetic field shielding is:

(1) The reflection of electromagnetic waves by the metal surface of the shield (in this regard, there is no requirement for the thickness of the shield).

(2) When electromagnetic waves that are not completely reflected enter the interior of the shield, they will be absorbed by the metal of the shield as they continue to propagate forward.

(3) When part of the electromagnetic waves that have not been absorbed penetrates the metal and reaches another surface layer of the shield, it will be reflected again at the interface between the metal and the air and return to the inside of the shield. As a result, multiple reflections and waves will be formed inside the shield. Absorption phenomenon (of course, a small amount of electromagnetic waves will eventually penetrate the shield and enter the protected space).

Therefore, electromagnetic shielding is based on the two functions of reflection and absorption of electromagnetic waves by metal materials.

3 filtering

Since the filter itself has a two-way effect (it can suppress the conducted interference from the equipment power line and reduce the conducted interference introduced from the power grid), therefore, the content of the filtering part is discussed together with the method of improving product immunity.