Electromagnetic disturbance includes two types of interference: conduction and radiation. The structure of EMI filter makes it an effective device to suppress conducted interference and radiation interference in power lines. Filtering not only directly suppresses the passage of interference signals, but also the structure of EMI filter can significantly suppress the radiation interference of circuit lines.

1 Structure of EMI filter

In order to eliminate the harm caused by EMI noise to the system, a dedicated EMI filter needs to be installed. The commonly used EMI filter is the through-type EMI filter (see Figure 1). Since the structure of the through-type EMI filter has the characteristics of axisymmetric structure, that is, the electrodes at both ends of the capacitor are 360° circular structures, its connection inductance is very small, close to the ideal non-inductive capacitor, while ordinary capacitors have parasitic inductance. For example, the larger the capacitance value, the larger the parasitic inductance value, and the filtering effect on high-frequency interference is not ideal. The through-type EMI filter composed of through-type capacitors completely solves the problem of broadband electromagnetic interference filtering, because the metal shell of the through-type capacitor is effectively grounded 360° in all directions with the metal panel on which it is installed, which can play a good bypass role for high-frequency signals, that is, it has a good filtering effect on high-frequency interference signals. The filter is installed through-type, so that its input and output ends are effectively isolated by metal plates, which plays a shielding role and avoids the coupling phenomenon of high-frequency interference signals. The adjustment of capacitance and inductance in the Structure of EMI filter and the structural changes can change its cutoff frequency to achieve the suppression of electromagnetic interference signals in the required frequency band.

Figure 1 Through-type EMI filter

The actual EMI filter can be used with inductors and capacitors alone, or with inductors, capacitors and resistors. Therefore, from the structural point of view, it forms C-type, T-type, L-type, π-type and even more complex multi-stage filter circuit forms, as shown in Figure 2.

Figure 2 Structure of EMI filter C-type (left 1), L-type (left 2), T-type (left 3), π-type (left 4)

The types Structure of EMI filter in actual circuit applications are too numerous to mention. The above basic structural forms can be combined into various filters. The specific categories are as follows:

1. Surface mount filter (for PCB installation): surface mount three-terminal capacitors, surface mount chip beads, sheet (surface mounted) LC filters, etc.
2. Signal filter (for through-type installation): magnetic rings and magnetic beads, through-hole capacitors, filter connectors (i.e. filter arrays), etc.;
3. Power supply filters used in AC and DC power supply parts: power supply filters, magnetic rings and magnetic beads, etc.;

Currently, there are many manufacturers producing EMI filters, such as SPECTORL and AVX in the United States, Murata and TDK in Japan, and Schaffner in Switzerland. These manufacturers have developed and produced a variety of EMI filter products, such as chip capacitors, three-terminal capacitors, chip LC composite devices, plate array filter capacitors, chip ferrite magnetic rings and magnetic beads, common mode chokes, and power supply filters, block combination filters, etc. The above new structural components reflect the design different Structure of EMI filter, which are specially designed to meet the needs of actual circuits and better applied to anti-electromagnetic interference filtering.

In fact, the circuits of electromagnetic interference filters manufactured by various manufacturers are basically the same, and can be equivalent to inductors, capacitors, resistors or their composite components. The only difference is the selection of devices and the internal structure. For example, if two filters have the same circuit, their filtering performance may be very different, and this difference is mainly reflected in the higher frequency band (above 10MHz). There are three reasons for this difference: the selection of components to realize the circuit; the internal structure of the filter and the installation method of the components; the installation method of the filter when in use, so the filters of different manufacturers will have a large difference above 10MHz.

In order to improve the high-frequency filtering characteristics of the filter, special treatments must be taken in the selection of components, internal installation structure and external installation structure, so as to achieve excellent high-frequency filtering characteristics.

2 Working principle of EMI filter to suppress electromagnetic interference

In addition to shielding and grounding, the commonly used EMI suppression technology in practice is to install EMI filters. It should be the most effective method.

EMI filters can effectively suppress the conducted interference signals in the circuit. The quality of their working performance mainly depends on the internal components and overall design of the filter, that is, the Structure of EMI filter. The main technical indicators for evaluating its performance are: rated voltage, withstand voltage, rated current, leakage current, insulation resistance, DC resistance, operating temperature range, temperature rise Tr, insertion loss dB, external dimensions, weight, etc. The most important of these is insertion loss (also known as insertion attenuation), which is a key indicator for evaluating electromagnetic interference filters.

The input and output impedance characteristics of the filter directly affect its insertion loss characteristics (see Table 1). To achieve the best insertion loss characteristics for EMI filters, the impedance mismatch principle must be followed when designing the filter, that is, the filter impedance and source impedance are mismatched.

For example, a four-terminal network: ZS is the source impedance, ZL is the load impedance, and ZT is the transfer impedance four-terminal network.

Figure 3 EMI filter input/output four-terminal network diagram

The insertion loss is:

                                 (1)

When ZS=ZL=ZO

                                             (2)

For parallel C-type filter, we have:

                                                                      (3)

                                                      (4)

For series L-type filter, we have:

                                                          (5)

                                       (6)

For L-type connection:

                                    (7)

For п-type connection:

  (8)

For T-type connection:

  (9)

Figure 4 Various EMI filter circuit structure diagrams

The above L and C are ideal components. It is known that L and C can be used to calculate IL.

When ZS≠ZL≠ZO, it can be recalculated according to formula (1). The conclusion is: the series L provides high impedance and is suitable for cascading with a low impedance source or load; the parallel C provides low impedance and is suitable for cascading with a high impedance source or load. That is, the greater the mismatch, the greater the attenuation.

Inductors are usually used to suppress noise in low impedance circuits, and capacitors are used to suppress noise in high impedance circuits. Inductor filters (inductors) do not need to be grounded and can be directly connected in series in the line, while through-type capacitor filters can be connected across the signal line and the ground line. There are two grounding methods: screw mounting and welding. Input and output impedance characteristics of the filter (see Table 1)

Table 1 Input and output impedance characteristics of the filter

When the through-type EMI filter is actually used (Figure 5, Figure 6), its input and output ends are connected to the signal line, and its shell is connected to the metal cavity ground of the component.

Figure 5 Application example of through-type EMI filter

Figure 6 Application example of screw-mounted through-type EMI filter

The two ends of the component in the application example are the main channels, and there are four signal inputs on the fully sealed and shielded shell. In order to avoid the interference signal carried by the input signal causing erroneous processing results, the two ends of the metal lead of the through-type EMI filter are the signal input and output ends respectively. Due to the symmetry of the circuit principle, the input and output ends can be interchanged when used; the “ground” lead is connected by welding (the “ground” can be the earth or the circuit loop ground according to the user’s overall design) and the metal lead is connected to form a capacitor; this component is a radio frequency signal processing component, and the two ends of this component are the main channels. There are four signal inputs on the fully sealed and shielded shell. In order to avoid the interference signal carried by the input signal causing erroneous processing results, a through-type EMI filter is installed to filter out the conducted and radiated interference signals in the input signal line.

Application example Figure 5 is equipped with a welded EMI filter, while example Figure 6 is equipped with a screw-mounted EMI filter. Welded and screw-mounted filter products can be selected according to the specific conditions of installation and use—XUANSN from China also provides both welded and screw-mounted through-type EMI filters, adapting to grounding mounting plates made of different materials (silver-plated copper for welding, aluminum for screw mounting) to meet the diverse installation needs of different customers. There is no difference in performance between the two: the welded type is suitable for grounding mounting plates made of silver-plated copper or other easy-to-weld materials; the screw-mounted type is suitable for grounding mounting plates made of aluminum or other materials that are not easy to weld. These different forms reflect the diversity and adaptability of the Structure of EMI filter in practical applications.

The installation method of the filter also directly affects the use effect of the filter, because the filtering effect of the filter is not only determined by the filter circuit, but also has a lot to do with the type, installation method, and structural form of the device. In particular, whether the filter circuit is shielded has a great impact on the high-frequency characteristics of the filter. When the filter circuit is not shielded, the interference in the space will be directly coupled into the filter circuit, making the filter ineffective. The inductor in the filter circuit is composed of a coil wound on a high-permeability material. It is a very good magnetic receiving antenna. In the absence of shielding, it will receive interference around the filter circuit and even enhance the interference. The metal shell of the filter must be in direct conductive contact with the metal chassis of the equipment over a large area. Therefore, the following principles should generally be followed during installation:

① Select an appropriate installation location. Generally, the filter should be installed at the entrance of the equipment or shielded shell;

② If necessary, the filter should be shielded, and its shell should be connected to the ground wire of the filter. If the equipment has a metal shell, the filter shell is required to be well overlapped with the metal shell of the equipment and as close to the grounding point of the equipment shell as possible;

③ For screw-mounted EMI filters, in order not to damage the internal components of the filter during installation, a special tool with a torque scale should be used for installation. The recommended installation torque is 0.3N.m.

④ Try to make the contact resistance as small as possible to ensure the bypass effect of the filter capacitor. In order to reduce the contact resistance and reliable connection, it is best to install an inner tooth, outer tooth washer and a flat washer between the filter and the isolation board.

⑤ Avoid coupling between the external leads of the filter. Generally, it is required that the input and output leads are not led out on the same side. If necessary, a shielding isolation layer is added between the two sets of leads.

⑥ For soldering type EMI filter, soldering iron or hot air reflow soldering can be used. In order to avoid damaging the internal components of the filter during the shell installation and soldering, if soldering iron is used, the product is required to be preheated, the preheating temperature is 150℃, the soldering iron power is below 35w, the maximum soldering temperature does not exceed 260℃, and the soldering time at this temperature does not exceed 5s; if hot air reflow soldering is used, the optimal temperature curve should be adjusted.