Precise Design of EMI Filters

　　With the development of electronic technology, electromagnetic compatibility issues have become a very important and difficult problem for circuit design engineers. Based on years of engineering experience, it is generally believed that the two most important and difficult items in the electromagnetic compatibility standard are conducted emission and radiated emission. In order to meet the requirements of conducted emission limits, electromagnetic interference (EMI) filters are usually used to suppress the conducted noise generated by electronic products. But how to choose an existing filter or design a filter that can meet the needs? Engineers are blind and can only try based on experience. First, use a filter based on experience, and if it does not meet the requirements, modify the design or change to another new filter. Therefore, finding a suitable EMI filter becomes a time-consuming and costly task.

　　Characteristics of interference generated by electronic systems

　　To solve the problem, we must first understand the total interference generated by the electronic system. How much interference voltage needs to be suppressed to meet the standard requirements? What is the common mode interference and what is the differential mode interference? Only by clarifying these interference characteristics can we make requirements based on actual needs.

　　From the perspective of the current path of the object under test, the interference signal return path may pass through the ground wire or other power grids, as shown in Figure 1. The interference current passing through the ground wire generates a common-mode interference voltage with the same phase on the power grid. Through other wires, a differential-mode interference voltage with opposite phase is generated on the two power lines. The path of the interference current is shown in Figure 2.

　　Figure 1. Interference signal return path

　　Figure 2 a) Differential mode interference filtering and b) Common mode interference filtering

　　There are usually four technologies for power supply filtering to suppress interference noise. In actual use, two or even more of these technologies are often used in combination. They are:

　　Add capacitors between the positive and negative power lines, namely X capacitors;

　　Add capacitors between each power line and ground line, namely Y capacitors;

　　Common mode suppression (the suppression coils on the two power lines are wound in the same direction);

　　Differential mode suppression (each power line has its own suppression coil).

　　The function of the capacitor is to "short-circuit" the high-frequency interference voltage. In addition, when the interference signal frequency is very high, the suppression coil will generate a large AC impedance. Figure 2 shows the structure of two types of filters, among which LISN is a linear impedance stabilization network used for measurement purposes. If the interference is caused by a common mode problem, the X-type capacitor is basically useless because the interference voltage on the two lines is the same. Therefore, understanding the type of interference will play an important role in selecting a reasonable circuit structure and provide a technical basis for solving the problem.

　　In a standard electromagnetic compatibility test laboratory, the total interference of the equipment can be obtained, but the common mode interference and differential mode interference characteristics of the equipment cannot be understood. In order to distinguish the common mode or differential mode interference signal in the measurement, it is difficult to achieve with general instruments. Using a dedicated conduction tester, the total interference, common mode interference and differential mode interference of the equipment can be obtained. The test results are shown in Figure 3.

　　Figure 3 Total interference, common mode interference and differential mode interference power supply input impedance characteristics obtained by traditional testers

　　The filter insertion loss given by the filter manufacturer is the performance in a 50W standard impedance system. As we all know, the input impedance of the power supply has discontinuity as the frequency changes. The change in impedance causes a large change in the filter insertion loss characteristics.

　　As can be seen from Figure 4, in a 50W system, a 100mH filter provides about 18dB of attenuation, but in a 500W system it only provides about 4dB of attenuation. Similarly for a 100nF capacitor; in a 50W system, the attenuation at 1MHz is about 23dB, which drops to 7dB in a 5W system.

　　The above example shows that the reason why choosing a filter with very high insertion loss cannot effectively suppress the conducted noise is the influence of the impedance at the power input. Therefore, in addition to choosing a suitable filter, the designer also needs to understand the impedance characteristics, common mode impedance and differential mode impedance of the power supply. Impedance testing can be done with the help of a dedicated impedance tester or a conduction analyzer. The changes in the common mode impedance (a) and differential mode impedance (b) of a filter are shown in Figure 5.

　　Figure 4 a) Attenuation of 100uh inductor b) Attenuation of 100nF capacitor

　　Figure 5: Changes in a) common mode impedance and b) differential mode impedance

　　Filter Design

　　After knowing the interference characteristics and input impedance characteristics of the equipment, it becomes simple to design or select a filter. If you use a ready-made filter, you can call the filter database accumulated in the past, compare the filter parameters, and find a suitable filter. If there is no suitable one or you want to design a special filter, you can use special filter design software. After determining a filter mode, enter some simple constraints of the filter. The design software automatically calculates the most suitable component value according to the impedance characteristics, and provides the most suitable attenuation. (As shown in Figure 6)

　　Figure 6 An optimal filter designed by software

　　Design Results

　　After testing the interference characteristics and impedance characteristics of a product, a low-frequency interference problem below 5MHz needs to be solved. The dedicated filter design software combines the test data obtained above to give the filter component parameters: including 470nF X capacitor, 2.2nF Y capacitor and 15.1mH common mode inductor. However, experienced filter designers believe that a filter with a 13.5mH common mode inductor is sufficient. The emission of a 13.5mH filter including additional high-frequency components is shown in Figure 7.

　　Figure 7. Minimum 15mH system usage and test results at 18mH

　　To verify the software design data, off-the-shelf filters of 470nF, 2.2nF and 18mH were quickly connected to the system to obtain a center frequency of less than 5MHz without the need for high-frequency filters. The results clearly showed that the minimum 15mH limit was appropriate.

　　Conclusion

　　The design of EMI filters should fully consider the interference characteristics and impedance characteristics. Designing based on impedance test and interference characteristic test data is the only way to accurately design filters.