Research on EMI filter testing technology

With the development of science and technology and production, the number of electronic products is increasing, and the electromagnetic environment in space is becoming more and more complex. The harsh electromagnetic environment will have a serious impact on humans and various organisms. In addition, electronic products may interfere with each other, causing them to not work properly, so EMC testing has emerged.
In electronic equipment and electronic products, electromagnetic interference energy can be transmitted through radiative coupling or conductive coupling. Shielding is an effective measure to suppress the radiation interference of electromagnetic interference signals; EMI filters are very effective devices to suppress the conducted interference of electromagnetic interference signals. Since EMI filters are important devices for suppressing conducted interference, it is very important to study the test methods of EMI filters.

l Test indicators of EMI filter
1.1 Common mode interference and differential mode interference
In fact, EMI power supply filter plays two roles of low-pass filter: attenuating common mode interference and attenuating differential mode interference. Any conducted interference signal on the power line can be represented by common mode and differential mode interference signals. In addition, the common mode interference signal between L-E and N-E and the differential mode interference signal between L-N can be regarded as independent EMI sources, and L-E, N-E and L-N in the single-phase power supply can be regarded as independent network ports to analyze EMI signals and related filtering networks. In Figure 1, U1 and U2 are common mode interference signals, and U3 is a differential mode interference signal.

1.2 EMI filter network structure
Figure 2 shows the basic network structure of a single-phase power filter. It is a passive low-pass network composed of concentrated parameter components. The filter network is mainly composed of two inductors L1 and L2, and three capacitors CX, CY1, and CY2. If one end of the filter is connected to the power supply and the load end is connected to the interfered device, then L1 and CY1, L2 and CY2 will respectively form a low-pass filter between two pairs of independent ports L-E and N-E, which are used to suppress the common-mode EMI signal on the power line. In addition, L1, L2 and CX form a low-pass filter between the independent ports L-N to suppress differential-mode interference signals.
1.3 EMI filter leakage current performance test
Leakage current refers to the current flowing between the phase line and the neutral line and the filter housing (ground line) at a voltage of 250VAC. It mainly depends on the value of the grounding capacitor (common-mode capacitor). A larger common-mode capacitor CY can increase the insertion loss, but it will cause a larger leakage current. The leakage current test circuit is shown in Figure 3:

1.4 EMI filter withstand voltage performance test
In order to ensure the performance of the power filter and the safety of equipment and personnel, a withstand voltage test must be carried out. The withstand voltage test is a test under extreme working conditions. If the withstand voltage performance of the CX capacitor is poor, it may be broken down when the peak surge voltage appears. Although its breakdown does not endanger personal safety, it will cause the filter function to lose function or performance degradation. In addition to meeting the requirements of ground leakage current, the CY capacitor also has sufficient safety margin in electrical and mechanical performance to avoid breakdown and short circuit under extremely harsh environmental conditions. Therefore, the withstand voltage performance between the line and the ground is of great significance to protecting personal safety. Once the insulation protection measures of the equipment or device fail, it may cause casualties.
1.5 EMI filter performance evaluation
When using EMI power filters, the three most considered items are rated voltage and current value, withstand voltage performance, and leakage current, and the most important evaluation performance is the insertion loss performance of the filter.
The ability of EMI power filters to suppress interference noise is measured by insertion loss I. L. (Insertion Loss). Insertion loss is defined as the ratio of the power P1 transmitted from the noise source to the load when no filter is connected and the power P2 transmitted from the noise source to the load after the filter is connected, expressed in dB (decibel).

2.2 Loading test of insertion loss
In EMI filter products, due to the use of inappropriate materials, the common mode choke cannot guarantee complete symmetry, which will lead to saturation of the magnetic ring. At the same time, parasitic differential mode inductance may also cause saturation of the magnetic ring, making the actual use of the filter very different from the data provided by the manufacturer. Therefore, the filter must be subjected to loading test.

3 Time Domain Test of EMI Filter
Generally, for EMI power filter, we only care about its conventional performance and frequency domain suppression performance. For EMI signal line filter, since the transmission line itself will generate certain electromagnetic interference, the test signal will inevitably produce certain attenuation. At this time, we need to test its time domain transmission performance.
Using a 50kHz square wave to filter the filter pin with a capacitance value of 8000pF, it is found that the rising and falling edges in the time domain have obvious changes. In the frequency domain, after filtering, the high-frequency component of the square wave signal is filtered out.

For the same filter pin, the higher the frequency of the square wave, the greater the attenuation of its harmonic signal by the filter pin, and the longer the rise and fall time of the square wave waveform will be. Similarly, for the same frequency waveform, the larger the filter capacitance value, the slower the rise time of the square wave.

4 Design of EMI filter insertion loss automatic test system
In recent years, as the content of EMC testing has become increasingly complex, the test workload has increased dramatically, and the requirements for test equipment in terms of function, performance, test speed, test accuracy, etc. have also increased. In this case, traditional manual testing has been difficult to meet the requirements. In addition, the current national standard (GB) and national military standard (GJB) both require that electromagnetic compatibility testing must be performed automatically, and have strict requirements for data post-processing. Therefore, the development of EMC automatic testing has become an inevitable path. The automatic test system established in this paper uses virtual instrument technology to perform insertion loss testing on EMI power filter based on signal source-spectrum analyzer.
4.1 Test system program flow
This system is to build a virtual test platform on the computer, and use virtual instrument technology to directly program the spectrum analyzer on the computer through the communication bus GPIB to reduce the impact of the test fixture on the filter insertion loss test results. The program flow chart is shown in Figure 6.
4.2 Hardware interface and driver
To realize the function of the automatic test system, it is necessary to solve the problem of realizing computer control of instrument movement and reading measured data. The first problem to be solved in this step is the communication problem between the instrument and the computer. Here, an instrument with a GPIB (IEEE-488) interface is selected.
In addition, the system also needs to implement the driver of the test hardware. The software implements the driver of the signal generator and the driver of the spectrum analyzer respectively. The program control of the signal generator is written in the VB environment. The software statement refers to the writing method of the driver in the manual. The basic settings of the signal generator are completed, set to RF output mode, the frequency of the RF signal calls the frequency point list generated by the information input interface, and the transmission frequency is set by the user in the software. In the program, the basic parameters used in the command are first defined, and then the program is written. The corresponding code is shown in Figure 7.

4.3 Interface design of test software
The user interface is the medium for users to interact with the application. The user interface is the most important part of the application and is also the intuitive real world. For users, the interface is the application.
This software consists of six interfaces, namely the function selection interface, the input information interface, the calibration interface, the test interface, the result display interface and the historical data comparison interface. In the function selection interface, the user selects the function implemented by the software to perform insertion loss test or query historical test data. In the input information interface, the user fills in the basic test information and determines the range of the test frequency point to form a frequency point list, and the test frequency point list is stored in the database. In the calibration interface, first drive the signal generator to generate a signal for calibration, set the transmission power and store it in the database, and then drive the spectrum analyzer to calibrate the test filter, generate the calibration results and graphs, and display them. In the test interface, drive the signal generator again to generate the required test signal, call the transmission power in the database, and drive the spectrum analyzer again to test the filter insertion loss performance, generate the test results and graphs and display them. The insertion loss test result table and the differential mode and common mode insertion loss curves are displayed in the result interface. Historical data comparison adds the insertion loss results of different filter models in the database for comparison, and generates a comparison curve chart to facilitate users to compare test results.
4.4 Test database
In the input information interface, the user enters the basic test information and generates a management database. The database stores the user-defined test frequency points and the corresponding common mode and differential mode calibration, test and insertion loss results. If the file name is the same, the original database is overwritten. After determining the test frequency range, an insertion loss sub-database is generated, and the calibration and test results are stored in the sub-database, and the insertion loss test value is calculated. As shown in Figure 9.
4.5 Test data processing
Since the calibration value and test value generated by the spectrum analyzer are both processed calculated values, the unit is dB. Therefore, according to formula (1), it can be seen that the insertion loss value is the difference between the calibration value and the test value. Through the mixed programming of VB and LabVIEW in the software, the calibration and test graphs generated by the spectrum analyzer can be displayed on the interface.
In addition, the software will automatically store the calibration results and test results in the insertion loss database, and obtain the common mode insertion loss data and differential mode insertion loss data through the insertion loss calculation formula of this software and save them. A data report can be generated by calling the insertion loss database. This software uses the DataReport data report designer and the data source (Data Environment data environment designer) to create a printable report and export the report to HTML or text files.

5 Conclusion
With the emergence and rapid development of electronic devices and various electrical appliances, electromagnetic interference (EMI) between electronic devices has become a serious public hazard. As the most powerful means to suppress electromagnetic interference, EMI filters are increasingly known and widely used. Therefore, the study of EMI filter testing technology has become very important. This paper briefly analyzes the testing principles and methods of various aspects of EMI filters, and compiles insertion loss automatic testing software based on virtual instrument technology on the basis of manual testing of insertion loss. It avoids some defects of manual testing, realizes the automation of testing, and has simple functions and flexible operation.