Anti-interference and impedance characteristics testing of EMI filters
Interference characteristics 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 requests based on actual needs.
From the current path of the measured object, the interference signal return path may pass through the ground line or through other power grids, as shown in Figure 1. The interference current through the ground line produces an in-phase common mode interference voltage on the power grid. Inverted differential mode interference voltage is generated on the two power lines by other lines. The path of the interference current is shown in Figure 2.
Figure 1 Return path of the interference signal
The total interference of the equipment is available in the standard electromagnetic compatibility test laboratory, but the common mode interference and differential mode interference characteristics of the equipment cannot be known. In order to distinguish common mode or differential mode interference signals in measurements, general-purpose instruments are difficult to implement. Total interference, common mode interference and differential mode interference of the device can be obtained using a dedicated conduction tester. The test results are shown in Figure 3.
Figure 2 Total interference, common mode interference, and differential mode interference obtained by a conventional tester
The filter insertion loss given by the filter manufacturer is the performance in a 50W standard impedance system. It is well known that the input impedance of a power supply has discontinuities as a function of frequency. The change in impedance causes a large change in the insertion loss characteristics of the filter.
Knowing the interference characteristics and input impedance characteristics of the device makes it easy to design or select a filter. If you use an off-the-shelf filter, you can call the filter database accumulated in the past and compare the filter parameters to find a suitable filter. If you don't have the right or want to design a dedicated filter, you can use a dedicated filter design software. After determining a filter mode, the input filter has some simple constraints. The design software automatically calculates the most appropriate component values based on the impedance characteristics and provides the most appropriate attenuation. (As shown in Figure 3)
Figure 3 An optimal filter designed by software
After testing the interference characteristics and impedance characteristics of a product, it is necessary to solve a low frequency interference problem below 5MHz. The dedicated filter design software combines the previously obtained test data to give the filter's component parameters: a 470nF X capacitor, a 2.2nF Y capacitor, and a 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 filter using an additional 13.5 mH including additional high frequency components is shown in Figure 4.
Figure 4 Test results with a minimum 15mH system and 18mH
To validate the software design data, 470nF, 2.2nF, and 18mH non-customized filters were quickly connected to the system to achieve a center frequency of less than 5MHz and no high frequency filters were needed. The results clearly show that a minimum of 15 mH is suitable.
The design of the EMI filter should fully consider the interference characteristics and impedance characteristics. Designing based on the impedance test and interference characteristic test data is the only way to accurately filter the design.