A new method for measuring electromagnetic interference characteristics of power electronic devices and Jun Ping, Jiang Jianguo, Chen Yu (Tsinghua University, Beijing 100084, China) Equivalent interference source and equivalent internal impedance characteristics of magnetic interference, effectively guiding the design of power EMI filters . In this paper, the measurement principle and measurement steps are introduced in detail, and the AC/DC converter and its EMI filter are designed.
Through the above measurement and derivation, it can be obtained that the main circuit of the converter under a certain load reflects the equivalent conducted interference model of worse conduction interference emission conditions. In an actual device, the auxiliary power supply and the drive circuit also generate conduction disturbances externally, but it is relatively small compared to the main circuit, and if necessary, it can be refined by the above-mentioned test method, so the present embodiment ignores its influence.
It can be seen from the figure that the differential mode internal impedance ZZ2 indicated by the dotted line is mainly inductive, and the common mode internal impedance Z3 is capacitive below 20 MHz. At 25 MHz, the stray capacitance of the main circuit resonates with the inductance of the power input lead and then appears as Sensibility. Conduction interference emission measurement requires adding LISN. In order to obtain phase information, Vi and V2 waveforms are measured using a high-precision memory oscilloscope Tek75 4C. The MOSFET turn-on time in the circuit is only 40ns. To effectively extract the transient information of the switch, the sampling rate is taken as 500Ms/S.. where the plus sign indicates Vcm and the diamond symbol indicates Vdm. Through the above processing, the offline AC/DC converter is obtained. Equivalent interference source and equivalent internal impedance model.
To verify the validity of the above model, the LISN 50 resistor was changed to 25 to measure the voltage waveform on the rectifier diode when it was diagonally conducting. At the same time, the predicted voltage waveform on the 25 resistor was also obtained from the equivalent model. a and b are the time domain waveforms predicted and measured by V2 when the main circuit power transistor is turned off and turned on. It can be seen that both waveforms are similar.
The converter also gives the V2 predicted voltage waveform of a switching period and the amplitude-frequency curve of the measured waveform. It can be seen that the spectral envelopes of the two are below 5MHz, and the difference between 5MHz and 25MHz is less than 3dB, indicating that the device The equivalent model of conducted interference is more accurate.
Actual Measurement - V2 Predicted and Measured Waveform Amplitude Spectrum Comparison EMI Filter Design Based on the derived conducted interference characteristics and the known grid-side high-frequency impedance of 50 amps, combined with the CISPR Class B conducted emission interference emission limits, EMI can be determined. Filter attenuation. It can be seen that the common mode components dominate the conducted interference, so the filter design is dominated by common-mode attenuation, taking 3dB margin, the design common-mode filter inductance is 2.6mH, the common-mode filter capacitance is 4700pF. Similar to available The differential mode filter inductance is 10PH and the capacitance is 0.22F. The circuit is finally designed as shown. To ensure the high frequency performance of the filter, maintain proper spacing during assembly to reduce unwanted stray coupling. The designed filter is added to the converter and its conducted interference emissions are measured at the rated load. The conductive interference emissions of the converter measured by the ESA1500 spectrum analyzer without and with the added filter are plotted. Because of the actual filter's magnetic loss and stray capacitance, the actual filtering performance has been reduced. Although this design meets the CISPR22B level conduction interference emission requirements, the actual attenuation in the higher frequency range is smaller than expected. .
EMI Filter Design Circuit Diagram Converter without Filter and Conduction Interference Emission after Filtering 4 Conclusion Measurement of the line-side lead terminal impedance of the offline converter and the LISN conduction interference voltage of the linear impedance stabilization network lead to the equivalence of the device to the grid side. The method of interference source and equivalent impedance can accurately reflect the characteristics of conducted interference in the device, which helps designers to understand the nature of conducted interference and guide the design of EMI filters, thereby reducing the external conductive interference emission of the device and achieving EMC international standards. . The experimental results prove the effectiveness of the method and provide a convenient method for the EMC analysis and design of medium and low power off-line converters.
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