A New Method for Measuring Electromagnetic Interference Characteristics of Power Electronic Devices and Jun Ping, Jiang Jianguo, Chen Wei (Tsinghua University, Beijing 100084, China) The 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 procedure are introduced in detail, and the design of AC/DC converter and its EMI filter is carried out.
Through the above measurement and derivation, the equivalent conduction interference model of the converter main circuit reflecting the harsh conduction interference emission condition under a certain load can be obtained. In the actual device, the auxiliary power supply and the drive circuit also generate conductive interference, but it is relatively small compared with the main circuit, and if necessary, it can be refined by the above test method, so the influence is ignored here.
As can be seen from the figure, the differential mode internal impedance ZZ2 indicated by the broken line is mainly inductive, and the common mode internal impedance Z3 is mainly capacitive below 20 MHz. At 25 MHz, the main circuit stray capacitance resonates with the inductance of the power input lead, and then appears as Sensual. The LISN is added according to the measurement requirements of the conducted interference emission. To obtain the phase information, the Vi and V2 waveforms are measured by the high-precision memory oscilloscope Tek75 4C. The MOSFET turn-on time in the circuit is only 40 ns. To effectively extract the switching transient information, the sampling rate is 500 Ms/S. 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.
In order to verify the validity of the above model, the voltage resistance of the LISN is changed to 25, and the voltage waveform on the rectifier diode is turned on. The predicted voltage waveform on the 25 resistor can also be obtained from the equivalent model. a and b are the time-domain waveforms of V2 prediction and actual measurement when the main circuit power tube is turned off and on transient. It can be seen that the waveforms of the two are similar.
The converter also gives a V2 predicted voltage waveform of the switching cycle and the amplitude-frequency curve of the measured waveform. It can be seen that the spectral envelopes of the two are below 5 MHz, and the difference between 5 MHz and 25 MHz is less than 3 dB, indicating that the device The equivalent model of conducted interference is more accurate.
Measured - V2 prediction and measured waveform amplitude spectrum comparison EMI filter design based on the resulting conducted interference characteristics and known grid side high frequency 50 impedance, combined with CISPR specified Class B conducted interference emission limits, can determine EMI The amount of attenuation of the filter. It can be seen that the conducted interference is dominated by the common mode component, so the filter design is dominated by common mode attenuation, taking 3dB margin, the common mode filter inductance is 2.6mH, and the common mode filter capacitor is 4700pF. The differential mode filter inductor is 10PH and the capacitor is 0.22F. The final design circuit is shown. To ensure high frequency performance of the filter, proper spacing is maintained during assembly to reduce unwanted spurious coupling. The designed filter is applied to the converter and its conducted interference emissions are measured at rated load. The conducted interference emissions of the converter measured by the ESA1500 spectrum analyzer at the time of no filter addition and after adding the filter are plotted. Due to the magnetic loss and stray capacitance of the actual filter, the actual filtering performance is degraded. Although the design meets the CISPR22B level conducted interference emission requirements, the actual attenuation in the higher frequency band is smaller than expected. .
EMI filter design circuit diagram converter without filter and filter after conduction interference emission 4 conclusions use the measurement of off-line converter grid side lead terminal impedance and linear impedance stability network LISN conducted interference voltage, and then the device is equivalent to the grid side The method of interference source and equivalent impedance can accurately reflect the conducted interference characteristics in the device, which helps the designer to recognize the nature of conducted interference and guide the design of the EMI filter, thereby reducing the external conduction interference emission of the device and meeting EMC international standards. . The experimental results demonstrate the effectiveness of the proposed method and provide a convenient method for EMC analysis and design of medium and small power off-line converters.
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