Abstract: This paper respectively studied the detection techniques of the current, voltage and power conduction interference in the switches power supply, and discussed the measurement methods and experience. Finally, as an example, the harmonic contents of the current, voltage and power in a few switches power supply were measured. Keywords: conduction interference; detection technique; harmonic content 1 Introduction In power lines and switching power supplies, electromagnetic interference (EMI) mainly manifests as conducted interference. The interference signals are mainly current and voltage harmonic components [1-3]. Power lines, signal lines and control lines are the carriers of conducted interference [4]. Conducted interference detection technology is complex. In the measurement , not only should the shielding technology of electromagnetic radiation interference be considered, but the correct use of the measuring instrument and the correctness of the measurement method directly affect the measurement results. This section primarily discusses the detection technology for conducted interference in electrical equipment. 2. Measurement of Conducted Voltage Interference from Switching Power Supplies When measuring conducted interference from switching power supplies, a linear impedance stabilizing network (LISN) must be connected between the AC mains power supply and the equipment under test (EUT). The LISN serves two purposes: firstly, it provides a standard linear impedance to the EUT's power input port during high-frequency harmonics, ensuring that changes in other devices connected to the same power supply do not affect the EUT's input impedance; secondly, the LISN filters out EMI from the mains power supply, providing a "clean" power supply to the electrical equipment and not affecting the measurement results of conducted interference to the EUT itself. Figure 1 shows the principle circuit for measuring voltage harmonics based on the LISN. The LISN is a three-port RLC network. Port 12 is connected to the mains voltage; ports 3 and 4 are connected to the EUT; and ports 5 and 6 are connected to a radio noise meter with an input impedance of 50Ω, used to measure the conducted voltage interference (voltage harmonic components) of the switching power supply. The principle circuit for measuring the voltage harmonic components of the EUT based on the LISN is shown in Figure 1. The impedance frequency characteristics seen from port 12 are shown in Figure 2. If a wider frequency range of stable impedance characteristics is desired for the LISN, a more complex network structure is required. Figure 2 shows the impedance characteristic curve of the LISN. If the power capacity of the LISN is insufficient, or if the LISN cannot be installed in the actual device, a voltage probe (VP) can be used for measurement. The principle circuit of the VP is shown in Figure 3. In Figure 3, the sum of R and Rm is R + Rm = 1500Ω (1) In equation (1), Rm is the input impedance of the radio noise meter, and the typical data is 50Ω. The relationship between the actual interference voltage Vi and the measured voltage Ui is: Vi = (1500/Rm) × Ui (2) Figure 3 shows the voltage probe circuit diagram. In equation (2), the subscript i indicates the i-th voltage harmonic. When using the VP to measure the voltage conducted interference of the EUT, the following two points should be noted: (1) When using the VP to measure conducted EMI, it should be ensured that the EMI level from the mains power supply is more than 20dB lower than the EMI from the device under test; (2) There is generally an impedance mismatch between the mains power supply and the VP, or between the VP and the device under test, so the measurement error cannot be eliminated. Therefore, the accuracy of measuring voltage conducted interference using VP is lower than that based on LISN. 3. Measurement of Current Conducted Interference in Switching Power Supplies Measuring EMI current noise using a current probe (CP) is very simple. The CP surrounds the current-carrying conductor to measure the corresponding current noise. When the CP surrounds a single wire, differential-mode conducted current can be measured; when the CP surrounds the entire cable, common-mode conducted current can be measured. During the measurement, the CP should be positioned between the EUT and the LISN, and as close to the LISN as possible to minimize measurement error. The insertion impedance of the CP is less than 0.5Ω and the working frequency can reach 50MHz. 4 Measurement of power conducted interference of switching power supply The power conducted interference measuring instrument generally uses a power probe (PP). The PP can measure the conducted interference power of 5MHz-300MHz. The principle of the PP is shown in Figure 4. In Figure 4: A is the CP, and the input of A is the conducted current noise of the EUT; B and C are ferrite bushings, which surround the power line and the signal line connected to the measuring instrument, respectively, and are used to attenuate the current in the frequency range of interest; D is an additional absorber, which is used to attenuate the current conducted interference from the power grid. Since the interference signal with a frequency greater than 30MHz is radiated, the power supply leads must be well shielded when measuring the conducted interference power. The maximum intensity of this conducted interference exists at the connection point between the power line and the ETU [4]. Therefore, the PP is placed at this position for measurement. In addition, the precise measurement point varies with the frequency, and the measurement location can be adjusted by observing the maximum value of the instrument. The PP provides an impedance of approximately 100Ω-250Ω to the EUT, and its inductive reactance component is no greater than 2% of the impedance. Only by paying attention to the above points can the measurement of conducted interference power obtain relatively accurate results. 5. Detection Examples and Analysis The harmonic components of the current and voltage at the output ports of various models of 29-inch color TVs, 15-inch color monitors, and UPS power supplies were measured using CP, VP, and LISN-based methods. Representative measurement results are shown in Table 1. The fundamental component current (voltage) value is considered as 100%, and the remaining data are percentages of the fundamental component. In Table 1: I-THD represents the total harmonic content of the current; V-THD represents the total harmonic content of the voltage. Table 1 shows that current harmonic interference is the main problem in switching power supplies, while voltage harmonic interference is much smaller than current harmonic interference. Therefore, suppressing conducted interference in switching power supplies mainly involves suppressing current harmonic interference. Table 2 shows the measurement data of harmonic power at the output ports of color TVs, color monitors, and UPS power supplies (the power value of the fundamental component is considered as 100%, and the remaining data are the percentage of the fundamental component). P-THD represents the harmonic content of each harmonic power. 6 Conclusion The conducted interference between power networks and electrical equipment is the main form of electromagnetic interference. Research on the measurement technology of conducted interference has very important practical significance. In the process of measuring conducted interference, not only is a certain electromagnetic compatibility theoretical foundation required, but also rich practical measurement experience is needed. The research on the conducted interference detection technology of switching power supply electrical equipment in this paper has practical application significance. References [1] Cai Rengang: Electromagnetic Compatibility Principle Design and Prediction Technology [M], Beijing: Beijing University of Aeronautics and Astronautics Press, 1997, 132-138. [2] Cao Caikai, Wang Ren: Virtual Instrument Measurement of EMI of Switching Power Supply [J], Shenzhen: Power World, No. 8, 2002, 53-54, 38. [3] Li Fengxiang: Research and application of harmonic suppression and power compensation technology [J], Shanghai: Electrical Automation, February 2002, 44-46. [4] Liu D: Wide Band AC Power Line Characterization, IEEE Trans on Consumer Electronics, 1999, 45(4) [R] 1087-1097.