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Application of artificial power network in EMC test

Release time:

2022-03-01 00:00

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　　1 Introduction

　　With the development of modern science and technology, the number and types of electrical and electronic equipment continue to increase, and the miniaturization, densification and operating frequency of electronic equipment continue to increase, and the application of electronic equipment is also becoming more and more extensive. Use, resulting in a more complex and harsh electromagnetic environment, such as power harmonics, voltage fluctuations, pulse noise, electromagnetic field radiation, static interference, etc. When electromagnetic interference occurs, it may cause degradation of the function of sensitive electronic equipment affected by the interference, or cause its function to fail, which may cause serious consequences. Therefore, in order to enable electronic equipment to coexist harmoniously and work normally, relevant international and domestic industries have stipulated the normal emission level of electronic equipment and the level that can withstand interference. After research and analysis, the background noise of different test sites is not consistent. The impedance of the AC power system at the test site changes greatly, and the change of the grid impedance directly affects the magnitude of the noise current conducted from the power line, thus making the test results inconsistent. Therefore, in the conduction interference voltage test, the impedance value of the power grid is very important. In order to ensure the consistency of the conduction emission test, an auxiliary device called artificial power supply network appears. The artificial power network can provide a standard 50 ohm impedance for the measurement of the disturbance voltage within a given frequency range, and isolate the equipment under test (EUT) from the high-frequency interference on the power grid, and couple the interference voltage to the receiver. This measures the disturbance voltage emitted by the equipment under test (EUT) along the power line to the grid.

　　2 Classification of Artificial Power Networks

　　There are two basic types of artificial mains networks: V-type artificial mains network (V-AMN) for coupling asymmetric voltages and delta-type artificial mains networks (Δ-AMN) for coupling symmetrical and asymmetrical voltages. V-type artificial power network is used to measure the common mode voltage on unshielded symmetrical signal lines.

　　According to different application frequency ranges, the internal circuit structure of the artificial power supply network is also different. GB/T 6113.102-2018 gives three frequency circuit structures.

　　1) Figure 1 is a 50Ω/50μH+5Ω V-type artificial power network. Its working frequency range is: 9kHz~150kHz. If it can meet the impedance of 50Ω/50μH type, this type of artificial power network can also be applied to 150kHz~30MHz Frequency Band Measurements. The impedance calculation formula is: z=(jw×50uH+5) //50. The main standards for its application are:

　　l GJBl51B-2013 "Requirements for Electromagnetic Emission and Sensitivity of Military Equipment and Subsystems";

　　l GB4343.1-2009 "Electromagnetic Compatibility Requirements for Household Appliances, Electric Tools and Similar Appliances Part I: Emission";

　　l GB 9254-2008 "Radio Disturbance Limits and Measurement Methods of Information Technology Equipment".

Figure 1

　　2) Figure 2 is a 50Ω/50μH V-type artificial power network, and its working frequency range is: 150kHz~30MHz. The impedance calculation formula is: z=(jw×50uH) //50. The main standards for its application are:

　　l GB 4343.1-2009 "Electromagnetic Compatibility Requirements for Household Appliances, Electric Tools and Similar Appliances Part 1: Emission";

　　l GB 4824-2019 "Radio Frequency Disturbance Characteristics Limits and Measurement Methods of Industrial, Scientific and Medical Equipment";

　　l GB 9254-2008 "Radio Disturbance Limits and Measurement Methods of Information Technology Equipment"; GB/T 18387-2017 "Electromagnetic Field Emission Limits and Measurement Methods of Electric Vehicles".

Figure II

　　3) Figure 3 is a 50Ω/5μH+1Ω V-type artificial power network, and its working frequency range is: 150kHz~108MHz. The impedance calculation formula is: z=(jw×50uH+1) //50, and the main standards applied are:

　　l GJBl51B-2013 "Requirements for Electromagnetic Emission and Sensitivity of Military Equipment and Subsystems";

　　l GB/T 18655-2018 "Limits and Measurement Methods of Radio Disturbance Characteristics of Vehicles, Ships and Internal Combustion Engines for Protection of Vehicle Receivers".

Figure three

　　According to the impedance calculation formula, the impedance curves of the three circuits are drawn, as shown in Figure 4. Curve 1, curve 2, and curve 3 in the figure correspond to 50Ω/50μH+5Ω, 50Ω/50μH, and 50Ω/5μH+1Ω respectively. Types of artificial power network impedance, as can be seen from the figure: the impedance of each structure of artificial power network in their respective operating frequency bands is close to 50Ω, the impedance values ​​of curve 1 and curve 2 are basically consistent, and the impedance of curve 3 The frequency point tending to 50Ω is shifted backward. Therefore, the 50Ω/5μH+1Ω type artificial power network can be used for conduction tests in higher frequency bands.

Figure four

　　3 Working principle and function of artificial power network

　　The simplified equivalent circuit of the conducted interference voltage measurement is shown in Figure 5. From the simplified circuit diagram, we can see that the conducted interference voltage U2 measured from the grid port is not only related to the conducted interference source (U1, Z1), but also related to the impedance (Z2) of the grid itself. However, we found that the impedance (Z2) of the power grid is not constant, it is affected by various factors such as measurement time, measurement location and measurement frequency, and the line impedance of the power grid is unstable. Therefore, in order to make the measurement results repeatable and comparable, the Special International Radio Interference Committee has designed various types of artificial power networks and specified standard line impedances for different types of conducted interference voltage measurements.

Figure five

　　The conduction interference voltage measurement circuit connected in series to the artificial power network is shown in Figure 6. It can be seen from the circuit diagram that the impedance (Z2) of the artificial power supply network is connected in series in the grid, which is much greater than the impedance (Z1) of the grid itself. Therefore, the total equivalent impedance in the measurement grid is greatly reduced by the grid's own impedance (Z1); at the same time, for the EMI source, the artificial power network provides it with a pure impedance load R, further stabilizing the grid line impedance in . The impedance (Z2) and impedance (Z3) in the artificial power network also form a high-frequency suppression circuit, which can suppress the high-frequency noise signal from the power grid and further reduce the measurement error.

Figure six

　　Taking the 50Ω/50μH+5Ω V-type artificial power network as an example, the actual circuit diagram of the artificial power network is shown in Figure 7.

Figure seven

　　It can be seen from Figure 7 that the artificial power network has the following main functions:

　　1) The EUT is powered by an artificial mains network which provides it with a standard line impedance. At the same time, the high-frequency noise signal from the power grid is effectively suppressed, and the line impedance change caused by other equipment from the power grid is reduced;

　　2) If there is no 50μH inductance and 8μF capacitor in the artificial power network, any disturbance signal in the power grid will be coupled to the measurement receiver through the artificial power network, so it will be considered to be generated by the equipment under test. Therefore, the 50μH inductor and 8μF capacitor in the artificial power network are used to isolate and bypass the external noise from the grid, and can also effectively isolate the disturbance signal generated by the device under test from entering the public grid to interfere with other electrical equipment;

　　3) The 0.25μF DC blocking capacitor in the artificial power supply network can prevent the input terminal of the measurement receiver from being overloaded; the 1kΩ resistor provides an electrostatic discharge path for the 0.25μF capacitor, and at the same time, the 1kΩ resistor is used as the measurement signal output port of the artificial power supply network, which is compatible with the measurement The signal input terminal of the receiver is connected, and the signal interference signal generated by the equipment under test is coupled to the measuring receiver for conducted interference measurement.

　　4 Use and precautions of artificial power network

　　In order to ensure the safety and authenticity of test results when using artificial power network for conducted emission test, there are the following points for attention:

　　1) Regular measurement of the artificial power supply network is required to ensure that the specified impedance can be provided within the working frequency range, and it can continue to be used within the range of measurement requirements.

　　2) During use, the voltage division coefficient of the artificial power network will change. In order to ensure the accuracy of the test, it is necessary to input the voltage division coefficient after regular measurement into the test software for correction.

　　3) When conducting a conducted emission test using an artificial power network, it is necessary to add an attenuator (such as 20dB) or a pulse limiter at the input of the measuring receiver to protect the receiver. Because some products (especially in the initial prototype stage of product development) will cause transient spikes when switching or momentary power failure, the amplitude of which is far beyond the measurement range of the measurement receiver, and it is easy to damage the measurement receiver.

　　4) The artificial power network needs to be well grounded, preferably lapped with the ground plane. When using a high-voltage power supply, pay attention to wait for the discharge of the artificial power network after the power is cut off, and then touch the artificial power network to ensure personal safety.

　　5) There is a large capacitor inside the artificial power network, which generates a large leakage current. It is recommended to use an air switch without leakage protection for the air switch in the supporting test environment.

　　6) In order to determine the credibility of the conducted emission data, the background noise of the test environment should be checked before the test. The method is to remove the power line of the product from the artificial power network, retain the power supply line of the artificial power network and the RF connection cable between the artificial power network and the measuring receiver, and the interference value measured by the measuring receiver is equivalent to the background noise. The test is only valid when the background noise is more than 6 dB below the limit.

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