Pspice simulation of power input EMI filter

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Pspice simulation of power input EMI filter [Copy link]

1 Characteristics of switching power supply and causes of noise With the rapid development of electronic technology, the types of electronic equipment are increasing, and any electronic equipment cannot do without a stable and reliable power supply, so the requirements for power supply are getting higher and higher. With its advantages of high efficiency, low heat generation, good stability, small size, light weight, and environmental protection, the switching power supply has achieved rapid development in recent years, and its application field has been continuously expanded. The switching power supply works in a high-frequency switching state, which will interfere with the power supply equipment and endanger its normal operation; and external interference will also affect its normal operation. The interference of the switching power supply mainly comes from the rectified waveform and the switching operation waveform of the power frequency current. The current of these waveforms leaks to the input part and becomes conducted noise and radiated noise, and leaks to the output part to form a ripple problem. Considering the relevant requirements of electromagnetic compatibility, EMI power supply filters should be used to suppress the interference on the switching power supply. This paper mainly studies the EMI filter at the input end of the switching power supply. 2 Structure of EMI filter The EMI filter used at the input end of the switching power supply is a bidirectional filter. It is a low-pass filter composed of capacitors and inductors. It can not only suppress the external electromagnetic interference introduced from the AC power line, but also prevent the equipment from emitting noise interference to the outside. The interference of the switching power supply is divided into differential mode interference and common mode interference. The conducted interference signals in the line can be represented by differential mode and common mode signals. Differential mode interference is the interference generated between the live wire and the neutral wire, and common mode interference is the interference generated between the live wire or the neutral wire and the ground wire. The generally effective method to suppress differential mode interference signals and common mode interference signals is to install an electromagnetic interference filter in the input circuit of the switching power supply. The circuit structure of the EMI filter includes a common mode choke (common mode inductor) L, a differential mode capacitor Cx and a common mode capacitor Cy. The common mode choke is a coil with the same number of turns but opposite winding directions on the upper and lower half rings of a magnetic ring (closed magnetic circuit). The magnetic flux directions of the two coils are consistent. When common-mode interference occurs, the total inductance increases rapidly to produce a large inductive reactance, which can suppress common-mode interference, but has no effect on differential-mode interference. In order to better suppress common-mode noise, the common-mode choke should use a magnetic core with high magnetic permeability and good high-frequency performance. The inductance value of the common-mode choke is related to the rated current. The differential mode capacitor Cx usually uses a metal film capacitor, and the value range is generally 0.1~1μF. Cy is used to suppress higher-frequency common-mode interference signals, and the value range is generally 2200~6800pF. Ceramic capacitors with higher self-resonance frequency are often used. Due to grounding, leakage current Ii-d will be generated on the common-mode capacitor Cy. Because leakage current can cause harm to human safety, the leakage current should be as small as possible, usually <1.0mA. The value of the common-mode capacitor is related to the leakage current, so it should not be too large, and the value range is generally 2200~4700pF. R is the discharge resistance of Cx. The performance of the power filter depends largely on its terminal impedance. According to the signal transmission theory, the termination of the filter input terminal to the power supply terminal and the termination of the filter output terminal to the load terminal should follow the principle of maximum impedance mismatch. Therefore, the filter design should follow the following: (1) If the source internal resistance is high resistance (low resistance), the filter input impedance should be low resistance (high resistance); (2) If the load is high resistance (low resistance), the filter output impedance should be low resistance (high resistance). For EMI signals, the inductor is high resistance and the capacitor is low resistance, so there are 4 types of filters to choose from as shown in Figure 1.

Power filters are generally used to suppress noise in the frequency range below 30MHz, but they also have a certain inhibitory effect on radiated emission interference above 30MHz. According to the characteristics of common mode and differential mode interference of switching power supplies. It can be roughly divided into 3 frequency bands according to the distribution of interference: O. 15～0.5MHz is mainly differential mode interference; 0.5～5MHz differential mode and common mode interference coexist; 5～30MHz is mainly common mode interference.

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3 Insertion loss Insertion loss is the main indicator for evaluating filter performance, and it is a function of frequency. Insertion loss is defined as the ratio of the power P1 transmitted from the noise source to the load when no filter is connected to the power P2 transmitted from the noise source to the load after the filter is connected, expressed in dB. The greater the insertion loss, the stronger the filter's ability to suppress interference. The circuit diagrams before and after the filter is connected are shown in Figure 3(a) and Figure 3(b). The insertion loss of the filter is expressed by formula (1).

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4 Three-terminal capacitors In high-frequency circuits, the lead wires of general capacitors have inductance components, which affects their high-frequency characteristics. However, the three-terminal capacitor can achieve a very small residual inductance component in series with the capacitor in terms of structure, so its insertion loss characteristics are better than those of two-terminal capacitors, thereby improving the high-frequency characteristics of the capacitor. There are two types of three-terminal capacitors: lead type and sheet type.

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6 Pspice simulation (1) Comparison of insertion loss of circuit using three-terminal capacitor and insertion loss of conventional circuit. The differential mode capacitor Cx is 0.1μF, the common mode capacitor Cy is 2200pF, and the common mode inductor L is 8mH. The equivalent series inductance ESL of the three-terminal capacitor is 0.36nH. In the 50Ω/50ΩQ system, the insertion loss of the general structure EMI filter and the EMI filter using the three-terminal capacitor are simulated by PSpice. As shown in Figure 7, when the EMI filter uses the three-terminal capacitor, the insertion loss after the resonance point is significantly better than the insertion loss of the two-terminal capacitor in the filter. The performance of the filter in the high frequency band is improved.

(2) Different Cy values, fixed ESL. In the filter circuit using a three-terminal capacitor, when the input impedance and output impedance are both 50, the common-mode capacitance Cy is taken as 4700pF, 3300pF and 2200pF respectively, and other parameters remain unchanged, and the influence of the common-mode capacitance Cy on the insertion loss is observed. The simulation results of Figure 8 show that with the increase of the common-mode capacitance, the insertion loss in the high-frequency band increases, and the resonance point of the filter decreases; while there is basically no change in the low-frequency band. Therefore, the insertion loss of the filter in the high-frequency band can be improved by selecting a larger common-mode capacitor. Since the common-mode capacitor needs to be grounded, there is leakage current, and the existence of Iid poses a threat to personal safety. The larger the common-mode capacitance, the greater the leakage current, so when selecting the common-mode capacitor, it is necessary to select the value when the leakage current meets the safety conditions.

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(3) Fixed Cy value, different ESL. The influence of the equivalent series inductance ESL of the three-terminal capacitor in series with the signal line on the insertion loss is investigated. The common-mode capacitance Cy is taken as 3300pF, and the ESL is taken as 0.03nH, 0.36nH and 0.72nH respectively, and the other parameter values remain unchanged. From the simulation results of Figure 9, it can be seen that as the ESL decreases, the resonance point increases, and the insertion loss after the resonance point decreases.

7 Conclusion Based on the general performance EMI filter, the original filter is improved by using a three-terminal capacitor as the common-mode capacitor. The simulation results show that it has a better insertion loss effect in the high frequency band. Since the impedance of the device in actual use and the parasitic effects of the components at high frequencies will affect the insertion loss of the EMI filter, the filter needs to be optimized and designed according to the actual situation.

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