Design and simulation of switching power supply input EMI filter

Abstract: EMI filters are commonly used in switching power supplies to suppress common-mode interference and differential-mode interference. Three-terminal capacitors have good performance in suppressing high-frequency interference in switching power supplies. Based on the general performance EMI filter circuit structure of switching power supplies, this paper gives a filter structure that uses a three-terminal capacitor to suppress high-frequency noise. PSpice software is used to simulate the insertion loss and the simulation results are given.

Keywords: switching power supply; EMI filter; three-terminal capacitor; insertion loss

1 Switching power supply characteristics and noise generation causes

With the rapid development of electronic technology, the types of electronic devices are increasing day by day, and any electronic device 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 switching power supply interference mainly comes from the rectified waveform and 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, thereby suppressing common mode interference, but not 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 2200~6800 pF. Ceramic capacitors with higher self-resonance frequency are often used. Due to grounding, a leakage current Ii-d will be generated on the common-mode capacitor Cy. Because the leakage current will cause harm to human safety, the leakage current should be as small as possible, usually <1.0 mA. The value of the common-mode capacitor is related to the size of the leakage current, so it should not be too large, and the value range is generally 2200~4700 pF. R is the discharge resistance of Cx. The performance of the power supply filter depends largely on its terminal impedance. According to the signal transmission theory, the termination of the filter input end and the power supply end, and the termination of the filter output end and the load end should follow the principle of maximum impedance mismatch. Therefore, the filter design should follow the following principles: (1) If the source internal resistance is high (low), the filter input impedance should be low (high); (2) If the load is high (low), the filter output impedance should be low (high). For EMI signals, the inductor is high and the capacitor is low, so there are four types of filters to choose from as shown in Figure 1.

Power supply filters are generally used to suppress noise in the frequency range below 30 MHz, but they also have a certain inhibitory effect on radiated emission interference above 30 MHz. According to the characteristics of common-mode and differential-mode interference of switching power supplies, the distribution of interference can be roughly divided into three frequency bands: 0.15-0.5 MHz differential-mode interference is dominant; 0.5-5 MHz differential-mode and common-mode interference coexist; 5-30 MHz common-mode interference is dominant.

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).