MOS power IC full range

Filter 18.432M

Abstract: Commonly used EMI filters in switching power supplies suppress common mode interference and differential mode interference. Three-terminal capacitors have good performance in suppressing high-frequency interference from switching power supplies. Based on the general performance EMI filter circuit structure of switching power supply, the filter structure using three-terminal capacitor to suppress high frequency noise is given. The insertion loss is simulated using PSpice software, and the simulation results are given.
Key words: switching power supply; EMI filter; three-terminal capacitor; insertion loss

1 Switching power supply characteristics and noise generation With the rapid development of electronic technology, the variety of electronic equipment is increasing, and any electronic equipment is inseparable from a stable and reliable power supply, so the requirements for power supply are getting higher and higher. Switching power supply has achieved rapid development in recent years due to its high efficiency, low heat generation, good stability, small size, light weight and environmental protection. The application field is expanding. The switching power supply works in the high-frequency switching state, which itself will cause interference to 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 rectification waveform of the power frequency current and the switching operation waveform. The currents of these waveforms leak into the input area and become conductive and radiated noise, which causes ripple problems when leaking to the output. In consideration of electromagnetic compatibility requirements, EMI power filters should be used to suppress interference on the switching power supply. The main research in this paper is the EMI filter at the input of the switching power supply.

2 EMI filter structure 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 a capacitor and an inductor. It can suppress external electromagnetic interference introduced from the AC power line. Avoid the noise interference from the device itself to the outside. The interference of the switching power supply is divided into differential mode interference and common mode interference, and 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 line and the neutral line. The common mode interference is the interference generated between the live line or the neutral line and the ground line. A commonly effective method for suppressing differential mode interference signals and common mode interference signals is to install an electromagnetic interference filter in the switching power supply input circuit. 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 coil is wound on the upper and lower half rings of a magnetic ring (closed magnetic circuit), respectively winding the same number of turns but opposite turns. The magnetic flux directions of the two coils are the same, and when the common mode interference occurs, the total inductance rapidly increases to generate a large inductive reactance, thereby suppressing common mode interference and having 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 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 to 1 μF. Cy is used to suppress higher frequency common mode interference signals, and the range is generally 2200-6800 pF. Ceramic capacitors with a high self-resonant frequency are often used. Due to the grounding, a leakage current Ii-d is 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 <1.0 mA. The value of the common mode capacitor is related to the magnitude of the leakage current, so it should not be too large. The value range is generally 2200~4700 pF. R is the bleeder resistance of Cx. The performance of the line filter depends largely on its terminal impedance. According to the signal transmission theory, the termination of the filter input and the power supply, and the termination of the filter output and the load should follow the principle of impedance mismatch. Therefore, the filter should be designed to follow: (1) the source internal resistance is high resistance (low resistance), the filter input impedance should be low resistance (high resistance); (2) the load is high resistance (low resistance), Then the filter output impedance should be low impedance (high impedance). For EMI signals, the inductor is high impedance and the capacitor is low impedance. There are four filter types in Figure 1.

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