Introduction
In switching power supplies, EMI filters play a crucial role in mitigating both common-mode and differential-mode conducted noise. This paper explores a method for independently analyzing and modeling these two types of signals, leading to a more effective approach for designing EMI filters. The proposed design procedure takes into account the unique characteristics of each noise type, ensuring better performance and compliance with electromagnetic compatibility (EMC) standards.
High-frequency switching power supplies are widely used across various industries, including industrial, military, and consumer electronics, due to their compact size, high power density, and efficiency. However, when connected to the AC grid, the rectifier circuit often causes input current to be discontinuous, which not only lowers the power factor but also introduces numerous harmonic distortions. Additionally, the rapid switching of power transistors—ranging from tens of kHz to several MHz—creates an EMI disturbance source. According to existing research, the primary forms of interference in switching power supplies include conducted and near-field radiation interference, with conducted noise potentially affecting other equipment connected to the same grid.
To reduce conducted interference, several strategies can be employed, such as proper grounding techniques, star-shaped layout, avoiding circular ground loops, minimizing shared impedance, and optimizing buffer circuits to reduce stray capacitance. In addition, EMI filters are commonly used to suppress noise between the power grid and the switching power supply.
EMI disturbances are often complex and difficult to model precisely. As a result, EMI filters are typically designed through an iterative process, gradually refining parameters to meet the required specifications. This paper starts by examining the principles of EMI filtering and then presents a practical method for designing filters by analyzing both common-mode and differential-mode noise models. A step-by-step example is provided to illustrate the process.
1. EMI Filter Design Principle
The main source of EMI in switching power supplies is the dv/dt and di/dt generated by the switching action of power semiconductor devices. This results in broadband noise that spans from the switching frequency up to several MHz. International and national standards, such as IEC 61000-6-3, define the conducted EMI measurement range from 0.15 to 30 MHz. Therefore, the goal of EMI filter design is to provide sufficient attenuation for the switching frequency and its higher harmonics. Typically, it is necessary to reduce EME above 150 kHz to an acceptable level.
The concept of a low-pass filter, widely used in digital signal processing, is equally applicable to power electronics. In general, EMI filter design should meet the following criteria:
1) Required stopband frequency and corresponding attenuation;
2) Low attenuation at grid frequency;
3) Cost-effective implementation.
1.1 Commonly Used Low-Pass Filter Models
An EMI filter is usually placed at the input of the power supply, acting as a low-pass filter composed of a series inductor and shunt capacitor. The filter's performance is influenced by the impedance mismatch between the noise source and the grid. Different orders of low-pass filters (first-order, second-order, or third-order) can be used to achieve the desired stopband attenuation. At high frequencies, the transfer function is often approximated by considering only the dominant terms, ignoring higher-order effects.
1.2 EMI Filter Equivalent Circuit
Conducted EMI noise includes both common-mode (CM) and differential-mode (DM) components. CM noise exists between the AC lines and the ground, while DM noise occurs between the live and neutral lines. These two types of noise have different sources and propagation paths, necessitating separate filtering for each mode. Understanding this distinction helps in accurately determining the noise levels and tailoring the filter design accordingly.
For example, in a typical EMI filter configuration, the common-mode choke suppresses CM noise, while the differential-mode capacitors suppress DM noise. However, parasitic elements such as leakage inductance and coupling capacitance can affect both modes. Proper modeling and parameter selection are essential to ensure optimal performance.
2. Practical Method for EMI Filter Design
2.1 Considerations in Design
The effectiveness of an EMI filter depends not only on its internal components but also on the impedance of the noise source and the grid. The grid impedance is usually measured using a LISN (Line Impedance Stabilization Network), which ensures standardized measurements. The noise source impedance varies depending on the converter topology, making accurate modeling challenging. Therefore, actual measurements are often used to determine the source impedance during the design process.
2.2 Design Steps
Designing an EMI filter involves several key steps, starting with measuring the source and grid impedances. Next, the noise spectrum is analyzed to separate CM and DM components. Based on the measured values and standard limits, the required attenuation is calculated. Parameters such as capacitors and inductors are then selected based on the desired performance, taking into account factors like leakage current and cost.
3. Conclusion
This paper focuses on low-frequency modeling of EMI filters, acknowledging that real-world components introduce parasitic effects that may require multiple iterations to optimize. High-frequency performance often demands additional adjustments, such as using low-ESR and ESL capacitors. Due to the complexity of power converter topologies and PCB layouts, EMI filters are generally not one-size-fits-all solutions. Instead, they require tailored designs and iterative testing to meet specific requirements. Future work could explore advanced modeling techniques and adaptive filtering methods to improve efficiency and performance.
Paste Type Rotor,Dc Variable Frequency Motor Rotor,Dc Motor Rotor,Adhesive Dc Variable Frequency Rotor
Suzhou Mountain Industrial Control Equipment Co., Ltd , https://www.szmountain.com