Switching power supplies typically employ pulse width modulation (PWM) technology, which is known for its high frequency, efficiency, power density, and reliability. However, the switching device in these power supplies operates in a high-frequency on-off state, allowing for efficient energy transfer. Unfortunately, this fast transient process also generates electromagnetic interference (EMI). The EMI produced has a broad frequency range and high amplitude, potentially disrupting the operation of other nearby electronic devices.
EMI filter circuits are essential to mitigate these disturbances. The main source of interference comes from the switching frequency and its harmonics, which affect the power line—this is known as conducted interference. Conducted interference can be further divided into common-mode and differential-mode types. Common-mode interference arises due to a potential difference between the current-carrying conductor and ground, with both lines experiencing the same phase and voltage. In contrast, differential-mode interference results from a potential difference between the two current-carrying conductors, with signals on each line being out of phase. To address these issues, EMI filters are designed specifically for either common-mode or differential-mode interference. Figure 1 illustrates the typical filter circuit used.
In Figure 1, LC1 and LC2 along with Cy1 and Cy2 form the common-mode filter, while Ld1, Ld2, Cx1, and Cx2 constitute the differential-mode filter. The performance of the common-mode and differential-mode inductors is crucial, as it directly impacts the effectiveness of the EMI filtering. These inductors' performance is largely determined by the characteristics of their magnetic cores, making the analysis of core materials highly important.
Magnetic materials are generally categorized into soft magnetic, hard magnetic, and magnetostrictive materials based on their properties and applications. Soft magnetic materials are the most widely used, especially in inductive components like inductors and transformers. For filter inductors, soft magnetic materials are commonly employed. When selecting magnetic materials, factors such as saturation flux density (Bs), initial permeability (μi), and Curie temperature (Tc) must be considered, along with electrical characteristics like resistivity and impedance.
For EMI filter applications, the core material should meet several criteria: a high initial permeability (μi > 2000), low coercivity (Hc) to minimize hysteresis loss, high resistivity (Ï) to reduce eddy current losses at high frequencies, a high cutoff frequency (ωc) for broader bandwidth, a high Curie temperature (Tc) to ensure stability across different environments, and a specific loss frequency response that allows effective EMI suppression while minimizing signal loss.
Common-mode inductors play a key role in suppressing EMI over a wide frequency range, typically between 10 kHz and 50 MHz. The total impedance of the common-mode core consists of inductive reactance (Xs) and resistive impedance (Rs). At lower frequencies, the impedance is mainly inductive, but as the frequency increases, the resistive component becomes dominant. This ensures that the core provides adequate attenuation throughout the entire frequency range.
The common-mode inductor coils (Lcl and Lc2) in Figure 1 are wound around a single magnetic core with opposite turns, causing the magnetic flux to cancel out and preventing core saturation. This design allows the core to operate in a low magnetic field region, requiring a material with high initial permeability. While higher μi values improve insertion loss, other factors such as frequency response, Curie temperature, and physical shape must also be considered.
Figure 3 shows the frequency vs. impedance curves for various high-μi soft magnetic materials under similar conditions. Curve IV represents Mn-Zn ferrite PC40, commonly used in foreign countries for common-mode filtering. Curve III corresponds to domestic ferrite R4 KB. Ferrites have high resistivity, resulting in lower AC resistance and higher inductive reactance at low frequencies, which limits their effectiveness in certain bands. Ultrafine crystal and metal magnetic thin film alloys (curves II and I) have lower resistivity, leading to increased eddy current losses at higher frequencies. Their impedance is significantly higher than that of ferrites, offering better EMI suppression in the 10–100 kHz range.
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