Application of magnetic beads in EMC design of switching power supply
1 Ferrite electromagnetic interference suppression component
Ferrite is a ferrimagnetic material with a cubic lattice structure. Its manufacturing process and mechanical properties are similar to ceramics, and its color is gray-black. A type of magnetic core often used in electromagnetic interference filters is ferrite material. Many manufacturers provide ferrite materials specifically used for electromagnetic interference suppression. This material is characterized by very high losses at high frequencies. For ferrites used to suppress electromagnetic interference, the most important performance parameters are magnetic permeability μ and saturation magnetic flux density Bs. Magnetic permeability μ can be expressed as a complex number, the real part constitutes the inductance, and the imaginary part represents the loss, which increases with the increase of frequency. Therefore, its equivalent circuit is a series circuit consisting of an inductor L and a resistor R. Both L and R are functions of frequency. When a wire passes through this ferrite core, the inductive impedance formed increases in form as the frequency increases, but its mechanism is completely different at different frequencies.
In the low frequency band, the impedance is composed of the inductive reactance of the inductor. At low frequency, R is very small and the magnetic permeability of the magnetic core is high, so the inductance is large. L plays a major role. The electromagnetic interference is reflected and suppressed; and at this time, the magnetic The core loss is small, and the entire device is an inductor with low loss and high Q characteristics. This inductor is easy to cause resonance. Therefore, in the low frequency band, interference may sometimes be enhanced after using ferrite beads.
In the high frequency band, the impedance is composed of resistance components. As the frequency increases, the magnetic permeability of the magnetic core decreases, causing the inductance of the inductor to decrease and the inductive reactance component to decrease. However, at this time, the loss of the magnetic core increases and the resistance component increases. , causing the total impedance to increase. When high-frequency signals pass through the ferrite, the electromagnetic interference is absorbed and converted into heat energy and dissipated.
Ferrite suppression components are widely used in printed circuit boards, power lines and data lines. If a ferrite suppression component is added to the inlet end of the power line of the printed circuit board, high-frequency interference can be filtered out. Ferrite magnetic rings or magnetic beads are specially used to suppress high-frequency interference and spike interference on signal lines and power lines. It also has the ability to absorb electrostatic discharge pulse interference.
2 Principles and Characteristics of Magnetic Beads When current flows through the wire in the central hole, a circulating magnetic track will be generated inside the magnetic bead. Ferrites used for EMI control should be formulated to dissipate most of the magnetic flux as heat in the material. This phenomenon can be simulated by a series combination of an inductor and a resistor. as shown in picture 2
The numerical values of the two components are proportional to the length of the magnetic beads, and the length of the magnetic beads has a significant impact on the suppression effect. The longer the length of the magnetic beads, the better the suppression effect. Since the signal energy is magnetically coupled to the magnetic beads, the reactance and resistance of the inductor increase as the frequency increases. The efficiency of magnetic coupling depends on the magnetic permeability of the bead material relative to air. Usually the loss of the ferrite material that makes up the magnetic beads can be expressed as a complex quantity by its magnetic permeability relative to air.
Magnetic materials often use this ratio to characterize the loss angle. The components used for EMI suppression require a large loss angle, which means that most of the interference will be dissipated without being reflected. The variety of available ferrite materials currently available provides designers with a wide range of choices for using magnetic beads in different situations.
