Introduction to the application of magnetic beads in switching power supply EMC design
EMC has become a hot and difficult issue in today's electronic design and manufacturing. The EMC problem in practical applications is very complex and cannot be solved by theoretical knowledge alone. It relies more on the practical experience of electronic engineers. In order to better address the EMC issue of electronic products, the main considerations include grounding, circuit and PCB board design, cable design, shielding design, and other related issues.
This article explains the importance of magnetic beads in the EMC aspect of switch mode power supplies by introducing their basic principles and characteristics, in order to provide more and better choices for switch mode power supply product designers when designing new products.
1. Ferrite electromagnetic interference suppression component
Ferrite is a ferromagnetic material with a cubic lattice structure. Its manufacturing process and mechanical properties are similar to ceramics, and its color is gray black. A commonly used type of magnetic core in electromagnetic interference filters is ferrite material, and many manufacturers provide ferrite materials specifically designed for electromagnetic interference suppression. The characteristic of this material is very high frequency loss. The most important performance parameters for ferrite used to suppress electromagnetic interference are magnetic permeability μ and saturation magnetic flux density Bs. The magnetic permeability μ can be expressed as a complex number, with the real part forming the inductance and the imaginary part representing the loss, which increases with frequency. Therefore, its equivalent circuit is a series circuit consisting of an inductor L and a resistor R, both of which are functions of frequency. When the wire passes through this ferrite core, the impedance of the inductance formed increases with the increase of frequency in form, but the mechanism is completely different at different frequencies.
In the low frequency range, impedance is composed of the inductive reactance of inductance. At low frequencies, R is very small and the magnetic permeability of the magnetic core is high, resulting in a large inductance. L plays a major role, and electromagnetic interference is reflected and suppressed; And at this time, the loss of the magnetic core is relatively small, and the entire device is a low loss, high-Q characteristic inductor, which is prone to resonance. Therefore, in the low frequency range, interference enhancement may sometimes occur after using ferrite beads.
In the high frequency range, impedance is composed of resistance components. As the frequency increases, the magnetic permeability of the magnetic core decreases, resulting in a decrease in the inductance of the inductor and a decrease in the inductive impedance component. However, at this time, the loss of the magnetic core increases, and the resistance component increases, leading to an increase in the total impedance. When high-frequency signals pass through ferrite, electromagnetic interference is absorbed and converted into heat energy for dissipation.
Ferrite suppression components are widely used in printed circuit boards, power lines, and data lines. If ferrite suppression components are added to the power line inlet end of the printed circuit board, high-frequency interference can be filtered out. Ferrite magnetic rings or beads are specifically designed to suppress high-frequency interference and spike interference on signal lines and power lines, and they also have the ability to absorb electrostatic discharge pulse interference.
