Application of Magnetic Beads in EMC Design of Switching Power Supplies
EMC issues have become a hot and challenging issue in electronic design and manufacturing today. The EMC problem in practical applications is very complex and cannot be solved solely by theoretical knowledge. It relies more on the practical experience of electronic engineers. In order to better address the issue of EMC in electronic products, it is mainly necessary to consider grounding, circuit and PCB board design, cable design, shielding design, and other issues.
This article explains the importance of magnetic beads in the EMC aspect of switching power supplies by introducing their basic principles and characteristics, in order to provide more and better choices for switching power supply product designers when designing new products.
1 Ferrite electromagnetic interference suppression element
Ferrite is a type of ferromagnetic material with a cubic lattice structure. Its manufacturing process and mechanical properties are similar to ceramics, with a gray black color. A type of magnetic core commonly used in electromagnetic interference filters is ferrite material, and many manufacturers provide ferrite materials specifically for electromagnetic interference suppression. The characteristic of this material is its high frequency loss. The most important performance parameter for ferrite used for suppressing electromagnetic interference is magnetic permeability μ And saturation magnetic flux density Bs. Magnetic permeability μ It 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 composed of inductance L and resistance R, both of which are functions of frequency. When the wire passes through this ferrite core, the inductance impedance formed increases formally with increasing frequency, but the mechanism is completely different at different frequencies.
In the low frequency range, impedance is composed of the inductive reactance of the inductor. At low frequencies, R is very small, and the magnetic permeability of the magnetic core is high. Therefore, the inductance is large, and L plays a main role. Electromagnetic interference is suppressed by reflection; And at this point, 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, sometimes there may be interference enhancement after using ferrite magnetic 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 and inductance components of the inductance. However, at this time, the loss of the magnetic core increases, and the resistance component increases, resulting in an increase in the total impedance. When the high-frequency signal passes through the 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 a ferrite suppression component is added to the power line inlet of the printed circuit board, high-frequency interference can be filtered out. Ferrite magnetic rings or beads are specifically designed to suppress high-frequency and peak interference on signal and power lines, and they also have the ability to absorb electrostatic discharge pulse interference.
The principle and characteristics of magnetic beads: When current flows through a wire in the core hole, it becomes a magnetic track that circulates inside the magnetic bead. When preparing ferrite for EMI control, it should be possible 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.
The signal energy is magnetically coupled to the magnetic beads, so the reactance and resistance of the inductor increase with increasing frequency. The efficiency of magnetic coupling depends on the permeability of the magnetic bead material relative to air. The loss of ferrite materials that typically make up magnetic beads can be expressed as a complex quantity by their relative permeability to air.
Magnetic materials are often characterized by the loss angle using this ratio. The use of EMI suppression components requires a large loss angle, which means that most interference will be dissipated without being reflected. The various available ferrite materials currently available provide designers with a wide range of options for using magnetic beads in different situations.
