Application of magnetic beads in switching power supply EMC design
1 Ferrite EMI Suppression Element
Ferrite is a ferrous magnetic material with a cubic lattice structure. Its manufacturing process and mechanical properties are similar to ceramics, the colour is grey-black. EMI filters are often used in a class of magnetic core is ferrite material, many manufacturers provide ferrite material specifically for EMI suppression. This material is characterised by a very large high-frequency loss. For ferrite for EMI suppression, the most important performance parameters are permeability μ and saturation flux density Bs. The permeability μ can be expressed as a complex number, with the real part constituting the inductance and the imaginary part representing the loss, which increases with frequency. Thus, 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 is passed through this ferrite core, the resulting inductive impedance formally increases as frequency
The inductive impedance is formally increased with increasing frequency, but the mechanism is quite different at different frequencies.
In the low-frequency band, the impedance by the inductance of the inductance, low-frequency R is very small, the magnetic permeability of the core is high, so the inductance is large, L plays a major role in the electromagnetic interference is reflected and suppressed; and this time, the loss of the core is small, the entire device is a low-loss, high-Q characteristics of the inductance, which is prone to resonance, so in the low-frequency band, there may be times when the use of ferrite beads after the interference enhancement of the phenomenon.
In the high-frequency band, the impedance is composed of resistive components, as the frequency rises, the magnetic permeability of the core decreases, resulting in a decrease in the inductance of the inductor, the inductive component decreases However, this time, the loss of the core increases, the resistive component increases, resulting in an increase in the total impedance when the high-frequency signals through the ferrite, electromagnetic interference is absorbed and converted into the form of thermal energy dissipation.
Ferrite suppression components are widely used in printed circuit boards, power lines and data lines. If a ferrite suppression element is added to the inlet end of the power line of a printed circuit board, high frequency interference can be filtered out. Ferrite magnetic ring or bead dedicated to inhibit signal lines, power lines on the high-frequency interference and spike interference, it also has the ability to absorb electrostatic discharge pulse interference.
2 The principle and characteristics of the magnetic bead when the current flows through its centre hole in the wire, it will be the magnetic bead internal circulation flow of magnetic channels. When a ferrite is formulated for EMI control, it should be possible to dissipate most of the magnetic flux as heat in the material. This phenomenon can be modelled by a series combination of an inductor and a resistor. As shown in Figure 2
The numerical magnitude of the two components is directly proportional to the length of the bead, and the length of the bead has a significant effect on the suppression, with the longer bead length providing better suppression. As the signal energy is magnetically coupled to the beads, the inductor reactance and resistance increase with frequency. The efficiency of magnetic coupling depends on the magnetic permeability of the bead material relative to air. The loss of the ferrite material that normally makes up the bead can be expressed as a complex quantity by its permeability with respect to air.
Magnetic materials are often characterised by this ratio to the loss angle. A large loss angle is required for EMI suppression components, which means that most of the interference will be dissipated.
This means that most of the interference will be dissipated and not reflected. The wide variety of ferrite materials available today gives the designer a wide choice of beads to use in different applications.
