Components: Application of magnetic beads in switching power supply EMC design

EMC issues have become a hot and difficult issue in today's electronic design and manufacturing. EMC issues in practical applications are very complex and cannot be solved by relying on theoretical knowledge. They rely more on the practical experience of a large number of electronic engineers. In order to better solve the EMC problem of electronic products, it is mainly necessary to consider grounding, circuit and PCB board design, cable design, shielding design and other issues.

This article introduces the basic principles and characteristics of magnetic beads to illustrate their importance in switching power supply EMC, in order to provide designers of switching power supply products with more and better choices when designing new products.

Ferrite EMI Suppression Components

Ferrite is a ferrimagnetic material with a cubic lattice structure. Its manufacturing process and mechanical properties are similar to those of ceramics, and its color is gray-black. 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 that the high-frequency loss is very large. For ferrites used to suppress electromagnetic interference, the most important performance parameters are magnetic permeability μ and saturation flux density Bs. Magnetic permeability μ can be expressed as a complex number, the real part constitutes inductance, and the imaginary part represents loss, which increases with increasing frequency. Therefore, its equivalent circuit is a series circuit composed of inductance L and resistance R, and L and R are both functions of frequency. When the wire passes through this ferrite core, the inductor impedance formed increases in form with the increase of frequency, 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 frequencies, R is very small, and the magnetic permeability of the magnetic core is high, so the inductance is large, L plays a major role, and the electromagnetic interference is reflected and suppressed; and at this time, the loss of the magnetic core is small, and the entire device is an inductor with low loss and high Q characteristics. This inductor is prone to resonance. Therefore, in the low frequency band, sometimes the interference may 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, resulting in a decrease in the inductance of the inductor and a decrease in the inductive reactance component. 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, 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 power line inlet of the printed circuit board, high-frequency interference can be filtered out. Ferrite magnetic rings or beads are specially used to suppress high-frequency interference and spike interference on signal lines and power lines. They also have the ability to absorb electrostatic discharge pulse interference.

Principles and characteristics of magnetic beads

When current flows through the wire in the center hole, a magnetic track is generated inside the bead. When ferrite is formulated for EMI control, most of the magnetic flux should be dissipated as heat in the material. This phenomenon can be simulated by a series combination of an inductor and a resistor. As shown in Figure 2

The values ​​of the two components are proportional to the length of the magnetic bead, and the length of the magnetic bead has a significant effect on the suppression effect. The longer the magnetic bead is, the better the suppression effect is. Since the signal energy is added to the magnetic bead in the form of magnetic coupling, the reactance and resistance of the inductor increase with the increase of frequency. The efficiency of magnetic coupling depends on the magnetic permeability of the magnetic bead material relative to air. Usually, the loss of the ferrite material that makes up the magnetic bead can be expressed as a complex quantity through its magnetic permeability relative to air.

Magnetic materials are often characterized by the loss angle of this ratio. A larger loss angle is required for EMI suppression components, which means that most interference will be dissipated rather than reflected. The various available ferrite materials currently available provide designers with a lot of choices for using magnetic beads in different occasions.

Spike Suppressor

The biggest disadvantage of switching power supplies is that they are prone to noise and interference, which is a key technical problem that has long plagued switching power supplies. The noise of switching power supplies is mainly caused by the fast-changing high-voltage switching and pulse short-circuit current of the switching power tube and the switching rectifier diode. Therefore, using effective components to limit them to a minimum is one of the main methods to suppress noise. Nonlinear saturated inductors are usually used to suppress reverse recovery current spikes, and the working state of the iron core is from -Bs to +Bs. Based on the consistency of the high magnetic permeability and saturable ultra-small inductor components-magnetic beads on the switching power supply freewheeling diode, a spike suppressor was developed to suppress the peak current generated when the switching power supply is switched.

Performance characteristics of spike suppressor:

(1) The initial and maximum inductance values ​​are very high, and the nonlinearity of the residual inductance after saturation is extremely insignificant. After being connected in series to the loop, it shows high impedance at the moment the current rises, and can be used as a so-called transient impedance element.

(2) It is suitable for preventing transient current peak signals, impact excitation circuits and the noise associated with them in semiconductor circuits, and can also prevent semiconductor damage.

(3) The residual inductance is extremely small, and the loss is very small when the circuit is stable.

(4) The performance is completely different from that of ferrite products.

(5) As long as magnetic saturation is avoided, it can be used as an ultra-small, high-inductance inductor element.

(6) It can be used as a low-loss, high-performance saturable core for controlling and generating oscillations. [page]

The spike suppressor requires the core material to have a higher magnetic permeability to obtain a larger inductance; the high rectangular ratio can make the core saturate and the inductance should drop rapidly to zero; the coercive force should be small and the high-frequency loss should be low, otherwise the core will release heat and cannot work normally.

The purpose of spike suppressor is to reduce the current spike signal; reduce the noise caused by the current peak signal; prevent the damage of the switching transistor; reduce the switching loss of the switching transistor; compensate the recovery characteristics of the diode; prevent the high-frequency pulse current impact excitation. It can be used as an ultra-small line filter, etc.

Application in filters

a) Test results without adding magnetic beads

b) Test results with magnetic beads

c) L line plus magnetic beads test results

d) N-wire plus magnetic beads test results

Ordinary filters are composed of lossless reactive elements. Their function in the circuit is to reflect the stopband frequency back to the signal source, so this type of filter is also called a reflection filter. When the reflection filter does not match the signal source impedance, part of the energy will be reflected back to the signal source, causing an increase in the interference level. To solve this problem, ferrite rings or magnetic beads can be used on the incoming line of the filter, and the eddy current loss of the high-frequency signal by the ferrite rings or magnetic beads can be used to convert the high-frequency component into heat loss. Therefore, the magnetic rings and magnetic beads actually absorb the high-frequency components, so they are sometimes called absorption filters.

Different ferrite suppression elements have different optimal suppression frequency ranges. Generally, the higher the permeability, the lower the suppression frequency. In addition, the larger the volume of the ferrite, the better the suppression effect. When the volume is constant, a long and thin shape has a better suppression effect than a short and thick one, and the smaller the inner diameter, the better the suppression effect. However, in the case of DC or AC bias current, there is still the problem of ferrite saturation. The larger the cross-section of the suppression element, the less likely it is to saturate, and the greater the bias current it can withstand.

Based on the above magnetic bead principles and characteristics, they are applied in the filter of the switching power supply with obvious results. The test results show that the application of magnetic beads is obviously different. From the experimental results, it can be seen that due to the influence of the switching power supply circuit, structure layout, and power, sometimes it has a good suppression effect on differential mode interference, sometimes it has a good suppression effect on common mode interference, and sometimes it has no suppression effect on interference and increases noise interference.

When the EMI absorbing magnetic ring/bead suppresses differential mode interference, the current value passing through it is proportional to its volume. The imbalance between the two causes saturation, which reduces the performance of the component. When suppressing common mode interference, the two wires (positive and negative) of the power supply pass through a magnetic ring at the same time. The effective signal is a differential mode signal, and the EMI absorbing magnetic ring/bead has no effect on it, but it will show a large inductance for the common mode signal. Another good way to use the magnetic ring is to let the wire passing through the magnetic ring be repeatedly wound several times to increase the inductance. You can use its suppression effect reasonably according to its suppression principle for electromagnetic interference.

Ferrite suppression components should be installed close to the interference source. For input/output circuits, they should be as close to the entrance and exit of the shielding shell as possible. For absorption filters composed of ferrite magnetic rings and magnetic beads, in addition to selecting lossy materials with high magnetic permeability, attention should also be paid to its application. The resistance they present to high-frequency components in the circuit is about ten to several hundred Ω, so its effect in high-impedance circuits is not obvious. On the contrary, it will be very effective in low-impedance circuits (such as power distribution, power supply or radio frequency circuits).

Since ferrite can attenuate higher frequencies while allowing lower frequencies to pass almost unimpeded, it has been widely used in EMI control. Magnetic rings/beads used for EMI absorption can be made into various shapes and are widely used in various occasions. For example, on PCB boards, they can be added to DC/DC modules, data lines, power lines, etc. It absorbs high-frequency interference signals on the line, but does not generate new zeros and poles in the system, and does not destroy the stability of the system. It can be used in conjunction with power supply filters to well supplement the shortcomings of the high-frequency end performance of the filter and improve the filtering characteristics of the system. The majority of professional researchers in switching power supplies should give full play to their technical advantages and flexibly apply ferrite materials such as magnetic rings and beads to the development of switching power supplies, so that they can play a greater role in the design of switching power supplies, improve the EMC of products, and reduce volume and cost.