Application of Magnetic Beads in EMC Design of Switching Power Supply
This paper introduces the characteristics of ferrite bead, and according to its characteristics, analyzes and introduces its important application in EMC design of switching power supply, and gives the experimental and test results in power line filter.
EMC has become a hot and difficult issue in today's electronic design and manufacture. The EMC problem in practical application is very complicated, and it cannot be solved by relying on theoretical knowledge. It depends more on the practical experience of electronic engineers. In order to better solve the problem of EMC of electronic products, it is necessary to consider issues such as grounding, circuit and PCB board design, cable design, and shielding design.
This paper introduces the basic principles and characteristics of magnetic beads to illustrate its importance in switching power supply EMC, in order to provide switching power supply product designers with more and better choices when designing new products.
1 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. One type of magnetic core often used in EMI filters is ferrite material, and many manufacturers provide ferrite materials specially used for EMI suppression. This material is characterized by very large high-frequency losses. For ferrite used to suppress electromagnetic interference, the most important performance parameters are magnetic permeability μ and saturation magnetic flux density Bs. The 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 composed of an inductor L and a resistor R, both L and R are functions of frequency. When the wire passes through this ferrite core, the formed inductive impedance increases in form as the frequency increases, but the 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, and 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.
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 dissipated in the form of heat energy.
Ferrite suppression components are widely used on printed circuit boards, power lines and data lines. If a ferrite suppression element is added to the inlet end of the power line of the printed 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. The principle and characteristics of magnetic beads When the current flows through the wire in its central hole, it will be a magnetic track that circulates inside the magnetic bead. Ferrites for EMI control should be formulated such that most of the magnetic flux is dissipated as heat in the material. This phenomenon can be modeled by a series combination of an inductor and a resistor. as shown in picture 2
The numerical value of the two components is proportional to the length of the magnetic bead, and the length of the magnetic bead has a significant impact on the suppression effect. The longer the length of the magnetic bead, the better the suppression effect. Since the signal energy is magnetically coupled to the magnetic bead, the reactance and resistance of the inductor increase with the increase of the frequency. The efficiency of the 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 bead can be expressed as a complex quantity through its permeability relative to air.
Magnetic materials often use this ratio to characterize the loss angle . A large loss angle is required for EMI suppression components, which means that most of the interference will be dissipated and not reflected. The wide variety of ferrite materials available today provides designers with a wide range of options for using ferrite beads in different applications.
3 Application of magnetic beads
3.1 Spike suppressor
The biggest disadvantage of switching power supply is that it is easy to generate noise and interference, which is a key technical problem that has plagued switching power supply for a long time. The noise of the switching power supply 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 of suppressing noise. Non-linear saturated inductance is usually used to suppress the reverse recovery current peak, at this time the working state of the iron core is from -Bs to +Bs. According to the consistency of the high magnetic permeability and saturable ultra-small inductance element-magnetic beads on the freewheeling diode of the switching power supply, a spike suppressor used to suppress the peak current generated when the switching power supply is switched is developed.
Performance Characteristics of Spike Suppressors
(1) The initial and maximum inductance values are very high, and the nonlinearity of the residual inductance value after saturation is extremely unobvious. After being connected in series to the circuit, the current rises and shows high impedance instantly, which can be used as a so-called instantaneous impedance element.
(2) It is suitable for preventing the transient current peak signal in the semiconductor circuit, the impact excitation circuit and the accompanying noise, and it can also prevent the semiconductor from being damaged.
(3) The residual inductance is extremely small, and the loss is very small when the circuit is stable.
(4) It is completely different from the performance of ferrite products.
(5) As long as magnetic saturation is avoided, it can be used as an ultra-small, high-inductance inductance element.
(6) It can be used as a high-performance saturable iron core with low loss to control and generate oscillation.
The spike suppressor requires the iron core material to have a higher magnetic permeability to obtain a larger inductance; when the high square ratio can saturate the iron core, the inductance should drop to zero quickly; the coercive force is small and the high frequency loss is low, otherwise the heat dissipation of the iron core will not work normally.
The purpose of the spike suppressor is mainly to reduce the current peak 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 high-frequency pulse current shock excitation. Use as an ultra-small line filter, etc.
3.2 Application in filter a) Test result without magnetic beads b) Test result with magnetic beads c) Test result with L line and magnetic beads d) Test result with N line and magnetic beads
Ordinary filters are composed of lossless reactive components. Its 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 impedance of the signal source, part of the energy will be reflected back to the signal source, resulting in an increase in the interference level. In order to solve this disadvantage, ferrite magnetic ring or magnetic bead sleeve can be used on the incoming line of the filter, and the eddy current loss of the high-frequency signal by the ferrite ring or magnetic bead can be used to convert the high-frequency component into heat loss. Therefore, the magnetic ring and magnetic beads actually absorb high-frequency components, so they are sometimes called absorption filters.
Different ferrite suppression components have different optimal suppression frequency ranges. Generally, the higher the permeability, the lower the frequency suppressed. In addition, the larger the volume of the ferrite, the better the suppression effect. When the volume is constant, the long and thin shape has better suppression effect than the 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 will be saturated, and the greater the bias current it can withstand.
Based on the above principles and characteristics of magnetic beads, it is applied to the filter of switching power supply, and the effect is obvious. From the test results, it can be seen that the application of magnetic beads is significantly different. It can be seen from the experimental results that due to the influence of the switching power supply circuit, structural 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 does not have a suppression effect on interference but increases noise interference.
When the EMI absorbing magnetic ring/magnetic bead suppresses differential-mode interference, the current value passing through it is proportional to its volume, and 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, and the effective signal is a differential-mode signal. Another better method in the use of the magnetic ring is to make the wire passing through the magnetic ring repeatedly wound several times to increase the inductance. According to its suppression principle of electromagnetic interference, its suppression effect can be reasonably used.
Ferrite suppression components should be installed close to the source of interference. For the input/output circuit, it should be as close as possible to the inlet and outlet of the shielding case. For the absorption filter composed of ferrite magnetic ring and magnetic beads, in addition to choosing lossy materials with high magnetic permeability, attention should also be paid to its application occasions. Their resistance to high-frequency components in the line is about ten to hundreds of Ω, so its role 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).
