Magnetic components to solve EMC problems in electronic products - Magnetic beads (Part 2)

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Magnetic components to solve EMC problems in electronic products - Magnetic beads (Part 2) [Copy link]

"Magnetic Components for Solving EMC Problems in Electronic Products - Magnetic Beads (Part 1)"bbs.eeworld.com.cn/thread-1059913-1-1.html This article explains the basic parameters of magnetic beads. This article will focus on analyzing the methods of adjusting the damping of the filter system and the influence of magnetic beads under DC bias. 4. Methods for adjusting the damping of the filter system This section introduces several methods for adjusting damping, which can be used to reduce the resonance peak level. Jefferson Eco proposed three methods in his article: Method A is to add a series resistor in the decoupling capacitor path, which can suppress system resonance but reduce the effectiveness of high-frequency bypass. Method B is to add a small value parallel resistor across the ferrite bead, which also dampens the system resonance. However, the filter's attenuation characteristics are degraded at high frequencies. The figure below shows the impedance vs. frequency curve for the MPZ1608S101A with and without a 10Ω parallel resistor. The light green dashed line represents the total impedance of the bead with a 10Ω parallel resistor. The impedance of the bead and resistor combination drops significantly and is dominated by the 10Ω resistor. However, the 3.8MHz crossover frequency with a 10Ω parallel resistor is much lower than the crossover frequency of the bead itself at 40.3MHz. The bead behaves resistively at a much lower frequency range, which reduces the Q value and improves damping performance. When Changhui Instrument applied the methods described in the literature to the switching power supply, it was found that the first two methods could not reduce the noise amplitude of 0.6MHz, and the third method could effectively reduce all high-frequency harmonic noise. Digital display meteryunrun.com.cn/product/ Figure 24 The effects of three methods applied in energy-saving switching power supply Compared with Jefferson Eco, Ken Kundert not only introduced in detail how to deal with underdamping in his article, but also gave the calculation method for each case. Figure 25 Several methods of damping decoupling network and the value of resistance required for critical damping From the four methods given by the author in the above figure, it is not difficult to see that all of them have certain defects, sacrificing the filter insertion loss or signal power energy to a certain extent. For this reason, the author added a damping adjustment method that satisfies both RCL parallel and series connection. As shown in the figure below, it can be seen that this method is exactly the same as the method described by Jefferson Eco. The difference is that Ken Kundert further gave the relationship between , , and , that is, Figure 26 The preferred method to provide damping for the decoupled network Since there are many distributed parameters in the actual circuit that we cannot accurately measure, the above method is not always effective. For most engineers, they are not willing to try it. Here Changhui Instrument proposes a simple and crude method, that is, for similar problems encountered in low-frequency switching frequency, they can be filtered out by adding inductance. The inductance value should not be less than 50uH. Of course, the setting of this target parameter is not static. The following figure shows the result of adding an inductor on the basis of the magnetic bead, and the inductance value increases from 10uH to 50uH. It can be seen that after increasing to 50uH, the noise amplitude of all harmonic components is lower than that of the initial non-filter circuit. Figure 27 Add inductance to the circuit to adjust damping Because the duty cycle of 50% is a special case, when the load is reduced from the initial 10Ω to 2Ω (simulating the full load of the circuit), the duty cycle is about 64%. At this time, the even harmonics are fully reflected, and increasing the inductance is still applicable under full load. However, the method mentioned in the above literature will still encounter the situation where the low frequency exceeds the initial value. Figure 28 The effect of adjusting the inductance value remains after the power load increases Figure 29 The conducted noise measured under heavy load using the three methods mentioned in the literature Magnetic beadsyunrun.com.cn/tech/2254.html 5. The influence of magnetic beads under DC bias Selecting the right ferrite bead for power applications requires considering not only the filter bandwidth, but also the impedance characteristics of the bead with respect to the DC bias current. In most cases, manufacturers only specify the impedance of the bead at 100MHz and publish a data sheet with a frequency response curve at zero DC bias current. However, when using ferrite beads for power supply filtering, the load current through the bead is never zero, and these parameters change rapidly as the DC bias current increases from zero. As the DC bias current increases, the core material begins to saturate, causing the inductance of the ferrite bead to drop significantly. The degree of inductance saturation varies depending on the material used in the component core. The following figure shows the typical DC bias dependence of two ferrite beads. At 50% of the rated current, the inductance drops by up to 90%. Figure 30a The effect of DC bias on the inductance of the magnetic bead and the curve relative to the DC bias current Figure 30b Using TDKMPZ1608S101A magnetic bead Figure 30c uses Wurth Elektronik742 792 510 beads. If you need to filter power supply noise efficiently, as a design principle, you should use ferrite beads at about 20% of the rated DC current. As shown in these two examples, at 20% of the rated current, the inductance drops to about 30% (6A beads) and about 15% (3A beads). The current rating of the ferrite bead is the maximum current value that the device can withstand under the specified temperature rise, not the actual operating point for filtering. Additionally, the effect of the DC bias current can be observed as a reduction in impedance values over frequency, which in turn reduces the effectiveness of the ferrite bead and its ability to eliminate EMI. Figure 30b and Figure 30c show how the ferrite bead impedance changes with changes in DC bias current. With just 50% of the rated current applied, the effective impedance at 100 MHz drops significantly from 100 Ω to 10 Ω (TDK MPZ1608S101A, 100 Ω, A, 0603) and from 70 Ω to 15 Ω (Würth Elektronik 742 792 510, 70 Ω, 6 A, 1812).png[/img] Figure 30c uses Wurth Elektronik742 792 510 beads For efficient power supply noise filtering, as a design principle, ferrite beads should be used at about 20% of the rated DC current. As shown in these two examples, at 20% of the rated current, the inductance drops to about 30% (6A beads) and about 15% (3A beads). The current rating of the ferrite bead is the maximum current value that the device can withstand under specified temperature rise conditions, not the actual operating point for filtering. In addition, the effect of DC bias current can be observed by the reduction of impedance value in the frequency range, thereby reducing the effectiveness of the ferrite bead and its ability to eliminate EMI. Figure 30b and Figure 30c show how the impedance of the ferrite bead changes with changes in DC bias current. By applying only 50% of the rated current, the effective impedance at 100 MHz drops significantly from 100 Ω to 10 Ω (TDK MPZ1608S101A, 100 Ω, A, 0603), and from 70 Ω to 15 Ω (Würth Elektronik 742 792 510, 70 Ω, 6 A, 1812).png[/img] Figure 30c uses Wurth Elektronik742 792 510 beads For efficient power supply noise filtering, as a design principle, ferrite beads should be used at about 20% of the rated DC current. As shown in these two examples, at 20% of the rated current, the inductance drops to about 30% (6A beads) and about 15% (3A beads). The current rating of the ferrite bead is the maximum current value that the device can withstand under specified temperature rise conditions, not the actual operating point for filtering. In addition, the effect of DC bias current can be observed by the reduction of impedance value in the frequency range, thereby reducing the effectiveness of the ferrite bead and its ability to eliminate EMI. Figure 30b and Figure 30c show how the impedance of the ferrite bead changes with changes in DC bias current. By applying only 50% of the rated current, the effective impedance at 100 MHz drops significantly from 100 Ω to 10 Ω (TDK MPZ1608S101A, 100 Ω, A, 0603), and from 70 Ω to 15 Ω (Würth Elektronik 742 792 510, 70 Ω, 6 A, 1812).

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