Magnetic components to solve EMC problems in electronic products - Magnetic beads (Part 1)

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Magnetic components to solve EMC problems in electronic products - Magnetic beads (Part 1) [Copy link]

The digital display meter contains many magnetic components, which account for a large proportion of the product cost. If we pick a product at random, we can directly see various inductors, magnetic beads, transformers, etc. However, perhaps due to the complex and changeable parameters in magnetism, or perhaps because the magnetic components look too simple, most engineers are accustomed to ignoring them when designing products. We know that in the design of switching power supplies, in order to achieve higher conversion efficiency, designers need to fully master the design skills of parameters such as transformer windings, air gaps, and PFC inductors. When designing EMI filters, we often focus on the inductive reactance and impedance parameters of magnetic components, while ignoring many key parameters. Changhui Instrument will use a series of articles on the application of magnetic beads in digital display meters to let readers further understand the various characteristics of magnetic components, hoping to help readers more accurately select magnetic components in actual projects and analyze the causes of problems more quickly. Digital display meteryunrun.com.cn/product/ Magnetic beads are one of many magnetic components. Magnetic beads are divided into through-core magnetic beads and patch magnetic beads. The author personally believes that through-core magnetic beads are closer to inductors and are relatively rare in practical applications, especially in the current trend of product miniaturization. SMD magnetic beads have more advantages. Changhui Instruments focuses on the SMD magnetic beads used in instrument production in this article, hoping to be helpful to readers. Magnetic beadsyunrun.com.cn/tech/2244.html 1. Ferrite beads Analog and digital ICs with different frequency and power characteristics in PCBs are usually powered by different power networks. This helps prevent fast digital switching noise from coupling to sensitive analog power networks and reducing converter performance, but independent power supplies will increase system-level complexity and manufacturing costs. Ferrite beads are usually selected to take appropriate high-frequency isolation for the power network. Ferrite beads are passive devices that can filter high-frequency noise over a wide frequency range. They have resistance characteristics within the target frequency range and dissipate noise energy in the form of heat. Generally, ferrite beads are mainly used in PDN power networks. Capacitors of appropriate capacitance are usually connected to the ground on both sides of the beads to form a filtering network to reduce the switching noise of the PDN power network. The equivalent circuit model of ferrite beads is a circuit composed of resistors, inductors, and capacitors. As shown in the figure below. RDC corresponds to the DC resistance of the bead. CPAR, LBEAD, and RAC represent parasitic capacitance, bead inductance, and AC resistance (AC core loss) related to the bead, respectively. Figure 1 (a) Simplified circuit model of ferrite beads; Figure 1 (b) ZRX curve of ferrite beads measured by TycoElectronics BMB2A1000LN2 Jefferson Eco gave the results of four parameters in his article, and the calculation process is also very simple, so I won’t go into details here. The specific values are RDC=300, CPAR=1.678, LBEAD=1.208, RAC=1.082. From the calculation results, the equivalent model of the bead is an LCR parallel resonant circuit, and the contribution of RDC is negligible. The magnetic bead model is established using CST, and the impedance parameters are as shown below. It can be seen that the overall results are consistent. RAC is the most important parameter among the four parameters that make up the magnetic bead. It is because of the existence of RAC that the magnetic bead is called a magnetic bead. Otherwise, the model can only be called a resistor. It is also because of the existence of RAC that there is an impedance curve as shown in the figure below. We all know the calculation formula for the LC resonant frequency. If you calculate the resonant frequency of the resonant circuit composed of CPAR and LBEAD, you will find that its resonant frequency is exactly the highest point of the impedance curve. When the RLC parallel circuit resonates, the circuit admittance Y(jω) =G=1/R, that is, the impedance value corresponding to the resonance point is the value of RAC. Figure 2 Impedance curve of magnetic beads calculated by CST When the circuit works like a current source (we know that the characteristics of common-mode noise are similar to current sources), the voltage on the RLC (i.e. magnetic beads) circuit is =. At this time, the addition of magnetic beads will cause the circuit noise to increase. Similarly, because Q=== in the RLC parallel resonant circuit, the quality factor Q is directly related to . Rather than saying that the addition of magnetic beads in extreme cases causes the noise in the circuit to be raised, it is better to say that the value of  in the magnetic beads causes the noise in the circuit to increase. The actual magnetic bead is composed of multiple layers of ferrite dielectric and spiral electrodes. The conductivity of the ferrite dielectric material is about 10e-2, and the magnetic permeability is about 100. The conductivity of the dielectric and the size of the internal electrode jointly determine the values of ,  and R. The magnetic permeability and the size of the internal electrode jointly determine the value of . As shown in the figure below, a magnetic bead on a PCB has an outer dimension of 4×4.6×1.85mm, and the internal electrode has a total of 4 turns. Figure 3 Internal structure of magnetic beads From the results, we can see that the inductance of the magnetic beads is about 3.2uH, 3.6pF, and 1207Ω. Figure 4 Impedance curve of magnetic bead model Since the inner electrode of the magnetic bead is wrapped by ferrite material as a whole, the magnetic bead itself has complete magnetic shielding, and the leakage of its external magnetic field is small. Therefore, when making layout, there is no need to consider whether there are sensitive circuits around the magnetic beads, nor is there any need to deliberately hollow out the stratum under the magnetic beads. Figure 5 Magnetic field distribution of magnetic beads at the highest impedance frequency 2. Insertion loss of magnetic beads In the design of filter circuits, insertion loss is the parameter that best reflects the characteristics of filter circuits. In the rectification of products, when considering the selection of devices, we will first look at the insertion loss characteristics of the device. Insertion loss can comprehensively reflect the circuit system's ability to consume electromagnetic energy. This consumption can be reflected back to the source end or converted into another type of energy through the device's own heating. However, the insertion loss parameter does not reflect the damping characteristics of the circuit system, which is exactly what most designers are most worried about. Often, devices with correct parameters are used in circuits, but the results are similar to those described in the following sections, and the circuit noise does not decrease but increases. The following figure shows the test system of Changhui Instruments, which uses a network analyzer and a special fixture to test the S parameters of components such as magnetic beads. When a 600R magnetic bead is used for testing, the results are shown in the figure. Figure 6: Insertion loss of magnetic beads tested with a network analyzer Figure 7: Insertion loss curve of magnetic beads Because the impedance curve given is not clear enough, we roughly estimate =1.59, =0.7, R=600, =680, the insertion loss curve calculated using this parameter is shown in the figure below. It can be seen that the calculated result is lower than the test result at high frequencies. We assume that the test equipment has been accurately calibrated, so the difference in the test results is that the product impedance parameter curve is inaccurate! According to the lowest insertion loss point of the test, which is 90.74MHz and the insertion loss is 18.188dB, the parameters in the test system are written into the software. After recalculation, the =1.59, =2.1, R=600, =715 of the magnetic bead are obtained. The distribution of the corrected insertion loss curve and impedance curve is shown in the figure below. Looking at the corrected insertion loss curve, the insertion loss parameters of the Mark point are almost exactly the same as those of the actual test. Figure 8 Insertion loss curve calculated according to the product impedance curve Figure 9 Corrected insertion loss curve Figure 10 Corrected impedance curve Compared with the inductor, the insertion loss characteristic of the magnetic bead is relatively small. Using LC filtering alone in the circuit can achieve a larger insertion loss value. In actual measurement, even 80dB of insertion loss can be achieved. However, when using LC filtering alone, when the frequency is higher than the LC resonant frequency, its insertion loss value will decrease rapidly, which is what we don't want to see. At this time, magnetic beads can be used to improve high-frequency characteristics. Figure 11 Insertion loss characteristics of ferrite beads used in L-type filter circuits (calculated values) 3. Magnetic beads in power supplies Murata's materials have examples of using ferrite beads to suppress and improve noise. As shown in the figure below, it can be seen that magnetic beads have a significant isolation effect on IC noise. Figure 12: Example of using magnetic beads to suppress and improve noise. Below, ANSYS Simplorer is used to build a Buck circuit with an input of 10V, an output of 3.5V, a switching frequency of 200khz, and a duty cycle of 50%. We need to check the waveforms of each device and the conducted noise of the switch at the input. Figure 13 Buck circuit simulation model Figure 14 Voltage waveforms on various devices The following figure shows the conducted noise received by LISN. It can be seen that the harmonics of the switching power supply itself are relatively rich. The switching frequency of 200Khz, its harmonics extend to 20MHz and are still clearly visible. The duty cycle of the power switch tube is 50%. Theoretically, the amplitude of the even harmonics under a 50% duty cycle should be 0, but here it is a non-zero value. Why is this? This question is left for readers to think about. Figure 15 Conducted noise received by LISN. Add the equivalent circuit of the magnetic beads in the previous section to the power input end, as shown in the figure below, to see the filtering effect of the magnetic beads on the switching noise in the circuit. Someone must ask here why the magnetic beads with the highest impedance at high frequency (100MHz) are chosen to deal with low-frequency noise. Readers with similar ideas, please calm down and continue to read on. From the results, it can be seen that even high-frequency magnetic beads have a certain attenuation on power supply noise. It can be seen in the figure below that the voltage ripple on the inductor is significantly reduced, and the conducted noise received by LISN is also reduced to a certain extent (note that this is a linear value, and this drop can be ignored when converted to a logarithmic value). Figure 16 Buck circuit after adding magnetic beads to the equivalent circuit Figure 17 Voltage waveforms on each component after adding magnetic beads to the equivalent circuit Figure 18 LISN before and after adding magnetic beads Comparison of the conducted noise received Some people always use magnetic beads to replace inductors, put them at the power input end, and form a filtering circuit with capacitors, as shown in the figure below. Using a similar approach will cause a problem. The power input input end will have a very large anti-resonance in the low frequency band. As shown in the LISN results in the figure below, the noise after 1MHz has dropped significantly. However, the noise amplitude before 0.6MHz is even higher than the power supply noise without filtering measures. Disappointingly, a certain number of engineers will directly choose to increase the capacitance value because of their lack of understanding of inductance, but they don’t know that the larger the capacitance value, the higher the low-frequency noise, and rectification is often confusing. Figure 19 Buck circuit with 10nF capacitor and ferrite bead added to the input (equivalent circuit replacement) Figure 20 Conducted noise received by LISN in three cases When the resonant frequency of the low-pass filter network (composed of ferrite bead inductor and high-Q decoupling capacitor) is lower than the crossover frequency of the ferrite bead, a spike occurs. The filtering result is underdamped. The figure below shows the relationship between the measured impedance and frequency of the TDK MPZ1608S101A (provided by the literature). The resistive component (related to the dissipation of interference energy) has little effect until it reaches about 20 MHz to 30MHz. Below this frequency, the ferrite bead still has a very high Q value and acts as an ideal inductor. The LC resonant frequency of a typical ferrite bead filter is generally in the range of 0.1MHz to 10MHz. For typical switching frequencies in the range of 300kHz to 5MHz, more damping is required to reduce the filter Q value. An example of this effect is shown in Figure 21(a) A TDK MPZ1608S101A ZRX curve; Figure 21(b) The S21 response of a ferrite bead and capacitor low-pass filter; Figure 21(b) The S21 frequency response of a ferrite bead and capacitor low-pass filter shows the peaking effect. The ferrite bead used in this example is the TDK MPZ1608S101A (100Ω, 3A, 0603) and the decoupling capacitor used is a Murata GRM188R71H103KA01 low ESR ceramic capacitor (10 nF, X7R, 0603). The load current is in the microamp range. An undamped ferrite bead filter may exhibit peaking from about 10dB to about 15dB, depending on the filter circuit Q. In Figure 4b, the peak occurs around 2.5MHz with a gain of up to 10dB. In addition, signal gain is visible from 1MHz to 3.5MHz. If this spike occurs within the operating frequency band of the switching regulator, it may be a problem. It can amplify interfering switching artifacts and seriously affect the performance of sensitive loads, such as phase-locked loops (PLLs), voltage-controlled oscillators (VCOs), and high-resolution analog-to-digital converters (ADCs). The results shown in Figure 4b are for extremely light loads (microampere level), but this is a practical application for parts of the circuit that only require a few microamperes to 1mA of load current or that are turned off in certain operating modes to save power. This potential spike creates additional noise in the system and may cause undesirable crosstalk. Readers will continue to read about the application of magnetic beads in instruments, so please look forward to "Magnetic Components for Solving Instrument EMC Problems-Magnetic Beads (Part 2)".