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All Ferrite Beads Are Not Created Equal - Understanding the Importance of Ferrite Bead Material Behavior - In Compliance Magazine

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A common situation: a design engineer inserts a ferrite bead into a circuit that encounters EMC problems, only to find that the bead actually causes unnecessary noise, which is very bad.

The answer to this question is very simple, but it may not be widely understood outside of those who spend a lot of time solving EMI problems. In short, ferrite beads are not ferrite beads, nor are they ferrite beads, etc. Most ferrite bead manufacturers provide a table that lists their part numbers, impedance at a given frequency (usually 100 MHz), DC resistance (DCR), maximum rated current and some size information (See Table 1). All are standard things. What is not shown in the data sheet is the material information and the corresponding performance characteristics that vary with frequency.

Ferrite beads are a passive device that can remove noise energy from the circuit in the form of heat. Magnetic beads will generate impedance in a wide frequency range, thereby eliminating all or part of the undesired noise energy in this frequency range. For DC voltage applications (for example, the Vcc line of an IC), it is desirable to have a low DC resistance value so that there is no large power loss (I2 x DCR loss) in the required signal and/or voltage or current source. However, it is desirable to have high impedance in certain defined frequency ranges. Therefore, the impedance is related to the material used (permeability), the size of the ferrite bead, the number of windings and the winding structure. Obviously, the more windings using a particular material in a given housing size, the higher the impedance, but as the physical length of the inner coil becomes longer, this will also produce a higher DC resistance. The rated current of the part is inversely proportional to the DC resistance.

One of the fundamental aspects of using ferrite beads for EMI applications is that the component must be in the resistive phase. What does it mean? In simple terms, this means that "R" (AC resistance) must be greater than "X"

"(Inductive reactance). At the frequency of X

> R (lower frequency), the part behaves more as an inductor than a resistor. At the frequency of R> X

, The part acts as a resistor, which is the ideal characteristic of ferrite beads. "R" is greater than "X" frequency

"Is called the "crossover" frequency. As shown in Figure 1, the crossover frequency is marked as 30 MHz with a red arrow in this example.

Another way to look at this problem is the operation that the part actually performs during the inductance and resistance phase. Like the impedance mismatch of inductors in other applications, part of the incoming signal will be reflected back to the source. This can provide some protection for sensitive devices on the other side of the ferrite bead, but it can also introduce "L" into the circuit, which can cause resonance and oscillation (ringing). Therefore, when the magnetic beads are still inductance in nature, depending on the inductance and impedance values, a part of the noise energy will be reflected and a percentage will pass.

As mentioned, when the ferrite bead is in the resistance phase, the component behaves like a resistor, so it blocks noise energy and absorbs that energy from the circuit, and absorbs it in the form of heat. Although the construction method of ferrite beads is the same as that of some inductors, they use the same process, production line and technology, machinery and some of the same constituent materials, but ferrite beads use lossy ferrite materials , And the inductor uses ferrite material with lower loss. As shown in the curve in Figure 2.

This graph shows [μ''], which is used to reflect the performance of lossy ferrite bead materials.

The fact that the impedance is given at 100 MHz is also part of the selection problem. In many cases of EMI, the impedance at this frequency is irrelevant and misleading. Regardless of whether the material is still in the inductance phase or has been transformed into the resistance phase, if the impedance increases at this frequency, the impedance decreases, becomes flat, and the peak value reaches the peak value, the "point" value will not be declared. In fact, many ferrite bead suppliers use multiple materials for the same sensing ferrite bead, or at least as shown in the datasheet. See Figure 3. All five curves in this figure are for different 120 ohm ferrite beads.

The impedance curve that the user must obtain shows the frequency characteristics of the ferrite beads. An example of a typical impedance curve is shown in Figure 4.

Figure 4 shows a very important fact. The device is designated as a 50-ohm ferrite bead at 100 MHz, but its crossover frequency is about 500 MHz, and it reaches 300 ohms between 1 and 2.5 GHz. Similarly, just looking at the data sheet will not make the user aware of this and may be very misleading.

As shown in the figure, the properties of the materials vary. Various ferrites are used in the structure of ferrite beads. Some materials have high loss, high frequency, high frequency, low insertion loss, etc. Figure 5 shows the general grouping by application frequency and impedance.

Another common problem is that circuit board designers are sometimes limited by the selection of ferrite beads due to the contents of their approved component database. If the company has very few ferrite beads that have been approved for use in other products and deemed satisfactory, then in many cases there is no need to evaluate and approve other materials and part numbers. In the recent past, this has repeatedly led to some worsening effects of the original EMI noise problem described above. Previous work may or may not affect the next project. One cannot simply keep the EMI solution of the previous project, especially when the frequency of the required signal changes or the frequency of potential radiating components such as clock equipment changes.

If you look at the two impedance curves in Figure 6, you can compare the material effects of two similar designated parts.

For these two parts, the impedance at 100 MHz is 120 ohms. For the part on the left, using the "B" material, the maximum impedance is about 150 ohms and is reached at 400 MHz. For the right part, using the "D" material, the maximum impedance is 700 ohms, which is achieved at about 700 MHz. But the biggest difference is the crossover frequency. The ultra-high loss "B" material transitions at 6 MHz (R>XL), while the very high frequency "D" material remains inductive until around 400 MHz. Which is the correct part to use? It depends on each individual application.

Figure 7 shows that when the wrong ferrite bead is selected to suppress EMI, it is too common. The unfiltered signal shows an undershoot of 474.5mV under 3.5V, 1uS pulse.

In the results of using a high-loss type material (center map), the measured undershoot increased due to the higher crossover frequency of the part. The signal undershoot increased from 474.5 mV to 749.8 mV. An ultra-high loss material with a lower crossover frequency has sufficient performance and will be the right material to use in this application (picture on the right). With this device, the undershoot is reduced to 156.3 mV.

As the direct current through the magnetic beads increases, the core material begins to saturate. For inductors, this is called saturation current and is specified as the inductance value decreases by a certain percentage. For ferrite beads, when the part is in the resistance phase, the effect of saturation is reflected in the decrease in impedance value over the entire frequency range. The decrease in impedance will reduce the efficiency of ferrite beads and their ability to eliminate EMI (AC) noise. Figure 8 shows a typical DC bias curve for a set of ferrite beads.

In this figure, the ferrite bead is rated at 100 ohms at 100 MHz. This is the typical measurement impedance when no DC current is passed through the device. However, it can be seen that once a DC current is applied (for example, for IC VCC input), the effective impedance will drop sharply. In the above curve, at a current of 1.0 A, the effective impedance changes from 100 Ohms to 20 Ohms. 100 MHz. Maybe not too critical, but the design engineer must be aware of some things. Similarly, only using the electrical characteristic data of the part in the supplier's data sheet, the user will not understand this DC bias phenomenon.

Like high-frequency RF inductors, the winding direction of the coil inside the ferrite bead has a great influence on the frequency characteristics of the bead. The winding direction not only affects the relationship between impedance and frequency, but also changes the frequency response. In Figure 9, two 1000 ohm ferrite beads of the same size and made of the same material but with two different winding configurations are shown.

Compared with the right part wound on the horizontal plane and stacked in the vertical direction, the part where the coil is wound on the left and stacked in the horizontal direction produces higher impedance and higher frequency response. Part of the reason is due to the lower capacitive reactance (XC), which is related to the reduction of the parasitic capacitance between the termination terminal and the internal coil. A lower XC will produce a higher self-resonant frequency, and then the impedance of the ferrite bead will continue to increase until it reaches a higher self-resonant frequency. Compared with the standard structure of the ferrite bead, the obtainable impedance The value will be higher. The curves of the above two 1000 ohm ferrite beads are shown in Figure 10.

To further show the effect of correct and incorrect ferrite bead selection, a simple test circuit and test board were used to demonstrate many of the contents discussed above. In Figure 11, a test board is shown with three ferrite bead positions, and the test points are marked as "A" at the distances of 0 mm, 50 mm and 100 mm from the transmit output (TX), " B" and "C") equipment.

The signal conditions for this test are as follows:

The signal integrity is measured on the output side of the ferrite beads in each of the three positions, and is repeated with two ferrite beads made of different materials. The first material is a low-frequency lossy "S" material, tested at points "A", "B" and "C". Next, use a higher frequency "D" material. Figure 12 shows the point-to-point results using these two ferrite beads.

The "through" unfiltered signal is displayed in the middle row with overshoot and undershoot on the rising and falling edges, respectively. It can be seen that using the correct material under the above test conditions, the lower frequency, lossy material shows good overshoot and undershoot signal improvement on the rising and falling edges. These results are shown in the upper row of Figure 12. The use of high-frequency materials results in ringing, which increases the level of each ringing and increases the instability time. These test results are shown in the bottom row.

When looking at the improvement in the EMI frequency of the recommended upper part (Figure 12) in the horizontal scan shown in Figure 13, it can be seen that for all frequencies, this part can greatly reduce EMI spikes and reduce the overall noise level between 30 and 30 In the frequency range of about 350 MHz, the acceptable level is far below the EMI limit highlighted by the red line, which is the general regulatory standard for Class B equipment (FCC Part 15 in the United States). The "S" material used in ferrite beads is specifically used for these lower frequencies. It can be seen that once the frequency exceeds 350 MHz, the "S" material has little effect on the original, unfiltered EMI noise level, but it can indeed reduce a major spike at 750 MHz, about 6 dB. If the main part of the EMI noise problem is higher than 350 MHz, you need to study the use of a high-frequency ferrite material that has a higher maximum impedance in the spectrum.

Of course, the actual performance test and/or simulation software can usually avoid all the ringing shown in the bottom curve of Figure 12, but it is hoped that this article will enable readers to bypass many common mistakes and reduce the time required to select the correct ferrite bead , And provide a more "educated" starting point when ferrite beads are needed to help solve EMI problems.

In order to avoid misuse in the future needs of ferrite beads, it is recommended that you always:

Finally, I hope to approve a series or series of ferrite beads, not just a single part number, to have more choices and design flexibility. It should be noted that different suppliers use different materials, and the frequency performance of each supplier must be reviewed, especially when multiple purchases are made for the same project. The initial operation is easier, but once the parts are entered into a part database with a control number, they can be used anywhere afterwards. It is important that the frequency performance of parts from different suppliers must be similar. Eliminate problems that may arise in other applications in the future. The best way is to obtain similar data from various suppliers and at least get the impedance curve. This will also ensure that the correct ferrite beads are used to solve your EMI problems.

Remember, not all ferrite beads are the same.

Chris Burket has been working at TDK since 1995 and is now a senior application engineer, providing support for many passive components. He has been engaged in product design, technical sales and marketing. Mr. Burkert has written and introduced technical papers in numerous forums. Mr. Burket has obtained three US patents for optical/mechanical switches and capacitors.

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