A common situation: A design engineer inserts a ferrite bead into a circuit experiencing EMC problems and finds that the bead actually makes unwanted noise worse. How could this be? Shouldn't ferrite beads eliminate noise energy without making the problem worse?

The answer to this question is fairly simple, but it may not be widely understood except for those who spend most of the time solving EMI problems. Simply put, ferrite beads are not ferrite beads, not ferrite beads and so on. Most ferrite bead manufacturers provide a table that lists their part number, impedance at some given frequency (usually 100 MHz), DC resistance (DCR), maximum rated current and some dimensions Information (see Table 1). Everything is almost standard. What is not shown in the data sheet is the material information and the corresponding frequency performance characteristics.

Table 1: Data sheet of typical ferrite beads

Ferrite beads are a passive device that can remove noise energy from the circuit in the form of heat. The magnetic beads generate impedance in a wide frequency range, thereby eliminating all or part of the unwanted noise energy in this frequency range. For DC voltage applications (such as the Vcc line of an IC), it is desirable to have a low DC resistance value to avoid large power losses in the required signal and/or voltage or current source (I2 x DCR loss). 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, in a given housing size and specific material used, the more windings, the higher the impedance, but as the physical length of the internal coil is longer, this will also produce a higher DC resistance. The rated current of this component is inversely proportional to its DC resistance.

One of the fundamental aspects of using ferrite beads in EMI applications is that the component must be in the resistance phase. What does it mean? Simply put, this means that "R" (AC resistance) must be greater than "XL" (inductive reactance). At frequencies where XL> R (lower frequency), the component is more like an inductor than a resistor. At frequencies where R> XL, the part behaves as a resistor, which is a required characteristic of ferrite beads. The frequency at which "R" becomes larger than "XL" is called the "crossover" frequency. This is shown in Figure 1, where the crossover frequency is 30 MHz in this example and is marked by a red arrow.

Another way to look at this is based on what the part actually performs during its inductance and resistance phases. As with other applications where the impedance of the inductor is not matched, part of the incoming signal is reflected back to the source. This can provide some protection for the sensitive equipment on the other side of the ferrite bead, but it also introduces an "L" into the circuit, which can cause resonance and oscillation (ringing). Therefore, when the magnetic beads are still inductive in nature, part of the noise energy will be reflected and part of the noise energy will pass, depending on the inductance and impedance values.

When the ferrite bead is in its resistive 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 constructed in the same way as some inductors, using the same process, production line and technology, machinery, and some of the same component materials, ferrite beads use lossy ferrite materials, while inductors use low loss iron Oxygen material. This is shown in the curve in Figure 2.

The figure shows [μ''], which reflects the behavior of the lossy ferrite bead material.

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. The value of this "point" does not indicate whether the impedance increases, decreases, becomes flat, and the impedance reaches its peak value at this frequency, and whether the material is still in its inductance phase or has transformed into its resistance phase. In fact, many ferrite bead suppliers use multiple materials for the same ferrite bead, or at least as shown in the data sheet. See Figure 3. All 5 curves in this figure are for different 120 ohm ferrite beads.

Figure 3: 120 Ohm (100 MHz) ferrite beads

Then, what the user must obtain is the impedance curve showing the frequency characteristics of the ferrite bead. An example of a typical impedance curve is shown in Figure 4.

Figure 4: Typical impedance curve with /Z​​/, R, XL

Figure 4 shows a very important fact. This part is designated as a 50 ohm ferrite bead with a frequency of 100 MHz, but its crossover frequency is about 500 MHz, and it achieves more than 300 ohms between 1 and 2.5 GHz. Again, just looking at the data sheet will not let the user know this and may be misleading.

As shown in the figure, the properties of the materials vary. There are many variants of ferrite used to make ferrite beads. Some materials are high loss, broadband, high frequency, low insertion loss and so on. Figure 5 shows the general grouping by application frequency and impedance.

Figure 5: Material properties based on frequency 1

Another common problem is that circuit board designers are sometimes limited to the selection of ferrite beads in their approved component database. If the company has only a few ferrite beads that have been approved for use in other products and are deemed satisfactory, 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 aggravating effects of the original EMI noise problem described above. The previously effective method may be applicable to the next project, or it may not be effective. You cannot simply follow 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.

Figure 6: Impedance curves of B material (top) and D material (bottom)

For these two components, 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 it is realized at 400 MHz. For the part on the right, using the "D" material, the maximum impedance is 700 ohms, which is achieved at approximately 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 at around 400 MHz. Which is the correct part to use? It depends on each individual application.

Figure 7 demonstrates an all too common problem that arises when the wrong ferrite bead is chosen to suppress EMI. The unfiltered signal shows 474.5 mV undershoot on a 3.5V, 1 uS pulse.

Figure 7: Measured performance of high-loss and ultra-high-loss materials

In the results of using high-loss type materials (center plot), the measured undershoot increases due to the higher crossover frequency of the part. The signal undershoot increased from 474.5 mV to 749.8 mV. The Super High Loss material has a low crossover frequency and good performance. It will be the right material to use in this application (pictured on the right). The undershoot using this part is reduced to 156.3 mV.

As the direct current through the beads increases, the core material begins to saturate. For inductors, this is called saturation current and is specified as a percentage drop in the inductance value. For ferrite beads, when the part is in the resistance phase, the effect of saturation is reflected in the decrease in impedance value with frequency. This drop in impedance reduces the effectiveness of the ferrite beads and their ability to eliminate EMI (AC) noise. Figure 8 shows a set of typical DC bias curves for ferrite beads.

Figure 8: Influence of DC current on impedance

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

Like high-frequency RF inductors, the winding direction of the inner coil in 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 level, but also changes the frequency response. In Figure 9, two 1000 ohm ferrite beads are shown with the same housing size and the same material, but with two different winding configurations.

Figure 9: "Giga" beads on the left, standard beads on the right 2

The coils of the left part are wound on the vertical plane and stacked in the horizontal direction, which produces higher impedance and higher frequency response than the part on the right side wound in the horizontal plane and stacked in the vertical direction. This is partly due to the lower capacitive reactance (XC) associated with the reduced parasitic capacitance between the end terminal and the internal coil. A lower XC will produce a higher self-resonant frequency, and then allow the impedance of the ferrite bead to continue to increase until it reaches a higher self-resonant frequency, which is higher than the standard structure of the ferrite bead The impedance value. The curves of the above two 1000 ohm ferrite beads are shown in Figure 10.

Figure 10: Comparison of frequency response caused by winding configuration

To further show the effects of correct and incorrect ferrite bead selection, we used a simple test circuit and test board to demonstrate most of the content discussed above. In Figure 11, the test board shows the positions of three ferrite beads and the test points marked "A", "B" and "C", which are located at the distance from the transmitter output (TX) device.

Figure 11: Test setup and test board

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, a low-frequency lossy "S" material, was tested at points "A", "B" and "C". Next, a higher frequency "D" material was used. The point-to-point results using these two ferrite beads are shown in Figure 12.

Figure 12: Online performance test results

The "through" unfiltered signal is displayed in the middle row, showing some overshoot and undershoot on the rising and falling edges, respectively. It can be seen that using the correct material for 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 result of using high-frequency materials can cause ringing, which amplifies each level and increases the period of instability. These test results are shown on the bottom row.

When looking at the improvement of EMI with frequency in the recommended upper part (Figure 12) in the horizontal scan shown in Figure 13, it can be seen that for all frequencies, this part significantly reduces EMI spikes and reduces the overall noise level to approximately 30 to approximately In the 350 MHz range, 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 a limited impact on the original, unfiltered EMI noise level, but it does reduce a major spike at 750 MHz by about 6 dB. If the main part of the EMI noise problem is higher than 350 MHz, you need to consider the use of higher frequency ferrite materials whose maximum impedance is higher in the spectrum.

Figure 13: Radiated EMI noise (level) suppression

Of course, all ringing (as shown in the bottom curve of Figure 12) can usually be avoided by actual performance testing and/or simulation software, but I hope this article will allow readers to bypass many common mistakes and reduce the need to select the correct ferrite bead Time, and provide a more “educated” starting point when ferrite beads are needed to help solve EMI problems.

To avoid being misused in your future ferrite bead needs, it is recommended that you always:

Finally, it is best to approve a series or series of ferrite beads, not just a single part number, for 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. It’s a bit easy to do this the first time, but once the parts are entered into the component database under a control number, they can then be used anywhere. The important thing is that the frequency performance of parts from different suppliers is very similar to eliminate the potential for other applications. Future issues. The best way is to obtain similar data from different suppliers, and at least have an impedance curve. This will also ensure that the correct ferrite beads are used to solve your EMI problem.

Remember, not all ferrite beads are created equal.

Chris Burket has been working at TDK since 1995 and is now a senior application engineer, supporting a large number of passive components. He has been involved in product design, technical sales and marketing. Mr. Burket has written and published technical papers in many forums. Mr. Burket has obtained three U.S. patents on optical/mechanical switches and capacitors.

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