How Do Magnetic Beads Affect the Integrity of the Power Supply

1. ForewordThe full name of magnetic beads is ferrite magnetic bead filter (another is magnetic beads made of amorphous alloy magnetic materials). It is an anti-interference element with remarkable effect of filtering high-frequency noise.The main raw material of magnetic beads is ferrite. Ferrite is a kind of ferromagnetic material with cubic lattice structure. Ferrite material is iron magnesium alloy or iron nickel alloy. Its manufacturing process and mechanical properties are similar to ceramics, and its color is gray black.

2 Basic knowledge of magnetic beads

The magnetic bead has high resistivity and permeability. Its equivalent circuit is a DCR resistor in series, an inductor in parallel, a capacitor and a resistor. DCR is a constant value, but the latter three elements are all functions of frequency, that is, their inductive reactance, capacitive reactance and impedance will change with the change of frequency. Of course, their resistance, inductive value and capacitive value are very small. Magnetic beads have better high-frequency filtering characteristics than ordinary inductors, and show resistance at high frequency, so they can maintain high impedance in a wide frequency range, so as to improve the effect of frequency modulation filtering. The circuit symbol of magnetic beads is inductance, but it can be seen from the model that magnetic beads are used. In terms of circuit function, magnetic beads and inductors have the same principle, but their frequency characteristics are different.

It can be seen from the equivalent circuit that when the frequency is lower than FL (LC resonant frequency), the magnetic beads show inductance characteristics; When the frequency is equal to FL, the magnetic bead is a pure resistance, and the impedance of the magnetic bead is the largest; When the frequency is higher than the resonant frequency point FL, the magnetic beads show capacitive characteristics. The principle of selecting magnetic beads for EMI is that the impedance of magnetic beads is the largest at the EMI noise frequency. For example, if the maximum value of EMI noise is 200MHz, you should look at the characteristic curve of magnetic beads when you choose, and the maximum value of its impedance should be about 200MHz.

The figure below shows the actual characteristic curve of a magnetic bead. You can see that the peak point of this magnetic bead appears at about 1GHz. At the peak point, the value of impedance (z) curve is equal to that of resistance (R). In other words, the magnetic bead is a pure resistance at 1GHz, and the impedance value is the largest.Z: Impedancer: R (f) X1: l C the basic characteristic curve of EMI magnetic beads was briefly introduced earlier. From the impedance curve of magnetic beads, in fact, its characteristic is that it can be used as a high-frequency signal filter. It should be noted that the magnetic bead impedance curve provided by the manufacturer is usually measured without bias current. However, most magnetic beads are usually placed on the power line to filter the EMI noise of the power supply. In the case of bias current, the characteristics of magnetic beads will change.The following is the impedance curve of an 0805 500mA magnetic bead under different bias currents. As you can see, with the increase of current, the peak impedance of magnetic beads will become smaller, and the frequency of impedance peak point will also become higher.

Before further expounding the characteristics of magnetic beads, let's take a look at the definition of the main characteristic index of magnetic beads: Z (impedance): the sum of the impedance of all components in the circuit under magnetic beads, which is a function of frequency. Generally, we use the impedance value of magnetic beads at 100MHz as the impedance value of magnetic beads. DCR (Ohm): DC resistance of magnetic bead conductor. Rated current: the current value when the magnetic bead is installed on the printed circuit board and a constant current is added, and its temperature rises by 40C from room temperature. So there are thousands of kinds of EMI magnetic beads, and the impedance curves are also different. How should we choose the appropriate magnetic beads according to our practical application? Let's first look at the same impedance value 600ohm@100MHz , but the characteristics of magnetic beads with different sizes under different bias currents and operating frequencies.

Above are the impedance values of four magnetic beads of different sizes operating at 0A, 100mA bias current and 100MHz, 500MHz and 1GHz operating frequencies respectively.It can be seen from the test data in the above table that when the 1206 size magnetic bead works at low frequency 100MHz, its impedance value is only reduced from 600ohm at 0A to 550ohm at 100mA bias current, while the 0402 size magnetic bead impedance value is greatly reduced from 600ohm at 0A to 175ohm.Therefore, in the case of low-frequency and large bias current applications, large-size magnetic beads should be selected, and their impedance characteristics will be better. Let's take a look at the magnetic beads working at high frequency. The impedance of 1206 size magnetic beads at 1GHz is greatly reduced from 600ohm at 100MHz to 105ohm, while the impedance of 0402 size magnetic beads at 1GHz is only slightly reduced from 600ohm at 100MHz to 399ohm.

In other words, in the case of low-frequency and large bias current, we should choose larger magnetic beads, while in high-frequency applications, we should try to choose smaller magnetic beads.3 Magnetic beads applied to signal linesLet's take another look at the application of magnetic bead a and magnetic bead B with different curve characteristics to the signal line.

The impedance peaks of bead a and bead B are between 100MHz and 200MHz, but the impedance frequency curve of bead a is relatively flat and bead B is relatively steep. We put the two magnetic beads on the following 20MHz signal line to see what impact it will have on the signal output.The following is the waveform diagram of magnetic bead output measured by oscilloscopeFrom the output waveform, the output waveform distortion of magnetic bead B is significantly less than that of magnetic bead a. The reason is that the impedance frequency waveform of magnetic bead B is steep, and its impedance is higher at 200MHz, which only attenuates the signal near 200MHz, but has little impact on the square wave waveform with wide spectrum. The impedance frequency characteristic of magnetic bead a is relatively flat, and its attenuation spectrum to the signal is also relatively wide, so it also has a great impact on the waveform of square wave.

The following is the EMI test results corresponding to the above three cases. As a result, both magnetic beads a and B will greatly attenuate EMI noise. The attenuation of magnetic bead a in the whole EMI spectrum is slightly better than that of magnetic bead B.Therefore, when selecting magnetic beads, magnetic beads a with flat impedance frequency characteristics are more suitable for power lines, while magnetic beads B with steep frequency characteristics are more suitable for signal lines. When the magnetic bead B is applied to the signal line, it can produce the maximum attenuation to the noise near the EMI frequency as far as possible while maintaining the signal integrity as far as possible.4 Magnetic bead and capacitor circuit

In the data manuals or application documents of some devices, it is generally recommended to isolate some power pins with high requirements (such as vcca and vccpll), and it is recommended to use magnetic beads for isolation.It is generally recommended to place the capacitor closer to the power pin of the device (relative to the magnetic bead), as shown in the figure below. The capacitance of the capacitor is related to the power (voltage & current) of the power pin, the position of the capacitor from the pin, the package size of the capacitor and so on.There are also some concerns about the layout of the capacitor. When installing the capacitor, pull out the lower lead from the pad and connect it with the power plane through a via. The same is true for the grounding section. Then the current circuit of capacitor is: power plane via outgoing line pad capacitor pad outgoing line via low plane. As shown in the figure below:

The basic principle of placing vias is to minimize the loop area and reduce parasitic inductance. The following figure shows several installation methods:The first method leads a long wire from the pad and then connects it to the via, which will introduce a large parasitic inductance. This must be avoided.The second method makes holes at the two ends of the pad, which is much smaller than the first method, and the parasitic inductance is also smaller, which is acceptable.

The third method is to punch holes on the side of the pad to further reduce the loop area, and the parasitic inductance is smaller than the first one. It is a better method.The fourth method is to punch holes on the second side of the pad. Compared with the third method, each end of the capacitor is connected to the power supply and ground plane through parallel vias, which is smaller than the parasitic inductance of the third method. As long as space allows, use it as much as possible.The last method is to punch holes directly on the pad, with the minimum parasitic inductance, but there may be problems in welding.

5 Harm of "abusing" magnetic beads

A typical 8-layer or 6-layer board adopts the design of three power ground planes and relatively tight coupling between power and ground. At this time, the filter capacitor on the board presents "global characteristics", that is, the position of the capacitor is not very "important", and the capacitor plays a role in the global situation. The power supply of double-sided board, four layer board and six layer board is far from the ground. When the power supply is relatively loosely coupled, the filter capacitor on the board tends to "local characteristics", and the position of the capacitor is more important. It is best to place it close to the chip pin.

When the power supply network does not use the power supply ground plane to design, the capacitance tends to "local characteristics". For example, the capacitance of PLL power supply, such as the capacitance of VREF power supply in DDR3 design, all hope to strictly put the corresponding capacitance close to the chip pin. It is even better to specify that the power supply must enter the chip pin from the filter capacitor in the design.Similarly, for conventional digital power supplies, such as 3.3V, 2.5V and other IO power supplies, if we use magnetic bead isolation for each chip and supply power separately, the capacitor will lose its "global" function. One of the most direct negative effects is that more filter capacitors need to be added in the design. Or the number and type of capacitance of a chip are not enough, resulting in increased power track noise.Even if the number of capacitors is not a problem and the power supply noise is controllable, "abuse" magnetic beads will also cause other design problems. Many types of power supply are the current situation of design. "Abusing" magnetic beads will "make it worse" and make more types of power supply. Increase the difficulty of power ground plane design. In fact, the increased magnetic beads do not bring benefits to the power supply noise.

6 SummaryWhen the conventional digital power supply adopts the multi-layer board design and the power supply ground plane is tightly coupled, it is not recommended to "abuse" the magnetic beads and keep the "global" characteristics of the capacitor.There are two kinds of occasions where magnetic beads need to be used

1. "Special" to protect themselves, such as PLL power supply, SerDes analog power supply in FPGA, etc