PCB Tech - Analyze the magnetic bead grounding from the EMC application

Inductance and magnetic beads should be said to be two brothers. Many people have always thought that they are "passing through and blocking" and are easy to mix together. As the saying goes: One mother has nine children, each of which is different. In fact, there is still a big difference between inductance and magnetic beads.

The unit of inductance is Heng, and the model is also named with the inductance value, such as: GZ2012-100 refers to the inductance of 2012 (0805) packaged 10uH; the unit of the magnetic bead is ohms, and the general model of the magnetic bead is the resistance at 100MHz The value is named, it should be noted that this is the resistance value, not the equivalent inductance. For example: JCB201209-301 refers to the magnetic beads with a resistance of 300 ohms when packaged in 2012 (0805) at 100MHz.

The magnetic bead itself is an energy-consuming element in theory, and the inductance does not consume energy in theory. This is the biggest theoretical difference between the two types of components. The magnetic material of the inductor is not closed. The typical structure is a magnetic rod. Part of the magnetic line of force passes through the magnetic material (magnetic rod) and part is in the air. The magnetic material of the magnetic bead is closed, and the typical structure is a magnetic ring. Almost all the lines of magnetic force are in the magnetic ring and will not radiate into the air. The intensity of the magnetic field in the magnetic ring is constantly changing, which will induce current in the magnetic material. Choosing a magnetic material with high hysteresis and low resistivity can convert this high-frequency energy into heat and then consume it. Inductance is the opposite. Magnetic materials with low hysteresis coefficient and high resistivity should be selected to make the inductance exhibit a consistent inductance value in the entire frequency band as much as possible. Therefore, the difference in structure and magnetic material determines the essential difference between magnetic beads and inductors.

Inductors are mainly used in switching power supplies, resonance, impedance matching, and special filtering, and magnetic beads are mainly used to prevent radiation, and the improvement of EMC is much better than inductance.

The magnetic beads consume the high frequency, and there is no external "magnetic leakage", while the inductance, because the magnetic material is not enclosed, will transmit a large amount of high frequency signals to the external space, causing EMI problems...

Magnetic beads are usually recommended to be used on power or signal lines to enhance the decoupling effect, but you must be careful when using them between grounds, especially in applications where large energy interference signals flow through the beads.

I remember that when I was just learning DSP design, the master recommended a circuit for the DSP analog part application, which is to string two magnetic beads on the digital power supply and the analog power supply, and then add a filter capacitor. Classically, it was not until I came into contact with EMC design that I found out how badly it was wrong, and I also understood the crux of some of the problems at that time.

The magnetic beads exhibit resistance characteristics when the interference current passes. At this time, there will be a large △V on both sides of the magnetic beads, which is specifically reflected in the analog part of the DSP, and there will be a large fluctuation in the A/D measurement, no matter what you use What kind of filtering algorithm, under continuous interference, the measurement result is definitely a mess.

So in these simple applications, do not add magnetic beads on the ground, just short-circuit it.