| |
| Learn about magnetic beads, important electronic components in EMI design |
| 2020-07-06 |
| |
As a common electronic component, the main function of the magnetic bead is to suppress the high-frequency noise of the signal line. Because of its excellent electromagnetic interference suppression performance, it is widely used in computers, VCD and other fields. In EMI design, the importance of magnetic beads is self-evident.
The role of magnetic beads
1. The magnetic beads are specially used to suppress high-frequency noise and spike interference on signal lines and power lines, and also have the ability to absorb electrostatic pulses.
2. The magnetic bead has a high resistivity and permeability, which is equivalent to a series connection of resistance and inductance, but the resistance value and inductance value change with frequency.
3. The magnetic bead has better high-frequency filtering characteristics than ordinary inductors, and exhibits resistance at high frequencies, so it can maintain a high impedance in a relatively wide frequency range, thereby improving the effect of FM filtering.
The difference between magnetic beads and inductance
1. Inductance is mostly used in power supply filter circuits, focusing on suppressing conductive interference; magnetic beads are mostly used in signal circuits, mainly used in EMI.
2. The filter performance of the inductor is good in the low frequency band, but the filter performance is poor in the frequency band above 50MHz; the magnetic beads can fully utilize the high frequency noise by using its resistance component, and convert it into heat energy to completely eliminate the high frequency noise purpose.
3. From the EMC (electromagnetic compatibility) level, because the magnetic beads can convert high-frequency noise into heat energy, it has very good anti-radiation function and is a commonly used anti-EMI device, often used in user interface signal line filtering, single board Power filter for high-speed clock devices, etc.
4. When the inductance and the capacitor form a low-pass filter, since the inductance and the capacitor are both energy storage devices, the cooperation of the two may generate self-excitation; the magnetic bead is an energy-consuming device and does not generate self-excitation when working with the capacitor. .
5. In general, the rated current of the power inductor is relatively large. Therefore, the inductor is often used in power circuits that need to pass a large current, such as for power module filtering; while the magnetic beads are generally only used for chip-level power filtering ( However, magnetic beads with large current ratings have appeared on the market.
6. Both the magnetic bead and the inductor have DC resistance. The DC resistance of the magnetic bead is smaller than the inductance with the same filtering performance. Therefore, when used for power supply filtering, the voltage drop on the magnetic bead is smaller.
What magnetic beads and inductors have in common
1. Rated current. When the rated current of the inductor exceeds its rated current, the inductance value will decrease rapidly, but the inductive device is not necessarily damaged; and when the working current of the magnetic bead exceeds its rated current, it will cause damage to the magnetic bead.
2. DC resistance. When used in power lines, there is a certain current on the line. If the DC resistance of the inductor or the bead itself is large, a certain voltage drop will occur. Therefore, in the selection, it is required to select a device with a small DC resistance.
3. Frequency characteristic curve. The manufacturer information of the inductor and the magnetic bead are attached with the frequency characteristic curve of the device. In the selection process, you need to carefully refer to these curves to select the appropriate device.
Choice of magnetic beads
To correctly select the magnetic beads, you must pay attention to the following points:
1. In the case of noise interference in the circuit, you should roughly understand the frequency and intensity of the noise. The frequency impedance curve of different magnetic beads is different, and the one with a higher magnetic impedance at the center of the noise should be selected.
2. How much noise attenuation is required.
3. What are the environmental conditions (temperature, DC voltage, structural strength).
4. What is the circuit and load impedance?
5. Is there space for magnetic beads on the PCB?
Note: The unit of the magnetic bead is ohm, not Henry, this should be paid special attention to. Because the unit of the magnetic bead is nominally based on its impedance at a certain frequency, the unit of impedance is also ohms.
|
| |
|
| |
|
