EMC of three tools: capacitor/inductor/magnetic beads
For the role of the three in circuit, believe that there are a lot of people is not very clear, this article from the detailed analysis of the elimination of EMC in the design of the three principle of tool.
Although in the view of the high frequency noise filtering, the resonant capacitance is not hope, but the resonance capacitance are not always harmful.
In practical engineering, to filter out the noise frequency electromagnetic often as high as several hundred MHz, even more than 1 GHZ.
For such a high frequency electromagnetic noise must use wear heart capacitance can effectively filter out.
Capacitors are not effectively filter out high frequency noise, because of two reasons:
Another reason is that lead the parasitic capacitance between the coupling in the high frequency signal, reduces the filtering effect.
And wear heart capacitance can be installed directly on the metal panel, have the effect of high frequency isolation using metal panel.
Wear heart capacitance's biggest weakness is afraid of high temperature and temperature shock, this in capacitance will wear heart during welding caused great difficulties to the metal panel.
Facing due to EMC problem solving is mostly common-mode interference, so the common-mode inductor is one of our commonly used powerful components.
Common-mode inductor is a ferrite core of common-mode interference suppression devices, it is made up of two same size, same number of turns of the coil symmetrically on the same ferrite ring core, forming a four terminal device, to the common mode signal present a large inductance with inhibition, and for the differential mode signal present a little small leakage inductance.
Principle is through the common-mode current of the circular flux overlap, thus has the quite big inductance, inhibiting effect of common-mode current, and when two coil through the differential mode current, flux of circular and cancel each other out, almost no inductance, so the differential mode currents can be through with little attenuation.
Therefore common-mode inductor in balancing circuit can effectively inhibit common-mode interference signals, and differential mode signal with no impact on the normal transmission line.
Common-mode inductor should meet the following requirements: (when making
Normally, at the same time pay attention to choose the required filter band, common mode impedance is bigger, the better, so we need to look at when choosing common-mode inductance device information, mainly according to the impedance frequency curve to choose.
Also when the choice attention to consider the effect of differential mode impedance the signal, focusing on differential mode impedance, pay special attention to the port.
【Magnetic beads 】
In the process of product EMC of digital circuit design, we often use to magnetic beads, ferrite material is magnesium alloy iron or iron nickel alloy, this material has high magnetic permeability, he can is the inductance of the coil winding in the high frequency under the condition of high resistance between the minimum capacitance.