Limitations of traditional common-mode filters.
Usually we talk about noise and interference in EMC problems, mostly common mode noise and common mode interferenceTherefore, the common filtering and protection devices are mostly in the form of common mode, and the typical representative is the common mode inductor;Because of its high impedance characteristics to common mode interference and almost no loss of differential mode signals, common mode inductors are basically found at the power input and signal input and output terminals of various products.
Figure 1 Common-mode inductance filtering of the CAN interface.
Figure 2 Common mode inductance at the DC input port of a product.
However, the common mode inductor is an inductor in the final analysis, and the characteristics of its ferrite core determine that the impedance of the common mode inductor is very different under different flow conditionsAs shown in Figure 3 and Equation (1), a commonly used method for calculating the inductance of winding wires:
Figure 3 Schematic diagram of inductor and core.
Equation (1) shows that in addition to some definite parameters or constants, the inductance of the inductance will change with the change of the initial permeability of the core, and the initial permeability of the core is affected by a variety of factors, as shown in the figure, the initial permeability will change with temperature;At the same time, because its b-h characteristics are nonlinear, when the external magnetic field changes, its initial permeability will also change, and the inductance will also change accordinglyHowever, when the flow of the device changes, the permeability changes due to these two factors.
Fig.4 Initial permeability of a core as a function of temperature.
Fig.5. Part of the b-h curve of a certain core.
As shown in Figure 6, it is the impedance curve of a 102 common mode inductor with a nominal 4A flow under different flow conditions, as can be seen from the figure: the 100 Gigabit impedance has been attenuated by 50% at 1A, and the 100 Gigabit impedance has been attenuated by nearly 90% at 4A, and the filtering effect is basically goneAnd as can be seen from the figure, its maximum impedance point (resonance point) will also change with the flow (the inductance changes, the resonance point will also change accordingly), so the effective high-impedance filtering frequency band will also change, if it is tens of MHz noise, in the case of the device going to the rated current rate, the suppression effect of this noise of the device has been greatly reduced.
Fig.6. The common-mode impedance of a 102 common-mode inductor with a 4A flow at different currents.
a.Very low ESL means that it has good high-frequency filtering characteristics
b.At the same time, due to the characteristics of capacitor isolation and bypass filtering in parallel, it is insensitive to current
c.Compared with chip common mode inductance, it has the advantage of reliability of ceramic chip
d.The smaller size is conducive to the miniaturization of the product.
The above is the content of the common mode capacitor: another EMC filter artifact (above) introduced to you, I hope you will find it helpful after reading it!