The selection of magnetic core materials for common mode inductors used in electromagnetic interference filters requires careful consideration. Most electromagnetic interference filters must use common mode inductors.

Due to the high impedance of common mode inductors over a wide frequency range, they can suppress high-frequency noise generated by high-frequency switching power supplies.

Turning off the power will generate two types of noise: common mode and differential mode. The propagation path of differential mode noise (Figure 1a) is the same as the input current.
Common mode noise (Figure 1b) is manifested as noise that is equal and in phase with each other, and its propagation path is connected to the ground wire through the winding.
To suppress electromagnetic interference, a typical filter should include common mode inductors, differential mode inductors, and X and Y capacitors. Y capacitors and common mode inductors are used to attenuate common mode noise.
Inductors display high impedance to high-frequency noise and reflect or absorb noise. At the same time, capacitors become a low impedance path to ground, diverting noise from the main circuit (Figure 2).
In order to achieve the above functions, common mode inductors must provide appropriate impedance within the switching frequency range.
A common mode inductor consists of two sets of windings with the same number of turns. These two windings make the magnetic flux generated by the line current in each winding equal in magnitude, but opposite in phase. So this
The magnetic flux generated by two sets of windings cancels out each other, leaving the magnetic core in an unbiased state. Differential mode inductors only have one winding, and the magnetic core needs to withstand all line currents and be in working condition
The lower part cannot be saturated. So common mode inductors and differential mode inductors have
There is a significant difference. To prevent saturation of the magnetic core, the effective magnetic permeability of the differential mode inductor core must be low (gap ferrite or magnetic powder core). But common mode inductors can use high permeability materials
Material can be used, and a smaller magnetic core can be used to obtain a very large inductance.
Select materials
The noise generated by switching power supplies is mainly located at the fundamental frequency of the device and includes higher-order harmonics. That is to say, the noise spectrum generally includes the part between 10kHz and 50MHz.
In order to provide appropriate attenuation, the impedance of the inductor must be sufficiently high within this frequency band. The total impedance of a common mode inductor consists of two parts, one of which is the series impedance (Xs),
The other part is series resistance (Rs). At low frequencies, reactance is the main part of impedance, but as the frequency increases, the real part of magnetic permeability decreases and the core loss increases, as shown in Figure 3
Show. The combination of these two factors helps to achieve acceptable impedance (Zs) across the entire spectrum. In most cases, common mode inductors use ferrite. Ferrites can be divided into two types: nickel
Zinc and manganese zinc. The characteristic of nickel zinc materials is that their initial magnetic permeability is low (<1000&micro;), but they can maintain the same magnetic permeability at very high frequencies (>100MHz).

Most ferrite manufacturers will provide the inductance coefficient (A L) value of the produced magnetic core, making it much more convenient to calculate inductance. The relationship between turns and inductance is:
N=1000 (L/A L) 1/2
Among them:
N=number of turns
L=Inductance (milliHeng)
A L=inductance coefficient, in milliohms per 1000 turns
Table 1 lists typical design data and
Provide an example of using A L value for calculation:
For this example:
Using J material (5000&micro;), the A L value is 3020
N is 20 turns, so L=1.208 milliHeng
If this small inductance is too small for the design, you can choose a material with higher magnetic permeability or a larger magnetic core. However, if we calculate
If the inductance produced is much higher than the design limit, it can be replaced with a smaller magnetic core with fewer turns.
Design example:
A 100 Ω impedance is required at 10kHz. The input line current is 3A RMS.
1. Select wire diameter:
The current is 3A and the current density is 400A/cm;, So the cross-sectional area of the wire is 0.0075cm&sup2;.
Select # 19 AWG with a wire cross-sectional area of 0.007907cm&sup2; (with a diameter of 1mm), including insulation layer.
2. Calculate the small inductance:
L small=100 Ω/2 π (10000Hz)=1.59 milliHeng
3. Select the size and material of the magnetic core from the table:
Select J-42206-TC
A L=3020&plusmn; 20%
Inner diameter=13.72mm
&Plusmn; 0.38mm=13.34mm (small)
4. Calculate the inner circumference (I.C.) and possible actual
Current number of large turns:
I. C.=π (core diameter - wire)
Diameter)
I. C.=π (13.34mm-1 mm)
I. C.=38.76mm
Large turns=(160/360)*
(38.76mm)/(1mm/turn)
I. C.=17.2 turns, or 17 turns
5. Calculate the small inductance at 17 turns:
A L=3020&plusmn; 20%
N=1000 (L/A L) 1/2
17=1000 (L/3020-20%) 1/2
L=0.698 milliHeng (small)
This value is much lower than the required 1.59 milliliters
Heng, so modifications must be made. Modifiable
The parameters are magnetic core size, material permeability, and line
Diameter. The larger the magnetic core, the larger the inner diameter, therefore
More turns can be wound on the magnetic core (the larger the magnetic core)
The larger the A L value, the greater. The higher the magnetic permeability of the material,
The larger the inductance and the smaller the wire diameter, the more flexible it can be wound
The more turns are made. But this will also increase
Copper loss