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The design of the power filter can usually be considered from both common mode and differential mode. The important part of the common mode filter is the common mode choke coil. Compared with the differential mode choke coil, a significant advantage of the common mode choke coil lies in its extremely high inductance value and small size. The common mode choke coil is designed An important issue to consider when looping is its leakage inductance, that is, differential mode inductance.Usually, the way to calculate the leakage inductance is to assume that it is 1% of the common mode inductance. In fact, the leakage inductance is between 0.5% and 4% of the common mode inductance. When designing choke coils with optimal performance, the influence of this error may not be negligible.
The importance of leakage inductance. How is leakage inductance formed? Even if there is no magnetic core, all the magnetic flux of a tightly wound toroidal coil is concentrated in the "core" of the coil. However, if the toroidal coil is not wound one full circle, or the winding is not tight, then the magnetic flux will leak out of the core.This effect is proportional to the relative distance between the turns and the magnetic permeability of the spiral tube core. The common mode choke coil has two windings, these two windings are designed so that the current flowing through them is conducted in opposite directions along the coil core, so that the magnetic field is zero.
If the core has a differential mode inductance, then the differential mode current will cause the magnetic flux in the core to deviate from zero. If the deviation is too large, the core will be magnetically saturated, making the common mode inductance basically the same as the inductance without a magnetic core. Same. As a result, the intensity of common-mode radiation is as if there is no choke in the circuit.
Overview of common mode chokes
When designing the filter, it is assumed that the common mode and differential mode are independent of each other. However, the two parts are not truly independent, because common mode chokes can provide considerable differential mode inductance. This part of the differential mode inductance can be simulated by a discrete differential mode inductance.
In order to utilize the differential mode inductance, in the design process of the filter, the common mode and the differential mode should not be carried out at the same time, but should be done in a certain order. First, the common mode noise should be measured and filtered out. The differential mode rejection network (Differential Mode Rejection NETWORK) can be used to eliminate the differential mode component, so the common mode noise can be directly measured. If the common mode filter is designed so that the differential mode noise does not exceed the allowable range at the same time, then the mixed noise of common mode and differential mode should be measured. Since it is known that the common mode component is below the noise tolerance, only the differential mode component exceeds the standard, which can be attenuated by the differential mode leakage inductance of the common mode filter. For low-power power systems, the differential mode inductance of the common mode choke is sufficient to solve the differential mode radiation problem, because the source impedance of differential mode radiation is small, so only a very small amount of inductance is effective.
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