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2022-01-04

The design of power filter can usually be considered from two aspects: common mode and differential mode. The most important part of the common mode filter is the common mode choke. Compared with the differential mode choke, a significant advantage of the common mode choke is that its inductance is very high and its volume is small. An important problem to be considered in the design of the common mode choke is its leakage inductance, that is, the differential mode inductance. Generally, the method of calculating the leakage inductance is to assume that it is 1% of the common mode inductance. In fact, the leakage inductance is between 0.5% ~ 4% of the common mode inductance. When designing the choke with optimal performance, the influence of this error may not be ignored.

The importance of leakage inductance. How is the leakage feeling formed? Even if there is no magnetic core, all the magnetic flux of the tightly wound ring coil is concentrated in the "core" of the coil. However, if the ring coil is not fully wound, or the winding is not tight, the magnetic flux will leak out of the core. This effect is directly proportional to the relative distance between wire turns and the permeability of the spiral tube core. The common mode choke has two windings, which are designed so that the current they flow through is conducted in the opposite direction along the coil core, so that the magnetic field is 0. For the sake of safety, if the coil on the core is not double wound, there will be a considerable gap between the two windings, which will naturally cause flux "leakage", that is to say, the magnetic field is not really 0 at all points of concern. The leakage inductance of common mode choke is differential mode inductance. In fact, the magnetic flux related to the differential mode must leave the core at some point. In other words, the magnetic flux forms a closed loop outside the core, not only in the annular core.

If the core has differential mode inductance, the differential mode current will make the magnetic flux in the core deviate from the zero point. If the deviation is too large, the core will have magnetic saturation, making the common mode inductance basically the same as the inductance without magnetic core. As a result, the intensity of common mode radiation is the same as if there were no choke in the circuit.

Overview of common mode choke

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 really independent because the common mode choke can provide a considerable differential mode inductance. This part of differential mode inductance can be simulated by discrete differential mode inductance.

In order to make use of differential mode inductance, common mode and differential mode should not be carried out at the same time, but in a certain order. First, the common mode noise should be measured and filtered out. The differential mode component can be eliminated by using the differential mode suppression network, so the common mode noise can be measured directly. If the common mode filter is designed so that the differential mode noise does not exceed the allowable range at the same time, the mixed noise of common mode and differential mode should be measured. Because the common mode component is known to be below the noise tolerance, only the differential mode component exceeding the standard can be attenuated by the differential mode leakage inductance of the common mode filter. For low-power power supply system, the differential mode inductance of common mode choke is enough to solve the problem of differential mode radiation. Because the source impedance of differential mode radiation is small, only a very small amount of inductance is effective.

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