The design of power filter can be considered from 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 value is very high, and its volume is small. An important problem to consider when designing 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% and 4% of the common mode inductance. In the design of choke with optimal performance, the influence of this error may not be ignored.

The importance of leakage. How is the sense of leakage formed? A tightly wound loop with all the magnetic flux concentrated in the "core" of the loop, even if there is no core. However, if the loop is not fully wound or not tightly wound, the flux will leak out of the core. This effect is directly proportional to the relative distance between turns and the permeability of the core. The common mode choke has two windings, which are designed so that the current flowing through them is conducted in the opposite direction along the core, so that the magnetic field is 0. If for the sake of safety, the coil on the core is not wound by two wires, so there is a considerable gap between the two windings, which naturally causes the "leakage" of magnetic flux, that is to say, the magnetic field is not really zero at all points of concern. The leakage inductance of common mode choke is differential mode inductance. In fact, the flux related to the differential mode must leave the core at some point. In other words, the flux forms a closed circuit outside the core, not only within the ring core.

If the core has a differential mode inductance, the differential mode current will cause the magnetic flux in the core to 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 there is no choke in the circuit.

Review of common mode choke

In the design of the filter, it is assumed that the common mode and the differential mode are independent of each other. However, these two parts are not really independent, because the common mode choke can provide considerable differential inductance. This part of differential inductance can be simulated by discrete differential inductance.

In order to make use of differential inductance, common mode and 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. By using differential mode rejection network, differential mode components can be eliminated, so 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, then 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 difference mode component is beyond the limit. The common mode filter can be attenuated by the difference mode leakage sense. For low-power power 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, so only a small number of inductance is effective.