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China Amorphous explains in detail common mode inductors and differential mode inductors. What are the uses of each?

by:Catech      2023-04-28

The design of the power filter can usually be considered from two aspects of 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 is that its inductance value is extremely high, and the volume is small. The design of the common mode choke coil An important issue to consider when looping is its leakage inductance, which is the differential mode inductance. Typically, 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. The influence of this error may not be negligible when designing a choke coil for optimal performance.

The importance of leakage inductance. How is leakage inductance formed? Tightly wound toroidal coil with one full turn, even without a magnetic core, all of its magnetic flux is concentrated in the coil 'core'. However, if the toroid is not wound one full turn, or if it is not tightly wound, flux will leak out of the core. This effect is proportional to the relative distance between the turns and the magnetic permeability of the helical tube core. Common mode choke coils have two windings designed so that the currents they flow through are conducted in opposite directions along the coil core, thereby making the magnetic field zero. If, for safety reasons, the coils on the core are not bifilarally wound, then there is a considerable gap between the two windings, which naturally causes flux 'leakage', that is to say, the magnetic field at various points of interest is not really 0. The leakage inductance of a common mode choke is a differential mode inductance. In fact, the flux associated with the differential mode must leave the core at some point, in other words, the flux forms a closed loop outside the core, not just confined within the toroidal core.

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, so that the common mode inductance is basically the same as the inductance without a magnetic core. Same. As a result, the intensity of common-mode radiation is as if there were no chokes in the circuit.

Common Mode Choke Overview

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

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, common-mode noise should be measured and filtered out. 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, then the mixed noise of the 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 is exceeded, 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 the differential mode radiation is small, so only a very small amount of inductance is effective.

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