Overview of common mode choke
When the filter is designed, it is assumed that the common mode and the difference mode are independent of each other. However, these two parts are not really independent, because the common mode choke provides a considerable difference in mode inductance. This part of differential inductance can be simulated by discrete differential inductors.
In order to make use of differential mode inductance, the common mode and differential mode should not be carried out simultaneously in the design process of filter, but should be done in a certain order. First, the common-mode noise should be measured and filtered out. Differential Mode Rejection Network is adopted to eliminate the Differential Mode components, so the common-mode noise can be directly measured. If the common-mode filter is designed to keep the difference mode noise within the allowable range, the mixed noise between the common mode and the difference mode should be measured. Because the common-mode components are known to be below the noise tolerance, the over-qualified ones are only the difference mode components, which can be attenuated by the difference mode leakage sense of the common-mode filter. For a low-power power system, the differential mode inductance of the common mode choke coil is sufficient to solve the problem of differential mode radiation, because the source impedance of differential mode radiation is small, so only a very small amount of inductance is effective.
Although a small number of differential inductors is useful, too large differential inductors can saturate the choke. A simple calculation can be made according to formula (2) to avoid the occurrence of magnetic saturation.
It is often difficult to measure the saturation characteristics of the core (whole or part) of a common-mode coil. It can be seen from the simple test that the attenuation of the common-mode filter is to what extent influenced by the inductance reduction caused by the 60Hz current. This test requires an oscilloscope and a differential mode suppression network (DMRN). First, use oscilloscope to monitor line voltage. Input the signal from channel A of oscilloscope in the following way, set the time reference of oscilloscope to 2ms/div, and then add the trigger signal to channel A. When the ac voltage reaches the peak, the cable current will be generated. The input end of DMRN is connected to LISN, and the output end is matched with 50 impedance and connected to channel B of oscilloscope. When the common-mode choke operates in a linear zone, during the fluctuation of the input current, the emission increment detected by channel B shall not exceed 6-10db. FIG. 1 results of this test shown on the oscilloscope, the above curve is common-mode emission; The curve below is the line voltage. During the peak of the online voltage, the bridge rectifier is conducting and transmitting the charging current.
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