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The design of power filters can usually be considered from both common mode and differential mode perspectives. The most important part of a common mode filter is the common mode inductance coil. Compared with a differential mode inductance coil, a significant advantage of a common mode inductance coil is its extremely high inductance value and small size. An important issue to consider when designing a common mode inductance coil is its leakage inductance, which is the differential mode inductance. Usually, the method of calculating leakage inductance is to assume it is 1% of the common mode inductance, but in reality, the leakage inductance is between 0.5% and 4% of the common mode inductance. When designing the optimal performance inductance coil, the impact of this error may not be negligible. 1. How is leakage formed? A tightly wound and fully wound circular coil, even without a magnetic core, all its magnetic flux is concentrated within the core of the coil. However, if the circular coil is not fully wound or not tightly wound, magnetic flux will leak out of the core. This effect is proportional to the relative distance between wire turns and the magnetic permeability of the spiral tube core. The common mode inductance coil has two windings, which are designed to cause the current they flow through to conduct in the opposite direction along the coil core, resulting in a magnetic field of 0. If, for safety reasons, the coil on the core is not wound in two wires, there is a considerable gap between the two windings, which naturally causes magnetic flux leakage. This means that the magnetic field is not truly zero at the points of concern. The leakage inductance of a common mode inductance coil is a differential mode inductance. In fact, the magnetic flux related to differential mode must leave the core at a certain point, in other words, the magnetic flux forms a closed loop outside the core, rather than just limited to the annular core. If the core has a differential mode inductance, the differential mode current will cause the magnetic flux inside the core to deviate from zero. If the deviation is too large, the core will experience magnetic saturation, making the common mode inductance basically the same as the inductance without a magnetic core. When designing a common mode inductor coil review filter, it is assumed that the common mode and differential mode parts are independent of each other. However, these two parts are not truly independent, as the common mode inductance coil can provide a considerable amount of differential mode inductance. This part of differential mode inductance can be simulated by discrete differential mode inductors. In order to utilize differential mode inductance, in the design process of filters, common mode and differential mode should not be carried out simultaneously, but should be done in a certain order. Firstly, common mode noise should be measured and filtered out. By using a Differential Mode Rejection Network, the differential mode components can be eliminated, allowing for direct measurement of common mode noise. If a common mode filter is designed to simultaneously ensure that the differential mode noise does not exceed the allowable range, 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 differential mode component that exceeds the limit can be attenuated by the differential mode leakage inductance of the common mode filter. For low-power power systems, the differential inductance of the common mode inductance coil is sufficient to solve the problem of differential mode radiation, as the source impedance of differential mode radiation is small, so only a very small amount of inductance is effective. Although a small amount of differential mode inductance is very useful, a too large differential mode inductance can cause magnetic saturation of the choke. It is usually difficult to measure the saturation characteristics of the common mode coil core (whole or part) using the LISN principle to measure the saturation characteristics of the common mode inductor coil. Through simple experiments, it can be seen to what extent the attenuation of the common mode filter is affected by the inductance reduction caused by the 60Hz programmed current. This test requires an oscilloscope and a Differential Mode Rejection Network (DMRN). Firstly, use an oscilloscope to monitor the line voltage. According to the following method, input the signal from channel A of the oscilloscope, set the time reference of the oscilloscope to 2ms/div, and then add the trigger signal to channel A. When the AC voltage reaches its peak, a wired current will be generated. At this time, the degradation of filter efficiency is expected. 4. There is also a common mode inductance coil with an E-shaped iron core structure, which is easier to coil than a circular magnetic core coil, but has more radiation than a pot shaped iron core coil. The common mode magnetic flux of the E-shaped iron core coil connects the two sets of coils on the external leads together. In order to achieve high magnetic permeability, there should be no air gap on the external leads. On the other hand, differential mode flux connects the external lead with the central lead

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