The design of the power supply filter is usually considered from both common mode choke coil 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 extremely high and the volume is small. Eccentric to consider an important issue is its leakage inductance, that is, differential mode inductance. In general, the method of calculating the leakage inductance is assumed to be 1% of the common mode inductance, and the leakage inductance is between 0.5% and 4% of the common mode inductance. In the design of the best performance of the choke, the impact of this error may not be overlooked.
How does the leak feel form? Tightly wound, and around the week of the ring coil, even without the core, all of its magnetic flux are concentrated in the coil "core". However, if the toroidal coil is not covered for a week, or the winding is not tight, then the flux will leak out from the core. This effect is proportional to the relative distance between the turns and the magnetic permeability of the spiral core. The common mode choke has two windings which are designed so that the current they flow through is in the opposite direction when the coil is conducted, so that the magnetic field is zero. If for the sake of safety, the coil on the core is not double-wound, so there is a considerable gap between the two windings, naturally caused by magnetic flux "leakage", that is, the magnetic field in the various points of concern Is not really 0. The leakage inductance of the common mode choke is a differential mode choke coil . In fact, the magnetic flux associated with the differential mode must leave the core at a certain point, in other words, the magnetic flux forms a closed loop outside the core, not just within the toroidal core.
If the core has a differential mode inductance, then the differential mode current will make the magnetic flux within the core deviation from the zero, if the deviation is too large, the core will occur magnetic saturation phenomenon, so that the common mode inductance and coreless same. As a result, the intensity of the common mode radiation is like no choke in the circuit.
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