What are the uses of common mode inductors and differential mode inductors?
The design of power filter can usually be considered from two aspects of common mode and differential mode. The most 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 it has a very high inductance value and is small in size. An important issue to consider when running a coil is its leakage inductance, also known as differential mode inductance. Usually, 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 effect of this error may not be negligible when designing choke coils for optimum performance.u200bu200b
The importance of leakage inductance. How is the leakage inductance formed? A tightly wound toroidal coil, even if there is no magnetic core, has all the magnetic flux concentrated in the coil 'core'. However, if the toroid is not fully wound, or is not wound tightly, the magnetic flux will leak out of the core. This effect is proportional to the relative distance between turns and the magnetic permeability of the helical core. Common mode choke coils have two windings that are designed so that the currents through them travel in opposite directions as they travel down the coil core so that the magnetic field is zero. If, for safety reasons, the coils on the core are not double-wound, so that there is a considerable gap between the two windings, which naturally causes the magnetic flux to 'leakis 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 to the toroidal 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 non-magnetic The core inductance is the same. As a result, the common mode radiation is as strong as if there were no choke coils in the circuit. Summary of Common Mode Choke Coils When designing a filter, it is assumed that the two parts, the common mode and the differential mode, are independent of each other. However, the two parts are not really independent, since common mode choke coils can provide considerable differential mode inductance. This part of the differential mode inductor can be simulated by a discrete differential mode inductor.u200bu200b
In order to utilize the differential mode inductance, in the design process of the filter, the common mode and 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. Using the differential mode rejection network, the differential mode components can be eliminated, 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 at the same time, then the mixed noise of the common mode and the differential mode should be measured. Because it is known that the common mode component is below the noise tolerance, only the differential mode component exceeds the standard, 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 a common-mode choke is sufficient to address differential-mode radiation, because differential-mode radiation has a small source impedance, so only a very small amount of inductance is effective.
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