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Often called a ring-shaped inductor, a core common-mode choke inductor is a very effective device for suppressing common-mode noise. Unlike a toroidal or rod-shaped coil, a ring-shaped inductor does not radiate out into the surrounding circuitry. Instead, the current through the inductor is steered in the same direction, as illustrated in Figure 1. The result is a high impedance to the unwanted frequency and a reduction in the amount of noise.
The ring-shaped inductor has a small diameter and is a good choice for low cost projects. It is usually wound manually. However, a machine can be used for this process. For a higher cost, a two-in-hand inductor can be used, which provides better circuit balance and lower leakage inductance. The two-in-hand design also provides greater capacitance.
Common mode chokes are made from a magnetic core. Typically, the inductance is around 10-50 pF. The permeability of the core varies with temperature and dc bias. Crystalline cores are more compact and have a higher frequency range. They are eight to ten times more efficient than ferromagnetic cores.
To calculate the inductance of a core common mode choke, the number of turns is multiplied by the permeability of the core. The permeability of the core can be measured by measuring the flux between the windings. The permeability of the core reflects the number of turns.
Several factors affect the permeability of the core. The number of turns, the diameter of the ring, the material, the dc bias, and the temperature all affect the permeability of the core. Consequently, the permeability of the core is a critical element in predicting the performance of an EMI filter.
The permeability of the core varies between 0 and 90 degrees, depending on the type of material. The permeability of a crystalline core is about 8 to 10 times more efficient than that of a ferromagnetic core. When a crystalline core is at room temperature, the permeability is about 90 degrees. This indicates that crystalline cores are more compact and efficient.
A common mode choke inductor consists of a ring-shaped or E-shaped coil. The ring-shaped coil has a smaller diameter than the UF series and requires manual winding. The inductance is proportional to the number of turns squared. The inductance of the ring-shaped coils is about one-quarter of the inductance of a UF series. A core common mode choke inductor is usually wrapped with three or four wires.
A common-mode choke inductor will saturate if it passes a peak current. During a peak current, a strong magnetic field will enter the core. This will cause the core to saturate, resulting in the loss of attenuation. This can cause a problem for the filter if it isn't derated. To prevent this, the inductor must have a high enough coupling factor. The coupling factor is a measure of the magnetic flux coupling between the two windings. It should be greater than 100%.
Toroidal Ferrite Core Common Mode Choke
Different range of freuency from 50khz to 500khz
Compact desin,lighter weight with various mounting options
High rated current and high inductance values with a compact design
Better frequency response due to a low number of turns
Standard:
RoHS
UL 94 v-0
Compliant with RoHS
Plastic materials meet UL 94 v-0 requirements
