How to choose the inductance size of common mode filter inductor?

1. How to choose the inductance of common mode filter inductor?

Common mode inductor, also known as common mode choke coil, is a closed magnetic ring inductor with opposite directions and the same number of turns. It is an ideal common mode choke coil. It has a blocking effect on the common mode interference between L and N, but has no inductive suppression effect on the differential mode inductance between L and N.However, if in fact the winding of the inductor coil is not completely symmetrical, it will lead to the occurrence of differential mode leakage inductance. When the power supply current flows through the two windings, the direction is opposite. At this time, the magnetic flux cancels out, resulting in low impedance.The power supply current flows in the same direction in the two windings. At this time, the magnetic flux adds up and becomes a high impedance, which plays a role in reducing common mode noise.

1. Common mode filter inductor

Common mode inductors are an important part used in mobile power supplies, switching power supplies and other equipment. Working principle: When current flows through two coils wound in opposite directions, two mutually canceling magnetic fields h1 and h2 are generated. At this time, the working current It is mainly damped by the ohmic resistance of the inductor coil and the small leakage inductance at the operating frequency that can be ignored.If there is an interference signal flowing through the coil, the coil will show high impedance and have a strong damping effect, which will weaken the interference signal.

Common mode inductor manufacturers will consider the frequency band curve of the filter when selecting common mode filter inductors. The larger the common mode impedance, the better. They should also pay special attention to the impact of differential mode impedance on the signal, and pay attention to the high-speed port.When the rated current is large, the wire diameter of the common mode inductor must also be increased to withstand the larger current.In addition, if you want low-frequency attenuation characteristics, you can maintain an appropriate increase in inductance.

2. What is the calculation formula for selecting the inductance of common mode filter inductor?

According to the formula L=AL×N² (inductance = inductance × number of turns²)

Therefore, in addition to the number of turns, the inductance of the filter inductor is also related to the inductance coefficient. However, the inductance coefficient is also related to the material of the magnetic core!Filters such as UU type and EE type generally use magnetic cores made of high-conductivity materials. According to your test results, calculate the AL value of the magnetic core according to the formula L=AL×N², and use SMD chip power inductors to understand the magnetic core. material.

The material can basically be determined from the AL values ​​of the two cores. UU16 should be a high-permeability core R12K-15K, while EE35 is a power type core (but the AL of the core is too low, and the normal EE35 power type , such as PC40 has more than 3000)