Working principle and type selection of common mode inductor

For an electronic product , At the beginning of redesign , There are often many problems to consider . Such as cost and function realization . In the design process , There are also many issues to consider , among EMI The problem is one of them . Inhibition EMI, Designers often use common mode inductors .

Let's talk about common mode inductance today ！

Common mode inductance （Common mode Choke）, It is also called common mode choke . seeing the name of a thing one thinks of its function , That is, the coil that chokes the common mode current . The common mode inductor has four pins .

It is often used in differential signal scenarios . Simply put, two are the same size , Signals in the opposite direction , Transmit on two conductors respectively , The difference between the two signals , It is the differential signal .

for example USB Interface ,Camera Of MIPI Signals, etc , Are in the form of differential signals .

It is in the design , Also often choose the field common mode inductance to play the role of filtering , Suppress common mode current , And other electronic devices that generate radiation interference , To effectively reduce EMI problem .

The interference signal in the figure is blue , Common mode signal . Characteristic is , equal , In the same direction .

Normal differential signal , I said that before , Characteristic is , equal , In the opposite direction , Is the green part in the figure .

According to the right-hand screw rule , The magnetic field generated by the differential signal in the spiral coil is the red part ,H1,H2 Same size , In the opposite direction , Just offset , When the differential signal flows through the common mode inductor , Mainly affected by coil resistance .

However , Interference signal is common mode signal , There is a common mode current in the coil . The magnetic field it produces , Same size , In the same direction , Increases the magnetic field present in the coil , However, the function of inductance is to prevent the change of current , The essence is to prevent the change of magnetic flux , Therefore, it plays a role in suppressing common mode current . At the same time, the greater the inductance , The stronger the inhibitory effect .

Common mode inductance , Several noteworthy parameters

1. Package volume
   Common mode inductors are divided into patch type , And plug-in . Usually , The pad of a patch type common mode inductor can often be pasted with two 0201 Packaged 0 Ohmic resistance , This is also why in some projects , To reduce costs , The common mode inductor of the previous chip , Replace with 0 Ohmic resistance . because 0 Ohmic resistance , Under high-speed signal , It can also play the role of common mode inductance .
   
   Direct insertion
   
   Patch

Direct insertion

Patch
2. DC resistance DCR： Winding resistance value of common mode inductance coil .
3. Rated current ： The maximum current allowed to flow through the common mode inductor , When designing circuits , The working current should be less than the rated current of the common mode inductance .
4. Rated voltage ： The maximum voltage that the common mode inductor can withstand . In circuit design , Voltage on the signal line , It must be less than the rated voltage when the common mode inductance works .
5. Self resonant frequency ： Common mode inductance , Its impedance changes with the change of frequency . When making a choice , The signal frequency of the circuit cannot be higher than the frequency corresponding to the maximum impedance of the common mode inductance .