Many people should be familiar with the common film inductor, but do you really understand it completely? Is the common film inductor on your circuit really connected correctly? First, let's understand the common film inductance. Common film inductor, also known as common mode choke coil, is usually used in computer switching power supply to filter the common mode electromagnetic interference signal of the system.
If the common-mode current caused by the main control board does not pass through the attenuation coefficient filtering device, the common-mode interference current is easily based on the electromagnetic wave radiation caused by the charging line of the socket mobile phone-the common-mode radiation source caused by the current caused by the common-mode current. Standards and specifications such as Federal Communications Commission, International Radio Interference Special Committee CISPR22 and China GB9254 limit the common mode conducted interference and radiation emission of communication ports of information technology equipment.
In the design scheme of the main control panel, the common film inductor acts as an EMI filter, which is used to suppress the emission point of radio waves to the external radiation source caused by the high-speed power line. The chip on the computer board is not only the electromagnetic interference object in the working process, but also the electromagnetic interference source. Generally speaking, we can divide these electromagnetic interferences into two categories: serial mode interference and common mode interference.
In order to eliminate the interference signal and induced interference from the signal input online, the filter circuit must be reasonably configured to filter the interference of common mode and serial mode of the system. Common mode choke is the basic element of filter circuit.
The common film inductor is essentially a bidirectional filter: on the one hand, besides the common mode choke coil on the signal line, on the other hand, it is necessary to suppress electromagnetic interference and prevent interference. Normal operation of other filtering equipment in the same electromagnetic environment. When one end of the filter circuit is connected to the interference source and the other end is connected to the interference device, La and C1, Lb and C2 form two groups of low-pass filters, which can control the common mode EMI signal on the line at a lower electrical frequency. When the AC frequency is necessary, the larger the inductance is, the greater the blocking ability to AC current is. On the contrary, the smaller the inductance is, the smaller the blocking ability is. The common-mode data signal is suppressed in the power circuit, and the inductance value of the common mode choke can be measured correctly by the power bridge.
Before designing the co-film inductor, it is necessary to check whether the coil meets the following principles.
(1) under normal working conditions, the magnetic core will not be saturated due to the energized current;
(2) High-frequency interference signals should have sufficient impedance and a certain bandwidth, and should have inimum impedance to the signal current of working frequency;
(3) The smaller the temperature coefficient of inductor, the smaller the distribution capacity;
(4)DC resistance should be as small as possible;
(5) The induced electric induction should be as large as possible, and the induction value should be stable;
