Common mode radiation electromagnetic interference noise suppression analysis

Common mode radiation is due to the voltage drop in the grounding circuit (as shown below). Some parts have a high potential common mode voltage. When the external cable is connected to these parts, the common mode current will be generated under the common mode voltage excitation. An antenna that radiates an electric field. Most of this is due to the voltage drop in the grounding system. Common mode radiation usually determines the radiation performance of the product.

1. Common mode radiation field

Common mode radiation is mainly radiated from the cable. It can be simulated by a short monopole antenna with a length less than 1/4 wavelength excited by the ground voltage. The current on the ideal antenna is uniform, and the actual antenna tip current tends to zero. The actual cable has a device connected to the other end, which is equivalent to a capacitive load antenna. That is, the end of the antenna is connected with a metal plate, and a uniform current flows through the antenna. Let the antenna point to the maximum field strength, then the formula for calculating the maximum field strength is:

Where f is the signal frequency, ICM is the common mode current in the circuit, L is the length of the radiating circuit wire, and r is the test distance. It can be seen from the equation that the common mode radiation is proportional to the length L of the cable, the frequency f of the common mode current, and the common mode current intensity ICM, which is inversely proportional to the test distance r. The equivalent circuit of the common mode radiation model is shown in the figure below, where uCM is the common mode radiation voltage, ICM is the common mode radiation current, and ZCM is the line equivalent impedance.

2. Method for reducing common mode radiation

The common mode radiation is proportional to the frequency f of the common mode current, the common mode current ICM, and the length L of the antenna (cable). Therefore, reducing the common mode radiation should reduce the frequency f, reduce the current ICM, and reduce the length L, while limiting the common mode flow is the basic method to reduce the common mode radiation. To do this, you need to do the following.

(1) Minimize the source voltage that excites the antenna, that is, the ground potential;

The ground plane can be effectively reduced in the grounding system by using the ground plane as shown in the following figure.

(2) providing a common mode impedance in series with the cable, that is, increasing the common mode choke;

The two wires are wound in the same direction on the ferrite bead to form a common mode choke. The DC and low frequency time difference mode currents can pass, but for high frequency common mode currents, the impedance is large, and the common mode radiation The model equivalent circuit shows that the ZCm increases and the common mode radiation current ICM decreases, so that the common mode radiation field strength is suppressed.

(3) Bypassing the common mode current to the ground;

In order to bypass the common mode current to ground, the ground plane of the printed circuit board can be separated from the connector as a “noise-free” input/output ground. To avoid contamination of the input/output ground, only allow The decoupling capacitors of the input/output lines and the shield of the external cable are connected to the "noise", and the electrical inductance of the decoupling loop is as small as possible. In this way, the common mode current of the printed circuit board carried by the input/output line is bypassed to the ground by the decoupling capacitor, and the external interference is also bypassed to the ground by the decoupling capacitor when it has not reached the component area, thereby protecting the internal The normal operation of the components.

(4) reduce the length of the cable; (5) the cable shielding layer and the shielding shell are 360° terminated;