Principle of Conducted Interference Generation

The conducted interference of the switching power supply is transmitted outward through the input power line, including both differential mode interference and common mode interference. The test frequency range of conducted interference is 0.15～30kHz.

In the frequency range of 0.15～1MHz, the interference mainly exists in the form of common mode, in the frequency range of 1～10MHz, the interference is in the form of differential mode and common mode coexistence, and above 10MHz, the interference is mainly in the form of common mode.

1. Differential mode interference The generation of differential mode interference is mainly due to the switch tube working in the switch state. When the switch tube is turned on, the current flowing through the power line rises linearly, and when the switch tube is turned off, the current suddenly drops to zero. Therefore, the current flowing through the power line is a high-frequency triangular pulsating current, which contains rich high-frequency harmonic components. As the frequency increases, the amplitude of the harmonic component becomes smaller and smaller, so the differential mode interference decreases as the frequency increases . The filter circuit of the output loop is shown in the figure. Capacitor C6 and inductor L3 form a low-pass filter. The differential mode conduction interference mainly exists in the low frequency segment.

2. Common mode interference The main reason for common-mode interference is that there is distributed capacitance between the power supply and the earth (protective ground). interference.

As shown in the figure, L and N are power inputs, C1, C2, C3, C4, C5, L1, and L2 form an input EMI filter, DB1 is a rectifier bridge, and V2 is a switch tube. The switch tube is installed on the radiator, and the D pole of the switch tube is connected to the radiator, forming a coupling capacitor between the radiator and the radiator. As shown in G7 in the figure, the switch tube V2 works in the switching state, and the voltage of its D pole is a high-frequency square wave. The frequency of the square wave is the switching frequency of the switch tube. The harmonics in the square wave will form a loop through the coupling capacitor, L, and N power lines, generating common-mode interference.

The distributed capacitance between the power supply and the earth is relatively dispersed and difficult to estimate, but from the figure, the coupling capacitor between the D pole of the switch tube V2 and the radiator has the greatest effect. The voltage frequency from DB1 to the inductor L3 is 100Hz, and the voltage of the connection line from L3 to the D pole of D1 and V2 is a square wave voltage, which contains a large number of high-order harmonics. Secondly, the influence of L3 is also relatively large, but L3 is far away from the casing, and the distributed capacitance is much smaller than the coupling capacitance between the switch tube and the radiator. Therefore, we mainly consider the coupling capacitance between the switch tube and the radiator.