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.

在0.15～1MHz的频率范围内，干扰主要以共模的形式存在,在1～10MHz的频率范围内，干扰的形式是差模和共模共存，在10MHz以上，干扰的形式主要以共模为主。

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). The high-frequency harmonic components of the square wave voltage in the circuit are transmitted to the earth through the distributed capacitance, forming a loop with the power line, and generating common-mode interference.

如图所示，L、N为电源输入，C1、C2、C3、C4、C5、L1、L2。组成输入EMI滤波器，DB1为整流桥，V2为开关管，开关管安装在散热器上，开关管的D极与散热器相连，与散热器之间形成一个耦合电容，如图中的G7所示，开关管V2工作在开关状态，其D极的电压为高频方波，方波的频率为开关管的开关频率，方波中的各次谐波就会通过耦合电容、 L、N电源线构成回路，产生共模干扰。

电源与大地的分布电容比较分散，难以估算，但从图来看，开关管V2的D极与散热器之间耦合电容的作用最大，从DB1到电感L3，之间的电压频率为100Hz，而L3到D1和V2的D极之间的连线的电压均为方波电压，含有大量的高次谐波。其次L3的影响也比较大，但L3与机壳的距离比较远，分布电容比开关管和散热器之间的耦合电容小得多，因此，我们主要考虑开关管与散热器之间的耦合电容。