Why add an EMI low-pass filter to increase the insertion high frequency loss?
Key point 1
Common-mode interference: There is a distributed capacitance between the switching power supply (switching MOS, when the output power is large, the MOS will increase the heat sink design) and the earth (the reference ground plate of the test system), the high-frequency component of the square wave voltage in the circuit of the switching MOS and the output rectifier diode is transmitted to the ground (reference ground plate) through the distributed capacitance, so that a loop with the power line is formed, and the high-frequency component generates common mode disturbance through the distributed capacitance and the power line to form a loop.
Key point 2
The generation of differential mode interference: mainly the switch tube in the switching power supply is in the switching state, when the switch tube is turned on, the current flowing through the power line rises linearly, and the current is mutated to O when the switch tube is turned off, so the current flowing through the power line is a high-frequency repeated triangle wave pulsating current, which contains a rich high-frequency harmonic component, and the amplitude of the harmonic component will become smaller and smaller with the increase of frequency, so the differential mode disturbance is reduced with the increase of frequency.
Note: As the frequency increases, the distribution of capacitance between the ground of our switching devices becomes critical, and the common mode interference becomes higher and higher, and small common mode currents will produce large interference.
Loop paths for common-mode and differential-mode harassment:
As shown in the figure above, the noise generated by the switching power supply system includes common mode noise and differential mode noise. Common mode interference is caused by the potential difference between the current-carrying conductor and the earth, which is characterized by the noise voltage on the two lines is in the same direction of the potential, while the differential mode interference is due to the potential difference between the current-carrying conductors, and its characteristic is that the noise voltage on the two lines is opposite to the same potential.
It is important to note that usually these two components of the interfering voltage on the line exist simultaneously.
As you can see in the structure diagram of the internal circuit board of the electronic product & device, we cannot pass the test standard without a specific EMI low-pass filter due to spurious parameters affecting the characteristics of the coupling channel!
In the EMI conducted disturbance frequency band < 30MHz, the coupling channels of most switching power supply systems are generally analyzed by circuit network diagrams. However, any component in the switching power supply, such as resistors, capacitors, inductors, switches, and diodes, contains spurious parameters, and the wider the frequency band studied, the higher the order of the equivalent circuit.
As a result, the equivalent circuitry of a switching power supply, including the spurious parameters of each component and the coupling between components, will be much more complex, such as:
At high frequency, the spurious parameters have a great influence on the characteristics of the coupling channel, and the existence of distributed capacitance becomes the channel of electromagnetic disturbance.
When the power of the switch tube is large, the switch tube generally needs to add a heat sink, and the distribution capacitance between the heat sink and the switch tube cannot be ignored at high frequency, which can form a space-oriented radiation disturbance source and a common mode disturbance source of power line conduction.
In summary, we know that adding an EMI low-pass power filter to the input can enable an efficient design of conducted interference in the power system.