Input filter design

The low-frequency differential-mode noise component is the main component of the noise generated by the switching power supply. A filter should be added to the power input port, and the input filter should have the best attenuation performance for EMI signals. The filter impedance should be mismatched with the power supply impedance. The greater the mismatch, the more ideal the attenuation is, and the better the insertion loss characteristics are. In other words, if the internal resistance of the noise source is low impedance, the input impedance of the EMI filter connected to it should be high impedance (such as a series inductor with a large inductance); if the internal resistance of the noise source is high impedance, the input impedance of the EMI filter should be low impedance (such as a parallel capacitor with a large capacity). The switching power supply also contains common-mode noise and differential-mode noise. Common-mode interference is caused by the potential difference between the current-carrying conductor and the earth. Its characteristic is that the noise voltage on the two lines is at the same potential and in the same direction; while differential-mode interference is caused by the potential difference between the current-carrying conductors. Its characteristic is that the noise voltage on the two lines is at the same potential and in the opposite direction. Usually, these two components of the interference voltage on the line exist at the same time. Due to the imbalance of the line impedance, the two components will transform into each other during transmission, which is very complicated. A typical EMI filter includes two parts: common-mode noise suppression circuit and differential-mode noise suppression circuit, as shown in the figure.

Figure 1 Power filter

In the figure: differential mode suppression capacitors C1, C2 0.1～0.47 μF;
differential mode suppression inductors L1, L2 100～130 μH;
common mode suppression capacitors C1/C2 ＜10000 pF;
common mode suppression inductor L 15～25 mH.

The definition of insertion loss is shown in Figure 2. When the filter is not connected, the output voltage of the signal source is U1. When the filter is connected, the voltage of the signal source measured at the output end of the filter is U2. If the output impedance of the signal source is equal to the input impedance of the receiver, both are 50Ω, then the insertion loss of the filter is:

Figure 2 Definition of insertion loss

During design, the resonant frequency of the common-mode filter circuit and the differential-mode filter circuit must be significantly lower than the operating frequency of the switching power supply, generally lower than 10kHz, that is,

FIG3 is a schematic diagram of differential mode interference and common mode interference.

Figure 3 Schematic diagram of differential mode interference and common mode interference

In the actual circuit, since the common mode and differential mode components generated by each device are different, the filter circuit can appropriately increase or decrease the filter components. The adjustment of the specific circuit generally requires EMI testing to obtain satisfactory results. When installing the filter circuit, it is necessary to ensure good grounding and good isolation between the input and output ends, otherwise the filtering effect will not be achieved. Figure 4 shows two filter circuits, and their filtering effects are shown in the experimental curve of Figure 5.

Figure 4 Two filter circuits

Figure 5 Experimental curves of the effects of two filter circuits