DC power supply EMI filter design - Power Circuit - Circuit Diagram

Probe current voltage pin 420*4450 head diameter 5.0 over current current and voltage pin

MOS power IC full range

Filter 18.432M

Design principle, network structure and parameter selection of DC power supply EMI filter

1 Design principle - to meet the maximum impedance mismatch

The insertion loss should be as large as possible, that is, the reflection of the signal should be increased as much as possible. Set the output impedance of the power supply and the input impedance of the filter connected to it to ZO and ZI respectively. According to the signal transmission theory, when ZO≠ZI, reflection occurs at the input port of the filter, and the reflection coefficient

p=(ZO-ZI)/(ZO+ZI)

Obviously, the larger the difference between ZO and ZI, the larger the p, the larger the reflection from the port, and the harder it is to pass the EMI signal. Therefore, the filter input port should be mismatched with the output port of the power supply to reflect the EMI signal. Similarly, the filter output port should be in a mismatched state with the load, causing the EMI signal to reflect. That is, the filter should follow the following principles:

The source internal resistance is high impedance, then the filter input impedance should be low impedance, and vice versa.

If the load is high impedance, then the filter output impedance should be low impedance and vice versa.

For EMI signals, the inductor is high impedance and the capacitor is low impedance. Therefore, the termination of the power EMI filter with the source or load should follow the following principles:

If the source internal resistance or load is resistive or inductive, the filter interface terminated with it should be capacitive.

If the source internal resistance or load is capacitive, the filter interface terminated with it should be inductive.

2 EMI filter network structure

The EMI signal includes the common mode interference signal CM and the differential mode interference signal DM. The distribution of CM and DM is shown in Figure 1. It can be used to guide how to determine the network structure and parameters of the EMI filter.

[3]