Input filter interaction with SMPS (8 of 13)

Practical Placement of Input Filter

Placement. P5 is the most practical placement of the input filter.

Input L. In this location, added input inductance from a system EMI filter or long leads to the power source will not cause the power supply to go unstable since Ls will migrate to the left, which is safe, and will not increase the Q.

Output C. However, added output capacitance may cause Co to migrate causing instability. The input filter capacitance Cs should be greater than the reflected output capacitance Co, including any added load capacitance.

Degradation. Penetrating the Rd line will degrade regulator output impedance. This is often acceptable if the output impedance degradation is less than the maximum regulator output impedance. This maximum usually occurs near where the loop gain passes through unity.

Damping. A well-damped input filter simplifies the design and there are other compelling reasons to damp the input filter.

Do not use this information for design without independent verification of the information.

Editor: Jerrold Foutz