Input filter interaction with SMPS (7 of 13)

Placement of Input Filter

No Problem in the Old Days. When 20kHz switching-mode power supplies began to be used in the mid 1960's usually the only EMI specification invoked was for conducted emissions above 150kHz. This placed the input filter out to the right of location P1, beyond the negative input resistance frequency of the power supply, and adding a filter never caused instability.

Now a Problem. Later when MIL-STD-461 limits CE01 and CE03 were invoked and filters had to meet emission requirements down to 30Hz, the added filter often resonated at location P3, the absolute worst placement. At first, filters where often place at location P2, but to get sufficient attenuation at the switching frequency forced it close to Lo and adding inductance (from long input lines or added system filters) caused Ls to migrate left into the region of instability.

Placement. Placing the input filter resonance at P4 insures stability and no degradation, but may result in a filter larger than the regulator. P5 is usually the most practical placement.

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

Editor: Jerrold Foutz