Input filter interaction with SMPS (12 of 13)

Input Filter Interaction - Summary

Negative R. Switching-mode power supplies have an incremental negative input resistance. Adding an LC filter on their input can cause them to go unstable or suffer performance degradation. The problem and solution has been discussed in the current literature starting in 1971. Bode plots of the voltage-gain loop may give no warning of the impending oscillations in current-programmed control, and applying the full Nyquist criterion to the total loop may be necessary.

Voltage-Mode. A fairly simple criteria to insure stability was worked out in 1976 by R. D. Middlebrook and is generally considered correct for duty-ratio programmed converters in the continuous-current mode. It was found not to be appropriate for current-programmed control.

Current-Mode. After initial work by Erich-and-Polivka, and by Kohut, Jang and Erickson worked out an addition to the Middlebrook Criteria in 1991 for current-programmed converters.

Cascaded Converters Criteria for cascaded converters with intermediate filters has been developed by Choi and Cho and by Florez-Lizarraga and Witulski.

Bottom Line. Start with the Jang and Erickson paper and refer to other referenced papers as required to improve understanding or to apply to multiple converters on the same bus.

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

Editor: Jerrold Foutz