Input filter interaction with SMPS (5 of 13)

Graphical Techniques

Middlebrook Criterion The Middlebrook Criterion is a graphical method for determining if the input filter of a switching mode power supply will cause instability or degrade performance parameters of a duty-ratio (voltage) programmed dc-to-dc converter switching-mode power supply. As usually applied, the output impedance of the input filter is overlaid on the open-loop input impedance of the switching-mode power supply at the worse-case conditions of low-line and full-load and low-line with shorted output.

This graph is a simplification for this discussion only and does not contain any criteria on the current-programmed case. If possible, it is recommended that the reader view the curves in Jang and Erickson (both voltage-programmed and current-programmed controllers) or one of the Middlebrook papers (voltage-programmed controllers) while reading this.

Instability. If the output impedance of the filter is greater than the open-loop input impedance of the power supply at any frequency, sustained oscillations may be possible at that frequency and further analysis is necessary.

Degradation. If the output impedance of the input filter is greater than the short-circuit open-loop input impedance of the power supply, performance degradation, especially in output impedance, is possible.

More Information. More information is found in the Timeline of key papers, especially the Middlebrook '76 and Middlebrook '78 papers, and the Jang and Erickson paper.

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

Editor: Jerrold Foutz