Input filter interaction with SMPS (6 of 13)

Explanation of Middlebrook Criteria Plot

Assumptions. The circuit is the canonical model of a buck, boost, or buck-boost converter operating in the continuous current mode with duty-ratio (voltage-mode) control. The input filter is a damped LC filter.

Input Impedance. The heavy blue line is the criterion used to test for stability. Ro, Co, and Lo are the equivalent load resistance, filter capacitor, and inductor reflected through the regulator (these are not necessarily the physical values of these components, but are those derived in the canonical models). The low frequency input impedance is Ro, which then breaks with the output capacitor, Co, which then breaks with the output inductor, Lo. Ro then takes control again. Near the LC break point, the impedance is modified by the damping of the output filter.

Short Circuit. The light blue line is the reflected input impedance of the output filter with the load shorted. Rd is the reflected series damping resistance of the output filter.

Input Filter. The heavy red line is the output impedance of the input filter. Ls and Cs are the inductor and capacitor impedance. The LC peaking is controlled by filter damping.

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

Editor: Jerrold Foutz