Input filter interaction with SMPS (10 of 13)

Current-Programmed Criteria

Y-Parameters. Using y parameters, both the analytical and graphical criteria for determining stability and degradation with the addition of an input filter have been worked out by Erich-and-Polivka, and by Kohut. Erich-and-Polivka use impedance for their graphical criteria and Kohut uses admittance.

Modified. The above work has probably been overtaken by the work of Jang and Erickson on current-programmed control. They show that if the original Middlebrook criteria is passed, then one more criteria associated with the feed-forward loop of the current-programmed controller must be applied. This criteria is important near the boundary between continuous and discontinuous modes of conduction and at high frequency.

Important! A key difference between voltage-programmed and current-programmed converters is that current-programmed converters may show no degradation in gain or phase margins on Bode plots of the voltage loop just before going unstable. Either the current-programmed criteria or application of the full Nyquist criterion to the total loop gain of the filter and converter combination must be used to assure stability.

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

Editor: Jerrold Foutz