A New Way to Model Current-Mode Control, Part One

Focus:

This two-part article series introduces the author’s unified model for fixed-frequency continuous-conduction-mode current-mode control—a model that will help to explain operation of and enable design analysis of any of the peak- or valley-related forms of current-mode control. But here in part 1, the author is providing the background needed to understand the new model, which is introduced in part two. The explanations presented here are more intuitive than mathematical. The article begins by identifying three factors important to understanding current-mode control: the dependency of the ideal current-mode converter on average inductor current; the dependency of modulator gain on the effective slope of the ramp on the input to the modulator’s comparator; and slope compensation. Then there is an explanation of current-mode control that begins with a description of voltage-mode control and then how the addition of the current loop changes the converter’s operation. Next, the problem of subharmonic oscillation and the use of slope compensation (addition of a ramp to the current-sense signal) to prevent the oscillation are explained. Finally, there is discussion of slope compensation criteria including equations for the up slope and down slope of the ramp in buck, boost and buck-boost regulators.

What you’ll learn:

• How to understand current-mode control intuitively with an eye toward applying generalized models to the analysis of different current-mode control architectures
• How to understand essential relationships in current-mode-controlled converters
• How to understand the role of slope compensation and how it’s applied in current-mode-controlled converters

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Author & Publication:

Robert Sheehan, Principal Applications Engineer, National Semiconductor, Santa Clara, Calif., Power Electronics Technology, May 01 2007

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