A New Way to Model Current-Mode Control

May 1, 2007 12:00 PM
By Robert Sheehan, Principal Applications Engineer, National Semiconductor, Santa Clara, Calif.

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When it comes to understanding current-mode control, one thing becomes painfully obvious: nine out of ten experts do not agree. For fixed-frequency operation, the vast majority of theory and modeling has focused on the classic peak current-mode method with a fixed-slope compensating ramp. Some theory has been developed for average current-mode control but little exists for other methods.

Of the newer architectures developed, emulated peak current-mode control solves the problem of large stepdown ratios (high input voltage to low output voltage) while maintaining good noise immunity. While the classic peak current-mode theory can be used for design analysis with reasonable results, it doesn't explain all aspects of current-mode operation. A fresh approach to modeling fixed-frequency continuous conduction-mode, current-mode control provides the solution for any peak- or valley-derived architecture, including the emulated method.

In the first part of this two-part article, the basic operation of current-mode control is broken down into component parts, allowing a greater intuitive understanding for the practical designer. A comparison of the modulator gain is made with voltage-mode operation. A simple analogy allows the optimal slope-compensation requirement to be met without any complicated equations.

In the second part of this article, which will appear in the June 2007 issue, a unified model using general gain parameters is developed, along with simplified design equations. An in-depth treatment of the analysis and theory is presented for the advanced reader. This general modeling technique explains how previous models can coexist and complement each other on various aspects of the current-mode control theory.

Current-Mode Control Fundamentals

There are a great deal of misconceptions and misinformation about current-mode control within the power electronics industry. Papers on the topic that have been written at the graduate or Ph.D. level are hard to understand, and many of the concepts introduced are difficult to put into practical use. This article aims to demystify current-mode control, and cut through the myths and misconceptions of its operation.

For current-mode control, there are three factors to consider. First, an ideal current-mode converter is only dependent on the dc or average inductor current. The inner current loop turns the inductor into a voltage-controlled current source, effectively removing the inductor from the outer voltage control loop at dc and low frequency.

The second factor to consider is modulator gain, which is dependent on the effective slope of the ramp presented to the modulating comparator input. Each operating mode will have a unique characteristic equation for the modulator gain.

The third consideration is slope compensation. The requirement for slope compensation is dependent on the relationship of the average current to the value of current at the time the sample is taken. For fixed-frequency operation, if the sampled current were equal to the average current, there would be no requirement for slope compensation.

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