New Modeling Approach for Current-Mode Control

Constant on-time current-mode control has been widely used to improve light-load efficiency, because it can reduce the switching frequency to save switching-related loss. Therefore, an accurate model for constant on-time control is indispensable to system design. However, available models for constant on-time control are unable to provide accurate physical insight and predict the system response, especially the possible sub-harmonic oscillation in the V² type implementation. This paper introduces a new modeling approach for constant on-time control. The inductor, the switches and the PWM modulator are treated as a single entity and modeled based on the describing function method. The fundamental difference between constant on-time control and peak current-mode control is analyzed using the proposed model. The new modeling approach can be extended to other current-mode controls as well. A simple equivalent circuit representation is proposed for easy understanding of current-mode control. Simulation and experimental results are used to verify the proposed model.