Modeling of V2 Current-Mode Control

Recently, the V² type of constant on-time control has been widely used to improve light-load efficiency. In V² implementation, the nonlinear PWM modulator is much more complicated than usual, since not only is the inductor current information fed back to the modulator, but the capacitor voltage ripple information is also fed back to the modulator. Generally speaking, there is no sub-harmonic oscillation in constant on-time control. However, the delay due to the capacitor ripple results in sub-harmonic oscillation in V² constant on-time control. So far, there has been no accurate model to predict instability issue due to the capacitor ripple. This paper presents a new modeling approach for V² constant on-time control. The power stage, the switches and the PWM modulator are treated as a single entity and modeled based the describing function method. The model for the V² constant on-time control achieved by the new approach can accurately predict sub-harmonic oscillation. Two solutions are discussed to solve the instability issue. The extension of the model to other types of V² current-mode control is also shown in the paper. Simulation and experimental results are used to verify the proposed model.