A Ripple-Based Constant On-Time Control With Virtual Inductor Current and Offset Cancellation for DC Power Converters

In recent years, there has been a growing trend of mandating high-power conversion efficiency, for not only the heavy-load but also the light-load conditions. To achieve this purpose, a ripple-based constant on-time (RBCOT) control for dc-dc converters has received wide attentions because of its natural characteristic of switching frequency reduction under the light-load condition. However, a RBCOT control suffers from an output-voltage offset problem and a subharmonic instability problem. In this paper, a modified RBCOT buck converter circuit is proposed to solve both problems. The circuit uses the concept of virtual inductor current to stabilize the feedback, and an offset-cancellation circuit to eliminate the output dc offset. The modified circuit can be fabricated into an integrated circuit (IC) without adding any pin compared to conventional circuits. A control model based on describing function is developed for the modified converter. The small-signal characteristics and design criteria to meet stability are derived. From the model, it is also found out that it is much easier to accomplish adaptive voltage positioning using the proposed modified RBCOT scheme compared to a conventional constant-frequency controller. Simulation and experimental results are given to verify the proposed scheme.

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