Undershoot-Less Open-Loop Soft-Start Strategy for Digital-Controlled Power Converters Based on an Error-ADC and Initial Duty Ratio Estimator

Abstract

In digital control, an error-analog-to-digital converter (EADC) is used to increase the effective resolution of analog-to-digital converter and achieve stability. However, digital control with EADC involves challenges in achieving soft-start without an output voltage undershoot or using a complex circuit, which increases the circuit cost and power loss. To overcome the aforementioned problems, this brief proposes an open-loop soft-start strategy without undershoot for a digital-controlled power converter based on an EADC. The proposed strategy does not require a digital-to-analog converter and accurate wide-range analog front-end circuit. A duty ratio estimator and design considerations were established to achieve a seamless open-loop/closed-loop transition for avoiding a large output voltage undershoot. The proposed control can be applied to various types of power converters. It was verified through simulations with a buck converter. Finally, a buck converter experiment was performed to verify the proposed strategy, in which the output voltage was within a ${V} _{{o}}$ dynamic range of ± 5% after start-up.