This paper aims to address a new challenge associated with parallel operation of a large number of solar power optimizers in dc power grids. It is first shown that as the number of solar power optimizers increases the stability margin of the host dc power grid decreases. Then, to circumvent the stability problem, a novel active damping approach is presented which can improve the stability of dc grids dominated by a large number of solar power optimizers. In this approach, an inner feedback loop is added to the control system of a voltage source dc-dc converter regulating the dc grid voltage. The feedback transfer function is properly tuned to achieve the highest stability margin of dc grid. The presented approach works robustly for any number of solar power optimizers. Different simulation results are provided to confirm the effectiveness of the proposed approach. (C) 2017 Elsevier Ltd. All rights reserved.