The output power of long strings of photovoltaic (PV) modules are vulnerable to the effect of mismatching and partial shading among different level PV elements such as cells, sub-modules and modules. In this paper, a sub-module synchronous buck converter (SBC) with the distributed maximum power point tracking (DMPPT) control is presented in order to achieve optimal output power of the PV module, low system cost, and high efficiency even under partial shading conditions. Main shading patterns in a PV module are classified and their typical characteristics are illustrated. In order to improve the efficiency, a series-connected DC optimizer structure is implemented and a two-switch synchronous buck converter is delicately designed for each sub-module. A two-step perturb & observe based MPPT algorithm is adopted: firstly, a coarse tracking is implemented with large step size in order to improve the tracking speed and followed by a refined tracking process with a small step size with aim to minimizing the static oscillations. Furthermore, a bypass mode is triggered in order to maximize the system efficiency when no mismatch among sub-modules is detected. In the proposed sub-module DMPPT PV system, only the output voltage is sampled, which reduces the current sensor and simplifies the implementation difficulty. A PV system with the proposed sub-module DMPPT algorithm and SBC power interface was built in Matlab/Simulink. Main simulation results were provided for various shading patterns and working scenarios. A 100 W low-cost and high efficiency sub-module integrated synchronous buck converter was designed. The experimental measured efficiency of the SBC was high up to 98.7%. By comparing the actual power yield under shading conditions, an average 27.55% output power improvement was achieved with the proposed sub-module DMPPT algorithm. (C) 2016 Elsevier Ltd. All rights reserved.