As residential photovoltaic (PV) systems become more and more common and widespread, their system architectures are being developed to maximize power extraction while keeping the cost of associated electronics to a minimum. An architecture that has become popular in recent years is the "DC optimizer" architecture, wherein one DC-DC converter is connected to the output of each PV module. The DC optimizer architecture has the advantage of performing maximum power-point tracking (MPPT) at the module level, without the high cost of using an inverter on each module (the "microinverter" architecture). This work details the design of a proposed DC optimizer. The design incorporates a series-input parallel-output topology to implement MPPT at the sub-module level. This topology has some advantages over the more common series-output DC optimizer, including relaxed requirements for the system's inverter. An autonomous control scheme is proposed for the series-connected converters, so that no external control signals are needed for the system to operate, other than sunlight. The DC optimizer in this work is designed with an emphasis on efficiency, and to that end it uses GaN FETs and an active clamp technique to reduce switching and conduction losses. As with any parallel-output converter, phase interleaving is essential to minimize output RMS current losses. This work proposes a novel phase-locked loop (PLL) technique to achieve interleaving among the series-input converters.