The MPPT DC/DC power stage performs the functions of translating the string voltage to a level suitable for the inverter (typically 400 V for single phase and 800 V for three phase) and Maximum Power Point Tracking (MPPT). The current trend is towards increasing this DC link voltage to 1000 V or beyond to reduce power losses in the system and to allow more panels to be added in series. In certain cases, it will have two power stages – a boost converter stage working as MPPT and an isolated DC/DC power stage to have safety isolation between the grid and the solar panels and to reduce the DC link voltage variation. However, in most cases, all these functions are performed by a single power stage – a non-isolated one in case there is no safety isolation requirement or an isolated one if safety isolation is required.

The boost converter is the preferred non-isolated topology in string inverters. It will be more efficient to maintain the DC link voltage higher than the highest voltage expected from the panel. A buck or buck-boost stage will be less efficient due to the higher current to be supported with a lower DC link voltage. To increase power level and to reduce ripple currents, interleaving of multiple boost power stages is done.

The isolated power stages used can be PWM-controlled full-bridge topologies (hard-switched, phase-shifted, or dual active) or resonant topologies like LLC, CLLLC, and so forth. The trend is towards soft-switched topologies that can accept relatively wider input voltage range. The dual active bridge with multiple phase controls is an example.