A coupled-inductor multi-level ladder converter for sub-module PV power management

The viability of solar photovoltaic energy has increased in recent years due to continuing efficiency improvements and cost reductions. However, there remains a need for improvements in power electronic circuits and architectures, especially to deal with sources of mismatch loss in real-world environments. This work presents a circuit implementation and multi-objective control scheme for a four-level DC-DC converter that provides sub-module energy optimization for photovoltaic systems. The work builds on past approaches using switched-inductor (SL) topologies that manage power flow in parallel with series-connected PV strings. We describe the use of coupled-magnetics to reduce current ripple and improve efficiency compared to past SL approaches. The converter works by enforcing voltage ratios among adjacent PV sub-strings, allowing independent sub-module maximum power point tracking (MPPT). A state-space model of the switched-inductor topology is presented to provide a foundation for a PI control scheme. Circuit simulations are compared to measurement results for a four-stage prototype integrated in the junction-box of a 245 Wp PV module.