High performance DC-DC converters for isolated low voltage PV modules

The demand for increased renewable energy production has led to increased photovoltaic installations worldwide. While the cost of PV modules has reduced strongly, the Balance of System (BoS) costs, notably the costs of the power conditioning system (PCS), have not had a corresponding decrease. Module integrated converter (MIC) technology provides an alternative approach to PCS design. This thesis examines the optimisation of DC-DC converter topologies for PCS designs with an intermediate DC bus. From among various topologies, the LLC resonant converter is chosen. Photovoltaic module power conditioning requires wide operating voltage ranges. The operating range can be extended for the LLC topology by combining variable frequency control and phase shift control. Analytical solutions exist for the converter operating with variable frequency control but these have not been fully developed for phase shift control. This thesis provides a detailed analytical solution for the modes of the phase shifted LLC resonant converter. This thesis presents a loss optimization method for the phase shifted LLC converter that can be applied to a single load condition or to multiple loading condition using weighted efficiency optimisation. A cost function is developed that incorporates a multiple mode solver for the phase shifted LLC state equations to provide a comprehensive solution. This thesis also verifies the applicability of the optimization procedure. This thesis explores the integration of two control methods, variable frequency control and fixed frequency phase shifting control for the effective operation of LLC resonant converter over an extended voltage range.