Topologies and Control Techniques for Efficiency Improvements in Non-Isolated DC/DC Converters for Stand-Alone PV-Battery Powered Pump Systems

Abstract

This thesis is focused on the stand-alone photovoltaic (PV)-battery powered pump system. The converters for this system are responsible for Maximum power point tracking (MPPT), battery charging and load regulation. This thesis involves different topologies of non-isolated cascaded two-stage and integrated single-stage converters used in the system. The energy conversion efficiency, power loss modelling and control of these topologies are investigated in this work. With regard to the cascaded two-stage dc/dc converters, this thesis demonstrates the relationship between the overall conversion efficiency and the dc bus capacitor ripple current. The ripple current can be reduced by controlling the phase-shift of duty cycles of the active switches. By doing this, the efficiency can be improved and the capacitor lifetime can be prolonged. The analysis is verified experimentally. As for integrated single-stage converters, this thesis intends to investigate the integrations of the conventional cascaded two-stage converters. A family of non-isolated dc/dc single-switch topologies are derived. Particularly, a single-switch buck cascaded buck-boost converter is proposed for the system. The integration of the two stages results in reduced repeated power processing. A control scheme using pulse-frequency modulation (PFM) and pulse-width modulation (PWM) is adopted in the proposed converter. A laboratory prototype converter confirms the proposed design. An integrated single-stage boost full-bridge converter is also proposed for this system, which could perform MPPT and battery charging with the pump load stopped. Direct power transfer is achieved during operation. A PWM with PFM control technique is introduced to this system with extra phase-shift synchronization technique to optimize the battery and bus capacitor current.