Design and Development of High Step-Up Single-Switch DC/DC Converters for Micro-inverter Applications

Abstract

Concerns about climate change have accelerated the shift towards renewable energy sources (RESs). Distributed energy systems, especially solar photovoltaic (PV), batteries and fuel cells (FC), are now key players in sustainable power generation. Hence, these sources require efficient high-step-up DC–DC converters to interface low voltage inputs with high-voltage DC buses. However, traditional converters such as boost and buck-boost require extreme duty cycles to achieve high voltage gains. For instance, achieving a gain ratio of 8 to 10 requires a duty cycle of 0.87 to 0.9. Under these operating conditions, the switch and diode currents increase approximately by a factor of (1/(1-δ)), which significantly increases conduction losses and growth in diode reverse-recovery losses due to higher diode current and switching stress. The novel architecture of the Quadratic Boost Converter (QBC) involves cascading two standard boost converters controlled by a single switch, allowing for an enhanced voltage gain ratio than the conventional boost converter (CBC). However, this design results in significant voltage stress on the switch equal to the output voltage of the converter. This limitation becomes pronounced at extremely high duty cycles, thereby constraining the achievable voltage gain ratio. To overcome these limitations, this thesis proposes and experimentally validates three novel non-isolated quadratic boost derived DC/DC converter topologies: QBĆUK, QBC-SEPIC and 〖3QBC〗_((SL/VL)), which systematically address the fundamental limitations of conventional QBCs. Rather than providing modifications of a single design, these designs are intentionally developed as a family of structurally distinct architectures, each optimised for a specific design objective: 1) QBĆUK converter prioritises switch voltage stress reduction while maintaining continuous input and output currents. 2) QBC-SEPIC converter provides dual-output capability and limits switch voltage stress to approximately half of the output voltage. 3) 〖3QBC〗_((SL/VL)) converter is designed to achieve ultra-high voltage gain without requiring extreme duty cycles or coupled inductors. All three converters offer continuous input current with low ripple making them suitable for solar maximum power point tracking (MPPT) applications. Meanwhile, all of them also employ a single switch and share a common ground which simplifies control and design.