Design and Development of High Voltage Gain and High Efficiency DC-DC Power Converters with Reduced Voltage Stress

High voltage conversion gain DC-DC power converters are essential for many applications, such as power supply, renewable energy generation, DC nanogrids, energy storage systems (ESSs), electric vehicles (EVs), and a myriad of industrial applications. A high step-up/down power conversion presents several design challenges to realise the high-voltage gain, high efficiency, soft-switching, low component count, and to reduce voltage stress across devices for a wide range of operation. The conventional non-isolated converters, such as boost and buck-boost, are unable to achieve the above requirement. The isolated topologies typically employ a high-frequency transformer to realise a high conversion ratio of voltage. The voltage gain primarily depends on the turns ratio of the transformer, which becomes challenging when considering associated cost, size, power losses, and leakage inductance.

Several topologies were proposed in the past to achieve the high-voltage conversion ratio. However, they generally cannot maintain low voltage stress for a wide range of operation, resulting in power losses, escalating cost, and low power density. The lack of scalability and modularity in structures is another shortcoming that limits their wide application. Furthermore, the realisation of soft-switching generally requires high component count and complexity.

This dissertation proposes and investigates new topologies for high conversion gain DC-DC power converters to overcome the limitations present in past topologies and techniques. The proposed topologies optimally integrate switched capacitor and coupled inductor techniques to extract the benefits of both in attaining improved performance. The coupled inductor technique primarily supports the achievement of high conversion gain, while the switched capacitor technique facilitates the reduction of voltage stress on devices. The integrated clamp circuits with switched capacitor and coupled inductor techniques clamp and recycle the leakage energy, suppress the voltage spike and destructive ringing across the switching node, and further minimise the voltage stress. Thus, the proposed converters can effectively achieve a high voltage gain and realise low and steady voltage stress on the semiconductor devices for the entire duty cycle operation. This feature is particularly essential to realising high efficiency and potentially high power density converters. Therefore, a low voltage rating and on-state resistance switch can be utilised to improve the conversion efficiency. Scalability is another design feature, with the possibility to extend the structure controlled by a single switch by adding more cells to boost the voltage gain further. Furthermore, the concept of soft switching with a single switch is realised in one proposed high step-up converter. The operation principle, steady-state analyses, power loss, efficiency analysis, component selection, and performance comparison with state of the art are presented in detail for each converter.

The performance expectation of the proposed converters is substantiated by developing and testing 200–300 W hardware prototype circuits for 30 V to 380 V conversion in the laboratory. The proposed solution demonstrated the highest efficiency of 96.72% and the EU efficiency of 96.32%, and the switch voltage stress is significantly curtailed to 1/6th of the output voltage.