Non-isolated high step-up and high step-down DC-DC conversions are highly demanded in many applications, including renewable energy systems, uninterrupted power supplies, data centers, high-end computers, etc. The high voltage-conversion-ratio requirement poses challenges on energy efficiency, power density, cost and reliability. This dissertation explores new high step-up and high step-down topologies for various applications.
First, a new family of interleaved high step-up converters integrating coupled-inductor and switched-capacitor are introduced for high current and ultra-high step-up applications. By integrating coupled-inductor and switched-capacitor techniques, the proposed converter achieves ultra-high step-up voltage gain without the need of extreme duty cycle or high turns ratio. Also, very low switch voltage stress can be achieved, thus low-voltage-rating MOSFETs with small on-resistance can be used to lower the conduction loss. Moreover, thanks to the interleaved operation at the input side, the input current is shared and low input current ripple is obtained. Furthermore, the coupled-inductor leakage energy can be recycled, which helps alleviate reverse recovery problem.
Second, multiphase interleaved high step-up converter with diode-capacitor voltage multiplier stages is presented, which is an extension of the previously reported two-phase current-fed Cockcroft–Walton multiplier. The multiphase configuration has flexible structure. It achieves high current handling capability, high voltage gain, low component voltage stress, low input current ripple and automatic phase current balancing. A three-phase converter in the multiphase family was analyzed and evaluated in detail.
Third, a single-phase coupled-inductor boost converter is presented for low current high boost applications. The voltage gain can be extended by the coupled inductor. Due to a independent inductor at the input, continuous input current is obtained, which significantly reduces the input capacitor, leading to increased power density and reliability. Also, the new converter has low component count, leading to low cost. Furthermore, recycled leakage energy and alleviated diode reverse recovery can be achieved.
Fourth, an improvement is proposed based on the proposed single-phase coupled-inductor boost converter. The improved converter not only retains the advantages of high voltage gain and continuous input current, but also achieves low semiconductor voltage stresses, zero-voltage switching and zero DC magnetizing current, which helps increase efficiency and power density.
Extensions of the above high step-up converters to high step-down operation are also discussed in the dissertation. Additionally, a 48V-to-1V high step-down converter is presented, where high step-down conversion is realized by an energy transfer capacitor and built-in transformer. Due to the interleaved technique, small values of output inductance can be used, leading to fast transient response. Furthermore, the use of a coupled inductor not only reduces the number of cores but also reduces inductor current ripples, leading to lower losses. Four switches are used in the converter and all can achieve zero-voltage-switching. Grounded gate drivers and boost strap drivers can be employed, avoiding the use of complex isolated gate drivers. In addition, if one of the switches fails, the high input voltage is blocked from the output to keep the load safe.
