ZVS three-level phase-shift high step-down DC/DC converter with two transformers

A novel three-level phase-shift DC/DC converter with two series-connected transformers is proposed in this paper to meet the requirements for high step-down ratio and high input voltage applications. The three-level construction internally halves the voltage stress of the power switch compared with the traditional full-bridge topology, for which the low-voltage-rated MOSFETs are used to reduce the conduction losses. Also, the inclusion of the flying capacitor allows the phase-shift operation of this converter to achieve ZVS for all the primary power switches to improve the efficiency. Since the flying capacitor is in parallel with each of the input capacitor during the different operational modes of one switching cycle, the voltage imbalance of the input capacitors can be reduced. The two transformers in series both work as an inductor and a transformer alternately during different intervals of the period. As a result, not only the ZVS range of the lagging leg is extended but also the output inductor is omitted. The voltage-conversion-ratio is quartered by both the three-level structure and the two staggered transformers, which is suitable for high step down situations. The operational principles are presented in this paper. Besides, the state space functions and the design equations of the proposed converter are also derived. Finally a 240W/500V-24V prototype is built and tested to verify the effectiveness of this converter.