A Novel Isolated DC–DC Converter for High Step-Down Applications

This paper proposes a new isolated high step-down DC–DC converter based on the phase-shift full-bridge converter. The proposed converter has some properties very suitable for high step-down applications including a high step-down ratio, low transformer turns ratio, low voltage stress on power switches, wide soft-switching range, and no saturation problem. These attractive features are achieved by a simple pulse-width modulation control method and a unique split-capacitor structure on the input side. By split-capacitor structure, both switching voltage and stress voltage of the power switches are decreased significantly. As a result, the required ZVS energy is reduced greatly, which means that an extended ZVS range can be obtained. This structure also leads to an inherent high step-down ratio, low transformer turns ratio, and de-saturation feature because the primary transformer voltage is decreased to a quarter of the input voltage and there are capacitors in series with the primary winding in power modes. In this paper, the operation principle is presented; the ZVS range and de-saturation feature are discussed in detail. In addition, a comprehensive comparison with some new isolated high step-down converters in the literature is also completed. Finally, a simulation verification using the LTspice simulation program and an 800 W, 400 V input, and 48 V output prototype are developed and tested to verify the salient features of the proposed converter.