Study on design technique for multiple resonant LLC converter with parallel-input and series-output

Abstract

Recently, a type of railway propulsion system based on an energy-efficient hydrogen fuel cell has attracted attention. Because it is difficult to use hydrogen fuel cells to supply high DC voltage to inverter to drive the traction motor, due to the very high cost, a step-up DC/DC converter is required between a low DC voltage hydrogen fuel cell and the inverter. A resonant LLC converter, such a DC/DC converter, capable of achieving zero voltage/zero current switching can be advantageous in terms of efficiency and volume; however, a single-stage resonant LLC converter has a very large Q factor under low turns-ratio conditions. It is impossible to design with optimized efficiency and volume due to the above area operation. Therefore, in this paper, a new structure of a resonant LLC converter is proposed with specific features via which conduction loss is reduced by connecting the inputs in parallel, by connecting the outputs in series, by selecting a diode with a low rated voltage, and by increasing the turns-ratio to enable resonance and operation below the input voltage range, resulting in optimized efficiency and volume. In addition, for the proposed converter, an optimized design is derived by comparing the efficiency and volume according to the number of modules. ? 2020 The Korean Society for Railway. All rights reserved.