Transformer winding losses are an important factor limiting the efficiency of LLC converter in high-ratio high-current applications. This paper proposes an optimal design scheme for fractional-turn transformers applicable to high-ratio high-current output LLC converters to reduce winding losses

considering the influence of the number of secondary turns and the dimensional parameters on the transformer losses

and designing the fractional-turn transformer with optimal efficiency. The voltage and current equalization characteristics of input-series output-parallel transformer structure is analyzed

and the method to realize the secondary current equalization is proposed. An LLC converter prototype with 400 V input

12 V output

rated power of 3 000 W is built

achieving a power density of 300 W/in3

a half-load efficiency of 97.7%

and a full-load efficiency of 96.9%. The half-load and full-load efficiencies are higher than those of integer-turn transformer with the same power density

validating the optimized design.