Transformer Design Conditions that Reduce the Output Ripple Current for Hybrid-Type DC-DC Converters

We applied a four-legged transformer that integrates a transformer and an output choke function on a hybrid-type dc-dc converter that alternately operates forward and flyback conversions and successfully reduced the footprint of the magnetic devices used in the converters. To further miniaturize hybrid-type dc-dc converters, it is necessary to reduce the output smoothing capacitors' footprint. In turn, reducing the size of output smoothing capacitors requires the ripple current that flows though them to be reduces. As a result of analyzing the magnetic circuit of four-legged transformers, we found that a transformer has an air-gap length that reduces the ripple current of the output smoothing capacitors to zero during forward operations. In addition, we found that the initial value of the ripple current during flyback operations matched the last value under this condition of air-gap length. In this paper, we discuss the design conditions for a four-legged transformer and the experiments we conducted to verify the effects of the design.