Selective Secondary Phase-Shift Control for High Gain in LLC Converters with Matrix Transformers

Many applications such as datacenters and electric vehicles require well-regulated isolated DC-DC power converters. However, these often require high voltage gains for a wide input voltage range or when the input supply is lost. Although resonant converters, especially LLC converters, are great candidates for such dc-dc converters, their performance at the nominal operating conditions must be sacrificed to enable the converter to reach high gain levels with the conventional frequency modulation. Matrix transformers are commonly employed to distribute the high output currents evenly among the different elemental transformers. This paper proposes a control method to utilize matrix transformers to achieve high gains while operating at the resonant frequency. One or more secondary rectifiers (SRs) can be phase-shifted with respect to the other SRs to create periods where the transformer windings are partially shorted, resulting in a precise boost to the resonant tank energy, and hence, the resonant tank gain. This technique is implemented on a 400 V/12 V LLC converter for datacenters that requires high transformer gains during the hold-up period.