An Adjustable Turns Ratio Transformer Based LLC Converter for Deeply-depleted PEV Charging Applications

The charging of deeply-depleted plug-in electric vehicle (PEV) requires the dc/dc converter to have an ultra-wide output voltage range. However, the conventional LLC converter is unsuitable for such applications. To address this issue, a novel LLC converter with an adjustable turns ratio transformer is proposed. The transformer has three different effective turns ratios, which correspond to different output voltage ranges. Thus, three different operation modes are derived and the switching frequency range in each mode can be squeezed close to the resonant frequency. Therefore, the loss of the converter is reduced and the efficiency is improved. The operational principles, design considerations, and mode transition are analyzed. A 1 kW prototype that converts 390V input into a 126-420 V wide output is designed and tested to validate the concept. A smooth mode transition is achieved without transient issues.