A Resonant Frequency Tracking Technique for LLC Converter-Based DC Transformers

This is the authors' abstract. We don't add key points for this paper.

Large circulating current and reactive power exist for theLLC/CLLCconverter in the case thatfsfr, wherefsis the switching frequency andfris the resonant frequency; on the other hand, in the scenario offs>fr, the switching losses for both primary and secondary semiconductor devices are increased due to the large turn-offcurrent and highdi/dtrelated reverse recovery issue, respectively. When operating at the natural resonant frequency of inductor and capacitor orfs=fr, the regulation capability forLLC/CLLCconverter is lost since the voltage conversion ratio is constant. This application is also known as dc transformer (DCX), which has been widely adopted in energy storage systems as interconnection between different units. However, due to the variation of circuit parameters, the switching frequency is different from the resonant frequency, which results in converter efficiency degradation and loss of output voltage regulation capability for two-stage applications. In this article, by sensing the output voltage and input voltage, the converter voltage conversion ratio is computed on-line. Based on the comparison result between the calculated gain and unity, the switching frequency is adjusted accordingly and the resonant frequency tracking is achieved. In addition, multiple frequency adjustment step strategy is proposed to improve the tracking speed. Various scenarios are emulated in the experiment part to examine the effectiveness of the solution.