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

Large circulating current and reactive power exist for the LLC/CLLC converter in the case that f_s < f_r, where f_s is the switching frequency and f_r is the resonant frequency; on the other hand, in the scenario of f_s > f_r, the switching losses for both primary and secondary semiconductor devices are increased due to the large turn-OFF current and high di/dt related reverse recovery issue, respectively. When operating at the natural resonant frequency of inductor and capacitor or f_s = f_r, the regulation capability for LLC/CLLC converter 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.