Unit Gain Characteristic based Resonant frequency Tracking for DC Transformer Operation

Resonant frequency operation or DC transformer (DCX) operation is the optimal operation mode for LLC converter due to the reduction of both conduction loss and switching loss. However, due to the variation of circuit parameters, the switching frequency will deviate from the resonant frequency, which results in converter efficiency degradation. To achieve on-line automatic resonant frequency tracking, in this paper, by sensing the output voltage and input voltage, the conversion ratio is computed on-line. Based on the unit gain operation characteristic, the switching frequency is modified to track the resonant frequency. The proposed method can be applied to all LLC converter topologies and operating in both below frequency region and above frequency region. Comprehensive experimental results are presented.