A bi-directional resonant converter with automatic resonant frequency tracking for DC transformer operation

DC transformer plays an importance role in the DC microgrid applications, where the high frequency transformer is used to replace the traditional line frequency bulky transformer. Due to its high efficiency operation characteristic, the LLC resonant converter has been widely adopted in DC microgrid to operation as a DC transformer. For DC transformer operation, the LLC converter is operating at resonant frequency to achieve high efficiency. However, due to the circuit components variations, the LLC converter resonant frequency is varying with converter operation conditions. In addition, the aging effect will also make the components value variable. To ensure the resonant frequency operation for LLC converter, in this article, a simple yet effective resonant frequency tracking technique based on the unity gain characteristic is proposed for bi-directional LLC converter. The converter switching frequency is adjusted simply based on the comparison result between the measured voltage gain and unity.