High Efficiency Wide Input Voltage Range LCLC Resonant Converter Using Nonlinear Frequency Controller

In server and telecommunication applications, LLC resonant converter featuring high efficiency and high power density becomes an excellent candidate for the frontend DC/DC stage. But, for conventional LLC converter, the performance will be severely deteriorated once the converter is designed for accommodating regulation over wide input voltage range. A modified LLC topology with changeable magnetizing inductance, LCLC resonant converter, has been successfully proved to extend the input voltage range without sacrificing the normal efficiency. Moreover, due to the strong nonlinear feature of the LCLC resonant converter, conventional linear control strategies such as PI controller are very hard to optimize. This paper proposes a nonlinear frequency control method for LCLC converter to achieve better output voltage regulation over wide input voltage range. In the controller, the switching frequency is calculated from a quadratic function, which expresses the opposite variation of the voltage gain. Finally, a 250 -400V input 12V/500W output LCLC prototype is built and tested. Experimental results demonstrate the effectiveness of the proposed nonlinear frequency control strategy.