Analysis for High-frequency LLC Resonant Converter with Planar Transformer at Light-load Condition

A high-frequency LLC converter has been widely employed to achieve high power density. However, it suffers from output voltage regulation problem under light-load conditions, which is light-load regulation problem. In this paper, a new synchronous rectifier (SR) control method is presented to improve the light-load regulation capability of high-frequency LLC converter. The proposed control method applies the extended SR turn-on time and the output voltage is regulated by the negative current flowing through SRs. Based on the equivalent circuit modeling of the LLC converter, the voltage gain of LLC converter with proposed control method can be calculated. Therefore, by reducing the voltage gain by negative current, the output voltage can be regulated with fixed switching frequency. The validity of a proposed control method is confirmed by the prototype with 330-380 VDC input and 750W (12 V/62.5 A) output.