Derivation and Analysis of a Secondary-Side LLC Resonant Converter for the High Step-Up Applications

An isolated high step-up converter, which is derived from the reverse structure of a regular LLC resonant converter, is proposed in this article. The proposed converter consists of an inverter stage, a secondary-side resonant tank, and an active voltage-doubler rectifier. An additional input diode is introduced into the regular push–pull inverter so as to make the magnetizing inductance free from being always clamped by the input voltage and make up new voltage gain features. The leakage inductance, which always shows a significant value when reflected to the secondary side in a high step-up transformer, can be used to make up the resonant tank. Under a simple pulse-frequency-modulation control with 0.5 duty ratios for both the primary-side and secondary-side switches, a high step-up function can be easily realized by the converter with good load regulation ability. Meanwhile, synchronous rectification (SR) can be naturally achieved by the secondary-side switches. The additional input diode can be replaced by a MOSFET with an analog SR chip to further improve the efficiency, without changing the original control scheme. The validity of the study is confirmed by a 500-W prototype with 36–42 V input and 380-V output. The peak efficiency can reach 96.9%.