A High Efficiency and High Power Density Integrated Two-Stage DC-DC Converter Based on Bipolar Symmetric Phase Shift Modulation Strategy

The integrated two-stage buck-boost-LLC isolated dc-dc converter is suitable for wide-voltage regulation range, highfrequency and high-efficiency application. However, the choke inductor current of buck-boost stage injecting into LLC stage will cause nonsinusoidal resonant current and asymmetric secondary current under the conventional modulation strategy. This will result in synchronous rectification problems. Traditionally, a large resonant inductor is utilized to improve the situation, but it sacrifices power density and efficiency. Therefore, this article proposes a bipolar symmetric phase shift modulation strategy to reduce the resonant inductance and correct resonant current with high power density and high efficiency. Moreover, LLC stage is improved. The RMS current of primary switches is decreased by the entrance of the choke inductor current. Zero-voltage-switching is achieved by choke inductor current instead of magnetizing current, so that the transformer can be designed without air gap and has lower winding loss caused by leakage flux. With minimum magnetizing current, the resonant current is minimized. The conduction loss of primary switches and winding loss of transformer shows 75% and 26% reduction, respectively. Finally, a 500 kHz 400-W prototype with 97.6% peak efficiency and 142W/in3 power density is built up. The experimental results verify that the proposed modulation strategy corrects resonant current effectively and improves efficiency and power density.