High power density high efficiency wide input voltage range LLC resonant converter utilizing E-mode GaN switches

The LLC resonant power supply topology shows great promise for meeting the demands of increased power density while maintaining high efficiency. But there is a tradeoff between maximizing efficiency and accommodating regulation over a wide input voltage range. The most efficient designs have narrow input voltage range, therefore requiring a large DC bus capacitor to support the required holdup time, thus impeding the goal of improved density. This paper presents a design approach which enables optimization of a modified LLC resonant converter for highest efficiency, while simultaneously extending the input regulation range from 340V-400V to 280V-400V by adding a small capacitor in series with the magnetizing inductance. This so-called `LCLC' topology can then use a 2X smaller DC bus cap and still maintain the same holdup time. The primary side is driven with a full bridge to reduce ripple-current on the smaller DC bus cap. A 3 kW high-frequency, high-density example is shown using a GaN full-bridge on the input side to achieve 98.4% peak efficiency while operating at 350 kHz. Experimental results compare the LCLC to a conventional LLC under identical conditions and demonstrate the wide operating range and high-density while maintaining the same high efficiency over the normal operating range.