As the world continues to examine its energy consumption with strict scrutiny, the demand for higher power conversion efficiency is ever growing. Regulatory standards concerning AC/DC converter efficiency have progressively increased mandatory minimum efficiency requirements with the latest European standard, Ecodesign 2019/1782, released in April of 2020. A topology gaining popularity for applications needing >100W is the half bridge LLC resonant converter due to its ability to achieve efficiency when operating close to the resonant frequency. While the LLC design process is straightforward for a fixed output voltage, applications such as LED lighting and power tool chargers require operation over a wide output voltage range, making the power stage optimization more challenging. This session will examine the key loss mechanisms that occur in the LLC topology when operating above and below the resonant frequency. Key tradeoffs when selecting the LLC power stage gain curve will be introduced. This session will also cover practical implementations of achieving a variable output voltage.