1 Introduction LLC topology has been used sparingly in high-end applications that required the very highest efficiency possible. However， with industry demanding increased efficiency in more mainstream products and organizations such as Energy Star and 80 Plus posting ever higher “recommendations” for efficiency at any load-level， LLC topology is proliferating throughout product categories and power levels. While this topology handles load variarations extremely well， it requires substantial upfront effort to configure a design to handle line variations – this must be fully considered in the resonant tank design， and a wider input range design will have some specific trade-offs. Use LLC in off-line SMPS with a pre-regulated bus （PFC） when efficiency and power density are high priorities. This design note aims to detail the operation of the LLC converter in its three different modes as well as provide guidance on how to properly design an LLC stage. A detailed example is provided with component selection and worst-case loss calculations for all semiconductor components. 2 LLC Converter Topoology The LLC converter is part of the resonant converter family， which means regulation is not like conventional Pulse Width Modulation schemes. Running at 50% Duty cycle and fixed 180 degree phase shift， regulation is obtained through frequency modulation. All primary side MOSFETs turn on resonantly – Zero Voltage Switching – resulting in full recycling of the energy contained in the MOSFETs’ parasitic output capacitance. Furthermore， all secondary side switches turn off resonantly – Zero Current Switching – to minimize switching losses normally associated with hard switching. Resonant operation of all switching devices in the LLC converter results in minimized dynamic loss and thus increased overall efficiency; particularly at higher operating frequencies in the hundreds of kilo Hertz to Mega Hertz range.