The LLC stage design procedure outlined here follows the one given in the TI publication “Designing an LLC Resonant Half-Bridge Power Converter” which is available at
http://www.ti.com/lit/ml/slup263/slup263.pdf. The document contains a full explanation of the origin of each of the equations used. The equations given below are based on the First Harmonic Approximation (FHA) method commonly used to analyze the LLC topology. This method gives a good starting point for any design, but a final design requires an iterative approach combining the FHA results, circuit simulation and hardware testing. An alternative design approach is given in TI Application Note, LLC Design for UCC29950, Texas Instruments Literature Number SLUA733.