Difference series resonant and LLC DC/DC converter
Small magnetizing inductance + extra inductor = LC; 

Large magnetizing inductance + extra inductor = LLC; 

Large magnetizing inductance (air gap) + large leakage inductance (coils are separated) + no extra inductor = LLC

The best option is to use a transformer that is deliberately built as bad transformer, with windings separated a part, and with air gap. Then you get two extra inductances L_leak + Lm.


A quality built transformer for SMPS has low leakage, this is done by doing stacked partial windinigs: primary, secodary, primary, secondary,... What you get is low magenetizing, low leakage tranasformer. Adding an extra tank inductor Lr you get LC topology, adding an air gap due to saturation you get LLC.