DCDC converters are widely used in different fields of applications to interface between DC voltage buses. A common structure of such converters is based on a resonant topology in which the working frequency of the converter should be close to the concerning resonance frequency. Therefore, a control scheme with a suitable model to design a controller is needed. Within this manuscript a nonlinear model of a CLLC DCDC converter is derived by means of the extended describing function method and a dq decomposition. Afterwards, this model is linearized to allow for a small signal analysis of the system. Both models are compared to a simulation of the DCDC converter in MATLAB Simscape and show good agreement.