In the paper, a control-affine model for the active dual-bridge series resonant converter is presented. The model has square form and produces cycle averaged output current and two parameters of waveform alignment for the controllable achievement of ZVS and synchronous rectification. The output characteristics of the converter are obtained by using the first harmonic approximation of input voltage applied to the resonant tank. The maximum prediction error of this model is about 10% in comparison to the measured data. This model is suitable for both voltage buck and boost operations, and also demonstrates fundamental control properties of the converter namely high nonlinearity respect to the control inputs, a singularity of output current surface between buck and boost modes, and holonomic constraints for control variables.