Bifurcation set cross-section method is proposed which allows to construct all the possible, at least, two-dimensional phase diagrams of models described by multiparametric thermodynamic potentials. The method is applied to investigation of the de Gennes model of smectic A mesophase in external field. Among other results, the most important one is that all the critical (end critical, tri-critical etc.) points of phase diagrams are peculiar points of the corresponding bifurcation sets.