A general formulation is presented to derive the equation of motion and todemonstrate thermodynamic consistency for several classes of phase field modelsat once. It applies to models with a conserved phase field, describing eitheruniform or periodic stable states, and containing slow as well as fastthermodynamic variables. The approach is based on an entropy functionalformalism previously developed in the context of phase field models for uniformstates [P. Galenko and D. Jou, Phys. Rev. E {\bf 71}, 046125 (2005)] and thusallows to extend several properties of the latter to phase field models forperiodic states (phase field crystal models). In particular, it allows todemonstrate the concept of thermodynamic consistency for phase field crystalmodels with fast dynamics.
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