The direct minimization of the free energy is used to calculate the phase diagrams for microphase separation transition in two polyelectrolyte systems : (i) a mixture of weakly charged polyelectrolyte/neutral polymer; (ii) a poor solvent solution of weakly charged polyelectrolyte. The results obtained are valid throughout the phase diagram, and the use of the "weak segregation’ or ’strong segregation’ assumptions, which are common in the theory of block copolymers, is avoided. Only the microphase separation of lamellar type is considered. The final phase diagram a exhibit wide macroscopic phase separation regions, which is their main difference from the corresponding phase diagrams for block copolymer systems. The formation of microdomains is coupled with macroscopic phase separation : in most of the cases microdomain structure is formed in one of the coexisting phases after macroscopic phase separation takes place.