Buck, boost and buck-boost converters are three basic switching DC-DC converters. Current mode controlled switching DC-DC converters have two boundaries in a wide circuit parameters variation range. Based on the ramp up and ramp down slopes of the inductor current before and after the turn on of power switch for switching DC-DC converters, a unified model of the current mode controlled switching DC-DC converters with ramp compensation is established in this paper. Only three parameters appear in this model after dimensionless normalization, from which the dynamical behaviors of switching DC-DC converters in continuous conduction mode (CCM) and discontinuous conduction mode (DCM) can be effectively illustrated. By utilizing the proposed unified model, two orbit state shifting borderline equations are derived, from which three operation state regions namely the stable period-one region, CCM robust chaos region, and DCM weak chaos and strong intermittence region, of switching DC-DC converters can be determined. The two-dimensional parameter bifurcation diagrams of switching DC-DC converters and circuit experimental observations of current mode controlled buck converter verify the analysis results of the partitioning of operation state regions by two borderline equations.