The state-space average model is extended to buck-boost, and buck-boost topology switching mode DC/DC converters and modified to have higher precision without increment of computation. The modified model is used in continuous conduction mode(CCM) switching DC/DC converters and some significant conclusions are derived. This paper discusses the discontinuous conduction mode(DCM) modeling of DC/DC converter and critical characteristic using average model of switch. Average model of switch approach is expended to the modeling of boundary conduction mode DC/DC converters that operate at the boundary between continuous conduction mode(CCM) and discontinuous conduction mode(DCM). Frequency responses predicted by the average model of switch are verified by simulation and experiment. A prototype featuring 15[V] input voltage, 24[V] output voltage, and 24[W] output power using MOSFET.