The Energy Transfer Mode (ETM) in a Boost dc-dc converter is classified into two types, i.e., the Complete Inductor Supply Mode (CISM) and the Incomplete Inductor Supply Mode (IISM). The critical inductance and boundary condition of CISM and IISM are derived in the paper. Based on the classification, three operating modes are defined, and they are CISM, IISM in Continuous Conduction Mode (IISM-CCM) and IISM in Discontinuous Conduction Mode (IISM-DCM). Output voltage ripple (OVR) in three modes is analyzed, respectively. As for a Boost converter with certain load, output capacitance and switching frequency, the OVR in CISM is the lowest and independent of the inductance and OVR in IISM is higher and increases with the decrease of the inductance. It is concluded that the minimal inductance to guarantee the Maximum OVR (MOVR) to be the lowest is actually the critical inductance of CISM and IISM under the condition of the lowest input voltage and minimum load resistance. In addition, the lowest MOVR is independent of the inductance. The design method to find the minimal inductance is proposed in the paper. Experiment results are in positive to the analysis showing the feasibility of the proposed methods.