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The low component count, high full-load efficiency and simplicity of hard-switched dc-dc converters in the low to medium frequency range lead to a low cost and low weight/volume solution for high-power dc-dc converters. The choice of the magnetic material is critical in order to optimise the size of the inductor. In this paper, Fe-based amorphous metal, 6.5 % silicon steel, nanocrystaline, and low-frequency ferrite materials are analysed and investigated for use in a gapped CC-core inductor. A novel area product derivation method, which includes foil ac copper loss effects and core loss due to the gap, enables material comparisons over a range of frequencies and ripple ratios. As expected, inductor size for a given inductance decreases with increased frequency. Inductor size, however, can increase in the laminated materials due to increased air gap core loss effects. A 2.5 kW boost converter is built to verify the optimum material selection over the frequency range and ripple ratio of interest.