In the context of energy storage with supercapacitors, the so called round trip efficiency is an important factor that can be strongly affected by the characteristics or performances of the used converter. This paper presents a dedicated DC-DC converter, where the design and control are the result of considerations on energy efficiency. The analysis of the efficiency shows that an optimal operation can be defined where the typical losses at low power of a given structure can be avoided by using an adapted number of active channels in a multichannel structure. A so called graduated control automatically adapts the number of used channels to the power level, assuming that each channel is operated at its maximum efficiency. Based on an analysis of the efficiency of the classical Buck converter, a well adapted number of channels has been selected, and the control strategy has been defined. The paper presents results of simulation where the completed system is in operation.