Intermediate and light load efficiency improvement of a high-power density bidirectional DC-DC converter in hybrid electric vehicles with MR fluid gap inductor

High power-density bidirectional DC-DC converters consisting of buck and boost converters are widely used in hybrid electric vehicles due to their smaller size and low cost. With a conventional constant-value inductor, the light load efficiency of this converter is greatly reduced because of the large constant peak-to-peak current swing and relatively constant conduction losses that result in the entire load range. This paper proposes a new type of nonlinear inductor using magneto-rheological (MR) fluid to improve the intermediate and light load efficiencies. The proposed nonlinear inductor has high current capability, and is gradually changeable. For comparison, a conventional constant-value inductor and the proposed nonlinear inductor are fabricated using the same EE-type ferrite cores with the same number of turns and gap length. The only difference is that in the proposed inductor, the gap is filled with MR fluid instead of air. A 3.7 kW prototype converter is built, and the efficiency from heavy to light loads is measured using both inductors separately, to validate the advantages of the proposed MR fluid-gap inductor.