A novel approach to the problem of partial state estimation of a class of nonlinear systems is proposed. In this paper we apply the new adaptive observer design technique to power converters. For the sake of ease of exposition we preferred to concentrate on the specific example of the Ćuk power converter. The main idea is to translate the state estimation problem into one of estimation of constant, unknown parameters related to the systems initial conditions. The proposed observer is shown to be applicable for the reconstruction of the state of power converters for further using in the full-state feedback controller. Comparison with Immersion and Invariance technique for the observer design is made via numerical examples.