This paper mainly analyses the observability of large signal models of DC-DC converters and designs a nonlinear observer for the boost converter. A graph-theoretic method which has small computational complexity is proposed to verify observability properties of the boost converter and the superbuck converter. Furthermore, a full state nonlinear observer is designed based on the port-Hamiltonian system form of the large signal model of a DC-DC converter. The observer is applied to the boost converter and the observation of the inductor current tracks the actual value well.