This paper presents a new observer for a class of DC-DC power converters based on a hybrid system approach. The considered class of switched systems contains two modes. Interestingly, the observability of each mode is not required. An original observer is designed to estimate the state over all modes by using available information provided by current and previous observable states. We show that the stability of the observer is guaranteed independently of the switching sequence. An application to a Single-Ended Primary Inductor Converter (SEPIC) is presented in simulation.