This paper addresses the problem of the output feedback control of the DC-DC converters with an unknown output load. It is assumed that only the capacitors’ voltage and the input voltage are measured. A general lossless model of the converters is considered which relaxes the knowledge about parasitic elements, such as the input side parasitic resistance, however it makes the control problem hard to tackle by observer-based design due to the lack of detectability. To overcome the detectability obstacle, a filtered transformation based on immersion and invariance technique is proposed that immerses the system dynamics to a proper adaptive observer form. A reduced-order observer is designed based on the transformation that is used in conjunction with a dynamic controller to render the closed-loop system uniformly globally asymptotically stable.