This paper reports an extended version of GZAD technique for controlling DC-DC power converters with completely uncertain, but bounded time-varying loads. GZAD technique has been recently proposed as generalized version of Zero Average Dynamics strategy (ZAD) for controlling DC-DC buck power converter with known and fixed loads. However several application require control schemes for unknown and time-varying loads. In this work it is proposed to extend the GZAD strategy to control the buck converter, with unknown and time-varying loads using a nonlinear state observer. This paper numerically demonstrates high performance of the system when time-varying loads are estimated by nonlinear state observer.