The lifetime estimation of power converters is a crucial issue for the reliability of electrical generators with renewable sources. During normal operations, the switching devices show junction temperature cycles that are widely recognized as the main cause of their failures. Although in-line junction temperature monitoring is extremely difficult, its knowledge is essential in order to achieve a reliable power converter lifetime estimation. This work proposes an innovative realtime in-line monitoring strategy based on an accurate dynamic compact thermal model of the whole power module, capable of estimating the junction temperature from the measurement of the switching device dissipated powers. This model is suitable for FPGA implementation thus can be easily integrated in the control architecture of modern power converters. Experimental results on an IGBT power module demonstrate the reliability of the proposed method for lifetime estimation.