This work presents the system level modelling of a hybrid energy harvesting device incorporating piezoelectric and electromagnetic transducers in order to optimize its output performance. The coupled electromechanical analysis of the device dynamics shows that the strong impedance mismatch between piezoelectric and electromagnetic components restricts the potential of the hybrid generator which can only be improved through innovation in the power management circuit. We propose a novel interface circuit topology, which uses synchronous electric charge extraction technique to combine the extracted energy from piezoelectric and electromagnetic harvesters. The designed circuits are simulated for CMOS 180 nm process and the maximum conversion efficiency of the proposed technique is shown to be around 95%.