A weighted design optimization is introduced to minimize total loss of electric vehicle drivetrain power electronics over EPA standard drive cycles. It is shown that the net loss of the conventional boost converter can be reduced by a factor of 1.5 with this approach, while computational effort is reduced by three orders of magnitude. Even larger efficiency improvements are achieved by optimized boost composite converters: losses are reduced by factors of 4.5, 2.9, and 4.3 for US06, UDDS, and HWFET driving cycles, respectively. These design optimization results are experimentally verified with a 30 kW laboratory prototype boost composite converter, which demonstrates 98.4% average efficiency over the US06 driving cycle.