Power electronics are vital for the generation, conversion, transmission, and distribution of electrical energy. Improving the efficiency, power density, and reliability of power electronics is an important challenge that can be addressed with electro-thermal co-design and optimization. Current thermal management approaches utilize metallic heat sinks, resulting in parasitic load generation due to different potentials between electronic components on the printed circuit board (PCB). To enable electrical isolation, a thermal interface material (TIM) or gap pad is placed between the PCB and heat sink, resulting in poor heat transfer. Here, we develop an approach to eliminate TIMs and gap pads through modularization of metallic heat sinks. The use of smaller modular heat sinks (MHSs) strategically placed on high power dissipation areas of the PCB enables elimination of electrical potential difference, and removal of electrical isolation materials, resulting in better cooling performance due to direct contact between devices and the heat sink. By studying a gallium nitride (GaN) 2kW DC-DC power converter as a test platform for electro-thermal co-design using the modular approach, and benchmarking performance with a commercial off-the-shelf heat sink design, we showed identical power dissipation rates with a 54% reduction in heat sink volume and a 8°C reduction in maximum GaN device temperature. In addition to thermal performance improvement, the MHS design showed a 73% increase in specific power density with a 22% increase in volumetric power density.