The modular Smart Transformer (ST) is composed by several basic converters rated for lower voltage and power. In this paper the quadruple active bridge (QAB) is used as the basic block for the modular ST. In this application, the efficiency and cost are the most important design parameters. Therefore, the paper focus on the design of the converter, with the aim to optimize its efficiency, taking the cost into consideration. To do so, the losses of all components are carefully modeled and a computer-aided design is used, where an algorithm to calculate the losses and cost is developed, allowing to perform multi-objective optimization. Additionally, Silicon IGBTs and Silicon Carbide MOSFETs are considered for the design and the performance of the converter using both semiconductors technology is compared. Experimental results obtained for the optimized 20 kW QAB converter has shown an efficiency of 97.5%.