Finding the most appropriate architecture to achieve a function is a major engineering challenge. In the power conversion context, the question that arises is: what is the best trade-off between an architecture based on a single high-power converter and another based on the combination of multiple low-power converters? This article aims to answer this question in the specific case of the isolated interface between the low voltage (28 VDC) and high voltage (±270 VDC) buses of an aircraft. Towards this goal, the targeted power electronics converter is carefully modelled in order to obtain an accurate and fast computing model. A technological database of the different usable components is then used to feed an optimization algorithm. The execution of the latter achieves an attractive and robust result showing an excellent performance in terms of power-to-weight ratio, which is the key index of this study. The precision and speed of this computer-aided design is based on analytical models that are quick to run and therefore enable exploring almost exhaustively the search space. More specifically, the high-frequency transformer has a large relative mass and generates significant losses that are difficult to assess. A major modeling effort has been undertaken and has enable to define a simplified but convincing and accurate model (successfully assessed on a wide bandwidth using an experimental setup). It was used for the first time in this optimization process. According to the targeted aeronautical specifications, the two best solutions would make it possible to achieve a mass power density of 4.5 kW/kg, i.e. twice as high as traditional solutions. This result very clearly shows that the appropriate use of the new GaN transistors makes it possible to make a technological breakthrough. Considering the voltage and current ratings of these components, this shows that the combination of multiple partial converters is very promising and will make it possible in the future to achieve a significant increase in the compactness of electrical power conversion functions. This sizing study therefore clearly shows the potential of the standardized design of power electronics converters and the search for the best combinations (series, parallel) to meet any specific specifications. Additionnaly, the developed approach, based on a modelling effort, especially as far as the high-frequency transformer is concerned, and collection of manufacturer data, also makes it possible to limit the implementations and associated tests. Hence, the main contributions of this article are i) the assembly of the analytical models of the various components constituting an isolated DC-DC power supply, ii) the simplification of an analytical model of the planar HF transformer, its experimental validation and its first use in a loop of optimization, iii) the fact of showing supporting figures that GaN technology enables technological breakthroughs to achieve high power density, which opens the way to modular architectures based on partial converters of electrical power, iv) the comprehensive description of a powerful pre-study design tool enabling to greatly reduce power converters' time to market.