Optimizing transformer parasitics in an inductor-less resonant converter

In this paper, the optimization of a 10 kW serial MPPT DC/DC converter designed to inject solar power into a low-voltage DC-microgrid is presented. The prototype was designed as two cascaded stages; an interleaved multi-branch boost topology followed by an inductor-less resonant topology. Traditionally, resonant converters use an external inductor, separating the resonant tank from the transformer. This has practical reasons but also has the drawback of an additional component and added weight. For this reason, the design exploits the leakage inductance of the transformer. However, the first prototype was overheating even in no-load conditions.This paper presents the optimization study conducted to eliminate this overheating. The analysis and simulations performed allowed to identify and pinpoint the source of the issue, namely highlighting the role of the parasitic capacitances in the high di/dt current step in the transformer. Simulations where first done to rough out the problem and get a better understanding of the different interactions between various parasitic elements. A literature review was conducted, and a theoretical analysis method was then applied. This allowed to estimate the order of magnitude of the transformer’s parasitic capacitances and helped fine tune the simulation model. Based on the performed analysis, recommendations and specifications were prepared for a new transformer. An new optimized transformer was then built and tested. The results were better than expected. The parasitics were reduced by 90%, showing that the issue had correctly been identified and addressed.