A Two-Stage Isolated Resonant DC-DC Converter for Wide Voltage Range Operation

This paper proposes a high-efficiency two-stage isolated dc-dc converter for applications with wide variations of output voltages. It employs a first, pre-regulation stage and a second stage based on an LLC converter, integrated with the first. The second stage is always operated at resonance, ensuring maximum efficiency. The first pre-regulation stage is in charge of i) achieving the desired overall conversion ratio and of ii) always achieving soft-switching operation of all the switches, even in presence of wide output voltage variations. This allows to tackle the typical challenge of keeping conversion loss low even with input or output port voltages that may vary in a wide range. In this paper, the considered conversion structure is shown considering a preliminary experimental prototype that interfaces a 750-V dc-link with an output bus of nominal voltage in the range 250 V to 500 V, which is common in electric vehicle battery-charging applications. The considered preliminary prototype is rated 5 kW and achieves a peak efficiency of 97.5% at 3 kW output power.