The increasing interest in cloud-based services, the Internet-of-Things and the take-over of artificial intelligence computing require constant improvement of the power distribution network. As higher voltage distribution yields lower conduction losses on the rack power lines, vendors are moving from the classical 12 V rack to 48 V: this enables a 16-fold loss reduction for a given delivered power. In most cases, the server board is required to withstand input variations between 40 V and 60 V. As a consequence, the VRM (Voltage Regulator Module) must be designed to operate at these boundary conditions, which decrease this stage's efficiency also at nominal input voltage. This work addresses this issue by proposing a regulated Intermediate Bus Converter (IBC) which can maintain a fixed intermediate voltage for a wide input variation. As inductor-based, high-step-down IBCs would be inefficient, this solution exploits a non-isolated sigma connection (or ISOP, Input-Series Output-Parallel connection) between two sub-converters to maintain a high power density and efficiency. As a result, most of the power flows through a high-efficiency, soft-switched hybrid converter, while regulation is achieved with a fraction of the processed power through a boost converter. This paper shows results for a 1.2 kW solution performing a 40-60 V to 12 V semi-regulated conversion in a 25x 51 mm area, with a power density exceeding 1 kW/in3.
