99% Efficient 3-Phase PFC Rectifiers

Published on Monday, 06 July, 2020
Conventional AC based systems (top) require up to five power conversion stages between the AC input the CPUs. Using a 380 VDC distribution bus (bottom) allows to omit two stages, increasing efficiency and reliability, and reducing cost.

Minimizing Life Cycle Cost in
Data Center Power Distribution

The world’s data centers consume an estimated 300 TWh of electricity each year, which is about five times Switzerland’s annual consumption. In conventional ac data centers with an online UPS, five conversion stages exist between the AC mains input and the IT load (CPUs, memory, hard disks, etc.) and up to 50% of the total consumption is caused by conversion losses and the required cooling.

Significant reductions in losses, cost, rack space and improved reliability can be achieved using 380 VDC power distribution. In this case a Power Factor Correction (PFC) rectifier is required, which supplies the DC bus from the three-phase AC mains and achieves sinusoidal input currents. These rectifiers are also well suited as fast charger for electric vehicles. If total cost of ownership is considered, it can be shown that conversion efficiencies of 99% are economically feasible for 24/7 operation.

Instead of standard boost-type PFC rectifier systems, ultra-efficient isolated and nonisolated three-phase PFC rectifiers with buck-type characteristic can be used to achieve the desired performance. These produce a DC output voltage between 0 and 480 VDC from a conventional 400 Vrms three-phase mains in a single conversion stage, thus reducing conversion losses and volume.

A nonisolated 8 kW prototype rectifier ("SWISS" topology) with a groundbreaking full-load efficiency of 99.16% and a peak efficiency of 99.26% can be realized using commercially available 1200 V SiC MOSFETs, hard switching at 27 kHz, and a novel interleaving transformer with integrated common mode inductor. With a volume of 2 liters, it achieves a power density of 4 kW/l, surpassing any off-the-self solutions.

Another example is an isolated three-phase AC to 400 VDC PFC rectifier, designed for the highest possible full-load efficiency. It uses 900V SiC MOSFETs in an advanced circuit topology that uses a direct matrix converter to power the primary side of a dual active bridge. To further reduce losses, a complex modulation and control scheme is implemented on a Xilinx ZYNQ 7000 SoC, which achieves soft switching. Conduction losses are minimized by using a 3D lookup table with precomputed switching times optimized for minimal conduction losses under all operating conditions.

When comparing different PFC rectifiers in terms of volumetric power density and efficiency, it is evident that the systems desigend and implemented with our expertise achieve an outstanding combination of high power density and ultra-high efficiency. This includes commercially available systems as well as prototypes reported in scientific literature. In practice, this saves energy costs rack space at the same time, enabling data centers to optimize their operation and have a technological and financial advance to their competition.

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Lukas Schrittwieser

+41 44 244 27 50