The theoretical advantages of GaN-based power transistors are now being realized in mainstream system designs. Power supplies for data centers and telecom switching racks are two application areas where GaN transistors show significant improvement in comparison to systems using best-in-class Silicon-based Superjunction devices. When the system is designed to leverage the inherent advantages of the wide-bandgap material, suppliers and system users realize both system cost and operating benefits.
Specifically, we compared designs that use eMode High Electron Mobility Transistor (HEMT) devices to Silicon-based alternatives. The intent was to determine to what extent Figure-of-Merit (FOM) advantages such as order of magnitude lower gate charge and output charge, combined with near zero reverse recovery charge, would help meet efficiency and power density goals. Comparative data on several key parameters can be seen in Figures 1-3.
Characteristic output capacitance curves of three consecutive technology nodes of Superjunction device in comparison to an e-mode GaN HEMT’s
Trend for the energy stored in the output capacitance across three consecutive generations of Superjunction devices in comparison to GaN HEMTs
Comparison of QOSS versus voltage for an e-mode GaN HEMT (left) to CoolMOS C7 (right)
More compact, efficient server supplies
The first evaluation studied the effect of substituting GaN for Silicon transistors in a server supply topology. Managing power consumption in a data center helps meet the demand for higher compute performance without increasing footprint and, through higher efficiency, with reduced facility cooling costs. A typical high-efficiency supply (Figure 4 and Table 1) employs a totem pole AC-DC rectifier with two interleaved high-frequency bridge-legs. In this 12 V supply, an LLC DC converter with center-tapped transformer is used. A 48 V system in this topology might be implemented with full-bridge rectification.
Server supply comprising a totem pole AC-DC rectifier with two interleaved high frequency bridge legs and an LLC DC-DC converter with center-tapped transformer
Specification for server supplies
Use of Superjunction devices in this bridgeless topology mandates operation in Triangular Current Mode (TCM) at all times. However, GaN switches can operate in any of three modes: TCM, Continuous Current (CCM) or Optimal Frequency Modulation (OFM). The efficiency improvement of the PFC (rectification) stage ranged from 0.2 – 0.3% in power density up to 170 W/inch3 , increasing to .4% or more as power density advances beyond 200 W/inch3 . In the DC-DC converter stage, efficiency is improved between .2% to .4% up to 200 W/inch3 . A comparison between the high density server supply and a typical silicon based Platinum supply shows, that not only is the efficiency improved by around 4% on average but also the maximum power is increased from 1600W to 3kW in the same box dimensions.
As computational architectures move toward more parallel processing with GPUs, power consumption per rack is tripling to 20 kW or greater. For this use case, distribution losses using 12 V supplies are too great. Thus, on-rack 48 V supplies are growing in popularity, with an emphasis on efficiency.
The basis for this optimization exercise is a 3 kW output 48 V silicon-based supply with peak efficiency of 97.1% at half-rated power and density of 33 W/inch3 . As a first step, the AC-DC stage was changed to a totem pole rectifier with a high and low frequency bridge leg. The high frequency leg uses GaN switches, while the low frequency leg uses Superjunction MOSFETs, allowing the efficiency to increase up to 97.5%. For further efficiency improvements, a second high-frequency leg, interleaved with the first in the totem pole, was added.
Several further improvements are also achieved in the DC-DC stage by populating the primary side half-bridge with 35 mΩ GaN devices. Taking advantage of the order of magnitude lower Qoss charge, with associated adjustments in resonant frequency and magnetizing inductance, increases system efficiency by about 0.3%. Changing the transformer setup to a matrix structure, with series connected primary windings and parallel connected secondaries, yields another 0.3% improvement.
The combination of improvements in both the PFC and LLC stage results in an increase of peak efficiency at a power density of 30 to 35 W/inch3 system efficiency up to 98.5%. (Figure 5)
Optimization result for the totem pole PFC stage, including EMI filter, comparing system efficiency versus density for both GaN or Si-based power devices respectively
Wireless infrastructure power requirements
With the transition to 5G wireless communications, power distribution and overall power consumption of telecom base stations is increasingly important to operators modeling both CAPEX and OPEX for the build-out. Factors such as load profile and variable output voltage range vary between data centers. Thus, while the general supply topology is similar, optimization targets emphasize lower typical rated power (30-50% vs. 50-70% in servers).
As with the 48 V server supply, design optimization involves a combination of GaN devices for high-voltage switches and low-frequency Superjunction MOSFETs in areas such as the return path switch of the totem pole PFC and the secondary side of the LLC. Other parameters in the study that are varied for optimization included the number of high-frequency legs in the PFC stage, total number of parallel and LLC stages, and matrix configuration of transformers. Variations in switching frequencies were also implemented, with optimal results for the LLC resonant frequency typically around 150 kHz for GaN-based designs vs. 100 kHz for Superjunction.
The results (Figure 6) show that the GaN solutions achieve higher efficiency in the range of 0.3% in the popular commercial range of 30 to 40 W/inch3 power density. At this density, using GaN in the PFC stage also lowers costs by allowing a single HF totem pole leg rather than using interleaved totem pole branches in the optimal Si-based design.
Optimization results for the LLC stage showing efficiency versus power density both for Si and GaN based power devices
It is clear from these application studies that e-mode GaN HEMTs make it possible to both achieve efficiency and higher density in high output power supply designs. More importantly, these benefits do not come at a higher system cost. Considering all aspects of the system design and optimizing to take advantage of the varied parameters of both wide-bandgap and conventional materials makes it possible to lower both CAPEX and the customer OPEX.
About the authors
Dr. Gerald Deboy, Senior Principal Power Semiconductors and System Engineering, Infineon Technologies
Gerald Deboy received his M.S. and Ph.D. degrees in physics from the Technical University Munich, Germany, in 1991 and 1996, respectively. He joined Siemens Corporate Research and Development in 1992 and the Semiconductor Division of Siemens in 1995, which later became Infineon Technologies. From 2004 to 2009, he was the head of the technical marketing department for power semiconductors and integrated circuits at Infineon Technologies Austria AG. Since 2009, he heads a business development group specializing in new fields for power electronics. He has authored or co-authored more than 70 papers in national and international journals, including contributions to three textbooks. He holds more than 60 granted international patents and has more applications pending. He is a Senior Member of the IEEE.
Dr. Matthias Kasper, Senior Specialist Application Engineering, Infineon Technologies
Matthias Kasper (S’12) received his M.Sc. and Ph.D. degrees in electrical engineering from the Swiss Federal Institute of Technology (ETH) in Zurich, Switzerland, in 2011 and 2016, respectively. During his studies, he focused on power electronics, power systems, and control of mechatronic systems. In 2011, he joined the Power Electronic Systems Laboratory, ETH Zurich, where he worked as a Ph.D. student and later as a Postdoctoral Researcher in close collaboration with industry partners. His research interests include photovoltaic energy systems, single-phase telecom rectifiers, multicell converter systems and GaN wide-bandgap power devices. Since 2017 he is with Infineon Technologies Austria AG as part of the Systems Innovation Lab team.
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Editor’s note: See my EDN Exclusive article on Power for Data Centers 2019 where I discuss the best solutions in the industry including more details about Infineon’s offerings.