System Level Impact of GaN in Server Architectures

December 20, 2016 by Jeff Shepard

At this year's IEEE International Electron Devices Meeting (IEDM), A. Lidow, D. Reusch and J. Glaser from Efficient Power Conversion Corp. presented an analysis of the "System Level Impact of GaN Power Devices in Server Architectures." The authors compared four ways to get from 48V to 1 V: (A) a conventional isolated 48V to 12V converter followed by a non-isolated 12V to 1V point-of-of-load (POL) converter, (B) a two-stage non-isolated 48V to 12V to 1V POL converter approach, (C) a non-isolated single stage 48V to 1V POL converter, and (D) a transformer-based single stage 48V to 1V POL converter.

Each implementation has comparative strengths and weaknesses that were examined using the following four dimensions; Efficiency, Power Density, Transient responsiveness, and Cost. Due to the significantly lower input capacitance of a GaN transistor compared to a silicon device with similar voltage and on-resistance rating, coupled with low-parasitic chip-scale packaging, GaN transistors are significantly faster than their silicon MOSFET counterparts. This speed advantage not only translates into a reduction of switching losses and higher power density, but also allows the system designer to reexamine the overall power architecture.

The architecture most commonly used today is Scenario A. A GaN-based fully-isolated eighth-brick converter can produce 600W at over 96% efficiency at a 12V output with conventional topologies. The high-frequency second stage is comprised of 15 buck converters that use parallel EPC2100 monolithic half-bridge devices operating at 1MHz for fast transient response. The 12V to 1V conversion is about 88% efficient, but the high current running through the 12V power planes adds 2% to overall losses. The product of these three sources of loss is about 83% overall system efficiency at 580W. For a Si MOSFET-based implementation, the efficiency at 580W is limited to about 80%.

The Si-based implementation has reduced power density as the first 48V to 12V stage, operating at less than half the switching frequency of the GaN-based design to achieve similar first stage efficiency, requires a larger quarter-brick footprint to deliver 580W. The second stage, operated at the same 1MHz switching frequency as the GaN-based design to maintain fast transient capability and power density, is 3% less efficient.

The two-stage buck converter (Scenario B) is more efficient (85%), and has the advantage of the highest power density of 350 W/in3. The output stages can operate efficiently at 1MHz for fast transient response. In addition, this is a relatively low cost solution in that it requires only 4-layer PCBs and has the lowest number of GaN devices needed to deliver 580W at 1V.

Scenario C requires 40 buck converters, each using an EPC2105 monolithic half-bridge. In order to achieve acceptable efficiencies of 83%, the operating frequency was dropped to 300kHz, which may result in an unacceptable transient response. For some applications, this can be mitigated by system-level control of the increased number of phases. This solution, however, gives the system designer excellent layout flexibility and high overall power density.

The efficiency winner is the non-isolated single-stage 48V to 1V converter (Scenario D) using a transformer step-down and achieving a peak efficiency of 90%. This topology uses a half-bridge converter at the input with a current-doubler rectifier on the output. There is a five-to-one step down transformer between input and output stage that minimizes the current, and therefore device losses, for the input side devices, but at the expense of converter power density and cost. This solution requires a 10-layer PCB and 90 GaN transistors to achieve the 580W output with an estimated power density of only 90W/in3.

New power system architectures for 48V to 1V conversion have grown in importance as power densities in server farms increase at a quickening pace due to applications such as deep learning and artificial intelligence. All of the GaN-based solutions examined in this paper demonstrate efficiency and power density exceeding today’s silicon-based solutions. A non-isolated two-stage converter had the highest power density, lowest acquisition cost, fastest transient response, and second-best efficiency. If efficiency is paramount, a one stage transformer-based solution can achieve up to 90% efficiency, but with a sacrifice in cost, density, and transient response.