Recent technical advances in graphics processing units have accelerated the proliferation of high-power graphics processing units (GPUs) and multiple GPUs in high-end gamer PC applications. Characterized by very high heat loads, this application is causing increasing numbers of OEMs to investigate alternative methods, such as liquid cooling, to achieve the level of thermal management needed for dramatically higher system-power levels. Traditional GPU cooling strategies, such as those combining a heat pipe, heat sink, and fan, provide diminishing thermal performance at 120W per chip. Alternatively, the aggressive cooling requirements of gamer PCs and other high heat-flux processor applications are proving to be fertile ground for “non-traditional” approaches that offer at least 25% better thermal performance, as typified by advanced liquid-cooling systems (LCS) (see figure 1).

Typical gamer PC platforms, like the system shown in Figure 1, employ a dual-card configuration to support the operational requirements of extreme graphics computing. In this application, the power required by the GPU often exceeds that of the CPU itself, a trend that processor roadmaps predict will continue for the foreseeable future. Another technical issue impacting gamer PC operation and user enjoyment is excessive noise caused by the system’s numerous air-cooled fans. Liquid-cooling systems provide the ability to move heat over greater distances compared to standard heat-pipe solutions. This allows heat to be moved to locations that minimize the number of required system fans or allows the use of larger, quieter fans. Now available on the market, high-reliability, service-free LCS are capable of a very high level of cooling performance as a result of the system fan being used more effectively for cooling the CPU.

Liquid Solutions

Illustrated by a schematic of a closed-loop liquid-cooling solution (see Figure 2) used in the gamer PC system prototype, the cold water flows counter-clockwise into the CPU micro-coldplate at a specific volumetric flow rate driven by the mechanical pump and exits the micro-structure heat exchanger after picking up the CPU heat. The warm water then enters the parallel network of GPU coldplates, absorbs additional heat from the two GPUs, and returns to the radiator to repeat the process. As the water flows through the system, the radiator fins are cooled by a series of fans providing airflow at the ambient temperature. The cold water then continues through the loop back to the CPU micro-coldplate. The overall pressure drop of the liquid as it flows through the loop depends on the design of the micro-coldplates and the radiator. System components are carefully designed to operate within the pressuredrop capabilities of the pump for the required flow rate. The LCS shown in Figure 1 is designed for an operating flow rate of around 1200 ml/min, a rate sufficient to achieve overall total cooling of 450W. The 150W cooling capability for each of three processors — one CPU and two GPUs — is achieved as the fluid passes through the radiator accompanied by a fan-generated airflow of approximately 60 cfm. The radiator itself is a standard aluminum fin-tube type with an extruded-tubing microport for liquid flow and folded fins for airflow. Microstructure heat exchanger performance was modeled in the laboratory using Icepak, a computational fluid dynamics (CFD) software for simulating and analyzing fluid flow, heat, and other design characteristics. The test LCS provided a water inlet temperature of 55°C and a flow rate of 1200 ml/min. The die sizes assumed for the CPU coldplate and graphics heat source simulations were 16 × 12 mm and 20 × 20mm, respectively. To minimize the number of components, the same coldplate design was used for both the CPU and GPUs. The micro-coldplate and radiator utilized a geometry optimized to deliver the required thermal performance of dissipating 150W from the CPU and 300W from the two GPUs, while keeping CPU spreader temperature below 63°C. Cooling system performance, as shown in Table 1, indicates an overall case-to-ambient resistance (Rc-a) of 0.16C/W for the CPU and 0.21C/W for the GPUs.

Benchmark Testing

The prototype liquid-cooling system was developed for a standard gamer PC configuration using one AMD CPU chip and two ATI GPU processors. The maximum power rating for the chips in this benchmark test is lower than the designed maximum power from the chips in Table 1. Full performance testing was conducted using 3DMark, a 3D game performance benchmarking tool that exercises the GPU and CPU at 100% and 90%, respectively. When monitored, the GPU indicated a case temperature of 37°C at ambient room temperature. Compared to a normal air-cooling value of 70°C, these results indicate that thermal performance may be significantly improved by means of microstructure liquid- cooling technology. Designed to provide service-free, highly reliable operation, the closed-loop microstructure liquid cooling system described in this article demonstrates the ability to cool up to 450W, or 150W each for the system’s single CPU chip and dual graphics processors. Augmented by extremely quiet fan operation, outstanding thermal performance was obtained from a very low combined airflow of only 60 cfm for all three system fans. Further, benchmark test results indicated a GPU case temperature of only 37°C at ambient room temperature — a significant thermal performance improvement over commercially available air-cooled solutions used in similar applications.

This article was written by Girish Upadhya, Ph.D., director of applications engineering, and Fred Rebarber, director of sales and marketing, at Cooligy, Inc., a division of Emerson Network Power, Mountain View, CA. For information, contact Dr. Girish at This email address is being protected from spambots. You need JavaScript enabled to view it., Mr. Rebarber at This email address is being protected from spambots. You need JavaScript enabled to view it.., or visit http://info.hotims.com/10956-402.

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