Within the past two to three years, the personal computer market has seen the introduction of liquid cooling for both desktop and notebook applications. Some mainstream examples of this cooling technology can be found in the systems of Apple and NEC on the desktop side, as well as in Hitachi products on the notebook side.

Fig. 1. Operation of traditional liquid-cooling system.

The need for this kind of cooling technology is being driven by the thermal challenges that are associated with ever increasing processor speeds. As CPUs and associated electronics become faster and hotter, the ability to dissipate this heat with conventional air-cooling devices is becoming increasingly difficult. Even when the heat can be removed with active air-cooling devices, the resulting noisier acoustics due to the required high fan speeds is often unacceptable to both the system designer and the product end user.

With increased thermal efficiency provided by a liquid heat-transfer medium, liquid circulation cooling offers significant advantages over conventional air cooling. However, there are still two key limitations of this approach that have prevented its widespread adoption by the majority of the mainstream desktop and notebook computer makers:

• Cost: The structure of the traditional liquid-cooling system is shown in Fig. 1. Required components include a pump, heat exchanger, radiator, fan, reserve tank, and some form of conduit to connect all of the fluid-handling parts. Adding these components can result in an 8X to 10X increase in the costs of the cooling system when compared to conventional air cooling.
• Leakage: All of the fluid-handling components must be connected to each other with tubing, resulting in a minimum of eight connection joints. That means that the cooling fluid (typically water) that runs through this system has a minimum of eight different locations where a leak is possible, not to mention the possibility of leaks occurring somewhere in the tubing itself. The fear of water leakage in the computer is enough to prevent some users/designers from adopting this technology.

The greater cost of the system, coupled with the higher risk of a system failure due to leakage, are significant factors that can mitigate the thermal advantages offered by liquid cooling.

In an attempt to make liquid cooling more widespread, Sunon has developed a liquid-circulation cooling system called WaturboTM, that preserves the thermal advantages of liquid cooling while addressing its limitations.

Fig. 2. Operation of Waturbo liquid-circulation cooling system.

The structure of the Waturbo liquid circulation cooling system is illustrated in Fig. 2. The five key components of the traditional liquid cooling system are still represented: a pump, heat exchanger, radiator, fan, and reserve tank, but in the Waturbo system, four of those components (everything but the fan) are packaged in a single, sealed liquid-holding radiator device. This radiator device is sandwiched between the fan (the active cooler, which also drives the pump) and the CPU (the heat generating device in the system).

What is missing is all of the rubber tubing found in the traditional system that is used to connect the different components. By eliminating the rubber tubing, and all of the connection joints with the different components, the risk of leakage has been drastically reduced. The only possible sources for leakage now are the top and bottom of the radiator device, which interface with the fan and CPU respectively. These two junction points are more easily sealed with the use of standard O-Rings.

In the operation of the Waturbo, as seen in Fig. 2, the exterior fan drives the interior pump, which circulates the cooling water inside the sealed liquid reserve tank. This circulating water allows the heat exchanger at the bottom of the reserve tank to efficiently remove the heat from the CPU. The corresponding temperature increase of the cooling water is dissipated to the outside through the radiator fins.

Fig. 3. Waturbo performance compared to original air cooling system for temperature and acoustic noise.

The fact that the Waturbo system uses the fan motor to drive the liquid pump is significant, in that the dual-action of the fan motor eliminates the need for a separate motor to drive the pump, as in a traditional system. On the Waturbo, the driving torque of the fan impeller is transmitted to the liquid pump through a magnetic coupling. This means that no pump motor or other electronic parts are required in the cooling water. All of this results in a lower risk and lower costs.

In a conventional air-cooling system, the distance between the CPU heat source and the heat dissipating radiator fin is critical. In practice, a fin that is located at a further distance from the CPU heat source will have a lower fin temperature, resulting in a less efficient thermal system design.

By contrast, the Waturbo system utilizes forced liquid convection to transfer the CPU heat to the radiator fin. Therefore, there is no significant distance-effect between CPU and radiator fin, which makes it possible to keep the fin temperatures stable and obtain an overall increase in thermal efficiency.

Sunon has developed and tested the Waturbo technology in desktop computer liquid cooling systems, and applications are currently under development for both notebook and server applications.

Test results shown in Fig. 3 compare data between a desktop computer using a conventional air-cooling system and the same computer as cooled by the Waturbo cooling system. The data measured in this test include CPU temperature (in DegC) and system acoustic noise (in dBA). As indicated, the Waturbo liquid-circulation cooling system results in a lower CPU temperature than was provided by the original air-cooling system.

Significantly, this improved cooling performance also provided an overall decrease in system acoustic noise. This is due to the fact that the improved cooling efficiency and performance of the Waturbo system allows the active fan to run at a lower speed, thereby lowering the associated fan-speed acoustic noise. This has particular advantages in the consumer computer industry where acoustic noise performance has become a major issue due the increasingly higher fan speeds found in conventional air coolers.

Miyahara Masaharu is vice president, Cooling Module Business Group, Sunon, Kaohsiung, Taiwan; and John Fulmer is senior engineer, Sunon USA, Brea, Calif.