Cooling electronic systems has always been an important design consideration. As electronic systems continue to shrink and become more powerful, heat dissipation isn't just important—it's a critical design issue. Today's higher levels of IC performance and integration, coupled with smaller packaging and tighter form factors, are producing more heat in a given area than ever.
Making matters worse, electronics systems are being pushed to take on appearances that are more aesthetic and blend in with their surroundings. Unfortunately, aesthetics often occurs at the expense of natural air intakes/exhausts, so many of these systems will contain one or more fans to provide adequate cooling—frequently at the expense of acoustics.
Employing fans is contrary to the whole concept of silent PCs, unobtrusive office or home equipment, and electrical-only systems. That's especially true today because people are obsessed with minimizing noise. Minimizing acoustic pollution isn't just becoming a social requirement, but environmental and legal policy too.
Europe takes the lead in this area, as it tends to have much more stringent acoustic requirements than the U.S. Of course, this forces American companies to define and design better, quieter products for European markets, ultimately benefiting consumers on a global scale. Award standards such as the European Blue Angel Award solidify the effort by setting defined noise targets.
Adding a fan: When adding a cooling fan, very carefully select your vendor. Fan noise is related to blade pitch, number of blades, fan size, airflow volume, and rotational speed. Other factors include the physical construction, bearing type (e.g., ball or hydrodynamic bearing), and the internal electronic commutation circuitry. Also, due to resonance with the enclosure, a fan fitted into the system will make more noise than if it was operated in free air.
To simulate free air, the fan is suspended from a 150-mm piece of string, and measurements are taken with a microphone situated 50 cm from the fan. This typically gives best-case acoustic data (Fig. 1). In contrast, when that same fan is placed into a notebook chassis, the noise increases by up to 10 dB simply because the fan is inside the enclosure (Fig. 2). Clearly, the fan is noisiest at high speeds. To minimize acoustic noise, only run the fan at higher speeds when necessary.
Intelligent fans or intelligent control? Rather than specifying a very complex fan with myriad features, the key to using fans intelligently is to specify the most basic fan and make it act intelligently by adding an intelligent fan controller. With this approach, any 2- or 3-wire fan from any vendor can be made intelligent, allowing easy and inexpensive multiple sourcing of fans.
Measuring system temperature: Base the amount of cooling on system temperature. Cooling a system that's not hot will waste power and cause unnecessary noise. If the system requires cooling, but it's not extremely hot, the amount of cooling can be reduced. Slowing down the fan will minimize the cooling effect, but save power and reduce system noise.
Some fans incorporate a thermistor to measure the temperature at the fan so its speed can be increased as the air around it heats up. Due to the thermal lag associated with heating the air, these fans tend to have slow responses to changes in system temperature. Also, the fan is often located remotely from the temperature hot spot, so it might not sufficiently cool the system even if it's adequately rated. In many applications, this type of fan isn't very useful.
The ultimate measurement in temperature is to directly measure the die temperature of the hottest ICs. This technique eliminates inaccuracies and thermal lags associated with reading a temperature through a heatsink. For a number of years, CPU manufacturers have integrated a thermal diode into their products to facilitate measuring the die temperature. More recently, this thermal diode has been incorporated into chip sets and graphics controllers.
Because the technique used to determine temperature only requires a semiconductor junction, a simple transistor such as a 2N3904 will suffice. It can easily be placed in remote system hotspots to measure the temperature. Multiple thermal sensors, carefully located in different thermal zones, permit truer system profiling—leading to a distributed, more intelligent cooling approach. By employing an ADM1023 thermal-diode-monitor (TDM) IC, users can measure temperature remotely to an accuracy of within 1°C (Fig. 3).
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