[Engineering Feature]
Turn Down The Heat, Please!
Cutting-edge products help create a smart thermal-management game plan to cool down those power-packed systems.
First, you've got ICs and power semiconductors operating at higher-than-ever power rates. Now add in the rising power densities of their associated systems and power-management devices generating more heat to support higher power loads. What you wind up with is heat dissipation reaching stratospheric levels.
To keep pace, thermal management must work harder to cool these systems and maintain an operating temperature that optimizes reliability—in other words, the lower the operating temperature, the higher the reliability.
Implementing thermal management in an electronic system involves a combination of electronic engineering and mechanical, chemical, and metallurgical disciplines. Most designers understand that thermal management requires fast-switching, low on-resistance MOSFETs to minimize heat dissipation. Yet many designers aren't as aware of the other disciplines required to optimize thermal management. Several non-electronic thermal-management solutions exist.
CONVECTION COOLING Many systems use convection cooling with either a fan or centrifugal blower. For instance, there's Comair Rotron's 7.9-in. Mixed Flow DC fan. Targeted at telecom applications, it also cools computer cabinets and racks. Cabinets and racks have more equipment packed in to deliver increased capabilities, raising the resistance to airflow within the limited space that surrounds the equipment. This airflow resistance makes it essential to cool the electrical equipment efficiently, to eliminate overheating and equipment failure.
The compact Mixed Flow fan combines characteristics of fans and blowers to deliver increased pressure and airflow. It measures 7.90 in.2 by 2.75 in. deep and weighs 4.6 lb. A powerful and efficient cooling unit, the fan delivers 375 to 480 cubic feet per minute (CFM) and a static pressure of 1.4 to 1.9 in. of water while generating 59- to 72-dBA noise as a function of speed.
This fan meets UL approval ratings and is polarity protected. Its die-cast housing and the polypropylene impeller both meet the UL 94V-0 flammability rating. Operating temperature ranges from -10°C to 75°C. Also, the fan runs at either the standard 24 V dc (nominal) or 48 V dc (nominal). It's specified for a continuous duty life of 50,000 hours at 25°C.
When coated with Comair Rotron's EnviroShield, the Mixed Flow also can withstand degradation caused by harsh environmental conditions, moisture, and salt. Thus, it's well suited for telecom applications.
Other product offerings from Comair Rotron use the ThermaPro-V technology, which automatically adjusts airflow based on current temperature needs. This leads to minimized power consumption and noise, and will eliminate expensive circuit redesigns. Also available is an automatic restart feature that ensures resumption of operation after power failure or overheating. In addition, a tachometer output can be incorporated to monitor performance and protect essential components.
THERMAL INTERFACE MATERIALS Thermal interface materials improve thermal conduction and electrical isolation between a heat-producing semiconductor and its associated heatsink. When heated or compressed between two surfaces, the interface material flows and displaces air gaps, providing better heat conduction between the two joined surfaces.
One form of interface material, phase change material (PCM), is usually found in sheet form with melting points above room temperature, typically between 40°C and 65°C. At that temperature, they change from a solid to the semi-liquid state to fill in any air gaps between the heatsink's surface and semiconductor.
PCMs came into prominence when neither the dry joint nor the thermal attachment tapes provided sufficient thermal performance to cool semiconductor-heatsink combinations. Manufacturers wanted an interface material applied by the heatsink manufacturer to avoid handling messy silicone grease.
Chomerics' Multiphase T557 and T558 are both PCMs (Fig. 1). T557 is an inherently adhesive film of 0.005 in. thickness. T558 is the same PCM as T557, except for a conformal metal-foil carrier coating on one side. Thermflow T558 is recommended for applications requiring rework and ease of disassembly. This new material offers the high-performance properties that are typical of free-film phase change materials, with the added benefit of easy removal.
Another type of thermal interface material is the gap filler, such as Chomerics' Therm-A-Gap 570 and 580 highly conformal, thermally conductive gap-filler pads. The materials consist of an ultra-soft silicone elastomer filled with ceramic particles. The conformability of these materials enables them to blanket highly uneven surfaces, wetting out mating surfaces to efficiently transfer heat away from components.
Therm-A-Gap T630 is a thermally conductive gap filler that suits applications where typical gap-filling pads cause too much stress on component solder joints and leads, resulting in damage to the pc board. T630 is a highly conformable, one-component pre-cured silicone that can be dispensed to fill large and uneven gaps in electronics assemblies.
PCMs require heat to melt and flow. Different interface materials, gap fillers, insulating pads, and other factors only require pressure to displace the air. This is because they tend to be soft rubbers.
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