Power Electronics

Cooling Trends: Power Electronic Systems Rely on Thermal Management

New innovative and exotic components provide thermal solutions.

Power electronic systems are vulnerable to thermal problems - however, a variety of components keep them cool and reliable.

Heat is the major enemy of power electronic systems, so you must conquer it to ensure system reliability. Eliminating heat involves a broad array of components that include heat sinks, thermal gap fillers, heat pipes, heat spreaders, cold plates, card retainers, and fans as well as software that provides thermal solutions. These components are used at the individual device and system level.

Heat sinks continue to be the most widely used thermal management component. A recent introduction by Aavid Thermalloy is The Max Clip System[TM] (Fig. 1), a proprietary line of extruded fin heat sinks and clips for power semiconductors used in power supplies, UPS, motor drives, etc. Its key feature is the ability to reduce assembly costs by eliminating the need to bolt the heat sink to the power semiconductor. The Clip System provides substantial mounting force and thermal connectivity to heat sinks, which reduces thermal resistance, adds flexibility in moving or changing devices, and mounts discrete power devices that have no through-holes. The clips exert a constant pressure, improving thermal contact and component reliability by applying the right amount of force over the center of the device.

Many of today's power electronic systems require increased heat sink efficiency, which means expanding the number of extended surfaces (fins) for cooling. One approach for adding fins to an extruded aluminum base is a flat or augmented aluminum fin epoxy-bonded into grooves in the base. An approach from Aavid employs a proprietary cold-forming process (Fig. 2, on page 81) that securely attaches the fins into matching grooves extruded into the heat-spreading base. No joints impede the flow of heat through the base. These all-metal heat sinks show a greater than 12% increase in pull out strength over some epoxy-bonded heat sinks. This strength contributes to long-term reliability of the assembly and the thermal conductivity of each extended surface and, therefore, to the overall thermal resistance of the product.

Another type of heat sink from Cool Innovations (Fig. 3, on page 81) involves the insertion of an array of pins cut to length into a machined base plate. As a result of this technology, the manufacturer can customize UltraCool II pin-fin heat sinks to the user's needs.

Forced convection in the impingement-cooling mode is the most effective cooling scheme for pin-fin heat sinks. Blown directly at the top of the pins, forced air creates turbulent air between the pins, breaking the air boundary layers around the pins and enhancing the heat sink's high convectional thermal coefficients. The volumetric efficiencies in the impingement-cooling mode are very high. An additional advantage of the impingement-cooling mode is an evenly distributed airflow along the surface of the heat sink. This eliminates the temperature gradient that is typical to forced horizontal air-cooling - avoiding the "hot end" of the heat sink.

Heat Pipes A heat pipe consists of a sealed and evacuated tube with a porous wick structure and a small amount of working fluid inside (Fig. 4, on page 82). An internal porous wick structure lines the tube, leaving the center core of the tube open to permit vapor flow. The heat pipe has three sections: evaporator, adiabatic, and condenser.

As heat enters the evaporator section, the vaporization of the working fluid absorbs it. The generated vapor travels down the center of the tube through the adiabatic section to the condenser section where the vapor condenses - giving up its latent heat of vaporization. The condensed fluid returns to the evaporator section by gravity or by capillary pumping in the porous wick structure. Heat pipe operation is completely passive and continuous. Because there are no moving parts to fail, a heat pipe can be very reliable.

It's difficult to cool power modules and power transistors in sealed enclosures that protect them from dirt and other contaminants. One effective cooling means is heat pipes combined with a heat sink that allows the module to remain sealed while fins exposed to ambient air remove waste heat.

Preliminary designs from Noren Products include heat pipes with an "input" pad with mounted components. This removes the heat inside an enclosure directly through its walls to the outside where cooling fins exposed to the cooler ambient remove the heat from the pipes.

Several prototypes were built and tested with two and four heat pipes. The four-heat pipe design had very high performance.

The assembled arrangement has a flange that holds a set of finned heat pipes that enter the enclosure and terminate into a flat input plate. The flange seals into the enclosure and is leak tight. The fins are a high-density, efficient design surrounded by a fan housing with a built-in fan.

Fig. 5 shows the thermal performance with various power inputs. The temperature rise at the input pad was 28.5C above the ambient 20C air with a 1500W input. This heat sink thermal resistance is 0.19C/W.

Heat Spreaders As electronic devices decrease in size and increase in power, the required heat sinks have grown to be larger than the devices. Heat sinks are most efficient when there is a uniform heat flux applied over the entire base. "Spreading resistance" occurs when you attach a heat sink with a large base-plate area to a heat source of a smaller footprint area. The higher the power, the smaller the source - and the farther the source is off-center, the greater the thermal spreading resistance. The brute force approach to overcoming spreading resistance is to increase the size of the heat sink, increase the thickness of the base, increase the airflow, or decrease the incoming air. These steps increase weight, noise, system complexity, and expense.

To deliver higher performance by alleviating spreading resistance, Thermacore developed Therma-Base[TM] heat sinks. They operate in the same way conventional heat pipe heat sinks do - through a 2-phase, heat-transfer process.

The base of a Therma-Base heat sink is a vapor chamber, a vacuum vessel with a saturated wick structure lining the inside walls. As heat is apply to the base, the working fluid at that location immediately vaporizes, and the vapor rushes to fill the vacuum. Wherever the vapor comes into contact with a cooler wall surface it condenses, releasing its latent heat of vaporization. The condensed fluid returns to the heat source via capillary action in the wick structure. It's this capillary action that enables the Therma-Base heat sink to work in any orientation with respect to gravity. As in a heat pipe, the thermal resistance associated with the vapor spreading is negligible, providing an effective means of spreading the heat from a concentrated source to a large surface.

High-powered electronic systems may require even more exotic cooling techniques. A common cold plate design for power electronics consists of an aluminum-extruded plate with copper or stainless steel tubing pressed into grooves on the plate. The cooling fluid flows through the tubing removing heat from the cold plate's surface. Lytron's cold-plate technology offers a 300% improvement in thermal conductance over traditional designs. A new design uses 1/4-in. tubing instead of the traditional 3/8-in. tubing to decrease thermal resistance and improve cold-plate performance. Cold-plate applications include heat removal from high-power electronics required in many industries.

Thermal Gap Fillers A continuing trend is the replacement of thermal silicone grease with thermal gap fillers. Thermagon's T-mate[TM] is a reworkable phase-change material, designed to interface between a chip or component and a heat-spreading device. T-mate is a special malleable metal alloy combined with a high-performance, phase-change material. Available in any thickness from 0.005 in. to 0.020 in., T-mate offers low-thermal resistance. When heated to 70C and under pressures as low as 5 psi, it will flow to fill surface irregularities - without exhibiting excessive flow characteristics. T-mate can also conform to any nonparallel surface conditions. Its smooth metal alloy surface allows the heat sink to separate from the device cleanly and easily with no residue left on the electronic part.

Another company supplying thermal gap fillers is Fujipoly. It offers SARCON[R] materials, a silicone rubber with high thermal conductivity and excellent flame-retardancy. It combines the inherent silicone rubber properties of heat resistance, electrical insulation, and long-term aging into one compound. These gap filler pads cover the 0.040-in. to 0.200-in. thickness range. One version has high heat conductivity characteristics and another has a hardened surface on one side for easier disassembly. The high heat conductivity pad has a thermal conductivity of 2.30 W/m K.

For efficiently transferring heat from components into various heat spreading devices, Chomerics introduced THERM-A-GAP[TM]F574 thermally conductive elastomer. The material replaces air gaps between hot power devices and their heat spreaders. Its thermal impedance ranges from 0.60C to 1.60C in.2/W for a thickness from 0.020 in. to 0.100 in. The flexible nature of this elastomer allows it to blanket uneven surfaces across components and circuit boards. It's reinforced with an internal fiberglass mesh and consists of an extremely soft silicone elastomer, loaded with a blend of ceramic particles. This is the softest of the family of thermal gap fillers, deflecting up to 35% under compression as low as 35 psi. The material has an inherently tacky surface and doesn't require any mounting adhesive. This property keeps the material in place during assembly and allows simple repositioning.

Chomerics also recently introduced its PowerSites thermal interface technology. This automated process selectively bonds electrically isolated copper patches to aluminum heat sinks. With the copper placed only where power devices are located, no etching processes are necessary - which allows you to apply it to heat sinks of any size or shape. You can then mount power devices to the PowerSites using conventional soldering methods.

Yet another type of thermal management material is a thermally conductive adhesive from Thermshield. The adhesive family has one and two-part systems that provide thermal conductivity of 1.2 to 1.5 W/m K. This family has high dielectric strength (430V/mil to 460V/mil), excellent bond strength, and is available with various cure cycles.

System Cooling The thermal components described above are all oriented toward individual power devices, but there are also thermal management techniques intended for power electronic systems. An example of a system-related approach is IERC's ZIF circuit board retainers. The ZIF retainer is a self-contained, precision assembly that provides an effective thermal interface between a circuit board and cold wall, as you can see in Fig. 6. The retainer consists of an aluminum housing, rod/cam assembly constructed from an aluminum or stainless steel double flat rod extrusion, and beryllium copper spring. Either the pin, hex-head, or screwdriver slot drives the ZIF rod assembly.

ZIF retainers mount to any flat metal surface. The standard configuration attaches with machine screws. However, you can supply the retainer housing with tapped holes for metric hardware, or leave it undrilled with only index pins for vacuum brazing, dip brazing, or epoxy bonding.

Thermal Management Software Probably the biggest changes in power electronic system design over the last five years have involved the use of computers and the Internet. Using these design tools, Flomerics and National Semiconductor have combined to provide WebTHERM[TM], the first online thermal simulator that generates rapid thermal predictions for designers as they select parts from the Web (power.national.com).

WebTHERM allows designers to perform online thermal analysis of their power supply boards, and assess the performance of various National Semiconductor parts. After finalizing the design, the designer can buy a part, a kit, or an entire evaluation board and have it delivered the next day.

The software helps engineers create power supply layout iterations, perform environmental and airflow adjustments, measure heat dissipation tradeoffs, run board-level simulations, and view a full-color thermal image in minutes. It's powered by "SmartPart" technology from Flomerics, which generates accurate thermal models of IC packages through high-level interpretive language.

Another thermal software company, Fluent, has incorporated Computational Fluid Dynamics (CFD) into its thermal management software. CFD enables the study of dynamics of things that flow. Using CFD, you build a computational model that represents a system or device you want to study. Then, you apply the fluid flow physics to this virtual prototype, and the software outputs a prediction of the fluid dynamics. CFD is a sophisticated analysis technique. It not only predicts fluid flow behavior, but also the transfer of heat, mass, phase change, chemical reaction, mechanical movement, and stress or deformation of related solid structures.

Aavid Thermalloy, Concord, N.H.

Thermacore, Lancaster, Pa.

Cool Innovations, Concord, Ontario

Noren Products, Menlo Park, Calif.

Lytron, Woburn, Mass.

Thermagon, Cleveland

Fujipoly, Kenilworth, N.J.

Chomerics, Woburn, Mass.

Thermshield, Laconia, N.H.

IERC, Burbank, Calif.

Flomerics, Southborough, Mass.

Fluent, Lebanon, N.H.

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