Passive Thermal Technology:
Small Solutions for Big Challenges
It’s no secret that electronics systems have provided exponential increases in power and capabilities, along with steady decreases in size and footprint, for designers in applications such as power control systems, industrial drives, radar systems, avionics and many other semiconductor-based systems. Revolutionary increases in power and control have made possible ultra-precise performance in semiconductor devices such as thyristors, insulated gate bipolar transistors (IGBTs), symmetric gate commutated turnoff thyristors (SCGTs), silicon controlled rectifiers (SCRs) and intelligent power modules (IPMs).
Perhaps the most impressive and demanding arena for modern electronics is the world of military applications, where electronics must not only operate flawlessly in harsh environments but also meet demanding size, weight and power (SWaP) constraints in current and future generation equipment. Systems such as naval onboard power conversion systems and aerospace power electronics offer a wealth of tough challenges: rugged environments involving exposure to salt water, heavy weather, extreme temperatures and vibration; extremely high power loads (sometimes in excess of 3,000 W); high heat fluxes due to concentrations of electronic components; and sometimes the need to manipulate coefficients of thermal expansion (CTE).
The challenge, however, is that increases in power and decreases in size also require designers to deal with tremendous increases in heat generation and heat fluxes, all within the context of smaller and smaller areas for thermal solutions. Thus, the combination of smaller footprint (thanks to increased miniaturization) and greater power makes life much more difficult for thermal engineers who need to develop solutions that protect the life and performance of these small wonders of technology. The challenge of increased heat loads isn’t just based on the amount of space available for thermal solutions. The configuration and the location of powerful electronics systems also play a role in making solutions more difficult. Designers need to consider:
• Configurations that require an extremely low profile for thermal solutions
• Competing for available space with other electronic components nearby
• Operation in oddly shaped or situated spaces
• Near maintenance-free operation (in satellites, for example)
• Thermal solutions that can operate independently of gravity
• Resistance to shock and vibration
• Quiet operation
• Avoiding drawbacks associated with liquid-based cooling (pipes, storage of chemicals, short circuit potential, ongoing or continuous maintenance is required, high cost, etc.)
Military Applications: Pushing the Thermal Envelope
Military electronics cooling offers a full menu of challenges like these, which is why thermal solutions are constantly evolving in these applications. The unique demands of cooling military equipment have all but removed a number of potential solutions from the thermal engineer’s “tool kit.” This includes conventional extruded heat sinks and traditional copper or aluminum heat spreaders, which may not be optimal under demanding conditions due to the need to isothermalize and remove “hot spots.” Space and configuration limitations also reduce the options available to thermal architects, because it may not be feasible to alter the geometry of the heat sink or increase the size of airside fins, which means the efficiency of the fin must be increased to compensate for relatively small size.
Additional challenges facing designers of state-of-the-art (SOA) heat sinks include low air penetration into heat sink surfaces, low thermal conductivity of many heat sink polymers and thermal barrier development on heat sink surfaces at high temperatures. Another option that creates its own set of challenges is pumped liquid or refrigerant-based cooling systems. These can be very efficient but add complexity, weight and the potential for failure, particularly in the context of six-sigma mission failure rate requirements and applications where maintenance is difficult or impossible. Air cooling-based solutions are also more environmentally friendly and have less risk of chemical contamination or interactions.
Components such as pumps, compressors, nozzles and other liquid cooling components can also add significant cost. The emerging need is for more and more efficient thermal management to deal with increasing amounts and concentrations of heat, in situations where the operating environment is making increased thermal efficiency more and more difficult. The sheer number of these military applications is also increasing as military systems evolve toward more-electric, more electronic and more-miniaturized designs. Virtually every new generation of military equipment creates additional electronics cooling needs: more sensitive transmit/receive radar modules, more powerful infrared cameras that generate more heat, Humvee electronic displays that need to function perfectly in desert climates.
The challenge is further increased by the specific configuration requirements found in military specifications. The Navy specification for nuclear submarine power conversion demanded, for various design reasons, that cooling fins be placed above the heat sink to cool a 2,900 W electronic module. Clearly, the stage is set for a new generation of passive thermal solutions (heat sinks, heat pipes, heat spreaders, including APG) that combine a number of thermal concepts in imaginative ways.
The Passive Cooling Alternative
The traditional heat pipe, a two-phase heat transfer system with a high effective thermal conductivity, is still a solid solution for military cooling applications -- but as a starting point, a foundation for modified passive systems that combine two or more concepts for efficient cooling. Designers can benefit from the inherent advantages of passive cooling (lack of moving parts, capillary action to move condensed working fluid instead of relying on pumps, ability to work against gravity) while getting additional, specialized performance advantages by modifying and combining thermal technology. Some examples of these novel solutions include:
• Combining heat pipes with a remote fin stack. This solution uses standard heat pipes embedded in a metal plate under power semiconductors. The heat pipes are extended from the plate to the fin stack, which can be placed anywhere outside the electronics for added flexibility.
• Embedding the heat pipes. Designers can get up to 20 percent greater thermal performance compared with typical aluminum or copper base spreaders by embedding heat pipes within a heat sink. This approach is ideal for applications where the heat source is small relative to the area available for fins. Design options are maximized by the fact that embedding heat pipes does not require changing heat sink geometry.
• Changing the thermal interface. Some types of heat pipes can be mounted to a heat sink base in a variety of ways including direct soldering into grooves on the base (in direct contact with the heat source) or embedded into holes to provide an efficient metal-to-metal interface. Heat pipes using a sintered powder wick structure are well suited for this approach because they can be shaped and bent to fit the tight or irregular spaces found so often in military applications. Heat pipes can be soldered or adhesively bonded to the metallic sink for optimum thermal contact.
• Use a hollow plate as a heat pipe. A plate style heat pipe can be applied to the base of a heat sink, allowing vapor to transport heat uniformly throughout the entire base plate surface. One variation of this approach, particularly valuable for cooling increasingly-powerful IGBT modules, is to build the plate heat pipe/heat sink combination as the module’s base.
• Improve the performance of extruded heat sinks. If embedded heat pipes are not feasible, specially-designed extrusion methods can produce thinner fins with special high-performance surfaces and greater length-to-thickness ratios. This allows higher fin density within any given space and enables the designer to retain the extrude heat sink concept.
• Use advanced materials. The capabilities of annealed pyrolytic graphite (APG), used as an insert within an encapsulating structure, are still being explored in military, aerospace and many commercial applications. Conventional thermal management materials (aluminum and copper alloys, ceramics, composites) can be used to surround APG and provide a lightweight, passive heat transfer system. Highly conductive thermally, this allows designers to tailor coefficients of thermal expansion (CTE) as needed. APG-based systems provide gains in thermal conductivity (W/mK) when APG is added to almost any commonly used thermal material (See table).
• Add electronics to help cool electronics. One recently developed heat sink concept relies on piezoelectric driven polymer plate agitator (PEAT) technology. PEAT plate agitators are bonded or built onto the surface of a substrate to support thin polymeric microstructures that serve as agitator flaps. Piezoelectric components respond to electric impulses that cause the polymeric plates to vibrate rapidly between two of the thin metal fins. This rapid movement causes surface pressure fluctuations and turbulence inside the spaces enclosed by the heat pipe fins, which break up the thermal boundary layer that can reduce cooling efficiency. As demands for high-heat-flux cooling become more exacting, hybrid passive-electronic solutions like this are likely to become more common and even more creative.
In applications such as military electronics cooling, the demands created by increasing power, increased heat fluxes and greater miniaturization will continue to increase. Technological advances such as APG-based solutions and PEAT-based solutions can provide viable design options in many cases. But for a number of reasons, including the need to minimize cost and complexity, traditional passive cooling approaches such as heat pipes will still be the preferred solution. The key will be innovative approaches that combine the benefits of two or more passive thermal systems (heat pipes, heat sinks, heat spreaders, including APG) in new and different ways. Versatility, the ability to develop a wide range of solutions that combine passive thermal technologies and blend traditional with advanced materials, is likely to be the largest factor in helping thermal engineers stay ahead in the ongoing race between increasing power/heat generation and effectivecooling.
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