by Timothy Wooley© 2007 Payne Engineering Co.All Rights Reserved

An Aerospace Approach to Heatsink Design

With microprocessor technology heating up the manufacturing industry, heat dissipation is becoming the new rocket science. Unfortunately, many commercially available heatsinks are poor substitutes for a quality heat exchanger. Microprocessor heatsinks may need to dissipate 30 watts, but what if you need to dissipate 250 watts? Or 2400 watts? With 100% reliability?

All silicon devices experience a forward conducting voltage drop of roughly .5V to 2V across the semiconductor device. If not for this forward voltage drop, we would not have any heat to exchange. For microprocessors running low amperes, the heat can be discarded relatively easily. But what about power silicon? Thyristors switching and controlling hundreds of amps at 480V or higher can generate serious heat. This heat must be exchanged reliably and it will take more than an off-the-shelf heatsink to accomplish the task.

Thyristors are employed in solid-state power controllers from 1kW to 1.5 megawatts and have been in use since the 1960’s. SCR-based devices are rapidly replacing electro-mechanical contacts and motor starters in nearly every industry. Solid state power control is clean, precise, sophisticated technology with a central theme: heat dissipation. The heart of the SCR is the silicon pellet, the reason for temperature concerns. The ampere capacity of any SCR is approx. inversely

proportional to the temperature of the silicon itself(over 1000C case temperature). The critical task then becomes reliable cooling of the SCR.

Heat transfer can occur in three ways:

Radiation: heat transferred, usually into the atmosphere, by electromagnetic waves leaving the surface of an object. Conduction: heat transferred through a solid mass.Convection: heat transferred to a moving fluid, usually air, by a process of heated air being replaced by cooler air either naturally or by force.

Every heatsink requires conduction as the first step in dissipating heat. Clean contact with the device that is generating the heat, in this way the heatsink acts as a conveyor carrying the heat away from the silicon device. Radiation plays less of a role in many heatsink designs simply because convection is where most of the gains are made in heat dissipation technology. Heatsinks are rated on how efficiently they can dissipate heat. This rating is referred to as 0C/watt. Simply put, if a 50w thermal load is applied to a heatsink, which raises the heatsink temperature to 250C, then the heatsink rating equals 0.50C/watt. The higher the number, the more efficient the heatsink is at dissipating heat.

Our company’s research team has been designing and testing heatsinks since the

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1960’s. By applying laminar flow fluid dynamic principles, Payne Engineering has developed proprietary heatsinks for every single power control unit they produce. Extensive testing has yielded interesting results in the area of natural convection. The best heatsink designs flow up to 1 meter/sec of air, high speed in natural convection terms. Laminar flow describes the condition of the boundary layer of the air as it passes over the surface of the heatsink. As the term suggests, the airflow is smooth, undisturbed and flows in constant contact with the surface of the heatsink material or coating, therefore maximizing heat transfer. This flow pattern is very important to the operation of the unit and anything that could possibly disturb the flow of air is taken into

consideration in the design, layout and construction of every

unit manufactured with this form of heat transfer.

Surface coatings are often misunderstood or simply overlooked

with common industrial heatsinks. Many companies offer anodized coatings to improve appearance and in some cases anodizing is even touted to improve

performance. Although a black body will radiate heat most

efficiently3, anodizing actually inhibits heat dissipation by placing an aluminum/copper oxide barrier between the heatsink material and the air. Black oil-based lacquers actually improve heat transfer by placing a thin layer of thermally conductive paint on the exterior surfaces of the heatsink and improving surface emissivity2.

Laminar FlowPassive Cooling &

Forced convection is based on fans with

a limited operation of typically 30,000 to 60,000 hours in a favorable environment. Longer life fans can be purchased at much higher cost. 8,760 hours yields roughly 1 year of service. Our company’s units have been in service for 10, 20 or 40 years with no fans in place. If a solid state device is designed to operate reliably only in the presence of forced convection cooling, then you have just introduced the only moving part into a completely solid state device, with little or no margin for error, fans fail. If the unit is designed to run safely only with a working fan, you could easily overheat a semiconductor when the fan does eventually stop working. Natural convection on the other hand relies solely on the natural laws of physics to ensure safe and proper cooling of semiconductor devices, with no moving parts.

In 1988, the Boeing Company

experienced first-hand the dynamics of a natural vs. forced air cooled solid state system. Following the unscheduled replacement of roughly 4000 flight data computers due to high incidence of failure, Boeing determined that the cooling system, which relied on forced air, needed to be replaced by a passive cooling system, which circulated air by natural convection1. The old system, by relying on forced air cooling, had introduced far more components with no improvement in overall system reliability. Aircraft cooling systems were steadily increasing in size and complexity with early systems weighing just 30lbs. Later model cooling systems (1980’s) had reached nearly 600lbs. and required more electricity to run them. Boeing called this “new” passive cooling system “buoyancy-induced convection”. With much attention to detail, Boeing successfully determined the many design changes required when using passive cooling. Changes that are often overlooked are environmental changes, such as component placement and mounting all circuit boards vertically. By placing the hottest components at the bottom of a vertical circuit board, they have the first contact with cooler air. By paying attention to detail and creating efficient natural convection designs, Boeing successfully redesigned a critical flight control system to run cooler, longer, and with no fans. Designers are

no longer limited in placing avionics computers only where there is access to mechanically cooled or forced air. Boeing saw the advantages of natural convection cooling and realized its application to the next generation of “integrated avionics approach”.

Unfortunately most power control companies prefer to rely on limited-life, low-cost fans with lifetimes of 3-4 years to ensure continued operation of an SCR controlled device. The same companies that may rely on computer models such as Computational Fluid Dynamics(CFD) to tell them if their heatsink is adequate. Recently we put some of these companies offering CFD software to the test. All of our heatsinks are designed and tested in ambient temperatures of 200C – 240C in a typical industrial enclosure in an industrial atmosphere. We asked a number of these software companies, specializing in electronic heat transfer CFD, to put our simple 4 inch channel design heatsink(26A-2) in their computer to see how accurate their $10,000 per year software is. After 10 years, and 6 companies efforts, they are unable to come within 10-15% of actual temperatures of a 15 amp heatsink. Worse still some machine run times exceeded 10 minutes. That is too long. We can physically change a heatsink test in less time and at no cost. To use a computer in designing an aluminum or copper heatsink would be very helpful to ensure you did not have to spend the money for an extruder die change. The study of laminar airflow is more difficult because of the low natural convection velocities up to the transition from laminar to turbulent flow(this normally occurs at a Reynolds number less than 1,000,000). Most CFD heat transfer programs are assuming forced airflow.

One must be very careful with today’s heatsinks as sold in catalogs such as Allied and Newark, they should be tested to corroborate manufacturer’s advertised ratings. We have tested these devices, and some are not even close to advertised ratings. If you look at a picture of a

heatsink and the fins are rectangular, by inspection you can assume they are not efficient. This does not apply to closely spaced fins (>2mm) intended for forced air.

These well-known laminar flow

principles of construction have been developed over nearly 50 years(NACA laminar flow airfoils were first applied to the P-51 before WWII) of SCR power control manufacturing. Our

heatsinks have been developed right alongside our controls for minimum weight and maximum heat transfer. Our latest heatsink, the Model 26A-16, is the first heatsink design for our company to use CAD with iteration. This has allowed us to have nearly uniform heatsink surface temperatures top and bottom of a 12 inch long, 2 inch wide heatsink up to 150 amps. For more information on Payne Engineering and our products visit:

www.payneng.com.

Norwall, Bruce D. “Boeing Studies Passive Cooling For Next-Generation Avionics”. Aviation Week & Space Technology, January 4, 1988.General Electric. SCR Manual, 1979.McAdams, W.H. Heat Transmission, McGraw-Hill, 1954.

© 2007 Payne Engineering Co.All Rights Reserved

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