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Power Electronics

Proactive Thermal Management: The Right Time Is During Design

Superior thermal management has often been an afterthought when bringing a new product to market. It only rushes to the forefront when initial testing reveals product damage, performance degradation, or increased injury risk due to unsatisfactory heat and energy management. The usual response is to design a retrofit, which should be understood for what it is—a last minute compromise to make the product usable. Retrofits generally involve compromising one aspect of a well-designed solution leaving another aspect partially solved.

Design compromises may at first glance appear to be fairly minor, such as increasing either airflow or the size of the heat sink, relocating the heat sources, or requiring less power to be dissipated. However these presumably “easy fixes” have become daunting last-minute questions and challenges:

  • If more airflow is needed, is there physical space for bigger fans?
  • Does the electrical power budget handle them?
  • How can heat sources be relocated after packaging has been defined?
  • Is there space for a bigger heat sink?
  • Can wiring be rerouted to handle changes?
  • Does less power mean unacceptable performance loss?

Designers may be able to provide an adequate first option, but they are likely to encounter other issues that begin to cascade and propagate with each fix (Fig. 1).

Instead of implementing “best practices” in solving problems, retrofits are, in actuality, “best attempts” at lesser solutions. Retrofitting certainly can create technically adequate fixes, but incorporating them may create a burden in cost and time for manufacturing. There may be a negative impact on the “difference maker” that made the product appealing in the first place.

What is required instead is a greater focus on thermal management during the design stage well before product testing occurs. It is not enough for a product to perform a function; the product must perform the function efficiently. Accordingly, an increasingly solid design goal is to focus on the concentration of energy use that goes beyond simple dissipation of excess heat because products that do not efficiently use energy are viewed as inferior. The challenge is to repurpose energy as it changes state within the product. Designers are called upon to get as much useful work as possible out of the energy they put into a product. No longer is “blow more air over it and dissipate it to the environment” an acceptable design even for a retrofit. Engineers and designers have more sophisticated modeling and analysis techniques which allow them to nip heat and energy issues in the bud before they surface in the testing stages.

Avoid Thermal Management Issues

As countless test results have verified, product performance tends to vary, sometimes unpredictably, from original design requirements as the product transitions from drawing board to prototype to finished product (Fig.2). One of the more frequently encountered problems is degradation of performance or even catastrophic damage caused by heat because the system has failed to manage heat or excess energy sufficiently.

When a product is unable to perform at its optimum due to heat that can damage internal components, the device’s performance may be limited at best or rendered ineffective at worst. This can be a serious issue, which can lead to significant delay of the product launch.

Another common problem is an unanticipated change to the local environment, which places different conditions on the product than were originally accounted for during the design stage. Components of a product are sometimes designed and specified for overall system requirements, but independent of integration effects within the system. At the component level, all parts function properly, but when combined at the system level, these conditions tend to result in product failure. In fact, there are a number of common failure points occurring in either fit-up or testing. Among them are misalignments, shock, vibration, freezing effects, cycling effects and component designs that fail at the system level. The product’s environment size has also led to other unpredictable occurrences stemming from improper orientation, incorrect assumptions rendered during the design process, material incompatibilities, manufacturing variances and tolerance stack up issues. There are too many unknowns that could have and should have been considered had the designers and engineers given thermal management a greater priority during the design stages.

Advancements in the analysis capabilities of thermal management have enabled designers to come to grips with many of these issues before they become problems. The exponential growth of computer power has led to development of more sophisticated 3-D modeling as well as more powerful and reliable analytical tools (Fig. 3).

Flow Analysis

Detailed transient flow analysis is now commonly used in addition to static analysis at equilibrium conditions. This allows the designer to look for unexpected issues caused by time-related events, such as time lags between flipping on a system switch and that system being in thermal equilibrium.

What this means is that designers now have the technology to create “what if” scenarios rather quickly and with reasonable accuracy. By combining experience, improved analytical tools, and improved computer processing power, today’s designers should be well equipped to identify and prevent potential issues from occurring.

Yet there still appears to be hesitance if not reluctance to incorporate thermal management techniques during the earlier design periods. Why?

One reason is the cost factor for toolsets and hiring appropriate personnel who are thermal experts. Costs in the manufacturing environment tend to impact everything from setting tolerance limitations to reproducing a component that retains design intent and accuracy but costs less to manufacture. Yet compare the costs of the technology with those unexpected expenditures necessitated by a product retrofit that might have been avoided. In many cases, the latter represents a significant burden on a budget that is probably being squeezed due to increasing development costs, while investment in the former may save the company significant money in the long run.

One can reasonably argue that money is the principal driver behind the growth of utilizing thermal management early in the design process as designers recognize substantial cost savings by being proactive instead of reactive on the issues of heat and energy. It is certainly easier and cost-effective to modify a design and run those “what if” scenarios electronically than to conduct a trial and error experiment that could lead to a retrofit situation.

The virtual world in which the scenarios are created is far less costly than the real world and its numerous pitfalls: tooling issues in production and testing; schedule slippage due to time constraints; loss of time due to fabrication of an incorrect part, and material waste. The project timetable and probably the entire shop will be set back when tools have to be taken off line because an overlooked energy management issue caused the product’s performance to fall unacceptably short.

Examples of Thermal Management Design

Understanding the thermal issues of a product and managing them from the very beginning is not limited to the theoretical. Here are some actual examples of electronic planning resolving some serious heat and energy issues in the design stage before they became potentially costly and disastrous during testing.

Optimizing a large heat sink with multiple heat pipes connecting a large fin stack to an input pad. Each individual heat pipe is time consuming to process. This specific example is challenging because of high power dissipation levels. More pipes coming into the same base plate mean more difficulties with fabricating and soldering such as increased risk of solder voids and limited surface area contact into the heat pipes. The result is a greater possibility of an unacceptable and excessive heat level.

The thermal management solution was to run CFD analysis of the fin stack along with several “what if” scenarios (Fig. 4). Data from the analysis and subsequent models revealed that the heat sink could manage heat efficiently and effectively by using four less heat pipes. That decision not only reduced the cost and the fabrication time significantly but also increased the yield rate—an outcome certain to be welcomed by everyone from engineers to designers to chief financial officers.

Heat sink assembly and a limited space envelope: The production challenge for this product was to maximize the number of fins and fin spacing. In recent years, resolution would have not occurred in advance; it would have depended on the outcome of tests—always a costly option. However, as was the case with the previous example, the use of CFDs and hypothetical scenarios offered solutions for optimizing fin spacing and airflow without deleterious impact from either heat or energy. There is little doubt that without the CFDs, the heat sink assembly design might have had to return to the proverbial drawing board with increased cost as a likely consequence.

Implications of Thermal Failures

There are too many instances in which the inability to manage a product’s heat and energy through advance planning have led to degradation, failure and prohibitive costs. Here are few that occurred in recent months:

Three chips valued at $25,000 were consistently overheating and previous efforts at dissipating the heat were unsuccessful. Continuing efforts to cool the chips and resolve the product issues became a drain on the company’s budget. Eventually a retrofit was designed enabling efficient heat dissipation and energy transfer through thermal management technology—the same software that might have forecast the problem before it occurred.

In another case, a computerized part overheated leading to a fire that destroyed a vehicle valued in excess of $100,000. A retrofit had to be designed to cool the part since the other vehicles using it were at risk. The technology enabled a solution but the cost to the company was significant. Engineers revealed that heat and energy control had not been thoroughly examined during the part’s design. System level impacts had been overlooked.

Perhaps one of the worst and certainly most frightening examples of the consequences for overlooking the pitfalls of excessive heat occurred when a medical device overheated and failed during an operation on a patient who fortunately survived. The company that produced the device paid heavily not only for the emergency retrofit but for lost revenue due to machine failure. Investigation revealed inadequate planning for the impact of residual heat on the product’s performance.

The lesson from these three examples is clear. Thermal management has to be a goal from the very beginning and not an afterthought. Product failure is the worst possible time to explore thermal alternatives.

The Importance of Energy Transformation

One of the newest and most beneficial aspects of thermal management technology in the design stage is energy transformation, the most efficient use of heat because it is repurposed instead of dissipated into the environment. To understand its ramifications, one need look no further than the Second Law of Thermodynamics, which states that energy becomes less useful with each change of stat. With today’s emphasis on green energy, it makes little sense during design to accept dissipation of heat and energy as the ultimate goal. Perhaps that explains why public utilities, whose interest in energy transformation should be obvious, see the wisdom of using thermal management techniques to bring power into the grid.

However energy transformation as an important component of thermal management does not have to be limited to large-scale environments. The technology is advancing to the point that even smaller components offer opportunities for the dissipation of heat from a product component and its transformation elsewhere. It is another example in which green technology goes beyond the idealistic to the reality of reducing a carbon footprint through maximum product efficiency, a preferable alternative than simple dissipation and wasted energy. It is an area of energy dynamics that will offer even greater opportunities for heat management regardless of product size.

Conservation of energy is a primary design goal, but it’s just one of the reasons proactive thermal management is so important. The time and costs necessitated because of reactive thermal management, i.e. waiting for product testing before initiating changes and/or retrofits, are unnecessary and easily exceed the cost of incorporating heat dissipation and transformation as integral components in the design stage.

Customer Thermal Performance Requirements

Higher power requirements, smaller product footprints, more demanding environments and less access to traditional cooling methods have all made thermal management critical to the success of a project. Thermal success used to be based on the extent that a product was neither hindered nor damaged by excessive heat. Now, however, with greater emphasis on repurposing energy instead of just dissipating it, thermal management has become a positive differentiator demanded by the customer. Thus, engineers to account for thermal performance in the early stages of the design cycle.

The ability to manage all of these thermal issues from the beginning of a project is key to a successful, cost-effective product that does more than arrive in a timely manner to the marketplace; it differentiates the end user as someone who is energy-aware and a positive steward of our resources.

Dennis Scott is thermal solutions manager for Noren Products, Menlo Park, Calif., Tel: (650) 322-9500.

Related Articles:

Vapor Chamber Improve Cooling of Power Semiconductors

Thermal Management Tools Face Cooling Issues

How Hot is Too Hot?

An Active Approach to Cooling High-Power Applications

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