Advanced Cooling Technologies (ACT) was selected by a leading company to develop an innovative design that would cool twelve IGBT modules below their maximum juncture temperature. In addition to the junction temperature requirement, ACT had to work within strict size, noise and temperature uniformity requirements as well. As is often the case with paralleling IGBTs, load sharing can be difficult to achieve without intelligent thermal designs.
Using ACT’s in-house code and CFD simulations, the design team paired a custom heat sink design with a fan that easily integrated into the customer’s assembly. The wide fin pitch selected for this design allowed for higher air flow rates from the fans, which was critical to achieving the temperature uniformity requirement. After running thermal simulations on heat sinks with and without heat pipes, it was decided that at this time, the customer would be able to meet their thermal requirements with a custom heat sink without heat pipes.
Utilizing its engineering experience and thermal software capabilities, ACT was able to design the customer a low-cost heat sink with excellent thermal performance. The analysis was also able to show that if IGBT power levels increase in the future, the benefits gained by upgrading this heat sink design to include heat pipes, are already known, and the change can be affected without needing changes to the rest of the system.
The key benefits for the customer of this production ready, fully integrated solution provided by ACT, are the temperature uniformity, low operational noise, and small footprint. Providing guidance for the next level thermal solution, for when power levels rise, is the type of added value gained from working with ACT’s experienced engineering staff.
Rhombus Energy Solutions came to ACT looking to replace their liquid cooling system with a passive heat transfer solution. The primary heat source in the system was a large IGBT module, which produces a maximum of 4kW of heat. The goal was to design a solution that provides cooling to keep the IGBT module case temperatures below 85°C, while fitting inside Rhombus’s existing packaging.
To achieve the program goals, ACT developed a custom loop thermosyphon assembly comprised of a custom brazed aluminum evaporator, microchannel aluminum condenser, and fluid transport lines.
The fluid in a loop thermosyphon is driven around the loop by gravity; this means that the pressure drop of the loop has to be lower than in a pumped system. ACT iterated through multiple evaporator, condenser, and transport line designs to determine the best combination, taking into account the performance, cost, and packaging of each design. Ultimately, the choice of an angled condenser was able to provide a large airflow area, while keeping the overall height of the assembly to a minimum. ACT designed-in flexible lines between the evaporator and condenser to improve the installation of the cooling system. The final solution is predicted to keep the IGBT case temperature below 79°C, providing ample cooling to the customer.
The key benefits of ACT’s solution are an increase in both the reliability, and power consumption, of the cooling system. This will drive up Rhombus’s efficiency rating, and improve the reliability and maintenance requirements for their system.
The power grid is facing ever-mounting challenges as the demand for residential and commercial electricity increases. North Carolina State University is committed to modernizing the electric grid and creating energy leaders. NCSU’s FREEDM System Center was involved in a recent project developing a Medium Voltage Solid-State Transformer (SST). Their goal was to increase energy density, simplify system design, reduce costs, and improve efficiency system reliability. Of course, these drive use and dissipate kW levels of heat so finding a thermal solution that achieved their program objectives was a challenge.
A major thermal puzzle was how to effectively and efficiently dissipate 2-10Kw of heat from up to 6 power modules on a single heat sink assembly. The proposed layout is seen in Figure 1.
For the thermal experts at ACT, there was an obvious solution: ACT’s Loop Thermosyphon. This rugged, reliable and essentially passive solution was specifically designed for these Power Electronics applications.
Working with the NCSU team, ACT designed and developed the Loop Thermosyphon seen below in Figure 2.
The device was capable of dissipating 2 kW of heat and achieved a maximum device junction temperature of 85°C with a coolant temperature of 60°C.
NCSU was delighted with the results and pleased with the performance of working with the ACT. Anup Anurag, a key person on the team, commented:
“In my opinion, ACT was one of the best companies we have worked with (not just in terms of thermal design). We got the products delivered as promised (before the promised time) and they were very helpful and professional in keeping us updated about progress in the design and manufacturing. We will definitely do business with ACT in the future.”
Got a difficult thermal challenge? Give us a call. ACT’s thermal experts design solutions to thermal challenges on projects in diverse markets and has particular expertise with emerging technology for the power grid. ACT is proud to help to create a more reliable technology for the future. Contact ACT for your power electronics and power grid applications today.
Successful Silicon Valley tech startups have high aspirations, tight deadlines and limited resources. For over two years now, ACT has proven its agility in meeting the dynamic needs for one such company.
Smart Wires Inc., an innovative technology company located in the San Francisco Bay Area, has developed a revolutionary product, the Power RouterTM, which adjusts the flow of power across grid lines by controlling impedance levels. This control enables power companies to push or pull power as required, offering customers a more reliable source of electricity. The product utilizes power-electronics devices which need well-engineered thermal-management solutions for enhancing reliability. That’s where ACT came in.
In the beginning, the tech startup brought in 50 experts in some 30 different disciplines from all over the world for an on-site brainstorming session to review the many features of their initial design. ACT was invited as a thermal management expert. From the first brainstorming session, ACT’s role grew. ACT’s engineers took responsibility for both thermal and mechanical design and the manufacturing of critical components. With ACT’s support, the Smart Wires Product Development team completed 11 different prototype designs in just over 18 months. ACT has since delivered over 50 prototypes of the final design. A significant portion of ACT’s work was performed and completed on site at Smart Wires. ACT engineers became true members of the team, even surfing and eating Ethiopian food with Smart Wires engineers.
According to Haroon Inam, Chief Technology Officer at Smart Wires, “We knew in the early development stages that we would need to address thermal issues and it would require a partner with industry-leading thermal-management engineering expertise. Within days of our request, ACT sent out an engineering team to help us pinpoint any thermal issues. They then designed and delivered effective solutions and have become a key part of our team. ACT has been a highly reliable and responsive partner.”
According to Devin Pellicone, Lead Engineer for ACT, “Working with the Smart Wires team has been an exciting challenge for us. They set, and met ambitious deadlines and goals that required us to rapidly develop innovative thermal solutions. It’s been very rewarding to help them position themselves for great commercial success. We believe this work has established a strong collaboration that will only get stronger.”
As the power density of electronic devices and lasers continue to increase, new cooling strategies such as pumped two-phase (P2P) cooling are needed to maintain their temperatures below maximum operating limits and provide a necessary high degree of isothermality to optimize performance and lifetime. ACT has developed bench-scale and standalone two-phase cooling systems that can handle several hundred Watts per square centimeter of thermal power and provide excellent temperature control and uniformity.
Figure 1 shows a stand-alone two-phase cooling system developed at Advanced Cooling Technologies, Inc. (ACT) that is capable of handling heat loads upwards of 300W/cm2 while maintaining tight temperature control and isothermality over the heat transfer area. In addition, the unit can handle multiple, separate, non-uniform and transient heat loads on the different evaporators. As noted, the unit is self-contained and simply has to be plugged in, setpoint temperatures input and thermal loads mounted to the two-phase evaporators. This apparatus has been used to test advanced two-phase evaporators, some of which are quite large and some of which include the use of microporous coatings to improve boiling performance. For more information on two-phase evaporators and the use of advanced boiling enhancement coatings, kindly refer to the following boiling enhancements page.
In the design of the unit, particular attention has also been given to address flow maldistribution between multiple evaporators and flow and thermal instabilities internal to the evaporator and at the system level. Regarding flow maldistribution, it is important since adequate flow to and within each evaporator is essential to avoid local dry-out and overheating. In addition, internal controls automatically adjust the saturation conditions of the refrigerant prior to the evaporator inlet(s) such that the refrigerant boils as it enters the evaporators providing optimal cooling. A two-phase mixture of liquid and vapor exits each evaporator, which is eventually condensed onboard the unit.
For those unfamiliar with pumped two-phase cooling systems, it should be noted that the key components in the system include a pump, preheater, surge tank, condenser, accumulator and evaporator(s). The surge tank consists of vapor and liquid at saturation; by controlling the pressure in the tank, the saturation (boiling) temperature of the working fluid can be controlled. The preheater heats the subcooled liquid exiting the condenser prior to entry into the evaporator(s) to again adjust saturation conditions and achieve optimal cooling performance.
High Conductivity (HiK™) heat sinks can also improve the Size, Weight, and Power (SWAP) compared to standard heat sinks. Placing a discreet heat source on a large metal heat sink will produce large thermal gradients as the heat slowly conducts through the aluminum to the fins. Embedding heat pipes in a HiK™ heat sink can increase the thermal conductivity from around 180 W/m K to 500-1,200 W/m K, providing an opportunity to reduce heat sink plate thickness and fin area. This approach has been proven in a variety of weight/volume sensitive applications including: Ruggedized Electronics, UAVs, Handheld/Portable Devices, LEDs and optical devices.
Embedded heat pipes can improve the performance and reduce that mass of forced and natural convection heat sinks. ACT fabricated a HiK™ heat sink and an all-aluminum heat sink with the same performance; see Figure 1. The total heat dissipation is 150W in both cases. The conventional aluminum heat sink is 12 inches (30.5 cm) long, weighs 9.6 lbs. (4.4 kg) and has a base thickness of 0.6 inch (1.5 cm). Introduction of 5 heat pipes, 3 in close proximity to the heat source and another two a little further out for improved spreading reduced the length to 10 inches (25.4 cm), reduced the thickness to 0.28 in (0.7 cm), and reduced the mass to 6.3 lbs. (2.9 kg) for an overall material reduction of over 34%. Thermal images that demonstrate the improvement are shown in Figure 2. The Hi-K heat sink seen on the right maintains the same source temperature, even though the heat sink is shorter, lighter, and thinner. The improvement is directly attributable to the addition of heat pipes which can be seen as red lines in the picture on the right.
With support from the National Science Foundation, ACT has developed a pumped two-phase cooling system for high heat flux electronic components and laser diodes and is now working on packing the system in a compact, user friendly, stand-alone platform. The two-phase system efficiently handles fluxes on ~ 300-500W/cm2, requires little pumping power, maintains device temperatures below operating limits and provides for a high degree of isothermality over the heated surface. Meeting these requirements is important in many applications including lasers whose emission wavelengths are temperature-dependent.
Unlike single-phase cooling, two-phase systems take advantage of the phase change (latent heat) of the coolant which enables it to handle higher heat fluxes for a given heat load. Two-phase cooling systems can however be more complex and prone to flow and thermal instabilities. As such, techniques to effectively manage instabilities have been developed and characterized. These include the application of engineered microporous coatings on the heated surface(s), which enhance boiling performance by increasing the number of nucleation sites together with a capillary-driven resupply of the coolant to the heated surface, which prevents or postpones dry-out (extends CHF).
A schematic of a pumped two-phase cooling system is shown in Figure 1. Key components include a pump, preheater, surge tank, evaporator (the heat sink), condenser and accumulator. The surge tank and the preheater differentiate this system from a traditional liquid cooling loop. The surge tank consists of vapor and liquid at saturation; by controlling the pressure in the tank, the saturation (boiling) temperature of the working fluid can be controlled. The preheater heats the subcooled liquid exiting the condenser to a temperature close to the saturation temperature before it enters the evaporator. This is important as boiling heat transfer is most efficient at saturation (minimal subcooling).
Figure 1: Schematic of a pumped two-phase cooling system
Also shown in Figure 2 is a representative copper minichannel heat sink coated with a microporous coating.
Figure 2: Minichannel heat sink with porous sintered power coating
The pumped two-phase cooling system shown in Figure1 was fabricated and evaluated. The heat transfer coefficient (HTC) [W/m2K] and the Incipient Wall Superheat [K] were determined as a function of the input heat flux and coolant mass flux [kg/m2s] using refrigerant R134a and others. Representative results for the HTC for coated and uncoated minichannel heat sinks are shown in Figure 3. Clearly, the HTC is higher for the coated heat sinks and the CHF is extended.
Figure 3: Heat Transfer Coefficient (HTC) associated with Minichannel Heat Sinks noting the increase in CHF for coated heat sinks.
The incipient wall superheat is also shown in Figure 4 as a function of input heat flux on coated and uncoated heat sinks. Again, the microporous coating enhances the thermal performance of the heat sinks as evident by lower values of incipient wall superheat; in other words, a heat-generating device mounted on a two phase heat sink with microporous coating will be maintained at a lower temperature compared to one that is mounted on an identical uncoated heat sink.
Figure 4: Incipient wall superheat decreases with the application of microporous coatings
In addition to improving the heat transfer, the thin copper coatings help suppress two-phase flow instabilities. Figure 5 shows flow instabilities with a bare copper cold plate, where the vapor flow occasionally reverses and enters the input plenum. In contrast, instabilities are suppressed in the cold plate with a thin porous coating, see Figure 6.
Figure 5. High heat fluxes with uncoated mini- and micro-channel systems can have unstable flow. Note the intermittent vapor flow back into the inlet plenum.
Figure 6. The maximum heat flux in two-phase systems can be increased by adding a thin porous layer. An additional benefit is that the flow becomes more stable.
In short, pumped two phase cooling systems have been developed. Flow and thermal stability issues were well managed with the use of porous coatings, which increase the heat transfer coefficient and extend the CHF. Pumping power required is minimal and the application of the coating does not increase the pressure drop in a measurable way as the coating thickness is very small compared to the channel dimensions. Additional work on flow boiling heat transfer in minichannel heat sinks is ongoing with a focus on maximizing performance and understanding the parameters (i.e., the coating properties – thickness for a given particle size, etc.) that affect performance for specific applications.
For more information on Pumped Two Phase Cooling, contact The Thermal Experts at ACT.
ACT Offers Customized Heat Pipe Heat Sink Solutions
Advanced Cooling Technologies, Inc. can deliver a customized heat sink heat pipe solution for companies in industries such as aerospace, electronics and many others. As a premier thermal management solutions provider with a strong focus on research and development, we can provide heat pipe heat sinks for your specific applications.
We can also handle every aspect of your heat pipe heat sink project from conception through production. As an ISO9001:2008-certified company, you can also count on us to deliver a high-quality heat sink product that will exceed your expectations.
We Take a Problem-Solving Approach to Heatpipe Heatsink Design
At ACT, we take great pride in our ability to solve problems for our customers. Here’s just one example of how we have helped our customers overcome a challenge:
A customer was designing a power module consisting of a series of IGBTs. They were seeking a heat sink design but had many constraints and requirements. In addition to power dissipation (almost 6kW during transient, up to 2.4KW steady state), cooling methodology, heat sink size, IGBT orientation and overall ruggedness requirements were all fixed.
ACT Provided an Effective Heat Sink Pipe Solution
To address the problem, ACT used in house developed analytical software to identify the optimal fin solution for the given geometric and flow constraints. This modeling provided a convection resistance, which was then incorporated into the FEA thermal analysis that included the electronics, thermal interfaces, and heat spreader. After a series of design iterations using strategically located vertical and horizontal heat pipes, a final prototype design proved feasible, satisfying the various performance objectives. Customer was so impressed with the thoroughness of the study that a second project to design and build prototypes was initiated.
Temperature Profile of Power Module with IGBT’s mounted on proposed heat sink design platform, operating under defined power dissipation operating conditions.
Contact Us to Learn More About Our Heat Pipe Heat Sink Design Process
With more than a decade of experience in solving difficult problems for customers in a wide range of industries, ACT can help you meet thermal management challenges in your operation. We’ll work closely with you to develop a customized heat pipe heat sink solution for your applications. Contact us today for more information or to schedule an on-site consultation.